Time-resolved photocurrent spectroscopy of the evolution of the electric field in optically excited superlattices and the prospects for Bloch gain

Abstract

We report on photocurrent spectroscopy on undoped GaAs/AlGaAs semiconductor superlattices subjected to femtosecond optical excitation. The evolution of the carrier-drift-induced inhomogeneity of the electric field is studied by tracing the shifting and broadening of Wannier-Stark transitions as a function of delay time and bias field. Based on experimental data and results of numerical simulations, we find that the superlattice rapidly splits into two moving field regions, one with strong field gradient and low electron density, the other with partially screened field at low gradient and high electron density. Concerning future Bloch-gain measurements, we find that gain is expected in spite of the inhomogeneous field if the electron-rich region is not heavily screened. The time window during which Bloch gain exists is determined by the sweep out of the electrons (10-30 ps)

Bloch gain in dc-ac-driven semiconductor superlattices in the absence of electric domains

“…24 Modern ultrafast optical techniques were applied to measure the Bloch gain in undoped superlattices during a short time window after a femtosecond optical excitation of carriers in the SL. 25,26 Another interesting suggestion is to work with two-dimensional ͑2D͒ structures, where the electric domains are effectively more suppressed in comparison with the case of three-dimensional ͑3D͒ structures. 27,28 With this aim, a lateral surface SL shunted by another SL ͑Ref.…”

Time-resolved photocurrent spectroscopy of optically excited superlattices and the prospects for Bloch gain

“…With this in mind, we will investigate here the scenario where charge carriers are excited by femtosecond laser pulses in an undoped superlattice subject to a static electric field, and explore the ensuing carrier and field dynamics in order to identify measurement condition for the detection of transient Bloch gain. We have found earlier by time-resolved photocurrent spectroscopy that the time for the electrons to be swept out from the structure is short in comparison with the typical time of domain formation but still long enough for Bloch-gain measurements [12,13]. In this paper, we refine our previous theoretical analysis of the experimental data by a more rigorous semiclassical treatment of electron and hole dynamics using an ensemble Monte Carlo method.…”

“…1(b), one also observes the near-complete washing-out of the excitonic transition line after a few ten ps, which indicates that the field in the superlattice becomes very inhomogeneous. Our previous work on the spatio-temporal dynamics of the electric field and the carrier density distribution allowed us to attribute the initial line shift to the dynamics of electrons and the later relaxation back to the initial conditions to the 0 dynamics of holes [12,13]. The conclusions drawn then were limited, however, by the fact that we employed a simplified semiclassical model and especially did not make a distinction between heavy and light holes, nor did we treat the transport of the holes on the basis of a miniband model.…”

Strong Electric Field Driven Carrier Transport Non-Linearities in n-Type GaAs/AlGaAs Superlattices

“…Recently, it was suggested that SL can be successfully used as parametric gain environment for achieving room temperature THz emission [4], moreover, it was shown that parametric gain does not require NDR [5]. It is worth noting that the latter is an inherent feature of superlattices tuned to the Bloch gain regime [6].…”