UCLA

Optical absorption driven by intrinsic material loss is often viewed as a challenge to be overcome in photonic and metamaterials-based systems. In the context of applications such as optical communication, reducing absorption remains a key goal to maximize optical transmission. However, for many other applications related to sensing and detection, as well as heat transfer and energy, absorption and emission are essential. The overarching goal of this thesis is about taking advantage and embracing the loss in materials. Sometimes we have to lose to “gain”.In Chapter 1, we leveraged the loss in the epsilon near zero (ENZ) materials to build a broadband directional thermal emitter for the first time. We introduced and experimentally realized gradient epsilon-near-zero (ENZ) materials that enable broad spectrum directional control of thermal emission by supporting leaky electromagnetic modes that couple to free space at fixed angles over a broad bandwidth. We experimentally demonstrated two emitters consisting of multiple metal and semiconductor oxides in a photonic configuration that enable gradient ENZ behavior over long-wave infrared wavelengths. The structures exhibits high average emissivity (> 0.6 and > 0.7) in the p polarization between 7.7 and 11.5�m over an angular range of 70�-85�, and between 10.0 to 14.3�m over an angular range of 60�-75�, respectively. Outside these angular ranges, the emissivity reduces to 0.4 and lower for angles smaller than 50� and 40� respectively. The structures’ broadband thermal beaming capability enables radiative heat transfer only at particular angles and is experimentally verified through direct measurements of thermal emission. By decoupling conventional limitations on angular and spectral response, our approach opens new capabilities for applications such as thermal camouflaging, solar heating, radiative cooling and waste heat recovery. In Chapter 2, we further expanded our gradient ENZ thin film idea using doped III-V semiconductors to realize more continuous and precise control for the directional thermal radiation. Both experimentally and numerically we proved that, by increasing the total thickness of the gradient ENZ thin film made by Si-doped InAs, the high emissivity angle will move towards normal incidence. Also, by changing the doping level of the doped InAs film, we observed the functional bandwidth of the sample changes to a different range, which makes doped III-V semiconductors a more flexible directional thermal emitter candidate. In Chapter 3, we explored the beauty of loss in photonic crystals: in particular non-Hermitian photonic crystal where two materials only have a difference in the imaginary part of the permittivity. We found counter-intuitive behavior in this under-studied class of system, including loss-driven reflection. We characterized the band structure as well as the reflection response in both 1D and 2D non-Hermitian system. By introducing a lossy material in the crystal, a quasi-bandgap purely induced by loss opens up at the band edge. A sharp reflection peak is also observed within the quasi bandgap in both 1D and 2D non-Hermitian photonic crystals we proposed. In the end, we designed a selective reflector which consists a traditional 2D photonic crystal waveguide and a non-Hermitian photonic crystal. This selective reflector can guide light along the waveguide and have the light absorbed by the non-Hermitian part while only the light with certain wavelength can be reflected back, which distinguishes this selective reflector with the traditional reflectors such as a Bragg mirror. Finally in Chapter 4, lossy materials such as conductive polymers can potentially enable a revolution in building energy efficiency by dynamically controlling their thermal emissivity. We proposed a mechanism to decouple the mean radiant temperature from actual temperatures of interior surfaces by dynamically tuning the thermal emissivity of interior building surfaces. We quantitatively evaluated how much impact it will have on building energy saving in a building scale thought out a whole year. We showed that in cold weather, setting the emissivity of interior surfaces to a low value (0.1) can decrease the setpoint as much as 6.5�C from a baseline of 23�C. Conversely, in warm weather, low emissivity interior surfaces result in a 4.5�C cooling setpoint decrease relative to high emissivity (0.9) surfaces. EnergyPlus calculation shows that by implementing the tunable emissivity interior, more than 30% of the cooling and heating energy can be saved year-round in different climates.