Shinjiro Machida

Photon statistics in enhanced fluorescence from a single CdSe/ZnS quantum dot in the vicinity of silver nanoparticles

Single-photon emission behavior in the enhanced fluorescence from single colloidal quantum dots (QDs) near silver nanoparticles (AgNPs) was investigated using a single molecule fluorescence spectroscopy technique. It was found that the degree of fluorescence enhancement from single QDs with AgNPs increased with decrease in the lifetime and the probability of single-photon emission, that is, highly enhanced fluorescence with a shortened lifetime exhibited a low probability of single-photon emission. The present results yield new insights into fundamentals of QD-metal nanostructure interactions, and are also important to understand the mechanism of the fluorescence enhancement by localized surface plasmon of metal nanostructures.

Refractive-index patterning using near-field scanning optical microscopy

By using near-field scanning optical microscopy (NSOM), patterning of pure refractive-index contrast on a transparent polymer film has been demonstrated. A thin film of poly(methyl methacrylate) doped with 3-phenyl-2,5-norbornadiene-2-carboxylic acid is irradiated with 325 nm UV light from an NSOM probe, resulting in a refractive-index decrease without changes in complete transparency in the visible region and surface flatness. The transmission NSOM image was obtained by the same probe at an incident wavelength of 442 nm. The relative change of the transmission signal is in agreement with the value deduced from a simple model based on the Rayleigh scattering theory of dielectric media.

Frequency dependence of quantum efficiency for hole formation in photochemical hole-burning for dye-doped polymer systems

Abstract Marked frequency dependence of the quantum efficiency for hole formation was observed at 4 and 20 K for photochemical hole-burning of free-base tetraphenylporphin (TPP) and sulfonated TPP in poly(vinyl alcohol) (PVA) and other polymers as well as non-photochemical hole-burning of methylene blue in PVA. The quantum efficiency of hole formation at the lower-frequency foot of the lowest-energy absorption band is about 10 times larger than that at the higher-frequency foot, which could be due to the existence of several vibronic states in the lowest-energy absorption band with different molar extinction coefficients, hole formation efficiencies, and/or Debye—Waller factors.

Wavelength and polarization dependence of spectral hole-burning efficiency in highly oriented J-aggregates

Abstract From the concentration dependence of the dichroic spectrum of highly oriented J -aggregates as well as the strong dependences of the spectral hole-burning efficiency on the burning wavelength and polarization, a system of cyanine dye aggregates was concluded to have a hierarchic structure of mesoscopic and macroscopic aggregates. The observed low efficiency, by a factor of 10 −3 , on the higher-energy side of the J -band is explained in terms of the inefficient conformational change in a macroscopic aggregate, which has a large enough size to be aligned fluid-dynamically by the centrifugal force in the process of the spin-coating method.

Local free volume and structural relaxation studied with photoisomerization of azobenzene and persistent spectral hole burning in poly(alkyl methacrylate)s at low temperatures

The final extent of trans-to-cis photoisomerization of an azobenzene probe in various amorphous polymers has been used in previous studies for estimating local free volume and its fluctuation in polymer solids. However, there have been few studies on what kinds of molecular motion cause the fluctuation of local free volume at low temperatures. The onset of local structural relaxation (molecular motion) can be observed with temperature cycling experiments in persistent spectral hole burning (PSHB). Thus, in the present article, the fluctuation of local free volume observed in trans-to-cis photoisomerization of azobenzene is related to the local structural relaxation observed in PSHB for poly(alkyl methacrylate)s with various ester groups, i.e., methyl (PMMA), ethyl (PEMA), n-propyl (PnPMA), isopropyl (PiPMA), and isobutyl (PiBMA) groups. In the final cis fraction, rapid decrease, from 20 to 4 K in PEMA, PnPMA, and PiPMA, and from 86 to 20 K in PiBMA, is observed. These temperature regions of the rapid decrease in final cis fraction in these polymers agree well with those where the hole width in PSHB temperature cycling experiments begins to increase for the same polymers. For example, PEMA begins its ester ethyl group rotation at 17 K, which was primarily observed with PSHB, causing the drastic decrease in final cis fraction of azobenzene from 20 to 4 K. The final cis fractions at 4 K for these poly(alkyl methacrylate)s reflect the intrinsic sizes of the local free volume, except in the case of PMMA, and are compared with the reported results of positron annihilation lifetime measurements.

Photon Antibunching in the Emission from a Single Organic Dye Nanocrystal

We demonstrated that the emission from a single organic nanocrystal consisting of organic dye molecules shows photon antibunching. That is, an assembling structure consisting of many chromophores can also be made to behave as a single-photon source by controlling the size. We found that most of single nanocrystals within a size range from 35 to 85 nm show photon antibunching even when more than one exciton in a crystal is generated by an intense single excitation pulse. On the other hand, single micrometer-sized crystals showed no antibunching. The present results indicate that molecular assemblies can also be considered as candidates for new single-photon sources.

Nanoparticle Formation of Pentacene by Laser Irradiation in Ethanol Solution

Laser irradiation using a Nd3+:YAG laser (532 nm, 8 ns, 10 Hz) on an ethanol suspension of micrometer-sized pentacene crystals induced absorption spectral changes in the solution, as a result of the formation of pentacene nanoparticles. The nanoparticles were platelike crystals. The nanoparticles formed by irradiation with a fluence of 50 mJ/cm2 for 30 min were 4–13 nm in height and 10–70 nm in width. The threshold laser fluence for the formation was determined to be ca. 20 mJ/cm2, and irradiation with fluences above 80 mJ/cm2 induced the decomposition of the pentacene nanoparticles. The nanoparticle size decreased with an increase in the laser fluence, which was reflected in the longest-wavelength peak position of the absorption bands of the pentacene nanoparticles. On the basis of these results, the mechanism of pentacene nanoparticle formation by laser irradiation is discussed in connection with those of vanadyl phthalocyanine and quinacridone systems reported by Masuhara and coworkers [J. Phys. Chem. A 106 (2002) 2135; Jpn. J. Appl. Phys. 42 (2003) 2725; Jpn. J. Appl. Phys. 45 (2006) 384].

Photochemical hole burning of tetraphenylporphin in epoxy resin: Effect of crosslinked structure

Photochemical hole burning (PHB) of free‐base tetraphenylporphin (TPP) in epoxy resin was performed at 4.2–80 K. Effect of crosslinked structure of the epoxy resin on the dephasing time T2, low‐energy excitation modes, and the temperature dependence of Debye–Waller factor were studied through the PHB measurements. The excellent thermal stability of TPP in epoxy resin including the hole burning above liquid‐nitrogen temperature (80 K) was demonstrated.

Influence of molecular weight and conformation on single-photon emission from isolated conjugated polymer chains

The influence of molecular weight (MW) and chain conformation on single-photon emission from isolated chains of poly[2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene-vinylene] have been investigated. The probability of single-photon emission clearly increased with decreasing MW, and when the polymer chains adopted the “collapsed” conformation, this probability was larger than for the “extended” conformation. Even single polymer chains of high MW behaved as single-photon sources by adopting the collapsed conformation. These results indicate that multi-quantum systems consisting of a large number of chromophores can be made to behave as single-photon sources by appropriately controlling their size.

Photochemical hole burning of organic dye doped in inorganic semiconductor

We report a new type photochemical hole burning material; organic dye, zinc porphyrin, doped in inorganic semiconductor, titanium dioxide. The hole burning mechanism of this system is concluded to be photoionization via single‐photon process. The small temperature dependence of Debye–Waller factor was indicated by cyclic annealing experiment. A hole could be burned and observed at 140 K.