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Application of femtosecond laser pulses for microfabrication of transparent media
Applied Surface Science 197±198 (2002) 705±709
Application of femtosecond laser pulses for microfabrication of transparent media S. Juodkazisa, S. Matsuoa, H. Misawaa,*, V. Mizeikisb, A. Marcinkeviciusb, H.-B. Sunb, Y. Tokudac, M. Takahashic, T. Yokoc, J. Nishiid a
Department of Ecosystem Engineering, Graduate School of Engineering, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan b Satellite Venture Business Laboratory, The University of Tokushima, Tokushima 770-8506, Japan c Institute for Chemical Research, Kyoto University, Kyoto, Japan d Optical Materials Division, Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Abstract Femtosecond laser microfabrication of 3D optical memories and photonic crystal (PhC) structures in solid glasses and liquid resins are demonstrated. The optical memories can be read out from both transmission and emission images. The PhC structures reveal clear signatures of photonic bandgap (PBG) and microcavity formation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser microfabrication; Photonic crystals; Optical memories
Femtosecond laser microfabrication [1] is gaining more importance due to the increasing demand for mechanical, optical, and electronic microstructures. This technique is based on permanent photomodi®cation of mechanical and optical properties of materials. By focusing a laser beam into a transparent material (see the sketch in Fig. 1(a)), high light power density is reached in the focal spot region (the encircled area). By adjusting the laser power, it is possible to achieve beam propagation with negligible linear absorption towards the focal spot region, where it becomes effectively absorbed via multi-photon transitions. Thus, dense free carrier plasma is generated in a small spatial domain. Using high numerical aperture (NA) microscope objectives for the focusing, it is possible to *
Corresponding author. Tel.: 81-88-656-7389; fax: 81-88-656-7598. E-mail address: [email protected] (H. Misawa).
reduce the photoexcited region to the size comparable to laser wavelength. The processes following the initial plasma generation depend on the material. In solids, plasma absorption leads to avalanche ionization and heating. Plasma energy relaxation to the atomic lattice destroys it, creating a void with shape nearly spherical in average, but also having some rough edges. In some liquid resins, the effect of plasma generation is to create free radicals by the light-induced bond cleavage, which subsequently triggers the propagation of photopolymerization reaction. This renders nearly spherical solid regions, while the unexposed resin remains liquid and can be removed later. Due to the non-linear nature of the damage/ solidi®cation processes, and non-uniform intensity distribution at the focal spot, the damaged/solidi®ed volume element (or voxel) may be even smaller than the focal spot. By using visible and NIR lasers, voxels with diameters < 1 mm can be recorded. High spatial
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 9 7 - 5
706
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
Fig. 1. (a) Photomodi®cation of a transparent dielectric by a focused gaussian beam, (b) recording of complex patterns in laser microfabrication experiments.
resolution and access to the bulk of materials without mechanical or chemical contact are the main advantages of laser microfabrication. The instantaneous power density can be further increased through the use of ultrashort (fs or ps) laser pulses. In the case of fs pulses, laser damage occurs as a non-thermal microexplosion. In this work we use femtosecond laser microfabrication to obtain optical memory and photonic crystal (PhC) structures in solid glass and liquid resin, and characterize their properties. Complex 1D, 2D and 3D objects can be drawn from many voxels, recorded at precisely de®ned locations as shown schematically in Fig. 1(b). In our experiments microfabrication was performed by tpulse 250 fs pulses at the fundamental (lI 800 nm) or second harmonic (lII 400 nm) wavelengths of a Ti:Sapphire laser. Oil immersion 100 microscope objective with NA 1:35 was used for the focusing. The samples were mounted on a piezoelectric transducer (PZT) controlled stage which ensured positioning with precision better than 50 nm. The solid samples were thin slabs (thickness of 200±300 mm) of vitreous silica glass (EDC from Nippon Glass), and 30NbO5=2 70TeO2 glass (prepared by the Kyoto University group). In silica, photomodi®cation at lI (lII ) occurs due to six-photon (three-photon) absorption, while in 30NbO5=2 70TeO2 at lII two-photon absorption is required. The liquid resin sample was a drop of photopolymer Nopcocure 800 (from San Nopco) on a glass substrate. The microfabrication was performed at lII , at which two-photon absorption is the lowest non-linear absorption process. The fabricated structures were subsequently characterized
Fig. 2. (a) Optical microscopy images of the array of damaged voids in vitreous silica in transmission and luminescence (magni®ed, inset). Bit volume determined from the transmission image is V 1 1 2:5 mm3 , which at the in-plane bit separation of 2.5 mm corresponds to V 1 0:4 Tbits=cm3 information density, (b) spectral signatures of bits in the white light continuum generation under 100 fs pulsed excitation at 400 nm. The dashed line shows the spectral pro®le of Rayleigh scattering. Curves 1 and 2 were recorded using different size of confocal aperture.
