Overview
An active transmissive metasurface architecture has been theoretically demonstrated to achieve independent amplitude and phase modulation in the mid-infrared. This development addresses a challenge previously noted in active transmissive metasurfaces, where independent control of both light parameters had not been achieved. The proposed design incorporates high-quality (high-Q) cuboid germanium (Ge) Mie resonators integrated with graphene stripes.
Research Context
Active metasurfaces serve as a platform for spatial light modulators (SLMs) in applications such as holography, beam shaping, and light detection and ranging (LiDAR). These devices facilitate dynamic control over the amplitude and phase of light at subwavelength scales. Transmissive metasurfaces specifically offer a compact alternative to conventional SLMs due to their potential for monolithic integration with chip-scale light sources. While independent amplitude and phase modulation has been observed in active reflective metasurfaces, its realization in transmission has been challenging.
Approach
The metasurface design employs cuboid germanium (Ge) Mie resonators integrated with graphene stripes. Electrostatic gating is utilized to modulate the intraband and interband transitions of graphene, enabling control over transmittance and phase. To achieve both types of modulation at a single operating wavelength, the thermo-optic effect of Ge is employed as a global tuning mechanism. This global tuning spectrally aligns the resonant frequencies, which facilitates both transmittance and phase-only modulation at different base temperatures. The architecture is described as a dual-mechanism approach.
Findings
- The proposed metasurface theoretically demonstrates the ability to modulate transmittance from near 0% to 5% at one specific spectral frequency.
- The same device achieved 281 degrees of phase-only modulation at another frequency.
- Through the dual-mechanism architecture, which involves electrostatic gating of graphene and the thermo-optic effect of Ge, the system achieved a transmittance modulation efficiency of approximately 100%.
- The architecture also yielded 282 degrees of phase-only modulation.
- The efficacy of phase-only modulation was further demonstrated by individually addressing each unit cell to realize a beam-steering device.
- This beam-steering device exhibited relative diffraction efficiencies exceeding 90%.
Why This Matters
The development of an individually addressable active metasurface, as demonstrated, opens a route toward compact, dynamically reconfigurable metaphotonic devices. This capability is relevant for spatial light modulators in applications like holography, beam shaping, and LiDAR.