MASUMOTO Single Quantum Dot


Research Director: Yasuaki Masumoto
(Professor, Institute of Physics, University of Tsukuba)
Research Term: 1995-2000


A quantum dot is a 20-100-Angstrom, 3-dimensional structure involving 1,000 to 100,000 atoms. This size is interesting because it is smaller than the de Broglie wavelength of a slow electron, and thus involves various quantum phenomena. Another interesting point is the presence of surface-related phenomena, because the surface-to-volume ratio of a dot is large. The Masumoto single quantum dot project was aimed at understanding the basic physics of quantum dots, especially their optical properties.

Research Results

Quantum dot arrays: One-dimensional In0.45Ga0.55As quantum dot (QD) arrays were successfully fabricated on a GaAs (311) B substrate by metal-organic vapor-phase epitaxy (MOVPE). The spontaneous QD alignment along the [01-1] direction originates from the periodic strain field in a plane. The photoluminescence spectra show a clear polarization.

In situ ellipsometry: The first real-time observation of the formation of a QD by means of MOVPE was achieved in situ by an ellipsometry method. Since the ellipsometry is sensitive to the refractive index of the surface, changes in the monolayer are observed. An inflection point indicates the formation of QDs.

Spectroscopy of a single QD: The photoluminescence from a single InP QD was observed by using a microscope within a spectrometer. Sharp emission lines coming from excitons and multi-exciton states confined within a QD were observed.

Interface/Surface Effects of a QD: Anisotropic luminescence of a InP QD was observed due to uniaxial interface caused by an ordered embedding GaInP layer.

Blinking of a QD: One of several hundred dots blinked at an interval of between milliseconds and seconds due to defects near to the QD. The blinking mechanism was clarified.

Carrier relaxation by phonons: Electron relaxation by longitudianl optical (LO) phonon and acoustic phonons was viewed as clear structures in the luminescence spectra of site-selectively excited InP as well as InGaAs QDs under the electric field. Acoustic phonon mediated relaxation is faster than theoretically expected, which shows the minor contribution of phonon botteleneck effect in QDs.

Phonon renormalization in QDs: Sharp LO phonon sideband structure was observed in CuCl QDs using persistent hole-burning spectroscopy. The LO phonon frequency in the excited state of the QD is reduced by about 10% from the free value because of mixing with the excitons.

Biexciton and triexciton states: Antibonding biexciton and triexciton states in CuCl QDs were discovered by a time-resolved size-selective pump-and-probe technique. A clear induced absorption band observed is assigned to a transition from the exciton ground state to an antibonding biexciton state which were theoretically predicted to exist. Further, a triexciton state in QDs was observed for the first time.


(a)Schematic image of the QD sample structure

(b)The surface AFM image (1μm × 1μm) of QDs with 20 period of In0.45Ga0.55As/GaAs multi-layers.


·Single Quantum Dot Spectroscopy


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