Propagation of X-ray femtosecond pulses through tapered nanometer-scale capillary waveguides

It is shown that temporal broadening of an X-ray femtosecond pulse under propagation through a tapered nanometer-scale waveguide is negligible small for the wide region of the guide and pulse parameters. The output pulse has the step-like transverse intensity distribution. The conventional waveguide theories do not predict this phenomenon.     

Poincaré representation of polarization-shaped femtosecond laser pulses

The usage of Poincaré phase space for the representation of polarization-shaped femtosecond laser pulses is discussed. In these types of light fields the polarization state (i.e. ellipticity and orientation) changes as a function of time within a single laser pulse. Such deliberate variation can be achieved by frequency-domain femtosecond pulse shaping in which two polarization components are manipulated individually. Here it is shown how these light pulses can be represented as temporal trajectories through the ellipticity-orientation (Poincaré) phase space, whereas conventional light (either continuous-wave or pulsed) is determined by only one specific Poincaré location. General properties of parametric Poincaré trajectories are discussed, and their relation to experimentally accessible pulse-manipulation parameters (i.e. amplitudes and phases) determined. Specifically, it is shown how the maximum rate by which a given polarization state can be turned into a different one (at significant intensity levels) is limited by the spectral laser bandwidth. Apart from their direct usage in polarization-shaped pulse representation, Poincaré trajectories also form the basis for intuitive quasi-three-dimensional renderings of the electric field profile. There, the temporal evolution of polarization, intensity, and chirp is directly apparent in a single illustration. Received: 10 December 2002 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-931/888-4906, E-mail: brixner@physik.uni-wuerzburg.de     

Acousto-optical shaping of ultraviolet femtosecond pulses

This work demonstrates a simple method for ultraviolet (UV) acousto-optical pulse shaping of both spectral amplitude and phase. A fused-silica acousto-optical modulator is used to ensure high transmission and a high damage threshold at 400-nm center wavelength. The technique eliminates complications associated with the parametric transfer of the spectral phase of near-infrared pulses through a nonlinear process out to UV wavelengths, by separating the frequency doubling and shaping processes. Three illustrative applications of phase control are presented: the compensation of material dispersion, the generation of multiple pulse trains, and the generation of arbitrarily shaped pulse trains. Self-diffraction frequency-resolved optical gating is used to characterize the success of the technique.     

Self-compression and controllable guidance of multi-millijoule femtosecond laser pulses

Self-compression of multi-millijoule femtosecond laser pulses and dramatic increase of the peak intensity are found in pressurized helium and neon within a range of intensity in which the ionization modification of the material parameters by the pulse is negligible. The pulse propagation is studied by the (3 + 1)-dimensional nonlinear Schrödinger equation including basic lowest order optical processes - diffraction, second order of dispersion, and third order of nonlinearity. Smooth and well controllable pulse propagation dynamics is found. Construction of compressed pulses of controllable parameters at given space target point by a proper chose of the pulse energy and/or gas pressure is predicted.     

Efficient transmission in cascaded photonic crystal waveguides via a strong coupling effect

A design of cascaded photonic crystal waveguide is proposed in this paper inspired by the work of Tang et al. [D. Tang, L. Chen, W. Ding, Appl. Phys. Lett. 89 (2006) 131120]. In contrast to a conventional waveguide source, a plane wave source is applied in the current design. We show that an efficient guide mode in the photonic band gap can be achieved. The same idea also works for a slight variation by defects introduction in the photonic crystal. Finally, the strong coupling effect present in the cascaded waveguides is demonstrated by an analogy with photonic quantum wells.     

Nonlinear propagation dynamics of an ultrashort pulse in a hollow waveguide

We present a theoretical investigation of the self-focusing dynamics of femtosecond pulses in a hollow waveguide. We show that transverse effects play an important role in these dynamics, even for pulses that are significantly below the critical power for self-focusing in free space, and that excitation of higher-order modes of the waveguide results in the spreading of the pulse in time. Inclusion of self-steepening and space-time focusing in our model is necessary for properly capturing the pulse dynamics.     

