Strong Coherent Field Research Articles

Four-Wave Mixing in Metamaterials

Interaction of four counterpropagating waves in a cubic medium being “left” for the signal wave is considered in the constant-intensity approximation. Analytical expression for the signal wave intensity is derived for the general case of four-wave interaction in a metamaterial. The influence of different parameters on the signal wave amplification coefficient, efficiency of conversion into the signal wave, and the reflection coefficient of the mirror whose role is played by the metamaterial is analyzed. It has been obtained for the first time that the determining role in the backward signal wave amplification is played by the total length of the metamaterial and the intensities of all three forward waves. An analysis has shown that the optimal thickness of the metamaterial depends not only on the phase mismatch and the strong coherent pump field intensity, as in the constant-field approximation, but also on the intensity of the weak wave at the frequency ω2. It has been demonstrated for the first time that the efficiency of frequency conversion in metamaterials depends on the total metamaterial length, input intensities of all four interacting waves, phase mismatch, and losses in the medium. It is established that the maxima of the reflection coefficient of the metamaterial depend on the total metamaterial length and the intensities of all three forward waves.

Impact of electron-electron Coulomb interaction on the high harmonic generation process in graphene

Generation of high harmonics in a monolayer graphene initiated by strong coherent radiation field, taking into account electron-electron Coulomb interaction is investigated. A microscopic theory describing the nonlinear optical response of graphene is developed. The Coulomb interaction of electrons is treated in the scope of dynamic Hartree-Fock approximation. The closed set of integrodifferential equations for the single-particle density matrix of a graphene quantum structure is solved numerically. The obtained solutions show the significance of many-body Coulomb interaction on the high harmonic generation process in graphene.

Magnetic-field-dependent slow light in strontium atom-cavity system

Realizing and controlling a long-lived slow light is of fundamental importance in physics and may find applications in quantum router and quantum information processing. In this work, we propose a feasible scheme to realize the slow light in a strontium atom-cavity system, in which the value of group delay can be continuously adjusted within a range of different Zeeman splittings and vacuum Rabi frequencies by varying the applied static magnetic field and the atom number instead of a strong coherent field. In our scheme, the major limitations of the slow-light structure, namely, dispersion and loss, can be effectively resolved, and so our scheme may help to achieve the practical application of slow light relevant to the optical communication network.

Improvement on reflectivity of tunable photonic band gap with spontaneous generated coherence

The photonic band gap is a spectral range which cannot propagate in a periodic optical nanostructure, that is, the structure itself has a “forbidden band”. It has been successfully applied to the filters, amplifiers, mixers, etc. As is well known, dynamically tunable photonic band gaps in cold atomic lattices are of great importance in various research fields. However, the photonic band gaps of a traditional photonic crystal are non-tunable because the periodic structure is determined once the photonic crystal is grown. On the other hand, a majority of previous researches focused on improving the reflectivity of photonic band gap, which can only keep approaching to 1. Due to the action of the vacuum of the radiation field, near-degenerate lower level has an additional coherence term, the spontaneously generated coherence term. In this paper, we consider a three-level ∧-type atomic system driven by a strong coherent field, a weak coherent field and an incoherent pump, in which the two ground states are of hyperfine structure. The one-dimensional photonic band gaps are formed by cold atoms trapped in a one-dimensional-ordered optical lattice and this system may create two photonic band gaps (PBGs). The trapped cold atoms have a Gaussian density distribution in each period as determined by the optical potential depth and the average atomic temperature. We investigate in detail how the reflectivities of the two PBGs are influenced by the coherent effect of spontaneously generated coherence. Then, we find that the reflectivities of the two band gaps can be significantly improved by the spontaneously generated coherence. The reflectivities of such two band gaps can be dynamically manipulated by varying the intensity of incoherent driving field and the relative phase between the probe field and the coupling field, which cannot be realized in a conventional ∧-type atomic system. Besides, by adjusting the parameters appropriately, the reflectivities of these two band gaps can be higher than 1, which is because probe field gain stems from the spontaneously generated coherence. In the future, photonic transport properties can be investigated in the three-dimensional atomic lattices and this work is meaningful for the optical routing, photodiode and transistor.

