- Email: [email protected]
Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses
Radiation Measurements 38 (2004) 759 – 762 www.elsevier.com/locate/radmeas
Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses Toshio Kuroboria;∗ , Tetsu Kitaoa , Yukio Hirosea , Ken-ichi Kawamurab , Daijyu Takamizuc , Masahiro Hiranob , Hideo Hosonob; c a Department
of Materials Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan Transparent Electro-Active Materials(TEAM) Project, ERATO, Japan Science Technology Corporation, KSP C-1232, Sakato 3-2-1, Takatsu-ku, Kawasaki 213-0012, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b Hosono
Received 31 October 2003; received in revised form 5 November 2003; accepted 5 January 2004
Abstract We report a new technique to fabricate both laser-active F2 and F+ 3 colour centres in lithium :uoride and permanent periodic gratings with fringe spacings as ;ne as sub-micron size simultaneously by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties of such colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as X-rays. Moreover, the present technique is applied to the ;rst production of three-dimensional active channel waveguide and a pulsed distributed-feedback (DFB) laser at around 700 nm in LiF containing F2 colour centres with ;ne-pitched micro-grating structures. c 2004 Elsevier Ltd. All rights reserved. Keywords: DFB laser; Colour centres; Femtosecond laser; Lithium :uoride
1. Introduction Among the alkali halide crystals, lithium :uoride (LiF) is of particular interest because of the excellent thermal and optical stabilities of its laser-active colour centres even at room temperature (RT) as well as its good physical and chemical properties. In particular, the F2 and F+ 3 colour centres (two electrons bound to two or three neighbouring anion vacancies, respectively) in LiF are very promising candidates for producing visible laser action from the green-to-red spectral range (Kulinski et al., 1980; Kurobori et al., 1983; Ter-Mikirtychev and Tsuboi, 1996) at RT and the realization of miniaturized optical devices (Baldacchini and Montereali, 2001; Larciprete et al., 2002). However, due to it having the strongest ionic binding and a larger energy band gap (∼ 14 eV), and the low vapour pressure of lithium metal, LiF, in the form of bulk or ;lms, cannot be ∗ Corresponding author. Tel.: +81-76-264-5478; fax: +81-76-234-4132. E-mail address: [email protected] (T. Kurobori).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.006
coloured with the usual technique of additive colouration and colour centres are only produced with low-ionizing radiation such as electrons, - and X-rays. Recently, we successfully fabricated laser-active colour centres and ;ne-pitched micro-grating structures in LiF simultaneously by a holographic technique using two femtosecond (fs) laser pulses (Kurobori et al., 2003). In this paper, we report the optical properties of these centres in LiF crystals, which contain diFerent quantities of impurities, produced by fs laser pulses compared with those created by damage from X-ray radiation. Moreover, the present technique is, for the ;rst time, to our knowledge, applied to fabricate three-dimensional active channel waveguide inside bulk LiF up to ∼ 5 mm in depth by a single chirped (0.1–1 ps duration) laser pulse, and a DFB laser, based on F2 centres in LiF produced by two interfering fs pulses. 2. Experimental details Two LiF crystals with thickness of approximately 1:5 mm, which contained diFerent quantities of impurities,
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762
3. Experimental results and discussion Fig. 1(a) shows the optical absorption spectra of a LiF crystal (sample #2), kept in the dark for more than 24 h after irradiation with fs laser pulses (solid curve) and X-rays (dashed curve) at RT. The single pulse was a Fourier transform-limited one with 70 J=pulse energy and 100 fs duration. In this case, there was no interference with the beam and it was focused by a convex lens ( f = 60 mm) onto the sample. Two main absorption bands can be observed peaking at 250 nm, due to the F centres, i.e., an electron trapped in an anion vacancy, and at 450 nm, due to the unresolved F2 and F+ 3 centres, whose band was previously called an M band. ‘M band’ means both the F2 and the F+ 3 absorption bands hereafter. As clearly shown by the normalized M absorption band, the major diFerences between the absorption spectra for sample #2 coloured with fs laser pulses and X-rays were as regards their half-widths, i.e., full-widths at half-maximum (FWHMs) for the M band around 450 nm. In the case of fs irradiation, a much wider absorption band can be distinctly observed. The half-widths at RT were 0.17 and 0:28 eV for the irradiated with X-rays and fs laser pulses, respectively; these values agree quite well with the data (i.e., the values
