Polychromatic Field Research Articles

Quantum interference in a driven two-level atom

We show that a dynamical suppression of spontaneous emission, predicted for a three-level atom [S.-Y. Zhu and M. O. Scully, Phys. Rev. Lett. 76, 388 (1996)] can occur in a two-level atom driven by a polychromatic field. We find that the quantum interference, responsible for the cancellation of spontaneous emission, appears between different channels of transitions among the dressed states of the driven atom. We discuss the effect for bichromatic and trichromatic (amplitude-modulated) fields and find that these two cases lead to the cancellation of spontaneous emission in different parts of the fluorescence spectrum. Our system has the advantage of being easily accessible by current experiments.

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.

Theoretical analysis of resonances in the polarization spectrum of a two-level atom driven by a polychromatic field

Analytic solutions of the optical Bloch equations for a two-level atom interacting with a strong polychromatic field whose frequencies are symmetrically positioned with respect to the atomic frequency are used to obtain the polarization spectrum of the atom. The spectrum is found to consist of a series of discrete peaks or dips superimposed on the continuous part of the spectrum. Physical interpretation of resonances exhibited in the continuous part of the spectrum is given using a semiclassical dressed-atom approach.

Polychromatic radiation field model for a honeycomb monolith photocatalytic reactor

In this paper we extend our radiation field model previously developed to predict the monochromatic light intensity profile inside a photocatalytic square-channeled monolith reactor to predict the integral average polychromatic light intensity profile. In this extended polychromatic formulation, the mathematical representation of the model accounts for a source that emits a distribution of wavelengths and for wavelength-dependent optical properties (reflectivity) of the active photocatalytic thin film coated on the inner walls of the monolith channels. In the sense that these wavelength dependent properties of a given photocatalytic reactor system can be independently measured, the polychromatic radiation field model contains no adjustable parameters. Model predictions for the integral average light intensity transported through square monolith channels of varying length are in excellent quantitative agreement with experimental measurements employing Degussa P25 titania-coated monolith channels and near ultraviolet lamps emitting light primarily in the 300–400 nm range. The model reveals that axial light intensity gradients are severe, with the light intensity in the portion of the monolith channel beyond about 3–4 aspect ratios in length falling to below 1% of the incident light intensity. At a given axial distance down the monolith channel, the effective light flux that reaches the catalytic wall is approximately a factor of two lower than the total light flux through the channel cross section near the channel entrance, and an order of magnitude lower than the total cross section light flux near the channel exit.

Monte Carlo simulation of the radiation field in a reticulated foam photocatalytic reactor

AbstractThe 3‐D polychromatic radiation field for an annular packed‐bed photocatalytic reactor using alumina reticulated foams as a monolithic catalyst support was simulated using Monte Carlo methodology. Two distinct methods were used for simulating photon transport: (1) a “spatial” approach that tracks the flight of a photon in a predetermined reticulate structure; (2) a “temporal” approach that generates the random porous structure of the reticulate as the photon flies into it. The two approaches yield almost identical results, although the temporal approach is far more efficient computationally. Simulations for the integral axially‐averaged radial ultraviolet (UV) light profiles agree closely with experimental measurements for titania‐coated 10, 20 and 30 PPI alumina reticulates and near UV lamps. The simulations reveal that the local volumetric rate of energy absorption (LVREA) in the reticulate and the magnitude of the LVREA gradient both increase with decreasing reticulate pore size.

On the generalized radiance function for a polychromatic field

We find that the new generalized radiance function that was derived from Walther’s first generalized radiance function by the present authors for a polychromatic field [J. Opt. Soc. Am. A14, 3379 (1997)] cannot generally be identified with Walther’s first original function, even if the field is stationary in time. The identification of the new generalized radiance function with Walther’s first original function requires an additional condition of quasi-homogeneity of the field. In contrast, the new complex radiance function for a polychromatic field that was also derived in our previous paper can be derived from the complex version of Walther’s second generalized radiance function, regardless of the state of coherence of the field.

Generalized Floquet theoretical formulation of time-dependent density functional theory for many-electron systems in multicolor laser fields

We generalize the Floquet formulation of the time-dependent density . w . x functional theory TDDFT Telnov and Chu, Chem. Phys. Lett. 264, 466 1997 to the case of many-electron systems in multicolor or polychromatic time-dependent fields. It is shown that the time-dependent Kohn)Sham equations can be transformed into an equivalent time-independent infinite-dimensional Floquet Hamiltonian eigenvalue problem. For the case of bound-free transitions, we introduce the notion of complex density and present a non-Hermitian many-mode Floquet formulation of TDDFT. A procedure is presented for the determination of the complex quasi-energies and the calculation of the total and partial ionization rates from individual electron orbitals. The theory is illustrated by a case study of multiphoton ionization of He atoms in the presence of an intense 248 nm laser field and its third harmonic. Novel intensity- and phase-dependent behavior of multiphoton processes in two-color fields is reported.

