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Photoionization of silicate glasses exposed to IR femtosecond pulses
Journal of Non-Crystalline Solids 253 (1999) 58±67
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Photoionization of silicate glasses exposed to IR femtosecond pulses O.M. E®mov, L.B. Glebov *, S. Grantham, M. Richardson Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, FL 32826, USA
Abstract Photoionization of alkali-silicate, boro-silicate, lead-silicate and photosensitive multi-component silicate glasses has been studied under exposure to infrared femtosecond laser pulses at irradiance below the thresholds of laser-induced damage and catastrophic self-focusing. It is proved that the supercontinuum that is generated in all glasses studied as a result of the femtosecond laser pulses spectral broadening extends up to the short-wavelength part of the ultraviolet region of spectrum even if the glass is opaque in this region. It is shown that photoionization results from absorption of the short-wavelength component of this supercontinuum generated by infrared radiation. This ionization leads to the color center formation and luminescence in the bulk of glasses studied. Photoionization of photosensitive multicomponent glasses leads to the creation of latent image and to the refractive index changing in exposed area after thermodevelopment. Fused silica exhibits the same spectral broadening too but color center formation in the visible was not recorded at irradiance up to the laser damage threshold. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Photoionization of glasses leads to an appearance of mobile charge carriers and as a result of this process to the photoconductivity, color center formation and luminescence. In Ref. [1] (Chapter 5) it was mentioned that coloration of alkali-silicate glasses could be caused by exposure to ultraviolet (UV) radiation in the region of 200±230 nm. Later it was proven [2] that for sodium silicate glasses the generation of the color centers is a result of the photoinduced transition of electrons from the valence band to the levels situated above the electron mobility threshold, and established [3] that the fundamental (intrinsic) absorption edge of
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these glasses is in the region of 6 eV (200 nm). It can be observed in glasses if transition ions concentration does not exceed 1 ppm. In the other case, absorption band of trivalent iron, with a peak at 5.5 eV (225 nm) [4], and three absorption bands of divalent iron, with peaks at 4.4, 5.1 and close to 6 eV (280, 240, and 200 nm) [5] mask intrinsic absorption and form the UV absorption edge of glass. A model according to which the fundamental absorption edge of these glasses is formed by transitions between levels in complexes of ºSiÿOÿ . . . Na (L-centers), was developed in Ref. [6]. Excitation into the fundamental absorption region results in intrinsic luminescence with a peak in the 3.4 eV (365 nm) region and a decay time of 1 ls [7]. A study of color-center formation [2,8] and the internal photoelectric eect [9] have shown that the threshold of charge-carrier mobility in alkali-silicate glasses placed in the region of the fundamental
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 4 3 - 9
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absorption edge. Thus, a term `mobility threshold' in these glasses is similar to a term `bandgap' in crystals (Ei Eg ). Exposure of glasses to radiation with photon energy less than intrinsic absorption edge cannot cause photoionization of glass matrix, and the corresponding photochemical processes do not occur. The evidences of the glass matrix ionization are intrinsic luminescence and hole color centers generation. It should be emphasized that the photoinduced processes, which are not followed by these phenomena, are the result of ionization of dopants or impurities that only leads to their recharging and electron color center formation. It was shown in Ref. [10,11] that alkali- and boro-silicate glass exposure to powerful pulsed laser radiation with Ei /2 < hm < Ei causes twophoton ionization of glass matrix followed by intrinsic luminescence and the same color centers as after excitation in the intrinsic absorption range. It is important that in this case the color centers formation eect is de®nitely recorded at power density of q 10ÿ3 q0 (q0 is the optical breakdown threshold of glass), while the fundamental luminescence is recorded at q 10ÿ4 q0 [12]. Exposure of glasses studied to radiation with photon energy hm < Ei /2 does not cause these eects even at q 0.98q0 , i.e. it is impossible to observe threeand more photon ionization in alkali- and borosilicate glasses. It was found in Ref. [13] that the long-wavelength boundary of carrier mobility in lead-silicate glasses is placed considerably higher than the intrinsic absorption edge (Ei > Eg ) and was proven that coloration is resulted from three-photon excitation which occurs through virtual states that are located in the fundamental absorption region. This process in lead containing glasses was possible because of very high density of intrinsic levels of glass matrix below the mobility threshold, which increased dramatically the probability of three-photon absorption in comparison with other materials. Another type of ionization is observed in photosensitive silicate glasses doped with silver, cerium, ¯uorine, and bromine that named photothermo-refractive (PTR) glasses. These glasses are used for Bragg gratings recording in their bulk [14] because they can change their refraction index
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under exposure to UV radiation followed by thermal development. The simpli®ed description of processes that occur in these glasses and thermal development is following (for example, see [15,16]). The ®rst step is photoionization of cerium ions under exposure to UV radiation in region 300±400 nm (the band of Ce3 ion absorption). The electrons released from the cerium ions are then trapped by silver ions. As a result, silver is converted from a positive ion to a neutral atom. These atoms possess a very high diusion rate in silicate glasses and create tiny silver particles that can be considered similar to a latent image in a classic photographic process. The further, higher temperature heating of an exposed glass sample causes precipitation of sodium and ¯uorine ions on the nucleation centers (silver particles) yield crystals of NaF. The refractive index of crystals grown is signi®cantly smaller than that of the base glass (1.32 and 1.49, respectively). Consequently, the refractive index of the exposed fragment of glass is dierent from the unexposed region. Thus, photoinduced processes in PTR glasses are the result of photoionization of dopant ion of Ce3 . No two-photon absorption was found in similar glasses doped with Ce3 exposed to nanosecond laser pulses at 630 nm [17]. Thus, all silicate glasses mentioned have longwavelength boundary of linear or nonlinear photoionization placed in UV or visible region of spectrum: k < 400 nm for alkali-silicate, boro-silicate [10,11], and PTR glasses [14], and k < 600 nm for lead-silicate glasses [13]. Nevertheless it was reported recently [18±22] that photoionization of silicate glasses is possible under infrared (IR) high-power femtosecond pulses (850 nm). Authors of [20±22] supposed that multi-photon ionization was responsible for observed phenomena of photo-refraction in glasses. The same supposition was made in Refs. [23±26] to explain the features of laser-induced breakdown in glasses exposed to femtosecond IR pulses. According to [18,19] the photoionization in sodium silicate and boro-silicate glasses results from supercontinuum (`white light') generation in the bulk of exposed glass and are followed by single- and two-photon absorption of the short-wavelength part of this supercontinuum.
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The ®rst work on supercontinuum generation in crystals and glasses was performed in 1970 [27,28]. It was found that from ®ve to ten small-scale ®laments occurred in transparent dielectric under excitation by picosecond pulse at 530 nm. The irradiance in these ®laments could achieve 1013 W/ cm2 . In this case, intense radiation in a wide spectral range from 400 to 700 nm was observed. Similar phenomena were obtained under excitation with other wavelengths [29,30]. A description and explanation of the main mechanisms of this process were presented in [31,32]. White light generation was considered in Ref. [31,32] as a substantial spectral broadening of the propagating beam both in the long-wavelength and in the short-wavelength regions of the spectrum due to self-phase, induced-phase, or cross-phase modulation. Optical glasses, especially silicate glasses, are the principal optical media for laser applications that include femto- and picosecond devices and they are promising materials for photo-induced fabrication of optical elements. It is important to know the stability of the optical parameters of glasses under femtosecond IR irradiation as well as to understand the mechanism of their ionization under these conditions. In this paper we study alkali-silicate, boro-silicate, lead-silicate, and PTR glasses exposed to femtosecond IR pulses. The goal of this work is to examine the mechanisms, which are responsible for the photoionization and color centers formation in all mentioned glasses at laser pulse intensities below the threshold for catastrophic self-focusing and laser-induced damage.
