Intersubband absorption of light in heterostructures with double tunnel-coupled GaAs/AlGaAs quantum wells

Intersubband absorption of mid-IR light was studied in heterostructures with asymmetrical tunnel-coupled quantum wells in equilibrium conditions and under high-power pumping by picosecond pulses of light. The energy spectrum of electrons in tunnel-coupled quantum wells was found from an analysis of equilibrium and nonequilibrium intersubband absorption spectra. The dynamics of intersubband absorption under high-power optical pumping was studied using the pump-and-probe picosecond technique. The experimental data are compared with the results of calculations based on solving rate equations. The intersubband relaxation times are determined.     

Intersubband Absorption in Terahertz Lasers Based on Optically Pumped Quantum Well Structures

We study the effects of microscopic dynamics of electrons on the intersubband optical absorption in terahertz lasers based on optically pumped quantum wells (THz-OPQW). The ensemble Monte Carlo method is used to include different electron scattering mechanisms in the calculation, so that the dependence of intersubband absorption lineshape on microscopic dynamics of electrons can be easily investigated. We find that electron-electron scattering is the dominant factor to determine the absorption linewidth. The real distributions of hot electrons are included to study the temporal changes of the spectra lineshape. Our findings show that the dependence of spectral lineshape on electron population essentially results from the electron scattering and the non-Fermi electron energy distribution.     

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Light absorption and refraction due to intersubband transitions of hot electrons in coupled GaAs/AlGaAs quantum wells

Variation of the absorption coefficient and refractive index of a system of tunnel-coupled GaAs/AlGaAs quantum wells in a longitudinal electric field is discovered and investigated in the spectral region corresponding to intersubband electron transitions. The phenomena observed are explained by electron heating in the electric field and electron transfer in physical space. The equilibrium absorption spectra at lattice temperatures of 80 and 295 K are presented. Fiz. Tekh. Poluprovodn. 32, 852–856 (July 1998)     

The effect of interdiffusion on the intersubband optical propertiesin a modulation-doped quantum-well structure

The linear and nonlinear (based on optical field intensity) intersubband absorptions in conduction band, and its change in refractive index in AlGaAs-GaAs interdiffused quantum wells (QWs) are presented. The calculation of the electron energy levels and the envelope wave functions in a modulation doped interdiffused QWs with screening effects are considered. QW interdiffusion shows a wavelength tunability of the intersubband absorption peaks and refractive index dispersions. This shifting of the transition energies is also demonstrated here to be a useful technique for broad-band and multicolor photodetector application. In addition, it can serve to remove noise, such as minor peaks and dispersions, in the optical spectra     

The effect of resonant sublevel coupling on intersubband transitions in coupled double quantum wells

The effect of resonant sublevel coupling on intersubband transitions in double quantum wells is investigated using far-infrared spectroscopy. We study widely tuneable parabolic double quantum wells in which potential spikes of different energetic height and thickness provide tunnel barriers for the electron systems on either side of the barrier. The use of gate electrodes enables us to tune both the carrier densities as well as the respective sublevel spacings and allow for a manifold degeneracy scheme like in an artificial molecule where the atomic number of both partners can be intentionally changed. Depending on the actual experimental condition, we observe pronounced level anticrossings into a symmetric and antisymmetric state. This single-particle sublevel coupling manifests itself in a rich spectrum of the observed collective intersubband transitions which occur at the depolarization shifted intersubband energy.     

Electron energy spectra in neon, xenon, and helium-neon laser discharges

Absolute measurements of the electron energy spectrum in a helium-neon mixture and in pure neon and xenon have been obtained by an energy analysis of a sample of electrons extracted through a small hole in the anode. The spectrum appears to be nearly Maxwellian for the lower pressures but deviates markedly from a Maxwellian at higher pressures. At higher pressures, the energetic part of the spectrum drops off faster, and one can describe this part by a Maxwellian of lower temperature than that for the bulk of the distribution. The average energies agree with those obtained from microwave measurements of the radiation temperature of the electrons if corrections are made for nonthermal distribution. Several production rates are computed with the help of the measured spectra, and they are related to the wall current, the power dissipation, and the possible electron depopulation of helium metastables. The production and destruction rates for the different parts of the energy spectrum have been formulated mathematically. A theoretical formula, which describes the actual spectra, has been derived for the faster part of the spectrum. For the helium-neon laser discharge we can say definitely that the de-excitation of helium metastables by electrons is negligible.     

