Phase coherent photorefractivity in ZnSe single quantum wells

We observe an efficient phase coherent photorefractive effect in ZnSe single quantum wells for ultrashort light pulses resonant to the excitonic transition. The effect is attributed to the formation of an electron grating in the quantum well induced by the interference of coherent excitons that preserve phase and polarization of the incident light fields. All characteristic features of the diffracted signal are explained and reproduced by numerical calculations that are based on the optical Bloch equations for a three-level system.     

Negatively charged excitons and the optical properties of modulation-doped quantum wells

Optical and magneto-optical properties are studied for II-VI semiconductor multiple quantum wells (MQWs) doped with donors in the barriers to give electron concentrations of 2 10¹⁰ to 6 10¹¹ cm^-2 in the well layers. Following on from the recent identification of negatively charged excitons X^- (two electrons bound to one hole) in CdTe/Cd_1-xZn_xTe MQWs, this paper presents more specifically the circular polarisation of the luminescence associated with X^- and with the normal exciton X (one electron and one hole) in this type of structure. Very similar magneto-optical properties are observed for modulation doped Zn_0.9Cd_0.1Se/ZnSe MQWs, and X^- is identified in these wells with a binding energy as large as 7 meV for the second electron at 50Å, well-width.     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Linear and nonlinear optical properties of excitons in semiconductor quantum wells and microcavities

Linear and nonlinear light propagation in single and multiple quantum wells and in semiconductor microresonators are studied on the basis of Maxwell’s equations. The treatment includes radiative broadening of quantum-confined excitons, radiative coupling between quantum wells as well as coupling of quantum wells to the cavity field of a microresonator for steady state or ultrashort pulse excitation. The dynamical evolution of the coherent quantum-well polarization under the influence of many-body effects is studied within a microscopic model. The theory is used to investigate the influence of exciton saturation and dephasing on pulse propagation and excitonic normal-mode coupling.     

Terahertz emission with a time-dependent amplitude in asymmetric triple quantum wells

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an exciton in a GaAs/GaAlAs triple quantum well heterostructure. Considering a Coulomb interaction between the electron-hole pair in a triple quantum well, we have obtained a time-varying dipole moment in the system whose amplitude is also oscillating with time. In this way, we have shown the possibility of having a new kind of terahertz electromagnetic radiation emerging from a semiconductor device after an optical excitation of the system.     

Tunnel transfer of excitons due to a Fano disintegration and an optical phonon emission in biased double quantum wells

We consider theoretically and experimentally the tunnel transfer of excitons in an asymmetric double quantum well structure. We interpret quantitatively the measured tunnel transfer time in terms of two intrinsic mechanisms for excitons: an inter-well Fano disintegration for excitons photocreated in an excited state and the inter-well emission of optical phonons.     

Charged excitons in GaAs quantum wells

Il Nuovo Cimento D - We implement optical spectroscopy to study charged excitons (trions) in modulation-doped GaAs/AlGaAs quantum wells. We observe for the first time several new trions: the...     

Ultrafast time-resolved quantum beats in the polarization state of coherent emission from quantum wells

Dual-channel spectral interferometry is used to measure the ellipticity, the orientation of the polarization ellipse, and the sense of rotation of the four-wave-mixing signal from a GaAs-AlGaAs multiple quantum well. Each parameter is observed to oscillate dramatically at the heavy-hole-light-hole quantum beat frequency. During each beat period (in the strong quantum beat regime), the ellipticity oscillates twice between linear and almost-circular polarization, the orientation of the polarization ellipse rotates through a complete 180 degrees , and the sense of rotation changes from left- to right-circular polarization.     

Enhanced and inhibited spontaneous emission of free excitons in GaAs quantum wells in a microcavity

Spontaneous emission of the free excitons in GaAs quantum wells in a microcavity is enhanced (x 130) or inhibited (x ) by placing excitonic dipoles at either a resonant wavelength and anti-node position or an off-resonant wavelength and node position of the standing-wave vacuum field fluctuations. The resulting spontaneous radiation pattern is highly concentrated into the normal direction for the enhancement case and the spontaneous emission coupling efficiency into a single microcavity resonant mode is estimated to be 0.3. It is expected that semiconductor lasers with substantially reduced threshold currents can be constructed using such a structure.     

Bound excitons in p-doped GaAs quantum wells

Photoluminescence attributed to excitons bound to neutral impurities has been observed from GaAs quantum wells in Al_xGa_1?xAs-GaAs heterostructures grown by molecular beam epitaxy. The quantum wells were either doped with [Be] ≈ 10¹⁷ cm^-3 or Zn-diffused. At low temperatures both single and multiple quantum wells exhibited this extrinsic luminescence which is ascribed to the radiative recombination of the n=1 ground state heavy hole exciton E_1h bound to a neutral acceptor A^o. The dissociation energy E_D of the A^o-E_1h complex is obtained directly from the measured separation of this extrinsic peak from the intrinsic E_1h free exciton peak. For 46Å wide GaAs wells, E_D=6.5meV and E_D decreases with increasing well width.     

Ballistic transport of electronic excitations and coherent light propagation in semiconductor quantum wells

Il Nuovo Cimento D - The transport of radiative and electronic excitations in coherently driven semiconductor quantum wells is analysed. On ultrashort time scales the spatial dynamics is determined...     

Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.