synthesized of high purity materials by a laboratory method [33] which kept the concentration of absorbing impurities in the range of 1 ppm. The latter glasses were melted in an electrical furnace in fused silica crucibles. The samples were of 2 ´ 2 cm ´ 0.6 cm in size. All sides of the glass samples were polished in order to observe photoinduced coloration and luminescence in dierent directions. The laser used for this investigation was a 100 fs, Cr: LiSAF system operating at a wavelength of 850 nm [34]. The laser consists of a Kerr lens modelocked Ti: sapphire laser followed by several stages of ¯ashlamp-pumped Cr: LiSAF ampli®ers in chirped pulse ampli®cation architecture. For these experiments the single pulse output of the ®rst stage regenerative ampli®er was compressed to a pulsewidth of 100 fs. Pulse energies of 2±3 mJ were used for these experiments. The size of the beam entering the compression gratings was 25 mm diameter. Each compressed pulse was bandwidth-limited with a Gaussian (FWHM) spectral width of 11 nm. The laser operated at a frequency of 6 Hz. The output of the laser beam (Fig. 1) was focused in air by a lens L1 (from 10 to 100 cm focal length) to a point located at speci®c distance in front of the sample S to exclude the increase of irradiance with distance due to self-focusing. In some experiments the beam was focused in the bulk of samples and self-focusing was observed under high-power radiation. The spectrum of the light transmitted through the sample was monitored with a 25 cm grating monochromator M having a resolution of 10 nm, and a photomultiplier tube PM with a sensitivity which spanned
2. Experimental We studied samples of a few types of commonly used commercial silicate glasses: fused silica, leadsilicate glass TF10 (Russia), and boro-silicate glasses K8 (Russia) and BK7 (Schott). Besides, two types of specially synthesized silicate glasses were used: a photosensitive PTR glass of approximate composition (mol.%) 15Na2 O±5ZnO±4Al2 O3 ±70SiO2 ±5NaF±1KBr±0.01Ag2 O±0.01CeO2 [14], and a sodium-silicate glass 22Na2 O±3CaO±75SiO2
Fig. 1. Experimental setup for glass samples exposure to IR femtosecond laser pulses and measurement of spectra of emitted radiation: L1 , L2 ± lenses; S ± glass sample; M ± monochromator; PM ± photomultiplier.
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from the visible to the UV region of the spectrum. The signal from photomultiplier tube was recorded by a Tektronix TDS 640 digital oscilloscope. The absorption spectra of the original and irradiated glasses were measured by Perkin±Elmer 330 spectrophotometer. The transverse cross-section of the light beam in this spectrophotometer is about of 1 cm2 . This means that the absorption spectrum of the fraction of the glass sample that was colored by a single laser beam of area 10ÿ4 cm2 , cannot be measured in the usual way. Therefore, to measure a spectrum of photoinduced absorption in the glasses, a fraction with cross section about 1 cm ´ 1 cm of the sample with transverse size of about 2 ´ 2 cm was exposed to laser radiation. The sample was placed after the focal plane to prevent self-focusing and laser damage. It was slowly scanned in the plane perpendicular to the direction of the propagation of laser beam with multiple laser shots to produce a colored region of the size large enough to be detected in the spectrophotometer. The distribution of color centers in the direction of the laser beam propagation in each glass sample was measured by following technique. The laser beam was focused with the F 100 cm lens to produce an almost parallel beam near the focal plane. The sample was scanned perpendicular to the direction of the propagation of laser beam to produce a colored section of a few millimeters. A small wafer, 1 mm thick, was cut from the exposed sample in the plane of propagation of the laser beam that produced the color centers. This small wafer was then polished and analyzed along the optical axis with a scanning microphotometer that measured the local absorption of white light. The photoinduced optical density in the visible region was then measured as a function of the distance from the wafer edge (this distance is equal to the distance from the front surface of the original sample in the direction of beam propagation). The spatial resolution of this method was about 0.1 mm. However the accuracy of measurement was decreased with decreasing of irradiance because of lower contrast. Finally, total metering error was about 10%. This approach provided a value for the color centers absorption averaged over the visible spectrum.
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3. Results The following observations were made. Firstly, the laser beam was focused by the lens of 100 cm and the sample located at the beam waist 40 lm diameter. At high irradiances (above 1014 W/cm2 ), laser-induced breakdown was produced in all the glass samples. One could see radiation of laserinduced plasma emitted in all directions at the moment of breakdown, and coloration in the sample after exposure. Coloration was not observed in fused silica. In the case of breakdown, the plasma ®lament was prolonged in the direction of laser beam propagation, its size was up to several hundreds of microns, and size depended on irradiance. It is clear that self-focusing participates in the process of femtosecond laser-induced damage of glasses under these conditions. When irradiance was lower the threshold of catastrophic self-focusing and laser-induced breakdown, white light emission in the same direction as laser beam (contrary to isotropic plasma emission) occurred at irradiance >1012 W/cm2 . No sharp onset with intensity or threshold was detected. This white light was approximately collinear with the laser beam but its intensity distribution diered signi®cantly from the distribution of the laser beam intensity. In addition, the white light distribution drifted during prolonged irradiation of the same place by the train of laser shots. Simultaneously with white light generation, the coloration of the exposed region was observed in all glasses excluding fused silica. In these experiments focusing lenses with a range of focal lengths were used and the focal point was in front of the sample as shown in Fig. 1. The divergent beam excluded the increase of irradiance with distance due to self-focusing. The coloration in the glass bulk was generated in the form of the cone with a shape similar to the laser beam one (gray area in Fig. 1), i.e. the diameter of colored region was about several tens or a hundred microns. This dark track was directed along the laser beam propagation and appeared after the ®rst exposure to laser pulse. The photoinduced coloration increased with the number of multiple exposures. Glasses such as PTR, K8, BK7, and alkali-silicate exhibited a bright blue luminescence during ex-
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posure. It was found that both the luminescence intensity and induced optical density have shown smooth variation across the laser beam diameter. No thin threads (as would be expected if catastrophic self-focusing was occurring) were observed inside the area exposed to laser beam by optical microscopy. The heating of the samples up to 150°C for several minutes led to disappearance of photoinduced coloration of all samples excluding PTR glasses. No visible damage was observable in optical microscope after such heat treatment. No coloration in the visible region was detected in fused silica. The absorption spectra induced in samples by femtosecond laser pulses for specially synthesized sodium-silicate and PTR glasses are shown in Fig. 2, curves 1. In addition, Fig. 2 shows the normalized absorption of color centers produced in
the same glasses by c-radiation from a Co60 radioactive source (Fig. 2(a), curve 2) or UV radiation (Fig. 2(b), curve 2). The spectra of PTR glass were measured after exposure and treatment at 520°C during 1 h ± standard conditions for thermal development of these glasses [14]. We do not show the analogous spectra for lead-silicate glass
Fig. 2. Spectra of additional absorption of high purity alkalisilicate glass (a) and PTR glass; (b) exposed to c-radiation (a, curve 1) and UV (b, curve 1) or femtosecond laser radiation at 850 nm (a, b, curves 2). Thickness of the both samples was 6 mm.
Fig. 3. Microscope photographs of channels in the bulk of PTR glass after exposure to femtosecond laser radiation at 850 nm and thermal development: (a) front view of channels; (b) lateral view of channels. Photographs were made in the region of focal plane of lens (df 5 lm).
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TF10 and boro-silicate glasses K8 and BK7 because the spectral shapes of induced absorption in these glasses exposed to femtosecond laser pulses and c-radiation from a Co60 were absolutely the same. The pictures of channels with modi®ed refractive index in PTR glass arisen after exposure to focused femtosecond radiation and thermal development are shown in Fig. 3. These pictures were made in region of focal plane of lens (df 5 lm) in the direction of beam propagation (Fig. 3(a)) and in perpendicular one (Fig. 3(b)). It was found that the color centers and intrinsic luminescence starts only at some distance from the front surface of the sample (Fig. 4). This phenomenon was observed even for the sample placed immediately adjacent the focal plane of the lens F 10 cm when the laser beam was strong diverging into the bulk of the sample (Fig. 1). Besides, the study showed that the distance between the front surface and colored region (point A in Figs. 1 and 4) depends on irradiance of incident beam and it is increased when the irradiance is decreased (Fig. 5). The measurements were made of the spectrum of the output radiation (the supercontinuum) after the studied samples. It was found for the samples of K8 and BK7 that the output radiation is a continuum spreading from the IR to 220 nm. This result is surprising since it well known that these glasses are opaque for wavelength less than 330 nm.
Fig. 4. The dependence of additional absorption coecient on distance from the front surface of K8 glass (see Fig. 1) irradiated by femtosecond laser radiation at 850 nm. Lens focal length was 100 cm, sample thickness was 2 cm.
