150 years after the invention of the incandescent bulb the generation of light by metals remains intriguing. Silver nanoparticle aggregates offer extraordinary fidelity in single-molecule Raman spectroscopy provided that the photophysics of the analyte is adequately controlled [1]. Under irradiation with femtosecond infrared laser pulses, non-linear luminescence is observed from hot spots, which spatially anti-correlate with SERS [2]. In contrast to surface-enhanced SHG, which exhibits pronounced intermittency [3], these hot spots display ultrafast white-light emission [4] from sub-diffraction volumes. Hot spots can even be used as an alternative to conventional SNOM in broad-band superresolution transmission microscopy of photonic crystals [5] (see Figure).
Single continuum hot spots can be characterized both in time and frequency domain, by autocorrelation spectroscopy of the emission intensity [6] or recording time-integrated emission as a function of excitation wavelength [7]. Both approaches reveal hybridization of bright localized nanoparticle plasmons with propagating surface-plasmon polaritons, which can traverse in excess of 100 microns in the rough silver film, leading to pronounced interference phenomena which are manifested in the frequency and time response.
Wet-chemical growth provides a simple metric to control interparticle coupling, which is examined directly using correlated electron and optical microscopy [8]. While being heterogeneous between single hot spots, continuum generation exhibits a universal excitation power dependency, extending to the Stokes regime, which demonstrates that emission originates from thermal radiation of the electron gas that is heated by the nanoparticle plasmon [9]. The generality of these radiative intraband transitions is readily extended to other metals such as gold, copper and aluminium, and provides an extended framework to examine electrically excited hot spots in percolation films.
[1] Walter et al., PRL 98, 137401 (2007). [2] Walter et al., JACS 130, 16830 (2008). [3] Borys et al., PRB 80, 161407(R) (2009). [4] Borys et al., JPCC 115, 13645 (2011). [5] Chaudhuri et al., Nano Lett. 9, 952 (2009). [6] Klemm et al., PRL 113, 266805 (2014). [7] Borys et al., Sci. Rep. 3, 2090 (2013). [8] Borys et al., Science 330, 1371 (2010). [9] Haug et al., PRL 115, 067403 (2015).
Optical properties of nanostructures , Nonlinear nano-optics
