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The coherent control of optical phonons has been experimentally demonstrated in various physical systems. While the transient dynamics for optical phonons can be explained by phenomenological models, the coherent control experiment cannot be explained due to the quantum interference. Here, we theoretically propose the generation and detection processes of the optical phonons and experimentally confirm our theoretical model using the diamond optical phonon by the double-pump-probe type experiment., Comment: 11 pages, 5 figures

We have studied micro-photoluminescence spectra of self-assembled single GaAs/AlGaAs quantum dots (QDs). Under non-resonant optical excitation, a single GaAs QD exhibits several sharp peaks in the PL spectra whose origins are the recombination of neutral or charged multi-excitonic complexes. In single photon correlation measurements, photons emitted from charged biexcitons and charged excitons show bunching which indicates that photon pairs are generated from the cascade recombination of the charged biexcitons. The result of polarization correlation measurements shows that each photon of the pairs is polarized linearly and orthogonally. These polarization features can be understood by the superposition of elliptically polarized photons affected by the electron–hole exchange interaction in the anisotropic QD.

Spin Information Achieved by Energy Transfer Via Optical Near Fields Between Quantum Dots and Its Robustness A. Sato1, F. Minami1, H. Hori2 4, and K. Kobayashi3 4 ∗ 1Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan 2Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8551, Japan 3Department of Electrical and Electronics Engineering, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan 4Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, 4-3-11 Takeda, Kofu, Yamanashi 400-8551, Japan

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.

A simple but flexible 2D lattice model is proposed to discuss localization and tunneling of a dressed photon in disordered nanomaterial systems. It is shown that a 1D finite disordered system with site-dependent masses is obtained after the contraction from 2D to 1D. A dressed photon description for a photon-electronic polarization–phonon interacting system is used to examine the spatial distribution of a dressed photon associated with coherent phonons in such a system. The temporal and tunneling behaviors of a dressed photon between two closely separated nanosystems are also investigated numerically. The model proposed is applicable to an arbitrary shape of nanostructure, and photon localization and tunneling inherent in the shape of the nanostructure is expected to be applied to the local manipulation of quantum states of nanostructure in excitation transfer processes, including spin and phonon degrees of freedom.

Four-wave mixing (FWM) technique is used to study the phonon induced dephasing of the excitons in CdTe self-assembled quantum dots. The FWM signals show two different decay structures with fast and slow decay components, indicating a non-Lorentzian line-shape with a zero-phonon line and a broadband from exciton–acoustic phonon interaction in the spectral region. The dephasing of the zero-phonon line and the sideband components both show non-linear characteristics with respect to temperature. The experimental data are reproduced well by a theoretical model taking into account the carrier–acoustic phonon coupling.

We discuss the mechanism of the spin and excitation energy transfer in a quantum-dot pair system driven by optical near fields (ONFs). When one of the pair is excited either by an ONF or effectively by a far field, the energy and spin information of the induced exciton can be transferred to the other quantum dot via a near-field optical interaction. Moreover, it is possible to flip the electron spin during the transfer process, depending on the resonant state of the energy levels of the quantum dots. We present the electron-spin selection rules, and calculate the spin and excitation energy transfer rates between the quantum-dot pair and the spin polarization, in terms of the exciton–polariton basis for ONFs. A way of making experimental observations is also suggested.

The qubits can be entangled when they interact with a common Ohmic reservoir. We analyze how the reservoir-induced entanglement of qubits can be observed as the beat signal in the decay curve of the macroscopic polarization. The origin of this effect is the Lamb phase shift on the qubit array. We quantify the amount of the reservoir-induced entanglement and show how to experimentally evaluate it from the decay curve of the macroscopic polarization. We discuss how the beat signal can be discriminated from the other kinds of beat signals. We also show that our analysis can be used to estimate the reservoir characteristics.

We measured time-resolved photon echo signals by using the heterodyne detection technique on GaAs multiple quantum wells. Oscillation structure with a period of about 0.8 ps is observed when the time interval (delay time) of two pulses which generate the photon echo signal is shorter than 0.4 ps. The oscillating structure disappears when the delay time is longer than 0.6 ps. Temperature and excitation density dependence indicate that the origin of this oscillation is the interaction between a normal exciton and an exciton coupled with a specific acoustic phonon mode which is identified by calculating the Rytov model.

