H. Watabe

OBSERVATION OF A SINGLE PHOTOLUMINESCENCE PEAK FROM A SINGLE QUANTUM DOT

We observed a very sharp photoluminescence peak from a single GaAs/AlGaAs quantum dot structure by using a microphotoluminescence measurement technique. The spectral linewidth was more suppressed by decreasing the excitation laser power, which is mainly due to reduction of the filling effect of quantized energy levels. The minimal spectral linewidth with low excitation laser power was 0.9 meV.

One‐dimensional exciton diffusion in GaAs quantum wires

One‐dimensional diffusion of excitons in GaAs quantum wires was observed by using microphotoluminescence measurements at low temperature. The observed diffusion length increased with decreasing wire width from 30 to 15 nm, and decreased from 15 to 7 nm, where maximum diffusion length was about 4 μm for the 15 nm quantum wire, which is the largest value so far reported. It is considered that the change of diffusion length versus wire width is caused by the competition between one‐dimensional character and the interface fluctuation.

Nonlinear filters for multimedia applications

A new type of nonlinear image processing filters for multimedia applications is proposed. The filters remove small-amplitude fluctuations while keeping edges sharp. The filter algorithm is simply realized by combination of a usual linear filter and a comparator, which is suitable for hardware implementation. Here, two types of multimedia applications are considered, one is to post-process block-based encoded images which contain blocking artifacts, and the other to remove undesirable skin components in facial images, which is useful for facial beautification. Computer simulations verify the high performance of the proposed methods.

Fabrication and microscopic photoluminescence imaging of ridge‐type InGaAs quantum wires grown on a (110) cleaved plane of AlGaAs/GaAs superlattice

We have fabricated ridge‐type InGaAs quantum wire structures on a (110) cleaved plane of AlGaAs/GaAs superlattice using selective metalorganic chemical vapor deposition growth. The lateral width of the quantum wires was about 25 nm. Spatially resolved photoluminescence (PL) of the quantum wires was observed by using microscopic photoluminescence measurement at low temperature, showing PL from the quantum wire region and its polarization dependence.

Light emission from vertical-microcavity quantum dot laser structures

Abstract A vertical microcavity laser structure with an active layer of Stranski-Krastanow quantum dots was fabricated for the first time. The microcavity consists of an InGaAs quantum dot layer grown by MOCVD, located between two AlAs/Al0.2Ga0.8As distributed Bragg-reflector mirrors. The length of the microcavity was 4λ(λ = 884 nm). The cavity effect was evidenced by the difference of the PL linewidths of samples with and without the cavity.

Time and spatial resolved photoluminescence from a single quantum dot

Abstract We discuss experimental results on the spatial and time resolved photoluminescence in quantum wires and dots using a micro-photoluminescence technique. Particularly, we succeeded in measuring time resolved photoluminescence from a single quantum dot. Very fast diffusion dynamics of excitons in the quantum wires were observed as well as long photoluminescence decay times in a single quantum dot. These results demonstrate the one-dimensional and zero-dimensional characters of excitons in the quantum nanostructures.

Influence of carrier transport/capture and gain flattening in picosecond pulse generation of InGaAs microcavity lasers

We have fabricated and studied the dynamics of In0.16Ga0.84As λ‐cavity lasers focusing on the influence of carrier transport/capture and gain flattening by using optical resonant pumping and off‐resonant pumping. With the off‐resonant pumping, shoulders in the lasing pulse shape were observed, while no shoulder appeared with the on‐resonant pumping. From rate equation analysis and time‐resolved photoluminescence measurement, it was concluded that the shoulders result from carrier transport/capture effects. Saturation of the inverse rise time was observed at high pump levels. This results from gain flattening due to the steplike density of states of 2D carriers. The pulse width obtained from the laser was as narrow as 7.7 ps.

Photoluminescence from point contact structure — Direct observation of electron flow

Abstract We demonstrate visualization of electron flow from a point contact by using a micro-photoluminescence measurement technique. It was found that the photoluminescence intensity at the exit of the point contact was strongly reduced by the majority electron flow from the point contact. This is considered to be caused from that the majority electrons with large wave number reduce radiative recombination of the electron-hole pairs generated by the laser illumination, which was also supported by the time and spatial resolved measurement.

Direct Observation of Electron Jet from a Point Contact.

We report on the photoluminescence image around a point contact fabricated in an InGaAs/GaAs quantum well using a micro-photoluminescence measurement technique. The photoluminescence image was strongly influenced by majority electron flow from the point contact, where the majority electron flow greately reduced radiative recombination of electron-hole pairs generated by the laser illumination. Using this effect we demonstrated visualization of electron flow from the point contact.

The Electrical Properties of PrBaCuO in YBaCuO/PrBaCuO/YBaCuO Junction

We demonstrated that a single PrBaCuO film with low resistivity could be fabricated by changing a cooling process after the deposition. This film showed shorter c-axis length than that of normal PrBaCuO. This film had also anisotropic resistivitis of the directions parallel to and across CuO planes. The resistivity parallel to the CuO planes was very lower, and similar to the reported a-axis oriented YBa2Cu30x/PrBa2Cu30x/YBa2Cu30x junction resistance. We consider that the c-axis of PrBaCuO decreases owing to the lattice matching in an a-axis oriented junction. As a result, the resistivity of the junction becomes low.