Strong Pump Field Research Articles

A study of the ac Stark effect in doped photonic crystals

In this paper we present calculations of level populations and susceptibility for an ensembleof five-level atoms doped in a photonic crystal, using the master equation method. Theatoms in the ensemble interact with the crystal which acts as a reservoir and are coupledwith two strong pump fields and a weak probe field. It is found that, by manipulating theresonance energy associated with one of the decay channels of the atom, the system can beswitched between an inverted and a non-inverted state. We have also observed theac Stark effect in these atoms and have shown that due to the role played bythe band structure of the photonic crystal, it is possible to switch between anabsorption state and a non-absorption state of the atomic system. This is a veryimportant finding as techniques of rendering material systems transparent to resonantlaser radiation are very desirable in the fabrication of novel optical and photonicdevices.

Theory of a two-mode entanglement amplifier

We develop a theory of two-mode entanglement amplifier in a cascade three-level atomic system. Two photons of a strong external pump field induce atomic coherence between the top and bottom levels. We study the dynamics of the system in the presence of losses, concluding that the creation of strongly entangled states with very large photon numbers seems achievable.

Nonlinear optical effects in a doubly driven four-level atom

We have studied nonlinear quantum optical effects in a doubly driven four-level atomic system in a homogeneously broadened regime. On application of two strong fields the system can be made to display double electromagnetically induced transparency (EIT) because of constructive quantum interference giving rise to nonlinear absorption at three-photon resonance condition. This fact may be used to obtain other phenomena like the effect of photon switching in a pulsed regime and large cross phase modulation (XPM) effect. We also show that the behaviour of spatial modulation of transparency of the weak probe beam in a three-level system interacted with a strong standing wave pump field can be dramatically changed in our four-level ladder type system. Here, strong coupling field acting on the uppermost transition is represented by a standing wave whereas strong pump and weak probe fields acting successively on the lower transitions are represented by travelling waves. The effect of double EIT on nonlinear light generation based on the four-wave-mixing (FWM) technique is also investigated. We have examined that the four wave mixing efficiency can be effectively controlled by varying the strength of the pump field.

X-ray detected ferromagnetic resonance in thin films

We discuss the physical content of X-ray Detected Magnetic Resonance (XDMR), i.e. a novel spectroscopy which uses XMCD to probe the resonant precession of the local magnetization in a strong microwave pump field. We focus on the simplest case of a steady-state precession of elemental moments in the non-linear regime of angular foldover. Like XMCD, XDMR is element and edge selective and could become a unique tool to investigate how precessional dynamics can locally affect the spin and orbital magnetization of p- or d-projected DOS. This should be possible only in the limit where there is no overdamping due to ultrafast orbit-lattice relaxation.

Superluminal optical solitons in a four-level tripod atomic system

With one weak probe field and two strong pumping fields, superluminal optical solitons are formed in a lifetime-broadened four-level tripod atomic medium. With proper parameters, both dark and bright solitons can occur in the highly resonant medium. The corresponding group velocity of the solitons can be superluminal. Meanwhile, the conditions for superluminal solitons occurrence are given.

Laser-induced birefringence in a wavelength-mismatched cascade system of inhomogeneously broadened Yb atoms

We report the observation of laser induced birefringence (LIB) in a wavelength-mismatched cascade system (J=0 \leftrightarrow J=1 \leftrightarrow J=0 transitions) of inhomogeneously broadened ytterbium atoms with strong pump and probe fields. We investigate the transmission spectrum of two circular polarization (\sigma_p^+ and \sigma_p^-) components of strong probe field at fixed frequency, depending on the detuning of circularly polarized (\sigma_c^-) coupling field from two-photon resonance. We find that \sigma_p^+ (\sigma_p^-) polarized component exhibits a narrow electromagnetically induced absorption (transparency) spectrum. Numerical solutions of density matrix equations show qualitative agreement with experimental results. A Doppler-free dispersive LIB signal is obtained by detecting the Stokes parameter of the probe field, enabling us to stabilize the frequency of coupling laser without frequency modulation.

Coherent transient in dressed-state and transient spectra of Autler-Townes doublet

In this paper we present a theoretical study of the time-dependent probe response in the presence of a strong pump field in a three-level pump-probe configuration. Two situations are investigated: a cw pump with a pulsed probe field and a pulsed pump with a cw probe field. The results are explained as dressed-state nutation and nutation by dynamic Stark switching. Dressed-state quantum beats are also an important feature for both situations. Furthermore, when a 90 deg. phase shift after a {pi}/2 period is introduced in the pulsed probe field, there is a spin locking in the dressed-state transition. Our results give a satisfactory theoretical account of a previous experimental observation [Wei et al., Phys. Rev. Lett. 74, 1083 (1995)].

