One dimensional spatial localization of polychromatic stationary wave-packets in normally dispersive media.

In this paper we illustrate how the localization of the stationary two-dimensional solution of the propagation equation strongly depends on the features of its spatio-temporal spectral bandwidth. We especially investigate the role of the ultra-broad temporal support and of the spatial bandwidth of the spectrum on the high localization in one spatial dimension of "Bessel-like" or "blade-like" beams, quasi-stationarily propagating in normally dispersive materials, and potentially interesting for microfabrication applications.

Experimental observation of a skewed X-type spatiotemporal correlation of ultrabroadband twin beams.

This work presents the experimental observation of the nonfactorable near-field spatiotemporal correlation of ultrabroadband twin beams generated by parametric down-conversion, in an interferometric-type experiment using sum frequency generation, where both the temporal and the spatial degrees of freedom of parametric down-conversion light are controlled with high resolution. The revealed correlation is skewed in space-time in accordance with the X structure predicted by the theory.

Detection of the ultranarrow temporal correlation of twin beams via sum-frequency generation.

We demonstrate the ultranarrow temporal correlation (6 fs full width half maximum) of twin beams generated by parametric down-conversion by using its reverse process, i.e., sum-frequency generation. The result relies on an achromatic imaging of a huge bandwidth of twin beams and on a careful control of their spatial degrees of freedom. The detrimental effects of spatial filtering and imperfect imaging are shown, along with the theoretical model used to describe the results.

Generation of broadly tunable sub-30-fs infrared pulses by four-wave optical parametric amplification.

We report on the generation of sub-30-fs near-IR light pulses by means of broadband four-wave parametric amplification in fused silica. This is achieved by frequency downconversion of visible broadband pulses provided by a commercial blue-pumped beta-barium borate crystal-based noncollinear optical parametric amplifier. The proposed method produces the IR idler pulses with energy up to ∼20 µJ and tunable in wavelength from 1 to 1.5 µm. The shortest pulse duration is 17.6 fs, measured at 1.2 µm.

Space-time focusing of Bessel-like pulses.

We report on a space-time compression technique allowing for complete and independent control of the longitudinal dynamics and of the transverse pulse localization by means of spatial beam shaping. We experimentally observe both strong temporal compression and high transverse localization, of the order of a few wavelengths, along free-space propagation.

Spatiotemporal amplitude and phase retrieval of Bessel-X pulses using a Hartmann-Shack sensor.

We propose a new experimental technique, which allows for a complete characterization of ultrashort optical pulses both in space and in time. Combining the well-known Frequency-Resolved-Optical-Gating technique for the retrieval of the temporal profile of the pulse with a measurement of the near-field made with an Hartmann-Shack sensor, we are able to retrieve the spatiotemporal amplitude and phase profile of a Bessel-X pulse. By following the pulse evolution along the propagation direction we highlight the superluminal propagation of the pulse peak.

Long spatio-temporally stationary filaments in air using short pulse UV laser Bessel beams.

The formation of long stationary filaments resulting in uniform high density plasma strings in air using short pulse UV laser Bessel beams is shown. The length and the electron density of the plasma strings can be easily tuned by adjusting the conical Bessel wavefront angle. It is shown that in this regime the length of the plasma string can be extended over meter-long scales without any compromise in the string uniformity or any temporal evolution of the filamented laser pulse.

Spontaneous emergence of pulses with constant carrier-envelope phase in femtosecond filamentation.

We study the possibility to obtain high-intensity pulses that maintain a constant Carrier Envelope Phase (CEP) during propagation in dispersive media, i.e. pulses such that the carrier-wave offset with respect to the main intensity peak remains fixed. Our numerical experiments strongly suggest that pulse splitting and X-wave formation within femtosecond laser pulse filamentation leads to the formation of "constant-CEP" within well-defined regions inside the filament. We study the creation of "constant- CEP" pulses in both gasseous and condensed media showing that this is a generic feature of filaments.

Finite-energy, accelerating Bessel pulses.

We numerically investigate the possibility to generate freely accelerating or decelerating pulses. In particular it is shown that acceleration along the propagation direction z may be obtained by a purely spatial modulation of an input Gaussian pulse in the form of finite-energy Bessel pulses with a cone angle that varies along the radial coordinate.We discuss simple practical implementations of such accelerating Bessel beams.

Kerr-induced spontaneous Bessel beam formation in the regime of strong two-photon absorption.

We study the effect of Two-Photon Absorption (TPA) nonlinear losses on Gaussian pulses, with power that exceeds the critical power for self-focusing, propagating in bulk kerr media. Experiments performed in fused silica and silicon highlight a spontaneous reshaping of the input pulse into a pulsed Bessel beam. A filament is formed in which sub-diffractive propagation is sustained by the Bessel-nature of the pulse.

Axial emission and spectral broadening in self-focusing of femtosecond Bessel beams.