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Fig. 5. Dependence of the distance between the front surface of the sample and the colored region (depth, marked as A in Figs. 1 and 4) on the irradiance of the incident laser radiation. The solid line is a hyperbolic curve: y / xÿ0:93 :
4. Discussion 4.1. Induced absorption spectra In Fig. 2(a) (curve 1) one can see well known for sodium-silicate glass four maxima of the intrinsic color centers in the visible and UV regions [35,36]. Two long wavelength bands were attributed to the hole color centers [35]. It was indicated in the Introduction that this means that glass matrix was ionized by c-radiation. The same maxima can be observed in the femtosecond laser induced spectra of alkali-silicate glass both in the visible and the UV regions (Fig. 2(a), curve 2). As was mentioned for commercial lead-silicate (TF10) and boro-silicate (K8 and BK7) glasses the shapes of spectra induced by femtosecond laser pulses and c-radiation from a Co60 radioactive source were absolutely the same. Thermal bleaching of the photoinduced absorption in all these glasses was similar to thermal bleaching of color centers generated by ionizing radiation. These data allow us to conclude that the darkening of the glass samples exposed to femtosecond IR laser radiation is caused by the generation of the same color centers (including hole centers) as arising under the ionizing radiation. It means that glass matrix with bandgap of 5.4 eV was ionized under exposure to IR radiation with 1.46 eV photon energy. Similarity in behavior of PTR glass spectra is observed. An additional absorption of PTR glass
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undergoing UV or femtosecond laser pulses exposure can be detected only in the UV region. The small tail of the induced absorption spectrum in the blue region can be distinguished by the naked eyes as a slight yellow coloration of the exposed area. However, thermal development causes colloidal silver and sodium ¯uoride precipitation in glass matrix [15,16] and transparency of glasses are decreased (Fig. 2(b)). Fluoride crystals are colorless and can result in scattering if the size of crystals is too large (>100 nm). Shoulders near 470 nm in the additional absorption spectra after thermal treatment in Fig. 2(b) were ascribed to silver particles in glass matrix [16]. In addition, thermal development of exposed samples leads to formation of glass sites with modi®ed refractive index (Fig. 3). These phase structures are similar to phase structures observed in PTR glass exposed to UV radiation [37]. In accordance with the model [15,16] described above, these induced changes of absorption spectra and refractive index in PTR glasses mean that photoionization of Ce3 (>3.5 eV) was occurred after exposure to the IR radiation with photon energy of 1.46 eV. These data allow drawing conclusion that the photoionization of all studied glasses is observed under high-power IR femtosecond laser pulses. It was mentioned that photon energy should be more than the threshold of the electron mobility to ionize glass matrix (>5.4 eV in the case of the alkali-silicate glasses [6] and >5.8 eV in the case of the lead±silicate glasses [13]) and more than 3.5 eV to ionize Ce3 in PTR glass. Consequently, the process of photoionization under the excitation by photons with energy 1.46 eV (k 850 nm) requires from 3 to 4 times more energy. However, as has already been noted [12,13], four-photon ionization in alkali-silicate, boro-silicate, and lead-silicate glasses does not occur under exposure to nanosecond laser pulses. Thus, the next goal was to examine the opportunity of multi-photon ionization of glasses under femtosecond excitation. 4.2. Distribution of color centers It is clear that probability of multi-photon absorption increases in the sites which were undergone exposure to radiation with higher irradiance. Con-
sequently, the concentration of color centers must be maximal in sites with maximal irradiance. This phenomenon was demonstrated in Refs. [10,11] for two-photon ionization of glasses. Maximum coloration was then observed in the focal plane of focused laser beam. In this work, we used the same approach to check on the possibility of four-photon ionization of the glass matrix by the femtosecond IR laser pulses. The color center distribution in the glass sample exposed to IR laser radiation focused before the front surface of the sample is shown in Fig. 4. In case of multiphoton absorption the coloration and luminescence have to start immediately from front surface because the irradiance in that point is maximal (see condition of exposure in Fig. 1). Fig. 4 shows the gap between the front surface and colored site. This is not a feature of the single sample. Fig. 5 shows dependence of the distance between the front surface and colored site (point A in Figs. 1 and 4) on the irradiance of exciting laser beam. The absence of the photoinduced coloration close to the front surface, in the region of the maximum intensity of the laser beam, proves that coloration and luminescence in these glasses induced by femtosecond IR laser pulses cannot be ascribed to multiphoton absorption of laser radiation at 850 nm. On the other hand one can see in Fig. 5 that this distance is inversely proportional to the irradiance of incident laser beam. It is known (see, for example, [31]) that the degree of spectral broadening of laser pulse is proportional to the length of the interaction between the exciting radiation and the media. Similar data were obtained for all glasses studied. This result indicates that the ionization of these glasses is due to a two-step nonlinear process as it was found for alkali- and boro-silicate glasses in Refs. [18,19]. The ®rst step is a spectral broadening of transmitting laser radiation, and the second step is linear or multi-photon absorption of the short wavelength part of this broadened supercontinuum. Thus, the next goal is to examine the spectral broadening of exciting radiation in the process of interaction of the femtosecond IR laser pulses with glass. 4.3. Transmitted radiation UV component detection in transmitted light proves the idea of the principal role of spectral
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broadening in the process of interaction of femtosecond IR radiation with matter. Firstly, the irradiance of short wavelength component of supercontinuum with k < 400 nm (hm > 3.1 eV) can be rather high (>106 W/cm2 ) to generate color centers in accordance with two-photon [11,12] or three-photon [13] mechanism. Secondly, detection of the UV component with k < 230 nm (hm > 5.4 eV) shows that even single-photon ionization [2] of glass occurs under femtosecond IR irradiation. It should be mentioned that commercial borosilicate glasses used in this work are completely opaque at wavelengths below 330 nm. The only explanation for the observation of the UV light as short as 220 nm in the transmitted spectra after K8 or BK7 glasses, is that this radiation is generated at each point of the glass volume and immediately absorbed at the same point. Thus, only the thin layer near the rear surface contributes the short wavelength radiation (220±330 nm) that can be detected outside of the glass bulk. The absence of radiation detected with wavelength less than 220 nm can be caused both by decreasing the eciency of supercontinuum emitting in the far UV region and by sharp increasing the intrinsic absorption of glass at that wavelengths. The observation of transmitted radiation up to 220 nm indicates that an eective transformation of IR radiation occurred inside the samples up to the far UV region. It should be noted, that the intrinsic absorption boundary (at the level of 1 cmÿ1 ) of alkali-silicate and boro-silicate glasses is in the region of 210±220 nm. The maximum of the color center generation spectrum in silicate glasses is placed near the intrinsic absorption boundary and it extends to long wavelength side out to 230± 240 nm. Hence, the presence of such short wavelength radiation transmitted within the glass sample, in accordance with data obtained in works [2,6], proves that color center generation results from the excitation of the intrinsic states of glasses by the short wavelength part of supercontinuum. This excitation can be created by both a linear absorption of radiation in the region 220± 240 nm [6] and by non-linear absorption in the region 250±400 nm [10]. To clarify the relative role of these processes in the glass matrix excita-
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tion, additional experiments are necessary. One can see some decrease of the induced absorption at long distances from the front surface (Fig. 4). This phenomenon can be caused by several physical mechanisms and it is necessary to study this process separately. 4.4. The model of dielectric solid ionization by femtosecond IR pulses The results depicted above suggest the need for a new model to describe the processes that occur in multicomponent silicate glasses during exposure to femtosecond IR laser radiation with irradiance less than the threshold for catastrophic self-focusing and laser-induced breakdown. The model is illustrated in Fig. 6. Originally all the electrons in dielectric glass are located in the valence band and the energy levels of the conduction band and of the impurities are empty. The energy of incident photons (1.46 eV) is 4 times less that the longwave boundary of carrier mobility in silicate glasses and 3 times less than Ce3 ionization energy. It was found that no photoinduced coloration is occurred due to multiphoton ionization. Therefore no coloration one can see near the front surface of the sample (up to point A in Fig. 6). The spectral width of the laser beam is broadened in the process of the pulse transmission through the glass sample. At some distance from the front
Fig. 6. Diagram of photo-induced processes in a dielectric material exposed to femtosecond IR pulses.
