Indirect Optical Absorption of Single Crystalline beta-FeSi2

We investigated optical absorption spectra near the fundamental absorption edge of beta-FeSi2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031 +- 0.004 eV. A value of 0.814 eV was obtained for the exciton transition energy at 4K.Comment: 10 pages with 3 figure

Relaxation and Coherent Control of Quantum Dots

歪誘起InGaAs/GaAs量子ドットの共鳴二次発光の干渉を用いた位相緩和測定法を実証し、また、蓄積フォトンエコー法によりCdSeおよびCuBr量子ドットにおける励起子位相緩和を測定した。ドットにおける低温均一幅は、温度に依存しない成分、励起子一母体の二準位系間の相互作用、励起子一閉じ込め音響フォノン間の2フォノンラマン過程の3つの和で統一的・普遍的に記述できる。 InPやInGaAs自己形成量子ドットに負の電気バイアスをかけ、ドットから光励起された正孔を抜き取る非輻射緩和速度を制御し、定常・時間分解発光の両面から競合するフォノン緩和が明らかにされた。フォノンボトルネック効果は、予想に比べ十分速いフォノン緩和の観測により否定された。単層の歪み誘起GaAs量子ドットと電場印加InP量子ドットのヘテロダイン検出フォトンエコー測定に成功し、前者では井戸に比ベドット中の励起子分子の束縛エネルギーの増大、後者では、電場により正孔をトンネル過程でドットから引き抜く過程を観測した。トリオン発光の量子ビートをInP量子ドットで見出し、:更に、量子ドットの4種類の量子ビートを発見した。これらの研究はチャージチューナブル量子ドットの概念を生み出した。 InP量子ドット中のドープ電子のスピン緩和時間がサブミリ秒からミリ秒に達することを示した。正孔ドープInAs量子ドットにおいて、無磁場で500psの光励起電子スピンの緩和時間が、0.1Tの磁場で4nsに伸びる、核スピン揺らぎの凍結効果を実証した。(1) Dephasing of the lowest-energy electronic transition of stress-induced InGaAs quantum dots was measured by the interferometric double pulse excitation and time-integrated detection of optical-phonon sideband in their resonant-photoluminescence spectra. Homogeneous linewidth of confined excitons in CdSe and CuBr quantum dots was investigated at low temperatures by means of accumulated photon echo. Homogeneous width is universally given by the sum of "temperature-independent" term, "exciton-two level system interaction" term and "exciton-confined acoustic phonon interaction" term.(2) Spectral and temporal behavior of photoluminescence of site-selectively excited InP and InGaAs quantum dots in external electric field showed fast phonon mediated relaxation processes is much faster than predicted theoretically. This observation demonstrates the breakdown of the predicted phonon bottleneck effect.(3) Highly sensitive heterodyne-detected photon echo enabled us to observe the signal from one layer of strain-induced GaAs quantum dots and InP quantum dots. A biexcitonic beat observed in GaAs quantum dots formed in the quantum well shows the additional increase in the biexciton binding energy compared with that of the quantum well. The photon echo in InP quantum dots under the electric field showed tunneling-induced dephasing and decayed nonexponentially, reflecting its non-Markovian nature.(4) Trionic quantum beat was discovered in eletron-doped InP quantum dots and 4 kinds of quantum beats were observed in quantum dots. The study created the concept of change tunable quantum dots.(5) Electron spin relaxation in electron-doped InP quantum dots was investigated and found to reach the submillisecond range.(6) It was shown that electron spin relaxation time of 500 ps observed in p-InAs quantum dots was elongated to 4 ns under the magnetic field of 0.1 T. This shows the efficient suppression of electron spin relaxation caused by randomly oriented nuclear spins.課題番号:1385200

Dynamical Study of Photoexcited States in Semiconductors by Means of Femtosecond Pump-and-Probe Spectroscopy

〈CdSe中の非平衡電子分布の観測〉フェムト秒ポンプ・プローブ分光法によりCdSeの伝導帯中に高いエネルギーを持った電子系を励起し約50fsの間、非平衡な電子分布を観測した。〈AlGaAs-AlAsタイプII型量子井戸中の励起子ー正孔散乱〉井戸層の伝導バンド下端のГ点よりも障壁層のX点のほうがエネルギー的に低い位置にあるタイプII型量子井戸においては、電子と正孔を空間的に分離する事ができ、この事を利用して励起子と正孔の散乱速度を励起子の位相緩和を測定する事により決定した。〈BiI_3における超高速コヒーレント現象の観測〉層状結晶BiI_3の間接励起子吸収端近傍の透明領域には、積層欠陥による励起子吸収線、R,S,Tが見られる。フェムト秒レーザを用いた強励起条件下において、積層欠陥励起子R,S,Tのコヒーレントな光学応答ならびに、周期およそ300fsの振動を示すコヒーレントフォノン効果を観測した。周期300fsは、14meVのエネルギーに対応するが、これはBiI_3のTOモードのフォノンエネルギーに一致する。〈ZnP_2におけるフェムト秒時間分解分光〉黒色ZnP_2半導体結晶は、その吸収スペクトルにn=7にも及ぶワニエ励起子系列が観測される興味深い物質である。フェムト秒時間分解分光法によりZnP_2中ワニエ励起子において、励起子間干渉ならびにポラリトン分枝間干渉に起因する2つのタイプの量子ビート効果の観測に成功した。レーザースペクトルがn=2、3、4の励起子吸収線をカバーするように同調させて測定を行うと、時間原点に、大きなピークに加えて数ピコ秒に及ぶ振動波形が観測された。この振動波形は励起子間の干渉に起因する量子ビートである。レーザの中心波長をn=1励起子に同調させて測定する際には、周期がしだいに広がっていく奇妙な振動構造を示した。この振動構造は光パルスの伝搬効果を反映したポラリトン分枝間の干渉効果によるものである。 Femtosecond pump-and-probe spectroscopy was applied to highly excited CdSe. Nonequilibrium distribution of electrons was observed for 50 fs. Exciton-hole collision rate was measured by means of time-resolved degenerate four-wave mixing. Phase relaxation of excitons is dominated by the collision between the excitons and long-lived holes. Resonant coherent lattice vibrations in the vicinity of the indirect absorption edge in a BiI_3 lattice crystal are observed in a femtosecond pump-and-probe experiment. The coherent transverse optical lattice vibration, which has a period of 300 fs, continues for more than 100 cycles. Quantum beats of Wannier excitons and excitonic polaritons were observed in ZnP_2. Qunatum beat originates from Wannier excitons whose principal quantum numbers n are 2,3 and 4. Polariton quantum beat was also observed as a result of interference of both upper and lower polaritons around the n =1 resonance.課題番号:0340200

