Effect of quantum interference on a three-level atom driven by two laser fields

Abstract

We study the effect of quantum interference on the population distribution and absorptive properties of a V-type three-level atom driven by two lasers of unequal intensities and different angular frequencies. Three coupling configurations of the lasers to the atom are analysed: (a) both lasers coupled to the same atomic transition, (b) each laser coupled to different atomic transition and (c) each laser coupled to both atomic transitions. Dressed states for the three coupling configurations are identified, and the population distribution and absorptive properties of the weaker field are interpreted in terms of transition dipole moments and transition frequencies among these dressed states. In particular, we find that in the first two cases there is no population inversion between the bare atomic states, but the population can be trapped in a superposition of the dressed states induced by quantum interference and the stronger field. We show, that the trapping of the population, which results from the cancellation of transition dipole moments, does not prevent the weaker field to be coupled to the cancelled (dark) transitions. As a result, the weaker field can be strongly amplified on transparent transitions. In the case of each laser coupled to both atomic transitions the population can be trapped in a linear superposition of the excited bare atomic states leaving the ground state unpopulated in the steady state. Moreover, we find that the absorption rate of the weaker field depends on the detuning of the strong field from the atomic resonances and the splitting between the atomic excited states. When the strong field is resonant to one of the atomic transitions a quasi-trapping effect appears in one of the dressed states. In the quasi-trapping situation all the transition dipole moments are different from zero, which allows the weaker field to be amplified on the inverted transitions. When the strong field is tuned halfway between the atomic excited states, the population is completely trapped in one of the dressed states and no amplification is found for the weaker field.     

Quantum interference induced sub-wavelength atomic localization

We investigate how observation of upper state atomic populations localize atomic position distributions for three-level atoms within a classical standing wave light field. We consider a three-level atom, with the standing wave near-resonantly coupling one transition and a probe laser field near-resonantly coupling the second transition. Two different cases of localization are identified and we explore the dependence of localization on the parameters of the field–atom interaction.     

The Effect of Continuum-continuum Transitions on Laser-induced Continuum Structures

Abstract

Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.     

A Double Resonance Method for Studying Collision Rates in an Intense Laser Field

Abstract

We propose a method for studying the modification of atomic collision rates in the presence of an intense laser field. This method employs a three-level lambda-system driven by two laser pulses: one strong and the other weak. The weak laser is used to populate a particular dressed state produced by the intense laser at some time during the pulse history. We show that this point can be selected by tuning the frequency of the weak laser. The spectrum of the emission produced by the dressed state is then recorded. Measuring this spectrum, both in the presence and in the absence of collisions, enables one to deduce the dependence of the collision rates on laser intensity. The method has other advantages discussed in the text.     

Spectrum modification of XUV radiation in the presence of a delayed weak control field

We use a delayed weak laser beam to control the spectral features of extreme ultraviolet (XUV) pulses generated by a strong femtosecond laser beam through phase-matched high harmonic generation (HHG) in an atomic medium, e.g. krypton. The variation of the HHG spectrum reveals the influence of the free electrons on the propagation of the XUV field in the medium. In addition, a signature of the autoionization process is visible. Our findings provide a promising technique to study ultrafast dynamics of atomic and molecular gases.     

Phase shift caused by microwave field based on light storage

We have theoretically investigated the phase shift of a probe field for a four-level atomic system interacting successively with two fields tuned near an EIT resonance of an atom, a microwave field, and a coupling field. It has been found that the phase of retrieved signal has been shifted due to the cross-phase modulation when the stored spin wave was disturbed by a microwave. Because of the low relaxation rates of the ground hyperfine state, our proposed technique can impart a large phase rotation onto the probe field with low absorption of retrieved field and very low intensity of the microwave field.     

Intensity dependence suppression and enhancement of four-wave mixing in a micrometric thin vapour

We theoretically investigate dressed-four-wave mixing (dressed-FWM) spectroscopy of rubidium atoms in a micrometric thin vapour. It is found that Dike-narrowing type Autler–Townes (AT) spectroscopy with high resolution can be achieved in a reverse Y-type four-level atomic system due to the phase-conjugated configuration of laser beams and the transient effects of atom–wall collision in the thin vapour. We also show that controllable suppression and enhancement of the dressed-FWM signal due to the evolution of atomic coherence can be obtained by selecting different coupling field intensities at the proper detuning of the probe and the coupling fields. This control of FWM processes can be interpreted by dressed state analysis and probably used in the design of optical switch and the enhancement of FWM processes for frequency conversion.     

