Finite-difference time-domain simulation of a liquid-crystal optical phased array

Model-based compensation of pixel crosstalk in liquid crystal spatial light modulators

Spatial light modulators (SLMs) based on liquid crystals are widely used for wavefront shaping. Their large number of pixels allows one to create complex wavefronts. The crosstalk between neighboring pixels, also known as fringing field effect, however, can lead to strong deviations. The realized wavefront may deviate significantly from the prediction based on the idealized assumption that the response across a pixel is uniform and independent of its neighbors. Detailed numerical simulations of the SLM response based on a full 3D physical model accurately match the measured response and properly model the pixel crosstalk. The full model is then used to validate a simplified model that enables much faster crosstalk evaluation and pattern optimization beyond standard performance. General conclusions on how to minimize crosstalk in liquid crystal on silicon (LCoS) SLM systems are derived, as well as a readily accessible estimation of the amount of fringing in a given SLM.

Coupled-wave analysis of vector holograms: effects of modulation depth of anisotropic phase retardation

The diffraction properties of thick vector holograms were analyzed with the use of a simple coupled-wave theory. Two eigenpolarizations in the holograms were determined based on the dielectric perturbation, and diffraction efficiencies for the polarizations were calculated by applying the Kogelnik method. The results were compared with those simulated by the finite-difference time-domain method. As a result, it was demonstrated that the diffraction efficiencies calculated by the two methods are in good agreement for any incident polarization when the modulation depth of the anisotropic phase retardation is substantially smaller than the mean retardation. In addition, we confirmed that coupled-wave analysis provides reasonable accuracy for relatively large modulation in the case of Bragg incidence with eigenpolarization.

Correction of aberration in holographic optical tweezers using a Shack-Hartmann sensor

Optical aberration due to the nonflatness of spatial light modulators used in holographic optical tweezers significantly deteriorates the quality of the trap and may easily prevent stable trapping of particles. We use a Shack-Hartmann sensor to measure the distorted wavefront at the modulator plane; the conjugate of this wavefront is then added to the holograms written into the display to counteract its own curvature and thus compensate the optical aberration of the system. For a Holoeye LC-R 2500 reflective device, flatness is improved from 0.8λ to λ/16 (λ=532 nm), leading to a diffraction-limited spot at the focal plane of the microscope objective, which makes stable trapping possible. This process could be fully automated in a closed-loop configuration and would eventually allow other sources of aberration in the optical setup to be corrected for.

Mode purities of Laguerre-Gaussian beams generated via complex-amplitude modulation using phase-only spatial light modulators

We investigate output mode purities of Laguerre-Gaussian (LG) beams generated from four typical simultaneous amplitude and phase modulation methods with phase-only spatial light modulators (SLMs). Numerical simulations supposing the practical SLM, i.e., stepwise phase modulation with a pixelated device, predict an output mode purity of beyond 0.969 for the LG beams of less than radially and azimuthally fifth order. Experimental results of generating LG beams are also shown to demonstrate the effects of the simultaneous phase and amplitude modulation.

Experimental study of an ultrasmall pixel, one-dimensional liquid-crystal device

A one-dimensional, ultrasmall pixel liquid-crystal (LC) device is experimentally demonstrated. The device has a one-dimensional array of ten 1 mm long, interdigitated, reflective gold electrodes on a glass substrate and a common transparent electrode on the opposite substrate. The interdigitated electrodes are 2 microm wide, separated by a 1 microm interelectrode gap. Operating as a dynamic, reflective, 3 microm pitch diffractive grating, the device simulates the performance of a reflective, ultrasmall, 3 microm pixel, spatial light modulator (SLM). It was shown that, for a proper choice of LC cell thickness (less than 2 microm), LC material (Merck's BL006 high-birefringence mixture), and driving conditions, the device can attain relatively high diffraction efficiency, thus demonstrating the practical feasibility of a 3 microm pixel, LC SLM.

Generation of high-quality higher-order Laguerre-Gaussian beams using liquid-crystal-on-silicon spatial light modulators

We report on the high-quality holographic generation of higher-order Laguerre-Gaussian (LG) beams using a liquid-crystal-on-silicon spatial light modulator. The effects of the input beam pattern on the output LG beam quality are investigated in detail through theoretical discussions and experiments. Correlation analyses between observed beam patterns and theoretical mode profiles reveal that higher beam quality is achieved for output LG beams generated from a top-hat input beam than from a Gaussian input beam.