0.15 irradiated with fs pulses irradiated with x-rays Optical Density
were obtained from Earth Chemical Co., Ltd. (abbreviated as sample #1) and supplied by the Institute of Geochemistry, Academy of Science of Russia, Irkutsk (abbreviated as sample #2), respectively. The magnesium concentrations of each crystal as the main impurities were determined by means of an inductively coupled plasma (ICP) method and estimated to be about 1 and 119 wt ppm, respectively. In our experimental design, a regenerative ampli;ed fs pulse from a mode-locked Ti:sapphire laser (wavelength: 800 nm, pulse duration: 100 fs, repetition: 10 Hz) was split into two beams; then these were crossed on the surface of LiF plates to give a spot size of approximately 30 m diameter. The pulse energy was varied from 0.01 to 3 mJ=pulse. The colliding angle between the two beams was varied from 15◦ to 158◦ , corresponding to the recorded fringe spacings of 3–0:4 m, respectively. An outcurve sketch of this experimental set-up has already been given elsewhere (Kawamura et al., 2000). In contrast, the colour centres produced by X-rays at RT were achieved with a Cu tube, operated at 35 kV and 25 mA for 5 –15 min. Optical absorption spectra at RT were measured with a Hitachi U-3500 spectrophotometer, while the emission spectra were taken with a Hitachi F-3010 spectro:uorometer. Moreover, all the optical spectra were best ;tted with a sum of single-Gaussian bands to obtain peak positions and spectral widths precisely. Irradiation-induced morphological changes on surface or near-surface regions of samples were observed with a diFerential interference optical microscope, a confocal laser-scanning microscope, and an atomic force microscope.
Sample #2
0.10 ×
0.3
0.05
0.00 200
300
(a)
Normalized Intensity (arb. units)
760
60
700
800
x-ray, sample #2 x-ray, sample #1 fs, sample #2 fs, sample #1
50 40
λ exc = 450 nm
30 20 10 0 500
(b)
400 500 600 Wavelength ( nm)
550
600 650 700 Wavelength ( nm)
750
800
Fig. 1. Absorption spectra at RT of a LiF crystal (sample #2) irradiated with fs laser pulses (solid curve) and X-rays (dotted curve). (b) Normalized luminescence spectra peaked at 530 nm of LiF irradiated with fs laser pulses for sample #1 (solid curve) and sample #2 (dashed curve) and with X-rays for sample #1 (dotted curve) and sample #2 (dashed–dot curve) excited at 450 nm.
of the half-width of the absorption band are 0.16 –0:19 eV for F2 centres and 0.29 –0:35 eV for F+ 3 centres, from the previous measurements (Baldacchini et al., 2000) on samples coloured by ionizing radiations at RT. Fig. 1(b) represents the normalized luminescence spectra that peaked at 530 nm for sample #1 and sample #2 after the irradiation with fs laser pulses and X-rays at RT, exciting the samples with the M band at 450 nm. Two separate broad emission bands due to the F2 and F+ 3 centres were observed at 650 nm (red component) and 530 nm (green component), respectively. It was found that the green component was dominant for the irradiation with fs laser pulses for both sample #1 and sample #2, while the red component was dominant for the irradiation with only X-rays for sample #2. It should be noted here that the addition of Mg impurity as a hole trap in LiF helped to stabilize F-hole pairs, thus increasing the F centre concentration by the radiation damage processes, and a LiF crystal with the oxygen impurity
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762 25
Sample #1
F3 +
20
Intensity (arb. units)
