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Dynamic control of spatial wavelength dispersion in holographic femtosecond laser processing Satoshi Hasegawa and Yoshio Hayasaki* Center for Optical Research and Education (CORE), Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan *Corresponding author: [email protected]‑u.ac.jp Received October 2, 2013; revised December 10, 2013; accepted December 14, 2013; posted December 18, 2013 (Doc. ID 198833); published January 20, 2014 Dynamic control of spatial wavelength dispersion is effective due to a potentially large spectral bandwidth of femtosecond pulses, in particular, when using sub-100-fs pulses. We demonstrate spatial wavelength dispersion control, which drastically reduces focal spot distortion in the reconstruction of a hologram, using a pair of spatial light modulators. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. The dynamic control of dispersion is also demonstrated in order to restrain unnecessary processing given by the zeroth-order pulse. © 2014 Optical Society of America OCIS codes: (090.1760) Computer holography; (090.2890) Holographic optical elements; (140.7090) Ultrafast lasers; (220.4000) Microstructure fabrication; (230.6120) Spatial light modulators; (260.2030) Dispersion. http://dx.doi.org/10.1364/OL.39.000478
Femtosecond laser processing with a spatial light modulator (SLM) displaying a dynamic computer-generated hologram (CGH), called holographic femtosecond laser processing [1,2], has the advantages of high-throughput pulsed irradiation and high energy-use efficiency of the pulse. In developing a holographic femtosecond laser processing system, three-dimensional parallel fabrication with single shots by using a Fresnel CGH has been demonstrated [3,4]. In order to develop a high-speed laser processing system and fabricate aperiodic structures, the CGH has been used in combination with high-speed galvanometer scanners [5] and a microlens array [6]. Optimization methods for obtaining a high-quality CGH have also been proposed [7]. Thus, holographic femtosecond laser processing has been widely used in many applications, for example, two-photon polymerization [8–11], optical waveguide fabrication [12,13], fabrication of volume phase gratings in polymers [14], surface structuring of silicon [15], and cell transfection [16]. Spatial wavelength dispersion gives a focal spot distortion and spatiotemporal spreading of the pulse in holographic femtosecond laser processing [17]. In particular, when using sub-100-fs pulses, the wavelength dispersion should be controlled for the generation of high-quality parallel beams. Some spatial and temporal dispersion compensation techniques for parallel femtosecond laser processing using special optical arrangements have already been reported [17–22]. These methods, based on a pair of fixed diffractive optical elements (DOEs), are useful techniques. In one report, a pair of fixed DOEs has performed the generation of a femtosecond optical vortex with static control of spatial wavelength dispersion [18]. In this Letter, we demonstrate dynamic control of spatial wavelength dispersion in our femtosecond laser processing system composed of a pair of SLMs displaying two types of CGH, one for diffracting an incoming pulse and the other for compensating for angular separation of the diffraction pulse. The CGH is changed flexibly according to an arbitrary CGH for diffracting the incoming pulse by taking advantage of the rewritable capability of the SLMs. The demonstrated dispersion control 0146-9592/14/030478-04$15.00/0
drastically improved focal spot distortion in CGH reconstruction. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. In parallel laser processing with a high-numerical-aperture lens, dispersion control produced higher spatial resolution compared with laser processing without control. Finally, the dynamic dispersion control was demonstrated in order to restrain unnecessary processing given by the zeroth-order pulse while reducing focal spot distortion in dynamic CGH reconstruction. Figure 1 shows an optical system for explaining the principle of spatial wavelength dispersion control. In an ordinary system, when a collimated femtosecond laser pulse with center wavelength λc , spectral width Δλ, and an incident angle θin of 0 deg is irradiated onto a Fourier CGH with center spatial frequency νcgh , the diffraction angle for λc is expressed as θc sin−1 νcgh λc in the case of paraxial analysis. Normally, the angular separation for Δλ is equal to Δθ sin−1 νcgh Δλ, which results in spatial broadening Δr of the focal spot at the diffraction position r on the focal plane, where r is defined as the horizontal distance from the zeroth-order pulse. With the paraxial approximation, r and Δr can be written as r f νcgh λc and Δr Δλ∕λc r, respectively, where f is the focal length of the Fourier lens. The broadening Δr increases linearly with increasing r and Δλ. To compensate for Δr due to wavelength dispersion, a spectrally dispersed femtosecond pulse with a different incidence angle θin for each λ is radiated onto the CGH, as shown in Fig. 1. The incidence angle for each λ is expressed using the grating equation: θin λ sin−1 νgrat λ − λc sin−1 νgrat λc .
