Group III-nitride materials have drawn a great deal of renewed interest due to their versatile characteristics as quantum emitters including room-temperature operation, widely tunable wavelengths from ultraviolet to infrared, and a high degree of linear polarization. However, most reported results for III-nitride-based quantum emitters show large inhomogeneous line width broadening in comparison to their lifetime-limited values, which is detrimental to achieving indistinguishability with high visibility. To overcome this, we propose an unprecedented InGaN quantum dot formation technique at the apex of GaN nanopyramid structures, which significantly suppresses inhomogeneous line width broadening. Using high-resolution transmission electron microscopy, a site-controlled InGaN quantum dot with small height (<2 nm) was estimated. No measurable screening effect or frequency jitter of the single-photon emission was observed, which leads to the narrow homogeneous emission line width (64 ± 8 μeV) beyond the spectral resolution limit via Fourier-transform spectroscopy. The emitted photons exhibited superb antibunching characteristics with a near-unity degree of linear polarization, which is highly relevant for polarized nonclassical light sources for applications in quantum information processing.

White light emitting InGaN nanostructures hold a key position in future solid-state lighting applications. Although many suggested approaches to form group III-nitride vertical structures have been reported, more practical and cost effective methods are still needed. Here, we present a new approach to GaN/InGaN core-shell nanostructures at a wafer level formed by chemical vapor-phase etching and metal-organic chemical vapor deposition. Without a patterning process, we successfully obtained high quality and polarization field minimized In-rich GaN/InGaN core-shell nanostructures. The various quantum well thicknesses and the multi-facets of the obelisk-shaped core-shell nanostructures provide a broad spectrum of the entire visible range without changing the InGaN growth temperature. Due to their high crystal quality and polarization field reduction, the core-shell InGaN quantum wells show an ultrafast radiative recombination time of less than 200 ps and uniformly high internal quantum efficiency in the broad spectral range. We also investigated the important role of polarization fields in the complex recombination dynamics in InGaN quantum wells.

Emission control of a quantum emitter made of semiconductor materials is of significance in various optical applications. Specifically, the realization of efficient quantum emitters is important because typical semiconductor quantum dots are associated with low extraction efficiency levels due to their high refractive index contrast. Here, we report bright and unidirectional emission from a site-controlled InGaN quantum dot formed on the apex of a silver-coated GaN nanopyramidal structure. We show that the majority of the extracted light from the quantum dot is guided toward the bottom of the pyramid with high directionality. We also demonstrate that nanopyramid structures can be detached from a substrate, thus demonstrating great potential of this structure in various applications. To clarify the directional radiation, the far-field radiation pattern is measured using Fourier microscopy. This scheme will pave the way toward the realization of a bright and unidirectional quantum emitter along with easy fabrication and large-area reproducibility.

Silicon (Si) nanostructures that exhibit a significantly low reflectance in ultraviolet (UV) and visible light wavelength regions are fabricated using a hydrogen etching process. The fabricated Si nanostructures have aperiodic subwavelength structures with pyramid-like morphologies. The detailed morphologies of the nanostructures can be controlled by changing the etching condition. The nanostructured Si exhibited much more reduced reflectance than a flat Si surface: an average reflectance of the nanostructured Si was approximately 6.8% in visible light region and a slight high reflectance of approximately 17% in UV region. The reflectance was further reduced in both UV and visible light region through the deposition of a poly(dimethylsiloxane) layer with a rough surface on the Si nanostructure: the reflectance can be decreased down to 2.5%. The enhancement of the antireflection properties was analyzed with a finite difference time domain simulation method.

Although the study of single quantum dot (QD) properties without the background noise and dephasing processes caused by surrounding carriers is a crucial issue, the spatial-selective excitation of a single QD is still challenging, due to the diffraction nature of light. Here, we demonstrate a deep subwavelength excitation of a single QD using twophoton plasmonic nanofocusing. Self-aligned plasmonic nanofocusing on a single QD was achieved using metal coated nanopyramid structures. The highly enhanced local electric field generated by the plasmonic nanofocusing gives rise to a large increase in the optical nonlinear effect (i.e., two-photon excitation). As a result of the enhanced field enhancement on the metal-pyramid hybrid structure, the two-photon luminescence intensity was enhanced by a factor of 5000, and the selective excitation of a single QD enabled us to observe InGaN QD emission at near room temperature, due to the large suppression of the background emission. Our approach opens promising perspectives for quantum optics experiments with highly reduced background emissions.