  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.
