Interwell Excitons Research Articles

Intrawell and interwell phonon-excitons in a CdS/CdSe/CdS asymmetric quantum well

Binding energies of intrawell and interwell excitons are studied in the CdS/CdSe/CdS asymmetric quantum well by varying right well sizes under polaronic effect. The transition energy, transition life time, oscillator strength and absorption coefficients are measured. The above properties are obtained in the influence of image charge potential included in the Hamiltonian. The optical absorption coefficients with the function of incoming photon energy are obtained. The results bring out that the peak of absorption coefficient is larger for the direct exciton than for the indirect exciton and the radiative lifetime for the indirect exciton is found to be higher.

Spectrally Resolved Ultrafast Exciton Transfer in Mixed Perovskite Quantum Wells.

Solution-processed perovskite quantum wells have been used to fabricate increasingly efficient and stable optoelectronic devices. Little is known about the dynamics of photogenerated excitons in perovskite quantum wells within the first few hundred femtoseconds-a crucial time scale on which energy and charge transfer processes may compete. Here we use ultrafast transient absorption and two-dimensional electronic spectroscopy to clarify the movement of excitons and charges in reduced-dimensional perovskite solids. We report excitonic funneling from strongly to weakly confined perovskite quantum wells within 150 fs, facilitated by strong spectral overlap and orientational alignment among neighboring wells. This energy transfer happens on time scales orders of magnitude faster than charge transfer, which we find to occur instead over 10s to 100s of picoseconds. Simulations of both Förster-type interwell exciton transfer and free carrier charge transfer are in agreement with these experimental findings, with theoretical exciton transfer calculated to occur in 100s of femtoseconds.

Darkening of interwell excitons in coupled quantum wells due to a stress-induced direct-to-indirect transition

In previous studies, an inhomogeneous strain field was used as a trapping mechanism for interwell excitons in coupled GaAs/${\mathrm{Al}}_{0.45}{\mathrm{Ga}}_{0.55}\mathrm{As}$ quantum wells. Photoluminescence measurements in this system demonstrated the presence of a dark population of excitons at trap center in the low-temperature, high-stress, high-density regime. The dramatic appearance of this effect at low temperature and high density initially suggested that it may be indicative of a Bose-Einstein condensation phase transition. Further experiments revealed that this effect appears more readily in wider quantum wells and occurs at strain values near the heavy-hole/light-hole crossover point. In this paper, it will be shown that this effect occurs in a regime where the heavy-hole valence band maximum shifts away from ${k}_{||}=0$, which is expected to strongly suppress the recombination rate at low temperatures. Simulations based on this assumption will be shown to match the experimentally observed behavior, without requiring the appearance of a Bose-Einstein condensate.

Effects of hydrostatic pressure on intrawell and interwell excitons in a strained GaAs/GaAlAs double quantum well system

Binding energies of intrawell and interwell excitons are investigated in a GaAs/GaAlAs double quantum well system in the presence of hydrostatic pressure applied in the z-direction. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of pressure. The pressure dependent photoionization cross section for a charged exciton placed at the center of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross section on photon energy is carried out for the charged excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The results show that the charged exciton binding energy, interband emission energy and the photoionization cross section depend strongly on the well width and the hydrostatic pressure. Our results are compared with the other existing literature available.

Temperature dependence of indirect-exciton luminescence in in-plane magnetic field

We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one.

Tunnel coupling in asymmetric semimagnetic double quantum wells

AbstractThe influence of the tunnel coupling on the steady‐state photoluminescence has been studied in Cd(Mg,Mn)Te‐based asymmetric double quantum wells (ADQWs) in a magnetic field up to B = 8 T. As grown structures were annealed to introduce Mn and Mg atoms from the opposite barriers inside pure CdTe quantum wells, resulting in a formation of the magnetic (MW) and nonmagnetic (NMW) wells, respectively. Radiative recombination in the ADQW with a wide barrier (9 nm) shows two bands, corresponding to excitons, localized in two decoupled QWs with a markedly different spin splitting: small splitting in the NMW due to the positive exciton Landé band g ‐factor and large splitting in the MW due to the giant Zeeman effect. Contrary, in the ADQW with a narrow barrier (3 nm) an unusual exciton splitting is observed with a negative value of the exciton g ‐factor and a strong circular polarization of the exciton bands. Simultaneously, a complete reduction of the MW emission is found, indicating strong interwell exciton relaxation. Calculations show that it is a strong electron coupling between the wells which is responsible for the observed behaviour. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct and interwell excitons in magnetic nanostructures

The energies of direct and interwell excitons in superlattices based on europium and lead sulfides have been calculated. It is established that these excitons have higher oscillator strengths and binding energies due to the indirect exchange. This circumstance can be used in semiconductor devices operating on exciton transitions.