Impact ionization and light emission in AlGaAs/GaAs HEMT's

Impact ionization and light emission phenomena have been studied in AlGaAs/GaAs HEMTs biased at high drain voltages by measuring the gate excess current due to holes generated by impact ionization and by analyzing the energy distribution of the light emitted from devices in the 1.1-3.1 eV energy range. The emitted spectra in this energy range can be divided into three energy regions: (i) around 1.4 eV light emission is dominated by band-to-band recombination between cold electrons and holes in GaAs; (ii) in the energy range from 1.5 to 2.6 eV energy distribution of the emitted photons is approximately Maxwellian; and (iii) beyond 2.6 eV the spectra are markedly distorted due to light absorption in the n⁺ GaAs cap layer. The integrated intensity of photons with energies larger than 1.7 eV is proportional to the product of the drain and gate currents. This suggests recombination of channel electrons with holes generated by impact ionization as the dominant emission mechanism of visible light     

Optically stimulated intersubband absorption in undoped amorphous Si/SiO2 qiantum well

Intersubband absorption has been observed in undoped amorphous multiple-quantum-well (MQW) strucctures under interband excitation. The transitions take place between the first and second subbands of the conduction band and involve non-equilibrium electrons excited into the first subband by optical pumping. The absorption band FWHM of 0.1 eV is much larger than for crystalline MQWs, a fact which reflects the relaxation of the electron momentum conservation in the QW plane. The high joint density of states and oscillator strength for the intersubband transitions in amorphous QWs allow one to observe the absorption at low electron concentrations (N_e ≈ 10¹³ cm^?3) in the ground conduction subband.     

Calculation of linear and nonlinear intersubband optical absorptions in a quantum well model with an applied electric field

Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.     

Electrical and spectroscopic studies of space-charged buildup, energy relaxation and magnetically enhanced bistability in resonant-tunneling structures

Photoluminescence measurements and magnetoquantum oscillations in the differential capacitance are used to measure space-charged buildup and study electron thermalization in a double-barrier resonant tunneling structure based on n-type (AlGa)As. The intrinsic bistability observed in the I(V) characteristics is also seen in the linewidth and photon energy of the photoluminescence. The spectroscopic data reveal clearly the importance of intersubband transitions in the voltage range at which electrons tunnel resonantly into the second bound state of the quantum well. A novel field-induced enhancement of the intrinsic bistability effect is reported for .     

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.     

Comparison of hole and electron intersubband absorption strengthsfor quantum well infrared photodetectors

It is well known that the hole intersubband absorption of normally incident (TE polarized) radiation is nonzero for p-doped quantum well infrared photodetectors (p-QWIP's) which have been fabricated without an optical grating. This present paper shows from k&oarr;·p&oarr; theory that, for typical p-QWIP designs, this hole intersubband absorption of TE polarized radiation (without the help of an optical grating) is significantly smaller than the electron intersubband absorption of TE polarized radiation in those n-doped QWIP's (n-QWIP's) fabricated with an optical grating. A second result of this present work is that, even when there is significant mixing of the light and heavy hole states, the p-QWIP absorption of TE polarized radiation (without the help of an optical grating) is still much smaller than the n-QWIP absorption of TE polarized radiation (with the help of an optical grating). The reason is that the mixing of light and heavy hole states never increases the amount of |S〉-symmetry in the mixed hole wave function beyond the amount of |S〉-symmetry which was present in the unmixed, purely light hole state. Finally, this present paper shows from k&oarr;·p&oarr; theory that strained layer growth on an (001) substrate does not significantly affect the strength of the hole intersubband absorption. The reason is that the Hamiltonian describing uniaxially strained quantum wells has precisely the same (tetragonal) symmetry as the Hamiltonian describing carrier confinement in unstrained quantum wells. All of these results are important in choosing a QWIP device design     

Photoluminescence from amorphous carbon grown by laser ablation of graphite

Photoexcitation and photoluminescence (1–7 eV) spectra of amorphous carbon films grown by laser ablation of a graphite target have been measured. The experimental data obtained are analyzed using the sum rule, and the energy dependence of the effective electron density of states is determined. Characteristic threshold energies in the effective density of states are revealed at ～1.4 and ～4 eV, and the conclusion is made that the energy distribution of the density of states is nonuniform, which is attributed to different contributions from the σ and π states of electrons. The temperature of the electron system excited by light is estimated to exceed the lattice temperature by a factor of ～40. This circumstance is attributed to heating of electrons by light.     