Wavelength conversion and pulse reshaping through cascaded interactions in an MZI configuration

An optical wavelength converter based on second-order cascaded interactions in a waveguide is presented. The conversion mechanism relies on the combination of amplification and nonlinear dephasing of a signal field in the presence of a strong pump field at the phase-matching wavelength for second-harmonic generation. The considered scheme of the device is that of a Mach-Zehnder interferometer. The performance is numerically evaluated in the pulsed regime. A wide conversion bandwidth and high conversion efficiency is found. An operating regime can be found in which a significant reshaping of the output pulses, with respect to input pulses, can be observed.

Nonstationary parametric amplification of polychromatic radiation propagating in an extended absorbing resonant medium

An experimental and theoretical study of the two-wave interaction (of a probe wave and a pumping wave) of wide-band multimode laser pulses with an optically dense extended resonant medium (metastable neon atoms in a positive glow-discharge column) is presented. Depending on the pumping power and characteristics of the medium, various regimes of interaction between the field and the matter, leading to qualitatively different types of amplification spectra, are realized: a cooperative interaction regime at low pumping powers corresponds to amplification spectra consisting of two peaks, which lie on both sides of the absorption line; in the regime of nonstationary Rabi oscillations (Rabi flopping), the amplification appears in the form of an isolated peak at a frequency close to the resonance in the substance; under certain interaction conditions, a dispersive shape of amplification spectra is observed. A significant gain in the probe field (by several times at a length of 10 cm) is obtained as a result of a nonstationary population inversion upon rotation of the Bloch vector of the resonant medium under the action of a strong pumping field in the course of propagation of the two waves.

Level-splitting effects in resonant four-wave mixing

Resonant spectral structures of four-wave mixing (FWM) in molecular sodium are investigated. For a double- ? configuration with strong-weak-strong-weak fields, split components in FWM spectra induced by the strong pump fields are observed. It is shown that the split components merge into a single peak for a certain ratio of the strong field intensities. A correlation between the level-splitting effects and the saturation behavior of the FWM signal is experimentally demonstrated.

The nonlinear optical response of interacting charge-transfer excitonswith a weak probe in a strong pump field

The nonlinear optical response of interacting charge-transfer excitons isinvestigated theoretically by means of pseudospin operators for the modelHamiltonian. The generalized optical Bloch equations including the staticdipolar interaction of charge-transfer excitons are derived in terms ofthe Heisenberg equation of motion and the mean-field approximation. It isshown that the intrinsic optical bistability due to the strong interactionof charge-transfer excitons may exist in the interacting charge-transferexciton system. The absorption spectrum of interacting charge-transferexcitons associated with a weak probe field in the presence of a strongpump field is also calculated using the linear-response theory. The resultsshow that a gain without population inversion can be observed in theinteracting charge-transfer exciton system.

Exciton absorption in semiconductor quantum wells driven by a strong intersubband pump field

Optical interband excitonic absorption of semiconductor quantum wells (QW’s) driven by a coherent pump field is investigated on the basis of semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW’s. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron–hole pairs in the QW structure is low, the pump field induces an Autler–Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron–hole pairs is increased, the split contrast (the ratio between the maximum and the minimum values) is decreased, because the exciton effect is suppressed at higher densities owing to the many-body screening.

Nonlinear optical response of a dense two-level atom system embedded in a linear dielectric medium

The nonlinear optical (local) response of a dense collection of two-level atoms embedded in a linear dielectric medium is investigated theoretically. The nonlinear response of absorption and dispersion of two-level atoms associated with a weak probe field in the presence of a strong pump field is calculated in terms of the linear response theory. The numerical results show that the absorption and dispersion spectra are different from those in vacua because of the local-field effect enhancement and the local cooperative effect.

Amplification of a probe field without population inversion in a two-level atom driven by a strong bichromatic pump field

The polarization spectrum of a two-level atom driven by a symmetrical bichromatic pump field is examined. It is shown that the heterodyne parametric four-wave interactions result in the amplification of a weak probe field without population inversion at the line center in a wide range of pump and probe field intensities.

Absorption spectra of two-level atoms interacting with a strong polychromatic pump field and an arbitrarily intense probe field

A numerical method is introduced that solves the optical Bloch equations describing a two-level atom interacting with a strong polychromatic pump field with an equidistant spectrum and an arbitrarily intense monochromatic probe field. The method involves a transformation of the optical Bloch equations into a system of equations with time-independent coefficients at steady state via double harmonic expansion of the density-matrix elements, which is then solved by the method of matrix inversion. The solutions so obtained lead immediately to the determination of the polarization of the atomic medium and of the absorption and dispersion spectra. The method is applied to the case when the pump field is bichromatic and trichromatic, and the physical interpretation of the numerically computed spectra is given.

