The optical properties of conventional optical components vary with wavelength. This leads to chromatic aberrations of optical components, and affects the accuracy and effectiveness of optical systems operating in wideband severely, especially in the visible band. The traditional optical design achieves achromatic result by bonding a plurality of lenses of different dispersion properties. The diffractive optical element (DOE) is characterized by miniaturization and light weight, which can realize functions such as array, integration, and arbitrary wave-front conversion. However, it is difficult to perform in conventional optical devices. In this paper, we can use diffractive lenses to achieve achromatic effects. A method of designing multi-wavelength achromatic lenses by using genetic algorithms combined with scalar diffraction theory is proposed. We use the focusing performance of different light wavelengths in the focal plane as the optimization condition. After several iterations, we finally get the optimal lens structure, and use this lens to do the simulation focusing experiment based on RGB three-color light. The simulation results show that the achromatic lens can focus the red, green and blue light to a point in space. In addition, the achromatic lens can be on the order of microns in diameter. Therefore, such a lens has a smaller volume, which is difficult to achieve with conventional lenses.
A deep sub-wavelength metal grating is used to replace the top electrode of liquid crystal on silicon (LCOS) to form a new structure of liquid crystal (LC) phase spatial light modulator (SLM), which to meet large spatial bandwidth product of dynamic holographic video display. Although the structure of this (gold) deep sub-wavelength grating-LC-metal electrode is similar to the geometry of the current LCOS, the physical mechanism is completely different, which we called it G-LCOS. In order to study the feasibility of the new G-LCOS for phase modulation of digital holographic display, based on previous calculation and simulation, we fabricate deep sub-wavelength gratings by using EBL and obtain a conceptual verification device by referring to the traditional LCOS process flow. In this paper, we present experimental investigations based on Michelson's interference principle on the phase modulation performance of this proof-of-concept device. The results show that the phase modulation of the structure can reach 1.2π. The slight disagreement between the theoretically predicted and the experimentally measured values for the G-LCOS phase modulation could be caused by the errors in the preparation process and measurement.
Metasurfaces, a type of metamaterials with ultrathin thickness, have drawn tremendous attention in recent years due to their extraordinary flexibility to manipulate the light at subwavelength scale. It is useful in implementing various optical functions with a set of elements. A typical application of the metasurface is the holographic imaging, and one key parameter for the realization of holographic imaging is its optical efficiency. In this paper, we demonstrate the optimized holographic imaging by using the metasurface coded with a combined phase distribution. Firstly, the phase hologram is generated by Gerchberg-Saxton (GS) algorithm and the blazed grating is formed by introducing a periodic linear phasegradient distribution. Then the phase profile of the hologram is superimposed with the phase of blazed grating to generate a new phase distribution. Benefiting from the advantage of high efficiency for the desired light-manipulation, the metasurface based on the metal-insulator-metal (MIM) structure with different geometric parameters was utilized to cover the phase shift of 0 to 2π for encoding the generated phase distribution. The structure consists of a four-level quantized metallic Au nanorods elements separated by dielectric layers of SiO2 with the Au substrate, so a macro cell of our metasurface consists of 16 (=4× 4) subwavelength meta-atom, which are made of the Au nanorods with different width. The simulated far-filed patterns are calculated by finite-difference time-domain (FDTD) method. Compared to previous metasurface, our structure preferentially steer incident energy into the desired first order diffracted beam with the help of the equivalent of the blazed grating. And the optimized holographic imaging results could be achieved.