Pump-probe Spectroscopy Study of Ultrafast Temperature Dynamics in Nanoporous Gold

We explore the influence of the nanoporous structure on the thermal relaxation of electrons and holes excited by ultrashort laser pulses ($\sim 7$ fs) in thin gold films. Plasmon decay into hot electron-hole pairs results in the generation of a Fermi-Dirac distribution thermalized at a temperature $T_{\mathrm{e}}$ higher than the lattice temperature $T_{\mathrm{l}}$. The relaxation times of the energy exchange between electrons and lattice, here measured by pump-probe spectroscopy, is slowed down by the nanoporous structure, resulting in much higher peak $T_{\mathrm{e}}$ than for bulk gold films. The electron-phonon coupling constant and the Debye temperature are found to scale with the metal filling factor $f$ and a two-temperature model reproduces the data. The results open the way for electron temperature control in metals by engineering of the nanoporous geometry.Comment: 6 pages, 3 figures, submitted to Physical Review

Control of excitonic absorption by thickness variation in few-layer GaSe

We control the thickness of GaSe on the level of individual layers and study the corresponding optical absorption via highly sensitive differential transmission measurements. Suppression of excitonic transitions is observed when the number of layers is smaller than a critical value of 8. Through ab-initio modelling we are able to link this behavior to a fundamental change in the band structure that leads to the formation of a valence band shaped as an inverted Mexican hat in thin GaSe. The thickness-controlled modulation of the optical properties provides attractive resources for the development of functional optoelectronic devices based on a single material

Ultraschnelle Ladungsträgerdynamik in elektronischen Systemen mit eingeschränkter Geometrie

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Sub-three-cycle pulses at 2 µm from a degenerate optical parametric amplifier.

peer reviewedIn this work we present a compact two-stage optical parametric amplifier (OPA) pumped at degeneracy by the fundamental of a Yb:KGW laser system. The output pulses span from 1.7 to 2.5 µm (120-176 THz) and are compressed to a sub-20 fs duration. This parametric amplifier exploits the broad phase-matching bandwidth at the degeneracy point in bismuth triborate (BiBO) and periodically poled lithium tantalate (PPLT). The result drastically expands the availability of ultrashort pulses with few-microjoule energy from near-infrared (NIR) to even longer wavelengths in the mid-infrared (MIR) spectral region

Control of excitonic absorption by thickness variation in few-layer GaSe

We control the thickness of GaSe on the level of individual layers and study the corresponding optical absorption via highly sensitive differential transmission measurements. Suppression of excitonic transitions is observed when the number of layers is smaller than a critical value of 8. Through ab-initio modelling we are able to assign this behavior to a fundamental change in the band structure that leads to the formation of a valence band shaped as an inverted Mexican hat in thin GaSe. This intrinsic modulation of the optical properties of GaSe provides attractive resources for the development of functional optoelectronic devices based on a single material.accepte

Pump-probe spectroscopy study of ultrafast temperature dynamics in nanoporous gold

We explore the influence of the nanoporous structure on the thermal relaxation of electrons and holes excited by ultrashort laser pulses (similar to 7 fs) in thin gold films. Plasmon decay into hot electron-hole pairs results in the generation of a Fermi-Dirac distribution thermalized at a temperature T-e higher than the lattice temperature T-1. The relaxation times of the energy exchange between electrons and lattice, here measured by pump-probe spectroscopy, is slowed down by the nanoporous structure, resulting in much higher peak T-e than for bulk gold films. The electron-phonon coupling constant and the Debye temperature are found to scale with the metal filling factor f and a two-temperature model reproduces the data. The results open the way for electron temperature control in metals by engineering of the nanoporous geometry

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 poly­(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO₂) 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