| dc.description.abstract | We have successfully demonstrated electrically switchable wavelength and focal length tunable reflective Fresnel lenses using liquid crystal spatial light modulators (SLM) based on a liquid crystal on silicon (LCoS) device. The SLM device can display various Fresnel zone plates (FZP) with different grayscales, focal lengths, wavelengths, and others. On the basis of the real time switches of FZP with suitable parameters, the white light focusing based on color sequential and additive color techniques can be realized.
First, the corresponding FZPs, plotted by the software of Matlab, for different focal lengths and wavelengths with appropriate ranges of parameters were elucidated. By using such an LCoS device, the phase difference between these two adjacent ring areas of each FZP can be electrically tuned to be about π to approach the maximum focusing efficiency. After analyzing the focusing efficiency of light utilization, the experimentally obtained maximum diffraction efficiencies reached about 36-38%, close to the theoretical limit value of ~40.5%. Second, regarding the FZPs having characteristics of several orders of focal points, we successfully removed the unnecessary rings of FZPs to achieve a new focal point, which was absent in the original FZP. Finally, we also used FZPs to separate the white light, the mixing light of three wavelengths of 632.8 nm, 532 nm, and 450 nm, to the desired color light at their focal points. This experiment proved that the FZP patterns are able to focus the light with the desired wavelength, while others are out of focus or even defocusing. Furthermore, the switching of different FZPs, which correspond to the wavelengths of 632.8 nm, 532 nm, and 450 nm, with its frame time shorter than the limitation of humans’ eyes was adopted to mix these three lights according to the additive color technique. Experimentally, the spectra of focusing lights at their focal points were consistent with the theoretical analyses. The color of the combined lights at the focal points were contributed from the main wavelengths of the incident light source. In other words, we can obtain a white focal point by using various reflective Fresnel lenses. | en_US |