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surface (`A' in Figs. 1, 4 and 6), the spectral broadening at the UV edge of the supercontinuum reaches half or third the threshold of carrier mobility (Ei ) for alkali- and boro-silicate or for leadsilicate glasses, respectively. Two-photon ionization of alkali- and boro-silicate glass [10] and three-photon ionization of lead-silicate glass [13] occur if the intensity is more than 106 W/cm2 . Coloration of the glass bulk then results from electrons and holes trapping in the region Z > A (Fig. 6). The process of single-photon ionization of glass begins at Z > B (Fig. 6), which corresponds to the distance from the front surface that is required for the UV edge of supercontinuum to reach a maximum photon energy equal to the energy of the boundary of carrier mobility or Ce3 ionization energy. In this case photoionization occurs with high quantum eciency, independently on irradiance. It should be noted that glass is opaque for these photons and only a thin layer near the back surface of the sample can emit this radiation outside of the glass. This is why the intensity of the short part of the UV radiation measured outside of the glass sample is very low. We emphasize that spectral broadening of femtosecond pulses is general process and was observed in all studied condensed substances. Therefore the proposed model of ionization through the spectral broadening of laser radiation should be rather common and can be applied for dierent photoinduced processes in dierent dielectrics (glasses and crystals, solids and liquids, etc.) The absence of the visible coloration in fused silica under multiple irradiation by femtosecond laser pulses at 850 nm with irradiance up to the damage threshold can be a result of two possible causes. Firstly, the band gap energy in fused silica is much greater than in multicomponent glasses and therefore its intrinsic absorption begins only in the vacuum UV region [38]. The amount of IR radiation that is converted into supercontinuum radiation in this short wavelength UV region is consequently much less and no signi®cant ionization of fused silica occurs. Secondly, color centers produced in high purity fused silica have low absorption in the visible region and may not be seen [38].
4.5. The role of spectral broadening in femtosecondlaser-induced processes In previous studies [23±26] of laser-induced damage of crystals and glasses under femtosecond IR laser irradiation, multiphoton absorption of the laser radiation is considered as the mechanism of initial free electron generation for avalanche ionization. The results obtained in the present paper show that photoionization of dielectrics with large bandgap energies, such as silicate glasses, is caused by absorption of the short wavelength part of the supercontinuum that is generated in the all materials, and not by multiphoton absorption of the laser radiation. This suggests that the mechanisms responsible for laserinduced damage of these materials under femtosecond radiation are not connected with multiphoton absorption of the laser radiation and demand the further investigation. Several recent publications [17,20±22] reported about photoionization of dierent glasses exposed to femtosecond IR laser radiation. This ionization allowed writing waveguides in a number of glasses [20,21] or creation of sites with modi®ed refractive index in PTR-type glasses [17,22]. All observed phenomena were explained as a result of multi-photon ionization. The results of this work show that discussed phenomena can be the result of spectral broadening at least for silicate glasses. 5. Conclusion Photoionization of multicomponent silicate glasses has been observed under exposure to infrared femtosecond laser radiation at irradiance below the threshold of catastrophic self-focusing and laser-induced damage. This ionization of glasses leads to the color center formation and luminescence as a result of the consequent trapping and recombination of electrons and holes, correspondingly. The same process in photosensitive PTR glasses leads to latent image formation and the refractive index changing after thermal development in exposed area. The photoionization results from absorption of the short-wavelength
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component of the supercontinuum that is generated in the bulk of glass due to the femtosecond laser radiation spectral broadening. The supercontinuum extends up to the short wavelength UV region even if the glass is opaque in this region. Fused silica exhibits the same spectral broadening too but color center formation in the visible was not observed at irradiance up to the laser damage threshold.
References [1] W.A. Weyl. Colored Glasses, Society of Glass Technology, Sheeld, 1951. [2] L.B. Glebov, A.A. Grubin, M.N. Tolstoi, Fiz. Khim. Stekla 1 (1975) 313. [3] L.B. Glebov, L.B. Popova, M.N. Tolstoi, Sov. J. Opt. Technol. 42 (1975) 214. [4] F.N. Steele, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 246. [5] L.B. Glebov, V.G. Dokuchaev, M.A. Petrov, G.T. Petrovskii, Sov. J. Glass Phys. Chem. 13 (1987) 310. [6] A.N. Trukhin, M.N. Tolstoi, L.B. Glebov, V.L. SavelÕev, Phys. Stat. Sol. (B) 99 (1980) 155. [7] A.N. Trukhin, V.L. SavelÕev, M.N. Tolstoi, L.B. Glebov, Sov. J. Glass Phys. Chem. 5 (1979) 634. [8] L.B. Glebov, M.N. Tolstoi, Sov. J. Glass Phys. Chem. 2 (1976) 340. [9] V.L. SavelÕev, A.N. Trukhin, M.N. Tolstoi, L.B. Glebov, in: Physics and Chemistry of Glass-Forming Systems, P. Stuchki Lit. Gos. Univ., Riga, 1979, p. 20. [10] A.P. Gagarin, L.B. Glebov, O.M. E®mov, O.S. E®mova, Sov. J. Glass Phys. Chem. 5 (1979) 337. [11] L.B. Glebov, O.M. E®mov, G.T. Petrovskii, P.N. Rogovtsev, Sov. J. Glass Phys. Chem. 10 (1984) 55. [12] L.B. Glebov, O.M. E®mov, G.T. Petrovskii, Sov. J. Quantum Electron. 16 (1986) 1245. [13] O.M. E®mov, A.M. Mekryukov, J. Non-Cryst. Solids 191 (1995) 94. [14] L.B. Glebov, N.V. Nikonorov, E.I. Panysheva, G.T. Petrovskii, V.V. Savvin, I.V. Tunianova, V.A. Tsekhomskii, Sov. Phys. Dokl. 35 (1990) 878. [15] S.D. Stookey, Ind. Eng. Chem. 41 (1949) 856.
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[16] S.D. Stookey, G.H. Beall, J.E. Pierson, J. Appl. Phys. 49 (1978) 5114. [17] Y. Kondo, T. Suzuki, H. Inouye, K. Miura, T. Mitsuyu, K. Hirao, Jpn. J. Appl. Phys. 37 (1998) L94. [18] O.M. E®mov, L.B. Glebov, S.V. Garnov, M.J. Soileau, M. Richardson, K. Gabel, S. Grantham, Proc. SPIE 2796 (1996) 247. [19] O.M. E®mov, K. Gabel, S.V. Garnov, L.B. Glebov, S. Grantham, M. Richardson, M.J. Soileau, J. Opt. Soc. Am. B 15 (1998) 193. [20] K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 (1996) 1729. [21] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 71 (23) (1997) 3329. [22] Y. Kondo, T. Suzuki, H. Inouye, K. Miura, T. Mitsuyu, K. Hirao, Proc. XVIII International Congress on Glass C11 (1998) 36. [23] D. Du, X. Liu, G. Korn, J. Squier, G. Mourou, Appl. Phys. Lett. 64 (1994) 3071. [24] B.C. Stuart, S. Herman, M.D. Perry, Proc. SPIE 2428 (1995) 568. [25] M.D. Feit, A.M. Rubenchik, B.W. Shore, B.C. Stuart, M.D. Perry, Proc. SPIE 2428 (1995) 469. [26] D. Du, X. Liu, G. Mourou, Appl. Phys. B 63 (1996) 617. [27] R.R. Alfano, S.L. Shapiro, Phys. Rev. Lett. 24 (1970) 584. [28] R.R. Alfano, S.L. Shapiro, Phys. Rev. Lett. 24 (1970) 592. [29] W. Yu, R.R. Alfano, C.L. Sam, R.J. Seymour, Opt. Commun. 14 (1975) 344. [30] P. Corkum, P. Ho, R. Alfano, J. Manassah, Opt. Lett. 11 (1986) 624. [31] P.L. Baldeck, P.P. Ho, R.R. Alfano, Revue Phys. Appl. 22 (1987) 1677. [32] R.R. Alfano, P.P. Ho, IEEE J. Quantum Electron. QE-24 (1988) 351. [33] L.B. Glebov, L.B. Popova, V.V. Rusan, M.N. Tolstoi, Sov. J. Glass Phys. Chem. 2 (1976) 547. [34] P. Beaud, M. Richardson, E.J. Miesak, B.H.T. Chai, Opt. Lett. 18 (1993) 1550. [35] L.B. Glebov, V.G. Dokuchaev, G.T. Petrovskii, Sov. J. Glass Phys. Chem. 11 (1985) 61. [36] J.H. Mackey, H.L. Smith, A. Halperin, J. Phys. Chem. Solids 27 (1965) 1759. [37] O.M. E®mov, L.B. Glebov, L.N. Glebova, K.C. Richardson, V.I. Smirnov, Appl. Opt. (AT&BO) 38 (1999) 619. [38] A.R. Silin, A.N. Trukhin. Point Defects and Elementary Excitations in Crystalline and Vitreous SiO2 , Zinatne, Riga, 1985.