using conventional and laser scanning optical microscopy (LSM), scanning electron microscopy (SEM), and by linear transmission and photoluminescence spectroscopy. More details about the experimental setups and techniques can be found elsewhere [2,3]. In glass, voids can be seen both in transmission (due to light scattering from the void boundaries) and photoluminescence. Fig. 2(a) shows images of the void array. Single pulses at lII and 4.5 mW average power (at 1 kHz repetition rate), were used for recording. The void sites were luminescent under 250 nm excitation, resonant with oxygen vacancy defects VO created by the damage. The emission became suppressed after annealing at 400 C for 30 min, because the annealing is known to reconstruct damaged silica bonds. The void size found from the emission image (FWHM) is 0.6 mm. By stacking void arrays, 3D optical data storage can be facilitated. Earlier, we have demonstrated readout of the data, recorded at the density up to 73 Gb=cm3 , in transmission. Alternatively, the data can be read out using defect-related broadband continuum emission (see Fig. 2(b) and explanations in the caption) [4]. Voids, arranged periodically in a solid, can form a PhC. In PhCs, periodic spatial modulation of refractive index leads to the formation of allowed photonic energy bands, separated by photonic bandgaps (PBGs) for electromagnetic waves [5]. The PBG
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
707
Fig. 3. (a) and (b) Structure and optical transmission in a PhC with opal lattice in silica glass, (c) and (d) in a PhC with diamond lattice in 30NbO5=2 70TeO2 glass.
central wavelength is comparable to the PhC lattice period. Fig. 3(a) and (c) shows images of PhCs with opal and diamond structures recorded in silica, and 30NbO5=2 70TeO2 glass, respectively. The images indicate good structural quality of the samples. Fig. 3(b) and (d) shows their respective optical transmission spectra. Transmission dips are seen in both spectra at wavelengths close to the PBG spectral regions (as inferred from numerical simulations of photonic band structures). However, the transmission in the dips does not drop to 0, indicating incomplete PBGs. PBG formation depends crucially on the type of PhC lattice, and the amplitude of the refractive index modulation. In silica, n 1:47 is too low for the formation of complete 3D PBGs. However, opal PhCs microfabricated in silica are still interesting because they resemble widely investigated natural and arti®cial opal PhCs. 30NbO5=2 70TeO2 has much higher refractive index n 2:14 at l 1:5 mm. Some early
numerical simulations have suggested that diamond lattice of dielectric spheres in air have complete PBG at n > 2 [6]. We have simulated numerically photonic band diagrams of diamond lattice of air spheres in dielectric, and have found that complete PBG will also open in such inverse diamond PhC if n > 2. Hence, 30NbO5=2 70TeO2 glass in principle promises the complete PBG. Weakness of the transmission dip should therefore be explained by reasons, other than low refractive index contrast. We have examined re¯ection from the voids under scanning laser microscope, and found that void radius in this glass r 100 nm is somewhat smaller than that determined in transmission with conventional microscope (about 250 nm). Furthermore, the void size did not depend considerably on the laser power. At the given lattice constant a 1:35 mm, the ratio r=a 0:08 is low, indicating small photomodi®ed volume, which cannot contribute suf®ciently to the periodic light scattering,
708
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
