Su-Hyun Gong

Squeezing Photons into a Point-Like Space.

Confining photons in the smallest possible volume has long been an objective of the nanophotonics community. In this Letter, we propose and demonstrate a three-dimensional (3D) gap-plasmon antenna that enables extreme photon squeezing in a 3D fashion with a modal volume of 1.3 × 10(-7) λ(3) (∼4 × 10 × 10 nm(3)) and an intensity enhancement of 400 000. A three-dimensionally tapered 4 nm air-gap is formed at the center of a complementary nanodiabolo structure by ion-milling 100 nm-thick gold film along all three dimensions using proximal milling techniques. From a 4 nm-gap antenna, a nonlinear second-harmonic signal more than 27 000-times stronger than that from a 100 nm-gap antenna is observed. In addition, scanning cathodoluminescence images confirm unambiguous photon confinement in a resolution-limited area 20 × 20 nm(2) on top of the nano gap.

Ultrafast single photon emitting quantum photonic structures based on a nano-obelisk

A key issue in a single photon source is fast and efficient generation of a single photon flux with high light extraction efficiency. Significant progress toward high-efficiency single photon sources has been demonstrated by semiconductor quantum dots, especially using narrow bandgap materials. Meanwhile, there are many obstacles, which restrict the use of wide bandgap semiconductor quantum dots as practical single photon sources in ultraviolet-visible region, despite offering free space communication and miniaturized quantum information circuits. Here we demonstrate a single InGaN quantum dot embedded in an obelisk-shaped GaN nanostructure. The nano-obelisk plays an important role in eliminating dislocations, increasing light extraction, and minimizing a built-in electric field. Based on the nano-obelisks, we observed nonconventional narrow quantum dot emission and positive biexciton binding energy, which are signatures of negligible built-in field in single InGaN quantum dots. This results in efficient and ultrafast single photon generation in the violet color region.

Electrically driven, phosphor-free, white light-emitting diodes using gallium nitride-based double concentric truncated pyramid structures

White light-emitting diodes (LEDs) are becoming an alternative general light source, with huge energy savings compared to conventional lighting. However, white LEDs using phosphor(s) suffer from unavoidable Stokes energy converting losses, higher manufacturing cost, and reduced thermal stability. Here, we demonstrate electrically driven, phosphor-free, white LEDs based on three-dimensional gallium nitride structures with double concentric truncated hexagonal pyramids. The electroluminescence spectra are stable with varying current. The origin of the emission wavelength is studied by cathodoluminescence and high-angle annular dark field scanning transmission electron microscopy experiments. Spatial variation of the carrier injection efficiency is also investigated by a comparative analysis between spatially resolved photoluminescence and electroluminescence.

Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes

Significance Control and optimization of interaction between light and single quantum emitters are a crucial issue for cavity quantum electrodynamics studies and quantum information science. Although considerable efforts have been made, reliable and reproducible coupling between quantum emitter and cavity mode still remains a grand challenge due to the uncertainty of the size, i.e., the emission wavelength, and position of the quantum emitter. Here, we demonstrate an unprecedented approach of the self-aligned deterministic coupling of single quantum dots (QDs) to nanofocused plasmonic modes on an entire wafer. Spatial precision is better than any nanopositioning techniques, and almost all processed QDs exhibit outstanding spontaneous emission rate enhancement. This reliable approach eliminates a major obstacle in the implementation of practical solid-state quantum emitters. The quantum plasmonics field has emerged and been growing increasingly, including study of single emitter–light coupling using plasmonic system and scalable quantum plasmonic circuit. This offers opportunity for the quantum control of light with compact device footprint. However, coupling of a single emitter to highly localized plasmonic mode with nanoscale precision remains an important challenge. Today, the spatial overlap between metallic structure and single emitter mostly relies either on chance or on advanced nanopositioning control. Here, we demonstrate deterministic coupling between three-dimensionally nanofocused plasmonic modes and single quantum dots (QDs) without any positioning for single QDs. By depositing a thin silver layer on a site-controlled pyramid QD wafer, three-dimensional plasmonic nanofocusing on each QD at the pyramid apex is geometrically achieved through the silver-coated pyramid facets. Enhancement of the QD spontaneous emission rate as high as 22 ± 16 is measured for all processed QDs emitting over ∼150-meV spectral range. This approach could apply to high fabrication yield on-chip devices for wide application fields, e.g., high-efficiency light-emitting devices and quantum information processing.

Giant Rabi Splitting of Whispering Gallery Polaritons in GaN/InGaN Core–Shell Wire

The hybrid nature of exciton polaritons opens up possibilities for developing a new concept nonlinear photonic device (e.g., polariton condensation, switching, and transistor) with great potential for controllability. Here, we proposed a novel type of polariton system resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core-shell GaN/InGaN hexagonal wire. High quality, nonpolar InGaN multiple-quantum wells (MQWs) were conformally formed on a GaN core nanowire, which was spatially well matched with whispering gallery modes inside the wire. Both high longitudinal-transverse splitting of nonpolar MQWs and high spatial overlap with whispering gallery modes lead to unprecedented large Rabi splitting energy of ∼180 meV. This structure provides a robust polariton effect with a small footprint; thus, it could be utilized for a wide range of interesting applications.

