Virtual reality (VR) and mixed reality (MR) devices are used to access applications in a growing number of fields, from entertainment to health care to manufacturing to tourism. These applications are transforming the way that people perceive and interact with digital information, and they underscore the need for near-eye displays that are both functional and comfortable for long-term use and wear. Researchers at the University of Central Florida, the College of Optics and Photonics (CREOL), are working to improve the user’s experience with VR and MR by developing a way to make these devices both comfortable and high-performing. To ensure an optimal experience with VR and MR displays, the form factor of the device must be ultracompact, and the device must be lightweight. Diffractive liquid crystal (LC) optical elements offer both an ultrathin form factor and a light weight, making them promising candidates for achieving ergonomically designed VR displays. (a) Imaging results of a single LC lens using a laser projector as the light source. (b) Imaging results of the proposed achromatic LC lens system using a laser projector as the light source. (c) Imaging results of a single LC lens using an OLED display panel as the light source. (d) Imaging results of the proposed achromatic LC lens system using an OLED display panel as the light source. Courtesy of Z. Luo et al. However, the severe chromatic aberrations found in diffractive LC optics pose a significant challenge for full-color display applications. The diffraction angle of diffractive LC optics depends on the wavelength, and this can lead to a severe chromatic aberration, making the diffractive LC optical elements unsuitable for imaging. The researchers addressed this issue by developing an achromatic diffractive LC optics system. The system consists of three stacked, diffractive LC optical elements with specifically designed spectral response and polarization selectivity. The transmission spectrum and phase pattern of each optical element are designed to control the polarization states and correct the chromatic aberrations. To eliminate the focal shift between blue and red light, the first component of the achromatic LC lens system is a broadband lens that shows high efficiency in the visible spectral region. The second component is a half-wave plate that is designed to switch the polarization state of blue light. The third component is an LC lens with a custom-designed transmission spectrum that is effective only for blue and red light. The three LC components are stacked together to form the achromatic LC lens system. Due to the use of polarization-selective Pancharatnam-Berry optical elements, the researchers’ approach enables good achromatic imaging performance while maintaining an ultrathin form factor. This approach can be used to build both achromatic grating and deflector systems. The researchers conducted proof-of-concept and simulation experiments to verify the effectiveness of their approach. The experimental results showed a significant improvement in imaging performance with two types of light engines: a laser projector and an OLED display panel. The results of the simulation indicated that the achromatic LC lens system reduced the lateral color shift by about 100 times, compared to a conventional, broadband diffractive LC lens at a 50° field angle, which corresponds to a 100° field of view for a VR headset. Diffractive LC optics offer several advantages for near-eye display applications, including nearly 100% diffraction efficiency, ease of fabrication, polarization selectivity, and dynamic switching. The method developed by the CREOL team overcomes the long-standing issue of chromatic aberration in diffractive optics, which could provide the impetus for greater use of diffractive elements for creating advanced display systems. The approach can be extended to other types of diffractive optics. This could lead to more compact optical components for beam steering, imaging, and display applications. The new approach to achieving a high-quality achromatic imaging performance while maintaining a compact form factor could have broad use in various MR-enabled applications, including apps for smart transportation, smart cities, smart health care, smart education, smart construction, and smart manufacturing. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-023-01254-8).
