Electromagnetic coupling between resonant plasmonic oscillations of two closely spaced noble metal particles can lead to a strongly enhanced optical near field in the cavity formed by the gap between the metal particles. However, discoveries in quantum plasmonics show that an upper limit is imposed to the field enhancement by the intrinsic nonlocality of the dielectric response of the metal and the tunneling of the coherently oscillating conduction electrons through the gap. Here, we introduce and experimentally demonstrate optical amplification by radiative relaxation of hot electrons in a tunneling junction of a scanning tunneling microscope forming an extremely small point light source. When electrons tunnel from the sample to the tip, holes are left behind. These can be repopulated by hot electrons induced by the laser-driven plasmon oscillation on the metal surfaces enclosing the cavity and lead to a much higher electron to photon conversion efficiency. The dynamics of this system can be described by rate equations similar to laser equations. They show that the repopulation process can be efficiently stimulated by the gap mode’s near field. Our results demonstrate how optical enhancement inside the plasmonic cavity can be further increased by a stronger localization via tunneling through molecules.
KEYWORDS: Confocal microscopy, Solar cells, Raman spectroscopy, Spectroscopy, Annealing, Near field, Near field optics, Organic photovoltaics, Mirrors, Signal detection
The poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) organic films are widely
employed as electronic donor and acceptor in the field of organic film solar cell because of their high photovoltaic
conversion efficiency. A home-built parabolic mirror assisted confocal and apertureless near-field optical microscope
was used to investigate the degradation behavior of the film and to distinguish the donor and acceptor domains both
topographically and optically. Under ambient condition, the degradation rates are decreased in the sequence of pristine
P3HT, blend P3HT:PCBM film and pristine PCBM. N2 protection dramatically slows down the film degradation rate.
Using confocal spectroscopic mapping, we are able to distinguish the local distributions of P3HT and PCBM.
Micrometer PCBM aggregates were observed due to the thermal annealing process. Our experimental methods show the
possibility to investigate morphology and the photochemistry properties of the organic solar cell films with high spatial
resolution.
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