Metasurfaces based on nanoresonator modes coupling to intersubband transitions

Metasurfaces, which are planar structures made of massive numbers of nanoresonators, have attracted significant attention due to their ability to control the frequency, intensity, and wavefronts of transmitted or reflected waves while providing a small spatial footprint. These advantages, together with the ability to efficiently couple light into the material in their nanocavities, make them promising candidate structures to harness the huge nonlinearities associated with intersubband transitions multi-quantum-well semiconductor. Optical transitions between electron subbands in semiconductor heterostructures are polarized along the material growth direction. This property makes it difficult to harness intersubband optical response using light incident or emitted normal to the surface of a semiconductor material. Our group have addressed this problem by coupling intersubband transitions to optical modes in metal antennas fabricated on top of multi-quantum-well heterostructures. This approach has led to the development of nonlinear metasurfaces with record-high nonlinear optical response, demonstrated by our group, as well as to the demonstrations of other photonic structures, e.g., quantum cascade laser metasurfaces that operate as vertical external cavity surface emitting lasers. However, there are also limitations which hinder their further developments. These include: (1) all successful intersubband metasurfaces demonstrated to date use metal nanoantennas, and thus are very lossy and vulnerable to the thermal damages; (2) most of the intersubband metasurfaces demonstrated to date are based on InP or GaAs materials systems and their operating wavelengths are limited to the mid-infrared and terahertz parts of the spectrum by the limited conduction band offset of InP- and GaAs-based semiconductor heterostructures; The focus of my PhD work was to investigate and overcome the limitations mentioned above using the following approaches: (1) the development of an all-dielectric nanoresonator metasurface based on Mie resonators for second harmonic generation; (2) investigation of intersubband transitions in GaN/AlGaN heterostructures grown on various non-polar substrates for extending to extend the intersubband metasurface material pool into higher energy range; (3) besides, I have also theoretically investigated the use of metasurfaces to create Purcell-enhanced mid-infrared light emitting devices using quantum cascade laser material that naturally has very low efficiency of spontaneous light emission.