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Photoionization of silicate glasses exposed to IR femtosecond pulses O.M. E®mov, L.B. Glebov *, S. Grantham, M. Richardson Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, FL 32826, USA
Abstract Photoionization of alkali-silicate, boro-silicate, lead-silicate and photosensitive multi-component silicate glasses has been studied under exposure to infrared femtosecond laser pulses at irradiance below the thresholds of laser-induced damage and catastrophic self-focusing. It is proved that the supercontinuum that is generated in all glasses studied as a result of the femtosecond laser pulses spectral broadening extends up to the short-wavelength part of the ultraviolet region of spectrum even if the glass is opaque in this region. It is shown that photoionization results from absorption of the short-wavelength component of this supercontinuum generated by infrared radiation. This ionization leads to the color center formation and luminescence in the bulk of glasses studied. Photoionization of photosensitive multicomponent glasses leads to the creation of latent image and to the refractive index changing in exposed area after thermodevelopment. Fused silica exhibits the same spectral broadening too but color center formation in the visible was not recorded at irradiance up to the laser damage threshold. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Photoionization of glasses leads to an appearance of mobile charge carriers and as a result of this process to the photoconductivity, color center formation and luminescence. In Ref. [1] (Chapter 5) it was mentioned that coloration of alkali-silicate glasses could be caused by exposure to ultraviolet (UV) radiation in the region of 200±230 nm. Later it was proven [2] that for sodium silicate glasses the generation of the color centers is a result of the photoinduced transition of electrons from the valence band to the levels situated above the electron mobility threshold, and established [3] that the fundamental (intrinsic) absorption edge of
* Corresponding author. Tel.: +1-407 823 6983; fax: +1-407 823 6880; e-mail: [email protected]
these glasses is in the region of 6 eV (200 nm). It can be observed in glasses if transition ions concentration does not exceed 1 ppm. In the other case, absorption band of trivalent iron, with a peak at 5.5 eV (225 nm) [4], and three absorption bands of divalent iron, with peaks at 4.4, 5.1 and close to 6 eV (280, 240, and 200 nm) [5] mask intrinsic absorption and form the UV absorption edge of glass. A model according to which the fundamental absorption edge of these glasses is formed by transitions between levels in complexes of ºSiÿOÿ . . . Na (L-centers), was developed in Ref. [6]. Excitation into the fundamental absorption region results in intrinsic luminescence with a peak in the 3.4 eV (365 nm) region and a decay time of 1 ls [7]. A study of color-center formation [2,8] and the internal photoelectric eect [9] have shown that the threshold of charge-carrier mobility in alkali-silicate glasses placed in the region of the fundamental
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 4 3 - 9
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absorption edge. Thus, a term `mobility threshold' in these glasses is similar to a term `bandgap' in crystals (Ei Eg ). Exposure of glasses to radiation with photon energy less than intrinsic absorption edge cannot cause photoionization of glass matrix, and the corresponding photochemical processes do not occur. The evidences of the glass matrix ionization are intrinsic luminescence and hole color centers generation. It should be emphasized that the photoinduced processes, which are not followed by these phenomena, are the result of ionization of dopants or impurities that only leads to their recharging and electron color center formation. It was shown in Ref. [10,11] that alkali- and boro-silicate glass exposure to powerful pulsed laser radiation with Ei /2 < hm < Ei causes twophoton ionization of glass matrix followed by intrinsic luminescence and the same color centers as after excitation in the intrinsic absorption range. It is important that in this case the color centers formation eect is de®nitely recorded at power density of q 10ÿ3 q0 (q0 is the optical breakdown threshold of glass), while the fundamental luminescence is recorded at q 10ÿ4 q0 [12]. Exposure of glasses studied to radiation with photon energy hm < Ei /2 does not cause these eects even at q 0.98q0 , i.e. it is impossible to observe threeand more photon ionization in alkali- and borosilicate glasses. It was found in Ref. [13] that the long-wavelength boundary of carrier mobility in lead-silicate glasses is placed considerably higher than the intrinsic absorption edge (Ei > Eg ) and was proven that coloration is resulted from three-photon excitation which occurs through virtual states that are located in the fundamental absorption region. This process in lead containing glasses was possible because of very high density of intrinsic levels of glass matrix below the mobility threshold, which increased dramatically the probability of three-photon absorption in comparison with other materials. Another type of ionization is observed in photosensitive silicate glasses doped with silver, cerium, ¯uorine, and bromine that named photothermo-refractive (PTR) glasses. These glasses are used for Bragg gratings recording in their bulk [14] because they can change their refraction index
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under exposure to UV radiation followed by thermal development. The simpli®ed description of processes that occur in these glasses and thermal development is following (for example, see [15,16]). The ®rst step is photoionization of cerium ions under exposure to UV radiation in region 300±400 nm (the band of Ce3 ion absorption). The electrons released from the cerium ions are then trapped by silver ions. As a result, silver is converted from a positive ion to a neutral atom. These atoms possess a very high diusion rate in silicate glasses and create tiny silver particles that can be considered similar to a latent image in a classic photographic process. The further, higher temperature heating of an exposed glass sample causes precipitation of sodium and ¯uorine ions on the nucleation centers (silver particles) yield crystals of NaF. The refractive index of crystals grown is signi®cantly smaller than that of the base glass (1.32 and 1.49, respectively). Consequently, the refractive index of the exposed fragment of glass is dierent from the unexposed region. Thus, photoinduced processes in PTR glasses are the result of photoionization of dopant ion of Ce3 . No two-photon absorption was found in similar glasses doped with Ce3 exposed to nanosecond laser pulses at 630 nm [17]. Thus, all silicate glasses mentioned have longwavelength boundary of linear or nonlinear photoionization placed in UV or visible region of spectrum: k < 400 nm for alkali-silicate, boro-silicate [10,11], and PTR glasses [14], and k < 600 nm for lead-silicate glasses [13]. Nevertheless it was reported recently [18±22] that photoionization of silicate glasses is possible under infrared (IR) high-power femtosecond pulses (850 nm). Authors of [20±22] supposed that multi-photon ionization was responsible for observed phenomena of photo-refraction in glasses. The same supposition was made in Refs. [23±26] to explain the features of laser-induced breakdown in glasses exposed to femtosecond IR pulses. According to [18,19] the photoionization in sodium silicate and boro-silicate glasses results from supercontinuum (`white light') generation in the bulk of exposed glass and are followed by single- and two-photon absorption of the short-wavelength part of this supercontinuum.
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The ®rst work on supercontinuum generation in crystals and glasses was performed in 1970 [27,28]. It was found that from ®ve to ten small-scale ®laments occurred in transparent dielectric under excitation by picosecond pulse at 530 nm. The irradiance in these ®laments could achieve 1013 W/ cm2 . In this case, intense radiation in a wide spectral range from 400 to 700 nm was observed. Similar phenomena were obtained under excitation with other wavelengths [29,30]. A description and explanation of the main mechanisms of this process were presented in [31,32]. White light generation was considered in Ref. [31,32] as a substantial spectral broadening of the propagating beam both in the long-wavelength and in the short-wavelength regions of the spectrum due to self-phase, induced-phase, or cross-phase modulation. Optical glasses, especially silicate glasses, are the principal optical media for laser applications that include femto- and picosecond devices and they are promising materials for photo-induced fabrication of optical elements. It is important to know the stability of the optical parameters of glasses under femtosecond IR irradiation as well as to understand the mechanism of their ionization under these conditions. In this paper we study alkali-silicate, boro-silicate, lead-silicate, and PTR glasses exposed to femtosecond IR pulses. The goal of this work is to examine the mechanisms, which are responsible for the photoionization and color centers formation in all mentioned glasses at laser pulse intensities below the threshold for catastrophic self-focusing and laser-induced damage.
