Effects of the piezoelectric field on the modulation of exciton–polaritons by surface acoustic waves

Surface acoustic waves (SAWs) provide a powerful tool for the modulation of polaritons in GaAs-based microcavities. In this contribution, we compare the modulation introduced by SAWs propagating along piezoelectric and non-piezoelectric crystal directions of the sample surface. Strain calculations reveal that the type-I band-gap modulation induced by the strain field is comparable for both SAW types. Piezoelectric SAWs have, however, an intrinsic longitudinal electric field, which can dissociate quantum well (QW) excitons and, thus, degrade the modulation. Images of the polariton far-field photoluminescence reveal this behavior for different excitation conditions.     

Manifestation of collective properties of spatially indirect excitons in GaAs/AlGaAs asymmetric double quantum wells

Low-temperature (T=1.8, 4.2 K) luminescence of GaAs/Al_0.3Ga_0.7As double-coupled asymmetric quantum wells (DQW) is observed under variation of an electric field V _dc applied along the normal to the DQW plane. The FWHM of the indirect exciton (IX) emission line was found to undergo, within a certain interval of V _dc, a strong (up to a factor 3.5) narrowing, accompanied by anomalously large IX intensity fluctuations in time within the V _dc region where a strong decrease (or increase) of the FWHM is observed to occur. The dependence of the FWHM on the pumping level I _p also exhibits a substantial decrease of the FWHM within a certain I _p interval. The results obtained are discussed in the frame of an assumption which relates the observed phenomena to the onset of a condensed state in the interacting ensemble of spatially indirect excitons in DQWs. Fiz. Tverd. Tela (St. Petersburg) 41, 325–329 (February 1999)     

Resonant Raman study of LO + acoustic phonon modes in CdSe

In studying resonant Raman scattering in the vicinity of the A and B excitons of CdSe, we have observed three new Raman peaks. Two of the peaks have been identified as two-phonon modes consisting of a longitudinal optical (LO) phonon plus respectively a transverse acoustic (TA) and a longitudinal acoustic (LA) phonon. A theory which involves the scattering of photoexcited B excitons to the A exciton by acoustic phonons via the piezoelectric exciton-phonon interaction was found to explain quantitatively the peak positions, lineshape and resonance enhancements of the observed peaks.     

Photoluminescence of n-doped double quantum well—electron subbands under influence of in-plane magnetic fields

We report on photoluminescence (PL) measurements of a GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. Measurements were carried out on a selectively contacted symmetric p-δn-DQW-δn-p structure, which allows a variation of the electron density in DQW by a p–n bias and simultaneously a tilting of DQW, when a p–p bias is applied. Attention was paid to phenomena in in-plane magnetic fields, theoretically studied by Huang and Lyo (HL), [Phys. Rev. B 59, (1999) 7600]. In this paper, we compare our results for both symmetric and asymmetric DQWs with the theoretical model made by HL. Whereas the spectra from a symmetric DQW fully confirmed the theoretical predictions, the results gained from DQW with an electric-field-induced asymmetry did not allow a proper study of anticipated effects. The reasons for that are discussed.     

Intersubband transitions and refractive index changes in coupled double quantum well with different well shapes

In this study, both the linear intersubband transitions and the refractive index changes in coupled double quantum well (DQW) with different well shapes for different electric fields are theoretically calculated within framework of the effective mass approximation. Results obtained show that intersubband transitions and the energy levels in coupled DQW can importantly be modified and controlled by the electric field strength and direction. By considering the variation of the energy differences, it should point out that by varying electric field we can obtain a blue or red shift in the intersubband optical transitions. The modulation of the absorption coefficients and the refractive index changes which can be suitable for good performance optical modulators and various infrared optical device applications can be easy obtained by tuning applied electric field strength and direction.     

Modulation spectroscopy study on additive-induced efficiency enhancement of PTB7:PC71BM organic photovoltaic devices

Additives have been known to play an important role for improving the photovoltaic performance of organic solar cells. However, the reasons why additives improve the performance have not been clearly known yet. We employed an electrical modulated optical spectroscopy to investigate the relationship between the photovoltaic performance and additive concentration in organic photovoltaic devices. Our measured modulation spectra of a sample without additive showed both first- (-α’) and second-derivative (α”) components of the absorption spectrum. The second-derivative (α”) component in the modulation spectrum increased with the additive concentration. These results indicate that the sample without additive contains both localized (or Frenkel type) and relatively delocalized excitons and the added additive results in a significant increase of the relatively delocalized excitons. We conclude that the increased delocalized excitons lead to a significant additive-induced improvement of the photovoltaic-device performance.     

