575 research outputs found
Real time vibronic coupling dynamics in organic conjugated systems
In this work we show the potentialities of applying impulsive coherent vibrational spectroscopy to conjugated systems relevant for applications. We studied films of sexithiophene, a candidate for large area molecular electronics, poly-phenylene vinylene, a prototype electroluminescent material, and polydiacetylene, very promising for applications in photonic devices. These experiments demonstrate the possibility of studying coherent molecular dynamics in organic systems with extremely high time resolution
Optical nonlinearity goes ultrafast in 2D semiconductor-based nanocavities
: Hybrid systems of silver nanodisks strongly coupled to monolayer tungsten-disulfide (WS2) show giant room-temperature nonlinearity due to their deeply sub-wavelength localized nature, resulting in ultrafast modifications of nonlinear absorption in a solid-state system
Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon
The calculation of the equilibrium optical properties of bulk silicon by
using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a
cornerstone in the development of an ab--initio approach to the optical and
electronic properties of materials. Nevertheless calculations of the {\em
transient} optical spectrum using the same efficient and successful scheme are
scarce. We report, here, a joint theoretical and experimental study of the
transient reflectivity spectrum of bulk silicon. Femtosecond transient
reflectivity is compared to a parameter--free calculation based on the
non--equilibrium Bethe--Salpeter equation. By providing an accurate description
of the experimental results we disclose the different phenomena that determine
the transient optical response of a semiconductor. We give a parameter--free
interpretation of concepts like bleaching, photo--induced absorption and
stimulated emission, beyond the Fermi golden rule. We also introduce the
concept of optical gap renormalization, as a generalization of the known
mechanism of band gap renormalization. The present scheme successfully
describes the case of bulk silicon, showing its universality and accuracy.Comment: 14 pages, 13 figure
Charge carrier generation in a conjugated polymer studied via ultrafast pump-push-probe experiments
Conjugated polymers find rapidly growing application in electroluminescent displays and are extensively studied for use in photovoltaics and laser diodes. For a wide range of conjugated materials ultrafast pump-probe experiments have revealed the excited state dynamics of singlet and triplet excitons as well as positively and negatively charged polarons. Charge carriers play a key role in all the above mentioned applications. However, there is yet no clear picture of the mechanisms which lead to their generation. Photocurrent excitation cross-correlation measurement on methyl-substituted ladder-type poly(para)phenyl (m-LPPP), a prototypical conjugated polymer with very appealing properties for the above mentioned applications, have suggested that charge carrier generation occurs preferentially from higher lying states during energy migration. Our approach to examining this mechanism consists of an innovative modification of the ultrafast time-resolved pump-probe technique
Scanning Fourier transform spectrometer in the visible range based on birefringent wedges
We introduce a spectrometer capable of measuring sample absorption spectra in the visible regime, based on a time-domain scanning Fourier transform (FT) approach. While infrared FT spectrometers typically employ a Michelson interferometer to create the two delayed light replicas, the proposed apparatus exploits a compact common-mode passive interferometer that relies on the use of birefringent wedges. This ensures excellent path-length stability (âŒÎ»/300) and accuracy, with no need for active feedback or beam tracking. We demonstrate the robustness of the technique measuring the transmission spectrum of a colored bandpass filter over one octave of bandwidth and compare the results with those obtained with a commercial spectrophotometer
Ultrafast hot electron dynamics in plasmonic nanostructures: experiments, modelling, design
Abstract
Metallic nanostructures exhibit localized surface plasmons (LSPs), which offer unprecedented opportunities for advanced photonic materials and devices. Following resonant photoexcitation, LSPs quickly dephase, giving rise to a distribution of energetic 'hot' electrons in the metal. These out-of-equilibrium carriers undergo ultrafast internal relaxation processes, nowadays pivotal in a variety of applications, from photodetection and sensing to the driving of photochemical reactions and ultrafast all-optical modulation of light. Despite the intense research activity, exploitation of hot carriers for real-world nanophotonic devices remains extremely challenging. This is due to the complexity inherent to hot carrier relaxation phenomena at the nanoscale, involving short-lived out-of-equilibrium electronic states over a very broad range of energies, in interaction with thermal electronic and phononic baths. These issues call for a comprehensive understanding of ultrafast hot electron dynamics in plasmonic nanostructures. This paper aims to review our contribution to the field: starting from the fundamental physics of plasmonic nanostructures, we first describe the experimental techniques used to probe hot electrons; we then introduce a numerical model of ultrafast nanoscale relaxation processes, and present examples in which experiments and modelling are combined, with the aim of designing novel optical functionalities enabled by ultrafast hot-electron dynamics
Deep reinforcement learning control of white-light continuum generation
White-light continuum (WLC) generation in bulk media finds numerous applications in ultrafast optics and spectroscopy. Due to the complexity of the underlying spatiotemporal dynamics, WLC optimization typically follows empirical procedures. Deep reinforcement learning (RL) is a branch of machine learning dealing with the control of automated systems using deep neural networks. In this Letter, we demonstrate the capability of a deep RL agent to generate a long-term-stable WLC from a bulk medium without any previous knowledge of the system dynamics or functioning. This work demonstrates that RL can be exploited effectively to control complex nonlinear optical experiments
Parametric Nonlinear Optics with Layered Materials and Related Heterostructures
Nonlinear optics is of crucial importance in several fields of science and technology with applications in frequency conversion, entangledâphoton generation, selfâreferencing of frequency combs, crystal characterization, sensing, and ultraâshort light pulse generation and characterization. In recent years, layered materials and related heterostructures have attracted huge attention in this field, due to their huge nonlinear optical susceptibilities, their ease of integration on photonic platforms, and their 2D nature which relaxes the phaseâmatching constraints and thus offers a practically unlimited bandwidth for parametric nonlinear processes. In this review the most recent advances in this field, highlighting their importance and impact both for fundamental and technological aspects, are reported and explained, and an outlook on future research directions for nonlinear optics with atomically thin materials is provided
Resonant optical control of the structural distortions that drive ultrafast demagnetization in CrO
We study how the color and polarization of ultrashort pulses of visible light
can be used to control the demagnetization processes of the antiferromagnetic
insulator CrO. We utilize time-resolved second harmonic generation
(SHG) to probe how changes in the magnetic and structural state evolve in time.
We show that, varying the pump photon-energy to excite either localized
transitions within the Cr or charge transfer states, leads to markedly
different dynamics. Through a full polarization analysis of the SHG signal,
symmetry considerations and density functional theory calculations, we show
that, in the non-equilibrium state, SHG is sensitive to {\em both} lattice
displacements and changes to the magnetic order, which allows us to conclude
that different excited states couple to phonon modes of different symmetries.
Furthermore, the spin-scattering rate depends on the induced distortion,
enabling us to control the timescale for the demagnetization process. Our
results suggest that selective photoexcitation of antiferromagnetic insulators
allows fast and efficient manipulation of their magnetic state.Comment: 7 pages, 5 figure
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