We develop a theory of active dephasing control of qubit systems in array structures by using the multi-wave mixing technique. Assuming an acoustic phonon with a cutoff frequency ω c as the reservoir boson, we evaluate the state evolution of the qubit–reservoir system under irradiation of an optical pulse sequence. The effect of active dephasing control by the optical π pulses would be observed as diffracted echo signals in the phase-matching direction. The analysis for the case of weak pulse control is also presented.

The signals due to six-wave mixing (SWM) processes are observed for two-dimensional excitons and biexcitons. The signal intensity of the SWM process strongly depends on the pulse area of the excitation light. The pulse area is estimated by comparing the intensities of four-wave mixing (FWM) signals in two different directions. The photon echo and the transient-grating signals due to the biexcitonic state of GaSe were observed by SWM. When we change the order of the excitation pulses in our SWM experiments, the excitonic phase relaxation process can be observed in the negative delay-time region.

The phase characteristics of transmitted optical pulses were investigated in the energy- and the temporal domain using the spectrally resolved cross-correlation technique. The temporal evolution of fs pulses passing through GaAs/AlGaAs quantum wells indicates a long tail due to the coherent emission from the exciton. The phase discontinuity was observed in the time-domain data. The phase of the coherent emission signal is found to differ by 180° from that of the incident light field. Furthermore, we report the pulse propagation effect when the pre-excited coherent exciton exists in the sample. The resultant data shows that the ultrashort optical pulses are strongly affected by the phase difference between the coherent emission and the coherent exciton polarization.

Time-resolved three-pulse photon echoes of the exciton in layered semiconductor GaSe are investigated by using up-conversion technique. The time-resolved echo signal depends on the separation time between the first and second pulses. The shape of the echo signal changes as the separation time increases. The time-resolved signal also depends on the waiting time between the second and third pulses. For a short waiting time, the temporal evolution of the echo signal shows an ideal photon-echo-like (Gaussian-like) shape. On the other hand, for longer waiting times, the temporal shape of the echo signal changes dramatically and has an extra peak at the arrival time of the third pulse. This behavior can be explained by spectral diffusion in the three-pulse photon echo process.

The coherent emission in the direction of the reflection has been investigated under the Brewster angle of incidence by using phase-locked pulse pairs to excite resonantly the excitons in the anisotropic crystal GaSe. The interferogram of the reflected pulse has a long tail corresponding to the free induction decay signal. We also observed the phase difference between the coherent emission from the Γ4 and Γ6 excitonic polarizations in the direction of the reflection.

Excitation intensity dependence of photoluminescence signals is observed in Cd 0.9 Mn 0.1 Te at 2 K with varying exciton density from 10 14 to 10 19 cm −3 . For intermediate density regime ( 16 cm −3 ) , the transient spectral shift reflecting the magnetic polaron formation is found to decrease with an increase in density. For high-density regime (>10 17 cm −3 ) , the polaron shift disappears, and the density-dependent red shift caused by the band-gap renormalization is observed. The experimental findings demonstrate that the magnetic polaron is unstable at high-density condition.

We demonstrate the decoherence control of excitons on a layered compound semiconductor GaSe by using successive femtosecond pulses. The intermediate pulses act as π pulses, which reverse the time evolution of non-Markovian dynamics. By changing the incident timing of the π pulses, we determine the effective pulse interval condition for suppressing the decoherence of excitons by optical pulse irradiation.

We performed a three-pulse six-wave mixing (SWM) measurement on a gallium arsenide (GaAs) multiple quantum well and observed the time-resolved SWM signal using the heterodyne detection technique. The second pulse acts as a π pulse that reverses the time evolution of the non-Markovian dynamics. By changing the pulse interval conditions, we confirmed the suppression of exciton decoherence by π pulse irradiation (bang–bang control).

Single-photon emitters and detectors are key devices for realizing secure communications by single-photon-based cryptography and single-photon-based quantum computing. For the establishment of these technologies, we need to understand the electronic structures of single and multiple excitons. Therefore, we have studied their emissions via the micro-photoluminescence (μ-PL) spectra of strain-free GaAs/AlGaAs single quantum dots, using excitation power dependence, time-resolved, and single-photon correlation measurements. Under pulsed excitation, we observed clear photon antibunching and bunching by auto- and cross-correlation measurements. From these results, we found that the emission peaks observed in the μ-PL spectra originated from exciton, charged exciton, and biexciton states.

