- Email: [email protected]
An optoelectronic device in bulk LiF with sub-micron periodic gratings fabricated by interference of 400 nm femtosecond laser pulses
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2762–2765 www.elsevier.com/locate/nimb
An optoelectronic device in bulk LiF with sub-micron periodic gratings fabricated by interference of 400 nm femtosecond laser pulses T. Kurobori a,*, Y. Obayashi a, M. Kurashima a, Y. Hirose a, T. Sakai b, S. Aoshima b, T. Kojima c, S. Okuda c a b
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Central Research Laboratory, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu 434-8601, Japan c Frontier Science Innovation Centre, Osaka Prefecture University, Gakuen-machi 1-1, Naka-ku, Sakai 599-8531, Japan Available online 22 March 2008
Abstract Sub-micron periodic gratings of transparent materials are holographically fabricated by interference with the second harmonic (400 nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. As one optoelectronic application, a pulsed, room temperature laser action in bulk lithium fluoride is demonstrated, for the first time, in the green spectral region based on the laser-active Fþ 3 colour centres utilizing a distributed feedback structure encoded by interference of 400 nm femtosecond laser pulses. A lasing output with a linewidth of 1 nm is obtained at approximately 539 nm, which value reflects the selective laser resonator. Realization of green and red distributed feedback colour centre laser action based on the Fþ 3 and F2 centres in LiF excited by a single wavelength can be expected. Ó 2008 Elsevier B.V. All rights reserved. PACS: 79.20.Ds; 61.72.Ji; 42.82.Bq; 78.55.Fv Keywords: Distributed-feedback laser; Femtosecond laser; Lithium fluoride; Colour centres; Micromachining
1. Introduction Recent advances in high-intensity femtosecond laser pulses have made it possible to encode various functional microstructures inside versatile materials including dielectrics, semiconductors, metals and polymers [1–3]. Among the alkali halides, LiF is of particular interest due to the excellent thermal and optical stabilities of its colour centres (CCs) even at room temperature (RT). In particular, the F2 and Fþ 3 CCs (i.e., two electrons bound to two or three neighbouring anion vacancies, respectively) in coloured LiF are excellent candidates [4] for the production of visible laser action at RT from the green-to-red spectral range and for the testing of their usefulness in optoelectronic and micro-optic applications.
*
Corresponding author. Tel.: +81 76 264 5478; fax: +81 76 234 4132. E-mail address: [email protected] (T. Kurobori).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.112
Regarding a distributed feedback (DFB) laser utilizing alkali halides, a pulsed DFB laser action at RT in LiF containing F2 CCs using a transient (i.e., non-permanent) grating formed by nanosecond (ns) interference fringes has been reported [5]. Moreover, DFB laser action in KCl containing N2 (i.e., F4) CCs [6] and in KCl:Li containing FA(II) [7] CCs has also been demonstrated in the near-IR region. However, these lasers require cryogenic temperature operation, complex photochromic conversion processes to obtain the laser-active CCs, and long exposures to obtain precisely registered gratings inside a crystal utilizing a Q-switched Nd:YAG laser. In the latter case, 260 min are required to obtain permanent spatial modulations (i.e., thick gratings) of the FA(II) centre concentrations over 13mm-long DFB laser gratings of the sample. Recently, we reported, for the first time, simultaneous fabrication [8] of laser-active CCs and micro-gratings in LiF by interference of femtosecond laser pulses from a ML Ti:sapphire laser system. In addition, we successfully
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
demonstrated visible DFB CC laser action [9,10] at RT based on the F2 CCs in LiF with sub-micron periodic, permanent microgratings written by a ‘‘hologram encoding system by high-intensity fs laser pulse” [11]. In this paper, we report laser action with a narrower oscillating linewidth in the green spectral region based on the Fþ 3 CCs in bulk LiF. In order to realize this, the DFB structure with fringe spacings as fine as approximately 390 nm was written inside the crystal utilizing the secondharmonic (400 nm) fs pulses from a ML Ti:sapphire laser system.