was characterized by enhancement of the red luminescence and reduction in the intensity of the green component. It has been con;rmed by absorption and luminescence measurements that the laser-active F+ 3 centres are mainly produced even at RT by fs laser irradiation. In general, one can hardly obtain F+ 3 centres in LiF with suPciently high density by ordinary colouration processes such as radiation damage at RT. In order to attain the selective production of the F+ 3 centres, the following procedure was reported to be used (Gu et al., 1989): the sample was coloured by ionizing radiation at liquid nitrogen temperature (LNT) and then there was irradiation with the UV radiation at the same temperature, and ;nally the sample was kept in the dark at RT. It may be considered that with fs pulses, energy is deposited into the material before any energy transfer to the lattice can occur. Fig. 2(a) shows the emission spectra excited at 450 nm of a LiF crystal (sample #1) produced by chirped pulses. The duration of a chirped pulse was varied from 100 fs to 1 ps. The laser used an ampli;er system based on the chirped-pulse-ampli;cation (CPA) technique. The CPA technique allows variable pulse width (Qt, FWHM) without changing other parameters such as pulse energy and spectral width (Qv, FWHM). For the case of transform-limited-pulse (TLP) assuming the gaussian-pulse envelope, the time-bandwidth product becomes QtQv = 0:441. For a pulse duration of 500 fs and 1 ps, QtQv = 2:21 and 4.41, respectively. It was found from the emission spectra that the colour centres are encoded at deeper layers inside the sample with increasing chirping degrees. Moreover, chirp-free pulses do not form colour centres inside the sample despite the peak power density (Kawamura et al., 2002). Fig. 2(b) shows a photograph of the :uorescent patterns of the F+ 3 (more) and F2 (fewer) colour centres written inside a LiF crystal with chirped infrared laser pulses. The width of the waveguide was about 10 m. It should be noted that each active waveguide was encoded at diFerent layers inside LiF bulk. We also examined the refractive index variation of the waveguide by using a He–Ne laser (633 nm). As a result, it was found that the value of the refractive index of the coloured region was 1.406 at 633 nm; this value was compared to that for the virgin area of the crystal, i.e., 1.3912 (Palik, 1997–1998) at 640 nm. The induced change in the refractive index was assessed to be about ∼ 1%. Fig. 3(a) shows the typical output spectra at RT of the sample pumped above threshold. The DFB output spectrum is compared to that of the ampli;ed spontaneous emission (ASE). As shown in Figs. 1(b) and 2(b), the F+ 3 centres are dominantly produced in the case of fs laser irradiation. If we intend to obtain the DFB laser action at around 530 nm (green component) due to the F+ 3 centres, we need a grating with periods as ;ne as 378 nm for the second-order Bragg re:ection. However, an infrared fs laser at 800 nm used in this work is not available for obtaining such a ;ne grating.
761
λ exc = 450 nm ∆t∆ν
15
100 fs ( 0.441 ) 500 fs ( 2.21 ) 1000 fs ( 4.41 )
10
F2
5 0 500
(a)
600
700
800
900
Wavelength ( nm )
(b) Fig. 2. (a) Emission spectra excited at 450 nm of a LiF crystal (sample #1) produced by chirped pulses. The duration of a chirped pulse is 100 fs (solid curve), 500 fs (dashed curve), and 1 ps (dotted curve). (b) A photograph of the :uorescent pattern of a three-dimensional active channel waveguide excited at 450 nm and a microscopic photograph of the waveguide at a larger magni;cation.
Therefore, in order to obtain the DFB laser action in the red spectral region, the sample with the periodic structures and the colour centres encoded by two interfering fs laser pulses had to be irradiated by X-rays to enhance the red component. As a result, the DFB laser action was achieved by placing the sample on a precisely positioning stage and transversely pumping with the 450 nm line (pulse duration: 5 ns, repetition: 10 Hz, energy: 0:8 mJ=pulse) of an OPO laser. The laser beam was focused by a cylindrical lens to realize homogeneous excitation of the entire grating region whose measured periods of the refractive-index modulation was 510 nm. The linewidth of the DFB output is less than 1 nm. The measured wavelength of the DFB output at 706:8 nm
Normalized Intensity (arb. units)
762
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762
30 25
The new technique, which is applicable not only for producing laser-active colour centres, but also for encoding non-erasable periodic structures in the refractive index inside the wide-gap material may well be a useful method in making miniaturized optical devices.
