Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger-Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al_xGa_1−xAs are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.     

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.     

Electric field effects on the linear and nonlinear intersubband optical properties of double semi-parabolic quantum wells

In this work, the effects of the electric field on the optical properties of the symmetric and 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. Our calculations for the asymmetric DSPQW show that the resonant peak values of the total refractive index change and total optical absorption coefficient are maximum for a certain value of the applied electric field, due to the anti-crossing effect. However, for the symmetric DSPQW, the resonant peak values of these optical properties decrease monotonically with increasing the applied electric field. Also, our results indicate that a larger value of the optical rectification coefficient of the symmetric DSPQW can be induced by applying a small electric field.     

Effect of position-dependent effective mass on linear and nonlinear optical properties of a cubic quantum dot

In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/Al_xGa_1−xAs cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.     

Intersubband transitions in an asymmetric double quantum well

The intersubband optical absorption in an asymmetric double quantum well for different barrier widths and the right well widths are theoretically calculated within the framework of effective mass approximation. The results obtained show that the intersubband transitions and the energy levels in an asymmetric double quantum well can be importantly modified and controlled by the barrier width and the well width. The sensitivity to the barrier and well widths of the absorption coefficient can be used in various optical semiconductor device applications.     

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.     

ZnSe／ZnS多量子阱激子光学双稳性

用MOCVD在CaF_2衬底上生长的ZnSe/ZnS多量子阱材料,在77K下用N_2激光泵浦染料获得的宽带光脉冲进行了非线性光学测量,首次观察到ZnSe/ZnS多量子阱的激子光学双稳性,据分析这是由激子的能带增宽效应引起的增强吸收光学双稳性.     

Intersubband optical absorption in InSb stepped quantum wells: Effect of spin sublevels crossing

We study linear and non-linear coefficients of the intersubband absorption in InSb-based stepped quantum wells subjected to an in-plane magnetic field. We consider also a transverse electric field to achieve near resonance conditions. Taking into account the two deepest conduction levels and their corresponding Zeeman spin splitting sublevels, we calculate dispersion relations by means of an improved version of Kane model. Besides the known anti-crossing between down and up spin split sublevels, we obtain an extra spin level crossing for some determined parameters. This crossing clearly modifies the absorption spectrum for transitions among the four sublevels considered. We study a low electron density case, when only the first deepest sublevel is occupied, and a high density case with only the highest sublevel empty. We find a similar behavior of the absorption spectrum in both cases.     

Intersubband absorption with difference-frequency generation in GaAs asymmetric quantum wells

An asymmetric quantum well(AQW) is designed to emit terahertz(THz) waves by using difference frequency generation(DFG) with the structure of GaAs/Al 0.2 Ga 0.8 As/Al 0.5 Ga 0.5 As.The characteristics of absorption coefficients are analysed under the parabolic and non-parabolic energy-band conditions in detail.We find that the absorption coefficients vary with the two pump optical intensities,and they reach the maxima when the pump wavelengths are given as λ p1 = 9.70 μm and λ p2 = 10.64 μm,respectively.Compared with non-parabolic conditions,the total absorption coefficient under parabolic conditions shows a blue shift,which is due to the increase in the energy difference between the ground and excited states.By adjusting the two pump optical intensities,the wave vector phase-matching condition inside the AQW is satisfied.     

Linear and nonlinear intersubband optical absorption in symmetric double semi-parabolic quantum wells

The linear and the nonlinear intersubband optical absorption in the symmetric double semi-parabolic quantum wells are investigated for typical GaAs/AlxGa1−xAs. Energy eigenvalues and eigenfunctions of an electron confined in finite potential double quantum wells are calculated by numerical methods from Schrödinger equation. Optical properties are obtained using the compact density matrix approach. In this work, the effects of the barrier width, the well width and the incident optical intensity on the optical properties of the symmetric double semi-parabolic quantum wells are investigated. Our results show that not only optical incident intensity but also structure parameters such as the barrier and the well width really affect the optical characteristics of these structures.     