EXTREMELY-NARROW LINEWIDTH OF THE CONFINED EXCITONS IN CuCl QUANTUM DOTS

Homogeneous linewidth of the excitons confined in CuCl quantum dots embedded in a NaClcrystal was studied by heterodyne-detected accumulated photon echo technique. At 2K, photonecho signal shows an exponential decay with a decay time of 160ps except for a slow rise. Thehomogeneous linewidth calculated from the dephasing time was found to be much smaller thanthat reported previously by the other methods. At 1.5K, we observed the linewidth of 1.6meV. Asfar as we know, this is the narrowest exciton linewidth in quantum dots.24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 199

Spin relaxation mechanism of strain-induced GaAs quantum dots studied by time-resolved Kerr rotation

We observed electron spin precession under magnetic field in single-layer quantum dots (QDs) by highly sensitive time-resolved Kerr rotation measurement. The spin lifetime is longer than that for the quantum well (QW). This is a result of the additional spatial confinement of electrons in QDs. Below 60 K, the spin lifetime is almost constant, and is 7 times shorter than the carrier lifetime. This suggests that the strong electron-hole exchange interaction dominates over the electron spin lifetime in QDs at low temperature

Tunneling and relaxation of photogenerated carriers in semiconductor quantum wells

The competition between tunneling (vertical transport) and relaxation (exciton formation) of photogenerated carriers in GaAs-Al0.29Ga0.71As multiple quantum wells in an electric field has been studied by means of nonlinear luminescence and photocurrent measurements and time-correlated single-photon counting. The nonlinearity is brought about by an exciton formed by bimolecular processes. The nonlinear luminescence and photocurrent signals are qualitatively explained in terms of the competition between tunneling and relaxation. Furthermore, the heavy-exciton luminescence shows two exponential decays when the electric field is 10–29 kV/cm. The slow component is ascribed to excitons formed by the bimolecular processes as a result of multiple successive processes of tunneling, relaxation, and exciton dissociation. Under the resonant electric field Fr=29 kV/cm, where electron sequential resonant tunneling occurs, the slow component diminishes because of the increase of the electron tunneling rate

Reply to ``Comment on `Inverse exciton series in the optical decay of an excitonic molecule' "

As a reply to the Comment by I.S. Gorban {\it et al.} (Phys. Rev. B, preceding paper) we summarize our criticism on their claim of the first observation of the $M$ series in $\beta$-ZnP$_2$. We support our analysis by reporting the first observation of inverse {\it polariton} series from the excitonic molecules selectively generated at ${\bf K}_m \simeq {\bf 0}$ in a CuCl single crystal. This observation and its explanation within the bipolariton model complete our proof of the biexcitonic origin of the inverse series.Comment: The Comment by I.S. Gorban et al. has been rejecte

Ultraviolet anti-Stokes photoluminescence in InxGa1-xN/GaN quantum-well structures

Ultraviolet anti-Stokes photoluminescence (PL) is observed in InxGa1-xN/GaN multiple quantum wells. The observed anti-Stokes PL exhibits a quadratic dependence on the excitation energy density. Anti-Stokes PL excitation spectrum is proportional to the optical absorption spectrum of the InxGa1-xN quantum wells. Time-resolved PL measurement shows that a decay of the anti-Stokes PL is slower than that of the GaN PL under the excitation above the band gap of the GaN barrier, and it is half the time constant of the InxGa1-xN PL decay. A two-step two-photon absorption process is directly observed by means of two-color pump-and-probe experiment. It is considered that the anti-Stokes PL is caused by a two-step two-photon absorption process involving a localized state in the InxGa1-xN quantum wells as the intermediate state, and that the second absorption step is provided by photon recycling of the InxGa1-xN PL

Disorder-induced transition from Gaussian to dispersive carrier transport in molecularly doped polymers

Dynamics of hopping charge transport in a polymer matrix doped with two different charge-transport molecules was studied by means of time-of-flight (TOF) photoconductivity measurements. In polymers doped with two molecules of little different ionization potential, the Gaussian (near-rectangular) TOF signal was observed over all compositions and the tail of the Gaussian TOF signal is broadened by a wide distribution of hopping time among molecules due to off-diagonal disorder. On the other hand, in polymers doped with two molecules of different ionization potential, the transition from the Gaussian to the dispersive TOF signal was observed and this is mainly caused by diagonal disorder. It is found that the energetic and spatial distribution of hopping sites play a key role in carrier transport in molecularly doped polymers