OPO laser
Sample #2
λ exc = 450 nm 0.8 mJ/pulse
20
DFB ASE
15
Acknowledgements
10 5 0 680
(a)
690
700 710 720 730 Wavelength (nm)
740
750
One of the authors (TK) would like to thank E. Radzhabov (Russian Academy of Sciences) for providing me with some crystals used in the work and for valuable discussions. This work was carried out within the Collaborative Research Project of the Materials and Structures Laboratory, Tokyo Institute of Technology. References
(b) Fig. 3. (a) The DFB (solid curve) and ASE (dotted curve) output spectra of LiF at RT. These curves were obtained by pumping the sample containing F2 centres with ;ne-pitched grating with a pulsed OPO laser at 450 nm. (b) A photograph of the DFB laser action at around 700 nm.
is in good agreement with the calculated value of 708:9 nm predicted from the grating period and index of refraction (n = 1:39) of the LiF host for the second-order Bragg re:ection. The misalignment angle of 0:2◦ results in an experimental error of about 1 nm for the oscillating wavelength. Fig. 3(b) shows a photograph of the DFB laser action at 709 nm in LiF at RT. In conclusion, we demonstrate, for the ;rst time, three-dimensional active channel waveguide inside bulk LiF produced by a chirped pulse and a pulsed DFB laser, based on the F2 colour centres with ;ne-pitched micro-grating structures, fabricated by two interfering fs laser pulses.
Baldacchini, G., Montereali, R.M., 2001. New perspectives of coloured LiF for optoelectronic devices. Opt. Mater. 16, 53–61. Baldacchini, G., Nicola, E.De., Montereali, R.M., Scacco, A., Kalinov, V., 2000. Optical bands of F2 and F+ 3 centers in LiF. J. Phys. Chem. Solids 61, 21–26. Gu, H.E., Qi, L., Wan, L.F., Guo, H.S., 1989. The two-step creation and tunable laser operation of F+ 3 centers in LiF crystals. Opt. Commun. 70, 141–144. Kawamura, K., Sarukura, N., Hirano, M., Hosono, H., 2000. Holographic encoding of permanent gratings embedded in diamond by two beam interference of a single femtosecond near-infrared laser pulse. Jpn. J. Appl. Phys. 39, L767–L769. Kawamura, K., Hirano, M., Kamiya, T., Hosono, H., 2002. Holographic writing of volume-type microgratings in silica glass by a single chirped laser pulse. Appl. Phys. Lett. 81, 1137–1139. Kulinski, T., Kaczmarek, F., Ludwiczak, M., Blaszczak, Z., 1980. Emission characteristics of an LiF : F2 colour centre laser in the visible. Opt. Commun. 35, 120–124. Kurobori, T., Inabe, K., Takeuchi, N., 1983. Room temperature visible distributed-feedback colour centre laser. J. Phys. D: Appl. Phys. 16, L121–L123. Kurobori, T., Kawamura, K., Hirano, M., Hosono, H., 2003. Simultaneous fabrication of laser-active colour centres and permanent microgratings in lithium :uoride by a single femtosecond pulse. J. Phys.: Condens. Matter 15, L399–L405. Larciprete, R., Gregoratti, L., Danailov, M., Montereali, R.M., Bon;gli, F., Kiskinova, M., 2002. Direct writing of :uorescent patterns on LiF ;lms by X-ray microprobe. Appl. Phys. Lett. 80, 3862–3864. Palik, E.D., (Ed.), 1997–1998. Handbook of Optical Constants of Solids. Academic Press, New York. Ter-Mikirtychev, V.V., Tsuboi, T., 1996. Stable room-temperature tunable color center lasers and passive Q-switchers. Prog. Quantum Electron. 20, 219–268.
