Programmable Metasurface Creates Dozens of Holograms Simultaneously

Engineers have long created devices to generate holograms, which are 3D or 2D images formed by controlling light waves. Holograms are used for visual representations, physical property measurements, authentication, and educational tools. While hologram quality has improved, most current technologies can only produce one hologram at a time. To generate multiple independent holograms simultaneously, existing methods require increasing holographic channels, which often degrades image quality or reduces refresh speed.

A team at Southeast University in China has engineered a new programmable metasurface, an ultrathin material designed to manipulate waves in novel ways. This metasurface, detailed in Nature Electronics, features 6,000 individually controllable elements that can be adjusted spatially and temporally. The development was driven by the increasing need for high-capacity holographic systems in areas like intelligent displays, virtual reality, and advanced communications.

Traditional holography often relies on multiplexing dimensions such as polarization or wavelength, but these approaches have limitations in capacity and reprogrammability. Programmable metasurfaces offer dynamic control over wavefronts, but most prior systems could only generate a single high-quality holographic image. The inspiration for this new work came from space-time coding optimization algorithms, particularly the efficient convergence of the Gerchberg-Saxton (GS) algorithm when applied to space-time coding. This suggested that the space-time domain offers numerous parallel dimensions, with each harmonic order capable of carrying independent information.

The researchers leveraged a space-time coding mechanism, which splits a single-frequency wave into multiple harmonic beams, to combine and process multiple information streams for holography. They developed a theoretical framework and a new algorithm, dubbed the space-time Gerchberg-Saxton (ST-GS) algorithm, to enable the simultaneous generation of thousands of independent holographic channels in real time on a single programmable metasurface. The metasurface is modulated not only in space but also in time, with each meta-unit switching between two phase states according to a time-coding sequence. This space-time modulation generates multiple harmonic frequencies in the reflected beam from a single-frequency incident wave.

The fabricated metasurface has 6,144 independently controlled elements, achieving a frame refresh rate exceeding 1 MHz and an equivalent coding bandwidth of 6 Gb/s. A key advantage is its instantaneous parallelism; unlike sequential time-multiplexing, all channels are generated simultaneously without blind zones and with resilience to frequency distortions. Using this system, the researchers successfully produced up to 62 distinct holographic images at once, demonstrating the potential of space-time coding for generating dozens of images from thousands of holographic channels.