ASYMMETRIC DOUBLE QUANTUM WELLS AS EXCITON SPIN SEPARATOR

Exciton recombination in asymmetric CdMgTe / CdTe / CdMgTe / CdTe / CdMnTe double quantum wells (ADQW) with different barrier widths is studied in magnetic field up to 10 T. As grown structures were subjected to temperature annealing to introduce Mn and Mg atoms from the barriers into the CdTe quantum wells (QW). At low fields exciton transition in QW with magnetic impurity ( Mn ) is higher in energy than that in QW with nonmagnetic impurity ( Mg ), and interwell exciton relaxation is fast independently on the spin state. In contrast, at high fields, when the energy order reverses, an unexpectedly low relaxation rate of σ- polarized excitons from nonmagnetic QW to the ground σ+ polarized state in magnetic QW has been observed. Effect strongly correlates with hh–lh splitting value Δhh – lh and discussed in terms of valence band mixing. Such a slowing down of relaxation allows separation of oppositely polarized excitons in different QWs.

COHERENCE AND SPIN RELAXATION OF INTERWELL EXCITONS IN GaAs/AlGaAs COUPLED QUANTUM WELLS

The kinetics of spin orientated interwell excitons gas has been investigated in GaAs / AlGaAs coupled quantum wells (n–i–n heterostructures). In these structures, the electrons and the holes are spatially separated between neighboring quantum wells by a narrow AlAs barrier. The time evolution and kinetics of photoluminescence spectra of interwell excitons have been measured under the pulse resonance excitation of intrawell 1sHH excitons using a pulsed tunable laser. The temporal formation of collective excitons phase and the temperature dependence of its spin relaxation rate have been studied. It was observed that spin relaxation rate of interwell excitons in the collective phase is strongly reduced. The observed effect is an evidence of the coherence of indirect excitons' collective phase at the temperatures below the critical one.

Interwell exciton relaxation in semimagnetic asymmetric double quantum wells

The possibility of magnetic field control of the spectral and polarization characteristics of exciton recombination is examined in Cd(Mg, Mn) Te-based asymmetric double quantum wells. At low fields, the exciton transition in a semimagnetic well is higher in energy than that in a nonmagnetic well and the interwell exciton relaxation is fast. In contrast, when the energy order of the exciton transitions reverses at high fields, unexpectedly slow relaxation of σ− polarized excitons from the nonmagnetic well to the σ+-polarized ground state in the semimagnetic well is observed. Strong dependence of the total circular polarization degree on the heavy-light hole splitting Δhh-lh in the nonmagnetic well is found and attributed to the spin dependent interwell tunneling controlled by exciton spin relaxation. Such a slowing down of the relaxation allows separation of oppositely spin-polarized excitons in adjacent wells.

Bose condensation of interwell excitons and spatial structure of luminescence in lateral traps

Russian Academy of Sciences (RAS) dedicated to the 60th anniversary of the Zavoisky Kazan Physical-Technical Institute of the Kazan Scientific Center, Russian Academy of Sciences, was held in the Conference Hall of the Zavoisky Kazan Physical-Technical Institute on 19 December 2005. The following reports were presented at the session: (1) Krokhin O N (Lebedev Physics Institute, RAS, Moscow) ``50 years of quantum electronics''; (2) Gorbunov A V, Timofeev V B (Institute of Solid State Physics, RAS, Chernogolovka, Moscow region) ``Bose condensation of interwell excitons and spatial structure of luminescence in lateral traps''; (3)Chekalin SV (Institute of Spectroscopy, RAS, Troitsk, Moscow region) ``The unique femtosecond spectrometric complex as an instrument for ultrafast spectroscopy, femtochemistry, and nanooptics''; (4) Salikhov K M (Zavoisky Kazan Physical-Technical Institute of the Kazan Scientific Center, RAS) ``Timeresolved EPR spectroscopy of nonequilibrium spin systems produced during spin-dependent photophysical and photochemical processes in condensed media''; (5) Manenkov A A (Prokhorov General Physics Institute, RAS,Moscow) ``The role of electron paramagnetic resonance in the development of quantum electronics: facts and comments''; (6) Smirnov A I (Kapitza Institute for Physical Problems, RAS, Moscow) ``Magnetic resonance modes in spin-gap magnets''; (7) Kochelaev B I (Kazan State University) ``Evolution of antiferromagnetic cuprates in high-temperature superconductors''; (8) Garifullin I A (Zavoisky Kazan Physical-Technical Institute of the Kazan Scientific Center, RAS) ``The superconductor/ferromagnet proximity effect and its potential application in spintronics.'' The main propositions of O N Krokhin's report were published in Usp. Phys. Nauk 174 1117 (2004) [Phys. Usp. 47 1045 (2004)]. A brief presentation of reports 2 ± 6 and 8 is given below. PACS numbers: 03.75.Nt, 71.35. ± y, 78.55. ±m DOI: 10.1070/PU2006v049n06ABEH006043