Laser-active colour centres with functional periodic structures in LiF fabricated by two interfering femtosecond laser pulses Toshio Kuroboria;∗ , Tetsu Kitaoa , Yukio Hirosea , Ken-ichi Kawamurab , Daijyu Takamizuc , Masahiro Hiranob , Hideo Hosonob; c a Department
of Materials Science and Engineering, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan Transparent Electro-Active Materials(TEAM) Project, ERATO, Japan Science Technology Corporation, KSP C-1232, Sakato 3-2-1, Takatsu-ku, Kawasaki 213-0012, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b Hosono
Received 31 October 2003; received in revised form 5 November 2003; accepted 5 January 2004
Abstract We report a new technique to fabricate both laser-active F2 and F+ 3 colour centres in lithium :uoride and permanent periodic gratings with fringe spacings as ;ne as sub-micron size simultaneously by two interfering infrared femtosecond (fs) laser pulses. In particular, the optical properties of such colour centres produced by a single fs laser pulse are compared with those created by damage from radiation such as X-rays. Moreover, the present technique is applied to the ;rst production of three-dimensional active channel waveguide and a pulsed distributed-feedback (DFB) laser at around 700 nm in LiF containing F2 colour centres with ;ne-pitched micro-grating structures. c 2004 Elsevier Ltd. All rights reserved. Keywords: DFB laser; Colour centres; Femtosecond laser; Lithium :uoride
1. Introduction Among the alkali halide crystals, lithium :uoride (LiF) is of particular interest because of the excellent thermal and optical stabilities of its laser-active colour centres even at room temperature (RT) as well as its good physical and chemical properties. In particular, the F2 and F+ 3 colour centres (two electrons bound to two or three neighbouring anion vacancies, respectively) in LiF are very promising candidates for producing visible laser action from the green-to-red spectral range (Kulinski et al., 1980; Kurobori et al., 1983; Ter-Mikirtychev and Tsuboi, 1996) at RT and the realization of miniaturized optical devices (Baldacchini and Montereali, 2001; Larciprete et al., 2002). However, due to it having the strongest ionic binding and a larger energy band gap (∼ 14 eV), and the low vapour pressure of lithium metal, LiF, in the form of bulk or ;lms, cannot be ∗ Corresponding author. Tel.: +81-76-264-5478; fax: +81-76-234-4132. E-mail address: [email protected] (T. Kurobori).
c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2004.01.006
coloured with the usual technique of additive colouration and colour centres are only produced with low-ionizing radiation such as electrons, - and X-rays. Recently, we successfully fabricated laser-active colour centres and ;ne-pitched micro-grating structures in LiF simultaneously by a holographic technique using two femtosecond (fs) laser pulses (Kurobori et al., 2003). In this paper, we report the optical properties of these centres in LiF crystals, which contain diFerent quantities of impurities, produced by fs laser pulses compared with those created by damage from X-ray radiation. Moreover, the present technique is, for the ;rst time, to our knowledge, applied to fabricate three-dimensional active channel waveguide inside bulk LiF up to ∼ 5 mm in depth by a single chirped (0.1–1 ps duration) laser pulse, and a DFB laser, based on F2 centres in LiF produced by two interfering fs pulses. 2. Experimental details Two LiF crystals with thickness of approximately 1:5 mm, which contained diFerent quantities of impurities,
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762
3. Experimental results and discussion Fig. 1(a) shows the optical absorption spectra of a LiF crystal (sample #2), kept in the dark for more than 24 h after irradiation with fs laser pulses (solid curve) and X-rays (dashed curve) at RT. The single pulse was a Fourier transform-limited one with 70 J=pulse energy and 100 fs duration. In this case, there was no interference with the beam and it was focused by a convex lens ( f = 60 mm) onto the sample. Two main absorption bands can be observed peaking at 250 nm, due to the F centres, i.e., an electron trapped in an anion vacancy, and at 450 nm, due to the unresolved F2 and F+ 3 centres, whose band was previously called an M band. ‘M band’ means both the F2 and the F+ 3 absorption bands hereafter. As clearly shown by the normalized M absorption band, the major diFerences between the absorption spectra for sample #2 coloured with fs laser pulses and X-rays were as regards their half-widths, i.e., full-widths at half-maximum (FWHMs) for the M band around 450 nm. In the case of fs irradiation, a much wider absorption band can be distinctly observed. The half-widths at RT were 0.17 and 0:28 eV for the irradiated with X-rays and fs laser pulses, respectively; these values agree quite well with the data (i.e., the values
0.15 irradiated with fs pulses irradiated with x-rays Optical Density
were obtained from Earth Chemical Co., Ltd. (abbreviated as sample #1) and supplied by the Institute of Geochemistry, Academy of Science of Russia, Irkutsk (abbreviated as sample #2), respectively. The magnesium concentrations of each crystal as the main impurities were determined by means of an inductively coupled plasma (ICP) method and estimated to be about 1 and 119 wt ppm, respectively. In our experimental design, a regenerative ampli;ed fs pulse from a mode-locked Ti:sapphire laser (wavelength: 800 nm, pulse duration: 100 fs, repetition: 10 Hz) was split into two beams; then these were crossed on the surface of LiF plates to give a spot size of approximately 30 m diameter. The pulse energy was varied from 0.01 to 3 mJ=pulse. The colliding angle between the two beams was varied from 15◦ to 158◦ , corresponding to the recorded fringe spacings of 3–0:4 m, respectively. An outcurve sketch of this experimental set-up has already been given elsewhere (Kawamura et al., 2000). In contrast, the colour centres produced by X-rays at RT were achieved with a Cu tube, operated at 35 kV and 25 mA for 5 –15 min. Optical absorption spectra at RT were measured with a Hitachi U-3500 spectrophotometer, while the emission spectra were taken with a Hitachi F-3010 spectro:uorometer. Moreover, all the optical spectra were best ;tted with a sum of single-Gaussian bands to obtain peak positions and spectral widths precisely. Irradiation-induced morphological changes on surface or near-surface regions of samples were observed with a diFerential interference optical microscope, a confocal laser-scanning microscope, and an atomic force microscope.