Focus shaping of cylindrically polarized vortex beams by a high numerical-aperture lens

The focus-shaping technique of a cylindrically polarized vortex beam by a high numerical-aperture lens is reported. Such a polarized vortex beam is decomposed into radial and azimuthal polarization. It is shown that the total intensity distribution in the focal region is dependent not only on the numerical-aperture maximal angle and the polarization rotation angle but also on the topological charge. By choosing the proper combination of parameters, the adjustably confined flat-topped focus and focal hole can be obtained. The focus-shaping technique may find wide applications, such as optical tweezers, laser printing and material processing.     

Propagation properties of anomalous hollow beams in a turbulent atmosphere

Propagation of circular and elliptical anomalous hollow beams in a turbulent atmosphere is investigated in detail. Based on the extended Huygens–Fresnel integral, analytical formulae for the average irradiance of circular and elliptical anomalous hollow beams propagating in a turbulent atmosphere are derived. The irradiance and spreading properties of circular and elliptical anomalous hollow beams in a turbulent atmosphere and in free space are studied numerically. It is found that circular and elliptical anomalous hollow beams at short propagation distance in turbulent atmosphere have similar propagation properties to those of free space, while at long propagation distance, circular and elliptical anomalous hollow beams eventually become circular Gaussian beams in a turbulent atmosphere, which is much different from their propagation properties in free space. The conversion from an anomalous hollow beam to a circular Gaussian beam becomes quicker and the beam spot spreads more rapidly for a larger structure constant, a shorter wavelength and a smaller waist size of the initial beam.     

Concatenation of plasma filaments created in air by femtosecond infrared laser pulses

We report the first observation of the attachment of two single plasma filaments created collinearly in the atmosphere by IR femtosecond laser pulses. The linked filamentary structure is electrically conductive and emits sub-THz radiation over its entire length. Concatenation is achieved only for a specific time ordering between the two initial laser pulses. The pulse producing the filament closer to the laser source must be retarded with respect to the other pulse. This special time ordering is attributed to the acceleration of light in a self-guided pulse. Received: 4 March 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/6931-9996, E-mail: stzortz@ensta.fr     

Phase-matched four-wave mixing of isolated waveguide modes of high-intensity femtosecond pulses in a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers with a special dispersion profile are shown to allow phase-matched nonlinear optical interactions of isolated air-guided modes of high-intensity femtosecond laser pulses confined in the hollow fiber core. We present theoretical and experimental studies of the four-wave mixing of fundamental and second-harmonic pulses of a Cr:forsterite laser with an initial pulse duration of about 50 fs and an intensity on the order of 10¹⁴ W/cm² in waveguide modes of a hollow photonic-crystal fiber with a core diameter of about 13μm.     

Depolarization characteristics of incompletely polarized and partially coherent laser beams in slant atmospheric turbulence

Based on the cross-spectral density function of transmission theory and the unified theory of coherence and polarization, the depolarization characteristics of incompletely polarized and partially coherent laser propagation in slant atmospheric turbulence are investigated. According to extended Huygens–Fresnel principle, the analytical expressions for intensity and degree of polarization in the slant path are derived. The effects of the wavelength, the initial spot size and the transmission distance on the intensity and degree of polarization are described. The results show that a more stable distribution of the degree of polarization at the receiver is obtained with increasing wavelength for a certain receiver height. The conclusions play an important role in optical communications and target recognition.     

Confined micro-explosion induced by ultrashort laser pulse at SiO2/Si interface

Ultrashort laser pulses tightly focused inside a transparent material present an example of laser interaction with matter where all the laser-affected material remains inside the bulk, thus the mass is conserved. In this paper, we present the case where the high intensity of a laser pulse is above the threshold for optical breakdown, and the material is ionised in the focal area. We consider in detail a special case where a micro-explosion is formed at the boundary of a silicon surface buried under a 10-micron-thick oxidised layer, providing the opportunity to affect the silicon crystal by a strong shock wave and creating new material phases from the plasma state. We summarise the main conclusions on ultrafast laser-induced material modifications in confined geometry and discuss the prospects of confined micro-explosion for forming new silicon phases.     

Stochastic electromagnetic vortex beam and its propagation

The recent theory formulated in terms of the 2×2 cross-spectral density matrix and the propagation law of cross-spectral density are employed to investigate the stochastic electromagnetic vortex beam and its propagation characterization. Based on these, we derived the general formulae for the intensity distribution, degree of coherence and degree of polarization for stochastic electromagnetic vortex beam while propagating in free space. It is shown that the intensity distribution and the degree of polarization of the stochastic electromagnetic vortex beam propagating in free space depend on the correlation length and the topological charge of the vortex beam.     

