Intersubband and Interband Absorptions in Near-Surface Quantum Wells Under Intense Laser Field

The intersubband transitions in quantum wells have attracted much interest due to their unique characteristics: a large dipole moment, an ultra-fast relaxation time, and an outstanding tunability of the transition wavelengths (Asano et al., 1998; Elsaesser, 2006; Helm, 2000). These phenomena are not only important by the fundamental physics point of view, but novel technological applications are expected to be designed. Many important devices based on intersubband transitions in quantum well heterostructures have been reported. For example: farand near-infrared photodetectors (Alves et al., 2007; Levine, 1993; Li, S.S. 2002; Liu, 2000; Schneider & Liu, 2007; West & Eglash, 1985), ultrafast all-optical modulators (Ahn & Chuang, 1987; Carter et al., 2004; Li, Y. et al., 2007), all optical switches (Iizuka et al., 2006; Noda et al., 1990), and quantum cascade lasers (Belkin et al., 2008; Chakraborty & Apalkov, 2003; Faist et al., 1994). It is well-known that the optical properties of the quantum wells mainly depend on the asymmetry of the confining potential experienced by the carriers. Such an asymmetry in potential profile can be obtained either by applying an electric/laser field to a symmetric quantum well (QW) or by compositionally grading the QW. In these structures the changes in the absorption coefficients were theoretically predicted and experimentally confirmed to be larger than those occurred in conventional square QW (Karabulut et al., 2007; Miller, D.A.B. et al., 1986; Ozturk, 2010; Ozturk & Sokmen, 2010). In recent years, with the availability of intense THz laser sources, a large number of strongly laser-driven semiconductor heterostructures were investigated (Brandi et al., 2001; Diniz Neto & Qu, 2004; Duque, 2011; Eseanu et al., 2009; Eseanu, 2010; Kasapoglu & Sokmen, 2008; Lima, F. M. S. et al., 2009; Niculescu & Burileanu, 2010b; Ozturk et al., 2004; Ozturk et al., 2005; Sari et al., 2003; Xie, 2010). These works have revealed important laser-induced effects: i) the confinement potential is dramatically modified; ii) the energy levels of the electrons and, to a lesser extent, those of the holes are enhanced; iii) the linear and nonlinear absorption coefficients can be easily controlled by the confinement parameters in competition with the laser field intensity. The intersubband transitions (ISBTs) have been observed in many different material systems. In recent years, apart from GaAs/AlGaAs, the InGaAs/GaAs QW structures have