Function Of Incident Intensity Research Articles

Scattering of intense femtosecond laser radiation at water aerosol in backward direction

We investigated the scattering of femtosecond laser pulses with intensities within the 1012–1013W/cm2 range at a water aerosol jet at 20° to backward direction. The scattered energy and spectra transformation as a function of incident intensity obtained in experiment show good agreement with the results of extensive numerical modeling based on self-consistent solution of Maxwell equations using a nonlinear 3D FDTD code and balance equation for plasma density.

Femtosecond laser pulse-induced breakdown of a single water microdroplet

The interaction of femtosecond laser radiation with incident intensities in the 0.1–10  TW/cm2 range and a single 30 μm water microdroplet is investigated experimentally and numerically. The experimental dependencies of backscattered energy, spectrum, and acoustic signal as a function of incident intensity are compared with the results of extensive numerical modeling based, for the first time, on self-consistent solution of Maxwell equations using a nonlinear 3D finite-distance time-domain (FDTD) code and a balance equation for plasma density.

Study of UV Cure Kinetics Resulting from a Changing Concentration of Mobile and Trapped Radicals

UV cure kinetics of the system bisphenol A dimethacrylate and a phenylphosphine oxide initiator are studied as a function of incident intensity (I) and initiator concentration (PI). The rate of polymerization, d(ln[M(t)])/dt, is found to be proportional to the intensity and initiator concentration raised to an exponential power of approximately 0.7 rather than the classical value of 0.5. The system does not obey classical steady-state kinetics but rather the rate of the reaction reaches a maximum shortly after gel and then decreases rapidly well before the reaction is quenched during the glass transition. These nonclassical results are proposed to be due to the coexistence of a varying ratio of the traditional bimolecular kinetics and a unimolecular trapped radical termination process due to the changing spatial/ dynamic heterogeneity arising from microgel formation in these systems. A model which focuses on the changing concentration of mobile active radicals is proposed. It uses four fitting parameters to describe changes in the total number of radicals minus the number of trapped immobile radicals. The model assumes a constant propagation rate constant, kp, as is generally assumed in traditional crosslinking reactions up to the reaction quench in the final stage of the cure. The model is similar in mathematical form to a more complex multiparameter free volume molecular approach in which both kp, kt, and a trapping rate constant change with reaction advancement. Our model is based on the changing mobile radical concentration, which reflects the changes in the spatial heterogeneity of these systems due to the formation of microgels at the beginning of the reaction. It predicts the initial rapid buildup of the total radical concentration to a constant value and then the existence of an increasing proportion of trapped radicals up to reaction quenching in the glass transition. It accurately describes the UV cure kinetics at varying incident intensities and varying initiator concentrations and its predictions of the changes in the total and trapped radical concentrations are similar to ESR measurements on other systems.

Pulse reshaping in photonic crystal waveguides and microcavities with Kerr nonlinearity: Critical issues for all-optical switching

We delineate critical issues for ``controlling light with light'' in photonic crystal (PC) waveguides coupled to Kerr-nonlinear microresonators. These arise from (a) fundamental trade-off between switching speed and switching intensity threshold inherent in high-quality $Q$-factor cavities and (b) the dynamical nonlinear oscillation of such cavities in response to incident light pulses. Using finite-difference time-domain simulations of electromagnetic pulse propagation, we consider both (i) a nonlinear Fabry-Perot microresonator (embedded within a PC waveguide) exhibiting a narrow transmission resonance and (ii) a nonlinear point defect (side-coupled to a PC waveguide) exhibiting a narrow reflection spectrum. We describe self-induced switching from transmission to reflection induced by pulse intensity tuning as well as control of pulse transmission induced by the secondary, continuous (cw) laser field propagating through the same PC waveguide. For the Fabry-Perot microresonator, a well-defined self-switching threshold is obtained. However, this is accompanied by considerable temporal and spectral distortion of the pulse caused by the oscillatory nonlinear response of the microresonator. When the quality factor of the microresonator is increased, the switching intensity threshold can be lowered but the pulse transit (switching) time and the pulse distortion are increased. For the side-coupled microresonator, a gradual (not sharp) self-switching behavior as a function of incident intensity is obtained. For both the Fabry-Perot and side-coupled nonlinear microresonators, control of pulse transmission can be achieved by means of a secondary cw laser field. The cw power required for switching with realistic Kerr nonlinearities is in excess of $1\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕\ensuremath{\mu}{\mathrm{m}}^{2}$ and may cause optical damage to the semiconducting PC backbone. Both instantaneous and noninstantaneous Kerr-response models are considered. Our results underscore the limitations and trade-offs inherent in the possible control of light with light using Kerr-nonlinear microresonators.

Analysis of higher-order self-diffraction of low-power continuous wave based on nonlinear transmission of bR film

Higher-order self-diffraction in bacteriorhodopisn (bR) film has been investigated. The diffraction efficiencies of higher-order self-diffraction as a function of incident intensity and order are analysed theoretically. The increase in diffraction efficiency at higher orders is due to the nonlinear transmission property of bR film, which leads to a non-sinusoidal grating shape and induces an increase in harmonic components. Calculated results for diffraction efficiencies taking into account the contribution of harmonic components are in agreement with measured results.

Analysis of higher-order self-diffraction of low-power continuous wave based on nonlinear transmission of bR film

Abstract Higher-order self-diffraction in bacteriorhodopisn (bR) film has been investigated. The diffraction efficiencies of higher-order self-diffraction as a function of incident intensity and order are analysed theoretically. The increase in diffraction efficiency at higher orders is due to the nonlinear transmission property of bR film, which leads to a non-sinusoidal grating shape and induces an increase in harmonic components. Calculated results for diffraction efficiencies taking into account the contribution of harmonic components are in agreement with measured results.

