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Femtosecond laser irradiation on YAG and sapphire crystals
ARTICLE IN PRESS
Journal of Crystal Growth 260 (2004) 181–185
Femtosecond laser irradiation on YAG and sapphire crystals Chengyong Jiang*, Guoqing Zhou, Jun Xu, Peizhen Deng, Fuxi Gan Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Science, R&D Center for Laser & Optoelectronic Materials No. 390, Qinghe Road, Jiading, Shanghai 201800, China Received 20 June 2003; accepted 21 August 2003 Communicated by M. Schieber
Abstract YAG and sapphire crystals were irradiated by 800 nm and 120 fs laser pulses. Crystals changed to dark and absorption increased after irradiation. Irradiation induced cracks were found in SEM photograph of CZ grown sapphire. After annealing at 500 C for an hour, absorption of YAG decreased and absorption of sapphire increased. Darkness of YAG crystal was attributed to formation of color centers, and the darkness of sapphire was caused by irradiation-induced micro-cracks. Irradiation resistance of YAG is better than sapphire according to micrograph and absorption spectrum. r 2003 Elsevier B.V. All rights reserved. PACS: 78.20. e; 76.30.Mi; 79.20.Ds Keywords: A1. Absorption; A1. Color center; A1. Femtosecond laser; B1. Sapphire; B1. YAG
1. Introduction Studies on mechanisms of interaction between wide band gap materials and laser are dated back to the 1960s [1,2]. With the development of laser technology, especially the appearance of femtosecond pulse laser, irradiation effect of femtosecond pulse laser on optical materials attracts more attention for its high power density and ultrashort pulses width. Many papers are about interaction between optical glass and femtosecond laser. Glass change to black after femtosecond laser pulses irradiation and it was attributed to formation of color centers during irradiation [3,4].
But studies on structural change of crystal irradiated by ultra-short laser pulses are less available. Compared with optical glass, formation of color centers in oxide crystal is more difficult. Study on irradiation effect on crystals by ultrashort laser is attractive because of its wide application. Both Yttrium Aluminum Garnet (YAG) and sapphire are important laser host and windows materials for their excellent optical and mechanical properties [5]. This work focused on structural and optical changes of sapphire and YAG crystals irradiated by femtosecond laser pulses. 2. Experimental procedure
*Corresponding author. Tel.: +86-21-59915634; fax: +8621-59928755. E-mail address: [email protected] (C. Jiang).
Sapphire and YAG crystals used in this work were grown by Czochralski method (CZ) and
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.029
ARTICLE IN PRESS 182
C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
temperature gradient technique method (TGT). Both samples were cut to 10 10 1 mm3 and given mechanically polished with diamond paste. Sapphire surface is paralleled (0 0 0 1) plane and YAG surface is paralleled (1 1 1) plane. The irradiation procedure was performed in Photo Craft Lab, Shanghai Institute of Optics and Fine Mechanics. The system produces 800 nm laser with pulse duration of 120 fs at a repetition of 1 kHz. The laser spot was focused interior of the crystal. The sample was cleaned with acetone before mounting onto a three-axis translation stage, and then the crystal was irradiated by
Fig. 1. Schematic view of crystals irradiated by femtosecond laser pulses.
translating it across tightly focused laser beam at linear speed of 1000 mm/s. As shown in Fig. 1, a plane was scanned in the sample. The irradiated specimens were observed by optical microscope and JEOL JSM-6360 scanning electron microscope. Absorption spectra were taken with JASCO V—570 UV/VIS/NIR spectrometer.
3. Results and discussion Unlike ablation on crystals, the laser spot was focused into the sample to avoid effects of sample surface and atmosphere. A dark plane in crystal was formed and some cracks were found in CZ grown sapphire after irradiation. Micrograph of front (ABCD plane in Fig. 1) and side (ABEF plane in Fig. 1) of YAG and sapphire were shown in Figs. 2 and 3. It can be found that the dark plane is composed with black lines irradiated by laser pulses. Compare two kinds of crystals, color of sapphire is darker than YAG sample Side of the crystals (ABEF plane in Fig. 1) was observed under SEM. Although the irradiated region can be seen distinctly under optical microscope, nothing was found under SEM except some
Fig. 2. Front micrograph of crystals after femtosecond laser irradiation: (A) YAG grown by CZ method, (B) YAG grown by TGT method, (C) Sapphire crystal grown by CZ method, and (D) Sapphire crystal grown by TGT method.
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
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Fig. 3. Side micrograph of crystals after femtosecond laser irradiation: (A) YAG grown by CZ method, (B) YAG grown by TGT method, (C) Sapphire crystal grown by CZ method, and (D) Sapphire crystal grown by TGT method.
