On negative dispersion without absorption in bichromatically driven two-level systems

Abstract

The possibility of obtaining strong negative dispersion without absorption in the central resonance structure of the spectrum of a bichromatically driven two-level system is investigated. A nonlinear analysis is performed using the technique of continued fractions with respect to the dressedstate basis of a strong pumping laser. We discuss the transition between the two regimes of the monochromatic driving case with a weak probing laser and of nearly equally strong lasers in terms of the contributions from coherences and populations of these dressed states. In contrast with the strong dependence of the Rabi side bands on probe field intensity, we find a remarkable stability of the negative dispersion in the central resonance structure.     

Coherent control of spontaneous emission in a four-level system

Abstract

In this article we study the potential for coherently control spontaneous emission in a four-level scheme. Our control parameter is the relative phase of two laser fields having the same angular frequency. We investigate in detail the trapping conditions, the population dynamics and the behaviour of the (long time) spontaneous emission spectrum, which are now phase dependent. Inhibition of spontaneous emission and extreme spectral narrowing are shown as the relative phase is varied. Our results are interpreted using a dressed state analysis of the dynamics.     

Resonance fluorescence in intense laser fields

Abstract

We consider the resonance fluorescence of a two-level atom in a vacuum with a high intensity driving field, such that the Rabi frequency of the field is non-negligible up to frequencies comparable to the transition frequency of the atom. At these high intensities, the conventional rotating wave approximation cannot be applied to the atom-laser Hamiltonian and the modal densities of the vacuum at the dressed transition frequencies differ significantly from each other. On treating the counter-rotating part of the Hamiltonian as a perturbation and carrying out the Weisskopf-Wigner approximation in the dressed states basis, we find that the high driving intensity leads to significant deviations from the traditional Mollow resonance fluorescence spectrum. In particular we find unequally intense Mollow side-bands and that the generation of higher order triplets and harmonics is substantially enhanced by the high vacuum modal density at the corresponding high resonance frequencies.     

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.     

Two-electron Excitation and Ionization in an Intense Laser Field

Abstract

We present a simple model for two-electron excitation and ionization of an atom in the presence of an intense laser field. We show, in particular, how the Coulomb interaction, via configuration interaction, prevents the excitation and ionization from being collective in all but the most intense fields. By collective we mean that each electron has the same spatial wavefunction: this implies that the electron pair may be described by a restricted time-dependent Hartree-Fock wavefunction. The configuration interaction prevents collective behaviour not only through the process of auto-ionization but also through the mixing of doubly excited independent electron states into the ground state. We are able to give criteria for establishing true collective excitation and ionization of our model two-electron system in an intense field.     

Generation of Optical Harmonics by Intense Pulses of Laser Radiation

Abstract

We discuss effects of optical depth, beam focusing and non-perturbative atomic response on the spectrum of optical harmonics generated by an intense beam of laser radiation (up to 10¹⁴W cm^?2) in a small volume of atomic vapour. We find remarkable enhancement of high harmonics at intensities beyond the perturbative limit.     

Narrow absorption lines induced by electron shelving

Abstract

We investigate the absorption spectrum of a laser-driven V system in which one of the upper levels is metastable. If the Rabi frequency of the laser driving the metastable transition is sufficiently small we find an extremely narrow peak in the stationary absorption spectrum of a weak probe on the strong transition. We give analytical expressions approximating the stationary absorption spectrum. The absorption spectrum of a two-level system is investigated for the case where an upper limit for the time separation of successive emissions is given. The absorption spectrum then exhibits a delta-function-like peak and using this we then show that the origin of the narrow peak in the absorption spectrum can be traced back to electron shelving, i.e. to the existence of light and dark periods in the resonance fluorescence emitted on the strong transition.     

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACT

We present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.     

Harmonic Generation by Strongly Driven Classical Anharmonic Oscillators

Abstract

We discuss, through classical simulations, some general properties of the response of anharmonic oscillators to an intense external oscillating force. We address in particular the question of the presence of hyper-Raman or ‘atomic’ components in the harmonic spectrum.     

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.     

Generalized master equation for a two-level atom in a strong field and tailored reservoirs

Abstract

The generalized master equation for a two-level atom driven by a strong classical field and damped into a ‘tailored’ reservoir with a non-flat density of modes is derived under the Born-Markov approximation. To derive the master equation the dressing transformation on the atomic operators is performed first and next the dressed operators are coupled to the reservoir and the corresponding damping rates are calculated. The modifications introduced by a strong field and/or by the reservoir with the non-flat density of modes lead to non-standard terms in the master equation, some of which are reminiscent of terms known for squeezed vacuum reservoirs. The optical Bloch equations based on this generalized master equation are obtained and solved for the steady state. The solutions are discussed from the point of view of both bare and dressed atoms. Analytical formulas for the fluorescence and probe absorption spectra are obtained and illustrated graphically.     

Multiple spontaneously generated coherence and phase control of fluorescence photon correlation in a driven four-level atom

Abstract

The fluorescence photon correlations in a driven four-level atom consisting of three upper closely lying excited states and a single lower state are investigated. It is found that in the presence of triple spontaneously generated coherence (SGC) effects, which are created by three degenerate levels coupling to the common ground state via the same vacuum mode, the correlation properties of the three fluorescence fields can be switched from strong correlation to anticorrelation or vice versa by modifying the detunings. In addition, such a system is very sensitive to the relative phases of the three fields, which can provide an effective way to control the photon correlation. Physically, these interesting phenomena can be interpreted in terms of dressed state analysis.     

