Kazunori Iwamitsu

Stress effects on nP yellow excitons in Cu2O thin films recrystallized epitaxially in a sample gap between paired MgO substrates

We investigated the stress effects on nP yellow excitons in Cu2O thin films recrystallized epitaxially in a sample gap between paired MgO substrates. In such samples, it is expected that a two-dimensional compressive stress acts on Cu2O because of the slightly larger lattice constant of Cu2O than of MgO. To clarify such stress effects, we measured the X-ray diffraction and nP absorption transitions of the yellow excitonic system and analyzed the strain and stress effects. Although the detected lattice strain and the energy shift of the yellow excitonic band gap are smaller than the values expected from the lattice mismatch at the heterointerface, this can be explained self-consistently by considering strain and stress relaxations in Cu2O thin films with departing from the MgO heterointerface. Consequently, we can find that shallow trapping potentials for the yellow excitons are formed in the ∼1.3-µm-thick region interfaced with the MgO substrates.

Bayesian Analysis of an Excitonic Absorption Spectrum in a Cu2O Thin Film Sandwiched by Paired MgO Plates

We analyzed the absorption spectrum of a Cu2O thin film sandwiched by paired MgO plates with the Metropolis algorithm of Bayesian estimation to estimate the energy shift of a yellow excitonic band gap. In the absorption spectrum, discrete excitonic resonant transitions with homogeneous and inhomogeneous widths are superimposed upon the tail parts of excitonic continuum and band-to-band absorption bands. On the basis of the Metropolis samplings with 2.5 million steps after sufficient burn-in steps, the spectral parameters of the excitonic resonant transitions can be estimated satisfactorily with a high degree of accuracy. In particular, we demonstrated that the yellow excitonic band gap in the Cu2O thin film shifts to the lower energy side than that of Cu2O bulk crystals. This result is consistent with the stress relaxation model for such Cu2O thin films, which was reported in Eur. Phys. J. B 86, 194 (2013).

A wavelength modulation system for highly sensitive absorption spectroscopy.

We developed a newly designed wavelength modulation (WM) system for highly sensitive absorption spectroscopy. In our system, the WM is realized by yawing an output mirror in a monochromator. In order to control an amplitude Δλ of the WM in a wide range, we employed a forced vibration of a permanent magnet driven by a magnetic field of a solenoid. Our system has an advantage of that the WM amplitude Δλ can be adjusted in extensively wide range from 0.08 nm to 11 nm only by tuning a driving frequency of the applying current to the solenoid, because we utilize a resonance phenomenon of the forced vibration for adjustment of the WM amplitude. By using our system, we measured WM absorption spectra of a Cu(2)O thin film and found clearly spectral structures for weak 2-4P excitonic resonances in the WM absorption spectra.

High precision modeling of a damped oscillation in coherent phonon signals by Bayesian inference

By Bayesian inference with Metropolis algorithm, we have succeeded highly accurate estimation of a vibrational frequency as well as an initial phase for a damped oscillation contained in coherent phonon signals. Although a rise and damping profile of such vibrating signal impedes high precision estimation in conventional methods based on plane-waves expansion, the Bayesian inference makes it possible to obtain posterior probability distributions of all parameters in an appropriate physical model. On coherent phonon signals with a signal-to-noise ratio of ∼16dB, the probability distribution width of the vibrating frequency becomes two-orders of magnitude smaller than the Fourier spectral width. In addition, we can also estimate the initial phase with an accuracy on the order of 10 milli-radians as well as other parameters.By Bayesian inference with Metropolis algorithm, we have succeeded highly accurate estimation of a vibrational frequency as well as an initial phase for a damped oscillation contained in coherent phonon signals. Although a rise and damping profile of such vibrating signal impedes high precision estimation in conventional methods based on plane-waves expansion, the Bayesian inference makes it possible to obtain posterior probability distributions of all parameters in an appropriate physical model. On coherent phonon signals with a signal-to-noise ratio of ∼16dB, the probability distribution width of the vibrating frequency becomes two-orders of magnitude smaller than the Fourier spectral width. In addition, we can also estimate the initial phase with an accuracy on the order of 10 milli-radians as well as other parameters.

Coherent Phonons in a 1,3,5-Tri-Phenylbenzene Crystal

We studied low frequency vibrational modes in a light-harvesting (LH) antenna component molecule (1, 3, 5-tri-phenylbenzene: tri-ph) of a LH dendrimer by measuring a coherent phonon (CP) signal and molecular vibration analyses with semi-empirical molecular orbital calculations. In a Fourier transform spectrum obtained from the transient CP signal, we found three vibrating components with the frequencies of 35 ± 13, 60 ± 10, and 84 ± 14 cm−1. In the lower frequency range than 100 cm−1, the molecular vibration analyses give the following three kinds of vibrational modes: torsional vibrations, a butterfly vibration and bending vibrations, in which peripheral aromatic rings librate against the central ring in different manners. We identified the lowest frequency vibrating component (35 ± 13 cm−1) as the torsional vibrational mode changing dihedral angles and co-planarities between the central and the peripheral aromatic rings. In addition, it was also found that the vibrational frequencies obtained by our measurements are higher than those of the vibrational mode analyses. These hardenings are considered to account for steric hindrance between the neighbor molecules after the crystallization.

Thickness Dependence of Excitonic Spectra in Cu2O thin Films Sandwiched by MgO Plates

We investigated lattice distortion effects and their thickness dependence on the excitonic transitions in Cu2O thin films recrystallized in a small gap between paired MgO plates. Recently, Yoshiokaet al. reported the importance of exciton trapping into shallow potential minima for realizing the excitonic Bose-Einstein Condensation (BEC) in Cu 2O and it was also reported that uniaxial stresses are very useful to form the exciton trapping potentials. [2] In our investigation, we adopt Cu 2O thin films sandwiched by MgO plates because a small lattice mismatch between Cu 2O (4.273Å) and MgO (4.210Å) is expected to introduce compressive stresses in the Cu 2O thin films, which form the trapping potentials for the Cu 2O excitons. Figure 1 shows a schematic diagram of the cross section in our samples and the variation of the compressive stresses (horizontal arrows). As shown in this figure, compressive stresses due to the lattice mismatch are considered to relax gradually departing from the interfaces of MgO plates. Since it is expected that the degree of the lattice distortion effects varies with the sample thickness, we investigated thickness dependence of the excitonic spectra in Cu 2O thin films to clarify the lattice distortion effects and the formation of the exciton trapping potentials. Figure 2 shows the thickness dependence of the band gap energy shifts of the yellow excitonic system in Cu2O thin films from that in bulk crystals. In thick films B, C, and D, the red-shifts of the band gap are rather small. On the other hand, in a thin film A, one can recognize a larger red-shift. Consequently, by controlling their thickness, we can form an operative trapping potential for the yellow excitons to realize the excitonic BEC in the Cu 2O thin films.