Three-photon electromagnetically induced absorption and transparency in an inhomogeneously broadened atomic vapour

We study both theoretically and experimentally three-photon electromagnetically induced transparency and electromagnetically induced absorption resonances in inhomogeneously broadened 85 Rb atomic vapour driven by probe and drive laser radiations. We observe narrow Doppler-free absorption as well as transmission resonances for the probe field when the driving laser field is redshifted from the D₁ or D₂ lines of ⁸⁵Rb; the frequency difference between the drive and probe fields is equal to the hyperfine splitting of the ground state of the atoms, and the probe field is tuned to the centre of the Doppler broadened atomic transition. We theoretically study the spectroscopic effect in both homogeneously and inhomogeneously broadened media. Our numerical simulations are in good agreement with the experimental results.     

Population trapping in the Jaynes-Cummings model with a Kerr nonlinearity in the cavity

Abstract

An electromagnetic field state is found which maintains the population inversion of the atom stationary during the interaction with the field through a Jaynes-Cummings model (JCM) with a Kerr type nonlinearity in the cavity. The condition of stationarity of the population inversion includes the phase coupling of atomic dipole with the field. We have shown that the Kerr nonlinearity in the cavity field significantly modifies the photon statistics of the trapped field state through an intensity dependent detuning in the field compared to the normal JCM trapping state. We have also demonstrated the novel features of sub-Poissonian character and the squeezing of the trapped field state. The dynamics of the initial trapped field is studied in terms of squeezing and the Q-function.     

N-level Atom Interacting with Single-mode Radiation Field: An Exactly Solvable Model with Multiphoton Transitions and Intensity-dependent Coupling

Abstract

We study the dynamics of an N-level atom coupled in a lossless cavity to a single-mode near-resonant quantized field. The atomic levels are coupled by the multiphoton transitions and the coupling constants between the field and the atomic levels are supposed to be intensity dependent. We find the exact solution for the state vector describing the dynamics of the atom-plus-field system. As an illustration we use the model for studying (i) the time evolution of the atomic occupation probability with the initially coherent field and (ii) the light squeezing, when the cavity field is initially in the vacuum state and the atom is prepared in the atomic ‘coherent state’ (a superposition of atomic states).     

Effect of a Squeezed Vacuum on Coherent Population Trapping in a Three-level Lambda System

Abstract

Master equation methods are used to investigate the effects of a broad-band squeezed vacuum on a three-level atom of the lambda configuration. The two-mode squeezed vacuum is treated as a Markovian reservoir in a non-stationary phase-dependent state. In addition to the squeezed vacuum the atom is driven by two coherent laser fields each of which, depending on the polarization, can couple to one or both of the atomic transitions. We show that in general the optical Bloch equations for the atomic density matrix elements have oscillatory coefficients, thereby necessitating the use of Floquet methods. For the case of equal laser frequencies, which are also equal to the carrier frequency of the squeezed vacuum, the coefficients of the Bloch equations become time independent and stationary solutions for the populations and coherences are obtained by standard matrix methods. For the ordinary vacuum the usual coherent population trapping effect at two-photon resonance is obtained, with the upper state population being zero. An unsqueezed thermal field partially destroys the trapping effect as the upper state population is no longer zero at two-photon resonance. The squeezed vacuum has the effect of improving the trapping in that the coherence hole becomes more pronounced for some values of the relative phase between the squeezed vacuum and the driving fields. The additional effects of a coherence transfer rate between the two optical coherences, which occurs for special choices of angular momentum quantum numbers are also studied. For the case of equal laser frequencies, the inclusion of this coherence transfer process destroys population trapping and reduces the lambda system to a two-level system. However, for the case of unequal laser frequencies, the coherence transfer process in combination with the squeezed vacuum can restore to some extent the population trapping. We show that other features that do not occur for two-level atoms, such as stationary population inversions between pairs of the atomic levels, also depend on the relative phase and can be enhanced in the squeezed vacuum. In the case of unequal frequencies of the driving fields the population in the upper state depends on the relative phase only when the carrier frequency of the squeezed vacuum is equal to one of the two frequencies of the driving fields. When the carrier frequency of the squeezed vacuum is slightly detuned from both frequencies of the driving fields, the population in the upper state is insensitive to the relative phase but is dependent on the degree of squeezing. For large detunings, the population does not show any dependence on the degree of squeezing and its distribution in function of the two-photon detuning is similar to that in the thermal vacuum field.     

Magnetoelectric coupling at metal surfaces

Magnetoelectric coupling allows the magnetic state of a material to be changed by an applied electric field. To date, this phenomenon has mainly been observed in insulating materials such as complex multiferroic oxides. Bulk metallic systems do not exhibit magnetoelectric coupling, because applied electric fields are screened by conduction electrons. We demonstrate strong magnetoelectric coupling at the surface of thin iron films using the electric field from a scanning tunnelling microscope, and are able to write, store and read information to areas with sides of a few nanometres. Our work demonstrates that high-density, non-volatile information storage is possible in metals.     

