Broadband and resonant nanophotonic structures to enhance the emission from quantum emitters

Abstract

Quantum emitters (QEs) are fundamental to quantum optics and information science, driving technologies of the second quantum revolution for secure communications, quantum computations, and sensing. The key characterization factor of ideal QEs is that they emit single photons on demand with high coherence. However, theseatomic defects do not interact readily with an excitation source, leading to weak light-matter interactions. It is a highly sought-after limiting factor of QEs responsible for a low spontaneous decay rate. After the pioneering work of Purcell, it is well established that light-matter interactions can be modulated to enhance the emission intensity and rate by changing the emitter’s surrounding environment. Hence, the limitations involved with solid-state QEs can be mitigated by properly engineering the environment around them to enhance light-matter interactions. This thesis provides an in-depth exploration of the enhancement and manipulation of emission dynamics of QEs using nanophotonic structures underpinned by the development and utilization of advanced experimental setups. The work commenced with designing and building a stage scanning confocal microscopy system, which formed the backbone of all experimental investigations in this thesis by enabling precise spatial mapping, identification, and characterization of individual QEs. This setup allows for the study through photoluminescence spectroscopy in frequency and time domain and photon correlation spectroscopy, which are critical in understanding the emission dynamics at the single-emitter level. The emission modification is initially investigated using random dielectric cavities comprising silicon pyramids, where dye molecules served as model emitters due to their high quantum efficiency. These studies demonstrated significant emission intensity and rate enhancement, supported by finite difference time domain simulations that validated the experimental observations. The exploration extended to plasmonic cavities in nanoporous gold, where the emission dynamics of nitrogen-vacancy (NV) centers in nanodiamonds are examined. By systematically varying the pore sizes of nanoporous gold, the study established that the plasmonic cavities offer a superior enhancement to dielectric structures, highlighting the tunability and efficacy of disordered nanostructures in engineering light-matter interactions. Subsequently, an ordered resonant cavity such as the metal-dielectric-metal (MDM) cavity is investigated, where the structural parameters are optimized to achieve selective emission enhancement of NV’s zero phonon line while suppressing the phonon side band emissions. This design offered high tunability, making it adaptable for emitters with varying spectral properties. The work culminated in studying the Tamm and asymmetric Tamm cavities, demonstrating a higher quality factor than MDM cavities, further amplifying the interaction between light and NV centers. These systematic investigations collectively revealed the pivotal role of cavity quality factor, mode volume and local density of optical states in governing the spontaneous decay rates of QEs. Furthermore, this work underscored the importance of light collection efficiency in maximizing photon extraction, which is critical for applications in quantum technologies. The findings from this thesis provide a robust framework for understanding and engineering light-matter interactions at the nanoscale. The demonstrated methodologies for emission enhancement are promising for improving the performance of QEs and hold significant implications for applications in sensing, imaging, and solid-state quantum technologies. The combination of experimental and theoretical insights bridges the gap between fundamental studies and practical implementations, offering strategies for designing nanophotonic systems that cater to specific emission properties and applications. Integrating emitters with ordered and disordered nanostructures has unveiled new avenues for broadband and spectrally selective enhancement, making this work a valuable contribution to nanophotonics. The approaches and insights presented in this thesis can be further extended to other types of emitters and cavity designs, further advancing the frontiers of quantum light-matter interactions for devices and applications.