Chirped femtosecond pulse scattering by spherical particles

Generalized Lorentz-Mie formulas are used to study the scattering characteristics when a chirped femtosecond pulse illuminates a spherical particle. For a linear chirped Gaussian pulse with the envelope function g(τ) = exp[-π(1 + ib)τ(2)], dimensionless parameter b is defined as a chirp. The calculation illustrated that even for pulses with a constant carrier wavelength (λ(0) = 0.5 μm) and pulse-filling coefficient (l(0) = 1.98), the efficiencies for extinction and scattering differ very much between the carrier wave and the different chirped pulses. The slowly varying background of the extinction and the scattering curves is damped by the chirp. When the pulse is deeply chirped, the maxima and minima of the background curves reduce to the point where they disappear, and the efficiency curves illustrate a steplike dependence on the sphere size. Another feature is that the chirped-pulse scattering seems blind: it depends only on the amount of chirp (|b|), regardless of upchirp (b > 0) or downchirp (b< 0).     

Elastic light scattering from single microparticles on a femtosecond time scale

Experimentally measured and theoretically calculated elastic light-scattering spectra from single microparticles illuminated by 100-fs pulses are presented. Although in the theoretical calculation only a single incoming femtosecond laser pulse was used, the spectral behavior of scattered light shows all the features seen in the experimental spectrum from many femtosecond pulses, including morphology-dependent resonances (MDR's). The good agreement between experimental and theoretical elastic light-scattering data has stimulated a theoretical investigation of the time-dependent behavior of the elastically scattered light from a single microparticle on a femtosecond time scale. Since the spatial pulse length of the incoming laser pulse is smaller than the particle circumference, the temporal behavior of reflection, diffraction, refraction, and coupling into MDR's can be distinguished. Since the time-dependent scattering is strongly dependent on particle size, refractive index, and pulse chirp, it may be possible to encode several bits of information into a single laser pulse and therefore to increase optical data communication rates.     

Change of ultra-short laser pulse shape after scattering by a nanosphere in a dielectric matrix

The scattering of ultra-short laser pulses by a metallic nanosphere embedded in a dielectric matrix was studied theoretically in the frame of the dipole approximation. Calculations were performed for the corrected Gaussian shape of the incident laser pulse, which enabled us to consider both multi-cycle and sub-cycle pulses. Analysis of the influence of the scattering process on the shape of scattered pulse is made for various durations and carrier frequencies of the incident pulses.     

Effect of laser-radiation polarization on the nonlinear scattering of light in nanodiamond suspensions

The effect of laser radiation polarization on the nonlinear scattering of light in aqueous suspensions of detonation nanodiamonds (DNDs) in a regime of optical power limiting (OPL) has been studied. It is established that the nonlinear transmission coefficient of DND suspension in the OPL regime in a field of nanosecond laser pulses with a wavelength of 532 nm is independent of the polarization of incident radiation. The nonlinear scattering of light observed at an angle of 90° in the plane perpendicular to the plane of polarization of the incident radiation depends on the polarization angle in accordance with a trigonometric law. It is shown that the ratio of the signals of scattered radiation for the vertical and horizontal polarizations exhibits nonmonotonic dependence on the laser-beam power density. The results are explained by the Rayleigh-Mie scattering and a change in the size of scattering centers as a result of the effect of a laser upon the DND suspension.     

Mie scattering in the time domain. Part 1. The role of surface waves

We computed the Debye series p=1 and p=2 terms of the Mie scattered intensity as a function of scattering angle and delay time for a linearly polarized plane wave pulse incident on a spherical dielectric particle and physically interpreted the resulting numerical data. Radiation shed by electromagnetic surface waves plays a prominent role in the scattered intensity. We determined the surface wave phase and damping rate and studied the structure of the p=1,2 surface wave glory in the time domain.     

Stimulated Raman Scattering and Self-phase Modulation of Ultrashort Light Pulses in Optical Fibres

Stimulated Raman scattering of picosecond pulses in optical fibre is investigated, with account taken of pulse walk-off by group velocity dispersion, pump depletion and chirp production by the intensity-dependent refractive index. An analytical solution of the basic equation is given by which the characteristic parameters of the Stokes and pump pulses, such as pulse half-width, intensity, mid-frequency and chirp, are calculated. It is shown that the chirp of the Stokes pulse does increase faster than that of the pump pulse with increasing fibre length and may surpass it by nearly three times after the pulse walk-off.     

