Kensuke Miyajima

Superfluorescent pulsed emission from biexcitons in an ensemble of semiconductor quantum dots

Picosecond time-resolved photoluminescence from biexcitons in CuCl quantum dots (QDs) embedded in a NaCl matrix has been measured using an optical Kerr gate method. Ultrafast pulsed emission from the biexciton states was observed for the first time, only under resonant two-photon excitation of biexcitons. This implies that complete population inversion between the biexciton and exciton states is necessary in order to trigger the pulsed emission. In addition, the nature of the dependence of the time profiles of the pulsed emission on the excitation intensity reveals that the peak intensity is directly proportional to the square of the number of excited QDs. We conclude that this phenomenon is caused by superfluorescence, that is, the cooperative spontaneous radiative decay of many isolated excited states coupled by a resonant electromagnetic wave. Such a phenomenon has been observed for the first time in an ensemble of semiconductor QDs in this study. The results presented in this paper show that it is possible to control the microscopic coherent dynamics of electronic excited states in a QD ensemble.

Carrier recombination process and magneto-photoluminescence in Zn1−xCoxO layers

High-quality Zn1−xCoxO layers were obtained with isolated Co ions in the host (x no more than 0.008). The photoluminescence properties of excitonic and Co-related emissions in the Zn1−xCoxO (x=0.0015) layer were mainly investigated in both zero and up to 7 T magnetic fields in order to clear the energy transfer process from excitonic states to Co centers. It was found that Co emissions originated from 3d intraemissions in the Zn1−xCoxO (x=0.0015) layer were remarkably enhanced with the suppression of excitonic emissions, indicating the presence of efficient energy transfer to the Co centers. Assignment of absorption and magnetoluminescence spectra revealed that the energy transfer to the Co centers was not derived from the excitonic states but from charge transfer (CT) states formed on the Co centers after a band-to-band excitation. Finally, we consider that the primary role for the energy transfer process of Zn1−xCoxO (x=0.0015) layer is related to the phenomenon of impurity Auger recombination acting as...

Theory of Superfluorescence in Highly Inhomogeneous Quantum Systems

Superfluorescence originates from quantum coherence between polarizations in a radiation field. In general, system homogeneity is essential for superfluorescence because inhomogeneity causes decoherence. However, superfluorescence has been recently realized using solid-state materials with large inhomogeneity, with regard to the application for a coherent-light source, etc. This study proposes theoretical criteria for classifying superfluorescence in highly inhomogeneous systems into two regimes on the basis of competition between the generation of coherence due to the strong light-matter coupling in solid-state materials and decoherence caused by the system inhomogeneity. These criteria supports controlling and designing the system inhomogeneity with limited available fabrication technologies.

The photoexcited spin-aligned state of high-density exciton magnetic polarons and the effect of magnetic field in semimagnetic semiconductor Cd0.8Mn0.2Te

In Cd(0.8)Mn(0.2)Te, nonlinear photoluminescence (PL) appears only when localized excitons are selectively excited to high-density states. Here, the effect of a magnetic field is compared between nonlinear PL and PL due to localized magnetic polarons. Nonlinear PL shows a shift towards lower energy under an applied magnetic field, whereas PL of a localized magnetic polaron band shows a slight shift towards higher energy. The experimental results support the hypothesis that the origin of the nonlinear PL is a spin-aligned state of high-density exciton magnetic polarons. In the spin-aligned state, most spins of electrons (holes) in many magnetic polarons point in the same direction. In this new high-density photoexcited state, the s, p-d exchange interaction between photoexcited electrons (holes) and magnetic ions plays an important role.

Quantum-confined biexcitons in CuCl quantum dots and their unconventional optical properties

The optical properties of quantum-confined biexcitons in CuCl quantum dots (QDs) under the novel method of two-photon resonance excitation of biexcitons are reviewed. First, we briefly overview the optical properties of excitons and biexcitons in bulk CuCl single crystals and also those in quantum dots, and then we cover the biexcitonic properties of CuCl quantum dots in NaCl matrices in detail: realization of two-photon resonant excitation of confined biexcitons with different angular momenta, studies on the energy-level structure of biexcitonic states by size-selective two-photon resonant excitation of biexcitons, on subsequent efficient lasing of biexcitonic luminescence even in low-Q cavity, on ultrafast radiative decay of biexcitons, and, finally, the observation of excited states of confined biexcitons studied by infrared transient absorption. All these unconventional features are inherent in the biexcitons in CuCl QDs affected by quantum confinement effects.

Biexciton generation processes for CuCl quantum dot ensembles

We investigated various excitation-energy-dependent biexciton generation processes for CuCl quantum dot (QD) ensembles by observing photoluminescence (PL) bands originating from free excitons, bound excitons, and biexcitons as a function of the excitation photon energy. The resonant two-photon absorption (TPA) band of the biexcitons was revealed by the PL excitation spectrum of the biexcitons. Three biexciton generation processes were identified in relation to the range of the excitation photon energy: resonant two-photon excitations, resonant excitations of the bound or free excitons, and nonresonant excitations of the lowest-energy free excitons. Under the resonant two-photon excitation, the size selective excitation of the biexcitons in the QDs was confirmed. From the obtained relation between the excitation photon energy and the excited dot radius, the TPA band was found to be affected not only by the average dot radius but also by the size distribution of the dots. In addition, for the relaxation process, we demonstrated that the initial level of the biexciton PL was different from the biexciton level excited directly by the excitation light, indicating the presence of a certain relaxation process of the biexciton before radiative relaxation. These results indicate that it is possible to increase the density of the excited dots with identical PL energy regardless of their size distribution, which is advantageous for generating superfluorescence from QD ensembles.

Infrared transient absorption spectra of excitons and biexcitons confined in CuCl quantum dots

We studied excited states of excitons and biexcitons in CuCl quantum dots by infrared transient absorption measurement. We successfully obtained their absorption spectra clearly to discuss the specific excited states of the confined biexcitons.

Transition from amplified spontaneous emission to superfluorescence from biexcitons in semiconductor quantum dots

A transition from amplified spontaneous emission to superfluorescnece from biexcitons in semiconductor quantum dots was observed with increasing the excitation density under resonant two-photon excitation of the biexcitons.

The effect of the etching process on the morphology and photoluminescence of porous amorphous SiC

Porous amorphous silicon carbide (P-aSiC) was prepared by an electrochemical etching method. The surface morphology of the samples was measured by an atomic force microscope (AFM). Photoluminescence (PL) and PL excitation spectra of the samples were measured at room temperature. We observed clear dependence of PL spectra and morphology on fabrication conditions. Based on these results, we propose the emitting mechanism of the PL from the samples.

Superfluorescent Pulsed Emission from Biexcitons in an Ensemble of CuCl Quantum Dots

Time-resolved photoluminescence spectra of an ensemble of CuCl quantum dots have been measured by an optical Kerr gate method. The excitation photon energy was tuned to resonant energy for two-photon excitation of biexcitons. We observed that the time profiles of biexciton bands were changed from an exponential fast decay to a pulsed shape. This result indicates a transition from amplified spontaneous emission to superfluorescence. These results will introduce a new field of coherent phenomena originating an ensemble of quantum dots.