Ultrafast Dynamics of Lasing Semiconductor Nanowires

Abstract

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double-pump approach. Wide bandgap gallium nitride (GaN), zinc oxide (ZnO), and cadmium sulfide (CdS) nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier-phonon coupling in ZnO leads to the fastest laser onset time of ∼1 ps in comparison to CdS and GaN exhibiting values of ∼2.5 and ∼3.5 ps, respectively. These values are constant between nanowires of different sizes implying independence from any optical influences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from ∼5 to ∼15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization.

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

“…The data indicate that extremely high maximum repetition rates Δ f >200 GHz are possible, corresponding to emitted pulse durations t pulse <3 ps. 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 pulses612. In contrast, in Fig.…”

“…In these respects, semiconductor nanowires (NWs) are of particular interest since they represent the ultimate limit of downscaling for photonic lasers with dielectric resonators5. By virtue of their unique one-dimensional geometry, NW lasers combine ultra-high modal gain, support low-loss guided modes and facilitate low threshold lasing tunable across the ultrviolet, visible and near infrared spectral regions6789. Recently, optically pumped NW lasers have been demonstrated at room temperature and they can now be site-selectively integrated onto silicon substrates1011.…”

“…Recently, optically pumped NW lasers have been demonstrated at room temperature and they can now be site-selectively integrated onto silicon substrates1011. While the fundamental carrier relaxation and gain dynamics of NW lasers have been explored612, the coherent dynamics have hitherto received comparatively little attention. Here we demonstrate that subsequently emitted ultrafast (≤3 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.…”

Nanolaser arrays based on individual waved CdS nanoribbons

“…When the excitation fluence was increased from 55 to 140 μJ/cm 2 , τ 1 decreased rapidly from 65.5 ±3.2 to 11.8 ±0.1 ps, while τ 2 decreased slowly from 344.5 ±5.7 to 268.0 ±20.3 ps (see Supplementary Table S1). The fast and slow decay times coincide with the electron–hole plasma (EHP) and exciton‐like radiative recombination, respectively . Moreover, with the increase of the pumping fluence, the proportion of the fast decay component in the total decay process increase from 34% to 97%.…”

Coupled-Mode Theory for Semiconductor Nanowires

“…1(b)), propagating with group velocity v g = 7.72×10 7 m s and propagation constant β 0 = 22.28 1 µm . To create a laser cavity, the wire is terminated by realistic endfacets with an amplitude reflectivity matrix as used in equations (25) and (26) R = 0.51 0.02 0.02 0.51 (27) extracted from a simple linear FDTD simulation of a wire endfacet [26]. The occupation probabilities for electrons and holes are initialized with Fermi-distributions at T = 300K for spatial densities of N el = 3 × 10 19 cm −3 , N h,a = N h,b = 1.5 × 10 19 cm −3 .…”