2,043 research outputs found
Probing anomalous relaxation by coherent multidimensional optical spectroscopy
We propose to study the origin of algebraic decay of two-point correlation
functions observed in glasses, proteins, and quantum dots by their nonlinear
response to sequences of ultrafast laser pulses. Power-law spectral
singularities and temporal relaxation in two-dimensional correlation
spectroscopy (2DCS) signals are predicted for a continuous time random walk
model of stochastic spectral jumps in a two level system with a power-law
distribution of waiting times . Spectroscopic
signatures of stationary ensembles for and aging effects in
nonstationary ensembles with are identified
Cascading and Local-Field Effects in Non-Linear Optics Revisited; A Quantum-Field Picture Based on Exchange of Photons
The semi-classical theory of radiation-matter coupling misses local-field
effects that may alter the pulse time-ordering and cascading that leads to the
generation of new signals. These are then introduced macroscopically by solving
Maxwell's equations. This procedure is convenient and intuitive but ad hoc. We
show that both effects emerge naturally by including coupling to quantum modes
of the radiation field in the vacuum state to second order. This approach is
systematic and suggests a more general class of corrections that only arise in
a QED framework. In the semi-classical theory, which only includes classical
field modes, the susceptibility of a collection of non-interacting
molecules is additive and scales as . Second-order coupling to a vacuum mode
generates an effective retarded interaction that leads to cascading and local
field effects both of which scale as
Causal vs. Noncausal Description of Nonlinear Wave Mixing; Resolving the Damping-Sign Controversy
Frequency-domain nonlinear wave mixing processes may be described either
using response functions whereby the signal is generated after all interactions
with the incoming fields, or in terms of scattering amplitudes where all fields
are treated symetrically with no specific time ordering. Closed Green's
function expressions derived for the two types of signals have different
analytical properties. The recent controversy regarding the sign of radiative
damping in the linear (Kramers Heisenberg) formula is put in a broader context
Photon Statistics for Single Molecule Non-Linear Spectroscopy
We consider the theory of the non-linear spectroscopy for a single molecule
undergoing stochastic dynamics and interacting with a sequence of two laser
pulses. General expressions for photon counting statistics are obtained, and an
exact solution to the problem of the Kubo-Anderson process is found. In the
limit of impulsive pulses the information on the photon statistics is contained
in the molecule's dipole correlation function. The selective limit where
temporal resolution is maintained, the semi-classical approximation and the
fast modulation limit exhibit general behaviors of this new type of
spectroscopy. We show how the design of the external field leads to rich
insights on dynamics of individual molecules which are different than those
found for an ensemble
Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR
Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear
optical response of excitons to sequences of ultrafast pulses, has the
potential to provide some unique insights into carrier dynamics in
semiconductors. The most prominent feature of 2DCS, cross peaks, can best be
understood using a sum-over-states picture involving the many-body eigenstates.
However, the optical response of semiconductors is usually calculated by
solving truncated equations of motion for dynamical variables, which result in
a quasiparticle picture. In this work we derive Green's function expressions
for the four wave mixing signals generated in various phase-matching directions
and use them to establish the connection between the two pictures. The formal
connection with Frenkel excitons (hard-core bosons) and vibrational excitons
(soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.
Analysis of 2D THz-Raman spectroscopy using a non-Markovian Brownian oscillator model with nonlinear system-bath interactions
We explore and describe the roles of inter-molecular vibrations employing a
Brownian oscillator (BO) model with linear-linear (LL) and square-linear (SL)
system-bath interactions, which we use to analyze two-dimensional (2D)
THz-Raman spectra obtained by means of molecular dynamics (MD) simulations. In
addition to linear absorption (1D IR), we calculated 2D Raman-THz-THz,
THz-Raman-THz, and THz-THz-Raman signals for liquid formamide, water, and
methanol using an equilibrium non-equilibrium hybrid MD simulation. The
calculated 1D IR and 2D THz-Raman signals are compared with results obtained
from the LL+SL BO model applied through use of hierarchal Fokker-Planck
equations with non-perturbative and non-Markovian noise. We find that all of
the qualitative features of the 2D profiles of the signals obtained from the MD
simulations are reproduced with the LL+SL BO model, indicating that this model
captures the essential features of the inter-molecular motion. We analyze the
fitted 2D profiles in terms of anharmonicity, nonlinear polarizability, and
dephasing time. The origins of the echo peaks of the librational motion and the
elongated peaks parallel to the probe direction are elucidated using optical
Liouville paths.Comment: 37 pages with 14 figures and 3 table
Multiple time scales hidden in heterogeneous dynamics of glass-forming liquids
A multi-time probing of density fluctuations is introduced to investigate
hidden time scales of heterogeneous dynamics in glass-forming liquids.
Molecular dynamics simulations for simple glass-forming liquids are performed,
and a three-time correlation function is numerically calculated for general
time intervals. It is demonstrated that the three-time correlation function is
sensitive to the heterogeneous dynamics and that it reveals couplings of
correlated motions over a wide range of time scales. Furthermore, the time
scale of the heterogeneous dynamics is determined by the
change in the second time interval in the three-time correlation function. The
present results show that the time scale of the heterogeneous dynamics
becomes larger than the -relaxation time at low
temperatures and large wavelengths. We also find a dynamical scaling relation
between the time scale and the length scale of
dynamical heterogeneity as with .Comment: 4 pages, 5 figures, to appear in Phys. Rev. E (Rapid Communications
Decoherence in quantum dots due to real and virtual transitions: a non-perturbative calculation
We investigate theoretically acoustic phonon induced decoherence in quantum
dots. We calculate the dephasing of fundamental (interband or intraband)
optical transitions due to real and virtual transitions with higher energy
levels. Up to two acoustic phonon processes (absorption and/or emission) are
taken into account simultaneously in a non-perturbative manner. An analytic
expression of acoustic phonon induced broadening is given as a function of the
electron-phonon matrix elements and is physically interpreted. The theory is
applied to the dephasing of intersublevel transitions in self-assembled quantum
dots.Comment: 8 pages, 4 figure
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