2763
e-
E g = 14 eV (LiF)
Conduction Band
eF +/ F
fs laser pulses
3.11 eV
2. Experimental Valence Band
e-
160
0.8 Emission
Absorption
120
0.6
80
0.4
0.2
40
+
F2 F3
0
0.0
3. Results and discussion Fig. 1 shows a schematic energy-level diagram with possible channels for the creation of free electrons (holes) in LiF for 400 nm (3.11 eV) fs-laser-excitation. In the case of fs-laser-excitation, the simplest F CCs (i.e., an electron trapped in an anion vacancy) are initially formed and then two, three, and four CCs are aggregated to form F2, F3, and F4 CCs, respectively. With fs pulses, electrons are basically excited to the conduction band via multiphoton absorption and various ionization processes. For the excitation wavelength in this study, we required a five photon-absorption to excite LiF with energy band-gaps of up to 14 eV for 3.11 eV excitation pulses. Fig. 2 shows the optical absorption and emission spectra of a LiF crystal after irradiation with an electron beam at a temperature of 200 K and then heating up to RT in the dark. The F band, not shown here, had much higher optical density at 250 nm. Specifically, LiF samples were sealed in a single layer of aluminium foil and exposed to a 500 keV electron beam on each 15 10 mm2 side at the current density of approximately 1 lA/cm2 for 90 min, while cooled to 200 K or lower by a stream of dry N2 gas [12].
Emission Intensity (arb. units)
Fig. 1. Schematic energy-level diagram with possible channels for creation of free electrons in LiF for 400 nm (3.11 eV) fs-laser-excitation. Vertical arrows represent the optical transitions, and inclined arrows represent the relaxation processes.
Optical Density
The pure samples used in this study were commercially available optically polished and near-parallel LiF crystals with a dimension of approximately 15 10 2 mm3. In the experiment using interferometric irradiation of femtosecond laser pulses, regenerative amplified fs pulses from a ML Ti:sapphire laser (wavelength: 800 nm, pulse duration: 100–130 fs for the nearly-transform-limited pulses and 500–520 fs for the positively or negatively chirped pulses, repetition rate: 10 Hz) were split into two beams. Frequency doubled wavelength (400 nm) radiation was obtained by directing the fundamental wavelength pulses through appropriately phase matched b-barium borate (BBO) crystals. These were then crossed on the surface of the LiF plate to give a spot size of approximately 120– 200 lm in diameter. Each beam energy used for fabricating microgratings was approximately 300–400 lJ/pulse for 400 nm femtosecond laser pulses. Details of the experimental setup for fs hologram encoding system are described in our previous paper [9,10].
400
500
600
700
Wavelength (nm) Fig. 2. Absorption and emission (excited at 450 nm) spectra of bulk LiF after irradiation with an electron beam at 200 K, and then heating up to RT in the dark. The emission curve (solid line) is fitted with the sum of the 523 (dashed line), 547 (dotted line), and 604 nm (dash-dotted line) Gaussian bands.
