Ultrafast ZnO Nanowire Lasers: Nanoplasmonic Acceleration of Gain Dynamics at the Surface Plasmon Polariton Frequency

Themistoklis Sidiropoulos Physics, Imperial College London, London, UK Robert Röder Physics, University of Jena, Jena, Germany Sebastian Geburt Physics, University of Jena, Jena, Germany Ortwin Hess Physics, Imperial College London, London, UK Stefan Maier Physics, Imperial College London, London, UK Carsten Ronning Physics, University of Jena, Jena, Germany Rupert Oulton Physics, Imperial College London, London, UK

Surface plasmon polaritons (SPP) are capable of increasing optical confinement and accelerating the otherwise slow interaction of light with matter. This enhancement becomes the largest at the surface plasmon frequency. Thus, a plasmonic laser operating close to the surface plasmon frequency could act as nanoscopic source of light which also has ultrafast modulations capabilities. However, as at the surface plasmon frequency the inherent plasmonic losses also become maximal, the demonstration of such a device is a challenging task.

Here, we exploit the strong optical confinement in a hybrid plasmonic geometry to present ultrafast room-temperature semiconductor nanowire lasers operating at UV frequencies. We show that optically excited ZnO nanowires sitting on a Silver substrate, emit very close the surface plasmon frequency, leading to accelerated spontaneous recombination, gain switching, and gain recovery compared to conventional – photonic – ZnO nanowire lasers.

By spectrally monitoring the nanowire emission, evidence for the lasing of plasmonic modes arises from a range of tests. Firstly, the optical modes of photonic ZnO nanowires do not lase for diameters smaller than around 150 nm, whereas their plasmonic counterparts operate for diameters as small as ~120 nm. We also verify surface plasmon lasing by polarization measurements of the emitted light. Furthermore, an observed blueshift in laser emission with decreasing nanowire diameter highlights an intricate interplay between increased plasmonic confinement and increased loss occurring closer to the surface plasmon frequency.

To obtain the temporal characteristics of these plasmonic lasers, we used a novel double-pump approach that exploits the non-linearity of the plasmonic laser process itself. The effect of SPP confinement becomes apparent as we monitor the temporal dynamics of the plasmonic and photonic nanowire lasers. We see extremely strong acceleration of the optical processes with decreasing nanowire diameter for plasmonic lasers consistent with increasing mode confinement. The photonic devices however, become slower with decreasing nanowire diameter, which is due the loss of mode confinement.

This study highlights the feasibility of surface plasmon lasers operating near the surface plasmon frequency, despite the extremely high losses associated with fast electron scattering of electrons in metals.

t.sidiropoulos10@imperial.ac.uk









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