Influence of strongly textured microstructure on the all-round shape memory effect of rapidly solidified Ni51Ti49 alloy

Ni51Ti49 alloy strip with all-round shape memory effect (ARSME) was obtained through rapid solidification followed by constraint-aging treatment. The high cooling rate and large temperature gradient along the thickness direction of Ni51Ti49 alloy strip yield fine columnar grains and strong fiber texture of <100>B2 during rapid solidification. Such fine-grained and strongly textured microstructure increases the nucleation, limits the orientation and restricts the growth of Ni4Ti3 precipitates during constraint-aging treatment. The fine and well-aligned Ni4Ti3 precipitates with dispersive distribution introduce strong coherent stress field and numerous interfaces between the matrix and precipitates in the rapidly solidified and constraint-aged Ni51Ti49 alloy. Homogeneous composition and massive interfaces in the B2 matrix promote the simultaneity of local phase transformations. The strong coherent stress field and the excellent deformability of <100>B2-oriented grains contribute to large deformation and high recovery ratio of the alloy during phase transformation. Thus, the strongly textured microstructure formed during rapid solidification allows the constraint-aged NiTi alloy to possess superior ARSME with narrow phase transformation temperature range.

Enhanced detection of a low-frequency signal by using broad squeezed light and a bichromatic local oscillator

We experimentally study a protocol of using the broadband high frequency squeezed vacuum to detect the low-frequency signal. In this scheme, the lower sideband field of the squeezed light carries the low frequency modulation signal and the two strong coherent light fields are applied as the bichromatic local oscillator in the homodyne detection to measure the quantum entanglement of the upper and lower sideband for the broadband squeezed light. The power of one of the local oscillators for detecting the upper sideband can be adjusted to optimize the conditional squeezing in the low frequency regime by subtracting the photocurrent of the upper sideband field of the squeezed light from that of the low sideband field. By means of the quantum correlation of the upper and lower sideband for the broadband squeezed light, the low frequency signal beyond the standard quantum limit is measured. This scheme is appropriate for enhancing sensitivity of the low frequency signal by the aid of the broad squeezed light, such as gravitational waes detection, and does not need directly produce the low frequency squeezing in the optical parametric process.

Quantum transport in a two-level quantum dot driven by coherent and stochastic fields

Abstract We study theoretically the current and shot noise properties flowing through a two-level quantum dot driven by a strong coherent field and a weak stochastic field. The interaction x(t) between the quantum dot and the stochastic field is assumed to be a Gaussian-Markovian random process with zero mean value and correlation function 〈 x ( t ) x ( t ′ ) 〉 = D κ e − κ | t − t ′ | , where D and κ are the strength and bandwidth of the stochastic field, respectively. It is found that the stochastic field could enhance the resonant effect between the quantum dot and the coherent field, and generate new resonant points. At the resonant points, the state population difference between two levels is suppressed and the current is considerably enhanced. The zero-frequency shot noise of the current varies dramatically between sub- and super-Poissonian characteristics by tuning the stochastic field appropriately.

Revisiting the four-level inverted-Y system under both Doppler-free and Doppler-broadened conditions: an analytical approach

We report the occurrence of electromagnetically induced transparency (EIT) in the simulated probe response signal for a four-level inverted-Y type system that is being acted upon by a weak coherent probe field, a strong coherent pump field and a coherent repump field. There are two ground energy levels, one intermediate energy level and one uppermost energy level. The weak probe field couples the lowest ground level to the intermediate level whereas the repump field connects the other ground level with the intermediate level. The strong control field couples the intermediate level with the uppermost energy level, thereby forming an inverted-Y type system. The density matrix based theoretical model has been developed and solved analytically for this four-level system and the probe response signal has been simulated at different values of the control and repump Rabi frequencies, control and repump frequency detunings and under both Doppler-free and Doppler-broadened conditions using the parameters of 87Rb D2 transition. Extremely low line width (few tens of kHz) for the EIT signal has been noticed under thermal averaging for copropagating probe, control and repump field configuration. The EIT signal is found to be immune to the variation in the control Rabi frequency.

