A spatial light modulator based on sub-wavelength gold grating structure, named GLCoS by us, has been devised and tested to meet the challenge of the high space-bandwidth product in holographic video display. A sample GLCoS has been made and its display color appears green. However, if it is to be a device of a display system, its color characteristics must be further investigated, that is, its ability to display RGB color and its color purity. There are two MIM (Metal-Insulator-Metal) in the GLCoS, sub-wavelength gold grating plus liquid crystal cell and aluminum electrode forms a hybridized MIM and sub-wavelength gold grating itself is a MIM acting as an open-end TM-FP (TM-Fabry-Pérot) resonator. Based on our previous study involving GLCoS theoretical model, simulation software platform (FDTD plus CST Studio Suite) and an experimentally verified sample of single color, we seek an engineering way with simplicity and compatibility in the sense of the technique to investigate the red-green-blue (RGB) color display with GLCoS. Among the huge parameter space including the geometry and size of the structure, material, wavelength and light incident angle. We choose to change width of the slit, incident angle, LC (liquid crystal) birefringence to find the appropriate GLCoS spectral characteristics. The simulation results show that the dynamic modulation of the RGB phase can be achieved in the visible light band, and its amount of phase modulation is close to 2π and high chromatographic purity.
An efficient design method based on the vector diffraction theory is presented for high numerical aperture achromatic imaging lenses (HADLs). Simultaneously achieving high numerical aperture and achromatic performance puts extremely demanding requirements on the design of HADLs. Therefor an efficient numerical technique is devised by combining the finite-difference time-domain (FDTD) method with improved binary search algorithm and thus sets up a routing to facilitate the smooth connecting between vector diffraction computing with optimizing iteration. Furthermore by taking advantage of lens symmetry when optimizing the phase transmission function, tremendous computational resources saves. By taking advantage of FDTD rigorous electromagnetic computing and choosing carefully parameters, search strategy and objective function, optimal solution is obtained and the search time is very short. To validate the proposed method, a HADLs with focal length f=3.5um and numerical apertures (NA) of 0.753 is designed and focusing efficiency of over 30% across the visible light wavelength is obtained by numerical evaluation. A brief comparing with scalar diffraction theory is also carried out.
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