Fig. 4. SEM picture of sapphire grown by CZ method at different magnification.
cracks in CZ grown sapphire. Different optical properties between irradiated region and unirradiated region made irradiated plane can be seen under optical microscope, but SEM just investigated surface information. Fig. 4 showed cracks in CZ grown sapphire at different magni-
fication. Cracks just appeared the irradiated region of sample and they should be induced by the laser irradiation. Absorption spectra of YAG and sapphire crystal were shown in Fig. 5. Both YAG and sapphire increased after femtosecond laser
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
184
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1.2 before irradiation after irradiation annealed at 500°C
0.8 0.6 0.4
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before irradiation after irradiation annealed at 500°C
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Fig. 5. Absorption spectra of crystals after femtosecond laser irradiation and annealing at 500 C: (A) YAG crystals grown by CZ method, (B) YAG crystals grown by TGT method, (C) Sapphire grown by CZ method and (D) Sapphire grown by TGT method.
irradiation. The 255 nm absorption peak in CZ grown YAG was weakened and no new absorption peak appeared after irradiation. Annealed at 500 C for an hour, absorption decreased remarkably and the 255 nm absorption peak reappeared in CZ grown YAG. Compared with YAG crystals, annealing effect on sapphire is reverse. Absorption of sapphire increased after annealing. Efimov had reported the darkness in optical glass irradiated by femtosecond laser and attributed it to color centers formation during irradiation [2]. Laser induced color centers in alkali halide were also reported [6,7]. Annealing in air can annihilate most of oxygen vacancy color centers. Absorption decreasing after anneal indicated that
the laser irradiation induced color centers caused absorption increasing in YAG crystals. The origin of 255 nm absorption peak in CZ grown YAG was attributed to Fe3+ ions [8]. The 255 nm peak disappeared after irradiated and reappeared after annealing in this work. The results indicated that Fe3+ ions changed to Fe2+ ions during femtosecond laser and Fe2+ ions were oxidized to Fe3+ ions after annealing in air. The situation in sapphire is different. Cracks were found in SEM after irradiation and the absorption increased after annealing. It can be suggested that the darkness in sapphire were related irradiation induced microcracks. Irradiated effects on sapphire and YAG crystals are different, although conditions of
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
femtosecond laser irradiation are same. Eliminable color centers were formed in YAG crystals during irradiation, while micro-cracks were formed in sapphire during femtosecond laser irradiation, then these micro-cracks expanded to macro-cracks in latter processes. So absorption of sapphire increased after annealing. In author’s opinions, the different effects of laser irradiation are caused by self-focusing in sapphire. When self-focusing took place, the energy density of laser increased. Material will be broken down when energy density is larger than damage threshold. Filiform struct, which is character of self-focus, can be found in sapphire side micrograph (D in Fig. 3). Micrograph and absorption spectrum results showed that YAG crystal has better anti-irradiation ability than sapphire. Compared crystals grown by different methods, CZ grown YAG sample is almost same with TGT sample, but anti-irradiation ability of TGT grown sapphire is better than CZ sample because of less cracks were found. This is related to less defects density in sapphire grown by TGT method [9].
4. Conclusion After femtosecond laser pulses irradiation, both YAG and sapphire crystals changed to dark. Irradiation-induced cracks and filiform struct were
185
found in sapphire sample. Fe3+ ions in CZ grown YAG crystals changed to Fe2+ ions during laser irradiation and Fe2+ ions were oxidized to Fe3+ ions after annealing in air. Absorption spectra indicated that color centers were formed in YAG during irradiation and darkness in sapphire was caused by irradiation-induced micro-cracks. Different irradiated effect between sapphire and YAG were attribute to self-focus in sapphire. Antiirradiation ability of YAG is better than sapphire according micrograph and absorption spectrum. Compared two growth method, TGT grown sapphire is better than CZ sample.
References [1] A.J. Pedraza, Nucl. Instr. and Meth. B 141 (1998) 709. [2] W. Yablonovitch, Appl. Phys. Lett. 19 (1971) 495. [3] O.M. Efinov, K. Gabel, S.V. Garnov, J. Opt. Soc. Am. B15 (1998) 193. [4] Jiang Xiong-Wei, Zhu Cong-San, Gan Fu-Xi, et al., Chin. J. Laser 28 (2001) 602. [5] N.C. Fernelius, G.A. Graves, SPIE 297 (1981) 188. [6] C.H. Chen, M.P. McCamm, Opt. Commun. 60 (1986) 296. [7] R.L. Webb, L.C. Jensen, S.C. Langford, et al., J. Appl. Phys. 74 (1994) 2338. [8] Chengyong Jiang, Peizhen Deng, Fuxi G, J. Synth. Crystals 31 (2002) 573. [9] Xu Jianwei, Zhou Yoongzhong, Zhou Guoqing, et al., J. Crystal Growth 193 (1998) 123.