Interplay of classical and quantum dynamics in a thermal ensemble of atoms

In a thermal ensemble of atoms driven by coherent fields, how does evolution of quantum superposition compete with classical dynamics of optical pumping and atomic diffusion? Is it optical pumping that first prepares a thermal ensemble, with coherent superposition developing subsequently or is it the other way round: coherently superposed atoms driven to steady state via optical pumping? Using a stroboscopic probing technique, here we experimentally explore these questions. A 100 ns pulse is used to probe an experimentally simulated, closed three-level, Λ-like configuration in rubidium atoms, driven by strong coherent (control) and incoherent fields. Temporal evolution of probe transmission shows an initial overshoot with turn-on of control, resulting in a scenario akin to lasing without inversion. The corresponding rise time is dictated by coherent dynamics, with a distinct experimental signature of half-cycle Rabi flop in a thermal ensemble of atoms. Our results indicate that, in fact, optical pumping drives the atoms to a steady state in a significantly longer time-scale that sustains superposed dark states. Eventual control turn-off leads to a sudden fall in transmission with an ubiquitous signature for identifying closed and open systems. Numerical simulations and toy-model predictions confirm our claims. These studies reveal new insights into a rich and complex dynamics associated with atoms in thermal ensemble, which are otherwise absent in state-prepared, cold atomic ensembles.

Gauge turbulence, topological defect dynamics, and condensation in Higgs models

The real-time dynamics of topological defects and turbulent configurations of gauge fields for electric and magnetic confinement are studied numerically within a 2+1D Abelian Higgs model. It is shown that confinement is appearing in such systems equilibrating after a strong initial quench such as the overpopulation of the infrared modes. While the final equilibrium state does not support confinement, metastable vortex defect configurations appear in the gauge field which are found to be closely related to the appearance of physically observable confined electric and magnetic charges. These phenomena are seen to be intimately related to the approach of a non-thermal fixed point of the far-from-equilibrium dynamical evolution, signalled by universal scaling in the gauge-invariant correlation function of the Higgs field. Even when the parameters of the Higgs action do not support condensate formation in the vacuum, during this approach, transient Higgs condensation is observed. We discuss implications of these results for the far-from-equilibrium dynamics of Yang-Mills fields and potential mechanisms how confinement and condensation in non-abelian gauge fields can be understood in terms of the dynamics of Higgs models. These suggest that there is an interesting new class of dynamics of strong coherent turbulent gauge fields with condensates.

Analysis of light propagation in an anomalously dispersive medium

We analyse the propagation of a probe laser pulse though a medium of atoms in the double Λ configuration with two closely spaced upper levels. The medium is irradiated with a strong coherent field and is additionally incoherently pumped. Depending on the parameters of the system the dispersion of the medium may change alternatively from normal to anomalous with absorption varying from positive to negative (gain). The incoherent optical pump was found to be the switch between the sub- and the superluminal regime. A laser field propagating through such an optical system can experience alternatively the negative or positive group velocity. The dynamics of propagation in the case of positive but larger than c group velocities is investigated in particular. In this paper we show how the description in terms of the group velocity breaks down in the superluminal regime.

Phase control of light amplification in steady and transient processes in an inverted-Y atomic system with spontaneously generated coherence

We investigate the effects of spontaneously generated coherence (SGC) on both the steady and transient gain properties in a four-level inverted-Y-type atomic system in the presence of a weak probe, two strong coherent fields, and an incoherent pump. For the steady process, we find that the inversionless gain mainly origins from SGC. In particular, we can modulate the inversionless gain by changing the relative phase between the two fields. Moreover, the amplitude of the gain peak can be enhanced and the additional gain peak can appear by changing the detuning of the coupling field. As for the transient process, the transient gain properties can also be dramatically affected by the SGC. Compared to the case without SGC, the transient gain can be greatly enhanced with completely eliminated transient absorption by choosing the proper relative phase between the two fields. And the inverted-Y-type system with SGC can be simulated in both atomic and semiconductor quantum well systems avoiding the conditions of SGC.

Enhanced index and negative dispersion without absorption in semiconductor quantum wells

We investigate the dispersive and absorptive properties in a three-level ladder-type SQW system that both cascade transitions are driven by the same strong coherent field and are probed by the same weak probe field. It is found that the positive or negative refractive index with vanishing absorption can be also easily obtained in this solid-state system. Our scheme shows many intrinsic characteristics that other schemes do not have, which may provide some possibilities for the technological applications in solid-state optoelectronics.

Anomalous baryon production and its interplay with jet energy loss at RHIC and LHC energies

Hadron production consists traditionally of two main channels in the intermediate-pT region at RHIC and LHC energies: parton coalescence/recombination and jet fragmentation. We have studied a new channel, namely hadron production connected to the appearance of strong time-dependent coherent field in heavy ion collisions. In the momentum window of 5 GeV/c <pT < 25 GeV/c, this channel could yield an anomalous baryon enhancement. This extra yield is able to explain the measured structure of nuclear modification factors for identified hadrons at RHIC energies. We give predictions for LHC energies.