Coherent spin dynamics of an interwell excitonic gas inGaAs∕AlGaAscoupled quantum wells

The spin dynamics of an interwell excitons gas has been investigated in n-i-n GaAs/AlGaAs coupled quantum wells (CQWs). In these heterostructures the electron and the hole are spatially separated in neighboring quantum wells by a narrow AlAs barrier, when an electric field is applied. The time evolution kinetics of the interwell exciton photoluminescence has been measured under resonant excitation of the 1sHH intrawell exciton, using a pulsed tunable laser. The formation of a collective exciton phase in time and the temperature dependence of its spin relaxation rate have been studied. The spin relaxation rate of the interwell excitons is strongly reduced in the collective phase. This observation provides evidence for the coherence of the indirect excitons collective phase at temperatures below a critical $T_c$.

Collective state in a bose gas of interacting interwell excitons

Experiments associated with direct observations of a collective state in a gas of interacting interwell excitons in GaAs/AlGaAs double quantum wells are discussed. The structures constitute Schottky photodiodes. In a metallic gate, circular windows of various sizes (diameters of 2 to 20 μm) are etched by means of electronic-beam lithography. Through these windows, the photoluminescence of interwell and intrawell excitons is excited and detected. A microscopic device allows the observation of the spatial structure of luminescence with a resolution of 1 μm through the windows of a sample placed in superfluid helium. Using optical interference filters, the spatial structure of the luminescence is analyzed selectively in the spectrum for interwell and intrawell excitons under the same experimental conditions. It is found that the photoluminescence of interwell excitons under certain conditions exhibits an axisymmetric spatial structure: along the perimeter of the windows through which the photoluminescence is observed, a regular ring pattern of equidistant bright spots of the luminescence of interwell excitons appears. This structure appears only above the photoexcitation power threshold and the number of equidistant bright spots in the ring increases with the pumping power. At high pumping powers, the structure of distinct periodic luminescence spots is smeared. At a fixed pumping power, the phenomenon exhibits explicit critical temperature dependence: the structure of regularly located luminescence spots is smeared at T > 4 K. Axisymmetric spatial configurations of equidistant luminescence spots are observed in windows of the diameters 2, 5, and 10 μm. For intrawell excitons, the spatial structure of luminescence is not observed under similar experimental conditions: the luminescence of intrawell excitons is spatially uniform in all the windows under investigation. The effect is a result of the collective behavior of interacting interwell excitons.

Boze'kondensatciya mezh'yamnih eksitonov i prostranstvennaya struktura lyuminestcentcii v lateral'nih lovushkah

Boze'kondensatciya mezh'yamnih eksitonov i prostranstvennaya struktura lyuminestcentcii v lateral'nih lovushkah

Temperature dependence of luminescence intensity under Bose condensation of interwell excitons

Photoluminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructure) containing large-scale random potential fluctuations in the planes of heteroboundaries is studied. The properties of excitons, in which a photoexcited electron and a hole are spatially separated in neighboring quantum wells, were investigated upon variation of the power density of off-resonance laser excitation and temperature (1.5–4.2 K), both under lateral (in the heteroboundary plane) confinement of the excitation region to a few micrometers and without such a limitation (directly from the region of laser-induced photoexcitation focused to a spot not exceeding 30 μ. Under low pumping (with a power smaller than a microwatt), interwell excitons are strongly localized due to small-scale random potential fluctuations and the corresponding photoluminescence line is nonhomogeneously broadened to 2.5–3.0 meV. With increasing pumping power, the narrow line of delocalized excitons with a width of approximately 1 meV emerges in a threshold manner (the intensity of this line increases superlinearly near the threshold with increasing pumping). For a fixed pumping, the intensity of this line decreases linearly upon heating until it completely vanishes from the spectrum. The observed effect is attributed to Bose condensation in a quasi-two-dimensional system of interwell excitons. Within the proposed model, we show that the linear mode in the behavior of the luminescence intensity until its disappearance in the continuum of the photoluminescence spectrum upon a change in temperature is observed only for the condensed part of interwell excitons. At the same time, the luminescence of the above-the-condensate part of excitons is almost insensitive to temperature variations in the temperature range studied.