synthesized of high purity materials by a laboratory method [33] which kept the concentration of absorbing impurities in the range of 1 ppm. The latter glasses were melted in an electrical furnace in fused silica crucibles. The samples were of 2 ´ 2 cm ´ 0.6 cm in size. All sides of the glass samples were polished in order to observe photoinduced coloration and luminescence in dierent directions. The laser used for this investigation was a 100 fs, Cr: LiSAF system operating at a wavelength of 850 nm [34]. The laser consists of a Kerr lens modelocked Ti: sapphire laser followed by several stages of ¯ashlamp-pumped Cr: LiSAF ampli®ers in chirped pulse ampli®cation architecture. For these experiments the single pulse output of the ®rst stage regenerative ampli®er was compressed to a pulsewidth of 100 fs. Pulse energies of 2±3 mJ were used for these experiments. The size of the beam entering the compression gratings was 25 mm diameter. Each compressed pulse was bandwidth-limited with a Gaussian (FWHM) spectral width of 11 nm. The laser operated at a frequency of 6 Hz. The output of the laser beam (Fig. 1) was focused in air by a lens L1 (from 10 to 100 cm focal length) to a point located at speci®c distance in front of the sample S to exclude the increase of irradiance with distance due to self-focusing. In some experiments the beam was focused in the bulk of samples and self-focusing was observed under high-power radiation. The spectrum of the light transmitted through the sample was monitored with a 25 cm grating monochromator M having a resolution of 10 nm, and a photomultiplier tube PM with a sensitivity which spanned
2. Experimental We studied samples of a few types of commonly used commercial silicate glasses: fused silica, leadsilicate glass TF10 (Russia), and boro-silicate glasses K8 (Russia) and BK7 (Schott). Besides, two types of specially synthesized silicate glasses were used: a photosensitive PTR glass of approximate composition (mol.%) 15Na2 O±5ZnO±4Al2 O3 ±70SiO2 ±5NaF±1KBr±0.01Ag2 O±0.01CeO2 [14], and a sodium-silicate glass 22Na2 O±3CaO±75SiO2
Fig. 1. Experimental setup for glass samples exposure to IR femtosecond laser pulses and measurement of spectra of emitted radiation: L1 , L2 ± lenses; S ± glass sample; M ± monochromator; PM ± photomultiplier.
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from the visible to the UV region of the spectrum. The signal from photomultiplier tube was recorded by a Tektronix TDS 640 digital oscilloscope. The absorption spectra of the original and irradiated glasses were measured by Perkin±Elmer 330 spectrophotometer. The transverse cross-section of the light beam in this spectrophotometer is about of 1 cm2 . This means that the absorption spectrum of the fraction of the glass sample that was colored by a single laser beam of area 10ÿ4 cm2 , cannot be measured in the usual way. Therefore, to measure a spectrum of photoinduced absorption in the glasses, a fraction with cross section about 1 cm ´ 1 cm of the sample with transverse size of about 2 ´ 2 cm was exposed to laser radiation. The sample was placed after the focal plane to prevent self-focusing and laser damage. It was slowly scanned in the plane perpendicular to the direction of the propagation of laser beam with multiple laser shots to produce a colored region of the size large enough to be detected in the spectrophotometer. The distribution of color centers in the direction of the laser beam propagation in each glass sample was measured by following technique. The laser beam was focused with the F 100 cm lens to produce an almost parallel beam near the focal plane. The sample was scanned perpendicular to the direction of the propagation of laser beam to produce a colored section of a few millimeters. A small wafer, 1 mm thick, was cut from the exposed sample in the plane of propagation of the laser beam that produced the color centers. This small wafer was then polished and analyzed along the optical axis with a scanning microphotometer that measured the local absorption of white light. The photoinduced optical density in the visible region was then measured as a function of the distance from the wafer edge (this distance is equal to the distance from the front surface of the original sample in the direction of beam propagation). The spatial resolution of this method was about 0.1 mm. However the accuracy of measurement was decreased with decreasing of irradiance because of lower contrast. Finally, total metering error was about 10%. This approach provided a value for the color centers absorption averaged over the visible spectrum.
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3. Results The following observations were made. Firstly, the laser beam was focused by the lens of 100 cm and the sample located at the beam waist 40 lm diameter. At high irradiances (above 1014 W/cm2 ), laser-induced breakdown was produced in all the glass samples. One could see radiation of laserinduced plasma emitted in all directions at the moment of breakdown, and coloration in the sample after exposure. Coloration was not observed in fused silica. In the case of breakdown, the plasma ®lament was prolonged in the direction of laser beam propagation, its size was up to several hundreds of microns, and size depended on irradiance. It is clear that self-focusing participates in the process of femtosecond laser-induced damage of glasses under these conditions. When irradiance was lower the threshold of catastrophic self-focusing and laser-induced breakdown, white light emission in the same direction as laser beam (contrary to isotropic plasma emission) occurred at irradiance >1012 W/cm2 . No sharp onset with intensity or threshold was detected. This white light was approximately collinear with the laser beam but its intensity distribution diered signi®cantly from the distribution of the laser beam intensity. In addition, the white light distribution drifted during prolonged irradiation of the same place by the train of laser shots. Simultaneously with white light generation, the coloration of the exposed region was observed in all glasses excluding fused silica. In these experiments focusing lenses with a range of focal lengths were used and the focal point was in front of the sample as shown in Fig. 1. The divergent beam excluded the increase of irradiance with distance due to self-focusing. The coloration in the glass bulk was generated in the form of the cone with a shape similar to the laser beam one (gray area in Fig. 1), i.e. the diameter of colored region was about several tens or a hundred microns. This dark track was directed along the laser beam propagation and appeared after the ®rst exposure to laser pulse. The photoinduced coloration increased with the number of multiple exposures. Glasses such as PTR, K8, BK7, and alkali-silicate exhibited a bright blue luminescence during ex-
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posure. It was found that both the luminescence intensity and induced optical density have shown smooth variation across the laser beam diameter. No thin threads (as would be expected if catastrophic self-focusing was occurring) were observed inside the area exposed to laser beam by optical microscopy. The heating of the samples up to 150°C for several minutes led to disappearance of photoinduced coloration of all samples excluding PTR glasses. No visible damage was observable in optical microscope after such heat treatment. No coloration in the visible region was detected in fused silica. The absorption spectra induced in samples by femtosecond laser pulses for specially synthesized sodium-silicate and PTR glasses are shown in Fig. 2, curves 1. In addition, Fig. 2 shows the normalized absorption of color centers produced in
the same glasses by c-radiation from a Co60 radioactive source (Fig. 2(a), curve 2) or UV radiation (Fig. 2(b), curve 2). The spectra of PTR glass were measured after exposure and treatment at 520°C during 1 h ± standard conditions for thermal development of these glasses [14]. We do not show the analogous spectra for lead-silicate glass
Fig. 2. Spectra of additional absorption of high purity alkalisilicate glass (a) and PTR glass; (b) exposed to c-radiation (a, curve 1) and UV (b, curve 1) or femtosecond laser radiation at 850 nm (a, b, curves 2). Thickness of the both samples was 6 mm.
Fig. 3. Microscope photographs of channels in the bulk of PTR glass after exposure to femtosecond laser radiation at 850 nm and thermal development: (a) front view of channels; (b) lateral view of channels. Photographs were made in the region of focal plane of lens (df 5 lm).
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TF10 and boro-silicate glasses K8 and BK7 because the spectral shapes of induced absorption in these glasses exposed to femtosecond laser pulses and c-radiation from a Co60 were absolutely the same. The pictures of channels with modi®ed refractive index in PTR glass arisen after exposure to focused femtosecond radiation and thermal development are shown in Fig. 3. These pictures were made in region of focal plane of lens (df 5 lm) in the direction of beam propagation (Fig. 3(a)) and in perpendicular one (Fig. 3(b)). It was found that the color centers and intrinsic luminescence starts only at some distance from the front surface of the sample (Fig. 4). This phenomenon was observed even for the sample placed immediately adjacent the focal plane of the lens F 10 cm when the laser beam was strong diverging into the bulk of the sample (Fig. 1). Besides, the study showed that the distance between the front surface and colored region (point A in Figs. 1 and 4) depends on irradiance of incident beam and it is increased when the irradiance is decreased (Fig. 5). The measurements were made of the spectrum of the output radiation (the supercontinuum) after the studied samples. It was found for the samples of K8 and BK7 that the output radiation is a continuum spreading from the IR to 220 nm. This result is surprising since it well known that these glasses are opaque for wavelength less than 330 nm.
Fig. 4. The dependence of additional absorption coecient on distance from the front surface of K8 glass (see Fig. 1) irradiated by femtosecond laser radiation at 850 nm. Lens focal length was 100 cm, sample thickness was 2 cm.
