Elsevier

Physics Letters A

Volume 375, Issue 6, 7 February 2011, Pages 1036-1042
Physics Letters A

Intense laser field effect on the interband absorption in differently shaped near-surface quantum wells

https://doi.org/10.1016/j.physleta.2010.12.077Get rights and content

Abstract

The 1S-exciton properties and interband absorption spectra in differently shaped near-surface quantum wells (NSQWs) with symmetrical/asymmetrical barriers, under intense laser field, are investigated taking into account the correct dressing effect for the confinement potential and electrostatic interaction between carriers and their image-charges. We found that: i) the 1S-exciton binding energy is significantly reduced by the laser intensity in InGaAs NSQWs of different asymmetrical shape; ii) the red-shift of the absorption peak induced by the asymmetry diminution or by increasing cap layer thickness can be effectively compensated using the blue-shift caused by enhancing laser parameter. Therefore, the optical properties of the differently shaped NSQWs could be tuned by proper tailoring of the heterostructure parameters (well shape, barrier asymmetry) and/or dielectric mismatch as well as by varying the laser field intensity.

Introduction

The optical absorption associated with the excitons in semiconductor quantum wells (QWs) have been the subject of a considerable amount of work for the reason that the exciton binding energy and oscillator strength in QWs are considerably enhanced due to quantum confinement effect [1], [2], [3], [4], [5]. In recent years, InGaAs/GaAs QW structures have attracted much interest because of their promising applications in optoelectronic and microelectronic devices [6], [7], [8], [9].

As a distinctive type of dielectric quantum wells, the near-surface quantum wells (NSQWs) have involved increasing attention due to their potential to sustain electro-optic operations under a wide range of applied electric fields [10]. In these heterostructures the QW is located close to vacuum and the semiconductor–vacuum interface, which is parallel to the well plane, introduces a remarkable discrepancy in the dielectric constant resulting in a significant enhancement of the exciton binding energy [11], [12]. More recently, a basic phenomenon such as dielectric mismatch effect on the electronic energy levels, impurity states and excitonic absorption spectra in various semiconductor nanostructures [13], [14], [15], including InGaAs/GaAs symmetrical NSQWs [16], has been investigated.

The electronic and optical properties of the semiconductor QWs are significantly modified by applying intense high-frequency laser fields and this effect provides new degrees of freedom for practical applications [17], [18], [19], [20], [21]. As the rapid advances in modern growth techniques and researches for InGaAs/GaAs QWs [22], [23] create the possibility to fabricate such heterostructures with well-controlled dimensions and compositions, the asymmetrical InGaAs/GaAs NSQWs become interesting and worth studying systems.

The aim of this Letter is to investigate the interband optical transitions in differently shaped NSQWs with symmetrical/asymmetrical barriers subjected to intense high-frequency laser fields. We took into account an accurate form for the laser-dressing confinement potential as well as the presence of the image-charges. Within the framework of a simple two-band model the consequences of the exciton–surface interaction and laser field intensity on the absorption spectra have been investigated. To the best of our knowledge this is the first research concerning the intense laser field (ILF) effect on the interband optical transitions in differently shaped NSQWs with asymmetrical barriers.

This Letter is organized as follows: in Section 2 the theoretical model for the ILF effect on the exciton ground energy and interband transitions in differently shaped NSQWs, taking into account the repulsive interaction between carriers and their image-charges, is described. The single-particle wave functions in square (SQW), graded (GQW), and semiparabolic (sPQW) near-surface QWs are obtained by using the matrix transfer method in the effective mass approximation. In Section 3 the numerical results for the 1S-exciton binding energy and interband absorption coefficient in differently shaped InGaAs NSQWs with symmetrical/asymmetrical GaAs barriers and different cap layer thicknesses are discussed. Finally, our conclusions are summarized in Section 4.

Section snippets

Theory

We consider an InGaAs NSQW embedded between asymmetrical GaAs barriers. According to the effective mass approximation, in the absence of the laser field, the exciton Hamiltonian isH=Hez+Hhz+P22M+p22μ+Ueh. HereHjz=22mjz2zj2+Vj(zj)+Vself(zj), are the single-particle 1D Hamiltonians. The symbol j=e,h denotes the electron (hole) and Vj is the confinement potential in the growth direction (band-offset potential).

For a square NSQW, Vj has the well-known form:VjSQW(z)={,z<Lc,Ve(h),Lc<z<0 and

Numerical results and discussion

The numerical calculations were carried out for In0.18Ga0.82As/GaAs differently shaped NSQWs with the well width Lw=80Å and various cap layer thicknesses, Lc=40Å, 80 Å, 120 Å, by using the material parameters [12] listed in Table 1 and a broadening parameter Γ=1meV [18].

For two nanostructures, GQW and sPQW, one of the GaAs barriers (the cap layer) has an unchanged height, Ve (electron) or Vh (hole), and the other barrier is variable, Vr=σVe/h. The following values for the asymmetry parameter

Conclusions

The electronic properties and absorption spectra for excitons located in differently shaped NSQWs with symmetrical/asymmetrical barriers, under a non-resonant high-frequency laser field, have been investigated. By using the effective mass approximation and a matrix transfer method the ground exciton state energy and absorption coefficients for interband transitions in a wide range of laser intensity, α0, and barriers asymmetry, σ, have been calculated. Our results reveal that, by tuning the

Acknowledgements

The author would like to express her special thanks to Professor E.C. Niculescu for helpful discussions and constant support.

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