Optical limiting properties of In 2O 3 nanoparticles under cw laser illumination

We have measured the effective intensity dependent nonlinear coefficient n 2 of In 2O 3 nanoparticles with surface modification by Z-scan technique. The optical limiting properties of coated In 2O 3 nanoparticles under cw laser illumination were investigated. Novel absorption at different wavelengths as a function of incident intensity was observed.

A hue preserving enhancement scheme for a class of colour images

This paper presents a method which enhances the contrast between two objects in a colour image. The method assumes that images are taken from a microscope that looks at a three-dimensional microstructure with a parallel beam of reflected light. Accordingly, the Phong model of illumination is chosen. The image intensity function is shown to contain an additive term to model specular reflection as a function of incident intensity, and a multiplicative factor due to attenuation. Thus a contrast enhancement problem reduces to the estimation and normalisation of these terms.

Optically induced birefringence in bacteriorhodopsin as an optical limiter

Experimental data are presented, which demonstrates an optical limiter based on a large birefringence which is optically induced in bacteriorhodopsin. The induced birefringence is observed to be a function of incident intensity, but saturates at a value of about 0.454 W/cm2. A measured value of Δn of 6.6×10−4 at a wavelength of 514 nm is reported. The observed birefringence is found to be in good agreement with a proposed model.

Optical bistability in nonlinear periodic structures

Optical switching and optical bistability and multistability are observed experimentally with a nonlinear periodic structure. The periodic dielectric is a colloidal crystal that exhibits a large electrostrictive nonlinearity. The transmission characteristics of these crystals as a function of incident intensity are presented for several different frequencies of light within the stopgap of the periodic structures.

Photoabsorption of BCl3 Gas under pulsed ArF excimer laser irradiation

The gas immersion laser doping (GILD) technique requires the measurement of the fraction of incident light absorbed in the gas phase during the irradiation with a pulsed laser. Here we report the absorption of boron trichloride (BCl3) gas at the wavelength of a pulsed ArF excimer laser (λ=193 nm). We have determined the one-photon (σ1) and two-photon (σ) absorption cross sections of this dopant gas for 193 nm. The values of σ1 and σ are 3.6×10−20 cm2 and 9×10−45 cm4·s, respectively. However, the distinction between simultaneous and sequential absorption has not been possible. Based on these results, we have established a relationship which allows the calculation of the fraction of incident light absorbed as a function of incident intensity and gas pressure.

Theoretical intensity-dependent response of nonlinear periodic structures

We have modeled the response of a nonlinear periodic structure by means of the Abelés 2×2 matrix method. Our structure differs from the usual rejection-band filter designs, in that we have chosen the filter elements to be index matched in the absence of radiation, providing a rejection band that both grows and shifts as a function of incident intensity. The intensity output function of the model not only directly demonstrates optical bistability, but also limiting, switching, self-pulsing, and chaos.

Comparison of nonlinear optical responses of periodic and quasiperiodic superlattices

For one-dimensional superlattices consisting of unit cells with two films, one of which exhibits nonlinear response to the electromagnetic field, the transmissivity as a function of incident intensity and frequency is calculated for periodic and quasiperiodic structures. It is found that gap-soliton-mediated resonances for periodic structures arise from the Fabry-P\'erot oscillations in the transmission band. Solitons in the stop gap are not found in the quasiperiodic case.

Determination of the branching of the 1P1 Ba

The resonance fluorescence from the $^{138}\mathrm{Ba}$ I 5535-\AA{} 6${\mathit{s}}^{21}$${\mathit{S}}_{0}$-6s6p $^{1}$${\mathit{P}}_{1}$ transition has been measured as a function of incident intensity using a Doppler-free atomic beam geometry and a single-mode cw dye laser. The upper $^{1}\mathrm{P}_{1}$ level has a weak branch to intermediate metastable D states. A long laser-atom interaction region was used, which ensured that essentially all atoms could be pumped into the D states during their transit through the laser beam. By analyzing the shape of the saturation curve a value for the branching ratio to the 6s5d $^{1}$,3D levels of (3.57\ifmmode\pm\else\textpm\fi{}0.38)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ has been obtained.

Fluorescence kinetics of emission from a small finite volume of a biological system

The fluorescence decay, apparent quantum yield and transmission from chromophores constrained to a microscopic volume using a single picosecond laser excitation were measured as a function of incident intensity. The β subunit of phycoeryhthrin aggregate isolated from the photosynthetic antenna system of Nostoc sp. was selected since it contains only four chromophores in a volume of less than 5.6×10 4 Å 3. The non-exponential fluorescence decay profiles were intensity independent for the intensity range studied (5 × 10 13 - 2 × 10 15 photon cm −2 per pulse). The apparent decrease in the relative fluorescence quantum yield and increase of the relative transmission with increasing excitation intensity is attributed to the combined effects of ground state depletion and upper excited state absorption. Evidence suggests that exciton annihilation is absent within isolated β subunits.

A superconducting niobium powder detector for submillimeter radiation

A report is given of an attempt to make a radiation detector for the submillimeter range using a superconducting niobium powder. The voltage across the detector was measured as a function of incident intensity at a fixed bias current. Sample fabrication is discussed, and the I--V and response characteristics of a typical detector are given. (PMA)