Engineering the Optical Properties and Temporal Responses of Conducting Oxides and Nitrides for Optically Switchable Metasurfaces

Soham Saha, Purdue University

Abstract

My dissertation involves the development and characterization of conducting oxides and nitrides and their application in tunable nanophotonics. Tunable nanophotonic devices are those with switchable optical properties. When an external stimulus is applied, the optical response of these devices changes. There are a wide variety of stimuli utilized in optical switching, encompassing electrical, optical, thermal, and mechanical impulses. The optical response controlled can be amplitude, polarization, or phase. In my work, I focused on an optical pulse as the stimulus, and amplitude modulation as the response. Optically induced permittivity change is one of the fastest methods of changing the permittivity of a material and has potential use in optical transistors, beam steering, and especially in photonic time-crystal design. In the first chapter, I investigated how much the optical properties of an emerging conducting oxide, cadmium oxide, can be influenced by using yttrium as a dopant. I also investigated the dominant recombination mechanisms in the doped oxide via pump-probe spectroscopy. In the second section, I investigated the transient permittivities of zinc oxide with a singlepump, broadband probe measurement technique. Designing fast, dynamic metasurfaces generally requires the optical properties of a medium in the photoexcited state. However, a detailed characterization of almost any oxide in its excited state is rare in literature. Most of the previously reported optical characterizations only deal with transient reflectance or transmittance measurements. The few that report the transient permittivity, are limited to a wavelength regime near the epsilon-near-zero (ENZ) point of the materials. Working with undoped zinc oxide, I determined the limits of permittivity modulation with optical pumping. The transient permittivities were used to design and experimentally demonstrate a metasurface for amplitude modulation at specific wavelengths. In the third section, I report one of the first demonstrations of switching time-control in a dynamic metasurface. Different materials have different relaxation dynamics, dictating their response times. What happens when an optical switch is made with two materials with distinctly different dynamics? Using titanium nitride, a material with a nanosecond response time, and aluminum doped zinc oxide, a material with a picosecond response time, I designed a bilayer absorber. The absorber has distinct resonances near the epsilon near zero wavelengths of the individual components. When probed near the resonances, the same absorber shows distinctly different switching speeds. This demonstrates that the same switch can have different speeds depending on the probe wavelength. The techniques developed and the findings from this dissertation work will help better characterize conducting oxides and nitrides, as well as other materials to better design optically switchable devices, and to better characterize the transient optical properties of tunable materials.

Degree

Ph.D.

Advisors

Boltasseva, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Electromagnetics|Optics|Physics

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