Fig. 1.
Control of spatial wavelength dispersion.
© 2014 Optical Society of America
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This dispersed femtosecond pulse with incidence angle θin is generated by a grating with spatial frequency νgrat arranged in a 2f − 2f setup. Due to the dispersed pulse irradiation, the angular separation of the diffraction pulse is compensated for and is effectively given away to the zeroth-order pulse. When νgrat and νcgh are changed simultaneously and dynamically, the focusing point of the zeroth-order pulse is also changed. The dynamical change of the zeroth pulse is very effective because it is difficult to perfectly eliminate undesired processing by the zeroth-order pulse. Figure 2 shows the holographic femtosecond laser processing system mainly composed of an amplified femtosecond laser system (Coherent, Micra and Legend Elite), liquid-crystal-on-silicon SLMs (Hamamatsu Photonics, X10468-07), laser processing optics, and a personal computer (PC). The femtosecond pulses from the laser system had a center wavelength λc of 800 nm, spectral width Δλ of 30 nm [full width at half-maximum (FWHM)], and a repetition frequency of 1 kHz. Pulse width was 50 fs at the sample plane. Δλ was regulated to 8 nm by inserting an interference filter (IF) in front of the beam-expanding (BE) optics. To compensate for the angular separation of the diffraction pulse, the pulse was first radiated onto SLM1 displaying CGH1 for generating a spectrally dispersed pulse. To reconstruct the parallel beams, the pulse was radiated onto SLM2 displaying CGH2 for diffracting the incoming pulse. SLM2 was located at the image plane of SLM1 with a 2f –2f setup. The reconstructed spot array was directed to the laser processing optics, composed of a 60× objective lens (OL) (numerical aperture, NA 0.85). In this setup, an optical transmittance from the femtosecond light source to a Fourier plane of SLM1, a Fourier plane of SLM2, the front of the objective, and behind the objective were 51%, 32%, 28%, and 17%, respectively. To observe the processing results, the sample was illuminated from behind with a halogen lamp (HL) and a chargecoupled device (CCD) image sensor captured images of the sample via a dichroic mirror (DM) and an infrared (IR) cut filter. The sample was an indium tin oxide (ITO) film (thickness of 10 nm) on a glass substrate and fused silica. Irradiation energy E was the total energy of each diffraction pulse at the sample plane, which was obtained as follows: first, the ratio between the energy split off by the DM and the energy at the sample plane was estimated. The total irradiation pulse energy was monitored continuously using a power meter (PM) as the product of the energy split off by the DM and the ratio.
Fig. 2. Holographic femtosecond laser processing system.
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Figures 3(a) and 3(b) show the reconstructed spot array without and with the wavelength dispersion control, respectively, when Δλ of the femtosecond pulse was set to 30 nm. The reconstructed spot array was captured by a cooled CCD image sensor at plane P in Fig. 2. In Figs. 3(a) and 3(b), r means diffraction position. The ratio r p between the peak intensity of the zeroth order without and with dispersion control was 4.5 when the irradiation pulse energy to the CGH was constant. In Fig. 3(b), the total energy of the diffraction pulses was approximately half compared with that without dispersion control [Fig. 3(a)]. Here, total energy means summation of the pixel value captured by the image sensor at each beam position. Loss of total energy depends on the diffraction efficiency of CGH1 for controlling the dispersion. However, the sum of the peak intensities of each diffraction pulse was 1.3 times higher than that without dispersion control. Figure 3(c) shows spot diameter d versus r in the spot arrays shown in Figs. 3(a) and 3(b). Spot diameter d was estimated as the FWHM of the diffraction spot. In Fig. 3(c), the filled diamonds and open diamonds represent experimental results without and with dispersion control, respectively. The solid line indicates the theoretical spot diameter, expressed as dr dAiry Δr 0.51λc ∕NA r − r grat Δλ∕λc , where dAiry 25.5 μm is the FWHM of the Airy disk diameter, NA is the numerical aperture of the Fourier lens, and r grat f νgrat λc is a parameter
Fig. 3. Reconstructed spot arrays (a) without and (b) with wavelength dispersion control, respectively, and (c) spot diameter d versus its diffraction position r, when the spectral width Δλ of the femtosecond pulse was set to 30 nm. Reconstructed spot arrays (d) without and (e) with control, respectively, and (f) d versus r, when Δλ was set to 8 nm.
