Sajan Shrestha

Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

Tunability of indium tin oxide materials for mid-infrared plasmonics applications

Transparent conductive oxides (TCOs) have emerged as alternative plasmonic materials in recent years to replace noble metals. The advantages of TCOs include CMOS compatibility, tunability of optical and structural properties, and reduced losses. In this work, we demonstrate how post-deposition annealing of indium tin oxide (ITO) films in oxygen atmosphere allows for tuning their optical dispersion properties to the mid-infrared spectral range while simultaneously reducing their absorption losses. In particular, we show a materials strategy that extends the epsilon-near-zero (ENZ) point of ITO from the near-infrared to the mid-infrared range. This is demonstrated by fabricating periodic arrays of ITO discs of varying diameters and characterizing their plasmonic resonances in the mid-infrared range from λ = 5 to 10 µm. The developed ITO plasmonic structures pave the way to the development of novel infrared active devices for sensing and spectroscopy on a silicon-compatible platform.

Dimerized high contrast gratings

Abstract Metasurfaces and planar photonic crystals are two classes of subwavelength diffractive optical devices offering novel functionalities. The former employ independently operating subwavelength “meta-units” as their building blocks, while the latter exploit the collective response of many periodic building blocks. High contrast gratings (HCGs) are an example of one-dimensional (1D) planar photonic crystals with large refractive index contrast, exhibiting large in-plane scattering even with a limited number of grating periods. They are best known for their broadband features. Low contrast gratings (LCGs) are known for their control over sharp spectral features but require many periods due to small in-plane scattering. We explore a class of symmetry-broken HCGs called dimerized high contrast gratings (DHCGs), which have a period-doubling perturbation applied. DHCGs support modes accessible by free-space illumination with a long, controllable photon lifetime (inversely proportional to the magnitude of the perturbation) and reduced lateral energy divergence (confined by the high index contrast of the grating). We catalogue and clarify the resonant modes introduced by the dimerizing perturbation in 1D DHCGs and briefly explore the increased in-plane scattering present in two-dimensional (2D) DHCGs. We introduce an approach maximizing lateral localization by band structure engineering in the unperturbed HCG and using the dimerizing perturbation to generate sharp spectral features in devices with small footprint. We confirm the simultaneous control of photon lifetime and lateral localization with full-wave simulations of finite-sized DHCGs. We conclude by numerically demonstrating two compact devices (an optical modulator and a refractive index sensor) benefitting from the unique design freedoms of DHCGs.

High-efficiency amplitude-phase modulation holograms based on dielectric metasurfaces

We report a high-efficiency dielectric metasurface with continuous and arbitrary control of both amplitude and phase. We experimentally demonstrated the advantages of complete wavefront control by comparing amplitude-phase modulation metasurface holograms to phase-only metasurface holograms.