Tunneling Injection Quantum-Dot Lasers
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Tunneling Injection Quantum-Dot Lasers S. L. Chuang, J. Kim, P. K. Kondratko, G. Walter, N. Holonyak, Jr., R. D. Heller1, X. B. Zhang1, and R. D. Dupuis1 Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign 1406 West Green Street, Urbana, IL 61801, U.S.A. 1 School of Electrical and Computer Engineering Georgia Institute of Technology 777 Atlantic Drive NW, Atlanta, GA 30332, U.S.A. ABSTRACT We investigate a tunneling injection InP quantum-dot (QD) laser theoretically and experimentally. The device consists of a single compressively strained InP QD layer coupled closely to two tensile strained InGaP quantum wells (QWs). While most tensile strained QW lasers in this wavelength (red) range lase in the transverse-magnetic (TM) polarization, our QD laser lases in the transverse-electric (TE) polarization from the first excited state of the compressively strained QDs, which is coupled to the ground state of the tensile-strained InGaP QWs. When we measure TE and TM modal gain spectra, a typical QW gain evolution behavior is observed at low injection currents, which can be theoretically explained by the quasiequilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarizationdependent photon-mediated carrier re-distribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier re-distribution, which depends on the optical transition energy and polarization. This polarization-dependent photon-mediated carrier redistribution explains the TE gain narrowing and TM gain pinning behavior. To quantitatively demonstrate the photon-mediated carrier re-distribution near the threshold current, a set of coupled rate equations are solved taking into account the polarization-dependent stimulated emission processes. The calculated polarization power ratio based on the coupled rate equations explains the experimental observations. INTRODUCTION Semiconductor QD lasers have been studied intensively due to their expected improvement in performance over QW lasers in terms of lower threshold current density, less temperature sensitivity, higher differential gain, and lower linewidth enhancement factor [1]. However, the performance of QD lasers so far is limited by poor carrier capture and relaxation efficiency of the self-assembled QDs due to the inhomogeneous size distribution and the low dot density. Recent studies of a QD-coupled-QW structure incorporating carrier tunneling injection scheme demonstrate significant improvement of modal gain, modulation bandwidth, and linewidth enhancement factor [2-4]. In this paper, we investigate theoretically and experimen
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