Filament-Based Spectroscopy

Spectrum after laser filamentation in air and dispersion by a prism

When a high energy femtosecond laser beam propagates through a transparent medium with a peak power exceeding the critical power for self-focusing filamentation can occur. Filamentation is characterized by a dynamic interplay between Kerr-induced self-focusing and dispersion, diffraction, and plasma generation. Loosely focusing <50 fs pulses containing a few millijoules of energy in air results in the generation of an extended (2-3 times the Rayleigh range) low-density plasma channel ~100 micrometers in diameter. In this region (the "filament") dramatic pulse reshaping occurs, resulting in dramatic pulse shortening and white-light continuum generation. In conjunction with the high pulse intensity in the filament channel (1013-1014 Wcm-2) these characteristics make femtosecond laser filaments an interesting tool for spectroscopy.

Pulse shortening during filamentary propagation can generate pulses as short as <7 fs, merely a few oscillations of the carrier electric field. This time scale is much shorter than the characteristic time scale for nuclear motion in molecules, resulting in the impulsive excitation of all the Raman-active modes of the molecules in the propagation region through intra-pulse Raman scattering. The Raman spectrum shown at left demonstrates the excitation of modes as low in energy as 357 cm-1 (corresponding to a rotational period of ~93 fs) and up to the energetic hydrogen stretching mode at 4155 cm-1, which has a period of 8 fs. Probing the quantum wake left by the filament using a narrowband probe pulse results in the generation of coherent Stokes and anti-Stokes Raman sidebands that can be collected in a single shot, allowing for rapid identification of the molecules in the sample region. Our research focuses on applications of filament-assisted impulsive Raman spectroscopy for stand-off spectroscopy.

Raman spectrum of air and hydrogen excited by a filament pulse and probed by a narrowband (20 cm-1) pulse.

Thermometry:

Time-resolved impulsively excited Raman signal of the nitrogen vibrational line and fit using expression under ambient (dark blue, 300 K) conditions and in a bunsen burner flame (light blue, 2950 K).

Time-resolved filament-assisted Raman spectroscopy of the nitrogen line traces the rovibrational wave-packet dispersion that results from the anharmonicity of the oscillator. An analytical expression derived for the signal intensity as a function of temperature (shown at left) is used to extract the vibrational temperature of the system, which is found to be at room temperature in the wake of a filament generated using a one kilohertz repetition rate laser system. We can conclude that no energy from the laser pulse is immediately depositied into the vibrational modes of air during filamentary propagation, and that no persistent local heating occurs in our setup due to the repetetive excitation and subsequent thermalization of the medium. Such effects could become more important as the repetition rate of the laser is increased. As a further demonstration of the method for applications in thermometry, a methane-air flame is placed under the filament-probe interaction region and the time-resolved signal shows that the increased temperature populates higher lying rotational states, leading to the rapid dispersion of the wave-packet.

-Odhner et al., Physical Review Letters 103, 075005 (2009)

Standoff Detection:

The ability of filaments to be launched remotely and to propagate at high intensities over extended distances makes them a promising source for standoff detection applications. Filament-assisted Raman spectroscopy, with its ability to excite and probe the entire vibrational spectrum, can provide molecule-specific signatures to identify potential threats in the environment. Spectra measured in the lab (left) show the utility and universality of filament-assisted Raman spectroscopy for exciting and identifying Raman spectra over a 4000 cm-1 range in a number of target compounds for detection.

-Odhner et al., J. Phys. Chem. A 115, 13407 (2011)

Time-resolved impulsively excited Raman signal of the nitrogen vibrational line and fit using expression under ambient (dark blue, 300 K) conditions and in a bunsen burner flame (light blue, 2950 K).

 

 

Levis Group, Department of Chemistry, Temple University, Beury Hall 244, 1901 N. 13th Street, Philadelphia, PA 19122    Tel: 215-204-5241     Fax: 215-204-6179
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