Light Propagation Effects Research Articles

An analytical model of surface mass densities of cold dark matter haloes - with an application to MACHO microlensing optical depths

The cold dark matter (CDM) scenario generically predicts the existence of triaxial dark matter haloes which contain notable amounts of substructure. However, analytical halo models with smooth, spherically symmetric density profiles are routine ly adopted in the modelling of light propagation effects through such objects. In this paper, we address the biases introduced by this procedure by comparing the surface mass densities of actual N-body haloes against the widely used analytical model suggested by Navarro, Frenk and White (1996) (NFW). We conduct our analysis in the redshift range of 0.0 - 1.5. In cluster sized haloes, we find that triaxiality can cause sc atter in the surface mass density of the haloes up to �+ = +60% and � = 70%, where the 1-� limits are relative to the analytical NFW model given value. Subhaloes can increase this scatter to �+ = +70% and � = 80%. In galaxy sized haloes, the triaxial scatter can be as high a s �+ = +80% and � = 70%, and with subhaloes the values can change to �+ = +40% and � = 80%. We present an analytical model for the surface mass density scatter as a function of distance to the halo centre, halo redshift and halo mass. The analytical description enables one to investigate the reliability of results obtained with sim plified halo models. Additionally, it provides the means to add simulated surface density scatter to analytical density profiles. As an example, we discuss the impact of our results on the calculation of microlensing optical depths for MACHOs in CDM haloes.

Linear and nonlinear effects of light propagation in low‐index photonic crystal slabs

AbstractWe investigate theoretically and experimentally photonic crystals realised in slab waveguides made of materials with a low refractive index. Although in such systems a two‐dimensional photonic bandgap for TE polarised light can be achieved, the low index imposes several restrictions influencing the performance. We investigate the transmission and dispersion properties of photonic crystal defect waveguides, high‐quality factor microcavities and unusual phenomena of light propagation and localisation in truly periodic systems without defects. This is compared to results obtained in high‐index materials. Furthermore we investigate theoretically frequency conversion and solitons in nonlinear photonic crystals. An optical parametric oscillator in a photonic crystal microcavity and strongly localised solitons in coupled microcavities are proposed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The True Surface Mass density of Cold Dark Matter Halos

AbstractThe cold dark matter (CDM) scenario generically predicts the existence of triaxial dark matter halos which contain notable amounts of substructure. However, analytical halo models with smooth, spherically symmetric density profiles are routinely adopted in the modelling of light propagation effects through such objects. In this paper, we report the biases introduced by this procedure by comparing the surface mass densities of actual N-body halos against the widely used analytical model suggested by Navarro, Frenk and White (1996) (NFW). We conduct our analysis in the redshift range of 0.0 − 1.5.In cluster sized halos, we find that triaxiality can cause scatter in the surface mass density of the halos up to σ+= +60% and σ−= −70%, where the 1-σ limits are relative to the analytical NFW model given value. Subhalos can increase this scatter to σ+= +70% and σ−= −80%. In galaxy sized halos, the triaxial scatter can be as high as σ+= +80% and σ−= −70%, and with subhalos the values can change to σ+= +40% and σ−= −80%.We have developed an analytical model for the surface mass density scatter as a function of distance to the halo centre, halo redshift and halo mass. The analytical description enables one to investigate the reliability of results obtained with simplified halo models. Additionally, it provides the means to add simulated surface density scatter to analytical density profiles. We have tested our model on the calculation of microlensing optical depths for MACHOs in CDM halos.