Optically pumped GaN vertical cavity surface emitting laser with high index-contrast nanoporous distributed Bragg reflector

Laser operation of a GaN vertical cavity surface emitting laser (VCSEL) is demonstrated under optical pumping with a nanoporous distributed Bragg reflector (DBR). High reflectivity, approaching 100%, is obtained due to the high index-contrast of the nanoporous DBR. The VCSEL system exhibits low threshold power density due to the formation of high Q-factor cavity, which shows the potential of nanoporous medium for optical devices.

Nonlinear Photonic Diode Behavior in Energy-Graded Core–Shell Quantum Well Semiconductor Rod

Future technologies require faster data transfer and processing with lower loss. A photonic diode could be an attractive alternative to the present Si-based electronic diode for rapid optical signal processing and communication. Here, we report highly asymmetric photonic diode behavior with low scattering loss, from tapered core-shell quantum well semiconductor rods that were fabricated to have a large gradient in their bandgap energy along their growth direction. Local laser illumination of the core-shell quantum well rods yielded a huge contrast in light output intensities from opposite ends of the rod.

Effect of varying pore size of AAO films on refractive index and birefringence measured by prism coupling technique.

Anodic aluminum oxide (AAO) films with different pore sizes were prepared to modulate the effective refractive index and birefringence. To investigate the relationship between the refractive index and the pore size of the AAO film, optical constants were obtained using a prism coupler with various lasers. With experimental results, the dispersion curve of alumina itself without pores was extracted using a theoretical anisotropic model. We demonstrated that AAO films could offer a wide range of refractive index and birefringence values for optical device applications. Furthermore, index profiles as a function of the thickness of the AAO films were obtained by inverse Wentzel-Kramer-Brillouin approximation to examine the optical homogeneity.

Optical waveguiding properties into porous gallium nitride structures investigated by prism coupling technique

In order to modulate the refractive index and the birefringence of Gallium Nitride (GaN), we have developed a chemical etching method to perform porous structures. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores density. GaN films are prepared on sapphire by metal organic chemical vapor deposition and the microstructure is characterized by transmission electron microscopy, and scanning electron microscope analysis. Optical waveguide experiment is demonstrated here to determine the key properties as the ordinary (n0) and extraordinary (ne) refractive indices of etched structures. We report here the dispersion of refractive index for porous GaN and compare it to the bulk material. We observe that the refractive index decreases when the porous density p is increased: results obtained at 0.975 μm have shown that the ordinary index n0 is 2.293 for a bulk layer and n0 is 2.285 for a pores density of 20%. This value corresponds to GaN layer with a pore size of 30 nm and inter-distance of 100 nm. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices.In order to modulate the refractive index and the birefringence of Gallium Nitride (GaN), we have developed a chemical etching method to perform porous structures. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores density. GaN films are prepared on sapphire by metal organic chemical vapor deposition and the microstructure is characterized by transmission electron microscopy, and scanning electron microscope analysis. Optical waveguide experiment is demonstrated here to determine the key properties as the ordinary (n0) and extraordinary (ne) refractive indices of etched structures. We report here the dispersion of refractive index for porous GaN and compare it to the bulk material. We observe that the refractive index decreases when the porous density p is increased: results obtained at 0.975 μm have shown that the ordinary index n0 is 2.293 for a bulk layer and n0 is 2.285 for a pores density of 20%. This value corresponds to GaN layer with a p...

Semiconductor nanopyramids for building high-yield quantum photonic devices

Semiconductor quantum dots (QDs) are thought to be a promising candidate for a single-quantum emitter in on-chip systems because of their well-developed growth and fabrication techniques.1 Semiconductor QDs, however, have a number of inherent limitations that need to be overcome before they can be used in practical applications. For example, QDs in semiconductors are strongly affected by elements (e.g., phonons) in the surrounding environment, which results in short nonradiative decay times and rapid dephasing processes. Despite the high intrinsic radiative decay rates of semiconductor QDs compared with those of other single-quantum emitters (such as atoms and ions), the radiative decay rate needs to be further increased so that these fast nonradiative and dephasing processes can be overcome. Furthermore, the collection efficiency of the light that is emitted from conventional QDs embedded in a highindex planar substrate is typically low (about 4%). Processes with which to control and optimize the radiative decay rate of QDs are therefore crucial for increasing their collection efficiency and for realizing efficient single-photon generation. Indeed, improving the collection efficiency would be incredibly useful for the successful application of QDs in quantum information devices. Over the past few decades extensive efforts have been made to try and realize highly efficient light sources from single QDs, i.e., by introducing various photonics structures.2 A critical problem arises, however, when photonics structures are combined with semiconductor QDs. That is, conventional semiconductor QDs become randomly distributed across the wafer and have nonreproducible sites. Moreover, these QDs have a broad spectral distribution. Achieving spectral overlap between a QD and a Figure 1. (a) Schematic diagram and scanning electron microscope (SEM) image (inset) of site-controlled single indium gallium nitride (InGaN) quantum dots (QDs). (b) Schematic diagram of self-aligned nanofocused plasmonic modes on a single QD array. Inset shows an SEM image of the pyramid array after silver (Ag) deposition. Scale bars indicate 500nm. (c) Conceptual illustration of how 3D nanofocused plasmonic modes and single QDs can be spatially matched in a self-aligned fashion.4