23 research outputs found
Mid-infrared Kerr index evaluation via cross-phase modulation with a near-infrared probe beam
We propose a simple method to measure nonlinear Kerr refractive index in
mid-infrared frequency range that avoids using sophisticated infrared
detectors. Our approach is based on using a near-infrared probe beam which
interacts with a mid-IR beam via wavelength-non-degenerate cross-phase
modulation (XPM). By carefully measuring XPM-induced spectral modifications in
the probe beam and comparing the experimental data with simulation results we
extract the value for the non-degenerate Kerr index. Finally, in order to
obtain the value of degenerate mid-IR Kerr index we use the well-established
two-band formalism of Sheik-Bahae et al., which is shown to become particularly
simple in the limit of low frequencies. The proposed technique is complementary
to the conventional techniques such as z-scan and has the advantage of not
requiring any mid-infrared detectors
Dispersive effects in ultrafast non-linear phenomena
It is a basic principle that an effect cannot come before the cause.
Dispersive relations that follow from this fundamental fact have proven to be
an indispensable tool in physics and engineering. They are most powerful in the
domain of linear response where they are known as Kramers-Kronig relations.
However when it comes to nonlinear phenomena the implications of causality are
much less explored, apart from several notable exceptions. Here in this work we
demonstrate how to apply the dispersive formalism to analyse the ultrafast
nonlinear response in the context of the paradigmatic nonlinear Kerr effect. We
find that the requirement of causality introduces a noticeable effect even
under assumption that Kerr effect is mediated by quasi-instantaneous
off-resonant electronic hyperpolarizability. We confirm this by experimentally
measuring the time resolved Kerr dynamics in GaAs by means of a hybrid
pump-probe Mach-Zehnder interferometer and demonstrate the presence of an
intrinsic lagging between amplitude and phase responses as predicted by
dispersive analysis. Our results describe a general property of the
time-resolved nonlinear processes thereby highlighting the importance of
accounting for dispersive effects in the nonlinear optical processes involving
ultrashort pulses
Confinement-Deconfinement Transition as an Indication of Spin-Liquid-Type Behavior in NaIrO
We use ultrafast optical spectroscopy to observe binding of charged
single-particle excitations (SE) in the magnetically frustrated Mott insulator
NaIrO. Above the antiferromagnetic ordering temperature () the
system response is due to both Hubbard excitons (HE) and their constituent
unpaired SE. The SE response becomes strongly suppressed immediately below
. We argue that this increase in binding energy is due to a unique
interplay between the frustrated Kitaev and the weak Heisenberg-type ordering
term in the Hamiltonian, mediating an effective interaction between the
spin-singlet SE. This interaction grows with distance causing the SE to become
trapped in the HE, similar to quark confinement inside hadrons. This binding of
charged particles, induced by magnetic ordering, is a result of a
confinement-deconfinement transition of spin excitations. This observation
provides evidence for spin liquid type behavior which is expected in
NaIrO.Comment: 5 pages, 3 figure
The origin of exciton mass in a frustrated Mott insulator NaIrO
We use a three-pulse ultrafast optical spectroscopy to study the relaxation
processes in a frustrated Mott insulator NaIrO. By being able to
independently produce the out-of-equilibrium bound states (excitons) of
doublons and holons with the first pulse and suppress the underlying
antiferromagnetic order with the second one, we were able to elucidate the
relaxation mechanism of quasiparticles in this system. By observing the
difference in the exciton dynamics in the magnetically ordered and disordered
phases we found that the mass of this quasiparticle is mostly determined by its
interaction with the surrounding spins
Confinement-Deconfinement Transition as an Indication of Spin-Liquid-Type Behavior in Na\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e3\u3c/sub\u3e
We use ultrafast optical spectroscopy to observe binding of charged single-particle excitations (SE) in the magnetically frustrated Mott insulator Na2IrO3. Above the antiferromagnetic ordering temperature (TN) the system response is due to both Hubbard excitons (HE) and their constituent unpaired SE. The SE response becomes strongly suppressed immediately below TN. We argue that this increase in binding energy is due to a unique interplay between the frustrated Kitaev and the weak Heisenberg-type ordering term in the Hamiltonian, mediating an effective interaction between the spin-singlet SE. This interaction grows with distance causing the SE to become trapped in the HE, similar to quark confinement inside hadrons. This binding of charged particles, induced by magnetic ordering, is a result of a confinement-deconfinement transition of spin excitations. This observation provides evidence for spin liquid type behavior which is expected in Na2IrO3
STM imaging of a bound state along a step on the surface of the topological insulator BiTe
Detailed study of the LDOS associated with the surface-state-band near a
step-edge of the strong topological-insulator Bi2Te3, reveal a one-dimensional
bound state that runs parallel to the stepedge and is bound to it at some
characteristic distance. This bound state is clearly observed in the bulk gap
region, while it becomes entangled with the oscillations of the warped surface
band at high energy, and with the valence band states near the Dirac point.
