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. 2017 May 23:8:15521.
doi: 10.1038/ncomms15521.

Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser

B Mayer  1   2 , A Regler  1   3 , S Sterzl  1 , T Stettner  1 , G Koblmüller  1 , M Kaniber  1 , B Lingnau  4 , K Lüdge  4 , J J Finley  1
Affiliations

Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser

B Mayer et al. Nat Commun. .

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.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Time-resolved pump–probe response of the NW laser.
(a) Schematic illustration of a NW laser emitting a coherent pulse pair. (b) Optical spectra recorded as a function of the pump–probe delay Δt. (c) Corresponding theoretical results obtained using the optical Bloch equation model described in the text. b,c depict situations for a fixed pump pulse power in the lasing regime and three different probe pulse powers: L-SE: probe pulse in the spontaneous emission regime (probe pulse power Pprobe∼0.6 × Pth), L-ASE: probe pulse in the amplified spontaneous emission regime (Pprobe∼1.9 × Pth) and L-L: probe pulse in the lasing regime (Pprobe∼4.7 × Pth). The data are plotted on a linear colour scale.
Figure 2
Figure 2. Fringe spacing versus pump–probe delay.
(a) Selected optical spectra recorded from the NW laser subject to a pump pulse in the lasing regime and a probe pulse in the ASE regime (L-ASE) for Δt=+10 ps (green), Δt=+20 ps (red) and Δt=+30 ps (blue). The inset shows a zoom-in of the spectrum, demonstrating that modulation is still observed and that the spacing is close to the resolution limit. (b) Repetition rate (red circles) and NW laser pulse separation (blue circles) as a function of Δt, measured from the separation of the interference fringes in the NW laser spectra and their inverse in a, respectively.
Figure 3
Figure 3. Experimental and theoretical pump–probe data in the time domain.
Time-dependent emission of the NW laser as a function of Δt after Fourier transforming the optical spectra. The data are plotted on a logarithmic colour scale that spans two orders of magnitude. (a) Pump pulse in the lasing regime and probe pulse in the spontaneous emission regime (pump pulse power Ppump∼5 × Pth and probe pulse power Pprobe∼0.6 × Pth). (b) Pump and probe pulse in the lasing regime (L-L)—(Ppump∼5 × Pth and Pprobe∼4.7 × Pth).
Figure 4
Figure 4. Coherent-phase information transfer between two subsequent NW laser pulses.
(a) Time dependence of the emitted intensity obtained by Fourier transforming the optical spectra of the NW laser after only the pump pulse with P/Pth=4 was injected (red line: simulations, black line: experimental data) and computed occupation of the lasing state (blue line) as a function of time. The schematic illustration depicts a NW laser that emits a single pulse. (b) Directly simulated time-dependent emission (red line) of the NW after a pump and probe pulse in the lasing regime (L-L) separated by Δt=20 ps were injected (thin dotted line). Blue line shows the inversion of the laser level and the hatched region depicts the spontaneous emission noise level. The red regions in a,b mark the short pulse emission during the first Rabi oscillation (TΩ marks the Rabi oscillation period and ton the time needed to reach inversion after the pump pulse arrived). The schematic illustration depicts a NW laser that emits a coherent pulse pair.
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