Effect of focusing element-induced aberrations on filamentation and supercontinuum emission in ambient air.

Femtosecond laser pulse induced filamentation in atmosphere is susceptible to a number of input laser, focusing optics and medium characteristics. Filamentation of fs pulses in atmosphere is an intense propagation regime where the focusing geometry used to focus the fs laser pulses play an important role influencing the filament intensity and the associated supercontinuum. We identified different optical elements used for focusing the fs laser pulses leading to filamentation in air and classified them according to the induced aberrations. To clearly identify the role of aberrations, all the optical elements were taken to have same focal length. The subsequent filament structure and emissions from the filament were correlated with the aberrations induced by optical element revealed stark differences. The onset of the filamentation, its longitudinal intensity and the associated supercontinuum emission (SCE) have varied drastically with the aberrations induced by the focusing optics. A systematic study directed to choose and identify suitable optical elements according to the usage of the fs pulses for a specific filamentation regime is presented.

Spatio-temporal characterization of ablative Cu plasma produced by femtosecond filaments.

We present the spatial and temporal characterization of the copper (Cu) plasma produced by the femtosecond laser filaments. The filaments of various lengths and intensities were generated with the aid of three different focusing lenses. Further, the filamentation induced breakdown spectroscopy (FIBS) measurements were carried out for each filament at three different positions along the length of the filament. The filaments were spatially characterized by estimating the plasma temperature and electron density. Our investigation has demonstrated that the centre of the filament is the best to obtain a maximum signal. Both the spectral line intensity and their persistence time are highest for the center of the filament. The enhanced persistence and the scalability of the spectral line intensity tested across different focusing geometries can boost the application of this technique in various fields.

Supercontinuum generation from zinc borate glasses: bandgap versus rare-earth doping.

We report an enhanced supercontinuum generation (SCG) from a rare-earth (Pr3+) doped low bandgap zinc borate glass when excited with 60 fs pulses from a Ti-sapphire laser. The emission associated with the absorption bands due to Pr3+ doping (around 420-500 and 580-600 nm) is observed to assist the enhanced SCG.

Identification of Genistein and Biochanin A by THz (far-infrared) vibrational spectra.

The temperature dependent vibrational spectra of Genistein and Biochanin A, major components in Mongolian medicines Agi, in the range of 0.5-4.5 THz (16.7-150 cm(-1)) are presented for the first time. Over the temperature range from 295 to 77K, 12 highly resolved spectral features for the Genistein and 13 features for Biochanin A were measured by THz-TDS and display strong linewidth narrowing and frequency blue-shift with cooling. Such narrow-line THz fingerprint spectra provide a rapid, nondestructive and reliable method for the identification of these Chinese traditional medicines.

Terahertz (far-infrared) characterization of tris(hydroxymethyl)aminomethane using high-resolution waveguide THz-TDS.

Waveguide THz-TDS using metal parallel plate waveguides is employed to obtain a high-resolution terahertz vibrational spectrum of tris(hydroxymethyl)aminomethane, commonly known as Tris or THAM, an important biological buffer component, a neuroinhibitor and an organic thermal energy-storing molecule. Dropcast and sublimated thin films are used to characterize the sample. The development (with cooling) of 3 broad resonances seen at room temperature into 12 highly resolved spectral features for the dropcast sample and 11 features for a sublimated sample at 13.6 K is tracked by measuring the temperature dependence of the transmission spectrum. Resonances with fwhm linewidths as narrow as 12 GHz are obtained. These results provide strong constraints on theoretical modeling.

Temperature dependent characterization of terahertz vibrations of explosives and related threat materials.

Waveguide terahertz time-domain spectroscopy (THz-TDS) is used to characterize the temperature dependent vibrational properties of three threat-related materials: 4-amino-dinitrotoluene (4A-DNT), pentaerythritol tetranitrate (PETN), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). These materials are characterized as thin polycrystalline layers deposited in the 50 micron gap of a metal parallel plate waveguide. For each material waveguide THz-TDS at least partially resolves the underlying vibrational spectrum and reveals new features that have not been observed in previous free space measurements of these materials. Strong experimental evidence for a phase transformation is observed for 4A-DNT as the polycrystalline layer on the waveguide surface is cooled to near 200 K. For PETN a highly resolved spectrum containing eleven vibrational lines is observed at 11 K with full-width at half maximum linewidths ranging from 7 GHz to 40 GHz. Based on comparison to measurements in the literature, our PETN measurement suggests that it is possible to produce narrow linewidths from a polycrystalline layer that approach those from a single crystal. Finally, for HMX, a highly resolved vibrational spectrum is measured that is assigned to the metastable gamma polymorph.

Narrow-line waveguide terahertz time-domain spectroscopy of aspirin and aspirin precursors.

Low frequency vibrational modes of pharmaceutical molecules are dependent on the molecule as a whole and can be used for identification purposes. However, conventional Fourier transform far-infrared spectroscopy (FT-IR) and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper spectral features. Waveguide THz-TDS consists of forming an ordered polycrystalline film on a metal plate and incorporating that plate in a parallel-plate waveguide, where the film is probed by THz radiation. The planar order of the film on the metal surface strongly reduces the inhomogeneous broadening, while cooling the waveguide to 77 K reduces the homogeneous broadening. This combination results in sharper absorption lines associated with the vibrational modes of the molecule. Here, this technique has been demonstrated with aspirin and its precursors, benzoic acid and salicylic acid, as well as the salicylic acid isomers 3- and 4-hydroxybenzoic acid. Linewidths as narrow as 20 GHz have been observed, rivaling single crystal measurements.

High-resolution waveguide THz spectroscopy of biological molecules.

Low-frequency vibrational modes of biological molecules consist of intramolecular modes, which are dependent on the molecule as a whole, as well as intermolecular modes, which arise from hydrogen-bonding interactions and van der Waals forces. Vibrational modes thus contain important information about conformation dynamics of biological molecules, and can also be used for identification purposes. However, conventional Fourier transform infrared spectroscopy and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper features. For this technique, an ordered polycrystalline film of the molecule is formed on a metal sample plate. This plate is incorporated into a metal parallel-plate waveguide and probed via waveguide THz-TDS. The planar order of the film reduces the inhomogeneous broadening, and cooling of the samples to 77K reduces the homogenous broadening. This combination results in the line-narrowing of THz vibrational modes, in some cases to an unprecedented degree. Here, this technique has been demonstrated with seven small biological molecules, thymine, deoxycytidine, adenosine, D-glucose, tryptophan, glycine, and L-alanine. The successful demonstration of this technique shows the possibilities and promise for future studies of internal vibrational modes of large biological molecules.

High-resolution waveguide terahertz spectroscopy of partially oriented organic polycrystalline films.

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.

Femtosecond supercontinuum generation in a quadratic nonlinear medium (KDP).

We report the first ever femtosecond supercontinuum generation in a KDP crystal using ~100 fs pulses at 790 nm irradiation. Due to the quadratic nonlinearities of the KDP crystal an enhanced bandwidth of 385 -960 nm is generated. The spatial and temporal coherence of the generated white light is demonstrated using Young's double slit interference configuration and a Michelson interferometer.