5 research outputs found
Nonlinear Photoluminescence Spectrum of Single Gold Nanostructures
We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence
Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells
In this work, we
investigate the exciton dissociation dynamics
occurring at the donor:acceptor interface in organic and hybrid blends
employed in the realization of photovoltaic cells. Fundamental differences
in the charge separation process are studied with the organic semiconductor
polymer polyÂ(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) or titanium dioxide (TiO<sub>2</sub>) acting
as the acceptor. By using ultrafast broad-band transient absorption
spectroscopy with few-fs temporal resolution, we observe that in both
cases the incoherent formation of free charges dominates the charge
generation process. From the optical response of the polymer and by
tracking the excited-state absorption, we extract pivotal similarities
in the incoherent energy pathways that follow the impulsive excitation.
On time scales shorter than 200 fs, we observe that the two acceptors
display similar dynamics in the exciton delocalization. Significant
differences arise only on longer time scales with only an impact on
the overall photocarrier generation efficiency
Dynamics of Four-Photon Photoluminescence in Gold Nanoantennas
Two-pulse correlation is employed to investigate the
temporal dynamics
of both two-photon photoluminescence (2PPL) and four-photon photoluminescence
(4PPL) in resonant and nonresonant nanoantennas excited at a wavelength
of 800 nm. Both 2PPL and 4PPL data are consistent with the same two-step
model already established for 2PPL, implying that the first excitation
step in 4PPL is a three-photon sp → sp direct interband transition.
Considering energy and parity conservation, we also explain why 4PPL
behavior is favored over, for example, three- and five-photon photoluminescence
in the power range below the damage threshold of our antennas. Since
sizable 4PPL requires larger peak intensities of the local field,
we are able to select either 2PPL or 4PPL in the same gold nanoantennas
by choosing a suitable laser pulse duration. We thus provide a first
consistent model for the understanding of multiphoton photoluminescence
generation in gold nanoantennas, opening new perspectives for applications
ranging from the characterization of plasmonic resonances to biomedical
imaging
Activated Singlet Exciton Fission in a Semiconducting Polymer
Singlet exciton fission is a spin-allowed
process to generate two
triplet excitons from a single absorbed photon. This phenomenon offers
great potential in organic photovoltaics, but the mechanism remains
poorly understood. Most reports to date have addressed intermolecular
fission within small-molecular crystals. However, through appropriate
chemical design chromophores capable of intramolecular fission can
also be produced. Here we directly observe sub-100 fs activated singlet
fission in a semiconducting polyÂ(thienylenevinylene). We demonstrate
that fission proceeds directly from the initial 1B<sub>u</sub> exciton,
contrary to current models that involve the lower-lying 2A<sub>g</sub> exciton. In solution, the generated triplet pairs rapidly recombine
and decay through the 2A<sub>g</sub> state. In films, exciton diffusion
breaks this symmetry and we observe long-lived triplets which form
charge-transfer states in photovoltaic blends
Origin of optical nonlinearity in plasmonic semiconductor nanostructures
The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits