Effective surface passivation of GaAs nanowire photodetectors by a thin ZnO capping.

The III-V nanowire (NW) structure is a good candidate for developing photodetectors. However, high-density surface states caused by the large surface-to-volume ratio severely limit their performance, which is difficult to solve in conventional ways. Here, a robust surface passivation method, using a thin layer of ZnO capping, is developed for promoting NW photodetector performance. 11 cycles of ZnO, grown on pure zinc blende high-quality GaAs NWs by atomic layer deposition, significantly alleviates the undesirable effect of the surface states, without noticeable degradation in NW morphology. An average 20-fold increase in micro-photoluminescence intensity is observed for passivated NWs, which leads to the development of detectors with high responsivity, specific detectivity, and optical gain of 9.46 × 105 A W-1, 3.93 × 1014 Jones, and 2.2 × 108 %, respectively, under low-intensity 532 nm illumination. Passivated NW detectors outperform their counterparts treated by conventional methods, so far as we know, which shows the potential and effectiveness of thin ZnO surface passivation on NW devices.

Stray light analysis and suppression method of a pancake virtual reality head-mounted display.

Pancake virtual reality head-mounted displays (VR-HMDs) have attracted the attention of researchers in both academia and industry because of the compact size and light weight. However, owing to the use of optical path folding, there exist various stray lights in the optical system, which seriously degrades user experience. In this study, we analyze the causes and effects of multiple types of stray light systematically and design a VR-HMD with low stray light, large exit pupil diameter (EPD), compact form and light weight. Subsequently, several effective stray light suppression solutions are proposed and implemented. Finally, a prototype of a compact pancake VR-HMD system is successfully demonstrated. The prototype has stray light of less than 2.3%, a diagonal field of view (FOV) of 96° and an EPD of 10 mm at an 11 mm eye relief (ERF).

Design method of a wide-angle AR display with a single-layer two-dimensional pupil expansion geometrical waveguide.

Waveguide near-eye displays (NEDs) consist of a planar waveguide combiner and a coupling-in projection system. A two-dimensional geometrical waveguide (TDGW) can achieve an ultra-thin, large exit pupil diameter (XPD), wide-angle NED. The design method of a single-layer TDGW is presented and discussed in detail in this paper. A high-precision processing technology that can effectively guarantee the parallelism accuracy is also presented. A miniature coupling-in projection optics is designed with a catadioptric structure and integrated with the waveguide accordingly. Finally, a TDGW with a thickness of 1.75 mm is designed and analyzed. The results show that the stray light over the normal light is less than 0.5%, and the illuminance uniformity is well optimized. The field of view is up to 55°, and the XPD exceeds 12mm×10mm at an eye relief (ERF) of 18 mm. A proof-of-concept prototype was fabricated and demonstrated.

Design, stray light analysis, and fabrication of a compact head-mounted display using freeform prisms.

It has been a challenge to design an optical see-through head-mounted display that is compact, lightweight, and stray-light-suppressed by using freeform optics. A new type of design based on freeform prisms is presented. The system consists of three optical elements and a micro-display. Two prisms serve as near-eye viewing optics that magnify the image displayed by the micro-display, and the other freeform lens is an auxiliary element attached to the main wedge-shaped prism to provide an undistorted see-through view of a real-world scene. The overall thickness of the optical system does not exceed 9.5 mm, and the weight is less than 9.8 g per eye. The final system is based on a 0.49-inch micro-display and features a diagonal field of view of 38°, an F/number of 1.8, with a 10 mm × 7 mm exit pupil diameter, and a 19 mm eye relief. The causes of stray light in the optical-mechanical system are investigated, and effective solutions or theoretical suppression of stray light are given. The freeform optical elements are successfully fabricated, and the system performance is carefully investigated. The results show that the performance of the optical see-through head-mounted display is adequate for practical applications.

Optical design and pupil swim analysis of a compact, large EPD and immersive VR head mounted display.

Virtual reality head-mounted displays (VR-HMDs) are crucial to Metaverse which appears to be one of the most popular terms to have been adopted over the internet recently. It provides basic infrastructure and entrance to cater for the next evolution of social interaction, and it has already been widely used in many fields. The VR-HMDs with traditional aspherical or Fresnel optics are not suitable for long-term usage because of the image quality, system size, and weight. In this study, we designed and developed a large exit pupil diameter (EPD), compact, and lightweight VR-HMD with catadioptric optics. The mathematical formula for designing the catadioptric VR optics is derived. The reason why this kind of immersive VR optics could achieve a compact size and large EPD simultaneously is answered. Various catadioptric forms are systematically proposed and compared. The design can achieve a diagonal field of view (FOV) of 96° at -1 diopter, with an EPD of 10 mm at 11 mm eye relief (ERF). The overall length (OAL) of the system was less than 20 mm. A prototype of a compact catadioptric VR-HMD system was successfully developed.

Augmented reality enabled Galilean sighting system with an adjustable reticle.

A sighting system based on the Galileo telescope structure generally suffers from the lack of a reticle, which poses great challenges to target aiming. In this work, a hybrid Galilean sighting system integrated with an augmented reality (AR) component is proposed, which produces an image of the reticle to serve as a reticle. The depth of the reticle can be adjusted flexibly to superpose with the image of the observed object. The designed system features a fourfold magnification, a large modulation transfer function (MTF)>0.3 at 40 lp/mm, and a small distortion less than 0.3% in the full field of view. The superb imaging performance combined with the targeting function of the AR reticle renders the proposed design a powerful tool in the applications of target tracking.

Compact integrator design for short-distance sharp and unconventional geometric irradiance tailoring.

