Coherent effects in ultrashort pulse propagation through an optically thick three-level medium

A theoretical study is made of the propagation of a weak ultrashort pulse through an optically thick, inhomogeneously broadened three-level medium of the V configuration driven by a self-induced transparency pulse on the coupled transition. The weak coherent pulse experiences a greatly enhanced transparency. The new transparency effect is discussed in detail and the results are found to be in good agreement with recent experimental observations. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 359–363 (10 September 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Numerical study of heat transfer in an optically thick semi-transparent spherical porous medium

In this paper, heat transfer by simultaneous convection, conduction and radiation in a semi-transparent spherical porous medium is investigated. The ROSSELAND approximation is adopted to take account of radiation in the heat transfer rate. The routine used here to solve the set of differential equations is taken from the IMSL MATH/LIBRARY. Various results are obtained for the dimensionless temperature profiles in the solid and fluid phases, the radiative, conductive, convective and total heat fluxes. The effects of some radiative properties of the medium on the heat transfer rate are examined.     

Ionization yield of two-step photoionization process in an optically thick atomic medium of barium

The kinetics of a two-step photoionization process in optically thick atomic medium of barium (Ba) is studied using the rate equation approach. In the first step, Ba atoms get resonantly excited by laser radiation from their ground state to an intermediate excited state and subsequently are ionized in the second step by another laser radiation. The absorption of exciting radiation is taken into account along its propagation direction (optically thick). However, the medium is assumed to be optically thin for the ionizing radiation. A numerical simulation is done to estimate the ionization yield for time-varying Gaussian shaped laser pulses. The required energy density of the laser pulse to saturate the excitation transition throughout the thick medium is calculated. The effect of optical delay between the laser beams on the ionization yield is simulated. The calculated ionization yield from the simulation is compared with the measured values.     

Quantum interference and Manley-Rowe relations in resonant four-wave frequency mixing in an optically thick Doppler-broadened medium

It is shown that under resonant interaction conditions certain notions in nonlinear optics which are based on the Manley-Rowe relations no longer hold because of the interference of elementary quantum-mechanical processes. This conclusion is illustrated by numerical examples corresponding to the experiments performed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 862–866 (25 June 1999)     

Optical multistability and ground-state coherence in a three-level system

The interaction of a three-level atomic medium with two degenerate sublevels in the ground state with a coherent radiation field in a ring cavity is investigated. We pay specific attention to atomic coherence within the ground-state doublet, and introduce separate decay rates for the ground-state coherence and the population difference. If the atomic density exceeds a critical value, the system exhibits symmetric bistability. In addition, in certain regions of parameter space we find symmetry-breaking branches, yielding optical multistability. We propose physical mechanisms for the symmetric as well as for the symmetry-breaking instability.Work supported by the Swiss National Science Foundation     

Solution of a multi-level model of an optically thick sodium plasma

A five level model consisting of the ground state, three excited states (3P, 3D, 4S) and the continuum is solved for an optically thick sodium plasma. Radiation trapping is taken into account and dealt with by means of an eigenfunction technique.     

Adiabatic preparation of a Heisenberg antiferromagnet using an optical superlattice

We analyze the possibility to prepare a Heisenberg antiferromagnet with cold fermions in optical lattices, starting from a band insulator and adiabatically changing the lattice potential. The numerical simulation of the dynamics in 1D allows us to identify the conditions for success, and to study the influence that the presence of holes in the initial state may have on the protocol. We also extend our results to two-dimensional systems.     

Photonic crystal nanocavity laser in an optically very thick slab

A photonic crystal (PhC) nanocavity formed in an optically very thick slab can support reasonably high-Q modes for lasing. Experimentally, we demonstrate room-temperature pulsed lasing operation from the PhC dipole mode emitting at 1324 nm, which is fabricated in an InGaAsP slab with thickness (T) of 606 nm. Numerical simulation reveals that when T≥800 nm, over 90% of the laser output power couples to the PhC slab modes, suggesting a new route toward an efficient in-plane laser for photonic integrated circuits.     