Spontaneous and Induced Atomic Decay in Photonic Band Structures

Abstract

We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either ‘bare’ or ‘dressed’ by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of non-decaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the induced decay of a resonantly driven atom into such a continuum exhibits a cascade of transitions down the ladder of dressed states, which are labelled by decreasing photon numbers of the driving mode. Remarkably, this cascade is terminated at the dressed-state doublet, from which all subsequent transitions to lower doublets are forbidden because they fall within the PBG. This doublet then becomes an attractor state for the populations of higher-lying doublets. As a result, the photon-number distribution of the driving mode becomes strongly sub-Poissonian.     

Disappearance of Trapped Yb+

Abstract

The storage time of Yb⁺ ions trapped in an r.f. trap was determined by the rf resonance absorption method in the presence of buffer gas, without and with laser radiation of a frequency equal to the transition from the S_1/2 ground state to the excited P_1/2 state. The storage time of the ions with the buffer gas (presure between 1 × 10^?4 and 4·2 × 10^?4 Pa) was several hours without the laser radiation; this period was shortened to several hundred seconds by irradiation with a 500 nW laser source. This phenomenon shows that the trapped Yb⁺ ions disappeared from the trap as a result of irradiation with the resonance laser light.     

Wavelet analysis of short harmonics generated in presence of a two colour laser field

Abstract

The Morlet wavelet spectrum of the radiation emitted by a two level atom in presence of two laser pulses with very close frequency is obtained. The wavelet spectrum gives information on the time evolution of the full spectrum and of a particular line. The beating condition stimulates the atom to emit pulses of harmonics with duration of the order of a few optical cycles of the pumping radiation. Pulse trains of 3 optical cycles (FWHM) are observed.     

Level Crossing in a Frequency-dependent Photon Reservoir

Abstract

The level-crossing effect for a three-level atom placed into the resonant cavity (waveguide) and illuminated by a strong monochromatic laser light is discussed. The cavity plays the role of a frequency-dependent photon reservoir that modifies the spontaneous decay of the atom. The effect is studied by considering the modifications of the level-crossing (Hanle effect) signals. The non-Markovian character of spontaneous emission in frequency-dependent reservoirs is taken into account. Calculated signals are interpreted in terms of a dressed atom picture.     

Magnus expansion method for two-level atom interacting with few-cycle pulse

Abstract

Using the Magnus expansion to the fourth order, we obtain analytic expressions for the atomic state of a two-level system driven by a laser pulse of arbitrary shape with small pulse area. We also determine the limitation of our obtained formulas due to limited range of convergence of the Magnus series. We compare our method to the recently developed method of Rostovtsev et al. (PRA 2009, 79, 063833) for several detunings. Our analysis shows that our technique based on the Magnus expansion can be used as a complementary method to the one in PRA 2009.     

Performance improvement of multipath interfered packet radios with an extended time‐out period in the presence of capture effect

Abstract

The effect of multipath interference on the performance of packet radios using ALOHA or slotted ALOHA protocols has been investigated previously in which only the success of line‐of‐sight packets are counted in the evaluation of throughput and delay. Possible improvement in performance due to the survival of delay versions of the direct packets in the presence of the capture effect is studied in this paper. The type of signal capture to be considered is the one in which a signal involved in a collision survives if its power level exceeds the sum of the other signals participating in the same collision.     

Charge-changing Collisions of Stored,Multiply-charged Ions

Abstract

The results of measurements of rate coefficients for electron transfer from several target gases to multiply-charged ions are summarized and discussed. These measurements were carried out primarily in Penning ion traps. The stored ions were produced at low energy by several methods, including electron impact multi-ionization of atoms and molecules, recoil ions from impact of fast, stripped, heavy ions with target gas atoms, and inner shell photoionization of atoms using synchrotron radiation. The measurements typically do not exhibit any simple variation of the rate coefficients with ion charge state at these low energies, consistent with the expected mechanism of electron transfer at avoided crossings of the equipotential surfaces of the quasi-molecule formed during the collision. The magnitudes of the rate coefficients, and their trends with charge, are compared for different ions interacting with the same target, and with calculations of the rates using Langevin theory, which predicts a linear dependence on charge state.     

Optical switching and normal mode splitting in hybrid semiconductor microcavity containing quantum well and Kerr medium driven by amplitude-modulated field

ABSTRACT

We theoretically investigate optical bistability/multistability for all optical switching signature in a hybrid semiconductor microcavity system comprising a quantum well and a Kerr nonlinear substrate. The system is essentially two optically coupled microcavities with one of the microcavity being driven by an external amplitude-modulated pump laser. We show that the switching between bistable and multistable behaviour is influenced by the modulated pump laser, Kerr nonlinearity and the optical coupling between the two microcavities. We further investigate the intracavity spectrum of quantum fluctuations which exhibit the well-known normal mode splitting (NMS). The NMS behaviour is also found to be influenced by the system parameters. These results demonstrate that the present hybrid nonlinear system can be used in designing sensitive optical devices.