Enhancing capacity of coherent optical information storage and transfer in a Bose-Einstein condensate

The coherent optical information storage capacity of an atomic Bose-Einstein condensate is examined. The theory of slow light propagation in atomic clouds is generalized to the short-pulse regime by taking into account group velocity dispersion. It is shown that the number of stored pulses in the condensate can be optimized for a particular coupling laser power, temperature, and interatomic interaction strength. Analytical results are derived for a semi-ideal model of the condensate using the effective uniform density zone approximation. Detailed numerical simulations are also performed. It is found that the axial density profile of the condensate protects the pulse against group velocity dispersion. Furthermore, taking into account the finite radial size of the condensate, multimode light propagation in an atomic Bose-Einstein condensate is investigated. The number of modes that can be supported by a condensate is found. The single-mode condition is determined as a function of experimentally accessible parameters including trap size, temperature, condensate number density, and scattering length. Quantum coherent atom-light interaction schemes are proposed for enhancing multimode light propagation effects.

Uniform illumination and rigorous electromagnetic simulations applied to CMOS image sensors

This paper describes a new methodology we have developed for the optical simulation of CMOS image sensors. Finite Difference Time Domain (FDTD) software is used to simulate light propagation and diffraction effects throughout the stack of dielectrics layers. With the use of an incoherent summation of plane wave sources and Bloch Periodic Boundary Conditions, this new methodology allows not only the rigorous simulation of a diffuse-like source which reproduces real conditions, but also an important gain of simulation efficiency for 2D or 3D electromagnetic simulations. This paper presents a theoretical demonstration of the methodology as well as simulation results with FDTD software from Lumerical Solutions.

Exact solutions for the propagation of light in nonlinear optical media with nonperiodic modulation

In this paper, exact solutions for the nonlinear Schrödinger (NLS) equation with nonperiodic modulation describing propagation of light in nonlinear media, under three sets of transverse modulation forms of the refractive index are presented. Unlike the reported cases with periodic modulation, the novel ones can accurately describe the behaviour of propagation for a light beam in nonperiodic structures. Moreover, a series of new nonlinear effects of light propagation are displayed. Furthermore, the stability of the solutions is discussed numerically, and the results reveal that these solutions are stable to finite initial perturbations.

Light propagation effects in QED: Effective action approach

The effects of light propagation in constant magnetic and electric backgrounds are considered in the framework of the effective action approach. We use the exact analytic series representation for the one-loop effective action of QED and apply it to the birefringence effect. Analytical results for the light velocity modes are obtained for weak and strong field regime. We present asymptotic formulae for the light velocity modes for the ultra strong magnetic field which can be realized in some neutron stars.

Radiative coupling of intersubband transitions in GaAs/AlGaAs multiple quantum wells

Radiative coupling of resonantly excited intersubband transitions in GaAs/AlGaAs multiple quantum wells can have a strong impact on the coherent nonlinear optical response, as is shown by phase and amplitude resolved propagation studies of ultrashort electric field transients. Upon increasing the driving field amplitude, strong radiative coupling leads to a pronounced self-induced absorption, followed by a bleaching due to the onset of delayed Rabi oscillations. A many-particle theory including light propagation effects accounts fully for the experimental results.

Functional optical detection based on pH dependent fluorescence lifetime

Detection of possible alterations of physiological parameters (e.g., pH and temperature), resulting from malignant transformation of initially healthy tissue, can be a powerful diagnostic tool for earlier cancer detection. Such variations can be observed by comparing these parameters with those of healthy tissue surrounding the abnormality. Time-resolved spectroscopy of specifically targeted fluorescent labeled antibodies can be sensitive to such variations and provide a high resolution functional image of the region of interest. The goal of this study was to establish a forward experimental setup for calibration of the lifetime dependencies of near-IR fluorescent dyes on physiological parameters, and to develop analytical solutions, taking into account the effects of light propagation in turbid media (e.g., tissue), that was able to extract an original lifetime fluorescence signal from time-of-flight intensity distributions, measured in vivo from a deeply embedded live organ for further analysis. Tissue-like phantoms with embedded fluorescent dyes and background optical properties simulating those of live tissues were designed and created. Fluorescence decay curves were measured for different fluorophore positions, and pH values. Those measurements were made with a system based on a time-correlated single photon counting (TCSPC) instrument and a tunable femtosecond Ti-Sapphire system built by our group. Decay curves were recorded for fluorophore depths of up to 5 mm and source-detector separation of 7 mm. It was shown that a forward model, based on the random walk theory, adequately described the experimental data. Measured pH dependencies of the fluorescence lifetime were characterized for two different dyes. Good correlation between experimental data and predictions of the theoretical model allows the use of close-form analytical solutions to separate the effects of photon time delays due to multiple scattering in tissues from the original intensity fluorescence time decay curve, determined by the fluorophore itself and its immediate surroundings. It is the latter dependence that can be diagnostically important. Experimentally obtained scaling between lifetime and a parameter of interest can be used in vivo to obtain a map of physiological parameter changes which can serve as a base for an in vivo specific diagnostic system.