As previously reported [8] in the case of fs-laser-irradiation, the Fþ 3 centres are produced with much higher density than the F2 centres even at RT. However, the concentration of the Fþ 3 centres created by fs-laser-irradiation is not large enough to realize laser action. Therefore, after encoding sub-micron periodic gratings with 400 nm fs pulses, the samples required subjection to the abovedescribed treatments in order to obtain higher and uniform þ Fþ 3 centres for lasing. Maximum concentrations of the F3 17 3 centres were approximately 5 10 centres/cm . It should be noted that there is a great difference in the emission spectrum excited by 450 nm between an electronirradiated LiF crystal at low- and room-temperature. As already reported in detail [13], two main emission bands in LiF peaking at 540 and 670 nm are formed under ioniz-
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
ing radiation at RT. These bands correspond to the Fþ 3 and F2 centres, respectively. As shown in Fig. 2, for low-temperature-irradiation three emission bands peaking at 523, 547, and 604 nm were observed and analyzed. The emission curve was best fitted with a sum of single-Gaussian bands. According to the report by Gu and Liu [14], they also observed three main emission bands, i.e., peaking at 530, 554, and 595 nm, from a LiF crystal bombarded by electron-beam at liquid-nitrogen temperature. They pointed out that the 530 nm emission band corresponds to the Fþ 3 CCs and that both the 554 and 595 nm emission bands correspond to the unidentified CCs. In order to clarify these bands, radiative lifetime measurements were performed by time-resolved spectrofluorometer. An outline of the apparatus was given in a previous work [15]. The typical lifetime values at the wavelengths of the 523, 547, and 604 nm bands were 7.65, 7.92, and 17.7 ns at RT, respectively. These values compare with those reported by us [15], i.e., 8.76 ns at 530 nm (due to Fþ 3 center) and 18.5 ns at 670 nm (due to F2 center) of an X-ray irradiated LiF crystal at RT. The lifetime values at each emission band are slightly different from one another, though the reason is not clear at present. Fig. 3 shows optical microscope images of the twodimensional micrograting arrays (a) and the micro-structure at a larger magnification (b) encoded inside bulk LiF at a depth of 100 lm by a pair of 520 fs chirped interference laser pulses with a wavelength of 400 nm. Since we aimed at lasing from the Fþ 3 CCs in LiF, the designed fringe spacing of the gratings was approximately 390 nm and was achieved, as shown in the inset of Fig. 3(b), by adjusting the laser pulse crossing angle to 64°, which corresponds to the theoretically predicted DFB oscillator wavelength of 540 nm for the second-order Bragg reflection. The pulse energies of each interference beam at the exposure point were 327 and 399 lJ for the fabrication of the 390 nmpitched microgratings inside bulk LiF. Fig. 4 shows an output spectrum in the green spectral region from a coloured LiF with fine-pitched and permanent microgratings excited with laser radiation at 450 nm from an optical parametric oscillator (OPO) laser (10 Hz,
1.0
Normalized Intensity
2764
0.8 0.6 0.4 0.2 0.0 525
550
575
600
625
650
Wavelength (nm) Fig. 4. DFB output spectrum based on the Fþ 3 CCs in bulk LiF with microgratings, transversely excited with a pulsed OPO laser at 450 nm. A photograph of the DFB laser action at 539 nm is shown in the inset.
1.5 mJ/pulse). The 450 nm pumping beam was focused by a cylindrical lens to form a line on the front surface of the sample. The one-dimensional array of the microgratings was written in a 13-mm line at a depth of 100 lm using a pair of interference 520 fs chirped pulses at 400 nm. It should be noted that the use of chirped laser pulses made it possible to register such a grating inside the transparent materials several millimeter below the surface [2] and suppress the generation of plasma near the surface of bulk LiF. Single green intense beam spots emerging from both sides of the crystal were distinctly observed on the screen, as shown in the inset of this figure. The lasing output linewidth was nearly 1.0 nm at a center wavelength of 539 nm, though the value is narrower than that of the amplified spontaneous emission from the F3+ CCs in LiF (typical linewidth: 10–20 nm) [4]. However, the linewidth of a DFB laser based on the F2 CCs in LiF [10], which was fabricated by the same techniques, in the red spectral region at wavelengths of 690 and 710 nm was nearly the resolution of our system, 0.1 nm. A DFB CC laser based on the Fþ 3 CCs in LiF has not been completely realized at present.
Fig. 3. Optical microscope images of micrograting arrays (a) and the micro-structure at a larger magnification (b) encoded inside bulk LiF at a depth of 100 lm by a pair of 520 fs chirped interference laser pulses with a wavelength of 400 nm.
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
Moreover, the RT laser action due to the Fþ 3 CCs as well as that of the F2 CCs in LiF shows limited stability under high pumping intensities. Therefore, the laser operation was performed at a repetition rate of 1 Hz.
