Well‐width dependence of coupled Bloch‐phonon oscillations in biased InGaAs/InAlAs superlattices

Abstract

The coupling between Bloch oscillations and longitudinal optical (LO) phonons is investigated in ternary In 0.53Ga0.47As / In0.52Al0.48As superlattices of different well widths. In femtosecond time-resolved studies, a strong increase of the coherent LO phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with the different optical phonon modes of the ternary semiconductor materials. In a narrow-well superlattice where electronic minibands are energetically shifted close to the confining barrier potential, the phonon amplitudes are asymmetrically enhanced on the high-frequency edge of the resonance. Here, at high electric fields, field induced tunneling into above-barrier continuum states leads to a rapid dephasing of Bloch oscillations. The associated polarization change provides an additional excitation process for coherent LO phonons.1 Introduction Time-resolved investigations of resonantly coupled excitations involving phonons, such as coupled plasmon-phonon modes or phonon-polaritons, are one of the most intriguing subjects of coherent phonon spectroscopy (for a review see, e.g., [1]).Besides the elementary excitations involved there (plasmons, light waves), the coherent excitation of Bloch oscillations (BO's) has become possible in semiconductor superlattices by use of femtosecond laser pulses (for a review see, e.g., [2]). In general, BO's are described as the spatial oscillation of carriers in a periodic potential subject to a static electric field [3,4]. In biased superlattices, the coherent superposition of electronic Wannier-Stark (WS) states by ultrashort laser pulses leads to the formation of an electronic wave packet oscillating with a frequency ν BO = eF d/h. This frequency is determined by the externally applied electric field F , the superlattice period d, the electron charge e, and Planck's constant h.Recently, the resonant coupling between optical phonons and BO's has been demonstrated for the first time in GaAs/AlGaAs superlattices [5]. Here, we present a study on coupled Bloch-phonon oscillations in In 0.53 Ga 0.47 As / In 0.52 Al 0.48 As superlattices of different well widths but constant electronic miniband widths. A strong increase of the coherent LO phonon amplitudes is observed when the BO's are subsequently tuned into resonance with the distinct phonon modes of the ternary superlattice semiconductors by variation of the applied electric field. Furthermore, narrow superlattice wells lead to a shift of electronic minibands to higher energies in the conduction band and, thus, to an enhancement of Zener tunneling into above-barrier continuum states at high electric fields [6]. This effect results in a rapid dephasing of coherent BO's that in turn induces an incoherent polarization exciting coherent LO phonons. This contribution manifests itself as an asymmetric enhancement of the LO phonon amplitudes on the high-frequency edge of the Bloch-phonon resonance.

Coherent and ultrafast optoelectronics in III–V semiconductor compounds

“…We achieved further insight into this phenomenon in a narrow-well superlattice which was specifically designed in the In 0.53 Ga 0.47 As/In 0.52 Al 0. 48 As material system for the observation of Bloch-phonon coupling in the presence of Zener tunneling [28,29]. At high electric fields, Zener tunneling is an efficient dephasing mechanism of coherent Bloch oscillations in superlattices where the first electronic miniband is energetically shifted close to the confining barrier potential [30].…”