Improvement of performance characteristics of deep violet InGaN DQW lasers using a strip DQW active region

The effect of the laser ridge width on the performance characteristics of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) has been numerically investigated. Simulation results indicated that threshold current of LDs is decreased and slope efficiency and differential quantum efficiency (DQE) are increased by decreasing ridge width, whereas output power is decreased. The results also showed that a decrease of more than 1 μm in the ridge width reduces the threshold current, whereas the slope efficiency, output power, and DQE are decreased. A new DQW LD structure with a strip active region has been proposed to obtain a lower current threshold and higher output power, slope efficiency, and DQE. The results showed the InGaN DQW LD with a strip DQW active region has the highest output power, slope efficiency, and DQE; it also has a lower threshold current compared with that of the original LD. The comparative study conducted for the LDs with output emission wavelengths of 390, 414 and 436 nm has also confirmed the enhancement in LD performance using the strip DQW active region structure.     

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK_|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK_|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K_||.     

Acoustic–excitonic effects in a two-dimensional gas of dipolar excitons

The theory of the interaction of a two-dimensional gas of indirect dipolar excitons with Rayleigh surface elastic waves has been developed. The absorption and renormalization of the phase velocity of a surface wave, as well as the drag of excitons by the surface acoustic wave and the generation of bulk acoustic waves by a twodimensional gas of dipolar excitons irradiated by external electromagnetic radiation, have been considered. These effects have been studied both in a normal phase at high temperatures and in a condensed phase of the exciton gas. The calculations have been performed in the ballistic and diffusion limits for both phases.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.     

Large signal analysis of double quantum well transistor laser

In this paper, we present an analytical model for the large-signal analysis of the double quantum well (DQW) transistor laser. Our model is based on solving the continuity equation and the rate equations which incorporate the virtual states as a conversion mechanism. By using the presented model, effects of barrier width on DQW transistor laser static and dynamic performances are investigated. Also the static and dynamic responses of DQW transistor lasers are compared with single quantum well ones. Simulation results are in agreement with the numerical and experimental results reported by other researchers.     

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.     

Acoustically induced stark effect for excitons in intrinsic semiconductors

A Stark effect for excitons parametrically driven by coherent acoustic phonons is proposed. Our scheme refers to a low-temperature intrinsic semiconductor or semiconductor nanostructure pumped by an acoustic wave (frequency band nu(ac) approximately equal to 1-40 GHz and intensity range I(ac) approximately equal to 10(-2)-10(2) W/cm(2)) and probed by low-intensity light. Tunable optical band gaps, which strongly change the spectral shape of the exciton line, are induced in the polariton spectrum by acoustic pumping. We develop an exactly solvable model of the acoustic Stark effect and apply our results to GaAs driven by bulk or surface acoustic waves.     

极化诱导的内建电场对Mn δ掺杂的GaN/AlGaN量子阱居里温度的调制

采用传输矩阵方法分析极化诱导的内建电场对Mn δ掺杂的GaN/Al_xGa_1-xN量子阱居里温度(T_C)的调制作用.通过解薛定谔方程计算出在不同的内建电场条件下半导体量子阱局域态内的基态空穴能级和波函数分布情况，并在此基础上确定量子阱内Mn δ掺杂情况下T_C随内建电场的变化趋势，分析了不同量子阱结构引起的内建电场分布变化及其对T_C的影响.在耦合双量子阱中通过调节左右阱的不对称性可以得到T_C近3倍的增长. 关键词： GaN 量子阱 内建电场 居里温度     

Vanishing and emerging of absorption quantum beats from electron spin coherence in GaAs quantum wells

We report experimental studies of absorption quantum beats induced by electron spin coherence in GaAs quantum wells. Absorption quantum beats occur for strongly localized excitons, but nearly vanish for mobile excitons in the third order nonlinear optical response. Pronounced quantum beats for mobile excitons emerge in an unusual fifth order process. These results, along with a qualitative analysis based on the use of N-exciton eigenstates, elucidate how the manifestation of electron spin coherence in the excitonic nonlinear optical response can differ fundamentally from that in an atomic system.     

Absorption of surface acoustic waves by a gas of two-dimensional indirect excitons

Absorption of a Bleustein-Gulyaev surface acoustic wave interacting with a gas of two-dimensional indirect excitons has been calculated. It has been shown that the absorption is changed considerably when the excitons undergo Bose-Einstein condensation. This can serve as experimental evidence of such a transition.     

Ultrasonic light storage in a quantum well: A photon assembly line

Surface acoustic waves are used to effectively convert photogenerated excitons in a piezoelectric semiconductor quantum well into long-lived electron–hole polarizations. The strong lateral piezoelectric fields ionize the excitons and spatially separate the resulting electrons and holes. The strongly reduced wave function overlap results in very long storage times for this polarization which propagates across the sample at the speed of sound. External and deliberate screening of the piezoelectric fields triggers radiative recombination after long delay times and at a remote location of the sample.