2765
Promotion of Science. T.K. would like to thank T. Oka of the FSIC, Osaka Prefecture University, for the support in the samples colouration. References
4. Conclusions We have demonstrated DFB laser action in the green spectral region based on the F3+ CCs in bulk LiF. In order to obtain periodic microgratings as fine as approximately 390 nm inside the crystal, the second-harmonic (400 nm) fs chirped pulses from a ML Ti:sapphire laser system were used. The lasing linewidth of the DFB laser was approximately 1.0 nm at 539 nm. By taking the results obtained here into consideration, realization of green and red dualbeam DFB or Distributed Bragg Reflector (DBR) CC lasers based on the Fþ 3 and F2 centres in LiF excited by a single wavelength of 450 nm can be expected. Acknowledgements The first stage of this work was carried out in part under the Collaborative Research Project of the Materials and Structures Laboratory, Tokyo Institute of Technology. T.K. would like to thank K. Kawamura and H. Hosono of the Tokyo Institute of Technology for providing him with the collaborative research program. This work was partially supported by a Grant-in-Aid for Scientific Research (B) (No.18360314) from the Japan Society for the
[1] Y. Shimotsuma, K. Hirao, P.G. Kazansky, J. Qiu, Jpn. J. Appl. Phys. 44 (2005) 4735. [2] K. Kawamura, M. Hirano, T. Kamiya, H. Hosono, Appl. Phys. Lett. 81 (2002) 1137. [3] M. Kamata, M. Obara, R.R. Gattass, L.R. Cerami, E. Mazur, Appl. Phys. Lett. 87 (2005) 051106. [4] V.V. Ter-Mikirtychev, T. Tsuboi, Prog. Quant. Electron. 20 (1996) 219. [5] T. Kurobori, K. Inabe, N. Takeuchi, J. Phys. D 16 (1983) L121. [6] T. Kurobori, H. Hibino, Y.Q. Chen, K. Inabe, Jpn. J. Appl. Phys. 34 (1995) L894. [7] G.C. Bjorklund, L.F. Mollenauer, W.J. Tomlinson, Appl. Phys. Lett. 29 (1976) 116. [8] T. Kurobori, K. Kawamura, M. Hirano, H. Hosono, J. Phys.: Condens. Matter 15 (2003) L399. [9] K. Kawamura, M. Hirano, T. Kurobori, D. Takamizu, T. Kamiya, H. Hosono, Appl. Phys. Lett. 84 (2004) 311. [10] T. Kurobori, T. Sakai, S. Aoshima, Phys. Status Solidi. A 204 (2007) 699. [11] K. Kawamura, N. Sarukura, M. Hirano, H. Hosono, Jpn. J. Appl. Phys. 39 (2000) L767. [12] L.F. Mollenauer, in: M.L. Stitch, M. Bass (Eds.), Laser Handbook, North-Holland, Amsterdam, 1985, p. 143. [13] G. Baldacchini, E. De Nicola, R.M. Montereali, A. Scacco, V. Kalinov, J. Phys. Chem. Solids 61 (2000) 21. [14] H. Gu, H. Liu, Opt. Commun. 201 (2002) 113. [15] T. Kurobori, T. Kanasaki, Y. Imai, N. Takeuchi, J. Phys. C 21 (1988) L397.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2762–2765 www.elsevier.com/locate/nimb
An optoelectronic device in bulk LiF with sub-micron periodic gratings fabricated by interference of 400 nm femtosecond laser pulses T. Kurobori a,*, Y. Obayashi a, M. Kurashima a, Y. Hirose a, T. Sakai b, S. Aoshima b, T. Kojima c, S. Okuda c a b
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Central Research Laboratory, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamakita-ku, Hamamatsu 434-8601, Japan c Frontier Science Innovation Centre, Osaka Prefecture University, Gakuen-machi 1-1, Naka-ku, Sakai 599-8531, Japan Available online 22 March 2008
Abstract Sub-micron periodic gratings of transparent materials are holographically fabricated by interference with the second harmonic (400 nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. As one optoelectronic application, a pulsed, room temperature laser action in bulk lithium fluoride is demonstrated, for the first time, in the green spectral region based on the laser-active Fþ 3 colour centres utilizing a distributed feedback structure encoded by interference of 400 nm femtosecond laser pulses. A lasing output with a linewidth of 1 nm is obtained at approximately 539 nm, which value reflects the selective laser resonator. Realization of green and red distributed feedback colour centre laser action based on the Fþ 3 and F2 centres in LiF excited by a single wavelength can be expected. Ó 2008 Elsevier B.V. All rights reserved. PACS: 79.20.Ds; 61.72.Ji; 42.82.Bq; 78.55.Fv Keywords: Distributed-feedback laser; Femtosecond laser; Lithium fluoride; Colour centres; Micromachining
