Nanophotonic Resonators for Quantum Information and Refractory Plasmonics Applications

Abstract

New opportunities for plasmonic applications at high temperatures have stimulated interest in refractory plasmonic materials that show greater stability at elevated temperatures than the more commonly used silver and gold (Au). Titanium nitride (TiN) has been identified as a promising refractory material, with deposition of TiN thin films through techniques ranging from plasma-enhanced atomic laser deposition to sputter deposition to pulsed laser deposition, on a variety of substrates, including MgO, polymer, $SiO_2$, and sapphire. An implicit metric for TiN behavior has been the comparison of its plasmonic performance to that of Au, in particular at various elevated temperatures. THE FIRST HALF OF THIS THESIS, explores the deposition of highly metallic (high negative Re($\epsilon$)) TiN thin films through reactive sputtering and carries out a one-to-one comparison of bowtie nanoantennas formed of TiN and Au (on both Si and MgO substrates), examining the far-field characteristics, related to the measured near-field resonances. In both cases, the measured optical constants of the TiN films were used to simulate the expected plasmonic responses and enjoyed excellent agreement with the experimental measurements. Furthermore, we examined the consistency of the plasmonic response and the morphological changes in the TiN and Au nanoantennas at different temperatures up to 800 °C in the atmosphere. This comparison of the measured plasmonic response from nanoscale resonances to the far-field response allows for anomalies or imperfections that may be introduced by the nanofabrication processes and provides a more accurate comparison of TiN plasmonic behavior relative to the Au standard. We further optimize the deposition process to produce low loss metallic TiN films with not only high negative Re($\epsilon$) but also low Im($\epsilon$). Such TiN films are ideally suited to demonstrate surface plasmon polariton (SPP) resonances with long propagation lengths. We demonstrate the high plasmonic quality of our TiN thin films through far-field measurements of the SPP resonances supported in the 775-825 nm wavelength range. Moreover, we are able to directly image the SPP fringes using scanning near field optical microscopy. This near-field assessment allows us to determine the effective wavelengths of the SPP modes, and to measure propagation lengths greater than 10 microns. We are able to extend SPP to much longer wavelengths (6 microns) through the creation of Spoof SPPs by patterning the TiN to alter its dispersion characteristics. To guide and calibrate our studies, we carried out a full simulation of the Air/TiN/MgO structures used to model and understand the dispersion relations of the plasmon modes. Similar to the first half of the thesis that investigates the use of nanophotonic resonators to probe a quantum phenomenon such as coherent electron cloud oscillations in a refractory material, the second part of the thesis investigates using nanophotonic crystal cavities to probe point defects in a wide bandgap semiconductor material. THE SECOND PART OF THIS THESIS, investigates the negatively charged silicon monovacancy $V_{Si}$ in 4H silicon carbide (SiC) as a spin-active point defect that has the potential to act as a qubit in solid-state quantum information applications. Spin-active point defects (color centers) in solid-state wide bandgap semiconductors such as 4H Silicon carbide (SiC) possess electronic spin states that can exhibit long coherence times in the millisecond range. Furthrmore, for certain defects (depending on the existence of a shelving state , with different rates of transition from excited spin states to the shelving state then to the ground state), the spin state can be read out and intialized optically. Therefore, they have the potential to be qubits and quantum memories for quantum computation. We fabricate Photonic crystal cavities (PCCs) to augment the optical emission of the $V_{Si}$, yet fine-tuning the defect–cavity interaction proved to be challenging. We implement PCC-enabled experiments that reveal insights into defect modifications and interactions within a controlled, designated volume and indicate pathways to improved defect–cavity interactions. We report on two postfabrication processes that result in enhancement of the $V_{Si}$ optical emission from our PCCs, an indication of improved coupling between the cavity and ensemble of silicon vacancies. Below-bandgap irradiation of the implanted PCCs at 785 nm and 532 nm wavelengths carried out at times ranging from a few minutes to several hours results in stable enhancement of emission, believed to result from changing the relative ratio of $V_{Si}^{0}$ (“dark state”) to $V_{Si}^{-1}$ (“bright state”). The much faster change effected by 532 nm irradiation may result from cooperative charge-state conversion due to proximal defects. Thermal annealing at 100 °C, carried out over 20 min, also results in emission enhancements and may be explained by the relatively low-activation energy diffusion of carbon interstitials $C_i$, subsequently recombining with other defects such as silicon-carbon divacancy $VV$ to create additional $V_{Si}$. The studies described here show that prolonged laser irradiation and thermal annealing can enhance the defect–cavity coupling of $V_{Si}$ defects embedded in silicon carbide optical cavities. These results provide important insights to researchers seeking to optimize cavity coupling to such defects, affording greater optical outputs, clearer spin signatures, and control of the integrated qubits. The cavities serve as “nanoscopes,” allowing us to 1) map out the neighborhood of defects, 2) understand the influence of neighboring defects on the optical and spin performance of the qubit of interest, and 3) ultimately provide pathways to creating strong cavity coupling to the single defects of interest.