149 research outputs found
High stability white light generation in water at multi-kilohertz repetition rate
Efficient supercontinuum (SC) generation featuring high spectral intensity
across a large bandwidth requires high peak powers of several megawatt from
pulsed lasers. Under these conditions and at multi-kilohertz (kHz) repetition
rates, the SC generated in most materials is unstable due to thermal effects.
In this work, we leverage the superior dispersion properties of water to
maximize the spectral width of the SC, while avoiding stability issues due to
thermal loading by means of a constant laminar flow of the liquid. This flow is
controlled by a differential pressure scheme that allows to precisely adjust
the fluid velocity to an optimum value for maximum stability of the SC. This
approach is successfully implemented for repetition rates of 50 kHz and 100 kHz
and two different pump wavelengths in the visible (VIS) and near infrared (NIR)
spectral region with stability of the SC signal only limited by the driving
pulses. The resulting water SC spans more than one octave covering the VIS to
NIR range. Compared to established materials, such as yttrium aluminum garnet
(YAG) and sapphire, the spectral bandwidth is increased by 60 % and 40 %
respectively. Our scheme has the potential to be implemented with other liquids
such as bromine or carbon disulfide (CS2), which promise even wider broadening
and operation up to the mid-infrared
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 (
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
higher than the lattice temperature . 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 than for bulk gold
films. The electron-phonon coupling constant and the Debye temperature are
found to scale with the metal filling factor 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
Germanium Plasmonic Nanoantennas for Third-Harmonic Generation in the Mid Infrared
We explore the nonlinear optical properties of plasmonic semiconductor antennas resonant in the mid infrared. The nanostructures are fabricated on silicon substrates from heavily doped germanium films with a plasma frequency of 30 THz, equivalent to a wavelength of 10 μm. Illumination with ultrashort pulses at 10.8 μm produces coherent emission at 3.6 μm via third-harmonic generation
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
Optical activation of germanium plasmonic antennas in the mid-infrared
Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recombination redshifts the plasma frequency. The full behavior of the structures is modeled by the electrodynamic response established by an electron-hole plasma in a regular array of antennas
Generation of 85-fs pulses at 13 μm for ultrabroadband pump-probe spectroscopy
We report on a near-infrared non-collinear optical parametric amplifier (NOPA) based on periodically poled stoichiometric lithium tantalate. The NOPA generates mu J-energy pulses with spectrum spanning the 1-1.7 mu m wavelength range, which are compressed to nearly transform-limited 8.5 fs duration by a deformable mirror. By synchronizing this source with a sub-10-fs visible NOPA, we demonstrate an unprecedented combination of temporal resolution and spectral coverage in two-colour pump-probe spectroscopy. (C) 2009 Optical Society of Americ
The use of silicon-germanium superlattices for thermoelectric devices and microfabricated generators
Low dimensional structures such as superlattices have the potential to improve the thermoelectric properties of materials by engineering the scattering of phonons to reduce the thermal conductivity and therefore improve the thermeoelectric performance. Here we demonstrate the reduction in thermal conductivity in Ge/SiGe superlattices using multiple barrier engineering to scatter acoustic phonons at the key wavelengths for thermal transport. The approach allows ZT to be increased in wide quantum well superlattices through the reduction of heterointerfaces which scatter both electrons and phonons
n-Ge on Si for Mid-Infrared Plasmonic Sensors
The detection and amplification of molecular absorption lines from a mustard gas simulant is demonstrated using plasmonic antennas fabricated from n-Ge epitaxially grown on Si. Approaches to integrated sensors will be presented along with a review of n-Ge compared to other mid-infrared plasmonic materials
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