84 research outputs found
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
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
Temperature dependence of the thermal boundary resistivity of glass-embedded metal nanoparticles
The temperature dependence of the thermal boundary resistivity is
investigated in glass-embedded Ag particles of radius 4.5 nm, in the
temperature range from 300 to 70 K, using all-optical time-resolved
nanocalorimetry. The present results provide a benchmark for theories aiming at
explaining the thermal boundary resistivity at the interface between metal
nanoparticles and their environment, a topic of great relevance when tailoring
thermal energy delivery from nanoparticles as for applications in nanomedicine
and thermal management at the nanoscaleComment: 4 pages, 3 figure
Anisotropic complex refractive indices of atomically thin materials: determination of the optical constants of few-layer black phosphorus
In this work we briefly review the studies of the optical constants of
monolayer transition metal dichalcogenides and few layer black phosphorus, with
particular emphasis to the complex dielectric function and refractive index.
Specifically, we give an estimate of the complex index of refraction of
phosphorene and few-layer black phosphorus. We extracted the complex index of
refraction of this material from differential reflectance data reported in
literature by employing a constrained Kramers-Kronig analysis. Finally, we
studied the linear optical response of multilayer systems embedding phosphorene
by using the transfer matrix method.Comment: 11 pages, 3 figure
Strong enhancement of d-wave superconducting state in the three-band Hubbard model coupled to an apical oxygen phonon
We study the hole binding energy and pairing correlations in the three-band
Hubbard model coupled to an apical oxygen phonon, by exact diagonalization and
constrained-path Monte Carlo simulations. In the physically relevant
charge-transfer regime, we find that the hole binding energy is strongly
enhanced by the electron-phonon interaction, which is due to a novel
potential-energy-driven pairing mechanism involving reduction of both
electronic potential energy and phonon related energy. The enhancement of hole
binding energy, in combination with a phonon-induced increase of quasiparticle
weight, leads to a dramatic enhancement of the long-range part of d-wave
pairing correlations. Our results indicate that the apical oxygen phonon plays
a significant role in the superconductivity of high- cuprates.Comment: 5 pages, 5 figure
Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons
The understanding of how the sub-nanoscale exchange interaction evolves in macroscale correlations and ordered phases of matter, such as magnetism and superconductivity, requires to bridging the quantum and classical worlds. This monumental challenge has so far only been achieved for systems close to their thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics of the macroscale magnetic order parameter in the Heisenberg antiferromagnet KNiF 3 triggered by the impulsive optical generation of spin excitations with the shortest possible nanometre wavelength and femtosecond period. Our magneto-optical pump-probe experiments also demonstrate the coherent manipulation of the phase and amplitude of these femtosecond nanomagnons, whose frequencies are defined by the exchange energy. These findings open up opportunities for fundamental research on the role of short-wavelength spin excitations in magnetism and strongly correlated materials; they also suggest that nanospintronics and nanomagnonics can employ coherently controllable spin waves with frequencies in the 20 THz domain
Exciton-phonon coupling strength in single-layer MoSe2 at room temperature
Single-layer transition metal dichalcogenides are at the center of an ever
increasing research effort both in terms of fundamental physics and
applications. Exciton-phonon coupling plays a key role in determining the
(opto)electronic properties of these materials. However, the exciton-phonon
coupling strength has not been measured at room temperature. Here, we develop
two-dimensional micro-spectroscopy to determine exciton-phonon coupling of
single-layer MoSe2. We detect beating signals as a function of waiting time T,
induced by the coupling between the A exciton and the A'1 optical phonon.
Analysis of two-dimensional beating maps combined with simulations provides the
exciton-phonon coupling. The Huang-Rhys factor of ~1 is larger than in most
other inorganic semiconductor nanostructures. Our technique offers a unique
tool to measure exciton-phonon coupling also in other heterogeneous
semiconducting systems with a spatial resolution ~260 nm, and will provide
design-relevant parameters for the development of optoelectronic devices
Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe
The coupling of the electron system to lattice vibrations and their
time-dependent control and detection provides unique insight into the
non-equilibrium physics of semiconductors. Here, we investigate the ultrafast
transient response of semiconducting monolayer 2-MoTe encapsulated with
BN using broadband optical pump-probe microscopy. The sub-40-fs pump pulse
triggers extremely intense and long-lived coherent oscillations in the spectral
region of the A' and B' exciton resonances, up to 20% of the maximum
transient signal, due to the displacive excitation of the out-of-plane
phonon. Ab-initio calculations reveal a dramatic rearrangement of the optical
absorption of monolayer MoTe induced by an out-of-plane stretching and
compression of the crystal lattice, consistent with an -type
oscillation. Our results highlight the extreme sensitivity of the optical
properties of monolayer TMDs to small structural modifications and their
manipulation with light.Comment: 27 pages, 4 figures, supporting informatio
Coherent phonons and the interplay between charge density wave and Mott phases in 1<i>T</i>-TaSe<sub>2</sub>
1-TaSe is host to coexisting strongly-correlated phases including
charge density waves (CDWs) and an unusual Mott transition at low temperature.
Here, we investigate coherent phonon oscillations in 1-TaSe using a
combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES)
and time-resolved reflectivity (TRR). Perturbation by a femtosecond laser pulse
triggers a modulation of the valence band binding energy at the -point,
related to the Mott gap, that is consistent with the in-plane CDW amplitude
mode frequency. By contrast, TRR measurements show a modulation of the
differential reflectivity comprised of multiple frequencies belonging to the
distorted CDW lattice modes. Comparison of the temperature dependence of
coherent and spontaneous phonons across the CDW transition shows that the
amplitude mode intensity is more easily suppressed during perturbation of the
CDW state by the optical excitation compared to other modes. Our results
clearly identify the relationship of the in-plane CDW amplitude mode with the
Mott phase in 1-TaSe and highlight the importance of lattice degrees
of freedom.Comment: 7 pages, 4 figures, supplemental materia
Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2.
Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2D) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics, of pivotal importance for the understanding and optimization of device operation, is still lacking. Here, we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photoexcited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime
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