We present a new class of grating-integrated microdisk resonators that directly and efficiently couple to free space and can be excited by top illumination. We discuss the theory and design of such devices and present characterization results of 1530-nm-resonators with 0.8 µm to 1.2 µm radii, which are fabricated using amorphous silicon on glass. A 1.2-µm-radius resonator has a measured Q of ~16,000 and is efficiently excited by top illumination as evidenced by an observed thermally-induced bistability threshold of 0.7 mW. The small footprint and ease of coupling enable dense resonator arrays for applications in free space and flat optics.
The recent development of efficient dielectric metasurfaces has enabled practical optical components and systems composed of multiple cascaded metasurfaces. In this talk, I present an overview of our work on modeling, design, and implementation of cascaded metasurface components and systems. In particular, I present accurate system-level models for metasurfaces, techniques for designing efficient metasurfaces, multifunctional cascaded metasurfaces, and bilayer birefringent metasurfaces that provide the ultimate control over the wavefront and polarization of light. Furthermore, I will introduce a novel technique for engineering chromatic dispersion by cascading and briefly discuss a single-snapshot hyperspectral imager enabled by cascading multiple metasurfaces.
Hyperspectral imaging divides a scene into many spectral channels with narrow spectral width. Here we present a compact hyperspectral imaging system based on dielectric metasurfaces. Our system has nine channels spanning 795 nm to 970 nm, which are arranged in a rectangular array and acquired in a single snapshot, in contrast to many commercial systems. The system's narrowband filters, necessary for hyperspectral operation, also reduce chromatic aberration, a common problem in metasurface imaging systems. The small footprint of the device (2.5 mm × 2.5 mm × 1.5 mm) facilitates its potential integration into a handheld system (e.g., a mobile phone).
Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process.
Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.