Oscillation behavior with periods of 0.56 and 0.88 ps has been observed by time-integrated four-wave-mixing measurements in GaAs multiple quantum wells. Temperature and excitation density dependence shows that the oscillations are caused by the quantum kinetic interactions between excitons and phonons. The energies of the phonons correspond exactly to those of the folded acoustic phonons calculated by the Rytov model.

We report the demonstration of decoherence control of excitons on a layered compound semiconductor GaSe by using successive three femtosecond pulses, i.e., the six-wave mixing configuration. The second pulse acts as a π pulse which reverses the time evolution of non-Markovian dynamics. By changing the pulse interval conditions, we confirmed for the first time the suppression of exciton decoherence by π pulse irradiation.

We present results on the acoustical phonon dephasing for a single GaAs quantum dot. Micro photoluminescence ( μ -PL) is used to observe the temperature dependence of the exciton line in self-assembled GaAs semiconductor single quantum dots (QDs). The observation of the PL with temperature shows that there are two different states within the PL emission, namely the zero-phonon line and the phonon sidebands. When the temperature increases, the effect of the sidebands become clearer and show substantial broadening. In the present experiments, PL and μ -PL spectra have been measured at temperatures of 5 K≤ T ≤90 K for GaAs self-assembled QD samples.

The polarization dependence of the stimulated photon echo due to excitons has been studied in GaSe. A quantum beat modulation can be observed in the stimulated photon echo signals. The polarization dependence of the quantum beat is also studied. The quantum beat and the polarization interference are experimentally distinguished by the time-integrated stimulated photon echo.

We confirm the existence of a biexciton in ZnSe-based quantum wells using transient four-wave-mixing spectroscopy. It is found that a new spectral line appears below the heavy-hole-related exciton resonance, corresponding to the exciton-biexciton transition. Through the analysis, we have determined the dephasing time of the biexciton, as well as the biexciton binding energy.

We demonstrate polarization-correlated three-photons emitted from a positively charged triexciton in a self-assembled GaAs quantum dot (QD). The charged triexciton consists of three electrons and four holes. Several intermediate states appear in the cascade recombination because of the exchange interaction. Spin singlet and triplet states of charged biexcitons and charged excitons are respectively identified in the photoluminescence spectra. We found that the degree of polarization correlation of the three-photons is different according to which intermediate states are used. A pathway through an excited state of a charged biexciton shows a high degree of polarization correlation, because the polarizations of these photons are dominantly determined by electron spins, which have longer lifetime than the holes. The spin relaxation times were estimated for an electron and a hole by a rate equation analysis with cross-correlation measurements.

We have observed the excitonic wave‐packet motion through spectrally resolved four‐wave mixing measurements in a layered semiconductor GaSe, and demonstrated that the four‐wave mixing spectroscopy is a useful tool to observe the wave‐packet motion. We have found that the wave‐packet motion in a quasi‐two‐dimensional system is explained theoretically by using the configuration coordinate model, where the adiabatic potential curve is shifted due to the exciton‐phonon interaction.

We report two-photon-resonant phenomena of hyper-Raman-scattering by optic phonons in ${\mathrm{TiO}}_{2}$. It is found that the scattering efficiency of the allowed mode in the 90\ifmmode^\circ\else\textdegree\fi{}-scattering geometry increases remarkably as twice the incident photon energy, 2\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$, approaches the direct forbidden energy gap from below. In addition, the configuration-forbidden LO mode in the backward-scattering geometry is found to show a sharp two-photon-resonant behavior at the band edge. The prominent polarization dependence of the phenomena is observed, reflecting the anisotropic nature of ${\mathrm{TiO}}_{2}$. Further, the absolute value of the scattering efficiency is estimated as a function of 2\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$. It is shown that these results can be interpreted quantitatively with a model on the basis of the electronic band structure. In the course of the analysis, we have estimated the second-lowest-direct-gap energy as 3.10 eV. Finally, we emphasize that the resonant hyper-Raman spectroscopy would be useful as a method for studying the band structure in solids.

