Direct observation of polariton velocity in CuCl

Dynamic behaviour of the polariton-polariton resonant scattering process in CuCl was observed in a forward scattering configuration using a picosecond tunable source. The propagation times of the polariton in the crystal was estimated at various energies near the transverse exciton. They are well fitted with the theoretical group velocity calculated from the dispersion curve of the polariton.     

Evidence of an induced polariton branch in CuCl by hyper-Raman scattering

We study the renormalization of the dispersion of excitonic polaritons, induced by a strong light source at frequency ω_l. Since biexcitons with energy E_Bi show a giant oscillator strength, this renormalization effect is located around frequencies Ω such as $\text{h}\text{?}\text{Ω +}\text{h}\text{?}\text{ω}{}_{\mathrm{l}}\text{= E}{}_{\mathrm{Bi}}$. It gives rise to an induced polariton branch, the dispersion of which depends on the frequency of the exciting light source. It manifests itself through additional emission lines in hyper-Raman scattering processes.     

Raman scattering by the upper branch polariton in ZnO

The first observation of Raman scattering from the upper branch polariton in a semiconductor is reported. The significant reduction in collecting solid angle is the main cause that allows for this observation. The experimental polariton dispersion relation ω(K) can only be accounted for when dispersion is taken into consideration.     

Dynamics of polaritons resonantly created at the upper polariton branch

We have studied the polariton relaxation dynamics in a CdTe microcavity at low temperatures after resonant excitation into the upper polariton branch (UPB). Initially, we have set a negative exciton–cavity detuning, such that the energy difference between the two polariton branches coincides with that of an LO phonon. Our experimental results reveal a sublinear dependence of the integrated emission from the lower polariton branch (LPB) with excitation power. This evidences not only an inefficient LO phonon mediated relaxation from the UPB to the LPB but also a substantial inhibition of polariton relaxation along the LPB. After that, we have progressively reduced the negative detuning, approaching the exciton–cavity resonance. Under these conditions it is possible to observe a nonlinear emission arising from K0 LPB-states similar to that observed after nonresonant excitation. Marked oscillations are present in the time evolution traces, with a period that does not depend on excitation power or detuning.     

Excitonic-biexcitonic polariton interference in thin platelet of CuCl

The spectral position in Q-space of the transmission maxima of a 0.15 μm thick CuCl single crystal with parallel plates in Z₃-excitonic resonance region, measured by Mita and Nagasawa, has been interpreted as an indication for the mutual interference effect between two propagating excitonic-biexcitonic polariton modes.     

Broadening of upper polariton branch in GaAs, GaN, and ZnO semiconductor microcavities

The well-distinguished lower polariton branches (LPBs) and upper polariton branches (UPBs) are characteristics of strong coupling in semiconductor microcavities (MCs). In practice, however, the UPBs are often broadening especially in wide-bandgap material MCs. We present in detail the possible physical mechanisms for the broadening of UPBs for different designs of MCs by numerical simulations based on GaAs, GaN and ZnO materials. The calculated results show that the UPBs of the GaN- and ZnO-based MCs will become indistinct when the thickness of optical cavity is larger than λ and 0.25λ, respectively, mainly attributed to the larger product of the absorption coefficient and the active layer thickness. In wide-bandgap materials, it would be relatively easier to observe the UPB in the case of negative exciton-cavity mode detuning due to the exciton-like UPB and lower absorption of scattering states. In addition, the inhomogeneous broadening would be an important factor causing the invisible UPB in wide-bandgap semiconductor MCs. We demonstrate that in multiple quantum well embedded ZnO-based MCs, the UPB could be well defined due to the large 2D exciton binding energy and the small product of absorption coefficient and active layer thickness. These results show that the UPBs can be properly defined in wide-bandgap semiconductor MCs by appropriate design of the MC structures.     

Dynamics of “cold polariton” near a bottleneck region in CuCl

A “cold polariton” which has very sharp distributions of enery and momentum was created in the large kinetic energy region of the lower polariton branch by using a tunable picosecond laser in CuCl. The time behavior of the induced absorption from the “cold polariton” to the excitonic molecule was studied. From these experiments, the lifetime of the polariton at various energies in the bottleneck region was determined. A rather small decrease of the lifetime with the increase of the kinetic energy of the polariton was observed. This decrease was attributed to the increase of the group velocity of the polariton. The main scattering process of the polariton in the bottleneck region was discussed.     

Parametric scattering in a system of quasi-two-dimensional exciton polaritons under photoexcitation near the bottom of the upper polariton branch

Cascade processes of exciton-polariton scattering in a planar semiconductor microcavity taking place under resonance pumping near the bottom of the upper polariton branch are studied theoretically. When the conservation laws allow the decay of the resonantly excited state into two modes that belong to different (the upper and lower) polariton branches, the distribution of scattering directions has the general shape of two rings that correspond to the cross sections of the lower and upper polariton dispersion surfaces by constantenergy planes. Due to the interactions between the particles, instability develops in the system of scattered modes, which is accompanied by marked inhomogeneities in the distribution of the cavity photoluminescence signal. Self-organization in such a system leads to the appearance of solutions of an essentially collective nature. As the critical (threshold) pump power is attained, macroscopic occupancy of a predominant signal mode near the bottom of the lower polariton branch sets in. The characteristics of the signal for different powers and optical polarizations of the pump are analyzed.     