Collective behavior of interwell excitons laterally confined in GaAs/AlGaAs double quantum wells

The luminescence of interwell excitons laterally confined by long range potential fluctuations and with the use of inhomogeneous electric field in n-i-n GaAs/AlGaAs heterostructures double quantum wells has been investigated under variation of excitation power and temperature. Above mobility threshold very narrow interwell exciton line has been observed and its intensity decrease is linearly dependent on temperature growth. The observed phenomena, which were critical to exciton density and temperature, are attributed to the Bose-condensation in laterally confined quasi-two dimensional system of interwell excitons. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Collective behavior of a spin-aligned gas of interwell excitons in double quantum wells

The kinetics of a spin-aligned gas of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructure) is studied. The temperature dependence of the spin relaxation time for excitons, in which a photoexcited electron and hole are spatially separated between two adjacent quantum wells, is analyzed. For this purpose, use was made of pulsed circularly polarized resonant photoexcitation of intrawell 1sHH excitons by a femtosecond frequency-controlled laser. A sharp increase in the spin-relaxation rate is observed for interwell excitons upon a change in temperature from 2 to 3.6 K. This effect is associated with indirect evidence of the coherence of the collective phase of interwell excitons at temperatures below the critical value.

Interwell excitons in a lateral potential well in an inhomogeneous electric field

The luminescence of interwell excitons in double quantum wells based on GaAs/AlGaAs semiconductor heterostructures (n-i-n structures) in a lateral trap prepared with the use of an inhomogeneous electric field was studied at helium temperatures. A rather strong and inhomogeneous electric field occurred in the depth of the heterostructure when a current passed through the contact between the conducting tip of a tunneling microscope and the heterostructure surface to the bulk region containing a built-in gate. Because of the Stark shift of energy bands in the electric field, the photoexcited electrons and holes are spatially separated in neighboring quantum wells by a tunnel-transparent barrier and are bound into interwell quasi-two-dimensional excitons. These excitons have a dipole moment even in the ground state. Therefore, electrostatic forces in the inhomogeneous electric field cause the excitons to move in the plane of quantum wells toward the maximum field region and eventually accumulate in the lateral trap artificially prepared in such a way. The maximum trap depth achieved through the inhomogeneous electric field was 13.5 meV, and its lateral size was about 10 μm. It is shown that, in the traps prepared in this way, photoexcited interwell excitons behave with increasing concentration at sufficiently low temperatures (T=2K) in the same fashion as in the lateral traps caused by large-scale fluctuations of the random potential. At concentrations exceeding the percolation threshold, the interwell excitons condense into the lowest energy state in the trap.

Interwell excitons in semimagnetic semiconductor double quantum wells in an external magnetic field

The use of semimagnetic semiconductor double quantum wells in an external magnetic field is suggested for the separation of exciton charges. The exciton energies and wave functions, as well as the oscillator strengths, are calculated for (Zn, Be, Mg)Se/ZnSe/(Zn, Be, Mg)Se/(Zn, Mn)Se/(Zn, Be, Mg)Se double quantum wells for different interwell barrier widths and different external magnetic fields. It is shown that, in a certain range of magnetic fields, the lowest energy corresponds to the interwell (indirect) exciton. Moreover, its lifetime can be several orders of magnitude greater than the lifetime of an exciton localized in one well.

Bose condensation of interwell excitons in lateral traps: A phase diagram

The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructures) containing large-scale random-potential fluctuations was studied. The study dealt with the properties of an exciton whose photoexcited electron and hole are spatially divided between the neighboring quantum wells under density variation and at temperatures of down to 0.5 K. We investigated domains ∼1 µm in size, which act as macroscopic exciton traps. Once the resonance laser pump power reaches a certain threshold, a very narrow delocalized exciton line appears (with a width less than 0.3 meV), which grows strongly in intensity with increasing pump power and shifts toward lower energies (by approximately 0.5 meV) in accordance with the exciton buildup in the lowest state in the domain. As the temperature increases, this spectral line disappears in a nonactivated manner. This phenomenon is assigned to Bose condensation occurring in the quasi-two-dimensional system of interwell excitons. The critical exciton density and temperature were determined within the temperature interval studied (0.5 to 3.6 K), and a phase diagram specifying the exciton condensate region was constructed.