and complete PBG does not open. This conclusion was also supported by the numerical modeling of PhC photonic band diagrams for different r=a ratios. In laser microfabricated glass PhCs some amount of random scattering due to the void shape variations is also present, and tends to diminish the PBG effects. Currently, we are working on optimizing the fabrication conditions towards increasing r=a ratio and reducing random scattering. In resin, PhCs with log-pile structure were fabricated. Log-pile lattice shown in Fig. 4(a), has facecentered tetragonal symmetry, and favors large PBGs. The rods were drawn by slowly translating focal spot of the beam, with overlap between the neighboring laser shots larger than 99%. The unexposed resin was afterwards dissolved in acetone. The refractive index of solidi®ed resin, n 1:49, is similar to that of silica. SEM image of the fabricated 3D log-pile PhC structure edge is shown in Fig. 4(b). Optical transmission spectra of several PhC samples with different in-plane rod separations a is shown in Fig. 4(c) [7]. Red shift of the transmission dips with increasing distance between the rods indicates the PBG origin of the dips. The transmission does not vanish in the dips, hence the PBGs are incomplete, most likely due to low refractive
index contrast. Despite this de®ciency, the PBG effects are suf®cient for the formation of planar microcavities (MCs). A planar MC can be formed as shown in Fig. 4(a) by creating a defected layer in the middle of log-pile PhC. This is realized by, e.g., removing every second rod. We have fabricated a log-pile structure with total number of 20 layers, and defect at the 10th layer. The rod diameter in the PhC was 2r 0:8 mm, the in-plane distance a 1:3 mm, and the rod length l 40 mm. Fig. 4(d) shows the transmission spectrum of the sample along the layer stacking direction h0 0 1i. The MC mode is evident from a peak within the transmission dip. Origin of the peak can be explained in terms of multiple re¯ections of light between two PhC mirrors surrounding the defect layer. The MC properties were also con®rmed by numeric simulation of the transmission spectra using transfer matrix technique. From the experiments we have estimated quality factor of the cavity Q 130. In conclusion, we have microfabricated optical memories and PhCs in solid and liquid transparent materials using femtosecond laser pulses. Possibilities for recording and readout of 3D optical memories in silica have been demonstrated, and signatures of PBGs
Fig. 4. (a) Log-pile structure and its geometrical parameters, the rods drawn by dashed lines are absent in the microcavity sample, (b) SEM image of the structure edge region, (c) optical transmission of the log-pile PhCs with different in-plane distance between the rods a, (d) optical transmission of the microcavity sample with a 1:3 mm, and every other rod removed from the middle layer.
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
and microcavity effects in PhC structures were found. The presented ®ndings also show the possibility to fabricate other micrometric objects relatively easily and inexpensively using this technique.
[2] [3] [4]
References [1] H. Misawa, H. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, in: H. Helvajian, K. Sugioka, M.C. Gower, J.J. Dubowski (Eds.),
[5] [6] [7]
709
Laser Applications in Microelectronic and Optoelectronic Manufacturing V, Vol. 3933, SPIE, 2000, pp. 246±260. H. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1998) 786. V. Mizeikis, S. Juodkazis, A. Marcinkevicius, S. Matsuo, H. Misawa, J. Photochem. Photobiol. C 2 (2001) 35. M. Watanabe, S. Juodkazis, H.-B. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1999) 3957. E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. K. Ho, C. Chan, C. Sokoulis, Phys. Rev. Lett. 65 (1990) 3152. H. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, J. Nishii, Opt. Lett. 26 (2001) 325.
