Abstract
The ability to generate phase-stabilized trains of ultrafast laser pulses by mode-locking underpins photonics research in fields, such as precision metrology and spectroscopy. However, the complexity of conventional mode-locked laser systems has hindered their realization at the nanoscale. Here we demonstrate that GaAs-AlGaAs nanowire lasers are capable of emitting pairs of phase-locked picosecond laser pulses with a repetition frequency up to 200 GHz when subject to incoherent pulsed optical excitation. By probing the two-pulse interference spectra, we show that pulse pairs remain mutually coherent over timescales extending to 30 ps, much longer than the emitted laser pulse duration (≤3 ps). Simulations performed by solving the optical Bloch equations produce good quantitative agreement with experiments, revealing how the phase information is stored in the gain medium close to transparency. Our results open the way to phase locking of nanowires integrated onto photonic circuits, optical injection locking and applications, such as on-chip Ramsey comb spectroscopy.
Highlights
The ability to generate phase-stabilized trains of ultrafast laser pulses by mode-locking underpins photonics research in fields, such as precision metrology and spectroscopy
To check that the observed fringes do arise from the interference between two successively emitted laser pulses, the fringe separation in the frequency domain Df is plotted in Fig. 2 as a function of Dt
Two-pulse interference was recently reported for GaN, CdS and ZnO NW lasers in operation regimes corresponding to the direct temporal overlap between the emitted laser pulses[6,12]
Summary
The ability to generate phase-stabilized trains of ultrafast laser pulses by mode-locking underpins photonics research in fields, such as precision metrology and spectroscopy. Wavelength-scale coherent optical sources are vital for a wide range of applications in nanophotonics ranging from metrology[1] and sensing[2] to nonlinear frequency generation[3] and optical switching[4]. In these respects, semiconductor nanowires (NWs) are of particular interest since they represent the ultimate limit of downscaling for photonic lasers with dielectric resonators[5]. We demonstrate that subsequently emitted ultrafast (r3 ps duration) laser pulses emitted from incoherently pumped GaAs-AlGaAs core-shell NW lasers remain mutually phase coherent over timescales that are approximately 10 times longer than the emitted pulse duration. Numerical simulations performed by solving the optical Bloch equations produce good quantitative agreement with experiments supporting our identification of the mechanism that establishes mutual phase coherence
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