Mitsuru Sugisaki

Optical Properties of InP Self-Assembled Quantum Dots Studied by Imaging and Single Dot Spectroscopy

Optical properties of single InP quantum dots (QDs) were investigated by means of micro-photoluminescence (µ-PL) spectroscopy. The temperature dependence of the integrated µ-PL intensity revealed that each QD has different thermal activation energy ranging from 5 to 40 meV. The PL intensities of some QDs recovered when the temperature was raised beyond a certain value. This phenomenon was explained by competition between radiative and nonradiative processes. In this work, the mechanism of fluorescence intermittency was also investigated. It was found that the switching is triggered by a photo-induced process and that the confined excitons in the excited states play an important role. From the observation of the µ-PL spectra in the external electric field and their temperature dependence, we concluded that the large intensity change is caused by a local electric field generated by a carrier trapped at a localized center. These results indicate that the optical properties of the zero-dimensional system are strongly influenced by the local environment surrounding the QDs.

Temperature effects on quasi-isolated conjugated polymers as revealed by temperature-dependent optical spectra of 16-mer oligothiophene diluted in a sold matrix

Temperature dependences (4-300 K) of photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16 T) extremely diluted in polypropylene (PP) have been investigated in order to clarify temperature effects on quasi-isolated conjugated polymers. The PL and absorption spectra are found to blueshift with increasing temperature. The reason for the blueshift is discussed by comparing models based on the refractive index of the solvent (PP) and on the thermal conformational change of 16 T. The blueshift is concluded to result from the thermal conformational change. Time-resolved PL spectra show a redshift of PL band following photoexcitation (spectral migration). The amount of the migration is shown to increase with increasing temperature. The increased migration is concluded to be due to the thermal conformational change. The temperature dependence of the effective conjugation length (ECL) of 16 T is calculated for the absorption and PL transitions. The calculation suggests that ECL is reduced at room temperature to two-thirds of the intrinsic chain length. The activation energy of the conformational change is estimated to be 22.4 meV from the temperature dependence of ECL. We demonstrate that the steady-state PL spectra are well reproduced by simple Franck-Condon analyses using a single Huang-Ryes factor over a wide temperature range. The analyses reveal features of temperature dependence in important spectral parameters such as the Stokes shift, linewidth, and Huang-Ryes factor.

Many carrier effects in self-assembled InP quantum dots

Abstract Excitation power density dependence of InP self-assembled quantum dots (SADs) was investigated under band-to-band excitation in a Ga0.5In0.5P matrix by means of macro- (conventional) and micro-spectroscopy. State filling of confined excitonic states was studied in detail by micro-photoluminescence (μ-PL) spectra and images. The efficiency of the state filling differs from dot to dot, which depends on the inflow and outflow rates of the excitons. Sharp μ-PL lines from a single confined exciton and biexciton were clearly observed. The observed biexciton binding energy of an InP SAD is a few times larger than that of the bulk InP, suggesting that the multi-exciton states are stabilized because of the confinement.

Linear and nonlinear optical responses in bacteriochlorophyll a

Nonlinear optical responses of bacteriochlorophyll a (BChl a) were investigated by means of the three-pulse four-wave mixing (FWM) technique under the resonant excitation into the Qy band. The experimental results are explained by a theoretical model calculation including the Brownian oscillation mode of the solvent. We have determined the spectral density, which is the most important function with which to calculate optical signals. The linear absorption spectrum can be reproduced fairly well when the vibronic oscillation modes of the solvent together with those of BChl a are properly taken into consideration. The FWM signal was also calculated using the spectral density. It was found that a simple two-level model could not explain the experimental result. The effect of the higher-order interactions is discussed.

Photoluminescence and micro-imaging study of optically anisotropic InP self-assembled quantum dots

Abstract The optical properties of InP self-assembled quantum dots (SADs) were investigated under band-to-band excitation of the Ga 0.5 In 0.5 P matrix by means of macro- (conventional) and micro-photoluminescence (μ-PL) spectroscopy. We clearly observed that the number of bright spots in the μ-PL images of InP SADs depends on the detection energy, reflecting the size distribution of the SADs. The macro-photoluminescence spectra and μ-PL images of the InP SADs were found to exhibit a strong optical anisotropy with two-fold symmetry, which reflects the anisotropic structure of the Ga 0.5 In 0.5 P matrix due to the Cu-Pt B type long-range ordering.

Spin Quantum Beats in the Stokes Shifted Photoluminescence of InP Quantum Dots

Quantum beats observed in the photoluminescence kinetics of a single layer of self-assembled quantum dots in a magnetic field are studied as a function of the Stokes shift between the excitation and photoluminescence frequencies in the wide range of Stokes shifts from 7 to 70 meV. The quantum beats are attributed to beats between the fine-structure states of electron-hole pairs.

Quantum Beats in Photoluminescence of InP Quantum Dots in Electric Field

The photoluminescence (PL) kinetics of heterostructures with InP self-assembled quantum dots are studied under quasi-resonant pulse excitation in the presence of an external electric field. An oscillatory behavior of the PL kinetics is shown to arise due to quantum beats of the radiative states. A model of the coherent excitation of nonresonant PL is proposed.

Specific Channel of Energy Dissipation in Carotenoids: Coherent Spectroscopic Study

We investigate transient grating signals in β-carotene homologues by using sub-20-fs optical pulses. The results clearly show that shorter-chain carotenoids have a specific major channel of energy dissipation to the environment (the central C=C stretching mode) in the groundstate vibrational manifold, whereas the longer-chain carotenoids do not.

Coherent Spectroscopy of Carotenoid and Bacteriochlorophyll

Nonlinear Optical Responses Of A β-Carotene Homologue Having A Conjugation-Double Bond N = 15 And Bacteriochlorophyll A (Bchl A) Were Investigated In Order To Clarify The Dissipation Processes Of Excess Energy. The Experimental Results Were Explained By A Theoretical Model Calculation. We Have Determined The Spectral Density Which Is The Most Important Parameter To Calculate Optical Signals. We Found That The Linear Absorption Spectrum Can Be Fairly Well Reproduced When The Vibronic Oscillation Modes Of The Solvent Together Are Properly Taken Into Consideration. In Case Of The β;-Carotene Homologue, The Nonlinear Optical Response Can Be Well Reproduced By Properly Taking The S0, S1, And S2 Electronic Levels Into Consideration. In Case Of Bchl A, Our Computer Simulation Suggests The Involvement Of The Higher-Order Interaction, Such As The Two-Photon Process. The Role Of Coherence And The Efficient Energy Transfer In The Light-Harvesting Antenna Complexes Are Discussed.

Optical properties of InP self-assembled quantum dots studied by imaging and single dot spectroscopy

The temperature dependent optical properties of InP self-assembled quantum dots (QDs) were investigated by means of micro-spectroscopy. Successive thermal quenching with increasing temperature was clearly observed. A few QDs, however, showed anomalous temperature dependence with the photoluminescence (PL) intensity recovering when the temperature was raised beyond a certain value. This phenomenon is explained by competition between radiative and nonradiative processes. In this work, the mechanism of blinking behavior was also studied. Notably, the micro-PL spectra of a blinking QD were artificially reproduced by applying an electric field to a normal QD. We concluded that the blinking is due to the trapping and delocalization of carriers at localized states near the QD. These results indicate that the optical properties of the zero-dimensional system are strongly influenced by the local environment surrounding the QDs.