Nonstationary scattering of electromagnetic pulses by spherical particles

We have studied time variances of the shape of an electromagnetic pulse scattered by a spherical particle. General formulas are derived for a pulse with an arbitrary envelope, for momentary values of scattered-light fields and light intensity, and for efficiencies of extinction and scattering. It is possible, by the use of these formulas, to obtain by routine integration the sensitivity reaction of a receiver with any time dependence. The formulas are illustrated with examples of scattering of a Gaussian pulse with a carrier wave λ(0) = 0.6328 μm and of multisized water drops. Pulses of different durations are studied. However, only those pulses that have all significant values of the Fourier density in the domain of positive frequencies ω are considered.     

Characterization of chirped pulses with the fractional-order Fourier transformation.

We show that the fractional-order Fourier transformation (FRT) is a suitable method to describe chirped pulses submitted to group-velocity dispersion in a linear dispersive medium. Amplitudes exhibiting different chirp coefficients are easily separated with the FRT, although they are temporally superposed.     

Resonant scattering of light from a glass/Ag/MgF2/air system with rough interfaces and supporting guided modes in attenuated total reflection

We report experimental results of the resonant scattering of light from a prism-glass/Ag/MgF2/air system with use of the attenuated total reflection technique for p and s polarized light. Two MgF2 film thicknesses were used. The system with the thinner dielectric layer supports two transverse magnetic (TM) and two transverse electric (TE) guided modes at a wavelength of 632.8 nm, and the system with the thicker dielectric layer supports three TM and three TE guided modes. In both cases we found dips in the specular reflection as a function of incident angle that is due to excitation of guided modes in the MgF2 film. The scattered light shows peaks at angles corresponding to the measured excitation of the guided modes. These peaks are due to single-order scattering and occur for any angle of the incident light. All features in the scattering response are enhanced in resonance conditions, and the efficiency of injecting light into the guide is reduced.     

Geometrically enhanced morphology-dependent resonances of a dielectric sphere

The effect that geometrical resonances of orbiting internally reflecting rays have on the morphology-dependent resonances of microspheres is investigated heuristically and numerically using generalized Lorenz-Mie theory. Angularly resolved off-axis Gaussian beam elastic scattering spectra are presented. The results obtained show that the elastic scattering intensity of morphology-dependent resonances is noticeably enhanced in the vicinity of the geometrical resonance scattering angles.     

Evaluation of an ultrabroadband high-gain amplification technique for chirped pulse amplification facilities

Recently, an amplification technique for ultrashort pulses was explored in detail in a theoretical paper by Ross et al. [Opt. Commun. 144, 125 (1997)]. The technique, based on nonlinear optics, is called optical parametric chirped pulse amplification. It has a number of features that, in principle, make it highly attractive. It primarily offers extremely large gains simultaneously with extremely large bandwidths. Additional attractions are virtually no spatial and temporal phase distortion of the amplified pulse, high efficiencies and a low thermal loading, reduced amplified spontaneous emission levels, small optical material lengths, and an inherent simplicity of implementation. We present an evaluation of the technique as a front end amplifier for the ultrashort pulse amplification chain of the Vulcan laser system. Such a device could replace some of the existing amplification in Nd:glass and additionally have a wider effect as a direct replacement of Ti:sapphire regenerative amplifiers on large-scale chirped pulse amplification scale facilities.     

Investigation of a phase transition in a single optically levitated microdroplet by Raman-Mie scattering

Light-scattering measurements of optically levitated microdroplets containing three components, glycerin, water, and ammonium sulfate, are presented. Evaporation of the microdroplet is studied by means of morphology-dependent resonances observed in both Raman spectra as well as elastically scattered light and by the simultaneous measurement of the laser power. The phase transition from the liquid to the solid state of ammonium sulfate inside the microdroplet is observed by means of morphology-dependent resonances and Raman scattering.     

Quasi-stationary scattering of electromagnetic pulses by spherical particles

The Lorentz-Mie theory is generalized for the case of a spherical particle irradiated by a pulse with a finite length L that is transferred by a carrier wavelength λ(0). Two cases should be physically distinguished, depending on radiation-receiver properties: quasi-stationary scattering (a receiver integrates the entire signal over time) and nonstationary scattering, when a receiver is capable of recording scattered signal changes with time. General formulas that allow one to calculate optical characteristics for both scattering cases and for an arbitrary ratio L/λ(0) are derived. Quasi-stationary-scattering peculiarities and limiting cases of small and large particles are studied in detail. The formulas are illustrated with calculations of spherical-particle optical characteristics for pulses of different lengths, for differently sized particles, and for a case in which a scattered pulse has a Gaussian form. The results obtained should be taken into account when one is studying the passage of a pulse through scattering media.     

