We present our recent progress in the monolithic integration and epitaxial gain control of GaAs-based nanowire (NW) lasers on Si photonic platform. First, we describe the challenges in realizing vertical-cavity NW lasers on Si and SOI-based waveguides based on the stringent requirements for high gain, low-loss wave-guiding and coupling efficiency as illustrated by detailed numerical simulations. Consequently, we discuss bottom-up, epitaxial schemes for site-selective integration of individual GaAs NW lasers on planar Si and non-planar Si ridge waveguides under different geometries, demonstrating vertical-cavity NW lasers with remarkably low lasing threshold (< 20 µJ/cm2) and high spontaneous emission coupling factor (B > 0.2) under pulsed optical excitation. First experiments of individual NW-lasers with direct coupling of lasing emission into the underlying Si-ridge waveguides are also shown.
To further improve the threshold gain and lasing characteristics and achieve wavelength tunability, we further tailor the active gain media using low-dimensional systems embedded in the NW resonator cavity. Here, we particularly emphasize the challenges in the growth of GaAs-based cavities that incorporate coaxial GaAs-AlGaAs and InGaAs-AlGaAs multiple quantum well (MQW) heterostructures and illustrate even routes towards quantum-wire or quantum-dot based gain media. Finally, we provide a direct comparison of the lasing characteristics of 3D-bulk like GaAs NW lasers with coaxial NW-MQW heterostructure lasers, and show how the control of composition and structure of the MQW shifts the lasing emission to longer wavelengths.
Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic integration onto Si photonic circuits. An important milestone on the way towards novel on-chip photonic functionalities is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG). Subject to optical excitation, the observed lasing behavior shows clear “s-shape”-characteristics, linewidth narrowing and threshold values down to 19.8±1 µJ/cm², which is the lowest value reported to date for this kind of integrated lasing structure. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the spectral characteristics during mode pro-pagation in the WG, and in good agreement with Finite-Difference Time-Domain (FDTD) simulations. Using a WG structure with a series of mask openings along the central mode propagation axis, we further illustrate the out-coupling properties of both spontaneous and stimulated emission and demonstrate propagation of the lasing mode over distances > 60 µm, despite absorption in the silicon dominating the propagation losses. By replacing the bulk GaAs as the active gain medium by InxGa1-xAs quantum wells, the emission wavelength can be shifted towards telecommunication bands in order to reduce the absorption losses in silicon. These results pave the way for future on-chip monolithic integration of III-V NW lasers onto silicon based optoelectronic circuits.
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