Transport in Vortex State of d-Wave Superconductors at Zero Temperature: Wiedemann–Franz Violation

We show that the Wiedemann–Franz law is violated at zero temperature in the vortex state of a d-wave BCS superconductor with isotropic impurity scattering. We use a semiclassical approach to include the Doppler shift experienced by the quasiparticles due to the circulating supercurrents and consider as well the Andreev scattering from an array of vortices assumed to be randomly distributed. We also show that the vertex corrections to the electric conductivity, which can be large when there is significant anisotropy in the impurity scattering, become unimportant as the magnetic field is increased. For the thermal conductivity, the corrections remain negligible as in the absence of a magnetic field.     

A novel method to compensate for the effect of light shift in arubidium atomic clock pumped by a semiconductor laser

Precise measurement of the shift (i.e. microwave frequency shift induced by the electric field of the pumping light) in a rubidium atomic clock pumped by a semiconductor laser is discussed. The spectral lineshape of the microwave resonance, which is used as a frequency discriminator for the atomic clock in the optical microwave double resonance experiment, depends strongly on the spatial distribution of the laser beam intensity, laser frequency detuning, and modulation parameters of the microwave frequency. Based on measurements of the deformation of the resonance lineshape, a self-tuning system was built to compensate for the effect of light shift. As a result of controlling the laser frequency with this system, long-term drift of the microwave frequency as low as 6.3×10^-13/h was obtained     

Linear and nonlinear optical properties in an asymmetric double quantum well under intense laser field: Effects of applied electric and magnetic fields

In the present study, the effects of electric and magnetic fields on the linear and third-order nonlinear optical absorption coefficients and relative change of the refractive index in asymmetric GaAs/GaAlAs double quantum wells under intense laser fields are theoretically investigated. The electric field is oriented along the growth direction of the heterostructure while the magnetic field is taken in-plane. The intense laser field is linear polarization along the growth direction. Our calculations are made using the effective-mass approximation and the compact density-matrix approach. Intense laser effects on the system are investigated with the use of the Floquet method with the consequent change in the confinement potential of heterostructures. Our results show that the increase of the electric and magnetic fields blue-shifts the peak positions of the total absorption coefficient and of the total refractive index while the increase of the intense laser field firstly blue-shifts the peak positions and later results in their red-shifting.     

Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor

We demonstrate a robust method of stabilizing a diode laser frequency to an atomic transition. This technique employs the Zeeman shift to generate an antisymmetric signal about a Doppler-broadened atomic resonance, and therefore offers a large recapture range as well as high stability. The frequency of a 780-nm diode laser, stabilized to such a signal in Rb, drifted less than 0.5 MHz peak-peak (1 part in 10(9)) in 38 h. This tunable frequency lock can be constructed inexpensively, requires little laser power, rarely loses lock, and can be extended to other wavelengths by use of different atomic species.     

Transport in Vortex State of <Emphasis Type="Italic">d</Emphasis>-Wave Superconductors at Zero Temperature: Wiedemann–Franz Violation

We show that the Wiedemann–Franz law is violated at zero temperature in the vortex state of a d-wave BCS superconductor with isotropic impurity scattering. We use a semiclassical approach to include the Doppler shift experienced by the quasiparticles due to the circulating supercurrents and consider as well the Andreev scattering from an array of vortices assumed to be randomly distributed. We also show that the vertex corrections to the electric conductivity, which can be large when there is significant anisotropy in the impurity scattering, become unimportant as the magnetic field is increased. For the thermal conductivity, the corrections remain negligible as in the absence of a magnetic field.     

Lasing without population inversion in an Er3+-doped YAG crystal

Two atomic models are proposed for an Er³⁺-doped YAG crystal with application to lasing with and without population inversion. It is shown how an incoherent pumping field and coherent control coupling field can produce a laser in the presence and absence of population inversion.     

Irradiated bilayer graphene

We describe the gated bilayer graphene system when it is subjected to intense terahertz frequency electromagnetic radiation. We examine the electron band structure and density of states via exact diagonalization methods within Floquet theory. We find that dynamical states are induced which lead to modification of the band structure. We first examine the situation where there is no external magnetic field. In the unbiased case, dynamical gaps appear in the spectrum which manifest as dips in the density of states. For finite inter-layer bias (where a static gap is present in the band structure of unirradiated bilayer graphene), dynamical states may be induced in the static gap. These states can show a high degree of valley polarization. When the system is placed in a strong magnetic field, the radiation induces coupling between the Landau levels which allows dynamical levels to exist. For strong fields, this means the Landau levels are smeared to form a near-continuum of states.     

Permanently disentangled states of atom–field system via spontaneously generated coherence

Abstract

The effect of spontaneously generated coherence on evolution of the entanglement between a driven four-level Y-type atom and its spontaneous emission field is studied. We have shown that the atom will be entangled to its spontaneous emission field due to spontaneously generated coherence and coherent population trapping at the steady state. It is found that the degree of entanglement strongly depends on the initial atomic state. So, it can be controlled by the pumping laser pulses used for preparing an initial atomic system. More interestingly, the atom–field system can be found in a permanently disentangled state for a properly prepared atom.