Amplitude-Only Modulator Targets Photonic Device Efficiency

A thin-film, amplitude-only spatial light modulator made from phase-change-based material was developed by researchers at the University of Exeter and the Institute of Optics in Madrid.

The device operates in reflection and modulates the amplitude of light incident on its surface with virtually no changes to the optical phase. When the phase-change material is switched between its amorphous and crystalline states, there is no effect on optical phase.

Because phase-change materials can be switched on sub-microsecond timescales, the chalcogenide-based device could provide a route to the development of ultrafast spatial light modulators, with applications in fields such as wavefront shaping, communications, sensing, and imaging. It could enable a new class of amplitude-only spatial light modulators with megahertz switching rates for efficient optical wavefront control and manipulation.

The design of the device is based on a reflector with an embedded, switchable phase-change thin-film layer made from germanium telluride (GeTe). The simple design makes the device easy to fabricate and integrate with other devices.

A new spatial light modulator has been developed that can perform fast, amplitude-only modulation without modifying the optical phase. The technology is based on the use of chalcogenide phase-change materials, and achieves improvements that could be exploited in wavefront shaping experiments, communications, sensing, and imaging. Courtesy of IO-CSIC.

The device operates like a reconfigurable mirror to provide a tunable optical environment. By changing the phase-change material between the amorphous and crystalline states, a gradual change in the amplitude of the reflected light becomes accessible without the need to modify the optical phase.

The researchers fabricated and characterized the device and tested its performance in the areas of optical amplitude and phase modulation. They used a laser scanning system to write arbitrary patterns and images onto the device and measured the amplitude and phase response via optical spectroscopy and off-axis digital holography. They designed and built an off-axis digital holography interferometer to measure, in a single shot, the spatial phase distribution across the device and fully characterize its operation.

In experiments, the laser-written devices demonstrated absolute modulation depths of 38% upon GeTe crystallization, accompanied by near-zero changes in the optical phase (i.e., below ≈π/50). The cycling of GeTe and other chalcogenides is faster than current liquid crystal technologies, with transition rates in the order of nanoseconds or less.

In principle, the device can operate as a fast, solid-state, nonvolatile, energy-efficient, amplitude-only spatial light modulator. Discrete, amplitude-only light modulation could be used to increase the number of degrees of freedom that can be controlled in wavefront shaping, compared with phase-only modulation alone. This could be accomplished by combining amplitude-only spatial light modulators with their phase-only counterparts based on liquid crystals.

The amplitude-only spatial light modulator was designed to make in situ switching of the phase-change material layer possible. The device’s inherent rapid switching speeds, combined with its ease of fabrication, could make it possible to integrate the modulation device in electrically controlled pixelated devices in the future.

The research was published in Advanced Optical Materials (www.doi.org/10.1002/adom.202300765).