KEYWORDS: Near field, Antimony, Nanoparticles, Near field scanning optical microscopy, Dielectrics, Waveguides, Near field optics, Modulation, Nanostructures, Polarizability
We investigate the optical properties of nanostructures of antimony sulfide (Sb2S3), a direct-bandgap semiconductor material that has recently sparked considerable interest as a thin film solar cell absorber. Fabrication from a nanoparticle ink solution and two- and three-dimensional nanostructuring with pattern sizes down to 50 nm have recently been demonstrated. Insight into the yet unknown nanoscopic optical properties of these nanostructures is highly desired for their future applications in nanophotonics. We implement a spectrally broadband scattering-type near-field optical spectroscopy technique to study individual Sb2S3 nanodots with a 20-nm spatial resolution, covering the range from 700 to 900 nm. We show that in this below-bandgap range, the Sb2S3 nanostructures act as high-refractive-index, low-loss waveguides with mode profiles close to those of idealized cylindrical waveguides, despite a considerable structural disorder. In combination with their high above-bandgap absorption, this makes them promising candidates for applications as dielectric metamaterials, specifically for ultrafast photoswitching.
Image potential states are well established surface states of metallic films [1]. For a single metallic nanostructure these surface states can be localized in the near-field arising from illumination by a strong laser field. Thus single metallic nanostructures offer the unique possibility to study quantum systems with both high spatial and ultrafast temporal resolution. Here, we investigate the dynamics of Rydberg states localized to a sharp metallic nanotaper.
For this purpose we realized a laser system delivering few-cycle pulses tunable over a wide wavelength range [2]. Pulses from a regenerative titanium:sapphire amplifier generate a white light continuum, from which both a proportion in the visible and in the infrared are amplified in two non-collinear optical parametric amplification (NOPA) stages. Difference frequency generation (DFG) of both stages provides pulses in the near-infrared.
With a precisely delayed sequence of few-cycle pulses centered around 600 nm (NOPA#1 output) and 1600 nm (DFG output) we illuminate the apex of a sharply etched gold tip. Varying the delay we observe an exponential decay of photoemitted electrons with a distinctly asymmetric decay length on both sides, indicating the population of different states. Superimposed on the decay is a clearly discernible quantum beat pattern with a period of <50 fs, which arises from the motion of Rydberg photoelectrons bound within their own image potential. These results therefore constitute a step towards controlling single electron wavepackets released from a gold tip opening up fascinating perspectives for applications in ultrafast electron microscopy [3].
[1] Hofer, U. et al. Science 277, 1480 (1997)
[2] Vogelsang, J., Robin J. et al. Opt. Express 22, 25295 (2014)
[3] Petek, H. et al. ACS Nano 8, 5 (2014)
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