Application of femtosecond laser pulses for microfabrication of transparent media S. Juodkazisa, S. Matsuoa, H. Misawaa,*, V. Mizeikisb, A. Marcinkeviciusb, H.-B. Sunb, Y. Tokudac, M. Takahashic, T. Yokoc, J. Nishiid a
Department of Ecosystem Engineering, Graduate School of Engineering, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506, Japan b Satellite Venture Business Laboratory, The University of Tokushima, Tokushima 770-8506, Japan c Institute for Chemical Research, Kyoto University, Kyoto, Japan d Optical Materials Division, Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Abstract Femtosecond laser microfabrication of 3D optical memories and photonic crystal (PhC) structures in solid glasses and liquid resins are demonstrated. The optical memories can be read out from both transmission and emission images. The PhC structures reveal clear signatures of photonic bandgap (PBG) and microcavity formation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser microfabrication; Photonic crystals; Optical memories
Femtosecond laser microfabrication [1] is gaining more importance due to the increasing demand for mechanical, optical, and electronic microstructures. This technique is based on permanent photomodi®cation of mechanical and optical properties of materials. By focusing a laser beam into a transparent material (see the sketch in Fig. 1(a)), high light power density is reached in the focal spot region (the encircled area). By adjusting the laser power, it is possible to achieve beam propagation with negligible linear absorption towards the focal spot region, where it becomes effectively absorbed via multi-photon transitions. Thus, dense free carrier plasma is generated in a small spatial domain. Using high numerical aperture (NA) microscope objectives for the focusing, it is possible to *
Corresponding author. Tel.: 81-88-656-7389; fax: 81-88-656-7598. E-mail address: [email protected] (H. Misawa).
reduce the photoexcited region to the size comparable to laser wavelength. The processes following the initial plasma generation depend on the material. In solids, plasma absorption leads to avalanche ionization and heating. Plasma energy relaxation to the atomic lattice destroys it, creating a void with shape nearly spherical in average, but also having some rough edges. In some liquid resins, the effect of plasma generation is to create free radicals by the light-induced bond cleavage, which subsequently triggers the propagation of photopolymerization reaction. This renders nearly spherical solid regions, while the unexposed resin remains liquid and can be removed later. Due to the non-linear nature of the damage/ solidi®cation processes, and non-uniform intensity distribution at the focal spot, the damaged/solidi®ed volume element (or voxel) may be even smaller than the focal spot. By using visible and NIR lasers, voxels with diameters < 1 mm can be recorded. High spatial
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 3 9 7 - 5
706
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
Fig. 1. (a) Photomodi®cation of a transparent dielectric by a focused gaussian beam, (b) recording of complex patterns in laser microfabrication experiments.
resolution and access to the bulk of materials without mechanical or chemical contact are the main advantages of laser microfabrication. The instantaneous power density can be further increased through the use of ultrashort (fs or ps) laser pulses. In the case of fs pulses, laser damage occurs as a non-thermal microexplosion. In this work we use femtosecond laser microfabrication to obtain optical memory and photonic crystal (PhC) structures in solid glass and liquid resin, and characterize their properties. Complex 1D, 2D and 3D objects can be drawn from many voxels, recorded at precisely de®ned locations as shown schematically in Fig. 1(b). In our experiments microfabrication was performed by tpulse 250 fs pulses at the fundamental (lI 800 nm) or second harmonic (lII 400 nm) wavelengths of a Ti:Sapphire laser. Oil immersion 100 microscope objective with NA 1:35 was used for the focusing. The samples were mounted on a piezoelectric transducer (PZT) controlled stage which ensured positioning with precision better than 50 nm. The solid samples were thin slabs (thickness of 200±300 mm) of vitreous silica glass (EDC from Nippon Glass), and 30NbO5=2 70TeO2 glass (prepared by the Kyoto University group). In silica, photomodi®cation at lI (lII ) occurs due to six-photon (three-photon) absorption, while in 30NbO5=2 70TeO2 at lII two-photon absorption is required. The liquid resin sample was a drop of photopolymer Nopcocure 800 (from San Nopco) on a glass substrate. The microfabrication was performed at lII , at which two-photon absorption is the lowest non-linear absorption process. The fabricated structures were subsequently characterized
Fig. 2. (a) Optical microscopy images of the array of damaged voids in vitreous silica in transmission and luminescence (magni®ed, inset). Bit volume determined from the transmission image is V 1 1 2:5 mm3 , which at the in-plane bit separation of 2.5 mm corresponds to V 1 0:4 Tbits=cm3 information density, (b) spectral signatures of bits in the white light continuum generation under 100 fs pulsed excitation at 400 nm. The dashed line shows the spectral pro®le of Rayleigh scattering. Curves 1 and 2 were recorded using different size of confocal aperture.