Effect of magnetic fields on the linear and nonlinear intersubband optical absorption coefficients and refractive index changes in square and graded quantum wells

In this study, both the intersubband optical absorption coefficients and the refractive index changes as dependent on the magnetic field are calculated in square and graded quantum wells. Our results show that the position and the magnitude of the linear and total absorption coefficients and refractive index changes depend on the magnetic field strength and the shape of potential. The incident optical intensity has a great effect on the total absorption and refractive index changes.     

Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.     

Magnetic field dependence optical properties of GaN/AlN multiple quantum wells

In this work, we have investigated the optical properties of some multiple quantum wells under the influence of an external magnetic field and (for the first time) number of wells. We have retained the total length of the structure constant which is technologically important. Then we have detected a blue shift in the absorption peak positions due to the application of the magnetic field and a red shift due to the increasing of the number of wells. The red shift is only seen in the multiple quantum wells with odd number of wells and the absorption peaks of the multiple quantum well with even number of wells are not changed.     

Linear and nonlinear intersubband optical absorption coefficients and refractive index changes in a quantum box with finite confining potential

In this work, both the intersubband optical absorption coefficients and the refractive index changes are calculated exactly in a quantum box. Analytical expressions for the linear and nonlinear intersubband absorption coefficients and refractive index changes are obtained by using the compact-density matrix approach. Numerical results are presented for typical GaAs/Al_xGa_1−x As quantum box system. The linear, third-order nonlinear and total absorption and refractive index changes are investigated as a function of the incident optical intensity and structure parameters such as box-edge length and stoichiometric ratio. Our results show that both the incident optical intensity and the structure parameters have a great effect on the total absorption and refractive index changes.     

纤锌矿InGaN staggered量子阱中的激子态和光学性质

本文将基于有效质量近似下的变分法,理论研究了纤锌矿InGaN/GaN staggered量子阱中的激子态和光学性质.数值结果显示了InGaN量子阱中的量子尺寸和staggered受限垒对束缚于量子阱中的激子态和光学性质有着明显的影响.当阱宽增加时,量子受限效应减弱,激子结合能降低,带间发光波长增加.另一方面,当量子阱中staggered受限势增加时,量子受限效应增强,激子结合能升高,带间发光波长降低.本文的理论结果证明了可以通过调节staggered垒高和量子尺寸来调控纤锌矿InGaN staggered量子阱中的激子态和光学性质.     

纤锌矿InGaN staggered量子阱中的激子态和光学性质

本文将基于有效质量近似下的变分法,理论研究了纤锌矿InGaN/GaN staggered 量子阱中的激子态和光学性质。数值结果显示了InGaN量子阱中的量子尺寸和staggered受限垒对束缚于量子阱中的激子态和光学性质有着明显地影响。当阱宽增加时,量子受限效应减弱,激子结合能降低, 带间发光波长增加。另一方面,当量子阱中staggered受限势增加时,量子受限效应增强,激子结合能升高,带间发光波长降低。本文的理论结果证明了可以通过调节staggered垒高和量子尺寸来调控纤锌矿InGaN staggered 量子阱中的激子态和光学性质。     

a-Si ∶H/SiO2多量子阱材料制备及其光学性能和微结构研究

利用等离子体增强化学气相沉积技术制备了a-Si ∶H/SiO₂多量子阱结构材料.对a-Si ∶H/SiO₂多量子阱样品分别进行了3种不同的热处理,其中样品经1100 ℃高温退火可获得尺寸可控的nc-Si:H/SiO₂量子点超晶格结构,其尺寸与非晶硅子层厚度相当.比较了a-Si ∶H/SiO₂多量子阱材料与相同制备工艺条件下a-Si ∶H材料的吸收系数,在紫外/可见短波段前者的吸收系数明显增大,光学吸收边蓝移,说明该材料 关键词： 多量子阱 量子限制效应 光学吸收 能带结构     

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.