Polarization Control of Optically Pumped Terahertz Lasers
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Polarization Control of Optically Pumped Terahertz Lasers Gabriela Slavcheva1 and Alexey V. Kavokin2 1
Blackett Laboratory, Imperial College London and Centre for Photonics and Photonic Materials, University of Bath, U.K. 2 Spin Optics Laboratory, St. Petersburg State University, Russia and School of Physics and Astronomy, University of Southampton, U. K. ABSTRACT Two-photon pumping of excited exciton states in semiconductor quantum wells is a tool for realization of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited 2p and ground 1s exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarization of both photons. Variation of the threshold power for THz lasing by a factor of 5 is predicted by switching from linear to circular pumping. We calculate the polarization dependence of the THz emission and identify photon polarization configurations for achieving maximum THz photon generation quantum efficiency. INTRODUCTION Excitons in nanoscale semiconductor materials exhibit low-energy excitations in the range of the exciton binding energy, analogous to inter-level excitations in atoms, yielding infrared and terahertz (THz) transitions. Thus intra-excitonic transitions between excited exciton ladder states represent a natural system for generating THz radiation and coherence. Towards the goal of development of new compact and efficient coherent THz sources recently a new scheme of microcavity polariton-triggered THz vertical cavity surface emitting laser (VCSEL) has been proposed [1], whereby the 2p dark quantum well (QW) exciton state is pumped by two-photon absorption (TPA) using cw laser beam, and subsequently decays to 1s exciton-polariton state emitting THz radiation. The inverse process of a strong few-cycle THz pulses resonantly-driven intra-excitonic |1s²→|2p² transition in a QW has been experimentally observed and investigated [2, 3]. Generation of incoherent p-type exciton population and population inversion by single-photon pulsed excitation of 1s or 2s exciton resonances in a QW has been predicted in [4]. By contrast, within the framework of the proposed polariton-triggered THz laser scheme, we shall be interested in an optical cw pumping of the 2p exciton state by two-photon absorption mechanism. Although there has been a considerable body of theoretical work on two-photon absorption in QWs (see e.g. [5]), the polarization selection rules in a bosonic THz laser remain largely unknown. In the present study we demonstrate polarization control of THz emission and of the quantum efficiency for THz photon generation. We employ crystal symmetry point group theoretical methods [6] to calculate the polarization dependence of the excitonic TPA and intraexcitonic 2p to 1s THz transition radiative decay rates in GaAs/AlGaAs QWs. This enables us to calculate the polarization dependence of the quantum efficiency for THz photon generation and thus identify maximum efficiency regimes of operation. THEORY
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