Light projection displays are integral to modern life, with existing technologies like liquid crystal displays (LCDs) and digital micromirror devices (DMDs) providing spatial light modulation. Metasurfaces, ultrathin layers of subwavelength optical antennas, have revolutionized nanophotonics, offering exceptional control over light propagation. This research addresses the need for dynamic optical metasurfaces – those that allow active manipulation of light beams – which are crucial for real-world applications. While various methods exist for dynamically controlling metasurfaces (mechanical, chemical, electrical, thermal, magnetic), limitations include simultaneous tuning of all antennas, poor device control with low-intensity modulation, and narrow operating bands, especially at visible frequencies. This work presents a strategy for realizing dynamic optical metasurfaces by tailoring their spatial frequencies via modulation of both geometric and propagation phases at visible frequencies, specifically showcasing electrically-controlled DMSDs for light projection displays. The DMSD utilizes metasurface pixels arranged in an MxN array, with each pixel containing gold nanorods in a rectangular lattice. Selective columns are covered with dielectric material, and the entire structure is encapsulated in a liquid crystal (LC) cell. Electrical control of the relative phase between columns via LCs enables dynamic function on the millisecond timescale, allowing each pixel to generate a specific dynamic holographic pattern. For light projection, each pixel is addressable, switching between '1' and '0' states, creating an on/off anomalous reflection spot for programmable image generation.

The rapid advancement of nanophotonics necessitates innovative design principles for functional optical devices. Metasurfaces, offering exceptional control over light propagation, have shown promise in various applications, including beam deflectors, wave plates, flat lenses, displays, holograms, and surface wave couplers. However, the development of dynamic optical metasurfaces, crucial for real-world applications, lags. Previous attempts at dynamic control using mechanical, chemical, electrical, thermal, or magnetic stimuli have faced limitations, such as simultaneous tuning of all antennas and poor intensity modulation. This work aims to overcome these limitations, especially at visible frequencies.

The research employs a strategy of dynamic spatial frequency modulation by controlling both geometric and propagation phases. A specific device with ixj elements and a discrete phase distribution profile is described to generate anomalous reflection. A linear geometric phase gradient is introduced along each column by spatially varying the antenna orientation. The spacing between rows and columns is 300 nm, and the operating wavelength is 633 nm. A discrete Fourier transform reveals two frequency components: a low-frequency component representing anomalous reflection and a high-frequency component corresponding to evanescent waves. The phase factor Δφ, a generalized parameter encompassing geometric (Δφg) and propagation (Δφρ) phases, is used to control the transformation between propagating and evanescent waves. The intensity of the anomalous reflection is modulated by tuning Δφ. The experimental setup involves gold nanorods on a SiO2/gold mirror substrate embedded in a spacer with a refractive index of 1.5 and a thickness of 50 nm. Alternating columns are covered by dielectric materials with refractive indices na and no. The intensity of anomalous reflection is continuously modulated by tuning na and no. For the DMSD, an array of gold nanorods on a gold electrode is embedded in a PC403 layer. Alternating columns are covered by high-birefringence LCs (ne) or PMMA (nb) trenches. The LC cell is driven by a 1 kHz AC sine wave. Four independent metasurface pixels (M1-M4) are controlled by four electrodes. When the cell is off, ne=1.92 and nb=no=1.5 (PMMA). When the cell is on, ne varies, modulating the intensity of the anomalous reflection. A 4-bit DMSD is achieved, allowing for dynamic display of programmable optical information. Numerical simulations are conducted using COMSOL Multiphysics based on a finite element method. The fabrication involves multi-step electron beam lithography (EBL) to define electrode patterns and nanorods, followed by deposition of gold and PC403 layers. The LC cell is constructed using a rubbed-polyimide ITO-coated glass slide and ultraviolet-cured glue. The device for numeric indicator display consists of seven metasurface pixels, each generating a segment of the numeric indicator. For electrically controlled dynamic holography, the metasurface is multiplexed by two sets of gold nanorods to generate two independent phase profiles (using the Gerchberg-Saxton algorithm), creating switchable holographic images.

The research successfully demonstrates electrically controlled digital metasurface devices (DMSDs) for light projection displays. The devices achieve high-intensity contrast (105:1 modulation ratio) and fast switching speeds (millisecond timescale). The dynamic control is achieved by tailoring spatial frequencies through modulation of both geometric and propagation phases at visible frequencies. The anomalous reflection intensity is controlled by varying the phase difference (Δφ) between neighboring columns of gold nanorods. The phase is dynamically tuned using liquid crystals (LCs). The DMSDs demonstrate excellent reversibility, with no significant signal degradation after 100 cycles. A 4-bit DMSD is fabricated and used to demonstrate dynamic image display. Furthermore, a numeric indicator display is created using seven independently controlled metasurface pixels to generate holographic segments of numbers 0-9. Electrically controlled dynamic holography is also demonstrated, switching between different holographic images (“stop” and “walk” signs) by controlling the refractive index of the liquid crystals. The switching between images is achieved using two sets of gold nanorods and selectively coating them with PMMA or hafnium dioxide. The thickness of the DMSD can be subwavelength, unlike conventional liquid crystal displays.

The findings address the challenge of creating dynamic optical metasurfaces for real-world applications, particularly in visible light. The high contrast, fast switching speed, and excellent reversibility of the DMSD represent a significant advance over previous approaches. The ability to generate dynamic holographic patterns opens possibilities for various applications, including advanced displays, optical information processing, and holographic projection systems. The versatility of the approach, applicable to various active materials beyond liquid crystals, further broadens its potential. The subwavelength thickness of the device is a crucial advantage, leading to miniaturization and improved efficiency.

This work successfully demonstrates electrically-controlled digital metasurface devices (DMSDs) for light projection displays, achieving high-contrast, fast switching, and excellent reversibility. The approach, adaptable to various active materials, significantly expands the functionality of metasurface devices at visible frequencies. Future research could focus on further miniaturization, increasing pixel density, and exploring applications in augmented reality, virtual reality, and high-resolution displays.

The current study focuses on a relatively small number of pixels. Scaling up the device to larger arrays might introduce challenges in maintaining uniform control and addressing individual pixels efficiently. Further optimization of the liquid crystal material and device architecture could improve switching speed. The demonstrated devices operate at a specific wavelength; future work should investigate the potential for broadband operation.