Negative refraction without absorption via both coherent and incoherent fields in a four-level left-handed atomic system

This paper attempts a probe into negative refraction without absorption by means of an incoherent pump field and a strong coherent field coupling the dense four-level atomic system. With the application of the incoherent pump field to manipulate the populations in atomic levels and the variable strong coherent field to create quantum coherence, the constraint condition of two equal transition frequencies responding to the probe field in the atomic system isn't required. And these lead to the propagation transparency and strong magnetic response of the probe field, left-handedness with vanishing absorption in the atomic system. However, an excessive coherent field intensity would increase the absorption.

Electromagnetically induced left-handedness by both coherent and incoherent fields

We propose a scheme of electromagnetically induced left-handedness in a dense atomic vapor medium. By applying an incoherent pump field to manipulate the populations in atomic levels, and a strong coherent field to create quantum coherence, the magnetic part of the probe field can be amplified instead of strong absorption as usual materials. The system has negative permittivity and permeability simultaneously with transparent propagation of the electric part of the probe field. It is shown that the pumping rate of incoherent field, the intensity and frequency detuning of the coherent field, exert effects on the properties of the negative refractive index of left-handed material.

VIC Effect and Phase-Dependent Optical Properties of Five-Level K-Type Atoms Interacting with Coherent Laser Fields

Due to interaction with the vacuum of the radiation field, a K-type atomic system with near-degenerate excited and ground levels, which is driven by two strong coherent fields and two weak probe fields, has additional coherence terms — the vacuum-induced coherence (VIC) terms. In this paper, we find that, if the interference is optimized, the two-photon absorption properties of this atom system can be significantly modified and electromagnetically-induced transparency (EIT) is dependent on this interference. Furthermore, we find that in all the cases the coherence can suppress or enhance the partial two-photon transparency, while the complete transparency window is still strictly preserved, which means that it cannot be affected by the VIC. Another important result is the finding of the crucial role played by the relative phase between the probe and coupling fields: the relative height of absorption peaks can be modulated by the relative phase. The physical interpretation of the phenomena has been given.

Spectral feature with sub-natural linewidth due to quantum interference in a four-level system

This paper studies the narrow spectral feature appearing in a four-level system coupled by two strong coherent fields and probed by a weak laser field. The linewidth is examined as a function of the Rabi frequencies of coupling fields, and the result is explained by using the dressed-state formalism.

Lasing spaser

In 2003 Bergman and Stockman introduced the spaser, a quantum amplifier of surface plasmons by stimulated emission of radiation1. They argued that by exploiting a metal/dielectric composite medium it should be possible to construct a nanodevice, where a strong coherent field is built up in a spatial region much smaller than the wavelength1,2. V-shaped metallic structures, combined with semiconductor quantum dots, were discussed as a possible realization of the spaser1. Here we introduce a further development of the spaser concept. We show that by combining the metamaterial and spaser ideas one can create a narrow-diversion coherent source of electromagnetic radiation that is fuelled by plasmonic oscillations. We argue that a two-dimensional array of a certain class of plasmonic resonators supporting coherent current excitations with high quality factor can act as a planar source of spatially and temporally coherent radiation, which we term a ‘lasing spaser.’ The ‘spaser’ (surface plasmon amplification by stimulated emission of radiation) is a relatively new and exciting concept analogous to the laser. It involves amplifying specific surface plasmon modes using a nanoscale device. Zheludev and co-workers extend this concept by suggesting that metamaterials could be used to create a lasing spaser, that is, a spaser that can emit light with high spatial coherence.

Tunable all-optical bistability in a semiconductor quantum dot damped by a phase-dependent reservoir

We propose a method of frequency and phase control of optical bistability in a unidirectional ring cavity containing a semiconductor structure which is characterized as a ladder three-level system. The system interacts with a coherent probe field, and a control field which consists of a strong coherent field and a weak amplitude-fluctuating stochastic field. A perturbative solution of the master equation of the system allows to eliminate the stochastic field and provides a physical picture in terms of correlation properties of the stochastic field. We find that the bistable response can be modified strongly by means of the amplitude, the frequency and the phase of the stochastic field. In order to illustrate the feasibility of the results, we use parameter values corresponding to an semiconductor quantum dot (QD). This investigation may be used to optimize and control the optical switching process in the QD solid-state system, which is much more practical than that in atomic systems.