J. Kusano

Tailoring a high‐transmission fiber probe for photon scanning tunneling microscope

Transmission efficiency of a fiber probe used in photon scanning tunneling microscope is evaluated as a function of aperture diameter. The apertured probe has been fabricated by chemical etching technique and metal coating. By comparing two types of probes with different cone angles, we determine the most influential factor in the transmission property of the metal‐cladding tapered waveguide. A long tip with high efficiency is developed by a multistep etching method so as to be suitable during actual scanning operation. Photoluminescence imaging of lateral p‐n junctions on the GaAs substrate is demonstrated in the illumination‐collection hybrid mode operation of photon scanning tunneling microscope.

Alignment Control of a Liquid Crystal on a Photosensitive Polyvinylalcohol Film

The photoinduced optical anisotropy of an azo dye-doped polyvinylalcohol film has been utilized for controlling the azimuthal alignment of a liquid crystal. The dynamic behaviors of the optical transmission of the film and of a liquid crystal layer aligned on the film are studied using linearly polarized pumping (Ar+ laser) and probing (He-Ne laser) beams. The results show that the azo dye-doped polyvinylalcohol film memorizes the information on the polarization direction of the exciting laser beam and the resulting anisotropy induced in the film causes adjacent liquid crystal molecules on the film to rotate azimuthally.

Violet and Blue Light Emissions from Nanocrystalline Silicon Thin Films

Nanocrystalline silicon thin films with a grain diameter from three to seven nanometers were fabricated on silicon substrates. It is demonstrated for the first time that the thin films show intense violet and blue luminescence at room temperature. The luminescence spectra include three peaks at wavelengths of 415 nm, 437 nm and 465 nm. Anodizations of these thin films introduce additional green and red luminescence in the spectra. Fourier transform infrared spectroscopy indicates no hydrogen- or oxygen-related absorptions in the nanocrystalline silicon thin films. Only the anodized thin films show Si–Hx absorptions. The violet luminescence should be evidence of light emission from zero-dimensionally confined silicon structures.

Spatially resolved photoluminescence spectroscopy of lateral p-n junctions prepared by Si-doped GaAs using a photon scanning tunneling microscope

An accurate correspondence between the local optical responses and the structures of semiconductor light‐emitting devices is demonstrated by using an illumination‐mode photon scanning tunneling microscope with noncontact atomic force microscope technique. We study the novel‐structured lateral p‐n junctions grown on patterned GaAs(111)A substrate. Measuring the spatially resolved photoluminescence spectra with a 200 nm apertured probe, we precisely determine the position and the width of the transition region of p‐n junctions. The illumination‐collection hybrid mode is also employed to map the two‐dimensional emission efficiency with higher resolution, which is not affected by carrier diffusion.

Quantum size effect on the exciton polariton in GaAs thin films

Abstract Discrete fine structures were observed in a luminescence of the free exciton band in high quality GaAs thin films. The appearance of these structures is explained by the quantum size effect on exciton polariton due to the quantization of the center of mass motion. This experiment is a new approach to the additional boundary condition.

Determination of slant angle of p–n interface by multiwavelength near‐field photocurrent measurement

Near‐field photocurrent measurements with multiwavelength excitation sources are applied to the investigation of a lateral p–n junction grown on patterned GaAs (111)A substrate. In order to probe the internal properties of this device, propagation modes into the sample are utilized retaining high resolution with the contribution of a penetration depth smaller than the aperture diameter. By systematically varying the penetration depth over a wide range up to 900 nm, photocurrent signals due to internal optical response clearly appear. The capability of ‘‘tomographic’’ diagnostics is demonstrated and the slant angle of the p–n interface is determined to be 30±8°.

Two‐dimensional versus three‐dimensional excitons in wide GaAs quantum wells

Excitons confined in high‐quality GaAs/Al0.19Ga0.81As double heterostructures have been studied experimentally and theoretically with emphasis on phenomena associated with the transition from a two‐dimensional exciton in a quantum well to a three‐dimensional exciton in a thin film. Exciton luminescence and reflectance spectra are obtained for GaAs film widths between 750 and 5200 A. Exciton polariton luminescence is observed for the 5200 A sample. Highly resolved exciton peaks are obtained in the free‐exciton luminescence spectra for both the 990 and 2010 A samples. Magneto‐exciton spectra of the 990 A sample reveal an enhanced spin splitting of the ground‐level exciton. Experimental results of both the 750 and the 990 A samples are qualitatively explained by an effective‐mass theory which considers mixing of a large number of quasi‐two‐dimensional excitons. The large spin‐splitting of the 990 A sample is a distinctive feature of a wide quantum well which can be explained by the well width dependence of e...

Optical characterizations of undoped GaAs crystals grown by reduced pressure metalorganic vapor‐phase epitaxy

Undoped gallium arsenide crystals were successfully grown by the reduced pressure metalorganic vapor‐phase epitaxy technique using the (AsH3)/[Ga(CH3)3] mole ratio ranging from 10 to 128. The photoluminescence of the free exciton and sharp emission peaks due to the radiative recombination of bound excitons at 1.8 K indicated that this series of samples were high‐purity epitaxial layers. In particular, by using a picosecond pulsed laser and a decay measuring system, the radiative lifetime of the photoexcited carrier at 77 K was determined for various samples. The maximum decay time was found to be 2.8 ns at the (V/III) ratio of 32. The relationship between the radiative and nonradiative transition probabilities at 77 K was qualitatively estimated from the measurement of the photoluminescence intensity and the decay time. The nonradiative process, which consists of the deep impurity level, strongly depends on the (V/III) ratio. At the (V/III) ratio of between 32 and 45, the nonradiative transition probabili...

Laser irradiation effects on photoluminescence spectra of undoped GaAs grown by metalorganic vapor phase epitaxy

Photoluminescence spectra of undoped GaAs grown by metalorganic vapor phase epitaxy with Ar+ laser irradiation were measured at 1.8 K. The enhancement of the incorporation of the carbon acceptor and the increase of the luminescence intensity were recognized as laser irradiation effects. These experimental results suggest that the surface reaction between the radicals involving the Ga atom and the photoinduced carrier at the substrate surface is enhanced by the laser irradiation.

Dynamic behavior of two-dimensional exciton in GaAs single quantum well under a magnetic field

Abstract Time-resolved photoluminescence spectra of two-dimensional excitons in a GaAs single quantum well were investigated in a magnetic field. The radiative life-time was independent of the magnetic field up to 6 T. The high density excitation effect on the photoluminescence spectrum was observed. The exciton temperature was decreased by applying the magnetic field.