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Fig. 5. Dependence of the distance between the front surface of the sample and the colored region (depth, marked as A in Figs. 1 and 4) on the irradiance of the incident laser radiation. The solid line is a hyperbolic curve: y / xÿ0:93 :
4. Discussion 4.1. Induced absorption spectra In Fig. 2(a) (curve 1) one can see well known for sodium-silicate glass four maxima of the intrinsic color centers in the visible and UV regions [35,36]. Two long wavelength bands were attributed to the hole color centers [35]. It was indicated in the Introduction that this means that glass matrix was ionized by c-radiation. The same maxima can be observed in the femtosecond laser induced spectra of alkali-silicate glass both in the visible and the UV regions (Fig. 2(a), curve 2). As was mentioned for commercial lead-silicate (TF10) and boro-silicate (K8 and BK7) glasses the shapes of spectra induced by femtosecond laser pulses and c-radiation from a Co60 radioactive source were absolutely the same. Thermal bleaching of the photoinduced absorption in all these glasses was similar to thermal bleaching of color centers generated by ionizing radiation. These data allow us to conclude that the darkening of the glass samples exposed to femtosecond IR laser radiation is caused by the generation of the same color centers (including hole centers) as arising under the ionizing radiation. It means that glass matrix with bandgap of 5.4 eV was ionized under exposure to IR radiation with 1.46 eV photon energy. Similarity in behavior of PTR glass spectra is observed. An additional absorption of PTR glass
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undergoing UV or femtosecond laser pulses exposure can be detected only in the UV region. The small tail of the induced absorption spectrum in the blue region can be distinguished by the naked eyes as a slight yellow coloration of the exposed area. However, thermal development causes colloidal silver and sodium ¯uoride precipitation in glass matrix [15,16] and transparency of glasses are decreased (Fig. 2(b)). Fluoride crystals are colorless and can result in scattering if the size of crystals is too large (>100 nm). Shoulders near 470 nm in the additional absorption spectra after thermal treatment in Fig. 2(b) were ascribed to silver particles in glass matrix [16]. In addition, thermal development of exposed samples leads to formation of glass sites with modi®ed refractive index (Fig. 3). These phase structures are similar to phase structures observed in PTR glass exposed to UV radiation [37]. In accordance with the model [15,16] described above, these induced changes of absorption spectra and refractive index in PTR glasses mean that photoionization of Ce3 (>3.5 eV) was occurred after exposure to the IR radiation with photon energy of 1.46 eV. These data allow drawing conclusion that the photoionization of all studied glasses is observed under high-power IR femtosecond laser pulses. It was mentioned that photon energy should be more than the threshold of the electron mobility to ionize glass matrix (>5.4 eV in the case of the alkali-silicate glasses [6] and >5.8 eV in the case of the lead±silicate glasses [13]) and more than 3.5 eV to ionize Ce3 in PTR glass. Consequently, the process of photoionization under the excitation by photons with energy 1.46 eV (k 850 nm) requires from 3 to 4 times more energy. However, as has already been noted [12,13], four-photon ionization in alkali-silicate, boro-silicate, and lead-silicate glasses does not occur under exposure to nanosecond laser pulses. Thus, the next goal was to examine the opportunity of multi-photon ionization of glasses under femtosecond excitation. 4.2. Distribution of color centers It is clear that probability of multi-photon absorption increases in the sites which were undergone exposure to radiation with higher irradiance. Con-
sequently, the concentration of color centers must be maximal in sites with maximal irradiance. This phenomenon was demonstrated in Refs. [10,11] for two-photon ionization of glasses. Maximum coloration was then observed in the focal plane of focused laser beam. In this work, we used the same approach to check on the possibility of four-photon ionization of the glass matrix by the femtosecond IR laser pulses. The color center distribution in the glass sample exposed to IR laser radiation focused before the front surface of the sample is shown in Fig. 4. In case of multiphoton absorption the coloration and luminescence have to start immediately from front surface because the irradiance in that point is maximal (see condition of exposure in Fig. 1). Fig. 4 shows the gap between the front surface and colored site. This is not a feature of the single sample. Fig. 5 shows dependence of the distance between the front surface and colored site (point A in Figs. 1 and 4) on the irradiance of exciting laser beam. The absence of the photoinduced coloration close to the front surface, in the region of the maximum intensity of the laser beam, proves that coloration and luminescence in these glasses induced by femtosecond IR laser pulses cannot be ascribed to multiphoton absorption of laser radiation at 850 nm. On the other hand one can see in Fig. 5 that this distance is inversely proportional to the irradiance of incident laser beam. It is known (see, for example, [31]) that the degree of spectral broadening of laser pulse is proportional to the length of the interaction between the exciting radiation and the media. Similar data were obtained for all glasses studied. This result indicates that the ionization of these glasses is due to a two-step nonlinear process as it was found for alkali- and boro-silicate glasses in Refs. [18,19]. The ®rst step is a spectral broadening of transmitting laser radiation, and the second step is linear or multi-photon absorption of the short wavelength part of this broadened supercontinuum. Thus, the next goal is to examine the spectral broadening of exciting radiation in the process of interaction of the femtosecond IR laser pulses with glass. 4.3. Transmitted radiation UV component detection in transmitted light proves the idea of the principal role of spectral
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broadening in the process of interaction of femtosecond IR radiation with matter. Firstly, the irradiance of short wavelength component of supercontinuum with k < 400 nm (hm > 3.1 eV) can be rather high (>106 W/cm2 ) to generate color centers in accordance with two-photon [11,12] or three-photon [13] mechanism. Secondly, detection of the UV component with k < 230 nm (hm > 5.4 eV) shows that even single-photon ionization [2] of glass occurs under femtosecond IR irradiation. It should be mentioned that commercial borosilicate glasses used in this work are completely opaque at wavelengths below 330 nm. The only explanation for the observation of the UV light as short as 220 nm in the transmitted spectra after K8 or BK7 glasses, is that this radiation is generated at each point of the glass volume and immediately absorbed at the same point. Thus, only the thin layer near the rear surface contributes the short wavelength radiation (220±330 nm) that can be detected outside of the glass bulk. The absence of radiation detected with wavelength less than 220 nm can be caused both by decreasing the eciency of supercontinuum emitting in the far UV region and by sharp increasing the intrinsic absorption of glass at that wavelengths. The observation of transmitted radiation up to 220 nm indicates that an eective transformation of IR radiation occurred inside the samples up to the far UV region. It should be noted, that the intrinsic absorption boundary (at the level of 1 cmÿ1 ) of alkali-silicate and boro-silicate glasses is in the region of 210±220 nm. The maximum of the color center generation spectrum in silicate glasses is placed near the intrinsic absorption boundary and it extends to long wavelength side out to 230± 240 nm. Hence, the presence of such short wavelength radiation transmitted within the glass sample, in accordance with data obtained in works [2,6], proves that color center generation results from the excitation of the intrinsic states of glasses by the short wavelength part of supercontinuum. This excitation can be created by both a linear absorption of radiation in the region 220± 240 nm [6] and by non-linear absorption in the region 250±400 nm [10]. To clarify the relative role of these processes in the glass matrix excita-
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tion, additional experiments are necessary. One can see some decrease of the induced absorption at long distances from the front surface (Fig. 4). This phenomenon can be caused by several physical mechanisms and it is necessary to study this process separately. 4.4. The model of dielectric solid ionization by femtosecond IR pulses The results depicted above suggest the need for a new model to describe the processes that occur in multicomponent silicate glasses during exposure to femtosecond IR laser radiation with irradiance less than the threshold for catastrophic self-focusing and laser-induced breakdown. The model is illustrated in Fig. 6. Originally all the electrons in dielectric glass are located in the valence band and the energy levels of the conduction band and of the impurities are empty. The energy of incident photons (1.46 eV) is 4 times less that the longwave boundary of carrier mobility in silicate glasses and 3 times less than Ce3 ionization energy. It was found that no photoinduced coloration is occurred due to multiphoton ionization. Therefore no coloration one can see near the front surface of the sample (up to point A in Fig. 6). The spectral width of the laser beam is broadened in the process of the pulse transmission through the glass sample. At some distance from the front
Fig. 6. Diagram of photo-induced processes in a dielectric material exposed to femtosecond IR pulses.