Optical rectification of highly focused near-IR pulses in the plasmon waveguide

Generation of terahertz pulses via mixing the Fourier harmonics of strongly focused near-IR pumping laser pulses in a plasmon waveguide filled with a nonlinear medium is considered. At a certain choice of its parameters, this waveguide provides efficient transverse confinement of the terahertz mode and a low dispersion of its refractive index. As a result, the system becomes capable of generating short fairly intense terahertz pulses. Such an approach significantly (by a factor of 50) decreases the pump power compared with that in the conventional (waveguide-free) schemes, which generate terahertz pulses from single pumping pulses (optical rectification), and still yields the same intensity of the output terahertz pulse and conversion efficiency. For the Ag-GaP-Ag structure taken as an example, optimal parameters of the plasmon waveguide and pumping pulse are found and characteristics of the output terahertz pulse are calculated.     

Q-switch of a continuously pumped CO2 laser with a scanning coupled-cavity Michelson mirror

We report on the possibility of Q-switching a continuously pumped CO₂ laser using a scanning Michelson interferometer as an end mirror, instead of the habitual well-known strategies. This method, in addition to its simplicity, produces free tail relaxation pulses having duration of about 1.3 μs, which is comparable to what can be obtained when using a saturable absorber. A pulse repetition frequency as high as 90 kHz is obtained.     

Narrowing behavior of chirped pulses with small Fresnel number in focusing systems

Based on the diffraction integral and Fourier transform, the fields of chirped pulses passing through a thin lens are derived. Influences of Fresnel number and chirp parameter on the transverse intensity distributions of the chirped pulses at focal plane are illustrated with numerical results. Also, effect of pulse duration on the intensities is investigated. It is found that the width of transverse intensity distribution of the chirped pulses decreases with increasing the chirp parameter and the narrowing behavior presents more obvious with smaller Fresnel number. A physical explanation for the narrowing behavior is given on the basis of a simplified form of the field expression.     

Slow light pulse propagation in dispersive media

We present a theoretical and numerical analysis of pulse propagation in a semiconductor photonic crystal waveguide with embedded quantum dots in a regime where the pulse is subjected to both waveguide and material dispersion. The group index and the transmission are investigated by finite-difference-time-domain Maxwell–Bloch simulations and compared to analytic results. For long pulses the group index (transmission) for the combined system is significantly enhanced (reduced) relative to slow light based on purely material or waveguide dispersion. Shorter pulses are strongly distorted and depending on parameters broadening or break-up of the pulse may be observed. The transition from linear to nonlinear pulse propagation is quantified in terms of the spectral width of the pulse. To cite this article: T.R. Nielsen et al., C. R. Physique 10 (2009).     

Effects of the frequency chirp on the fields of a chirped Gaussian pulse passing through a hard-edged aperture

Based on the Huygens-Fresnel diffraction integral and Fourier transform, propagation expression of a chirped Gaussian pulse passing through a hard-edged aperture is derived. Intensity distributions of the pulse with different frequency chirp in the near-field and far-field are analyzed in detail by numerical calculations. In the near-field, amplitudes of the intensity peaks generated by the modulation of the hard-edged aperture decrease with increasing the frequency chirp, which results in the improving of the beam uniformity. A physical explanation for the smoothing effect brought by increasing the frequency chirp is given. The smoothing effect is achieved not only in the pulse with Gaussian transverse profile but also in the pulse with Hermite-Gaussian transverse profile when the frequency chirp increases.     

I-scan thermal lens experiment in the pulse regime for measuring two-photon absorption coefficient

We present a new pump-probe mode-mismatched thermal lens method for pulse excitation aimed to the measurement of nonlinear absorption coefficient in optical materials. We develop a theoretical model based on the Fresnel diffraction approximation and their predictions are verified experimentally with samples of Rhodamine 6G and Rhodamine B in ethanol solution. The principal advantage of this technique is that it does not require any mechanical movement during measurement. Below we perform the new type of thermal lens experiment in the pulse regime for the measurement of nonlinear absorption coefficient in transparent samples and we demonstrate the validity of theoretical predictions using an alternative method to the classical thermal lens technique.