using conventional and laser scanning optical microscopy (LSM), scanning electron microscopy (SEM), and by linear transmission and photoluminescence spectroscopy. More details about the experimental setups and techniques can be found elsewhere [2,3]. In glass, voids can be seen both in transmission (due to light scattering from the void boundaries) and photoluminescence. Fig. 2(a) shows images of the void array. Single pulses at lII and 4.5 mW average power (at 1 kHz repetition rate), were used for recording. The void sites were luminescent under 250 nm excitation, resonant with oxygen vacancy defects VO created by the damage. The emission became suppressed after annealing at 400 C for 30 min, because the annealing is known to reconstruct damaged silica bonds. The void size found from the emission image (FWHM) is 0.6 mm. By stacking void arrays, 3D optical data storage can be facilitated. Earlier, we have demonstrated readout of the data, recorded at the density up to 73 Gb=cm3 , in transmission. Alternatively, the data can be read out using defect-related broadband continuum emission (see Fig. 2(b) and explanations in the caption) [4]. Voids, arranged periodically in a solid, can form a PhC. In PhCs, periodic spatial modulation of refractive index leads to the formation of allowed photonic energy bands, separated by photonic bandgaps (PBGs) for electromagnetic waves [5]. The PBG
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
707
Fig. 3. (a) and (b) Structure and optical transmission in a PhC with opal lattice in silica glass, (c) and (d) in a PhC with diamond lattice in 30NbO5=2 70TeO2 glass.
central wavelength is comparable to the PhC lattice period. Fig. 3(a) and (c) shows images of PhCs with opal and diamond structures recorded in silica, and 30NbO5=2 70TeO2 glass, respectively. The images indicate good structural quality of the samples. Fig. 3(b) and (d) shows their respective optical transmission spectra. Transmission dips are seen in both spectra at wavelengths close to the PBG spectral regions (as inferred from numerical simulations of photonic band structures). However, the transmission in the dips does not drop to 0, indicating incomplete PBGs. PBG formation depends crucially on the type of PhC lattice, and the amplitude of the refractive index modulation. In silica, n 1:47 is too low for the formation of complete 3D PBGs. However, opal PhCs microfabricated in silica are still interesting because they resemble widely investigated natural and arti®cial opal PhCs. 30NbO5=2 70TeO2 has much higher refractive index n 2:14 at l 1:5 mm. Some early
numerical simulations have suggested that diamond lattice of dielectric spheres in air have complete PBG at n > 2 [6]. We have simulated numerically photonic band diagrams of diamond lattice of air spheres in dielectric, and have found that complete PBG will also open in such inverse diamond PhC if n > 2. Hence, 30NbO5=2 70TeO2 glass in principle promises the complete PBG. Weakness of the transmission dip should therefore be explained by reasons, other than low refractive index contrast. We have examined re¯ection from the voids under scanning laser microscope, and found that void radius in this glass r 100 nm is somewhat smaller than that determined in transmission with conventional microscope (about 250 nm). Furthermore, the void size did not depend considerably on the laser power. At the given lattice constant a 1:35 mm, the ratio r=a 0:08 is low, indicating small photomodi®ed volume, which cannot contribute suf®ciently to the periodic light scattering,
708
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