A compact microlens array (MLA) integral homogenizer composed of a projection MLA, a condenser MLA, and a subimage array mask based on Kohler illumination is presented herein. By adopting the optimal design of an aspheric projection sublens, a short-distance integrator for unconventional geometric irradiance tailoring can be acquired. Compared with the traditional integrator, the integral lens is removed in the proposed integrator. An incident beam with a larger divergence angle is permitted while a sharp illumination performance is maintained. In addition, the illumination distribution with a predefined geometrical profile can be obtained by introducing a subimage array mask without replacing the MLA elements. Through the introduced mask, the illumination attenuation area is cut, the distortion is caused by the large NA, and the peak effect caused by the integral lens of the traditional integrator can be eliminated. The subimage array mask is generated by combining ray tracing and the radial basis function image warping method. By introducing the array mask into the system, an 80% edge relative illumination and an 85% overall illumination uniformity are realized for a compact large-NA integrator with ${\rm NA} = {0.3}$ and thickness of 4.5 mm. An 88% edge relative illumination and a 92% overall illumination uniformity are realized for a small-NA integrator with ${\rm NA} = {0.15}$.

Design of an ultra-thin, wide-angle, stray-light-free near-eye display with a dual-layer geometrical waveguide.

The field of view (FOV) of a geometrical waveguide display is limited by the total internal reflection (TIR) condition (related with the index of glass) and the stray light generated inside the waveguide. A novel concept of an ultra-thin, wide-angle, stray-light-free, optical see-through near-eye display (NED) with a dual-layer geometrical waveguide is proposed in this paper. In the dual-layer waveguide, the two waveguides have different structures and are responsible for two different FOVs which are spliced together to form the entire FOV. The stray light of the dual-layer waveguide is analyzed and an optimized structure to suppress the stray light is designed. An optimized coupling-in structure is designed and a progressive optimization method is proposed for optimizing the illuminance uniformity of the entire FOV across the exit pupil. A dual-layer waveguide with a total thickness of 3.0 mm and stray light of less than 1% is designed. The FOV is 62° in the pupil-expanding direction, and the diameter of the exit pupil (EPD) is 10 mm at an eye relief (ER) of 18 mm. A compact projection optic is designed and finally is integrated with the dual-layer waveguide.

Design and evaluation of a biocular system.

In this study, a biocular system is established and a biocular analysis method is proposed by analyzing the biocular parallax principle to determine the relationship between convergence, dipvergence, azimuth, and elevation. Based on the biocular analysis method, a looped optimization method for reducing biocular parallax is presented. After optimizing the biocular system, convergence of the system is between -6 mrad-6 mrad, and dipvergence is between -1.5 mrad-1.5 mrad. This satisfies the requirements of the human eye for observation with a biocular system. Experimental results are approximately consistent with theoretical analysis. This proves the accuracy of the proposed biocular analysis method.

Design of a uniform-illumination two-dimensional waveguide head-up display with thin plate compensator.

Nonuniform illumination is one of the factors in degrading image performance of geometrical waveguide-based head-up display. The two-dimensional (2-D) waveguide expanding the exit pupil along two orthogonal directions makes illumination uniformization more intractable. To solve this problem, the reasons for nonuniform illumination in 2-D geometrical waveguide were probed and a novel waveguide structure incorporating illumination compensator was proposed. A bilayer illumination compensator was first presented to change the period of rays propagating in waveguide to improve the inter-pupil illumination uniformity and inter-field illumination uniformity. Then optimized coating design for different partially reflective mirrors in horizontal waveguide and the vertical one allowed further improvement of these two kinds of illumination uniformity, raising both up to 70%. A matched catadioptric projection optics using freeform surface was designed and integrated with the resultant 2-D geometrical waveguide, achieving a head-up display with an eye relief of 400 mm, an eyebox of 80 mm × 80 mm and a field of 24° × 15°. A prototype of the proposed 2-D geometrical waveguide display was developed and demonstrated.

Design of a two-dimensional stray-light-free geometrical waveguide head-up display.

Stray light is one of the factors exerting negative influences on the image quality of the head-up display based on the geometrical waveguide. Particularly, compared with the architecture used to expand a one-dimensional pupil, the geometrical waveguide expanding the two-dimensional pupil makes it difficult to remove the stray light due to the combined effect from two orthogonal directions. To overcome it, causes of stray light in the two-dimensional geometrical waveguide are analyzed based on the detailed calculation of the parameters that determine the geometry; the corresponding approach is put forward to decrease the percentage of the stray light across the eye box to below 0.1%. The relationship between the resultant combined waveguide and the projection optics is also clarified to obtain the pupil matching. The head-up display integrating the combined waveguide and the designed projection optics achieves the field of 24°×15° and the eye box of 80 mm×80 mm.

Stray light and tolerance analysis of an ultrathin waveguide display.

Waveguides are becoming increasingly popular in the field of near-eye display because of their low thickness and light weight. However, ghost stray light generated in the waveguide and the double-image problem caused by low parallelism precision seriously degrade the display quality. In this study, the causes of stray light are investigated, an effective solution is proposed, and a coupling-in structure is designed to suppress stray light. The Monte Carlo tolerances on the parallelism errors based on the visual acuity for the two substrates and the partially reflective mirror array (PRMA) of the waveguide are implemented. Results show that the fabrication accuracy of the substrates and the PRMA should be controlled within 6'' and 9'', respectively. A 2.4 mm thick waveguide with stray light of less than 1% is designed. The field of view is 36° in the pupil-expanding direction, and the diameter of the exit pupil is 11.6 mm at an eye relief of 20 mm. Finally, a proof-of-concept prototype is fabricated and demonstrated.