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of the grating used to control the wavelength dispersion. In this result, r grat was set to 3.9 mm. The experimental results agreed well with the theoretical predictions. A parameter dratio dmax ∕dAiry was introduced to estimate the spatial broadening Δr of the focal spot, where dmax means the maximum spot diameter in the reconstructed spot array. Here, dratio was significantly improved from 9.1 to 2.8 compared with the result without dispersion control. Figures 3(d) and 3(e) show the reconstructed spot arrays without and with wavelength dispersion control, respectively, when Δλ of the femtosecond pulse was set to 8 nm. r p was 3.7 when the irradiation energy to the CGH was constant. Figure 3(f) shows d versus r in the spot arrays shown in Figs. 3(d) and 3(e). In Fig. 3(f), the filled circles and open circles represent experimental results without and with dispersion control, respectively. The solid line represents the theoretical prediction. Here, dratio was improved from 3.2 to 1.4 compared with the result without control. In the result, each spot diameter was nearly equal to the theoretical dAiry , that is, nearly diffraction-limited. Parallel femtosecond laser processing was demonstrated to an ITO film using the reconstructed spot arrays without and with wavelength dispersion control. Figure 4(a) shows the spot array without dispersion control, when Δλ was set to 30 nm. The spot array showed distortion with increasing r. Figure 4(b) shows the structures fabricated with this spot array. Processing was performed by multipulse irradiation while scanning the sample. The scanning speed, pulse energy E, and pulse repetition rate were 20 μm∕s, 548 nJ, and 1 kHz, respectively. Due to spatial spreading of the optical energy at the sample plane due to wavelength dispersion, processing was performed only in a portion of the irradiation area. Furthermore, undesired processing by the zeroth
Fig. 4. (a) Reconstructed spot array without wavelength dispersion control and (b) structures fabricated with the spot array. (c) Reconstructed spot array with dispersion control and (d) structures fabricated with the spot array.
order appeared. Figure 4(c) shows the reconstructed spot array with wavelength dispersion control. The parameter r grat of the grating was set to 3.6 mm. Figure 4(d) shows the structures fabricated with this spot array. Processing was performed in the same manner as in Fig. 4(b), with E set to 555 nJ. The processing throughput was improved. In addition, undesired processing by the zeroth order was restrained effectively owing to the dispersion control. Another zeroth-order cancellation method [23] has been proposed that was based on fine-tuning of amplitude and phase magnitudes around the zeroth-order spot in the CGH design to reduce the zeroth-order intensity. In this method, the optical setup is quite simple because only one SLM is used; however, the computational cost of CGH may be higher than that of our cancellation method. Parallel femtosecond laser processing was also demonstrated to fused silica using the reconstructed spot arrays without and with dynamic control of spatial wavelength dispersion. Δλ was set to 30 nm. An OL with a NA of 0.45 was used in order to access an area larger than that in the result of Fig. 4. Figure 5 shows the transmission optical microscope images of the fabricated structure. The structure was composed of dot structures of 8151 points. Processing was performed by switching the CGHs displayed on SLM2 without mechanical scanning. Each CGH reconstructed 30 parallel pulses and simultaneously fabricated 30 dots with single-shot irradiation. The pulse repetition rate was set to 10 Hz. The processing time for each demonstration was about 27 s. Figure 5(a) shows the image of the fabricated
Fig. 5. Transmission optical microscope images of the fabricated structure. Structure fabricated with (a) a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged around the zeroth-order pulse; (b) a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged separately to the zeroth-order pulse; and (c) a pair of SLMs. The inserted square and rectangle areas indicate the position of zeroth-order pulse irradiation.