Optical dephasing of coherent intersubband transitions in a quasi-two-dimensional electron gas

We present a microscopic many-particle theory for the dephasing of coherent intersubband excitations in semiconductor quantum wells including carrier-carrier and carrier-phonon scattering and light propagation effects. The contributions of many-particle processes are nonadditive and thus cannot be treated separately. It is shown that due to nondiagonal correlation contributions, scattering rates alone cannot be taken as a measure for the dephasing of the intersubband polarization. Surprisingly, radiative damping is found to be important even at moderate carrier densities. Calculated absorption spectra are in excellent agreement with experiments on a high-quality sample.

Monte Carlo simulation of different TOF light readout schemes

We report the results of TOF detector resolution studies carried out using a simulation package which takes into account light generation, propagation, attenuation and readout effects. Time resolution has been studied for a classic TOF configuration as well as for the set ups based on light collection via WLS fiber and producing Cherenkov photons. Best resolution has been obtained for the classic TOF configuration.

Theory of the lineshape of quantum well intersubband transitions: optical dephasing and light propagation effects

AbstractWe outline a theoretical description of the absorption linewidth of quantum well intersubband transitions by solving Maxwell's equations for a non‐local susceptibility including many particle effects. We show that the intersubband absorption results from a complex interplay between mean‐field effects, dephasing contributions and light propagation effects, all being very sensitive to subband dispersion.

Anti-Shielding Effect and Negative Temperature in Instantaneously Reversed Electric Fields and Left-Handed Media

The connections between the anti-shielding effect, negative absolute temperature and superluminal light propagation in instantaneously reversed electric fields and in left-handed media are considered in the present paper. An instantaneous inversion of the exterior electric field may put the electric dipoles into a state of negative absolute temperature and therefore give rise to a negative effective mass term of the electromagnetic field (i.e., the electromagnetic field propagating inside the negative-temperature medium will acquire an imaginary rest mass), which is said to result in the potential superluminality effect of light propagation in this anti-shielding dielectric. In left-handed media, such phenomena may also arise.

The effect of laser light propagation through a self-induced inhomogeneous process gas on temperature dependent laser-assisted chemical etching

High resolution and precise surface morphology is a primary goal of laser-assisted chemical etching (LACE) for microfabrication. However, the interaction of the laser light with the process gas can change the intensity incident to the target surface due to self-induced, thermally developed inhomogeneities within the gas. The exponential relationship between LACE rates and surface temperature means that very small changes in intensity, and thus surface temperature distribution, has a very large effect on the etch profile. This paper models both the inhomogeneous light–process gas interaction and the etching to predict the regimes in time and absorption path length where this effect needs to be considered. Numerical calculations of LACE microfabrication of borosilicate glass in a sulfur hexafluoride process gas with a 10.6 μm wavelength laser beam are given that show how the surface morphology of the glass wafer is changed by the inhomogeneous interaction.

Transient Grating Induced by Excitonic Polaritons in Thin Films Semiconductors

Light propagation effects on nonlinear optical response of a size-controlled GaAs thin film at exciton resonance are investigated by transient gradient using picosecond pulses (1.5 ps). The signals are enhanced at the exciton resonance and the temporal profiles strongly depend on the excitation pulse energies. The result indicates that the nonlinear response is induced by excitonic polaritons and its decay is determined by the round-trip of the polariton in the film.

