Near-eye display (NED) systems for virtual reality (VR) and augmented reality (AR) have been rapidly developing; however, the widespread use of VR/AR devices is hindered by the bulky refractive and diffractive elements in the complicated optical system as well as the visual discomfort caused by excessive binocular parallax and accommodation-convergence conflict. To address these problems, an NED system combining a 5 mm diameter metalens eyepiece and a three-dimensional (3D), computer-generated holography (CGH) based on Fresnel diffraction is proposed in this paper. Metalenses have been extensively studied for their extraordinary capabilities at wavefront shaping at a subwavelength scale, their ultrathin compactness, and their significant advantages over conventional lenses. Thus, the introduction of the metalens eyepiece is likely to reduce the issue of bulkiness in NED systems. Furthermore, CGH has typically been regarded as the optimum solution for 3D displays to overcome limitations of binocular systems, since it can restore the whole light field of the target 3D scene. Experiments are carried out for this design, where a 5 mm diameter metalens eyepiece composed of silicon nitride anisotropic nanofins is fabricated with diffraction efficiency and field of view for a 532 nm incidence of 15.7% and 31°, respectively. Furthermore, a novel partitioned Fresnel diffraction and resample method is applied to simulate the wave propagations needed to produce the hologram, with the metalens capable of transforming the reconstructed 3D image into a virtual image for the NED. Our work combining metalens and CGH may pave the way for portable optical display devices in the future.

Metalens, a subcategory of metasurfaces, has been widely investigated by virtue of its miniature and ultrathin characteristics as well as versatile functionalities. In this study, a tunable bifocal metalens with two continuous-zoom foci is proposed and numerically verified. This design utilizes two cascaded layers of metasurfaces, and different phase profiles for incidences of opposite helicities are imparted on each layer by the combination of geometric phase and propagation phase. When two layers of metasurfaces are actuated laterally, focal lengths of both foci are tuned continuously, with the difference of both focal lengths increasing or decreasing. Additionally, the zoom range for each focus can be designed at will, and the relative intensity of both foci can be modulated by altering the ellipticity of incidence, with the focusing efficiency of the bifocal metalens varying from 19.8% to 32.7% for numerical apertures in a range from 0.53 to 0.78. The proposed device is anticipated to find applications in multi-plane imaging, optical tomography technique, optical data storage, and so on.

To overcome the retrieval problems in complex water, dual working wavelengths are required instead of a single wavelength in oceanic lidar. The wavelength optimization method of detecting chlorophyll a and Colored Dissolved Organic Matter (CDOM) absorption coefficient with a dual-wavelength lidar is studied in this paper. The inversion methods of chlorophyll a and CDOM absorption are developed based on the water absorption characteristics, which then lead to the inversion error equations. The effects of the wavelength on the inversion errors are studied. For the case in which λ1 and λ2 are both random, the errors are relatively small when λ1 is chosen between 420 and 560 nm and λ2 is selected under 420 nm. For the case in which λ1 is fixed at 532 nm, the errors generally decrease with decreasing λ2, with minimums around 300 and 356–360 nm under different water conditions. The wavelength optimization method discussed in this paper and the penetration depth criterion will be beneficial to the design of the dual-wavelength lidar.

Metasurfaces and metalenses have drawn great attentions since they can manipulate wavefront versatilely with a miniaturized and ultrathin configuration. Here we propose and numerically verify a tunable bifocal metalens with two continuous-zoom foci. This device utilizes two cascaded and circle layers of metasurfaces with different phase distributions for incidences of opposite helicities imparted on each layer by the combination of geometric phase and propagation phase. By relative rotation of both layers, focal lengths of both foci can be tuned continuously with the zoom range for each focus designed deliberately, and the relative intensity of both foci can be adjusted by changing the polarization state of incidence. The proposed device is anticipated to be applied in polarization imaging, depth estimation, multi-plane imaging, optical data storage, and so on.