We demonstrate enhanced photoluminescence (PL) from an optically pumped bias driven molecular tunneling junction (Au-substrate/self assembled molecular monolayer/Au-tip) with molecules chemically bound to the Au substrate. The gap between a sharp gold tip and a flat gold substrate covered with a self-assembled monolayer (SAM) of 5-chloro-2 mercaptobenzothiazole (Cl-MBT) molecules can be used as an extremely small optical gain medium. When a bias-voltage is applied between tip and sample such that electrons tunnel from the Cl-MBT’s highest occupied molecular orbital (HOMO) to the tip, holes are left behind in the molecules. These can be repopulated by hot electrons that are created by the laser-driven plasmon oscillation on the metal surfaces enclosing the molecule. Emission of photons occurs from the recombination of plasmon excited hot electrons with holes in the HOMO of surface bound molecules below the tip. Varying the laser pump power or alternatively the applied bias voltage shows in both cases a distinct threshold above which enhancement of the optical signal occurs. Solving the rate equations for this system shows that optical feed-back by the gap mode’s near field can efficiently stimulate the emission process. The system reflects many essential features of a superluminescent organic light emitting diode.
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.
Advanced optical setups are continuously developed to gain deeper insight into microscopic matter. In this paper
we report the expansion of a home-built parabolic mirror assisted scanning, near-field optical microscope (PMSNOM)
by introducing four complementary functions. 1) We integrated a scanning tunneling feedback function
in addition to an already existent shear-force feedback control mechanism. Hence a scanning tunneling
microscope (STM)-SNOM is realized whose performance will be demonstrated by the tip-enhanced Raman
peaks of graphene sheets on a copper substrate. 2) We integrated an ultrafast laser system into the microscope
which allows us to combine nonlinear optical microscopy with hyperspectral SNOM imaging. This particular
expansion was used to study influences of plasmonic resonances on nonlinear optical properties of metallic
nanostructures. 3) We implemented a polarization angle resolved detection technique which enables us to
analyze the local structural order of α-sexithiophene (α-6T). 4) We combined scanning photocurrent microscopy
with the microscope. This allows us to study morphology related optical (Raman and photoluminescence) and
electrical properties of optoelectronic systems. Our work demonstrates the great potential of turning a SNOM
into an advanced multifunctional microscope.
Individual small gold structures of different sizes and shapes are fabricated on planar substrates for subsequent
characterization of their optical properties. In the process, a combination of thin-film metallization, electron beam
lithography and ion milling is employed, where electron beam structured hydrogen silsesquioxane is used as an etch
mask for the underlying gold layer. Gold cones, vertical rods, cups and flat disks can be prepared with a typical height of
about 100 nm. Their optical properties are investigated by confocal optical microscopy using a parabolic mirror for both
laser focusing and signal collection. As an example, the photoluminescence signal collected from an array of gold cones
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