The electronic Raman scattering of both tetragonal ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ symmetries has been measured in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals as a function of the temperature. We observe that a redistribution of the electronic continuum spectrum below ${\mathit{T}}_{\mathit{c}}$ exhibits two aspects of the temperature dependence: One of them is associated with its energy scale, which has a significant temperature variation only just below ${\mathit{T}}_{\mathit{c}}$. This indicates an abrupt opening of the superconducting gap. The other is associated with the variation of the spectral weight, which shows a more gradual temperature dependence compared with the former. This indicates that a superconducting condensation of thermally excited quasiparticles occurs gradually with decreasing temperature. We also find that the ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ spectra exhibit a similar temperature dependence below ${\mathit{T}}_{\mathit{c}}$.

Angle-resolved inverse photoemission spectroscopy was performed on a Bi 2 Sr 2 CaCu 2 O 8 single crystal to study the electronic structure in the vicinity of the Fermi level. Inverse photoemission intensity shows two maxima at about 0.21 ΓX and 0.6ΓX from the Γ point along the ΓX direction, suggesting that there are two bands crossing the Fermi level. The present experimental result was compared with angle-resolved photoemission results and band structure calculations.

We report on photon coincidence measurement in a single GaAs self-assembled quantum dot (QD) using a pulsed excitation light source. At low excitation, when a neutral exciton line was present in the photoluminescence (PL) spectrum, we observed nearly perfect single photon emission from an isolated QD at 670 nm wavelength. For higher excitation, multiple PL lines appeared on the spectra, reflecting the formation of exciton complexes. Cross-correlation functions between these lines showed either bunching or antibunching behavior, depending on whether the relevant emission was from a biexciton cascade or a charged exciton recombination., Comment: 5 pages, 3 figures

We have studied the electronic structure of MnO by photoemission spectroscopy. Mn 3d--derived emission is found to be confined within \ensuremath{\sim}10 eV of the top of the valence band, and we find no evidence for an intrinsic satellite at higher binding energies. The photoemission intensity of the whole valence band is enhanced for photon energies in the Mn 3p\ensuremath{\rightarrow}3d core absorption region. These observations suggest that ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L photoemission final states (L denotes a ligand hole) are close in energy and are strongly hybridized with each other. The spectra are analyzed in terms of configuration-interaction theory using a ${\mathrm{MnO}}_{6}$ cluster model. We thus find that the ligand-to-Mn d charge-transfer energy \ensuremath{\Delta} is comparable to the intra-atomic Coulomb energy U\ensuremath{\simeq}7.5 eV. Namely, MnO is indeed close to the boundary between the Mott-Hubbard (U) and the charge-transfer (Ug\ensuremath{\Delta}) regimes in the Zaanen-Sawatzky-Allen phase diagram, and the ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states are nearly degenerate. The large \ensuremath{\Delta} value leads to a highly ionic character in the Mn-O bonding.

We show the coherent control of the non-Markovian oscillation of exciton-folded acoustic phonon scattering in GaAs multiple quantum wells by using four-wave mixing measurements. The extent of non-Markovian scattering depends on the delay time ${t}_{{1}^{\ensuremath{'}}1}$ between the phase-locked pulses. The phenomenological dephasing time in the long-time (Markovian) region does not depend on the scattering. The dephasing process is suppressed when ${t}_{{1}^{\ensuremath{'}}1}$ corresponds to half the period of the non-Markovian oscillation. The dephasing suppression is the result of a fifth-order nonlinear optical effect (a six-wave mixing process).

We have studied the micro‐photoluminescence spectra of strain‐free GaAs/AlGaAs single quantum dots, coupled with magnetic field dependence, time‐resolved, and photon correlation measurements. The experimental behavior can be well explained by a model calculation based on the successive relaxation of multi‐excitons.

We show the coherent control of the non‐Markovian oscillation of exciton‐folded acoustic phonon scattering in GaAs multiple‐quantum wells by using four‐wave mixing measurements. The extent of non‐Markovian scattering is suppressed when the delay time between the phase‐locked pulses corresponds to half the period of the non‐Markovian oscillation.

Time‐resolved four‐wave mixing technique (FWM) is used to study the phonon induced dephasing of the excitons in CdTe self‐assembled quantum dots. The FWM signals show two different decay structures with fast and slow decay components, indicating a non‐Lorentzian lineshape with a zero‐phonon line and a broadband from exciton‐acoustic phonon interaction in the spectral region. The dephasing of the zero‐phonon line and the sideband components both show non‐linear characteristics with respect to temperature. The experimental data are reproduced well by a theoretical model taking into account the carrier‐acoustic phonon coupling.