1. Introduction Recent advances in high-intensity femtosecond laser pulses have made it possible to encode various functional microstructures inside versatile materials including dielectrics, semiconductors, metals and polymers [1–3]. Among the alkali halides, LiF is of particular interest due to the excellent thermal and optical stabilities of its colour centres (CCs) even at room temperature (RT). In particular, the F2 and Fþ 3 CCs (i.e., two electrons bound to two or three neighbouring anion vacancies, respectively) in coloured LiF are excellent candidates [4] for the production of visible laser action at RT from the green-to-red spectral range and for the testing of their usefulness in optoelectronic and micro-optic applications.
*
Corresponding author. Tel.: +81 76 264 5478; fax: +81 76 234 4132. E-mail address: [email protected] (T. Kurobori).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.112
Regarding a distributed feedback (DFB) laser utilizing alkali halides, a pulsed DFB laser action at RT in LiF containing F2 CCs using a transient (i.e., non-permanent) grating formed by nanosecond (ns) interference fringes has been reported [5]. Moreover, DFB laser action in KCl containing N2 (i.e., F4) CCs [6] and in KCl:Li containing FA(II) [7] CCs has also been demonstrated in the near-IR region. However, these lasers require cryogenic temperature operation, complex photochromic conversion processes to obtain the laser-active CCs, and long exposures to obtain precisely registered gratings inside a crystal utilizing a Q-switched Nd:YAG laser. In the latter case, 260 min are required to obtain permanent spatial modulations (i.e., thick gratings) of the FA(II) centre concentrations over 13mm-long DFB laser gratings of the sample. Recently, we reported, for the first time, simultaneous fabrication [8] of laser-active CCs and micro-gratings in LiF by interference of femtosecond laser pulses from a ML Ti:sapphire laser system. In addition, we successfully
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
demonstrated visible DFB CC laser action [9,10] at RT based on the F2 CCs in LiF with sub-micron periodic, permanent microgratings written by a ‘‘hologram encoding system by high-intensity fs laser pulse” [11]. In this paper, we report laser action with a narrower oscillating linewidth in the green spectral region based on the Fþ 3 CCs in bulk LiF. In order to realize this, the DFB structure with fringe spacings as fine as approximately 390 nm was written inside the crystal utilizing the secondharmonic (400 nm) fs pulses from a ML Ti:sapphire laser system.
2763
e-
E g = 14 eV (LiF)
Conduction Band
eF +/ F
fs laser pulses
3.11 eV
2. Experimental Valence Band
e-
160
0.8 Emission
Absorption
120
0.6
80
0.4
0.2
40
+
F2 F3
0
0.0
3. Results and discussion Fig. 1 shows a schematic energy-level diagram with possible channels for the creation of free electrons (holes) in LiF for 400 nm (3.11 eV) fs-laser-excitation. In the case of fs-laser-excitation, the simplest F CCs (i.e., an electron trapped in an anion vacancy) are initially formed and then two, three, and four CCs are aggregated to form F2, F3, and F4 CCs, respectively. With fs pulses, electrons are basically excited to the conduction band via multiphoton absorption and various ionization processes. For the excitation wavelength in this study, we required a five photon-absorption to excite LiF with energy band-gaps of up to 14 eV for 3.11 eV excitation pulses. Fig. 2 shows the optical absorption and emission spectra of a LiF crystal after irradiation with an electron beam at a temperature of 200 K and then heating up to RT in the dark. The F band, not shown here, had much higher optical density at 250 nm. Specifically, LiF samples were sealed in a single layer of aluminium foil and exposed to a 500 keV electron beam on each 15 10 mm2 side at the current density of approximately 1 lA/cm2 for 90 min, while cooled to 200 K or lower by a stream of dry N2 gas [12].
