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Seminar 2/23/2011 - Nonlinear Dynamics and Mode Locking in Quantum Cascade Lasers

Professor Alexey Belyanin
Department of Physics & Astronomy
Texas A&M University

Nonlinear interaction in an ensemble of weakly coupled oscillators may give rise to the frequency and phase coupling of initially incoherent oscillations with random mutual phases. Under certain conditions, a completely synchronized, macroscopic coherent state emerges, either spontaneously or under periodic forcing. In lasers, elementary oscillators are eigenmodes of a laser cavity and periodic forcing is achieved by external or self-modulation of laser parameters. In quantum cascade lasers (QCLs), mode locking of longitudinal cavity modes belonging to the same transverse mode is very difficult to achieve because of ultrafast gain relaxation and strong hole burning effects [1,2]. Surprisingly, we found that the same feature of QCLs has a stabilizing effect for nonlinear coupling of combs of longitudinal modes belonging to different transverse modes: they can be spontaneously phase-locked over the broad range of injection currents and without any external forcing or nonlinear element in the cavity other than the gain transition itself [3]. In some cases a complete synchronization occurs in which combs of longitudinal modes belonging to different lateral modes merge into a single comb, whereas far-field pattern shows strong beam steering indicating the presence of several phase-locked lateral modes [4]. Transverse mode locking can lead to interesting dynamical regimes including bistability, beam steering, generation of frequency combs, and ultrashort pulse operation. I will discuss both theory and recent experimental results obtained in our collaboration with Harvard University.

[1] A. Gordon,et al., "Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole ", Phys. Rev. A 77, 053804 (2008).
[2] C.Y. Wang et al., "Mode-locked pulses from mid-infrared quantum-cascade lasers", Opt. Express 17, 12929 (2009).
[3] N. Yu et al., "Coherent coupling of multiple transverse modes in a quantum cascade laser", Phys. Rev. Lett. 102, 013901 (2009).
[4] A. Wojcik et al., "Self-synchronization of laser modes and multistability in quantum cascade lasers", Phys. Rev. Lett. Submitted.

Location: Bowen Hall Atrium

Date/Time: 02/23/11 at 12:00 pm - 02/23/11 at 1:00 pm

Category: PRISM/PCCM Seminar Series

Department: PRISM