Nonlinear optical response and optical coherence time of excitons localized to quantum islands in a $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ single quantum well are studied by the three-pulse photon echo technique. At low temperature, the optical coherence time is estimated to be $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ from the decay of the photon echo signal. The decay profile for short time delay shows the strong dependence on the input pulse area of the third pulse. Rabi oscillations are also observed. The oscillations show nonlinear behavior affected by the pre-excited polarization. Such behavior can be qualitatively explained by the local field effect due to the exciton-exciton interaction between the localized excitons.

We develop a theory to analyze the decoherence effect in a charged qubit array system with photon echo signals in the multiwave mixing configuration. We present how the decoherence suppression effect by the {\it bang-bang} control with the $\pi$ pulses can be demonstrated in laboratory by using a bulk ensemble of exciton qubits and optical pulses whose pulse area is even smaller than $\pi$. Analysis is made on the time-integated multiwave mixing signals diffracted into certain phase matching directions from a bulk ensemble. Depending on the pulse interval conditions, the cross over from the decoherence acceleration regime to the decoherence suppression regime, which is a peculiar feature of the coherent interaction between a qubit and the reservoir bosons, may be observed in the time-integated multiwave mixing signals in the realistic case including inhomogeneous broadening effect. Our analysis will successfully be applied to precise estimation of the reservoir parameters from experimental data of the direction resolved signal intensities obtained in the multiwave mixing technique., Comment: 19 pages, 11 figures

The dephasing control was performed for excitons in GaSe by using successive three femtosecond pulses. By changing the pulse interval conditions, we confirmed for the first time the suppression of the dephasing of the excitons by π pulse irradiation.

The energy levels of GaAs/AlGaAs quantum dots (QDs) are studied utilizing high pressure photoluminescence (PL), photoluminescence excitation (PLE), and single QD micro-PL measurements. It is found that the electron first excited state is situated 70 meV above the electron ground state, and the separation between the heavy hole ground state and its first excited state is found to be 20 meV. These values agree with effective-mass calculations. From the single QD micro-PL spectrum, the multi-electronic correlation energy is found to be of the order of 1 meV.

We have investigated the electronic stucture ofGaAs/AiGaAs/AlAs qantum wells by the resonance effects of scond-harmonic generation. Form the energy split between the 1S and 2P excition levels, the excition binding energies are determind. As aresult of the excition confinement effect,as a significant increase of the binding energy is found. as compared to bulk GaAs.

Subpicosecond coherent emission due to excitons in GaAs/AlGaAs quantum wells has been investigated by a new spectroscopic technique utilizing a two-pulse sequence of femtosecond duration laser pulses. By measuring the two-pulse interferograms of reflected pulses at the Brewster angle of incidence and that of a resonant Rayleigh scattering sequence, the polarization decay time can be obtained.

Photoemission from a single self-organized ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum dot (QD) is temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model which characterizes the successive relaxation of multiexcitons. Through the analysis we can determine the carrier relaxation time as a function of population of photoinjected carriers. Enhancement of the intradot carrier relaxation is demonstrated to be due to the carrier-carrier scattering inside a single QD.

The relaxation dynamics of a multiple exciton complex (multiexciton) confined in a semiconductor quantum dot has been investigated. Emission signals from a single self-organized GaAs/Al/sub 0.3/Ga/sub 0.7/As quantum dot are temporally resolved with picosecond time resolution. The emission spectra consisting of the multiexciton structures are observed to depend on the delay time and the excitation intensity. Quantitative agreement is found between the experimental data and the calculation based on a model describing the successive relaxation of multiexcitons.

We have studied the CW and time-resolved photoluminescence spectra of [001]-(GaAs)n/(AlAs)n (n=8−15) short-period superlattices. The superlattices with n⩽12 are of the type II and the lowest excited state is Xz exciton, while the samples with n⩾13 are type-I materials and the conduction band minimum occurs at Γ. It is found from decay pattern analysis that the significant Xz-Γ mixing occurs only near the direct-indirect crossover and dominantly at an odd number of AlAs monolayers.

Dynamics of quasi-two-dimensional excitons in the layered compound GaSe are investigated through temperature-dependent photon echo and luminescence decay measurements. A close correlation is found between the radiative lifetime τ and the phase coherence time T 2 of the free excitons. The relation between τ and T -1 2 (or the homogeneous linewidth) is compared to the prediction of a recent theoretical model.