Emission Intensity (arb. units)
Fig. 1. Schematic energy-level diagram with possible channels for creation of free electrons in LiF for 400 nm (3.11 eV) fs-laser-excitation. Vertical arrows represent the optical transitions, and inclined arrows represent the relaxation processes.
Optical Density
The pure samples used in this study were commercially available optically polished and near-parallel LiF crystals with a dimension of approximately 15 10 2 mm3. In the experiment using interferometric irradiation of femtosecond laser pulses, regenerative amplified fs pulses from a ML Ti:sapphire laser (wavelength: 800 nm, pulse duration: 100–130 fs for the nearly-transform-limited pulses and 500–520 fs for the positively or negatively chirped pulses, repetition rate: 10 Hz) were split into two beams. Frequency doubled wavelength (400 nm) radiation was obtained by directing the fundamental wavelength pulses through appropriately phase matched b-barium borate (BBO) crystals. These were then crossed on the surface of the LiF plate to give a spot size of approximately 120– 200 lm in diameter. Each beam energy used for fabricating microgratings was approximately 300–400 lJ/pulse for 400 nm femtosecond laser pulses. Details of the experimental setup for fs hologram encoding system are described in our previous paper [9,10].
400
500
600
700
Wavelength (nm) Fig. 2. Absorption and emission (excited at 450 nm) spectra of bulk LiF after irradiation with an electron beam at 200 K, and then heating up to RT in the dark. The emission curve (solid line) is fitted with the sum of the 523 (dashed line), 547 (dotted line), and 604 nm (dash-dotted line) Gaussian bands.
As previously reported [8] in the case of fs-laser-irradiation, the Fþ 3 centres are produced with much higher density than the F2 centres even at RT. However, the concentration of the Fþ 3 centres created by fs-laser-irradiation is not large enough to realize laser action. Therefore, after encoding sub-micron periodic gratings with 400 nm fs pulses, the samples required subjection to the abovedescribed treatments in order to obtain higher and uniform þ Fþ 3 centres for lasing. Maximum concentrations of the F3 17 3 centres were approximately 5 10 centres/cm . It should be noted that there is a great difference in the emission spectrum excited by 450 nm between an electronirradiated LiF crystal at low- and room-temperature. As already reported in detail [13], two main emission bands in LiF peaking at 540 and 670 nm are formed under ioniz-
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
ing radiation at RT. These bands correspond to the Fþ 3 and F2 centres, respectively. As shown in Fig. 2, for low-temperature-irradiation three emission bands peaking at 523, 547, and 604 nm were observed and analyzed. The emission curve was best fitted with a sum of single-Gaussian bands. According to the report by Gu and Liu [14], they also observed three main emission bands, i.e., peaking at 530, 554, and 595 nm, from a LiF crystal bombarded by electron-beam at liquid-nitrogen temperature. They pointed out that the 530 nm emission band corresponds to the Fþ 3 CCs and that both the 554 and 595 nm emission bands correspond to the unidentified CCs. In order to clarify these bands, radiative lifetime measurements were performed by time-resolved spectrofluorometer. An outline of the apparatus was given in a previous work [15]. The typical lifetime values at the wavelengths of the 523, 547, and 604 nm bands were 7.65, 7.92, and 17.7 ns at RT, respectively. These values compare with those reported by us [15], i.e., 8.76 ns at 530 nm (due to Fþ 3 center) and 18.5 ns at 670 nm (due to F2 center) of an X-ray irradiated LiF crystal at RT. The lifetime values at each emission band are slightly different from one another, though the reason is not clear at present. Fig. 3 shows optical microscope images of the twodimensional micrograting arrays (a) and the micro-structure at a larger magnification (b) encoded inside bulk LiF at a depth of 100 lm by a pair of 520 fs chirped interference laser pulses