Fast causal information transmission in a medium with a slow group velocity

It is widely believed that the velocity of information upsiloni encoded on an optical pulse is equal to the group velocity upsilong, at least when upsilong is less than the speed of light in vacuum c. On the other hand, several authors suggest that upsiloni=c, although the size of the signal traveling at this velocity may be small, thereby making it difficult to measure. Here, we measure upsiloni for pulses propagating through a resonant "slow-light" medium where upsilong approximately 0.006c. We find upsiloni=1.03c(+0.49c)-0.25c, or that upsiloni approximately 168upsilong, clearly demonstrating that the speed of information cannot be generally described by upsilong, but is characterized by its own velocity.     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

超声波在液体中相速与群速的测量

简要叙述了相速与群速的定义,并用实验测量了超声波在不同液体中的相速与群速.     

First measurement of the velocity of slow antihydrogen atoms

The speed of antihydrogen atoms is deduced from the fraction that passes through an oscillating electric field without ionizing. The weakly bound atoms used for this first demonstration travel about 20 times more rapidly than the average thermal speed of the antiprotons from which they form, if these are in thermal equilibrium with their 4.2 K container. The method should be applicable to much more deeply bound states, which may well be moving more slowly, and should aid the quest to lower the speed of the atoms as required if they are to be trapped for precise spectroscopy.     

Effect of kinesin velocity distribution on slow axonal transport

The goal of this paper is to investigate the effect that a distribution of kinesin motor velocities could have on cytoskeletal element (CE) concentration waves in slow axonal transport. Previous models of slow axonal transport based on the stop-and-go hypothesis (P. Jung, A. Brown, Modeling the slowing of neurofilament transport along the mouse sciatic nerve, Physical Biology 6 (2009) 046002) assumed that in the anterograde running state all CEs move with one and the same velocity as they are propelled by kinesin motors. This paper extends the aforementioned theoretical approach by allowing for a distribution of kinesin motor velocities; the distribution is described by a probability density function (PDF). For a two kinetic state model (that accounts for the pausing and running populations of CEs) an analytical solution describing the propagation of the CE concentration wave is derived. Published experimental data are used to obtain an analytical expression for the PDF characterizing the kinesin velocity distribution; this analytical expression is then utilized as an input for computations. It is demonstrated that accounting for the kinesin velocity distribution increases the rate of spreading of the CE concentration waves, which is a significant improvement in the two kinetic state model.     

Effect of disorder on slow light velocity in optical slow-wave structures

Slow-wave optical structures such as coupled photonic crystal cavities, coupled microresonators, and similar coupled-resonator optical waveguides are being proposed for slowing light because of the nature of their dispersion relationship. Since the group velocity becomes small, slow light and enhanced light-matter interaction may be observed at the edges of the waveguiding band. We derive a model of the effects of disorder on slow light in such structures, obtaining a relationship between the root-mean-square variation in the coupling coefficients and how slow the light is at the band edge.     

Complex branch points of the renormalization group function

Stationary points of the invariant charge Q(k²/k², g) as a function of k² are associated with specific branch points of β(g) in the complex g²-plane. In QCD, pairs of complex branch points of this type may be related to the apparent abrupt transition between weak and strong coupling regimes.     

Direct measurement of superluminal group velocity and signal velocity in an optical fiber

We present an easy way of observing superluminal group velocities using a birefringent optical fiber and other standard devices. In the theoretical analysis, we show that the optical properties of the setup can be described using the notion of "weak value." The experiment shows that the group velocity can indeed exceed c in the fiber; and we report the first direct observation of the so-called "signal velocity," the speed at which information propagates and that cannot exceed c.     

X-wave bullets with negative group velocity in vacuum

Propagation-invariant, X waves with negative group velocity are reported. Beam aperturing allows for a comprehensive analysis concerning the causality of the optical signal and forerunner formation.     

Controlling group velocity in a superconductive quantum circuit

We investigate the controllable group velocity of a microwave probe field in a superconductive quantum circuit(SQC) pumped by microwave fields,and the use of such a SQC function as an artificial Λ-type three-level atom.The exchange between the subluminal and the superluminal states of the probe field can be realized simply by sweeping the pumping intensity,and the superluminal state is usually realized with a lower absorption.This work is one of the efforts to extend the study of electromagnetically induced transparency and its related properties from the lightwave band to the microwave band.     

Low-temperature group velocity and elasticity in CaWO4

The CRESST experiment looks for evidence of dark matter particles colliding with nuclei in CaWO₄, using cryogenic bolometers sensitive to energy deposition (～10 keV) with high accuracy. Calibration of the energy deposited in the phonon system depends upon the details of the evolution of the non-equilibrium energy in the CaWO₄ absorber. Phonon images sensitively reveal variations in angular phonon flux in a single-crystal sample and provide accurate data for the group velocity along non-symmetry directions. Our measurements, over a wide range of propagation angles, provide new and precise values for phonon group velocities and uncover significant discrepancies between the experimentally observed transport of non-equilibrium energy and that predicted using values of the low-temperature elastic constants widely cited in the literature.