A. Seilmeier

Observation of intersubband real-space transfer in GaAs/AlAs quantum-well structures due to Γ–X mixing

The Γ2–Xz1 intersubband dynamics in GaAs/AlAs quantum-well structures is investigated by time-resolved infrared pump and probe experiments. In the studied structure, the second Γ level in GaAs is nearly resonant to the first Xz level in AlAs, resulting in elastic Γ–X scattering. A biexponential behavior of ground-state recovery has been found with a time constant in the order of 1 ps for the Γ2–Γ1 relaxation and a time constant of 7 ps (at 10 K), which represents the Xz–Γ2 transfer.

Carrier transfer in coupled asymmetric GaAs/AlGaAs double quantum wells after ultrafast intersubband excitation

Intersubband scattering processes are investigated in selectively doped asymmetric tunnel-coupled GaAs/AlGaAs quantum wells by infrared pump probe spectroscopy. Various subbands are excited by resonant absorption of a mid-infrared pump pulse and the carrier relaxation is observed via time- and frequency-resolved absorption changes monitored by a subsequent infrared probe pulse. The experimental data show that most of the carriers remain in the quantum well in which the excitation process mainly takes place. Only a minor fraction of the carriers is transferred between excited non-resonant subbands of the neighbouring quantum wells. The situation is different for electrons in the two lowest subbands which are separated only by 10 meV. Here a rapid and efficient transfer is observed. The scattering time is in the order of a picosecond at room temperature; it increases to values between 4 ps and 20 ps at 80 K. The data are interpreted with detailed model calculations taking into account phonon and impurity scattering. Both experimental and calculated data demonstrate the possibility of population inversion between excited states of quantum wells under optical or current electron injection.

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.

Intraband light absorption in InAs/GaAs quantum dots covered with InGaAs quantum wells

The spectral and polarization dependences of intraband light absorption in n- and p-doped structures with InAs/GaAs quantum dots covered by InGaAs layers were studied experimentally and theoretically. The transmittance of p- and s-polarized mid-infrared radiation was studied in a multi-pass sample geometry. Elastic strain, piezoelectric fields and a complex valence band structure were taken into account in calculations of the energy spectrum, the carrier wavefunctions and intraband optical matrix elements. The experimentally determined value of the absorption cross section corresponding to |0 0 0→|0 0 1 intraband electron transitions is 4.1 × 10−15 cm2. Light absorption related to intraband hole transitions was found to be significantly smaller. The dispersion of the dot sizes was evaluated using the experimentally determined spectral widths of intraband absorption peaks.

Optical phenomena in InAs/GaAs heterostructures with doped quantum dots and artificial molecules

Spectra of intraband absorption of polarized mid-IR light were investigated in undoped, p-, and n-doped InAs/GaAs quantum dots (QDs) covered with an InGaAs layer. Optical matrix elements for intraband electron and hole transitions in QDs have been calculated for different polarizations of light, and a good agreement with the experimental data is obtained. It is shown that the intraband absorption of light by electrons strongly exceeds the absorption by holes. Photoluminescence spectra and TEM images of structures with artificial molecules formed by pairs of QDs were studied.

Intersubband absorption of light in selectively doped asymmetric double tunnel-coupled quantum wells

The spectrum of equilibrium intersubband absorption has been studied in selectively doped asymmetrical double tunnel-coupled quantum wells designed for research into the modulation of IR light in a longitudinal electric field. The comparison of calculated and experimental spectra at different temperatures is carried out. In calculations, the influence of the space charge on the energy spectra of electrons and the difference in the electron effective mass in different subbands are taken into account. The data obtained on the intersubband absorption spectra in equilibrium conditions and under electron excitation by high-power picosecond pulses of light in the mid-IR range allow us to refine the energy spectrum of electrons in the actual structure.

Light absorption and emission in InAs/GaAs quantum dots and stepped quantum wells

The results of optical phenomena investigations in quantum dot and quantum well structures under interband optical pumping are presented. Interband and intraband light absorption in nanostructures with quantum dots has been studied experimentally and theoretically. Photoluminescence and interband light absorption in stepped quantum wells have been investigated including PL studies under picosecond optical pumping. Experimental results have been compared with results of calculation of energy spectrum and transition probabilities. It is shown that inversion of population exists between the third and second excited levels of stepped quantum well.

Optical phenomena connected with intraband carrier transitions in quantum dots and quantum wells

Optical phenomena in the mid-infrared range connected with interlevel and intersubband charge-carrier transitions in quantum dot and quantum well (QW) heterostructures under optical and electrical pumping were investigated. Spectra of interband photoluminescence are also presented. The existence of a metastable level in funnel-shaped QWs is experimentally confirmed. The intersubband transition dynamics in asymmetrical pairs of tunnel-coupled QWs was studied by means of pump-and-probe time-resolved spectroscopy. This paper was presented at the 3rd Russian Workshop on Nanophotonics, Nizhnii Novgorod, Russia, 26-29 March 2001.

Temperature dependence of intersubband scattering in multiple quantum well structures

The temperature dependence of intersubband scattering in n-modulation-doped GaAs/AlxGa1-xAs quantum well structures is systematically investigated for the first time by an infrared bleaching technique. In a highly doped sample, where the excited well subband is located above the bottom of the potential minimum of the barrier, a ground state recovery time of several picoseconds is found at T=300 K. At temperatures below 200 K an additional non-exponential component appears extending over approximately 100 ps. In samples where the energy of the excited well subband is lower than the energy of the subbands of the barrier potential minimum, the direct intersubband transition from the excited subband to the lowest subband is observed at T=10 K( tau approximately 2 ps). With rising temperature the measured time constants increase. The results are discussed taking into account the band structures of the samples.

Temperature Dependent Intersubband Dynamics in N — Modulation Doped Quantum Well Structures

Intersubband scattering in n-modulation doped GaAs/AlxGa1-xAs quantum well structures is systematically investigated by an infrared bleaching technique. The temperature dependence of the time constants is found to be sensitive to the self — consistent potential determined by the doping concentration and the well width. In samples with a shallow potential minimum in the barrier located above the excited subband of the well the direct intersubband transition from the excited to the lowest subband of the well is observed at T = 10K (τ ~ 2 ps). The measured time constants increase with rising temperature. In samples, where the energy of the excited subband is higher than that of the lowest barrier subbands (high doping concentration, narrow well width), ground state recovery times of several picoseconds are found at T = 300K. At low temperatures additional nonexponential components may appear extending over ~100 ps. The results are discussed taking into account transfer processes to the barrier.