We report on the experimental observations of on-axis spectral broadening arising from self-focusing of the axicon-generated femtosecond Bessel beam in water. The observed spectral broadening is interpreted by a nonlinearly phase-matched four-wave mixing process involving the intense conical pump, the axial signal and a conical idler wave.

Highly efficient four-wave parametric amplification in transparent bulk Kerr medium.

We report on highly efficient four-wave optical parametric amplification in a water cell pumped by an elliptically shaped, ultrashort pulsed laser beam under non-collinear phase-matching configuration. Energy conversion from pump to parametric waves as high as 25 % is obtained owing to the achievement of 1-dimensional spatial-soliton regime, which guarantees high intensity over a large interaction length and ensures high beam quality.

Spatio-temporal reshaping and X Wave dynamics in optical filaments.

Focus Serial: Frontiers of Nonlinear Optics

Temozolomide and thalidomide in the treatment of glioblastoma multiforme.

OBJECTIVE: The aim of this study was to assess efficacy and toxicity of temozolomide given alone or in combination with thalidomide, an anti-angiogenetic drug, in patients with newly diagnosed glioblastoma multiforme (GBM). PATIENTS AND METHODS: 46 patients with histologically proven GBM were eligible for inclusion. Twenty-three patients (15 males and 8 females) received temozolomide on a conventional schedule; 23 patients (12 males and 11 females) received temozolomide on the same schedule and thalidomide was dose-adjusted in each individual patient based on their tolerance. RESULTS: The median survival time was 12 months for temozolomide and 13 months for temozolomide + thalidomide. CONCLUSION: The administration of temozolomide in association with thalidomide after radiotherapy (RT) does not offer an advantage over temozolomide alone in adults with newly diagnosed GBM. The two therapeutic strategies produce similar results for survival, but the latter regimen shows a moderate increase in toxicity.

Nonlinear X-wave formation by femtosecond filamentation in Kerr media.

We investigate the formation of X waves during filamentation in Kerr media. From the standard model developed for femtosecond filamentation in liquids, solids, and gases, the influence of several physical effects and parameters is numerically studied in the strongly nonlinear regime where group velocity dispersion alone is insufficient to arrest collapse. The collapse is shown to be arrested by multiphoton absorption and plasma defocusing, but not by dispersion. The postcollapse dynamics takes the form of a pulse splitting, which induces large gradients in the near field and seeds the formation of X waves, appearing both in the near and far fields. We discuss the universal features of the X-wave patterns, among which the long arms in the far field that follow the linear dispersive properties of the medium [Conti, Phys. Rev. Lett. 90, 170406 (2003); Kolesik, Phys. Rev. Lett. 92, 253901 (2004)] and are accompanied by a strong modulated axial emission.

Near-field dynamics of ultrashort pulsed Bessel beams in media with Kerr nonlinearity.

The near-field dynamics of a femtosecond Bessel beam propagating in a Kerr nonlinear medium (fused silica) is investigated both numerically and experimentally. We demonstrate that the input Bessel beam experiences strong nonlinear reshaping. Due to the combined action of self-focusing and nonlinear losses the reshaped beam exhibits a radial compression and reduced visibility of the Bessel oscillations. Moreover, we show that the reshaping process starts from the intense central core and gradually replaces the Bessel beam profile during propagation, highlighting the conical geometry of the energy flow.

Near- and far-field evolution of laser pulse filaments in Kerr media.

Measurements of the spatio-temporal and far-field profiles of ultrashort laser pulses experiencing conical emission, continuum generation, and beam filamentation in a Kerr medium outline the spontaneous formation of wave packets with X -like features, thus supporting recent numerical results [M. Kolesik, E. Wright, and J. Moloney, Phys. Rev. Lett. 92, 253901 (2004)]. Numerical simulations show good agreement with experimental data.

Demonstration of vortex-induced beam shaping in seeded second-harmonic generation.

We report the experimental demonstration of a method that allows the simultaneous frequency-doubling and the shaping of a light beam into a variety of patterns. The technique is based on the controllable generation of multiple-vortex patterns in seeded schemes. Our observations were performed under conditions of second-harmonic generation pumped by picosecond pulses at 1055 nm, but the scheme can be extended to all parametric processes.

From quantum to classical images.

We display the results of the numerical simulations of a set of Langevin equations, which describe the dynamics of a degenerate optical parametric oscillator in the Wigner representation. The scan of the threshold region shows the gradual transformation of a quantum image into a classical roll pattern. An experiment on parametric down-conversion in lithium triborate shows strikingly similar results in both the near and the far field, displaying qualitatively the classical features of quantum images. c 1997 Optical Society of America.

Observation of quadratic optical vortex solitons

We report on the generation of stable dark-vortex solitons in large-phase-mismatched second-harmonic generation of self-defocusing type, sustained by a combined effect of transverse walk-off and finite beam size.