Shuichi Kinoshita

Mechanisms of structural colour in the Morpho butterfly: cooperation of regularity and irregularity in an iridescent scale

Structural colour in the Morpho butterfly originates from submicron structure within a scale and, for over a century, its colour and reflectivity have been explained as interference of light due to the multilayer of cuticle and air. However, this model fails to explain the extraordinarily uniform colour of the wing with respect to the observation direction. We have performed microscopic, optical and theoretical investigations, and have found that the separate lamellar structure with irregular heights is extremely important. Using a simple model, we have shown that the combined action of interference and diffraction is essential for the structural colour of the Morpho butterfly.

Wavelength–selective and anisotropic light–diffusing scale on the wing of the Morpho butterfly

We have found that cover scales on the wing of the butterfly Morpho didius possess specially designed microscopic structures for wavelength–selective reflection and contribute considerably to the brilliant blue colour of the wing. In addition, the cover scale functions as an anisotropic optical diffuser which diffuses light only in one plane, while it makes the range of reflection narrower in the orthogonal plane. The quantitative analyses for the wavelength–selection mechanism and the peculiar optical diffuser are given and the role of such a special optical effect is discussed from physical and biological viewpoints.

Structural or pigmentary? Origin of the distinctive white stripe on the blue wing of a Morpho butterfly.

A few species of Morpho butterflies have a distinctive white stripe pattern on their structurally coloured blue wings. Since the colour pattern of a butterfly wing is formed as a mosaic of differently coloured scales, several questions naturally arise: are the microstructures the same between the blue and white scales? How is the distinctive whiteness produced, structurally or by means of pigmentation? To answer these questions, we have performed structural and optical investigations of the stripe pattern of a butterfly, Morpho cypris. It is found that besides the dorsal and ventral scale layers, the wing substrate also has the corresponding stripe pattern. Quantitative optical measurements and analysis using a simple model for the wing structure reveal the origin of the higher reflectance which makes the white stripe brighter.

Dynamics of fluorescence of a dye molecule in solution

Transient and stationary fluorescence properties of rhodamine 6G in liquid ethanol have been investigated in a wide temperature range between 110 and 300 K. Various phenomena concerning the interaction between the dye molecule and the surroundings such as dynamic Stokes shift, stepwise temperature dependence of the fluorescence peak energy, and excitation‐wavelength dependent fluorescence peak shift, have been observed. To explain these phenomena in a unified manner, a stochastic theory based on the Onsager cavity model has been developed. It is shown that the fluorescence as well as the absorption characteristics are explained well by this treatment. Various parameters characterizing the optical properties of the molecule, i.e., the dipole moments in the ground and excited states, transition energy in vacuum, temperature‐dependent relaxation time of the dynamic Stokes shift and so on, can be derived from the comparison between the theory and experiment. Finally, the origin of the relaxation processes fou...

High‐performance, time‐correlated single photon counting apparatus using a side‐on type photomultiplier

Improvements of a previously reported simple fluorescence decay‐time measuring system are described. By the application of a high voltage between the photocathode and the first dynode of a side‐on photomultiplier, as well as by some modification of the electronic systems, the full width at half‐maximum of the response curve has been reduced to 160 ps and the wavelength dependence of the time response has become negligibly small. It is also demonstrated that the fluorescence decay time as short as several picoseconds can be analyzed by this system.

Theory of transient hole‐burning spectrum for molecules in solution

Theory based on a configuration coordinate model is presented to clarify solute–solvent relaxation effect on the line shape of a time‐resolved hole‐burning spectrum. The time‐resolved hole‐burning spectrum is affected by both a hole created in a ground‐state population and an excess excited‐state population. The solute–solvent relaxation causes thermalization in both populations and then the shift and broadening of the spectrum is expected observable. The line shapes of three typical cases are mainly considered: (1) The excited state is assumed to have an infinitely long lifetime; (2) the excited state has a finite lifetime, the population being assumed to relax toward an electronic state other than the ground state, and (3) the population created in the excited state relaxes to the ground state, where a hole‐filling effect is taken into account. Results obtained by numerical calculations show that a time evolution of the ground‐state hole is clearly observed when the low‐energy tail of the absorption ban...

Subnanosecond fluorescence‐lifetime measuring system using single photon counting method with mode‐locked laser excitation

A simple fluorescence decay‐time measuring system with single photon counting technique is described which uses, besides an undumped cw mode‐locked laser as an exciting source, a compact side‐on photomultiplier and IC comparators instead of a head‐on photomultiplier and a constant‐fraction discriminator. This system gives time behavior of fluorescence in the subnanosecond region or less with sufficient accuracy within a time as short as several seconds. The performance of this system is studied and factors to broaden the response time are discussed.

Effects of nonzero correlation time of system–reservoir interaction on the excitation profiles of second‐order optical processes in β‐carotene

Excitation spectra for narrow Raman lines and a broad luminescence band have been measured in β‐carotene in isopentane at 177 K over a wide spectral range on the low‐energy side of the 1Ag–1Bu absorption band. The result has been found to be well explained by a stochastic theory that takes into account the finiteness of the correlation time for the intermediate‐state interaction. Various parameters, such as the magnitude and the rate of the energy modulation in the intermediate state, have been determined from the analysis of the excitation profiles. The parameter values obtained indicate that the intermediate‐state interaction is far from the motional‐narrowing limit in this system.

Stochastic behavior of the dynamic stokes shift in rhodamine 6G

Abstract Transient behavior of the fluorescence spectrum has been investigated in rhodamine 6G in ethanol at low temperatures by means of a time-resolved luminescence technique. The width of the fluorescence spectrum as well as its peak position has been found to change with time. These characteristics are shown to be well explained by a stochastic model.

Single-scale spectroscopy of structurally colored butterflies: measurements of quantified reflectance and transmittance

Butterfly scales generally have very elaborate structures in submicrometer size, and some of them show distinctive optical effects through interaction with light. We describe two methods to quantitatively characterize the optical properties of the individual scales in those structurally colored butterflies. Owing to the small dimensions of the scale and to the fact that the reflection and transmission are very diffuse, it is generally difficult to accurately measure the reflectance and transmittance. To overcome these difficulties, we have carefully constructed an optical system including an integrating sphere and investigated variously colored nine kinds of scale. It is shown that the obtained spectra clearly characterize the optical differences among those structurally colored scales and also the differences between structural and pigmentary colors. Further, we have performed the angle-resolved measurement of the reflected light to characterize the spatial pattern of reflection, which is closely related to the mechanism of reflection.