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Lasers & Sources

Optimization methods improve holographic laser performance

Optimized parallel femtosecond-laser processing of computer-generated holograms allows for high-speed, high-light-use efficient 3D patterning.
17 December 2009, SPIE Newsroom. DOI: 10.1117/2.1200911.001591

Femtosecond-laser processing is important for simultaneously generating huge amounts of nanometer-sized structures. Combining a computer-generated hologram (CGH) with a spatial light modulator (SLM) provides important fabrication functions such as arbitrary and variable beam generation, spatial and temporal beam shaping, and adaptive wavefront correction. This approach is known as holographic femtosecond-laser processing.1–16 It offers high throughput and high light-use efficiency of the laser-pulse energy.

In this type of laser processing, precise control of diffraction peaks is essential for large-scale manufacturing. Computer-optimized holograms have highly uniform diffraction peaks a priori, but that uniformity decreases because of the optical system's inherent spatial and temporal properties. The optimization level needs to be improved to obtain the desired diffraction peaks.

We have studied two types of optical arrangements, characterized by their position between a hologram and a target. First, the target is placed onto the hologram's Fourier plane.3,8,9,11 The reconstruction does not depend on the undesired beam-intensity distribution. Second, the target is put onto the hologram's Fresnel plane.6,7,15,16 The zero-order beam is the weak background light of the diffraction peaks, and 3D parallel processing can be performed with a single shot.

Figure 1 shows holographic femtosecond-laser processing with the Fourier-transform hologram using an optimal rotation method.8 Fabrication on a glass sample was performed with 60 pulse irradiations (60 holograms), and the sample translated laterally. The hologram generated the maximum number of diffraction points (100), with spacings of 4.5μm. The stage speed was 90μm/s, while the maximum energy and repetition of the pulse irradiation were 20μJ and 2Hz, respectively.

Figure 1. Demonstration of holographic femtosecond-laser processing with a combination of 60 holograms and lateral sample movement.

CGHs can be optimized by taking into account the laser pulse's spatial distribution and the SLM's spatial properties.6–8,11 In practice, however, the uniformity of diffraction peaks decreases because of spatial nonuniformities and optical-system aberrations. Therefore, we performed an adaptive scheme to optimize the CGH using the optically reconstructed diffraction-peak intensities.15 The iterative adaptive optimization was carried out until a satisfactory output was achieved and the optical system's spatial properties were automatically incorporated into the hologram.

Figure 2 shows the operational system. The amplified pulse had a center wavelength of 800nm and a width of ~150fs. The CGH displayed on the liquid-crystal SLM (LCSLM) diffracted the pulse and formed a desired pattern on the sample through reduction optics composed of lenses and a 60× objective lens (numerical aperture = 0.85). A CMOS image sensor detected the diffraction pattern. A CCD image sensor, dichroic mirror, filter, and halogen lamp were used to observe the processing.

Figure 2. Holographic femtosecond-laser processing system.15 P1 and P2 are conjugate planes of the sample plane, L1–6 are lenses, and BS is a beam splitter. LCSLM: Liquid-crystal spatial light modulator.

Figure 3(a) shows the optimized CGH's phase distribution, while Figure 3(b) shows the optical reconstruction observed at P1 (plane 1) and its intensity profile. The uniformity, U, was 0.95. The theoretical and experimental diffraction efficiencies, η, were 98 and 58%, respectively. The experimental η was reduced by the LCSLM's spatial-frequency characteristics. Figure 3(c) shows an image and its profile of the fabricated area obtained by an atomic-force microscope. Focusing a diffracted laser pulse with an energy of E = 1.50μJ onto the glass surface produced pits surrounded by ring-shaped protrusions. The pits had minimum and maximum diameters of Dmin = 658 and Dmax = 779nm, respectively. The uniformity was Ud = Dmin/Dmax = 0.84. The pit diameter drastically increased with increasing irradiation energy near the threshold energy (~90nJ/pulse). Ud was lower than U because the mean energy per pulse (166nJ) was close to the threshold. Compared with our previous work,6,7U and Ud improved from 0.88 to 0.95 and from 0.80 to 0.84, respectively.15

Figure 3. (a) Phase distribution of the optimized CGH. (b) Optical reconstruction and its intensity profile. (c) Atomic-force-microscope image and profile of the fabricated area.

For actual scientific and engineering use, we need to further improve the hologram's diffraction uniformity and wavelength-dispersion control. We recently demonstrated an adaptive optimization method using second harmonics induced by parallel-pulse irradiation of a nonlinear optical crystal, taking into account pulse-duration changes for the diffraction positions. We also developed a spatiotemporal diffractive lens composed of a chirped diffractive lens and diffraction grating. The spatiotemporal lens yielded an ultrashort laser pulse with minimal duration only at the focal point.

By upgrading the holographic femtosecond-laser irradiation technique, we plan to work on 3D processing of transparent materials, microfabrication using photosensitive materials, access of multilayered optical memory and 3D volumetric displays, and developing a two-photon microscope with structured illumination for biological applications.

Yoshio Hayasaki, Satoshi Hasegawa
Utsunomiya University
Utsunomiya, Japan