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structure with a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged around zerothorder pulse. In the image, the square indicates the position of zeroth-order pulse irradiation, that is, an optical axis. E was 34.2 μJ. Unnecessary processing given by the zeroth-order pulse appeared at the center of the structure. To separate the processing by the zeroth-order pulse, the CGHs, which reconstruct the parallel pulses arranged separately to the zeroth-order pulse, were used. The CGHs were designed by superposing a grating with a spatial frequency ν of 9.8 lp∕mm and an orientation of 90 deg on the original CGHs used in Fig. 5(a). Figure 5(b) shows the image of the fabricated structure with a single SLM2 displaying the CGHs. In the image, the inserted square area indicates the position of zeroth-order pulse irradiation, that is, an optical axis. Although processing by the zeroth-order pulse was separated from the structure, the spatial resolution of the structure decreased significantly due to the influence of spatial wavelength dispersion in the reconstruction. In addition, the E required to fabricate the structure was 64.6 μJ and 1.9 times larger than in the result in Fig. 5(a). To control spatial wavelength dispersion and avoid processing by the zeroth-order pulse, two SLMs were used. SLM2 displayed the CGHs designed by superposing gratings with a ν of 9.8 lp∕mm and an orientation of 80– 100 deg on the original CGHs used in Fig. 5(a). SLM1 was used to control dispersion and displayed gratings with the same ν and opposite orientation to the gratings superposed in the CGHs displayed on SLM2. The orientation of the superposed grating determines the position relationship between the zeroth-order and first-order pulse. Therefore, gratings with a variety of orientations enable us to move the position of the zeroth-order pulse arbitrarily. Figure 5(c) shows the image of the fabricated structure with a pair of SLMs. In the image, the inserted rectangle area indicates the position of zeroth-order pulse irradiation. An optical axis was the center of the structure. E was 42.1 μJ. By dynamic control of spatial wavelength dispersion, the dispersion was effectively given away to the zeroth order. Furthermore, the position of zeroth-order pulse irradiation moved slightly from the bottom to the top part inside the rectangle area in each pulse shot. Therefore, the unnecessary processing given by the zeroth-order pulse was restrained perfectly. In conclusion, we have demonstrated dynamic control of spatial wavelength dispersion in a holographic femtosecond laser processing system composed of a pair of SLMs. The dispersion control drastically improved focal spot distortion in CGH reconstruction. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. In parallel laser processing with a high-numerical-aperture lens, dispersion control produced higher spatial resolution compared with laser processing without control. In addition, the processing throughput was improved with elimination of undesired processing by the zeroth order.
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Finally, dynamic control of wavelength dispersion performed position variation of zeroth-order pulse irradiation and restrained unnecessary processing given by the zeroth order while reducing focal spot distortion in dynamic CGH reconstruction. This work was supported by a Grant-in-Aid for Scientific Research (B) and Challenging Exploratory Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References 1. Y. Hayasaki, T. Sugimoto, A. Takita, and N. Nishida, Appl. Phys. Lett. 87, 031101 (2005). 2. N. Sanner, N. Huot, E. Audouard, C. Larat, J. P. Huignard, and B. Loiseaux, Opt. Lett. 30, 1479 (2005). 3. S. Hasegawa, Y. Hayasaki, and N. Nishida, Opt. Lett 31, 1705 (2006). 4. M. Yamaji, H. Kawashima, J. Suzuki, and S. Tanaka, Appl. Phys. Lett. 93, 041116 (2008). 5. Z. Kuang, W. Perrie, J. Leach, M. Sharp, S. P. Edwardson, M. Padgett, G. Dearden, and K. G. Watkins, Appl. Surf. Sci. 255, 2284 (2008). 6. P. S. Salter and M. J. Booth, Opt. Lett. 36, 2302 (2011). 7. S. Hasegawa and Y. Hayasaki, Opt. Lett. 36, 2943 (2011). 8. L. Kelemen, S. Valkai, and P. Ormos, Opt. Express 15, 14488 (2007). 9. K. Obata, J. Koch, U. Hinze, and B. N. Chichkov, Opt. Express 18, 17193 (2010). 10. S. D. Gittard, A. Nguyen, K. Obata, A. Koroleva, R. J. Narayan, and B. N. Chichkov, Biomed. Opt. Express 2, 3167 (2011). 11. Y. C. Li, L. C. Cheng, C. Y. Chang, C. H. Lien, P. J. Campagnola, and S. J. Chen, Opt. Express 20, 19030 (2012). 12. C. Mauclair, G. Cheng, N. Huot, E. Audouard, A. Rosenfeld, I. V. Hertel, and R. Stoian, Opt. Express 17, 3531 (2009). 