Sample #2
0.10 ×
0.3
0.05
0.00 200
300
(a)
Normalized Intensity (arb. units)
760
60
700
800
x-ray, sample #2 x-ray, sample #1 fs, sample #2 fs, sample #1
50 40
λ exc = 450 nm
30 20 10 0 500
(b)
400 500 600 Wavelength ( nm)
550
600 650 700 Wavelength ( nm)
750
800
Fig. 1. Absorption spectra at RT of a LiF crystal (sample #2) irradiated with fs laser pulses (solid curve) and X-rays (dotted curve). (b) Normalized luminescence spectra peaked at 530 nm of LiF irradiated with fs laser pulses for sample #1 (solid curve) and sample #2 (dashed curve) and with X-rays for sample #1 (dotted curve) and sample #2 (dashed–dot curve) excited at 450 nm.
of the half-width of the absorption band are 0.16 –0:19 eV for F2 centres and 0.29 –0:35 eV for F+ 3 centres, from the previous measurements (Baldacchini et al., 2000) on samples coloured by ionizing radiations at RT. Fig. 1(b) represents the normalized luminescence spectra that peaked at 530 nm for sample #1 and sample #2 after the irradiation with fs laser pulses and X-rays at RT, exciting the samples with the M band at 450 nm. Two separate broad emission bands due to the F2 and F+ 3 centres were observed at 650 nm (red component) and 530 nm (green component), respectively. It was found that the green component was dominant for the irradiation with fs laser pulses for both sample #1 and sample #2, while the red component was dominant for the irradiation with only X-rays for sample #2. It should be noted here that the addition of Mg impurity as a hole trap in LiF helped to stabilize F-hole pairs, thus increasing the F centre concentration by the radiation damage processes, and a LiF crystal with the oxygen impurity
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762 25
Sample #1
F3 +
20
Intensity (arb. units)
was characterized by enhancement of the red luminescence and reduction in the intensity of the green component. It has been con;rmed by absorption and luminescence measurements that the laser-active F+ 3 centres are mainly produced even at RT by fs laser irradiation. In general, one can hardly obtain F+ 3 centres in LiF with suPciently high density by ordinary colouration processes such as radiation damage at RT. In order to attain the selective production of the F+ 3 centres, the following procedure was reported to be used (Gu et al., 1989): the sample was coloured by ionizing radiation at liquid nitrogen temperature (LNT) and then there was irradiation with the UV radiation at the same temperature, and ;nally the sample was kept in the dark at RT. It may be considered that with fs pulses, energy is deposited into the material before any energy transfer to the lattice can occur. Fig. 2(a) shows the emission spectra excited at 450 nm of a LiF crystal (sample #1) produced by chirped pulses. The duration of a chirped pulse was varied from 100 fs to 1 ps. The laser used an ampli;er system based on the chirped-pulse-ampli;cation (CPA) technique. The CPA technique allows variable pulse width (Qt, FWHM) without changing other parameters such as pulse energy and spectral width (Qv, FWHM). For the case of transform-limited-pulse (TLP) assuming the gaussian-pulse envelope, the time-bandwidth product becomes QtQv = 0:441. For a pulse duration of 500 fs and 1 ps, QtQv = 2:21 and 4.41, respectively. It was found from the emission spectra that the colour centres are encoded at deeper layers inside the sample with increasing chirping degrees. Moreover, chirp-free pulses do not form colour centres inside the sample despite the peak power density (Kawamura et al., 2002). Fig. 2(b) shows a photograph of the :uorescent patterns of the F+ 3 (more) and F2 (fewer) colour centres written inside a LiF crystal with chirped infrared laser pulses. The width of the waveguide was about 10 m. It should be noted that each active waveguide was encoded at diFerent layers inside LiF bulk. We also examined the refractive index variation of the waveguide by using a He–Ne laser (633 nm). As a result, it was found that the value of the refractive index of the coloured region was 1.406 at 633 nm; this value was compared to that for the virgin area of the crystal, i.e., 1.3912 (Palik, 1997–1998) at 640 nm. The induced change in the refractive index was assessed to be about ∼ 1%. Fig. 3(a) shows the typical output spectra at RT of the sample pumped above threshold. The DFB output spectrum is compared to that of the ampli;ed spontaneous emission (ASE). As shown in Figs. 1(b) and 2(b), the F+ 3 centres are dominantly produced in the case of fs laser irradiation. If we intend to obtain the DFB laser action at around 530 nm (green component) due to the F+ 3 centres, we need a grating with periods as ;ne as 378 nm for the second-order Bragg re:ection. However, an infrared fs laser at 800 nm used in this work is not available for obtaining such a ;ne grating.
