Satoru Odashima

Bright single-photon source based on an InAs quantum dot in a silver-embedded nanocone structure

High photon-extraction efficiency is strongly required for a practical single-photon source. We succeed in fabricating metal (sliver)-embedded nanocone structure incorporating an InAs quantum dot. Efficient photon emission of ∼200 000 photons per second is detected and single-photon emission is demonstrated using autocorrelation measurements. The photon-extraction efficiency as high as 24.6% is obtained from the structure.

A Cooper-Pair Light-Emitting Diode: Temperature Dependence of Both Quantum Efficiency and Radiative Recombination Lifetime

A light-emitting diode (LED) in the optical-fiber communication band showed special features after replacing the n-type electrode with niobium (Nb) superconducting metal. Nb electrodes prepared on an InGaAs-based semiconductor surface formed a superconductor/semiconductor/superconductor junction, and the current-voltage characteristics exhibited both DC and AC Josephson junction properties. This was a result of the injection of electron Cooper-pairs into the n-InGaAs active layer of an LED. The drastic enhancement of the electroluminescence output observed below the Nb superconducting critical temperature, T c, demonstrates the active role of electron Cooper-pairs in radiative recombination. Lifetime measurements of this LED and accurate evaluation of the luminescence output made it possible to estimate the radiative recombination lifetimes. A theoretical formula derived for the Cooper-pair radiative recombination accurately describes both the measured steep reduction of the radiative recombination lifetime and the observed enhancement of the internal quantum efficiency below T c. This work will assist the development of interdisciplinary physics and new applications in superconductivity and optoelectronics.

Enhanced Photon Extraction from a Quantum Dot Induced by a Silver Microcolumnar Photon Reflector

We have fabricated silver microcolumnar photon reflectors to enhance the photon extraction efficiency from a quantum dot. The mechanically robust planar structure is favorable to couple to optical fibers for stable and efficient single-photon sources. A high photon extraction efficiency of up to 18% is achieved. Furthermore, strong suppression of multiphoton generation is confirmed. The proposed structure is quite promising toward the implementation of practical quantum key distribution systems with dot-fiber-coupled photon sources.

Metal-coated semiconductor nanostructures and simulation of photon extraction and coupling to optical fibers for a solid-state single-photon source

We have realized metal-coated semiconductor nanostructures for a stable and efficient single-photon source (SPS) and demonstrated improved single-photon extraction efficiency by the selection of metals and nanostructures. We demonstrate with finite-difference time-domain (FDTD) simulations that inclination of a pillar sidewall, which changes the structure to a nanocone, is effective in improving the photon extraction efficiency. We demonstrate how such nanocone structures with inclined sidewalls are fabricated with reactive ion etching. With the optimized design, a photon extraction efficiency to outer airside as high as ~97% generated from a quantum dot in a nanocone structure is simulated, which is the important step in realizing SPS on-demand operations. We have also examined the direct contact of such a metal-embedded nanocone structure with a single-mode fiber facet as a simple and practical method for preparing fiber-coupled SPS and demonstrated practical coupling efficiencies of ~16% with FDTD simulation.

Fiber-Based Bidirectional Solid-State Single-Photon Emitter Based on Semiconductor Quantum Dot

Fiber-based bidirectional photon extraction from nanoscale emitters and photon antibunching behavior between two outputs of two single mode optical fibers are experimentally demonstrated. Flakes of the epitaxial layer containing the InAs quantum dots (QDs) are fixed mechanically by both side with the edge faces of the single-mode-fiber (SMF) patch cables. The emitting photons from the single quantum dot are directly taken out of both side through the SMFs. Single-photon emission between two SMF outputs is confirmed by detecting non-classical antibunching in second-order photon correlation measurements with two superconducting single-photon detectors (SSPDs) and a time-amplitude converter (TAC). This simple opto-mechanical alignment-free single-photon emitter has advantage of robust stability more than 10 days and low-cost fabrication.Fiber-based bidirectional photon detection from nanoscale emitters and photon antibunching behavior between two outputs of two single-mode fibers (SMFs) are experimentally demonstrated. Flakes containing the epitaxially grown quantum dots (QDs) are mechanically fixed by both sides with the edge faces of the SMF patch cables. The emitting photons from a single QD are directly taken out of both sides through the SMFs. Single-photon emission between two SMF outputs is confirmed by detecting nonclassical antibunching in second-order photon correlation measurements. This simple optomechanical alignment-free single-photon emitter is advantageous because of its robust stability of more than three months and low-cost fabrication.

Cooper-Pair Radiative Recombination in Semiconductor Heterostructures: Impact on Quantum Optics and Optoelectronics

The injection of Cooper pairs into a normal medium such as a semiconductor is known as the proximity effect at the superconductor/normal interface. We confirm this injection as well as the contribution of Cooper pairs to a drastic enhancement of inter-band optical transitions in semiconductor heterostructures. In this paper we investigate and clarify the relation of internal quantum efficiencies and radiative lifetimes in Cooper-pair light emitting diodes (CP-LEDs). A quantitative description of the dynamic photon generation processes is given, and the contribution of the Cooper-pair recombination relative to normal-electron recombination in CP-LEDs is discussed in detail.

Two-photon interference and coherent control of single InAs quantum dot emissions in an Ag-embedded structure

We have recently reported the successful fabrication of bright single-photon sources based on Ag-embedded nanocone structures that incorporate InAs quantum dots. The source had a photon collection efficiency as high as 24.6%. Here, we show the results of various types of photonic characterizations of the Ag-embedded nanocone structures that confirm their versatility as regards a broad range of quantum optical applications. We measure the first-order autocorrelation function to evaluate the coherence time of emitted photons, and the second-order correlation function, which reveals the strong suppression of multiple photon generation. The high indistinguishability of emitted photons is shown by the Hong-Ou-Mandel-type two-photon interference. With quasi-resonant excitation, coherent population flopping is demonstrated through Rabi oscillations. Extremely high single-photon purity with a g(2)(0) value of 0.008 is achieved with π-pulse quasi-resonant excitation.

Optical control of spectral diffusion with single InAs quantum dots in a silver-embedded nanocone

We study the spectral diffusion in a single epitaxial InAs quantum dot placed in a silver-embedded nanocone structure. Making use of a series of stroboscopic detection of optical transitions, we demonstrate the temporal fluctuations in peak energy of photoluminescence. In particular, the photoluminescence fluctuations can be effectively suppressed by strong illumination. By analyzing the photon statistics, we find that the dot emission exhibits stable non-classical nature with the strong anti-bunching behavior after this stabilization.