Interferometry and radiometry

A general connection between interferometry and radiometry is studied on the basis of a new relationship between Walther’s first generalized radiance function and the mutual coherence function for polychromatic fields. New results include a definition of the generalized radiance function that is valid even for nonstationary fields and a principle of novel radiometry that retrieves three-dimensional radiance in the direction-frequency domain from the three-dimensional mutual coherence function. Radiometric interpretations of interferometric multispectral imaging are given as illustrative examples of this new principle.

Absorption and dispersion spectra of a polychromatic field in a two-level medium driven by a strong polychromatic pumping field.

Absorption and dispersion spectra of a polychromatic field in a two-level medium driven by a strong polychromatic pumping field.

Resonances of subnatural line width in iodine vapor driven by a polychromatic laser light field atλ=515 nm: a proposal

An initial study of the characteristics of sub-natural line width resonances in iodine elicited by polychromatic laser light has been made. Results of theoretical calculations are presented. Under proper experimental conditions, super-narrow resonances with high contrast ran be expected. >

Light pressure forces on multi-level atoms in intense polychromatic light fields: a continued fraction approach

Described is the application of a matrix form of the continued fraction technique to the calculation of the velocity dependent force experienced by a multi-level atom interacting with a complex optical field. Unlike other techniques, this approach provides a solution which is valid for a wide range of optical field configurations and atomoc velocities and which includes the effects of both single and multi-photon processes. As a specific example of this technique the force experienced by a three-level atom (V) in a bichromatic light field is calculated.

Gamma-ray spectral imaging using a single-shutter radiation camera

As part of a program to develop mobile robots for reactor environments, we are developing a radiation-imaging camera capable of operating in medium-intensity (<2 R/h), medium-energy (<8 MeV) gamma-ray fields. A systematic study of available detectors indicated the advisability of a high- Z scintillator. The raster-scanning camera uses a lead-shielded bismuth germanate (BGO) scintillator (1.25 cm×1.25 cm right-circular cylinder) coupled to a photomultiplier tube (PMT) operated in pulse mode. Measurements yielded an angular resolution of 2.5° and energy resolution of 12.9% at 662 keV. The camera motion is totally automated and controlled by stepping motors connected to a remote computer. Several 2D images of radioactive sources have been acquired in fields of up to 400 mR/h and energies up to 2.75 MeV. Some of the images demonstrate the ability of the camera to image a polychromatic field.

Fractal character of quasi-energy states in intense polychromatic fields

The quasi-energy eigenfunctions of N-level quantum systems driven by intense M-mode polychromatic fields are studied by means of the many-mode Floquet theory. It is found that the eigenfunctions exhibit striking self-similar (fractal) behavior in the temporal Fourier space. A method is presented for the calculation of the fractal ( D f) and entropy ( D s) dimensions of quasi-energy states and applied to the study of the behavior of two-level systems perturbed by bichromatic fields. The fractal dimensions are found to be (laser) intensity dependent and obey the relationship D f⩽ D s< M.

Floquet–Liouville supermatrix approach. II. Intensity-dependent generalized nonlinear optical susceptibilities

We present a practical nonperturbative method for exact treatment of intensity-dependent generalized nonlinear optical susceptibilities χ(ω) in intense polychromatic fields, valid for arbitrary laser intensities, detunings, and relaxation. By means of the many-mode Floquet theory, the time-dependent Liouville equation can be transformed into an equivalent time-independent infinite-dimensional Floquet–Liouville supermatrix (FLSM) eigenvalue problem. It is then shown that the nonlinear optical susceptibilities χ(ω) can be completely determined simply from the supereigenvalues and eigenfunctions of the Floquet–Liouvillian L̂F. In addition to this exact FLSM approach, we have also presented higher-order perturbative results, based on the extension of the Salwen’s nearly degenerate perturbation theory, appropriate for somewhat weaker fields and near-resonant multiphoton processes, but beyond the conventional perturbative or rotating wave approximation (RWA). In the case of two-level systems, for example, the implementation of Salwen’s method in the time-independent L̂F allows the reduction of the infinite-dimensional FLSM into a 4×4 dimensional effective Hamiltonian, from which essential analytical formulas for intensity-dependent χ(ω) can be obtained. These methods are applied to a detailed study of intensity-dependent spectral line shapes (such as hole burning and extra resonance peaks at the line center, and the effects of saturation, detuning, and radiative and collisional damping, etc.) and subharmonic structures in nonlinear multiple wave mixings χ[(m+1)ω1−mω2] for two-level systems in intense linearly polarized bichromatic fields.

