[Paper Review] Non-volatile Phase-only Transmissive Spatial Light Modulators

Zhuoran Fang, Rui Chen+15 more|arXiv (Cornell University)|Jul 22, 2023

Optical Polarization and EllipsometryEngineering3 citations

TL;DR

This paper presents a non-volatile, transmissive spatial light modulator based on a low-loss phase-change material (Sb2Se3) coupled with a high-Q diatomic metasurface, enabling electrically programmable, phase-only modulation of infrared light. The device achieves ~0.25π (simulated) and ~0.2π (experimental) phase shift per pixel with non-volatility, endurance over 1,000 cycles, and enables tunable far-field beam shaping via independent pixel control.

ABSTRACT

Free-space modulation of light is crucial for many applications, from light detection and ranging to virtual or augmented reality. Traditional means of modulating free-space light involves spatial light modulators based on liquid crystals and microelectromechanical systems, which are bulky, have large pixel areas (~10 micron x 10 micron), and require high driving voltage. Recent progress in meta-optics has shown promise to circumvent some of the limitations. By integrating active materials with sub-wavelength pixels in a meta-optic, the power consumption can be dramatically reduced while achieving a faster speed. However, these reconfiguration methods are volatile and hence require constant application of control signals, leading to phase jitter and crosstalk. Additionally, to control a large number of pixels, it is essential to implement a memory within each pixel to have a tractable number of control signals. Here, we develop a device with nonvolatile, electrically programmable, phase-only modulation of free-space infrared radiation in transmission using the low-loss phase-change material (PCM) Sb2Se3. By coupling an ultra-thin PCM layer to a high quality (Q)-factor (Q~406) diatomic metasurface, we demonstrate a phase-only modulation of ~0.25pi (~0.2pi) in simulation (experiment), ten times larger than a bare PCM layer of the same thickness. The device shows excellent endurance over 1,000 switching cycles. We then advance the device geometry, to enable independent control of 17 meta-molecules, achieving ten deterministic resonance levels with a 2pi phase shift. By independently controlling the phase delay of pixels, we further show tunable far-field beam shaping. Our work paves the way to realizing non-volatile transmissive phase-only spatial light modulators.

Motivation & Objective

• To overcome the limitations of volatile, high-voltage, bulky spatial light modulators in free-space optical systems.
• To enable non-volatile, electrically programmable phase-only modulation of infrared light for energy-efficient and stable operation.
• To integrate memory functionality at the pixel level to reduce control signal complexity in large-scale arrays.
• To achieve high-fidelity phase modulation with minimal crosstalk and low power consumption.
• To demonstrate tunable far-field beam shaping using independently controlled meta-molecules.

Proposed method

• The device uses a thin Sb2Se3 phase-change layer integrated with a high-quality-factor (Q ~ 406) diatomic metasurface to enhance phase modulation efficiency.
• The metasurface is engineered to support a high-Q resonance, significantly amplifying the phase shift induced by the PCM without increasing thickness.
• Non-volatile operation is achieved by leveraging the stable amorphous and crystalline phases of Sb2Se3, which retain their state after power removal.
• Pixel-level control is implemented by independently programming the phase state of each meta-molecule using localized electrical pulses.
• The device geometry is designed to support 17 independently controllable meta-molecules, each capable of achieving ten deterministic resonance levels with a 2π phase shift.
• Far-field beam shaping is demonstrated by spatially tailoring the phase profile across the array using programmable phase delays.

Experimental results

Research questions

• RQ1 Can a non-volatile, phase-only transmissive spatial light modulator be realized using a phase-change material and a high-Q metasurface?
• RQ2 Can the phase modulation depth be significantly enhanced beyond a bare PCM layer through metasurface coupling?
• RQ3 Can the device maintain stable phase states without continuous power, enabling low-energy operation?
• RQ4 Can independent pixel control be achieved in a scalable array format to enable complex wavefront shaping?
• RQ5 Can the device demonstrate tunable far-field beam shaping through programmable phase profiles?

Key findings

• The device achieves a simulated phase shift of ~0.25π and an experimental phase shift of ~0.2π per pixel, ten times larger than a bare Sb2Se3 layer of the same thickness.
• The metasurface exhibits a high quality factor (Q ~ 406), which significantly enhances the phase modulation efficiency of the PCM.
• The device demonstrates endurance over 1,000 switching cycles with stable phase states, confirming non-volatility and robustness.
• The device supports independent control of 17 meta-molecules, each capable of achieving ten deterministic phase levels with a 2π phase shift.
• Tunable far-field beam shaping is experimentally demonstrated by programming distinct phase delays across the array.
• The integration of Sb2Se3 with the high-Q metasurface enables low-loss, transmissive, phase-only modulation in the infrared spectrum.