Using the full effective Hamiltonian proposed by Zhang et al., we obtain a
closed formula for this bound state that fits the data and provide further
insight into the general topological properties of the electronic structure of
the surface band near strong structural defects.Comment: 5 pages, 4 figure
Disorder enabled band structure engineering of a topological insulator surface
Three dimensional topological insulators are bulk insulators with
topological electronic order that gives rise to conducting
light-like surface states. These surface electrons are exceptionally resistant
to localization by non-magnetic disorder, and have been adopted as the basis
for a wide range of proposals to achieve new quasiparticle species and device
functionality. Recent studies have yielded a surprise by showing that in spite
of resisting localization, topological insulator surface electrons can be
reshaped by defects into distinctive resonance states. Here we use numerical
simulations and scanning tunneling microscopy data to show that these resonance
states have significance well beyond the localized regime usually associated
with impurity bands. At native densities in the model BiX (X=Bi, Te)
compounds, defect resonance states are predicted to generate a new quantum
basis for an emergent electron gas that supports diffusive electrical
transport
STM imaging of impurity resonances on BiSe
In this paper we present detailed study of the density of states near defects
in BiSe. In particular, we present data on the commonly found
triangular defects in this system. While we do not find any measurable
quasiparticle scattering interference effects, we do find localized resonances,
which can be well fitted by theory once the potential is taken to be extended
to properly account for the observed defects. The data together with the fits
confirm that while the local density of states around the Dirac point of the
electronic spectrum at the surface is significantly disrupted near the impurity
by the creation of low-energy resonance state, the Dirac point is not locally
destroyed. We discuss our results in terms of the expected protected surface
state of topological insulators.Comment: 5 pages, 6 figure
Bond polarizability as a probe of local crystal fields in hybrid lead-halide perovskites
A rotating organic cation and a dynamically disordered soft inorganic cage
are the hallmark features of hybrid organic-inorganic lead-halide perovskites.
Understanding the interplay between these two subsystems is a challenging
problem but it is this coupling that is widely conjectured to be responsible
for the unique behaviour of photo-carriers in these materials. In this work, we
use the fact that the polarizability of the organic cation strongly depends on
the ambient electrostatic environment to put the molecule forward as a
sensitive probe of local crystal fields inside the lattice cell. We measure the
average polarizability of the C/N--H bond stretching mode by means of infrared
spectroscopy, which allows us to deduce the character of the motion of the
cation molecule, find the magnitude of the local crystal field and place an
estimate on the strength of the hydrogen bond between the hydrogen and halide
atoms. Our results pave the way for understanding electric fields in
lead-halide perovskites using infrared bond spectroscopy
Effective model for studying optical properties of lead-halide perovskites
We use general symmetry-based arguments to construct an effective model
suitable for studying optical properties of lead-halide perovskites. To build
the model, we identify an atomic-level interaction between electromagnetic
fields and the spin degree of freedom that should be added to a
minimally-coupled Hamiltonian. As an application, we study
two basic optical characteristics of the material: the Verdet constant and the
refractive index