We employ cross-phase modulation of a weak near-infrared pulse and an intense single-cycle terahertz field to extract the nonlinear refractive index of ZnTe near 1 THz (n2=4.36x10–14 cm2/W).

We use erbium and ytterbium femtosecond lasers to melt and shape semi-spherical nanostructure by high spatial frequency laser induced periodic surface structures into linear microstructures of 2 μm long in the direction of laser polarization.

The linear and nonlinear optical behavior of novel sulfur based polymer materials are evaluated at the optical communication wavelength, 1550 nm. These polymers are attractive for near-IR (NIR) and mid-IR applications. The two photon absorption (TPA) coefficient (beta) and second order refractive index (n(2)) of chalcogenide hybrid inorganic/organic polymers (CHIPs) from poly(sulfur-random-(1,3-diisopropenylbenzen) (poly(S-r-DIB)) are measured via the Z-scan technique. In this study, we investigated the linear and nonlinear optical behavior of two types of CHIPs where the weight percentage of sulfur is varied (poly(S-50%-r-DIB50%) and poly(S-70%-r-DIB30%)). The TPA coefficients for poly(S-50%-r-DIB50%) and poly(S-70%-r-DIB30%) obtained were 0.11 cm/GW and 0.063 cm/GW, respectively. The n(2) for poly(S-50%-r-DIB50%) and poly(S-70%-r-DIB30%) was measured and determined to be 2.45 x 10(-15) cm(2)/W and 3.06 x 10(-15) cm(2)/W, respectively, and are in good agreement with Miller's rule prediction. These materials exhibit low cost, low temperature processing, high transparency in the near to mid-IR range (except a few interval ranges) and relatively high refractive index, providing a unique set of properties for optics and photonics device applications. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

We present a Thulium fiber laser mode-locked bi-directionally with a carbon nanotube saturable absorber and demonstrate single shot dual-comb spectroscopy of H2O in the short-wave infrared (SWIR) with the laser.

We study the modification of indium semi-spherical nanostructures with radii of around 175 nm on silicon wafers into linear microstructures more than 2 μm long in the direction of polarization of laser pulses (1.56 μm, 150 fs, up to 7.5 nJ and 30 000 laser pulses with 8 MHz repetition rate). The experimental results and a rudimentary analysis confirm that melting occurs from intense laser pulses. In short, we demonstrate that melting of the indium droplet followed by trapping in high spatial frequency laser induced periodic surface structures on a silicon substrate cause nanostructure modification. The understanding of the modification process, melting, and moving in the nano-grating structured field, pave the way to design nanostructures of arbitrary shapes at the sub-wavelength scale.

We demonstrate the use of a bi-directional mode-locked thulium doped fiber ring laser as a single cavity free-running dual-comb source. We consider the effect of group-velocity dispersion on the repetition rate difference between the two combs and demonstrate a tunable repetition rate difference from 48-229 Hz. Dual-comb spectroscopy of atmospheric H2O near 1.87 mu m is performed over a 1.47-m air path in laboratory with a peak single-shot signal-to-noise ratio of 27.5.

We report a novel, polarization dependent, femtosecond laser-induced modification of surface nanostructures of indium, gallium, and arsenic grown on silicon via molecular beam epitaxy, yielding shape control from linear and circular polarization of laser excitation. Linear polarization causes an elongation effect, beyond the dimensions of the unexposed nanostructures, ranging from 88 nm to over 1 um, and circular polarization causes the nanostructures to flatten out or form loops of material, to diameters of approximately 195 nm. During excitation, it is also observed that the generated second and third harmonic signals from the substrate and surface nanostructures increase with exposure time.

We pump low-pressure nitrogen gas with ionizing femtosecond laser pulses at 1.5 mu m wavelength. The resulting rotationally excited N-2(+) 2 molecular ions generate directional, forward-propagating stimulated and isotropic spontaneous emissions at 428 nmwavelength. Through high-resolution spectroscopy of these emissions, we quantify rotational population distributions in the upper and lower emission levels. We show that these distributions are shifted with respect to each other, which has a strong influence on the transient optical gain in this system. Although we find that electronic population inversion exists in our particular experiment, we show that sufficient dissimilarity of rotational distributions in the upper and lower emission levels could, in principle, lead to gain without net electronic population inversion.