Evolution of an optically pumped ensemble of cold ground-state atoms in weak light fields

The evolution of multipole moments is analyzed for optically pumped cold ground-state atoms in the limit of low saturation of a closed j₀ → j₁ dipole transition. The longest multipole-moment relaxation times are analyzed as functions of ellipticity and frequency detuning from resonance for transitions with j₀ ? 5. The qualitative difference between the evolution toward steady-state Zeeman sublevel populations and dynamics of transient spontaneous emission is demonstrated for transitions of the following types: j → j?1, j → j with integer j, j → j with half-integer j, and j → j + 1.     

Quantum coherence and ground-state phase transition in a four-chain Bose–Hubbard model

We study the quantum coherence and ground-state phase transition of a four-chain Bose–Hubbard model with the long-range interaction. In a special four-chain Bose–Hubbard model,i.e., each chain only has one optical potential, four types of the ground-state phases are discovered. The effects of the disorder, the on-site interaction and the long-range interaction on the quantum coherence are studied. For the system without the long-range interaction, the quantum coherence changes from one periodic oscillation to two periodic oscillations as the onsite interaction increases. By considering the long-range interaction, the quantum coherence goes back to one periodic oscillation again. The on-site interaction itself suppresses the quantum coherence, both the on-site interaction and long-range interaction together enhance the quantum coherence with the weak disorder. If the disorder strength is increased beyond a critical value,they start to suppress the quantum coherence. In a regular four-chain Bose–Hubbard model, i.e.,each chain has many optical potentials, the ground-state phase transitions are obtained by using the cluster Gutzwiller mean-field method. Exotic ground-state phases are found, i.e., superfluid phase, integer Mott insulator phase, supersolid phase and loophole insulator phase. The combination of the loophole insulator phase and the supersolid phase expands the lobes with the half-integer filling per site for the small ratio β = t_■/t_⊥.     

Propagation of light through an optically thick flat homogeneous cloud layer

Abstract

Monte Carlo simulations of light pulse transfer through a flat homogeneous cloud layer are used to compute zero, first and second moments of the light power temporal distribution at the input of a receiver placed at the cloud bottom or removed from it to a distance H in a pure atmosphere. The upper boundary of a cloud layer is considered to be illuminated by an infinitely extended δ-pulsed light beam. Transmittance of the cloud layer T and radiance angular distribution B(θ) at its bottom (these data define the zero moment or the pulse-response energy under steady illumination W), as well as mean propagation time t¯ and variance (width) (Δt)² of the pulse-response are calculated for cloud optical thicknesses τ=0, …, 50, incidence angle θ₀=0, …, 60°, single scattering albedo ω₀=0.995, …, 1.0 and the receiver view angle 2 β=40° and 90°. Monte Carlo simulation results are compared with well known Rosenberg asymptotic formulae for T and B(θ) as well as with a simple model developed for calculating t¯ and Δt. The comparison reveals high accuracy of all formulae considered for the optical thickness τ>10, …, 15. A simple procedure for computing W, t¯ and Δt parameters for a three-layer medium (cloud/pure atmosphere/sea water) is outlined.     

Determination of the excited state density for an optically thick resonance line

The transverse profile of the monochromatic radiance of an optically thick resonance line from a cylindrical discharge is inverted exactly to give the radial distribution of radiating atoms. In contrast to the Abel transform, this result is valid for all optical depths.     

Chirped quantum cascade laser induced rapid passage signatures in an optically thick gas

We report observations of rapid passage signals induced in samples of N₂O and CH₄ present in a multipass cell with an optical path length of 5 m. The effect of laser power and chirp rate upon the signals has been studied by utilising two different chirped quantum cascade lasers operating around 8 μm. The rapid passage signals exhibit an increasing delay in the switch from absorption to emission as a function of increased gas pressure (up to 8 Torr of gas). By comparing a selection of transitions in N₂O and CH₄, we show that, unlike ammonia, this ‘pressure shift’ is independent of the transition dipole moment, spectroscopic branch probed and laser chirp rate. As the transition dipole moment is much larger in nitrous oxide than methane, we believe that this indicates that N₂O–N₂O collisions are more efficient at removing coherence from the polarised sample than CH₄–CH₄ collisions. We have also observed this pressure shift in a short path length of 40 cm, although with a much reduced value, indicating that propagation effects are important in this optically thick minimally damped system.