Semiconductor nanowire based coherent light sources : temporal dynamics and tunability
Semiconductor nanowires (NWs) offer exceptional optical properties such as efficient waveguiding of light despite their subwavelength diameter. These properties are strongly related to the semiconductor material and their morphology. Additionally, semiconductor NWs inherently offer all basic components of a laser system including robust, high optical gain and a beneficial Fabry-Pérot resonator structure. Thus, they are considered as one of the smallest coherent light sources available for different visionary concepts in recent research on nanophotonics, nanosensing and nanspectroscopy. Laser oscillations in single NWs have only been achieved in a pulsed operation mode so far, as the off times between the optical pump pulses allows the thermal budget introduced by the optical pumping to dissipate in order to avoid a successive material degradation. In order to establish the continuous wave (cw) emission mode in single NWs, a semiconductor material of exceptional high optical gain and an excellent NW resonator morphology is required. Both challenges can be accomplished by using CdS NWs synthesized by the vapor-liquid-solid (VLS) mechanism using Sn as catalyst. Knowing the fundamental laser dynamics of semiconductor NWs will enable the exploitation of their full potential as nanoscaled coherent light sources. The respective time scales for laser onset time and pulse width are in the range of few hundred femtoseconds up to few picoseconds and can be measured by a novel double-pump technique. In addition, both characteristic times can be adjusted by a proper choice of semiconductor material, pump energy and optical configuration. The field distribution within a NW laser can be adjusted between single and multi-transverse mode operation by varying the NW diameter. Thin NWs below the critical diameter for multimode waveguiding emit laser emission of the fundamental mode, whereas NWs with larger diameters clearly show multimode operation. Furthermore, the transverse mode structure strongly affects the light-matter interaction and the NW laser dynamics.