Studies on the generation of optical gain in the constituents of air are motivated by potential applications in air lasing [1], a concept, the realization of which would enable efficient single-ended remote sensing in the atmosphere. Various schemes of turning air into an active laser medium are being investigated [2-4]. One of the most promising approaches is based on pumping atmospheric nitrogen by ultra-intense femtosecond laser pulses propagating in the filamentation regime in air [5]. It has been suggested that air filamentation may result in the optical gain on the electronic transitions of N2 + at 391 nm and 428 nm emission wavelengths [6]. However, the gain mechanism responsible for those emissions has been controversial.

Through high-resolution spectroscopy of lasing on N2+ ions at 428nm wavelength, we quantify optical gain and population inversion in the gain medium. We show that molecular rotations are the enabling effect in this lasing process.

Filament-induced amplified spontaneous emission, ASE, in air–hydrocarbons (~2%) gas mixture, CH4, C2H2, and C2H4, was investigated by detecting fluorescence emitted from CH fragments prepared in the electronically excited A2Δ state in the filament. The fluorescence signal recorded from the side direction of the filament was linearly proportional to the length of the filament, while the fluorescence signal emitted in the backward direction of the laser propagation increased nonlinearly with the filament length. This difference showing that the filament acted as a gain medium in which the spontaneous emission from CH was amplified (ASE). This process realized by a small amount of hydrocarbon molecular species in air can be applied to remote sensing of pollutants in air.

We demonstrate the control of neutral fragmentation of methane (CH4) induced by a Ti:sapphire intense laser pulse (800 nm, 40 fs) by using a pump-probe spectroscopy. Enhancement of the fluorescence emission from the neutral radical CH (A2? ? X2?) induced by the intense laser field (~1014 W/cm2) is observed when the wavelength of the probe laser pulse is tuned to 400 nm. The phenomena are explained based on excitation enhancement of the super-excited state of the parent molecule resulting in an increase in neutral dissociation of the methane molecules.

We study the interference stabilization (population trapping) of Xe atoms using a fs Ti - Sapphire laser both experimentally and theoretically. The investigation is performed for two pulses of different duration. The signature of population trapping arising from the dynamic multiphoton resonance of the initial state and a group of Rydberg states of the atom is found to exist. The results obtained can be considered as the manifestation that population trapping is indeed a universal phenomena.

A femtosecond laser-induced filament in air was investigated by detecting the C3Πu–B3Πg (0,0) fluorescence of N2. The intensity of the backward fluorescence increased exponentially as a function of the filament length, showing the amplification of the spontaneous emission. The vibrational and rotational temperatures of N2 in the C state determined by spectroscopic analyses were found to take respectively almost the same values of 2800(200) and 450(100)K in the wide laser intensity range between 0.5 and 6mJ/pulse, which can be regarded as evidence of the clamping of the laser field intensity in the filament.

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.

This chapter discusses some experimental manifestations of interference stabilization or population trapping in atoms and molecules during the filamentation of strong 800-nm femtosecond laser pulses propagating in air and other gases. Particular emphasis is given to nitrogen molecules whose fluorescence induced by the 800-nm pump is probed respectively by 400 nm, 1,338 nm and THz radiations. Fluorescence enhancement and reduction were observed under different probe conditions and at the revival times of the rotational wave packet of nitrogen. Population trapping in the Rydberg states of the molecule is central to the explanation of these observations. We conclude that population trapping through interference stabilization in the multiphoton regime is a universal phenomenon in atoms and molecules in intense laser fields. This includes the excitation of super-excited states of molecules.

Using a femtosecond Ti:Sapphire laser we experimentally and numerically simulated in the laboratory vertical propagation and filamentation in the atmosphere for up to 10 km by changing the pressure in a gas cell. Filament diameter increased at reduced pressures in air. This is a manifestation of the invariance of intensity clamping. Filaments produced at low pressures corresponding to higher altitudes were shown to reveal excellent transport properties due to the wide low-density plasma column extended homogeneously over long propagation distances. (C) 2012 by Astro, Ltd.

We experimentally demonstrate the feasibility of controlling the fluorescence emission of nitrogen molecules in air induced by femtosecond laser filamentation by using a pump-probe method. An obvious enhancement or reduction in the filament-induced fluorescence signals of nitrogen molecules can be realized when a blue (400 nm) or an infrared (1338 nm) laser pulse is used as the probe. The completely opposite effect is ascribed to the excitation enhancement of ionization and population trapping of some highly excited states including Rydberg states of nitrogen molecules.

We report neutral dissociation of simple molecules in strong laser field, experimentally. Moreover, theoretical calculations of potential energy curves justify neutral dissociation through super excited states in all of the investigated gases.