13. M. Sakakura, T. Sawano, Y. Shimotsuma, K. Miura, and K. Hirao, Opt. Express 18, 12136 (2010). 14. D. Liu, Z. Kuang, W. Perrie, P. J. Scully, A. Baum, S. P. Edwardson, E. Fearon, G. Dearden, and K. G. Watkins, Appl. Phys. B 101, 817 (2010). 15. H. Imamoto, S. Kanehira, X. Wang, K. Kametani, M. Sakakura, Y. Shimotsuma, K. Miura, and K. Hirao, Opt. Lett. 36, 1176 (2011). 16. M. Antkowiak, M. L. Torres-Mapa, F. Gunn-Moore, and K. Dholakia, J. Biophotonics 3, 696 (2010). 17. J. Amako, K. Nagasaka, and N. Kazuhiro, Opt. Lett. 27, 969 (2002). 18. I. G. Mariyenko, J. Strohaber, and C. J. G. J. Uitenvaal, Opt. Express 13, 7599 (2005). 19. G. Mínguez-Vega, J. Lancis, J. Caraquitena, V. TorresCompany, and P. Andrés, Opt. Lett. 31, 2631 (2006). 20. G. Mínguez-Vega, E. Tajahuerce, M. Fernández-Alonso, V. Climent, J. Lancis, J. Caraquitena, and P. Andrés, Opt. Express 15, 278 (2007). 21. L. Martínez-León, P. Clemente, E. Tajahuerce, G. Mínguez Vega, O. Mendoza-Yero, M. Fernández-Alonso, J. Lancis, V. Climent, and P. Andrés, Appl. Phys. Lett. 94, 011104 (2009). 22. R. Martínez-Cuenca, O. Mendoza-Yero, B. Alonso, I. J. Sola, G. Mínguez-Vega, and J. Lancis, Opt. Lett. 37, 957 (2012). 23. A. Jesacher and M. J. Booth, Opt. Express 18, 21090 (2010).
OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
Dynamic control of spatial wavelength dispersion in holographic femtosecond laser processing Satoshi Hasegawa and Yoshio Hayasaki* Center for Optical Research and Education (CORE), Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan *Corresponding author: [email protected]‑u.ac.jp Received October 2, 2013; revised December 10, 2013; accepted December 14, 2013; posted December 18, 2013 (Doc. ID 198833); published January 20, 2014 Dynamic control of spatial wavelength dispersion is effective due to a potentially large spectral bandwidth of femtosecond pulses, in particular, when using sub-100-fs pulses. We demonstrate spatial wavelength dispersion control, which drastically reduces focal spot distortion in the reconstruction of a hologram, using a pair of spatial light modulators. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. The dynamic control of dispersion is also demonstrated in order to restrain unnecessary processing given by the zeroth-order pulse. © 2014 Optical Society of America OCIS codes: (090.1760) Computer holography; (090.2890) Holographic optical elements; (140.7090) Ultrafast lasers; (220.4000) Microstructure fabrication; (230.6120) Spatial light modulators; (260.2030) Dispersion. http://dx.doi.org/10.1364/OL.39.000478
Femtosecond laser processing with a spatial light modulator (SLM) displaying a dynamic computer-generated hologram (CGH), called holographic femtosecond laser processing [1,2], has the advantages of high-throughput pulsed irradiation and high energy-use efficiency of the pulse. In developing a holographic femtosecond laser processing system, three-dimensional parallel fabrication with single shots by using a Fresnel CGH has been demonstrated [3,4]. In order to develop a high-speed laser processing system and fabricate aperiodic structures, the CGH has been used in combination with high-speed galvanometer scanners [5] and a microlens array [6]. Optimization methods for obtaining a high-quality CGH have also been proposed [7]. Thus, holographic femtosecond laser processing has been widely used in many applications, for example, two-photon polymerization [8–11], optical waveguide fabrication [12,13], fabrication of volume phase gratings in polymers [14], surface structuring of silicon [15], and cell transfection [16]. Spatial wavelength dispersion gives a focal spot distortion and spatiotemporal spreading of the pulse in holographic femtosecond laser processing [17]. In particular, when using sub-100-fs pulses, the wavelength dispersion should be controlled for the generation of high-quality parallel beams. Some spatial and temporal dispersion compensation techniques for parallel femtosecond laser processing using special optical arrangements have already been reported [17–22]. These methods, based on a pair of fixed diffractive optical elements (DOEs), are useful techniques. In one report, a pair of fixed DOEs has performed the generation of a femtosecond optical vortex with static control of spatial wavelength dispersion [18]. In this Letter, we demonstrate dynamic control of spatial wavelength dispersion in our femtosecond laser processing system composed of a pair of SLMs displaying two types of CGH, one for diffracting an incoming pulse and the other for compensating for angular separation of the diffraction pulse. The CGH is changed flexibly according to an arbitrary CGH for diffracting the incoming pulse by taking advantage of the rewritable capability of the SLMs. The demonstrated dispersion control 0146-9592/14/030478-04$15.00/0