and complete PBG does not open. This conclusion was also supported by the numerical modeling of PhC photonic band diagrams for different r=a ratios. In laser microfabricated glass PhCs some amount of random scattering due to the void shape variations is also present, and tends to diminish the PBG effects. Currently, we are working on optimizing the fabrication conditions towards increasing r=a ratio and reducing random scattering. In resin, PhCs with log-pile structure were fabricated. Log-pile lattice shown in Fig. 4(a), has facecentered tetragonal symmetry, and favors large PBGs. The rods were drawn by slowly translating focal spot of the beam, with overlap between the neighboring laser shots larger than 99%. The unexposed resin was afterwards dissolved in acetone. The refractive index of solidi®ed resin, n 1:49, is similar to that of silica. SEM image of the fabricated 3D log-pile PhC structure edge is shown in Fig. 4(b). Optical transmission spectra of several PhC samples with different in-plane rod separations a is shown in Fig. 4(c) [7]. Red shift of the transmission dips with increasing distance between the rods indicates the PBG origin of the dips. The transmission does not vanish in the dips, hence the PBGs are incomplete, most likely due to low refractive
index contrast. Despite this de®ciency, the PBG effects are suf®cient for the formation of planar microcavities (MCs). A planar MC can be formed as shown in Fig. 4(a) by creating a defected layer in the middle of log-pile PhC. This is realized by, e.g., removing every second rod. We have fabricated a log-pile structure with total number of 20 layers, and defect at the 10th layer. The rod diameter in the PhC was 2r 0:8 mm, the in-plane distance a 1:3 mm, and the rod length l 40 mm. Fig. 4(d) shows the transmission spectrum of the sample along the layer stacking direction h0 0 1i. The MC mode is evident from a peak within the transmission dip. Origin of the peak can be explained in terms of multiple re¯ections of light between two PhC mirrors surrounding the defect layer. The MC properties were also con®rmed by numeric simulation of the transmission spectra using transfer matrix technique. From the experiments we have estimated quality factor of the cavity Q 130. In conclusion, we have microfabricated optical memories and PhCs in solid and liquid transparent materials using femtosecond laser pulses. Possibilities for recording and readout of 3D optical memories in silica have been demonstrated, and signatures of PBGs
Fig. 4. (a) Log-pile structure and its geometrical parameters, the rods drawn by dashed lines are absent in the microcavity sample, (b) SEM image of the structure edge region, (c) optical transmission of the log-pile PhCs with different in-plane distance between the rods a, (d) optical transmission of the microcavity sample with a 1:3 mm, and every other rod removed from the middle layer.
S. Juodkazis et al. / Applied Surface Science 197±198 (2002) 705±709
and microcavity effects in PhC structures were found. The presented ®ndings also show the possibility to fabricate other micrometric objects relatively easily and inexpensively using this technique.
[2] [3] [4]
References [1] H. Misawa, H. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, in: H. Helvajian, K. Sugioka, M.C. Gower, J.J. Dubowski (Eds.),
[5] [6] [7]
709
Laser Applications in Microelectronic and Optoelectronic Manufacturing V, Vol. 3933, SPIE, 2000, pp. 246±260. H. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1998) 786. V. Mizeikis, S. Juodkazis, A. Marcinkevicius, S. Matsuo, H. Misawa, J. Photochem. Photobiol. C 2 (2001) 35. M. Watanabe, S. Juodkazis, H.-B. Sun, S. Matsuo, H. Misawa, Appl. Phys. Lett. 74 (1999) 3957. E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. K. Ho, C. Chan, C. Sokoulis, Phys. Rev. Lett. 65 (1990) 3152. H. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, J. Nishii, Opt. Lett. 26 (2001) 325.