Journal of Crystal Growth 260 (2004) 181–185
Femtosecond laser irradiation on YAG and sapphire crystals Chengyong Jiang*, Guoqing Zhou, Jun Xu, Peizhen Deng, Fuxi Gan Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Science, R&D Center for Laser & Optoelectronic Materials No. 390, Qinghe Road, Jiading, Shanghai 201800, China Received 20 June 2003; accepted 21 August 2003 Communicated by M. Schieber
Abstract YAG and sapphire crystals were irradiated by 800 nm and 120 fs laser pulses. Crystals changed to dark and absorption increased after irradiation. Irradiation induced cracks were found in SEM photograph of CZ grown sapphire. After annealing at 500 C for an hour, absorption of YAG decreased and absorption of sapphire increased. Darkness of YAG crystal was attributed to formation of color centers, and the darkness of sapphire was caused by irradiation-induced micro-cracks. Irradiation resistance of YAG is better than sapphire according to micrograph and absorption spectrum. r 2003 Elsevier B.V. All rights reserved. PACS: 78.20. e; 76.30.Mi; 79.20.Ds Keywords: A1. Absorption; A1. Color center; A1. Femtosecond laser; B1. Sapphire; B1. YAG
1. Introduction Studies on mechanisms of interaction between wide band gap materials and laser are dated back to the 1960s [1,2]. With the development of laser technology, especially the appearance of femtosecond pulse laser, irradiation effect of femtosecond pulse laser on optical materials attracts more attention for its high power density and ultrashort pulses width. Many papers are about interaction between optical glass and femtosecond laser. Glass change to black after femtosecond laser pulses irradiation and it was attributed to formation of color centers during irradiation [3,4].
But studies on structural change of crystal irradiated by ultra-short laser pulses are less available. Compared with optical glass, formation of color centers in oxide crystal is more difficult. Study on irradiation effect on crystals by ultrashort laser is attractive because of its wide application. Both Yttrium Aluminum Garnet (YAG) and sapphire are important laser host and windows materials for their excellent optical and mechanical properties [5]. This work focused on structural and optical changes of sapphire and YAG crystals irradiated by femtosecond laser pulses. 2. Experimental procedure
*Corresponding author. Tel.: +86-21-59915634; fax: +8621-59928755. E-mail address: [email protected] (C. Jiang).
Sapphire and YAG crystals used in this work were grown by Czochralski method (CZ) and
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.029
ARTICLE IN PRESS 182
C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
temperature gradient technique method (TGT). Both samples were cut to 10 10 1 mm3 and given mechanically polished with diamond paste. Sapphire surface is paralleled (0 0 0 1) plane and YAG surface is paralleled (1 1 1) plane. The irradiation procedure was performed in Photo Craft Lab, Shanghai Institute of Optics and Fine Mechanics. The system produces 800 nm laser with pulse duration of 120 fs at a repetition of 1 kHz. The laser spot was focused interior of the crystal. The sample was cleaned with acetone before mounting onto a three-axis translation stage, and then the crystal was irradiated by
Fig. 1. Schematic view of crystals irradiated by femtosecond laser pulses.
translating it across tightly focused laser beam at linear speed of 1000 mm/s. As shown in Fig. 1, a plane was scanned in the sample. The irradiated specimens were observed by optical microscope and JEOL JSM-6360 scanning electron microscope. Absorption spectra were taken with JASCO V—570 UV/VIS/NIR spectrometer.
3. Results and discussion Unlike ablation on crystals, the laser spot was focused into the sample to avoid effects of sample surface and atmosphere. A dark plane in crystal was formed and some cracks were found in CZ grown sapphire after irradiation. Micrograph of front (ABCD plane in Fig. 1) and side (ABEF plane in Fig. 1) of YAG and sapphire were shown in Figs. 2 and 3. It can be found that the dark plane is composed with black lines irradiated by laser pulses. Compare two kinds of crystals, color of sapphire is darker than YAG sample Side of the crystals (ABEF plane in Fig. 1) was observed under SEM. Although the irradiated region can be seen distinctly under optical microscope, nothing was found under SEM except some
Fig. 2. Front micrograph of crystals after femtosecond laser irradiation: (A) YAG grown by CZ method, (B) YAG grown by TGT method, (C) Sapphire crystal grown by CZ method, and (D) Sapphire crystal grown by TGT method.
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
183
Fig. 3. Side micrograph of crystals after femtosecond laser irradiation: (A) YAG grown by CZ method, (B) YAG grown by TGT method, (C) Sapphire crystal grown by CZ method, and (D) Sapphire crystal grown by TGT method.