drastically improved focal spot distortion in CGH reconstruction. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. In parallel laser processing with a high-numerical-aperture lens, dispersion control produced higher spatial resolution compared with laser processing without control. Finally, the dynamic dispersion control was demonstrated in order to restrain unnecessary processing given by the zeroth-order pulse while reducing focal spot distortion in dynamic CGH reconstruction. Figure 1 shows an optical system for explaining the principle of spatial wavelength dispersion control. In an ordinary system, when a collimated femtosecond laser pulse with center wavelength λc , spectral width Δλ, and an incident angle θin of 0 deg is irradiated onto a Fourier CGH with center spatial frequency νcgh , the diffraction angle for λc is expressed as θc sin−1 νcgh λc in the case of paraxial analysis. Normally, the angular separation for Δλ is equal to Δθ sin−1 νcgh Δλ, which results in spatial broadening Δr of the focal spot at the diffraction position r on the focal plane, where r is defined as the horizontal distance from the zeroth-order pulse. With the paraxial approximation, r and Δr can be written as r f νcgh λc and Δr Δλ∕λc r, respectively, where f is the focal length of the Fourier lens. The broadening Δr increases linearly with increasing r and Δλ. To compensate for Δr due to wavelength dispersion, a spectrally dispersed femtosecond pulse with a different incidence angle θin for each λ is radiated onto the CGH, as shown in Fig. 1. The incidence angle for each λ is expressed using the grating equation: θin λ sin−1 νgrat λ − λc sin−1 νgrat λc .
Fig. 1.
Control of spatial wavelength dispersion.
© 2014 Optical Society of America
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This dispersed femtosecond pulse with incidence angle θin is generated by a grating with spatial frequency νgrat arranged in a 2f − 2f setup. Due to the dispersed pulse irradiation, the angular separation of the diffraction pulse is compensated for and is effectively given away to the zeroth-order pulse. When νgrat and νcgh are changed simultaneously and dynamically, the focusing point of the zeroth-order pulse is also changed. The dynamical change of the zeroth pulse is very effective because it is difficult to perfectly eliminate undesired processing by the zeroth-order pulse. Figure 2 shows the holographic femtosecond laser processing system mainly composed of an amplified femtosecond laser system (Coherent, Micra and Legend Elite), liquid-crystal-on-silicon SLMs (Hamamatsu Photonics, X10468-07), laser processing optics, and a personal computer (PC). The femtosecond pulses from the laser system had a center wavelength λc of 800 nm, spectral width Δλ of 30 nm [full width at half-maximum (FWHM)], and a repetition frequency of 1 kHz. Pulse width was 50 fs at the sample plane. Δλ was regulated to 8 nm by inserting an interference filter (IF) in front of the beam-expanding (BE) optics. To compensate for the angular separation of the diffraction pulse, the pulse was first radiated onto SLM1 displaying CGH1 for generating a spectrally dispersed pulse. To reconstruct the parallel beams, the pulse was radiated onto SLM2 displaying CGH2 for diffracting the incoming pulse. SLM2 was located at the image plane of SLM1 with a 2f –2f setup. The reconstructed spot array was directed to the laser processing optics, composed of a 60× objective lens (OL) (numerical aperture, NA 0.85). In this setup, an optical transmittance from the femtosecond light source to a Fourier plane of SLM1, a Fourier plane of SLM2, the front of the objective, and behind the objective were 51%, 32%, 28%, and 17%, respectively. To observe the processing results, the sample was illuminated from behind with a halogen lamp (HL) and a chargecoupled device (CCD) image sensor captured images of the sample via a dichroic mirror (DM) and an infrared (IR) cut filter. The sample was an indium tin oxide (ITO) film (thickness of 10 nm) on a glass substrate and fused silica. Irradiation energy E was the total energy of each diffraction pulse at the sample plane, which was obtained as follows: first, the ratio between the energy split off by the DM and the energy at the sample plane was estimated. The total irradiation pulse energy was monitored continuously using a power meter (PM) as the product of the energy split off by the DM and the ratio.
