Computer-Generated Hologram (LCoS SLM)

A chip that speaks in phase — steering light one pixel at a time to sculpt wavefronts that trick your eyes into seeing objects that are not there.

An LCoS spatial light modulator is a reflective display chip — resembling a tiny cinema projector panel — in which each pixel contains a steerable liquid-crystal layer sitting on a silicon backplane. Rather than emitting light, it reflects an incident laser beam while independently shifting the optical phase at every pixel by up to 2π (one full wave cycle). When a computer calculates the correct phase pattern — a computer-generated hologram — the reflected beam propagates and constructively interferes in free space at the locations that would match the wavefronts of a real 3D scene, producing a floating 3D reconstruction. No physical film, no photographic chemistry: the scene can be changed at the chip's update rate.

Silicon CMOS circuitry beneath each pixel applies a voltage to a thin nematic or ferroelectric liquid-crystal layer. The electric field rotates the crystal directors, changing the effective refractive index for the polarised laser illumination. Because the light reflects from the metallic backplane, it travels through the LC layer twice, doubling the effective optical path. By mapping a voltage level (0–255 on an 8-bit device) to a phase retardation of 0–2π at each pixel, the chip becomes a programmable diffraction grating. The phase pattern loaded on the chip is the computer-generated hologram — numerically derived via algorithms such as the Gerchberg–Saxton phase-retrieval iteration or direct angular-spectrum propagation. The diffracted beam reconstructs the target wavefront at a chosen propagation distance. Fill factor (the ratio of active pixel area to total pitch area) determines what fraction of reflected power contributes to the desired diffraction order; high fill factors, typically 90–97%, suppress zeroth-order leakage and stray light.

In plain terms

Imagine you have a lake, and you want ripples to look as if there's a rubber duck floating right in the middle — but there is no duck. If you had millions of tiny paddles on the lake floor, each one nudging the water at exactly the right moment, you could fake those ripples perfectly. An LCoS SLM is those paddles, but for laser light instead of water.

Also called: CGH display, phase-only SLM, liquid-crystal-on-silicon spatial light modulator, wavefront modulator.

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A short history

Dennis Gabor proposed wavefront reconstruction in 1948 — an optical two-step method initially aimed at correcting aberrations in electron microscopes — and coined the word 'hologram' in his 1949 Proceedings of the Royal Society paper. Practical laser-illuminated holography followed Leith and Upatnieks's 1963 demonstrations at the University of Michigan. The modern computer-generated hologram was independently developed in the late 1960s: Lohmann and Paris computed binary holograms in 1967, and the first kinoform (continuous-phase CGH) was described by Lesem, Hirsch, and Jordan at IBM in 1969. Liquid-crystal spatial light modulators began replacing photographic film for CGH in the 1990s; the first LCoS chip appeared commercially in the late 1990s in projectors before being adopted for phase-only holography research. Jasper Display (spun out of Himax Technologies in 2015) and Holoeye Photonics (Berlin, founded 1999) now supply the research-grade LCoS SLMs used in most academic holographic-display laboratories.

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