Microscopic Analysis of the Coherent Optical Generation and the Decay of Charge and Spin Currents in Semiconductor Heterostructures

Abstract

The coherent optical injection and temporal decay of spin and charge currents in semiconductor heterostructures is described microscopically, including excitonic effects, carrier LO-phonon and carrier-carrier scattering, as well as nonperturbative light-field-induced intraband and interband excitations. A nonmonotonous dependence of the currents on the intensities of the laser beams is predicted. Enhanced damping of the spin current relative to the charge current is obtained as a consequence of spin-dependent Coulomb scattering. PACS numbers: 72.25.Fe, 72.25.Rb One of the fundamental principles of quantum mechanics is that superpositions of wave functions lead to interference phenomena which depend on the relative phase differences. In the coherent regime, e.g., shortly after an external perturbation has generated a nonequilibrium situation, such quantum mechanical interference effects can be employed for the coherent control of dynamical processes in atomic, molecular, biological, and semiconductor systems [1,2,3,4,5,6]. Many of the measurements and proposals in this area make use of the coherent evolution of electronic excitations induced by specially designed optical laser pulses, e.g., sequences of phase-locked or suitably chirped beams.In semiconductors, the ultrafast coherent generation of photocurrents using two light beams with frequencies ω and 2ω has attracted considerable attention [7,8,9]. As shown in Ref. 10, the same type of interference scheme can also be employed to create pure spin currents which are not accompanied by any charge current. Such spin currents generated on ultrafast time scales have been observed in semiconductors [11,12] and could be useful for future applications in the area of spintronics [13,14]. As shown recently, it is even possible to control photocurrents via the carrier-envelope phase [15,16] which makes this scheme also interesting for optical metrology. Furthermore, for disordered semiconductors it has been predicted that sequences of temporally delayed excitation pulses can be used to induce current echoes [17].The coherent generation of photocurrents in semiconductors by two light fields with frequencies ω and 2ω satisfying 2hω > E gap >hω, where E gap is the band gap energy, has first been described in terms of nonlinear optical susceptibilities which have been obtained on the basis of band structure calculations [8,18]. In this framework, the optically-induced intra-and interband transitions are treated within Fermi's golden rule. The interference between intra-and interband excitations leads to electron and hole distributions which are not symmetric in k-space corresponding to a nonequilibrium situation with a finite current. The dynamics of the generation process has been analyzed using Bloch equations. This approach has been applied to disordered semiconductors within a two-band model [17] and to ordered quantum wells within a multiband formalism [19]. Although the relaxation of the photocurrent by carrier LO-phonon scattering bulk GaAs has been analyzed [20], ...

Generation and time‐resolved detection of coherently controlled electric currents at surfaces

“…This process works in centrosymmetric and noncentrosymmetric materials. For semiconductor nanostructures, a microscopic theory has been developed that provides a detailed description of the ultrafast dynamics of the optical excitations and the photocurrents including many-body effects and scattering processes [26][27][28]. This approach is based on the semiconductor Bloch equations [29] which are extended by the electric field induced intraband acceleration [26,30].…”

“…The most important aspect for the microscopic description of the optical current generation is the inclusion of not only resonant optical interband transitions between initially occupied and unoccupied bands, but also fieldinduced intraband accelerations [30,26] which are the standard components for the description of the electronic response of solids to static and low-frequency electric fields [1]. This process can often safely be neglected when lightmatter interaction is analyzed since optical frequencies do not resonantly generate such excitations.…”

Microscopic analysis of high‐harmonic generation in semiconductor nanostructures

“…For this purpose, we solve the dynamics of the microscopic interband coherence (polariza- λ,k are destruction (creation) operators for electrons in band λ with lattice momentum k. With the assumptions mentioned above, the equations of motion are given by [14][15][16] …”

Oscillatory excitation energy dependence of injection currents in GaAs/AlGaAs quantum wells

“…The dynamics of the system under optical excitation is described by the multi-subband semiconductor Bloch equations (SBE) [8,10] of populations are phenomenologically modeled by decay times of 150 fs in the SBE [11]. We include in the SBE up to the two energetically lowest conduction bands (e1 and e2) and five highest valence bands (hh1, hh2, lh1, hh3, and hh4).…”