Ultrafast dynamical response of the lower exciton-polariton branch in CdZnTe

Abstract

We investigate the transient optical response in high-quality Cd 0.88 Zn 0.12 Te crystals in the regime of slow light propagation on the lower exciton-polariton branch. Femtosecond photoexcitation leads to very substantial transmission changes in a ∼10-meV broad spectral range within the transparency window of the unexcited semiconductor. These nonlinear optical signatures decay on picosecond time scales governed by carrier thermalization and recombination. The temporal and spectral dependence indicate the dynamical optical response as arising from excitation-induced dephasing and perturbed free induction decay. Model simulations for the optical response taking into account the actual exciton-polariton dispersion and excitation-induced dephasing of a nonlinearly driven two-level system support this interpretation.

Spatially asymmetric transients of propagating exciton-polariton modes in a planar CdZnTe/CdMgTe guiding structure

“…[12], for detuned pumping two-photon excitations may dominate the nonlinear propagation and therefore we include n two-photon which is generated through two-photon absorption into our model. Further quantities are the resonance energy of the two-level systemhω R =hω exciton = 1.64 eV, the phenomenological dephasing constant γ p = 2/(1 ps), the relaxation rate γ n = 2/(10 ps), the dipole-matrix element μ, the phenomenological constant α representing the strength of the so-called excitation-induced dephasing process [25,[27][28][29] which here is caused by two-photon excitations [12]. Whereas the absolute values of μ and α are not relevant, their ratio, or more precisely the ratio between the different nonlinearities arising from one-and two-photon excitation is adjusted in order to describe the experimental results.…”

“…It is known for this particular material system that for far off-resonant excitation the so called excitationinduced dephasing [27] (EID) instead of Pauli blocking (PB) is the dominant nonlinearity. It has been suggested that for off-resonant excitation conditions EID is mainly caused by carriers that are generated via two-photon absorption [12]. For simplicity we model the density generated by two-photon absorption by an additional set of differential equations of type (1) and (2) which are resonantly tuned to the pump with a squared amplitude.…”

“…In CdZnTe bulk crystals, we observed nutation of the EP polarization due to strong enhancement of magneto-optical effects such as the Faraday rotation and the nonreciprocal birefringence [11]. Recently, long-distance EP propagation in bulk CdZnTe crystals was studied using pump-probe techniques, where an off-resonant pump pulse in the transparency region created excess carriers due to two-photon absorption [12].…”

“…Therefore, based on this motivation, we investigate nonlinear optical phenomena in planar wave guiding structures in this work. We focus on CdZnTe structures where large delays for pulse propagation were observed in our previous studies [10][11][12].…”