Y. Ota

Site-controlled formation of InAs/GaAs quantum-dot-in-nanowires for single photon emitters

We report the demonstration of site-controlled InAs/GaAs quantum dots (QDs) embedded in GaAs nanowires (NWs) by selective metalorganic chemical vapor deposition. InAs/GaAs quantum-dot-in-nanowires (QD-in-NWs) with various InAs thicknesses are realized on patterned GaAs(111)B substrates in the form of InAs/GaAs heterostructures and identified by structural analyses using scanning transmission electron microscopy and photoluminescence characterization. Sharp excitonic emission peaks at 10 K from single QD-in-NWs with the narrowest exciton linewidth of 87 μeV are observed. Light emission from the single QD-in-NW shows photon antibunching, along with biexciton-exciton cascaded emission process, which evidences single photon emission from high-quality QD-in-NWs.

Circularly polarized vacuum field in three-dimensional chiral photonic crystals probed by quantum dot emission

The quantum nature of light-matter interactions in a circularly polarized vacuum field was probed by spontaneous emission from quantum dots in three-dimensional chiral photonic crystals. Due to the circularly polarized eigenmodes along the helical axis in the GaAs-based mirror-asymmetric structures we studied, we observed highly circularly polarized emission from the quantum dots. Both spectroscopic and time-resolved measurements confirmed that the obtained circularly polarized light was influenced by a large difference in the photonic density of states between the orthogonal components of the circular polarization in the vacuum field.

Neutralization of positively charged excitonic state in single InAs quantum dot by Si delta doping

Positively charged excitonic state in single InAs quantum dot (QD) has been neutralized by Si delta doping and the optical properties of the fabricated samples are characterized by micro-photoluminescence measurements (μ-PL) at low temperature. An InAs QD sample having dominant emissions from positively charged excitonic states is prepared for a reference. By optimizing Si delta doping conditions, positively charged QDs are neutralized successfully and distinctive emissions from neutral exciton and biexciton from single QDs are achieved. For excessive doping condition, μ-PL spectra show optical degradation of lowering of peak intensity and broadening of peak width as doping concentration increases.

p-shell carrier assisted dynamic nuclear spin polarization in single quantum dots at zero external magnetic field

Repeated injection of spin polarized carriers in a quantum dot leads to the polarization of nuclear spins, a process known as dynamic nuclear spin polarization (DNP). Here, we report the first observation of p-shell carrier assisted DNP in single QDs at zero external magnetic field. The nuclear field - measured by using the Overhauser shift of the singly charged exciton state of the QDs - continues to increase, even after the carrier population in the s-shell saturates. This is also accompanied by an abrupt increase in nuclear spin buildup time as p-shell emission overtakes that of the s-shell. We attribute the observation to p-shell electrons strongly altering the nuclear spin dynamics in the QD, supported by numerical simulation results based on a rate equation model of coupling between electron and nuclear spin system. DNP with p-shell carriers could open up avenues for further control to increase the degree of nuclear spin polarization in QDs.

Observation of enhanced exciton decay rate of single InAs quantum dots in nanoscale metal-semiconductor-metal plasmonic structures

We observed exciton decay rates of single InAs quantum dots (QDs) in metal-semiconductor-metal (MSM) plasmonic structures for the first time. The decay rates were enhanced compared with that of as-grown QDs. Depending on the height position of the QDs in the MSM structures, we obtained enhancement factors in the decay rate ranging from 1.3 to 1.7. The field distribution of the surface plasmon polariton modes in the structures can explain the results qualitatively.

Wavelength tunable single-photon source with a side gate

T. Nakaoka1,2,3, Y. Tamura1, T. Miyazawa1, K. Watanabe1, Y. Ota1, S. Iwamoto1, and Y. Arakawa1 1 NanoQuine, Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan E-mail: nakaoka@sophia.ac.jp 2 PRESTO, JST, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan 3 Faculty of Science and Technology, Sophia University 7-1 Kioi, Chiyoda-ku, Tokyo 102-8554, Japan