In recent years, structural colors have attracted great attention in a wide variety of research fields. This is because they are originated from complex interaction between light and sophisticated nanostructures generated in the natural world. In addition, their inherent regular structures are one of the most conspicuous examples of non-equilibrium order formation. Structural colors are deeply connected with recent rapidly growing fields of photonics and have been extensively studied to clarify their peculiar optical phenomena. Their mechanisms are, in principle, of a purely physical origin, which differs considerably from the ordinary coloration mechanisms such as in pigments, dyes and metals, where the colors are produced by virtue of the energy consumption of light. It is generally recognized that structural colors are mainly based on several elementary optical processes including thin-layer interference, diffraction grating, light scattering, photonic crystals and so on. However, in nature, these processes are somehow mixed together to produce complex optical phenomena. In many cases, they are combined with the irregularity of the structure to produce the diffusive nature of the reflected light, while in some cases they are accompanied by large-scale structures to generate the macroscopic effect on the coloration. Further, it is well known that structural colors cooperate with pigmentary colors to enhance or to reduce the brilliancy and to produce special effects. Thus, structure-based optical phenomena in nature appear to be quite multi-functional, the variety of which is far beyond our understanding. In this article, we overview these phenomena appearing particularly in the diversity of the animal world, to shed light on this rapidly developing research field.
A quantum dot-based ratiometric pH sensor that responds to the pH range 6-8 was developed using FITC (fluorescein isothiocyanate) conjugated CdSe/CdZnS nanocrystals.
A new method to determine a coupling function in a phase model is theoretically derived for coupled self-sustained oscillators and applied to Belousov-Zhabotinsky (BZ) oscillators. The synchronous behavior of two coupled BZ reactors is explained extremely well in terms of the coupling function thus obtained. This method is expected to be applicable to weakly coupled multioscillator systems, in which mutual coupling among nearly identical oscillators occurs in a similar manner. The importance of higher-order harmonic terms involved in the coupling function is also discussed.
We investigate the dynamics of exciton hopping in a CdSe/ZnS quantum dot (QD) array composed of an inhomogeneously broadened ensemble. Time-and spectrally resolved fluorescence intensities are measured by varying the excitation photon energy at the absorption edge. This method allows us to observe fluorescence from only the subdistribution of the QD ensemble, thereby allowing the dynamics of exciton hopping, which depends on the initial (donor) exciton energy, to be elucidated. Experimental results along with numerical calculations using a model of a coupled QD array show that when high-energy QDs are selectively excited, exciton energy transfer occurs repeatedly to a site of low energy, leading to a large exciton hopping length. In contrast, when the low-energy end of the QD ensemble is excited, the exciton tends to be trapped in the initial QD. Furthermore, from the analysis of the decay time of fluorescence intensities, it is suggested that there are dark QDs associated with the defect and/or off state of blinking QDs in the ensemble and energy transfer to such a site is mainly followed by quenching.
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