Characterization of ultrashort pulses by a modified grating-eliminated no-nonsense observation of ultrafast incident laser light E fields (GRENOUILLE) method

The measurement and characterization of ultrashort laser pulses remains an arduous task. The most commonly used pulse-measurement method is known as frequency-resolved optical gating (FROG), and another version with great experimental simplification and low-priced setup is known as grating-eliminated no-nonsense observation of ultrafast incident laser light E fields (GRENOUILLE). Nevertheless, there is interest in elaborating other, more accessible or simpler and cheaper, setups with equal or better assets. We explored modification of the GRENOUILLE method in which we replaced the original Fresnel biprism with a beam splitter and two mirrors and used a cheap webcam to measure the pulse traces. We have evaluated our system, and we propose a method to correct border effects caused by the beam intensity's profile based on the characterization of three pulse classes: Fourier-transform limited, double, and chirped. We compare the recovered electric field with further spectral and second-order correlation data of the corresponding pulses.     

A circularly focused and scanned acoustic scanner

This paper describes a new type of electronically scanned and focused acoustic imaging device called the cylindrical grating acoustic scanner. With this device, a new transducer called the circular-edge-bonded transducer (CEBT) is devised that is bonded on one end of a cylindrical substrate and can generate a surface acoustic wave propagating along the cylindrical surface of the substrate. An array of grooves is fabricated on the cylindrical surface of the substrate to serve as a scattering grating. The periodical grooves scatter a chirped surface-wave pulse coherently into a focused bulk-wave "ring" that scans at the surface-wave velocity. The focal length and resolution can be adjusted by changing the chirp rate and time-bandwidth product of the chirp, respectively. A 2.3-MHz circular scanner with 25-cm field of view and 2.6-mm resolution has been constructed and studied. Acoustic image of an artificial defect in an aluminum pipe is obtained.     

Ultrafast, cross-correlated harmonic imaging through scattering media

A simple upconversion scheme utilizing 40-fs pulses is shown to permit high-contrast imaging of objects obscured by a highly scattering medium when no ballistic component is evident in the scattered light and imaging is performed with any portion of the scattered light pulse. We present a time-gated, inherently low-pass spatially filtered imaging method that minimizes signal-averaging requirements and greatly facilitates imaging under severe scattering (turbid) conditions.     

Influence of the wavefront mismatch,spatial chirp and multimode pump on the beam quality of OPCPA

The beam quality and wavefront distribution of the optical parametric chirped‐pulse amplification (OPCPA) front‐end system of the petawatt laser has become a focus for research. The influence of wavefront mismatch, spatial chirp and multimode pump on the beam quality of OPCPA is numerically simulated and analyzed. It is shown that the beam quality and wavefront distribution becomes worse as the mismatch angle or the order of the spatial chirp increases. Meanwhile, it is also found that the beam quality and conversion efficiency of the amplified signal becomes worse as the transverse distribution of the pump laser descends.     

Ultrashort pulse compression by using one-dimensional inhomogeneous photonic crystal

Ultrashort pulse compression by propagation in one-dimensional inhomogeneous photonic crystals with symmetric and asymmetric refractive index profiles are studied by using the transfer matrix method and Fourier analysis. The effect of different parameters, like chirp value of the incident pulse, angle of incidence, and number of unit cells on the compress factors, are investigated and temporal evolution of the reflected pulse is calculated. Compress factors as low as 32% for a sinusoidal profile are obtained using this structure.     

Nonlinear scattering of light in nanodiamond hydrosol

The nonlinear scattering of light under the conditions of optical limiting of nanosecond pulsed laser radiation at a wavelength of 1064 nm in a nanodiamond (ND) hydrosol has been experimentally studied. Superstable hydrosols were obtained from detonation NDs with a modified surface. Using an improved scheme of z scanning, it is shown that a decrease in the optical transmission coefficient of an ND hydrosol under optical limiting conditions is due to enhanced nonlinear scattering. It is established that the energy of pulsed radiation scattered at a right angle obeys a power law in dependence on the energy density of incident radiation pulses. Hydrosols of detonation NDs with the modified surface exhibit high stability with respect to the periodic laser action at high power density.     

Chirped optical X-shaped pulses in material media

We analyze the properties of chirped optical X-shaped pulses propagating in material media without boundaries. We show that such ("superluminal") pulses may recover their transverse and longitudinal shapes after some propagation distance, whereas the ordinary chirped Gaussian pulses can recover their longitudinal width only (since Gaussian pulses suffer a progressive transverse spreading during their propagation). We therefore propose the use of chirped optical X-type pulses to overcome the problems of both dispersion and diffraction during pulse propagation.