Intersubband absorption of TE and TM waves in p-type semiconductorsuperlattice including the effects of continuum states

We present theoretical results for the absorption coefficients for both TE and TM waves due to intersubband transitions within the valence subbands of a p-type superlattice. Both the bound and continuum states are included in our calculation. The continuum states are discretized by applying a periodic boundary condition. Numerical results are illustrated for different well and barrier widths. The results show that the intersubband transitions among the bound states result in sharp resonance absorption while the intersubband transitions between bound states and continuum states give a broad maximum at a photon energy larger than the barrier height. The TE and TM absorption coefficients are comparable in magnitude. Our theoretical results are in agreement with recent experimental observations on bound states to continuum states transitions     

Numerical analysis of the longitudinal electric current during resonance current flow in a n-GaAs/AlxGa1−x As superlattice with doped quantum wells

The electric current density in the resonance electron transport along the axis of a n-GaAs/Al_0.36Ga_0.64As superlattice with doped quantum wells at low temperatures was analyzed numerically. The energy spectrum of the conduction-band minibands was calculated in the Kane and effective-mass approximations. The current density was calculated using the well-known formula obtained on the basis of the density-matrix method for a uniform electric field ignoring heating of the electron gas. The effect of an interaction between the minibands in an electric field on the electrical conductivity and intersubband optical absorption was studied. Fiz. Tekh. Poluprovodn. 32, 607–612 (May 1998)     

新型高效双供体型有机染料的设计及理论研究

在实验合成染料WD8的基础上,设计了一系列新型双供体结构的染料ME301-ME306; 并利用密度泛函理论（Density Functional Theory, DFT）和含时密度泛函理论（Time-dependent Density Functional Theory, TDDFT）进一步研究了其物理和电子性质,包括几何结构、IR光谱、吸收光谱和光捕获效率（(Light Harvesting Efficiency, LHE）。结果显示,染料ME302中的供体芴基团是更有前途的官能团,特别是染料ME306与染料WD8相比,不仅具有较高的摩尔消光系数以及红移了50 nm, 而且覆盖整个可见光范围,具有较宽的吸收光谱。另外,染料ME306能够将电子有效地注入到TiO2导带中。这种新型双供体结构染料的设计可以为高效染料敏化太阳能电池的研究提供新的策略和指导。     

Specific features of the photoexcitation spectra of epitaxial InN layers grown by molecular-beam epitaxy with the plasma activation of nitrogen

The results of studies of the photoexcitation spectra of epitaxial InN layers formed by molecular-beam epitaxy with the plasma activation of nitrogen are reported. The concentration of free charge carriers in the layers is 10¹⁸–10¹⁹ cm^–3. The photoconductivity, photoluminescence, and absorption spectra exhibit a shift of the long-wavelength threshold of interband transitions in accordance with the Burstein–Moss effect for n-InN with different concentrations of equilibrium electrons. In the samples, absolute negative photoconductivity with a nanosecond relaxation time is observed. The results of photoelectric, absorption, and luminescence spectroscopy experiments are correlated with the technological parameters and electron microscopy data.     

Electron-phonon scattering engineering

We present calculations which show that independent quantization of electrons and phonons allows the intra-and intersubband electron-phonon scattering rate in two-dimensional structures to be changed. It is considered how the design of multi-heterostructure quantum well (QW) changes the electron mobility and population of subbands in the QW. It was shown that the insertion of the phonon wall (a few AlAs monolayers) into an AlAs/GaAs/AlAs double heterostructure allows the electron mobility in the QW to be enhanced and electron intersubband population to be inverted. Fiz. Tekh. Poluprovodn. 31, 85–88 (January 1997)