Floquet-Liouville supermatrix approach: Time development of density-matrix operator and multiphoton resonance fluorescence spectra in intense laser fields.

A Floquet-Liouville supermatrix (FLSM) approach is presented for nonperturbative treatment of the time development of the density-matrix operator of atoms and molecules exposed to intense polychromatic fields. By extending the many-mode Floquet theory (MMFT) recently developed, the time-dependent Liouville equation for the density matrix of quantum systems undergoing relaxations (due to radiative decays and collisional dampings, etc.) can be transformed into an equivalent time-independent non-Hermitian FLSM eigenvalue problem. This yields a numerically stable and computationally efficient approach for the unified treatment of nonresonant and resonant, one- and multiple-photon, steady-state and transient phenomena in nonlinear optical processes, much beyond the conventional rotating-wave-approximation (RWA) method. Connections of the FLSM approach to the MMFT in the limit of zero relaxations are also made, providing the understanding of the physical significance of FLSM supereigenvalues and eigenvectors. In addition to the exact FLSM formalism, we have also presented higher-order perturbative results, based on the extension of the generalized Van Vleck (GVV) nearly degenerate perturbation theory, appropriate for somewhat weaker fields and near-resonant processes, but beyond the RWA limit. The implementation of the GVV method in the time-independent Floquet-Liouvillian allows the reduction of the infinite-dimensional FLSM into a finite-dimensional GVV-Liouville matrix, from which essential analytical results are readily obtained. As an illustration of the usefulness of the new formalism, we extend both the FLSM and the GVV methods to a formal study of the multiphoton-induced resonance fluorescence spectra of two-level systems subject to purely radiation relaxations. Both the time-averaged power spectrum and the time-dependent physical spectrum are exploited in details, and novel new features in intense fields are pointed out.

Floquet-liouville super-matrix approach for multiphoton non-linear optical processes in intense laser fields

A practical non-perturbative approach is presented for the treatment of multiphoton non-linear optical processes in intense monochromatic or polychromatic field. By extending the many-mode Floquet theory recently developed by the authors, the time-dependent Liouville equation for the density matrix of atoms or molecules undergoing radiative and/or collisional relaxations can be transformed into an equivalent time-independent Floquet-Liouville super-matrix eigenvalue problem. The method is illustrated by a study of the multiphoton resonance fluorescence spectra of two-level systems.

Shape invariant propagation of polychromatic fields

When a polychromatic field propagates the transverse distribution of the power spectrum generally changes from one plane to another. Conditions are found under which the spectrum distribution maintains the same shape in any cross-section. Examples of fields satisfying these conditions are given.

The autocorrelation function of polychromatic laser speckle patterns near the image plane

The autocorrelation function of polychromatic speckle patterns produced near the image plane of a double diffraction imaging system is experimentally studied. The condition under which the polychromatic speckle field obeys Gaussian statistics is further investigated as a function of the numberN of scattering cells, using the average contrast of the speckle intensity fluctuations. The profile of the autocorrelation function is next investigated as a function ofN and the defocused distance from the image plane. It is found that the average contrast and autocorrelation function of the polychromatic speckle intensity fluctuations converge less rapidly with an increase inN than those of the monochromatic ones.

Expansion of Coherence Functions of Stationary, Partially Coherent, Polychromatic Fields

The self-coherence function of stationary, partially coherent, polychromatic optical fields is written as sum of totally coherent fields and an autocorrelation term. Also, it is shown that spatially fully coherent fields (whose mutual coherence function corresponds to the autocorrelation term of the former expansion) may be written as sum of cross-spectrally pure fields. As an illustrative example of these developments, a Young's interference arrangement is analysed. The behaviour of each term of the foregoing expansion with respect to an arbitrary optical system is examined.

5146. Variance of intensity for a discrete-spectrum, polychromatic speckle field after propagation through the turbulent atmosphere: Fred J Holmes and V S Rao Gudimetla,J Opt Soc Am,71 (10), 1981, 1176–1179

5146. Variance of intensity for a discrete-spectrum, polychromatic speckle field after propagation through the turbulent atmosphere: Fred J Holmes and V S Rao Gudimetla,J Opt Soc Am,71 (10), 1981, 1176–1179