Fig. 4. SEM picture of sapphire grown by CZ method at different magnification.
cracks in CZ grown sapphire. Different optical properties between irradiated region and unirradiated region made irradiated plane can be seen under optical microscope, but SEM just investigated surface information. Fig. 4 showed cracks in CZ grown sapphire at different magni-
fication. Cracks just appeared the irradiated region of sample and they should be induced by the laser irradiation. Absorption spectra of YAG and sapphire crystal were shown in Fig. 5. Both YAG and sapphire increased after femtosecond laser
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
184
1.2
1.2 before irradiation after irradiation annealed at 500°C
0.8 0.6 0.4
0.6 0.4
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before irradiation after irradiation annealed at 500°C
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Fig. 5. Absorption spectra of crystals after femtosecond laser irradiation and annealing at 500 C: (A) YAG crystals grown by CZ method, (B) YAG crystals grown by TGT method, (C) Sapphire grown by CZ method and (D) Sapphire grown by TGT method.
irradiation. The 255 nm absorption peak in CZ grown YAG was weakened and no new absorption peak appeared after irradiation. Annealed at 500 C for an hour, absorption decreased remarkably and the 255 nm absorption peak reappeared in CZ grown YAG. Compared with YAG crystals, annealing effect on sapphire is reverse. Absorption of sapphire increased after annealing. Efimov had reported the darkness in optical glass irradiated by femtosecond laser and attributed it to color centers formation during irradiation [2]. Laser induced color centers in alkali halide were also reported [6,7]. Annealing in air can annihilate most of oxygen vacancy color centers. Absorption decreasing after anneal indicated that
the laser irradiation induced color centers caused absorption increasing in YAG crystals. The origin of 255 nm absorption peak in CZ grown YAG was attributed to Fe3+ ions [8]. The 255 nm peak disappeared after irradiated and reappeared after annealing in this work. The results indicated that Fe3+ ions changed to Fe2+ ions during femtosecond laser and Fe2+ ions were oxidized to Fe3+ ions after annealing in air. The situation in sapphire is different. Cracks were found in SEM after irradiation and the absorption increased after annealing. It can be suggested that the darkness in sapphire were related irradiation induced microcracks. Irradiated effects on sapphire and YAG crystals are different, although conditions of
ARTICLE IN PRESS C. Jiang et al. / Journal of Crystal Growth 260 (2004) 181–185
femtosecond laser irradiation are same. Eliminable color centers were formed in YAG crystals during irradiation, while micro-cracks were formed in sapphire during femtosecond laser irradiation, then these micro-cracks expanded to macro-cracks in latter processes. So absorption of sapphire increased after annealing. In author’s opinions, the different effects of laser irradiation are caused by self-focusing in sapphire. When self-focusing took place, the energy density of laser increased. Material will be broken down when energy density is larger than damage threshold. Filiform struct, which is character of self-focus, can be found in sapphire side micrograph (D in Fig. 3). Micrograph and absorption spectrum results showed that YAG crystal has better anti-irradiation ability than sapphire. Compared crystals grown by different methods, CZ grown YAG sample is almost same with TGT sample, but anti-irradiation ability of TGT grown sapphire is better than CZ sample because of less cracks were found. This is related to less defects density in sapphire grown by TGT method [9].
4. Conclusion After femtosecond laser pulses irradiation, both YAG and sapphire crystals changed to dark. Irradiation-induced cracks and filiform struct were
185
found in sapphire sample. Fe3+ ions in CZ grown YAG crystals changed to Fe2+ ions during laser irradiation and Fe2+ ions were oxidized to Fe3+ ions after annealing in air. Absorption spectra indicated that color centers were formed in YAG during irradiation and darkness in sapphire was caused by irradiation-induced micro-cracks. Different irradiated effect between sapphire and YAG were attribute to self-focus in sapphire. Antiirradiation ability of YAG is better than sapphire according micrograph and absorption spectrum. Compared two growth method, TGT grown sapphire is better than CZ sample.
References [1] A.J. Pedraza, Nucl. Instr. and Meth. B 141 (1998) 709. [2] W. Yablonovitch, Appl. Phys. Lett. 19 (1971) 495. [3] O.M. Efinov, K. Gabel, S.V. Garnov, J. Opt. Soc. Am. B15 (1998) 193. [4] Jiang Xiong-Wei, Zhu Cong-San, Gan Fu-Xi, et al., Chin. J. Laser 28 (2001) 602. [5] N.C. Fernelius, G.A. Graves, SPIE 297 (1981) 188. [6] C.H. Chen, M.P. McCamm, Opt. Commun. 60 (1986) 296. [7] R.L. Webb, L.C. Jensen, S.C. Langford, et al., J. Appl. Phys. 74 (1994) 2338. [8] Chengyong Jiang, Peizhen Deng, Fuxi G, J. Synth. Crystals 31 (2002) 573. [9] Xu Jianwei, Zhou Yoongzhong, Zhou Guoqing, et al., J. Crystal Growth 193 (1998) 123.