Self-consistent theory of optical gain with and without inversion in semiconductor quantum wells

In a self-consistent field formalism we investigated the weak-probe optical intersubband response of a three-subband semiconductor quantum well pumped by a strong coherent laser beam. We showed that, when the pumping photon energy is somewhat above the energy separation between the two lowest subbands, a combination of hole-burning effects and coherent pump–probe wave interactions can lead to both optical gain without population inversion and an Autler–Townes-like doublet in the probe optical spectrum. These two absorption peaks, whose positions are blueshifted as the pump intensity is increased, are due to the two-photon absorption process and light-induced intersubband transitions between the two upper states. It is also demonstrated that, in a stepwise two-photon pumping scheme, the optical gain with population inversion can also occur in the quantum-well system when a modestly strong pump field is present.

Effect of a strong dressing field on the polarizability of atomic helium at harmonic frequencies

We investigate the effect of a strong dressing field on the polarizability of atomic helium at harmonic frequencies. This study is carried out in a time-dependent non-perturbative calculation by looking at the harmonic spectrum resulting from the interaction with the combination of two fields: a strong pump field at the fundamental frequency and a weak probe field at some odd harmonic frequency. We present results for harmonic frequencies extending over the characteristic spectrum of high-order harmonic generation.

Simultaneously forbidden resonances in the Autler-Townes effect with a modulated pump

It was shown recently that systems subject to a strong modulating interaction can exhibit a new property in their response to a probe field. Under certain conditions an infinite number of resonances are simultaneously forbidden [V. N. Smelyanskiy, G. W. Ford, and R. S. Conti, Phys. Rev. A 53, 2598 (1996)]. In the present paper we investigate this effect in the case of a three-state system in which a strong pump field with a periodic frequency modulation \ensuremath{\Omega} couples a pair of excited levels while the complex Autler-Townes spectrum is probed via a weak field that connects one of the coupled states to the ground state. Under certain conditions a half-infinite comb of spectral lines, spaced by \ensuremath{\Omega}, simultaneously disappear from the Autler-Townes spectrum. These lines are positioned above or below a unique edge frequency, which is that of the probe transition in the absence of the strong field. It is shown that the aforementioned effect results from a special factorization property of the corresponding Floquet Hamiltonian that describes the Autler-Townes spectrum. Detailed analysis of this property is presented. In particular, it is found that the subset of the parameter space of the system where the factorization occurs consists of an infinite number of quasiperiodic manifolds. These manifolds exhibit some universal features related to the degeneracy of the dressed states. The line shapes of the probe resonances near the degeneracy points are derived. The intensities of the probe resonances are investigated in the limit of \ensuremath{\Omega} small compared with the modulation depth and the strength of the pump field. In the latter case, effects related to the avoided crossings of the dressed-state levels are considered. The Floquet Hamiltonian that describes the Autler-Townes spectrum in the case considered, effectively corresponds to a special model of a periodically driven system (with period 2\ensuremath{\pi}/\ensuremath{\Omega}) in which an external perturbation has the form of an operator projecting onto a single-quantum state. We generalize this model to the case of an N-level periodically driven system where the simultaneous vanishing of a half-infinite number of the dressed-state Fourier harmonics are analyzed. Possible experimental tests of the effect are suggested.

Gain enhancement in lasing without inversion in an optically dense medium

Under suitable conditions, a Λ-type three-level system driven by a strong pump field on one of the transitions and interacting with a weak probe field exhibits gain in a probe amplitude without population inversion [A. Imamoglu, J.E. Field and S.E. Harris, Phys. Rev. Lett. 66 (1991) 1154]. We present a theory of gain without population inversion in an optically dense medium where the local-field corrections that arise due to near dipole-dipole interactions, become significant. Numerical calculations that account for strong pump induced nonlinearities in the dense medium, show evidence of a large enhancement of the probe gain without population inversion in an optically dense medium.

Nonlinear parametric phenomena in plasma during radio frequency heating in the ion cyclotron frequency range

Parametric phenomena in plasma which occur due to varying electric fields with the ion cyclotron frequency are reviewed. Beam-like lower hybrid instability emerges in strong pumping fields provided that the transverse relative velocity of particles is larger than the ion thermal speed . The resulting turbulence and the following numerous manifestations observed experimentally are addressed. The turbulence may prove important for experiments aimed at plasma production or radio frequency (RF) cleaning of metallic surfaces of vacuum chambers in stellarators, tokamaks and helicon devices. In contrast, for a weak field the kinetic parametric instabilities of ion cyclotron oscillations arise due to electrons. The issues of the turbulence, mathematical modelling, its role in turbulent heating observed on the torsatron Uragan-3M, decay instabilities associated with ion cyclotron oscillations and the triggering of ion quasimodes are considered.