761
λ exc = 450 nm ∆t∆ν
15
100 fs ( 0.441 ) 500 fs ( 2.21 ) 1000 fs ( 4.41 )
10
F2
5 0 500
(a)
600
700
800
900
Wavelength ( nm )
(b) Fig. 2. (a) Emission spectra excited at 450 nm of a LiF crystal (sample #1) produced by chirped pulses. The duration of a chirped pulse is 100 fs (solid curve), 500 fs (dashed curve), and 1 ps (dotted curve). (b) A photograph of the :uorescent pattern of a three-dimensional active channel waveguide excited at 450 nm and a microscopic photograph of the waveguide at a larger magni;cation.
Therefore, in order to obtain the DFB laser action in the red spectral region, the sample with the periodic structures and the colour centres encoded by two interfering fs laser pulses had to be irradiated by X-rays to enhance the red component. As a result, the DFB laser action was achieved by placing the sample on a precisely positioning stage and transversely pumping with the 450 nm line (pulse duration: 5 ns, repetition: 10 Hz, energy: 0:8 mJ=pulse) of an OPO laser. The laser beam was focused by a cylindrical lens to realize homogeneous excitation of the entire grating region whose measured periods of the refractive-index modulation was 510 nm. The linewidth of the DFB output is less than 1 nm. The measured wavelength of the DFB output at 706:8 nm
Normalized Intensity (arb. units)
762
T. Kurobori et al. / Radiation Measurements 38 (2004) 759 – 762
30 25
The new technique, which is applicable not only for producing laser-active colour centres, but also for encoding non-erasable periodic structures in the refractive index inside the wide-gap material may well be a useful method in making miniaturized optical devices.
OPO laser
Sample #2
λ exc = 450 nm 0.8 mJ/pulse
20
DFB ASE
15
Acknowledgements
10 5 0 680
(a)
690
700 710 720 730 Wavelength (nm)
740
750
One of the authors (TK) would like to thank E. Radzhabov (Russian Academy of Sciences) for providing me with some crystals used in the work and for valuable discussions. This work was carried out within the Collaborative Research Project of the Materials and Structures Laboratory, Tokyo Institute of Technology. References
(b) Fig. 3. (a) The DFB (solid curve) and ASE (dotted curve) output spectra of LiF at RT. These curves were obtained by pumping the sample containing F2 centres with ;ne-pitched grating with a pulsed OPO laser at 450 nm. (b) A photograph of the DFB laser action at around 700 nm.
is in good agreement with the calculated value of 708:9 nm predicted from the grating period and index of refraction (n = 1:39) of the LiF host for the second-order Bragg re:ection. The misalignment angle of 0:2◦ results in an experimental error of about 1 nm for the oscillating wavelength. Fig. 3(b) shows a photograph of the DFB laser action at 709 nm in LiF at RT. In conclusion, we demonstrate, for the ;rst time, three-dimensional active channel waveguide inside bulk LiF produced by a chirped pulse and a pulsed DFB laser, based on the F2 colour centres with ;ne-pitched micro-grating structures, fabricated by two interfering fs laser pulses.
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