Objective and subjective evaluations of the multi-channel auralization technique as applied to solo instruments

Auralizations are commonly used today by architectural acousticians as a tool to model acoustically sensitive spaces. This paper presents investigations employing an auralization methodology known as multi-channel auralizations, to determine the benefits of using an increasing number of channels in such auralizations. First an objective evaluation was conducted to examine how acoustic parameters, such as reverberation time, vary when using “quadrant” (one fourth of a spherical source) or “thirteenth” sources to create the binaural room impulse responses. Large differences in the values were found between the different sections of the sphere, on the order of several just noticeable differences. Two subjective studies were then pursued, first to determine if auralizations made with an increasing number of channels sound more realistic and have an increased perceived source size, using solo musical instruments of varying directivity indices as the sources. Overall, subjects perceived the auralizations made with an increasing number of channels as more realistic, whereas results for perceived source size are less clear. The second subjective study assessed the ease with which subjects could identify the source orientation from the auralizations as a function of number of channels. Results indicate that more channels made it easier for subjects to differentiate between source orientations.     

Phase dependence of cross-phase modulation in asymmetric quantum wells

We study the Kerr nonlinearity associated with cross-phase modulation based on electromagnetically induced transparency in asymmetric double quantum wells. It is shown that, different from atomic system, not only the nonlinear dispersion and absorption but also the linear absorption depends on the relative phase of the laser fields because of the Fano interference. By choosing the parameters appropriately, large cross-phase modulation with nearly vanishing two-photon absorption, even π phase shift with single photon level, could be achieved in the asymmetric quantum wells.     

Dynamic control of photonic bandgap mediated by vacuum-induced coherence

We theoretically examine the storage and retrieval of a light pulse in a medium comprised of four-level atoms of the V − Λ-type. The two intermediate levels are probed by a weak field and vacuum-induced coherence effects lead the system to transparency. The temporal variation of the intermediate levels' splitting is used as an external parameter which allows the transfer of the impinging field to a combination of spin coherences. An auxiliary and far-detuned control field in a standing-wave configuration is used to induce a variable photonic bandgap by cross-phase modulation. It is shown that dynamic control of such a bandgap can be used to coherently manipulate the previously stored probe pulse. We use a general scheme to take into account multiwave mixing effects and solve the combined Maxwell-Bloch equations for the relevant coherences. It is shown that the system acts as an all-optical router.     

Intersubband transitions in Al0.82In0.18N/GaN single quantum well

The influence of the width of a lattice-matched Al_0.82In_0.18N/GaN single quantum well (SQW) on the absorption coefficients and wavelength of the intersubband transition (ISBT) has been investigated by solving the Schrödinger and Poisson equations self-consistently. The wavelength of 1—2 ISBT increases with L, the thickness of the single quantum well, ranging from 2.88 μm to 3.59 μm. The absorption coefficients of 1—2 ISBT increase with L at first and then decrease with L, with a maximum when L is equal to 2.6 nm. The wavelength of 1—3 ISBT decreases with L at first and then increases with L, with a minimum when L is equal to 4 nm, ranging from approximately 2.03 μm to near 2.11 μm. The absorption coefficients of 1—3 ISBT decrease with L. The results indicate that mid-infrared can be realized by the Al_0.82In_0.18N/GaN SQW. In addition, the wavelength and absorption coefficients of ISBT can be adjusted by changing the width of the SQW.     

Intersubband absorption in strained AlGaN/GaN double quantum wells

The intersubband absorption of the four-energy-level system in strained AlGaN/GaN double quantum wells is calculated by considering the polarization effect and the strain modification on material parameters (e.g., the conduction band offset, the electron effective mass and the static dielectric constant). It is found that the electron wavefunctions mainly locate at the left well and penetrate into the left barrier. The absorption spectrum exhibits multiple peaks contributed by different transitions. The position and height of absorption peaks are not very sensitive to the structural parameters (i.e., composition and thickness) of the central barrier because of the strong built-in electric field. However, the coupling between two wells can be enhanced by strain modulation.     

Donor impurity states in zinc-blende GaN/AlGaN quantum well: Quantum confinement and laser-dressed effects

Within the framework of effective-mass approximation, the effects of a laser field on the ground-state donor binding energy in zinc-blende (ZB) GaN/AlGaN quantum well (QW) have been investigated variationally. Numerical results show that the donor binding energy is highly dependent on QW structure parameters and Al composition in ZB GaN/AlGaN QW. The laser field effects are more noticeable on the donor binding energy of an impurity localized inside the QW with small well width and low Al composition. However, for the impurity located in the vicinity of the well edge of the QW, the donor binding energy is insensible to the variation of Al composition, well width and laser field intensity in ZB GaN/AlGaN QW. In particular, the competition effects between laser field and quantum confinement on impurity states have also been investigated in this paper.     

Donor impurity states in coupled quantum well wires under hydrostatic pressure and applied electric field

In this work we study the binding energy of the ground state for a hydrogenic donor impurity in laterally coupled GaAs/Ga_1−xAl_xAs quantum well wires, considering the simultaneous effects of hydrostatic pressure and applied electric field. We have used a variational method and the effective mass and parabolic band approximations. The low dimensional structure consists of two quantum well wires with rectangular transverse section coupled by a central Ga_1−xAl_xAs barrier. Our results are reported for several sizes of the structure and we have taken into account variations of the impurity position along the growth direction of the heterostructure.     

Linear and nonlinear intersubband optical absorption and refractive index changes of asymmetric double semi-parabolic quantum wells

The optical properties of the asymmetric double semi-parabolic quantum wells (DSPQWs) are investigated numerically for typical GaAs/Al_xGa_1−xAs. Optical properties are obtained using the compact density matrix approach. In this work, effects of the structure parameters such as the barrier width and the well widths on the optical properties of the asymmetric DSPQWs are investigated. The results show that the linear and nonlinear optical properties of asymmetric DSPQW are non-monotonic functions of these structure parameters. The behavior of the refractive index changes of asymmetric DSPQW with the variation of the barrier width is different substantially with that of symmetric DSPQW. Results reveal that the resonant peak values of the total absorption coefficient of asymmetric DSPQW is usually greater than that of symmetric DSPQW. Our calculations also show that the total absorption coefficient of asymmetric DSPQW is larger than that of asymmetric double square quantum well.     

Effect of interdiffusion on band structure and intersubband absorption coefficient of GaAs/GaAlAs double quantum well

The influence of Ga and Al atoms interdiffusion on the band structure and absorption coefficient of coupled double quantum wells are investigated on the basis of a potential composed of four modified Wood–Saxon potentials. Wave functions of the real potential are expressed as a superposition of the stationary state wave functions of the “basal” potential. It is shown that interdiffusion leads to the smearing of the rectangular profile of the potential and to the disappearance of the barrier between the wells, as a result of which, the degradation of doublet character of the electron energetic spectrum is observed. It is also shown that interdiffusion leads to the significant change in oscillator strength and in the enhancement of the thermal stability of the absorption spectrum of the considered heterostructure.