Title: Functional Metasurfaces towards Applications
Description: Metasurfaces, a new class of artificial media attracting great research interest, are composed of a two-dimensional ensemble of designer optical antennas arranged with subwavelength separation that introduce spatially-varying optical properties (e.g., amplitude, phase and polarization). By engineering the subwavelength optical antennas and integrating with functional materials, metasurfaces can manipulate light at one’s will and have led to the demonstration of many exotic electromagnetic phenomena. Metasurfaces have the potential to replace bulky optical components and devices as they are ultra-thin (subwavelength thickness), light weight, and able to provide new functionalities and overcome the limitations of their conventional counterparts. There are a number of promising areas in fundamental research and practical applications where metasurfaces could have a significant impact. In this dissertation, I studied the fundamental physics of the strong interaction between light and metasurfaces and explored passive and active nanophotonic devices based on metasurfaces.

I demonstrated metasurface-based devices showing record-breaking or completely novel functionalities; these devices include optical modulators for dynamic control of light propagating in free space over an unprecedented broad wavelength range, photonic integrated devices with record-small footprints, and metasurface sensors orders of magnitude more sensitive than the state-of-art sensing techniques. Strongly correlated perovskites possess widely tunable electronic structure that can host a variety of phases. Nickelates, in particular, undergo electric-field-tunable phase transitions with dramatic changes in the optical properties. In Chapter 2, I will describe my discovery of a new optical phase-transition material SmNiO3 and experimental demonstration of strong optical modulation utilizing the large and non-volatile optical refractive index change associated with electron-doping induced phase transition of SmNiO3. Large electrical modulation of light over a broad wavelength range, from the visible to the mid-infrared,  = 0.4 m – 17 m, is demonstrated using thin-film SmNiO3.

By integrating SmNiO3 and plasmonic metasurface structures, modulation of a narrow band of light that resonantly interacts with the metasurfaces is realized. Furthermore, solid-state electro-optic modulators are demonstrated by integrating SmNiO3 and solid polymer electrolytes. Correlated perovskites with tunable and non-volatile electronic phases create a new platform for active photonic devices, such as optoelectronic modulators, electrically programmable optical memories, smart windows, and variable emissivity coatings. Research on metasurfaces has so far focused on controlling wavefronts of light propagating in free space, and the implication of metasurfaces on integrated photonics has not been explored. I conducted initial work on using metasurfaces to control light propagation on a chip. In chapter 3, I will show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nano-antennas provide a platform to control guided waves via strong optical scattering at subwavelength intervals.

Such gradient metasurfaces enable the creation of small-footprint, broadband, and low-loss photonic integrated devices. I will describe experimental demonstration of waveguide mode converters, polarization rotators, and asymmetric optical power transmission in waveguides patterned with plasmonic gradient metasurfaces. I will also describe experimental demonstration of all-dielectric on-chip polarization rotators that are based on phased arrays of Mie resonators and have negligible insertion losses. Metasurfaces emerge as a new promising photonic platform for biosensing because they offer strong optical confinement and tunable optical resonances. In chapter 4, I will show that metasurface-based biosensors consisting of gold nano-antenna arrays loaded with graphene and working in the mid-infrared spectral range can achieve simultaneous high-sensitivity and high-specificity detection of biomolecules. The biosensors support a hybrid plasmon-phonon resonant mode that concentrates incident light into deeply subwavelength optical spots with local light intensity enhancement by a factor of 104.

Strong light-molecule interactions in these optical spots allow for determing protein molecule concentrations via spectral shifts of the plasmon-phonon resonance. A combination of passive and active tuning of the metasurface sensors allows for spectrally overlapping the plasmon-phonon resonance and the vibrational modes of protein molecules, so that I can identify protein molecules via their characteristic mid-infrared “fingerprints”. The high sensitivity and specificity of the metasurface sensors enable the detection of the secondary structure of protein immunoglobulin (IgG) molecules with a sensitivity four orders of magnitude higher than that of conventional attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).