with a wavelength of 400 nm. Since we aimed at lasing from the Fþ 3 CCs in LiF, the designed fringe spacing of the gratings was approximately 390 nm and was achieved, as shown in the inset of Fig. 3(b), by adjusting the laser pulse crossing angle to 64°, which corresponds to the theoretically predicted DFB oscillator wavelength of 540 nm for the second-order Bragg reflection. The pulse energies of each interference beam at the exposure point were 327 and 399 lJ for the fabrication of the 390 nmpitched microgratings inside bulk LiF. Fig. 4 shows an output spectrum in the green spectral region from a coloured LiF with fine-pitched and permanent microgratings excited with laser radiation at 450 nm from an optical parametric oscillator (OPO) laser (10 Hz,
1.0
Normalized Intensity
2764
0.8 0.6 0.4 0.2 0.0 525
550
575
600
625
650
Wavelength (nm) Fig. 4. DFB output spectrum based on the Fþ 3 CCs in bulk LiF with microgratings, transversely excited with a pulsed OPO laser at 450 nm. A photograph of the DFB laser action at 539 nm is shown in the inset.
1.5 mJ/pulse). The 450 nm pumping beam was focused by a cylindrical lens to form a line on the front surface of the sample. The one-dimensional array of the microgratings was written in a 13-mm line at a depth of 100 lm using a pair of interference 520 fs chirped pulses at 400 nm. It should be noted that the use of chirped laser pulses made it possible to register such a grating inside the transparent materials several millimeter below the surface [2] and suppress the generation of plasma near the surface of bulk LiF. Single green intense beam spots emerging from both sides of the crystal were distinctly observed on the screen, as shown in the inset of this figure. The lasing output linewidth was nearly 1.0 nm at a center wavelength of 539 nm, though the value is narrower than that of the amplified spontaneous emission from the F3+ CCs in LiF (typical linewidth: 10–20 nm) [4]. However, the linewidth of a DFB laser based on the F2 CCs in LiF [10], which was fabricated by the same techniques, in the red spectral region at wavelengths of 690 and 710 nm was nearly the resolution of our system, 0.1 nm. A DFB CC laser based on the Fþ 3 CCs in LiF has not been completely realized at present.
Fig. 3. Optical microscope images of micrograting arrays (a) and the micro-structure at a larger magnification (b) encoded inside bulk LiF at a depth of 100 lm by a pair of 520 fs chirped interference laser pulses with a wavelength of 400 nm.
T. Kurobori et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2762–2765
Moreover, the RT laser action due to the Fþ 3 CCs as well as that of the F2 CCs in LiF shows limited stability under high pumping intensities. Therefore, the laser operation was performed at a repetition rate of 1 Hz.
2765
Promotion of Science. T.K. would like to thank T. Oka of the FSIC, Osaka Prefecture University, for the support in the samples colouration. References
4. Conclusions We have demonstrated DFB laser action in the green spectral region based on the F3+ CCs in bulk LiF. In order to obtain periodic microgratings as fine as approximately 390 nm inside the crystal, the second-harmonic (400 nm) fs chirped pulses from a ML Ti:sapphire laser system were used. The lasing linewidth of the DFB laser was approximately 1.0 nm at 539 nm. By taking the results obtained here into consideration, realization of green and red dualbeam DFB or Distributed Bragg Reflector (DBR) CC lasers based on the Fþ 3 and F2 centres in LiF excited by a single wavelength of 450 nm can be expected. Acknowledgements The first stage of this work was carried out in part under the Collaborative Research Project of the Materials and Structures Laboratory, Tokyo Institute of Technology. T.K. would like to thank K. Kawamura and H. Hosono of the Tokyo Institute of Technology for providing him with the collaborative research program. This work was partially supported by a Grant-in-Aid for Scientific Research (B) (No.18360314) from the Japan Society for the
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