RADIATION ELEMENT METHOD FOR TRANSIENT HYPERBOLIC RADIATIVE TRANSFER IN PLANE-PARALLEL INHOMOGENEOUS MEDIA

In this study the radiation element method is formulated to solve transient radiative transfer with light radiation propagation effect in scattering, absorbing, and emitting media with inhomogeneous property. The accuracy of the method is verified by good agreement between the present calculations and Monte Carlo simulations. The sensitivity of the method against element size, ray emission number, and time increment size is examined. The transient effect of radiation propagation is essential in short-pulse laser radiation transport when the input pulse width is not considerably larger than the system radiation propagation time. The transient characteristics of radiative transfer are investigated in the media subject to collimated laser irradiation and/or diffuse irradiation withtemporal Gaussian and/or square profiles. The inhomogeneous profile of extinction coefficient of the medium affects strongly the transient radiative flux divergence inside the medium.

Sub-Microarcsecond Astrometry and New Horizons in Relativistic Gravitational Physics

AbstractAttaining the limit of sub-microarcsecond optical resolution will completely revolutionize fundamental astrometry by merging it with relativistic gravitational physics. Beyond the sub-microarcsecond threshold, one will meet in the sky a new population of physical phenomena caused by primordial gravitational waves from the early universe and/or different localized astronomical sources, space-time topological defects, moving gravitational lenses, time variability of gravitational fields of the solar system and binary stars, and many others. Adequate physical interpretation of these yet undetectable sub-microarcsecond phenomena cannot be achieved on the ground of the “standard” post-Newtonian approach (PNA), which is valid only in the near-zone of astronomical objects having a time-dependent gravitational field. We describe a new, post-Minkowskian relativistic approach for modeling astrometric observations having sub-microarcsecond precision and briefly discuss the light-propagation effects caused by gravitational waves and other phenomena related to time-dependent gravitational fields. The domain of applicability of the PNA in relativistic space astrometry is outlined explicitly.

Theory of Optically Nonlinear Effects in Ultrafast Spatio-Temporal Dynamics of Semiconductors

Recently, the dynamics of excitons, biexcitons and the electron-hole plasma after excitation with femtosecond light pulses (1 fs = 10 -15 s) in semiconductor microand nanostructures has been studied extensively [1, 2]. Coherent nonlinear effects such as the optical Stark-effect, four-wave mixing (FWM), photon echoes, and Rabi oscillations have been observed [3]. Whereas the linear optical susceptibility is dominated by electron-hole Coulomb (exciton) effects, at moderate intensities, biexcitons may form and carrier-carrier scattering takes place for the excitation of continuum states. For sufficiently high electron-hole densities in a semiconductor, negative absorption, i.e., optical gain appears [4]. The purpose of this paper is to give a short introduction in the theoretical treatment of spatiotemporal effects in semiconductor optics and to review some recent results. Our analysis includes light propagation effects in optically thick samples as well as optically induced electron and exciton transport.

Design of Talbot array illuminators for planar optics

The planar optical integration of Talbot array illuminators is investigated. The fractional Talbot distances are calculated from the parabolic approximation of light propagation. In addition, we discuss distortions of the diffraction patterns caused by aberrations due to off-axis light propagation and edge effects due to the finite size of the diffraction gratings. Using computer simulations, we estimate the efficiency of array generation in terms of light concentration ability.

Light propagation and disorder effects in semiconductor multiple quantum wells.

Propagation of femtosecond light pulses in semiconductor multiple quantum wells with interface roughness is investigated theoretically and experimentally. It is shown that the nonlocal susceptibility causes exciton broadening, decreases the relative magnitude of the 1s and 2s resonances, and leads to multiple reflections. The time resolved signal exhibits modulations that are sensitive to the barrier thickness. The absorption spectrum and time dependence of the beat frequency are influenced by static disorder.