Fig. 2. Holographic femtosecond laser processing system.
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Figures 3(a) and 3(b) show the reconstructed spot array without and with the wavelength dispersion control, respectively, when Δλ of the femtosecond pulse was set to 30 nm. The reconstructed spot array was captured by a cooled CCD image sensor at plane P in Fig. 2. In Figs. 3(a) and 3(b), r means diffraction position. The ratio r p between the peak intensity of the zeroth order without and with dispersion control was 4.5 when the irradiation pulse energy to the CGH was constant. In Fig. 3(b), the total energy of the diffraction pulses was approximately half compared with that without dispersion control [Fig. 3(a)]. Here, total energy means summation of the pixel value captured by the image sensor at each beam position. Loss of total energy depends on the diffraction efficiency of CGH1 for controlling the dispersion. However, the sum of the peak intensities of each diffraction pulse was 1.3 times higher than that without dispersion control. Figure 3(c) shows spot diameter d versus r in the spot arrays shown in Figs. 3(a) and 3(b). Spot diameter d was estimated as the FWHM of the diffraction spot. In Fig. 3(c), the filled diamonds and open diamonds represent experimental results without and with dispersion control, respectively. The solid line indicates the theoretical spot diameter, expressed as dr dAiry Δr 0.51λc ∕NA r − r grat Δλ∕λc , where dAiry 25.5 μm is the FWHM of the Airy disk diameter, NA is the numerical aperture of the Fourier lens, and r grat f νgrat λc is a parameter
Fig. 3. Reconstructed spot arrays (a) without and (b) with wavelength dispersion control, respectively, and (c) spot diameter d versus its diffraction position r, when the spectral width Δλ of the femtosecond pulse was set to 30 nm. Reconstructed spot arrays (d) without and (e) with control, respectively, and (f) d versus r, when Δλ was set to 8 nm.
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of the grating used to control the wavelength dispersion. In this result, r grat was set to 3.9 mm. The experimental results agreed well with the theoretical predictions. A parameter dratio dmax ∕dAiry was introduced to estimate the spatial broadening Δr of the focal spot, where dmax means the maximum spot diameter in the reconstructed spot array. Here, dratio was significantly improved from 9.1 to 2.8 compared with the result without dispersion control. Figures 3(d) and 3(e) show the reconstructed spot arrays without and with wavelength dispersion control, respectively, when Δλ of the femtosecond pulse was set to 8 nm. r p was 3.7 when the irradiation energy to the CGH was constant. Figure 3(f) shows d versus r in the spot arrays shown in Figs. 3(d) and 3(e). In Fig. 3(f), the filled circles and open circles represent experimental results without and with dispersion control, respectively. The solid line represents the theoretical prediction. Here, dratio was improved from 3.2 to 1.4 compared with the result without control. In the result, each spot diameter was nearly equal to the theoretical dAiry , that is, nearly diffraction-limited. Parallel femtosecond laser processing was demonstrated to an ITO film using the reconstructed spot arrays without and with wavelength dispersion control. Figure 4(a) shows the spot array without dispersion control, when Δλ was set to 30 nm. The spot array showed distortion with increasing r. Figure 4(b) shows the structures fabricated with this spot array. Processing was performed by multipulse irradiation while scanning the sample. The scanning speed, pulse energy E, and pulse repetition rate were 20 μm∕s, 548 nJ, and 1 kHz, respectively. Due to spatial spreading of the optical energy at the sample plane due to wavelength dispersion, processing was performed only in a portion of the irradiation area. Furthermore, undesired processing by the zeroth
Fig. 4. (a) Reconstructed spot array without wavelength dispersion control and (b) structures fabricated with the spot array. (c) Reconstructed spot array with dispersion control and (d) structures fabricated with the spot array.