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surface (`A' in Figs. 1, 4 and 6), the spectral broadening at the UV edge of the supercontinuum reaches half or third the threshold of carrier mobility (Ei ) for alkali- and boro-silicate or for leadsilicate glasses, respectively. Two-photon ionization of alkali- and boro-silicate glass [10] and three-photon ionization of lead-silicate glass [13] occur if the intensity is more than 106 W/cm2 . Coloration of the glass bulk then results from electrons and holes trapping in the region Z > A (Fig. 6). The process of single-photon ionization of glass begins at Z > B (Fig. 6), which corresponds to the distance from the front surface that is required for the UV edge of supercontinuum to reach a maximum photon energy equal to the energy of the boundary of carrier mobility or Ce3 ionization energy. In this case photoionization occurs with high quantum eciency, independently on irradiance. It should be noted that glass is opaque for these photons and only a thin layer near the back surface of the sample can emit this radiation outside of the glass. This is why the intensity of the short part of the UV radiation measured outside of the glass sample is very low. We emphasize that spectral broadening of femtosecond pulses is general process and was observed in all studied condensed substances. Therefore the proposed model of ionization through the spectral broadening of laser radiation should be rather common and can be applied for dierent photoinduced processes in dierent dielectrics (glasses and crystals, solids and liquids, etc.) The absence of the visible coloration in fused silica under multiple irradiation by femtosecond laser pulses at 850 nm with irradiance up to the damage threshold can be a result of two possible causes. Firstly, the band gap energy in fused silica is much greater than in multicomponent glasses and therefore its intrinsic absorption begins only in the vacuum UV region [38]. The amount of IR radiation that is converted into supercontinuum radiation in this short wavelength UV region is consequently much less and no signi®cant ionization of fused silica occurs. Secondly, color centers produced in high purity fused silica have low absorption in the visible region and may not be seen [38].
4.5. The role of spectral broadening in femtosecondlaser-induced processes In previous studies [23±26] of laser-induced damage of crystals and glasses under femtosecond IR laser irradiation, multiphoton absorption of the laser radiation is considered as the mechanism of initial free electron generation for avalanche ionization. The results obtained in the present paper show that photoionization of dielectrics with large bandgap energies, such as silicate glasses, is caused by absorption of the short wavelength part of the supercontinuum that is generated in the all materials, and not by multiphoton absorption of the laser radiation. This suggests that the mechanisms responsible for laserinduced damage of these materials under femtosecond radiation are not connected with multiphoton absorption of the laser radiation and demand the further investigation. Several recent publications [17,20±22] reported about photoionization of dierent glasses exposed to femtosecond IR laser radiation. This ionization allowed writing waveguides in a number of glasses [20,21] or creation of sites with modi®ed refractive index in PTR-type glasses [17,22]. All observed phenomena were explained as a result of multi-photon ionization. The results of this work show that discussed phenomena can be the result of spectral broadening at least for silicate glasses. 5. Conclusion Photoionization of multicomponent silicate glasses has been observed under exposure to infrared femtosecond laser radiation at irradiance below the threshold of catastrophic self-focusing and laser-induced damage. This ionization of glasses leads to the color center formation and luminescence as a result of the consequent trapping and recombination of electrons and holes, correspondingly. The same process in photosensitive PTR glasses leads to latent image formation and the refractive index changing after thermal development in exposed area. The photoionization results from absorption of the short-wavelength
O.M. E®mov et al. / Journal of Non-Crystalline Solids 253 (1999) 58±67
component of the supercontinuum that is generated in the bulk of glass due to the femtosecond laser radiation spectral broadening. The supercontinuum extends up to the short wavelength UV region even if the glass is opaque in this region. Fused silica exhibits the same spectral broadening too but color center formation in the visible was not observed at irradiance up to the laser damage threshold.
References [1] W.A. Weyl. Colored Glasses, Society of Glass Technology, Sheeld, 1951. [2] L.B. Glebov, A.A. Grubin, M.N. Tolstoi, Fiz. Khim. Stekla 1 (1975) 313. [3] L.B. Glebov, L.B. Popova, M.N. Tolstoi, Sov. J. Opt. Technol. 42 (1975) 214. [4] F.N. Steele, R.W. Douglas, Phys. Chem. Glasses 6 (1965) 246. [5] L.B. Glebov, V.G. Dokuchaev, M.A. Petrov, G.T. Petrovskii, Sov. J. Glass Phys. Chem. 13 (1987) 310. [6] A.N. Trukhin, M.N. Tolstoi, L.B. Glebov, V.L. SavelÕev, Phys. Stat. Sol. (B) 99 (1980) 155. [7] A.N. Trukhin, V.L. SavelÕev, M.N. Tolstoi, L.B. Glebov, Sov. J. Glass Phys. Chem. 5 (1979) 634. [8] L.B. Glebov, M.N. Tolstoi, Sov. J. Glass Phys. Chem. 2 (1976) 340. [9] V.L. SavelÕev, A.N. Trukhin, M.N. Tolstoi, L.B. Glebov, in: Physics and Chemistry of Glass-Forming Systems, P. Stuchki Lit. Gos. Univ., Riga, 1979, p. 20. [10] A.P. Gagarin, L.B. Glebov, O.M. E®mov, O.S. E®mova, Sov. J. Glass Phys. Chem. 5 (1979) 337. [11] L.B. Glebov, O.M. E®mov, G.T. Petrovskii, P.N. Rogovtsev, Sov. J. Glass Phys. Chem. 10 (1984) 55. [12] L.B. Glebov, O.M. E®mov, G.T. Petrovskii, Sov. J. Quantum Electron. 16 (1986) 1245. [13] O.M. E®mov, A.M. Mekryukov, J. Non-Cryst. Solids 191 (1995) 94. [14] L.B. Glebov, N.V. Nikonorov, E.I. Panysheva, G.T. Petrovskii, V.V. Savvin, I.V. Tunianova, V.A. Tsekhomskii, Sov. Phys. Dokl. 35 (1990) 878. [15] S.D. Stookey, Ind. Eng. Chem. 41 (1949) 856.
67
[16] S.D. Stookey, G.H. Beall, J.E. Pierson, J. Appl. Phys. 49 (1978) 5114. [17] Y. Kondo, T. Suzuki, H. Inouye, K. Miura, T. Mitsuyu, K. Hirao, Jpn. J. Appl. Phys. 37 (1998) L94. [18] O.M. E®mov, L.B. Glebov, S.V. Garnov, M.J. Soileau, M. Richardson, K. Gabel, S. Grantham, Proc. SPIE 2796 (1996) 247. [19] O.M. E®mov, K. Gabel, S.V. Garnov, L.B. Glebov, S. Grantham, M. Richardson, M.J. Soileau, J. Opt. Soc. Am. B 15 (1998) 193. [20] K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 (1996) 1729. [21] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 71 (23) (1997) 3329. [22] Y. Kondo, T. Suzuki, H. Inouye, K. Miura, T. Mitsuyu, K. Hirao, Proc. XVIII International Congress on Glass C11 (1998) 36. [23] D. Du, X. Liu, G. Korn, J. Squier, G. Mourou, Appl. Phys. Lett. 64 (1994) 3071. [24] B.C. Stuart, S. Herman, M.D. Perry, Proc. SPIE 2428 (1995) 568. [25] M.D. Feit, A.M. Rubenchik, B.W. Shore, B.C. Stuart, M.D. Perry, Proc. SPIE 2428 (1995) 469. [26] D. Du, X. Liu, G. Mourou, Appl. Phys. B 63 (1996) 617. [27] R.R. Alfano, S.L. Shapiro, Phys. Rev. Lett. 24 (1970) 584. [28] R.R. Alfano, S.L. Shapiro, Phys. Rev. Lett. 24 (1970) 592. [29] W. Yu, R.R. Alfano, C.L. Sam, R.J. Seymour, Opt. Commun. 14 (1975) 344. [30] P. Corkum, P. Ho, R. Alfano, J. Manassah, Opt. Lett. 11 (1986) 624. [31] P.L. Baldeck, P.P. Ho, R.R. Alfano, Revue Phys. Appl. 22 (1987) 1677. [32] R.R. Alfano, P.P. Ho, IEEE J. Quantum Electron. QE-24 (1988) 351. [33] L.B. Glebov, L.B. Popova, V.V. Rusan, M.N. Tolstoi, Sov. J. Glass Phys. Chem. 2 (1976) 547. [34] P. Beaud, M. Richardson, E.J. Miesak, B.H.T. Chai, Opt. Lett. 18 (1993) 1550. [35] L.B. Glebov, V.G. Dokuchaev, G.T. Petrovskii, Sov. J. Glass Phys. Chem. 11 (1985) 61. [36] J.H. Mackey, H.L. Smith, A. Halperin, J. Phys. Chem. Solids 27 (1965) 1759. [37] O.M. E®mov, L.B. Glebov, L.N. Glebova, K.C. Richardson, V.I. Smirnov, Appl. Opt. (AT&BO) 38 (1999) 619. [38] A.R. Silin, A.N. Trukhin. Point Defects and Elementary Excitations in Crystalline and Vitreous SiO2 , Zinatne, Riga, 1985.