T. Saiki

Near-field optical fiber probe optimized for illumination–collection hybrid mode operation

The structure of a near-field probe was optimized for illumination-collection hybrid mode (I–Cu2009mode) operation. We developed a highly sensitive probe with a sharp-edged aperture and a double-tapered structure fabricated by a chemical etching technique. Through measurement of the photoluminescence (PL, λ=1 μm) image of single quantum dots, the performance of many versions of the probe with different aperture diameters, ranging from 80 to 500 nm, was systematically evaluated. In addition to the throughput and spatial resolution, the absolute value of the PL collection efficiency of the probe in (I–Cu2009mode) was also estimated for the first time. A collection efficiency several times greater than that of an objective lens with a numerical aperture of 0.8 was achieved with high spatial resolution in the 130–200 nm (λ/8–λ/5) range.

Photoluminescence intermittency in an individual single-walled carbon nanotube at room temperature

We described the photoluminescence (PL) properties of individual micelle-encapsulated single-walled carbon nanotubes (SWNTs) at room temperature. Single PL peak from isolated individual SWNT with a chiral index of (6, 5) showed a linear increase and saturation behavior of the PL intensity. Unusual PL intensity fluctuation in the temporal evolutions of the PL intensity, referred to as PL intermittency, was seen with some SWNTs, while the PL intensity with most SWNTs remained at a constant amplitude. The mechanism of the PL intermittency was discussed.

Near-field photoluminescence imaging of single semiconductor quantum constituents with a spatial resolution of 30 nm

High-resolution photoluminescence (PL) imaging of semiconductor quantum dots (QDs) was demonstrated using a low-temperature near-field scanning optical microscope. We systematically evaluated the spatial resolution for various fiber probes with different aperture diameters ranging from 30 to 135 nm. We achieved a spatial resolution of 30 nm (∼λ/30:λ=930u2009nm) in the PL imaging of self-assembled InAs QDs due to both improvement in probe preparation and optimization of the sample structure. The spatial resolution obtained in this study is on the scale of semiconductor quantum constituents and will make it possible to map out and manipulate the wave function in quantum-confined systems.

Near-field photoluminescence study of GaNAs alloy epilayer at room and cryogenic temperature

We have measured the spatial distribution of the optical properties of a GaNAs (N∼0.8%) epilayer to investigate the carrier recombination mechanism at both room temperature and cryogenic temperature using a near-field scanning optical microscope. A difference between the macro and near-field photoluminescence (PL) spectra at room temperature was not observed. At low temperature, we found spatial inhomogeneity of the optical properties and sharp features in the near-field PL spectrum. These findings indicate that the dominant emission mechanism changes from recombination of delocalized carriers at room temperature to recombination of localized carriers (excitons) trapped in the local potential minimum due to compositional fluctuation at low temperature.

Near‐field fluorescence imaging of single molecules with a resolution in the range of 10 nm

We demonstrate fluorescence imaging of single molecules, by near‐field scanning optical microscopy (NSOM), using the illumination‐collection mode of operation, with an aperture probe. Fluorescence images of single dye molecules were obtained with a spatial resolution of 15u2003nm, which is smaller than the diameter of the aperture (20u2003nm) of the probe employed. Such super‐resolution may be attributable to non‐radiative energy transfer from the molecules to the coated metal of the probe since the resolution obtained in the case of conventional NSOM is limited to 30–50u2003nm due to penetration of light into the metal.

Photoluminescence properties of single Mn-doped CdS nanocrystals studied by scanning near-field optical microscopy

We have fabricated Mn-doped CdS (CdS:Mn) nanocrystals embedded in Al2O3 matrices by sequential ion implantation and studied their photoluminescence (PL) properties by a scanning near-field optical microscope (SNOM). In the PL spectra of CdS:Mn nanocrystals measured by the SNOM, several sharp PL lines and a broad PL band were observed. The sharp PL lines are related to bound excitons at shallow impurities in CdS nanocrystals. The Mn-related PL spectrum is very broad even in single nanocrystals at low temperatures, and both the peak energy and the spectral width of the PL band depend on the excitation laser intensity. The PL properties of single CdS:Mn nanocrystals are discussed.

Design and optimization of tapered structure of near‐field fibre probe based on finite‐difference time‐domain simulation

The finite‐difference time‐domain method was employed to simulate light propagation in tapered near‐field fibre probes with small metal aperture. By conducting large‐volume simulations, including tapered metal‐cladding waveguide and connected optical fibre waveguide, we illustrated the coupling between these guiding modes as well as the electric field distribution in the vicinity of the aperture. The high collection efficiency of a double‐tapered probe was reproduced and was ascribed to the shortening of the cut‐off region and the efficient coupling to the guiding mode of the optical fibre. The dependence of the efficiency on the tapered structure parameters was also examined.

Observation of compositional fluctuations in GaNAs alloys grown by metalorganic vapor-phase epitaxy

Abstract Photoluminescence (PL) spectra and scanning near-field optical microscope (SNOM) images have been measured at low temperature to investigate the compositional fluctuations in GaN x As 1− x epilayers grown on GaAs by metalorganic vapor-phase epitaxy. Time-resolved PL has been employed to study the optical transitions and their dynamic processes. Our results suggest that the PL emission from GaN x As 1− x epilayer at low temperature is dominated by localized exciton recombination induced by compositional fluctuations. Furthermore, we directly observed the distribution of the compositional inhomogeneity in the GaN x As 1− x samples even with a small N-content of 0.5% by measuring SNOM images. The emission spot size of less than 1xa0μm is estimated for the GaN x As 1− x (N=0.5%) epilayer, and its size tends to decrease with increasing nitrogen concentration.

Low-temperature near-field nonlinear absorption spectroscopy of InGaAs single quantum dots

Nonlinear absorption spectroscopy of InGaAs single quantum dots (QDs) was realized by means of a low-temperature near-field optical microscope. The spatial distribution of the nonlinear absorption change in single QDs was determined. The dependence of the nonlinear absorption change on the pump power density demonstrates that the nonlinearity originates from the state filling of the ground state. The nonlinear absorption spectrum showed a homogeneous broadening of the ground state of about 5 meV. Furthermore, the change in the absorption cross section of the single QD when the ground state is saturated with carriers is estimated to be 2.8±0.6u200anm2, which agrees with the result predicted on the basis of theoretical consideration.

Sharp photoluminescence of CdS nanocrystals in Al2O3 matrices formed by sequential ion implantation

We report on photoluminescence (PL) experiments in CdS nanocrystals fabricated by sequential ion implantation in Al2O3 matrices. The PL spectrum and the spatial image of the PL intensity have been studied at 8 K using a scanning near-field optical microscope. The PL spectrum at each bright spot has been found to consist of narrow lines of various energies, although the spectrum measured by conventional optics shows a single and broad band locating below the free-exciton absorption energy. The origin of the sharp PL lines in CdS nanocrystals will be discussed.