order appeared. Figure 4(c) shows the reconstructed spot array with wavelength dispersion control. The parameter r grat of the grating was set to 3.6 mm. Figure 4(d) shows the structures fabricated with this spot array. Processing was performed in the same manner as in Fig. 4(b), with E set to 555 nJ. The processing throughput was improved. In addition, undesired processing by the zeroth order was restrained effectively owing to the dispersion control. Another zeroth-order cancellation method [23] has been proposed that was based on fine-tuning of amplitude and phase magnitudes around the zeroth-order spot in the CGH design to reduce the zeroth-order intensity. In this method, the optical setup is quite simple because only one SLM is used; however, the computational cost of CGH may be higher than that of our cancellation method. Parallel femtosecond laser processing was also demonstrated to fused silica using the reconstructed spot arrays without and with dynamic control of spatial wavelength dispersion. Δλ was set to 30 nm. An OL with a NA of 0.45 was used in order to access an area larger than that in the result of Fig. 4. Figure 5 shows the transmission optical microscope images of the fabricated structure. The structure was composed of dot structures of 8151 points. Processing was performed by switching the CGHs displayed on SLM2 without mechanical scanning. Each CGH reconstructed 30 parallel pulses and simultaneously fabricated 30 dots with single-shot irradiation. The pulse repetition rate was set to 10 Hz. The processing time for each demonstration was about 27 s. Figure 5(a) shows the image of the fabricated
Fig. 5. Transmission optical microscope images of the fabricated structure. Structure fabricated with (a) a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged around the zeroth-order pulse; (b) a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged separately to the zeroth-order pulse; and (c) a pair of SLMs. The inserted square and rectangle areas indicate the position of zeroth-order pulse irradiation.
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS
structure with a single SLM2 displaying the CGHs, which reconstruct the parallel pulses arranged around zerothorder pulse. In the image, the square indicates the position of zeroth-order pulse irradiation, that is, an optical axis. E was 34.2 μJ. Unnecessary processing given by the zeroth-order pulse appeared at the center of the structure. To separate the processing by the zeroth-order pulse, the CGHs, which reconstruct the parallel pulses arranged separately to the zeroth-order pulse, were used. The CGHs were designed by superposing a grating with a spatial frequency ν of 9.8 lp∕mm and an orientation of 90 deg on the original CGHs used in Fig. 5(a). Figure 5(b) shows the image of the fabricated structure with a single SLM2 displaying the CGHs. In the image, the inserted square area indicates the position of zeroth-order pulse irradiation, that is, an optical axis. Although processing by the zeroth-order pulse was separated from the structure, the spatial resolution of the structure decreased significantly due to the influence of spatial wavelength dispersion in the reconstruction. In addition, the E required to fabricate the structure was 64.6 μJ and 1.9 times larger than in the result in Fig. 5(a). To control spatial wavelength dispersion and avoid processing by the zeroth-order pulse, two SLMs were used. SLM2 displayed the CGHs designed by superposing gratings with a ν of 9.8 lp∕mm and an orientation of 80– 100 deg on the original CGHs used in Fig. 5(a). SLM1 was used to control dispersion and displayed gratings with the same ν and opposite orientation to the gratings superposed in the CGHs displayed on SLM2. The orientation of the superposed grating determines the position relationship between the zeroth-order and first-order pulse. Therefore, gratings with a variety of orientations enable us to move the position of the zeroth-order pulse arbitrarily. Figure 5(c) shows the image of the fabricated structure with a pair of SLMs. In the image, the inserted rectangle area indicates the position of zeroth-order pulse irradiation. An optical axis was the center of the structure. E was 42.1 μJ. By dynamic control of spatial wavelength dispersion, the dispersion was effectively given away to the zeroth order. Furthermore, the position of zeroth-order pulse irradiation moved slightly from the bottom to the top part inside the rectangle area in each pulse shot. Therefore, the unnecessary processing given by the zeroth-order pulse was restrained perfectly. In conclusion, we have demonstrated dynamic control of spatial wavelength dispersion in a holographic femtosecond laser processing system composed of a pair of SLMs. The dispersion control drastically improved focal spot distortion in CGH reconstruction. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. In parallel laser processing with a high-numerical-aperture lens, dispersion control produced higher spatial resolution compared with laser processing without control. In addition, the processing throughput was improved with elimination of undesired processing by the zeroth order.
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