S. Varma

Direct Measurement of the Electron Density of Extended Femtosecond Laser Pulse-Induced Filaments

We present direct time- and space-resolved measurements of the electron density of femtosecond laser pulse-induced plasma filaments. The dominant nonlinearity responsible for extended atmospheric filaments is shown to be field-induced rotation of air molecules.

Single-shot, space- and time-resolved measurement of rotational wavepacket revivals in H(2), D(2), N(2), O(2), and N(2)O.

Femtosecond laser-induced alignment and periodic recurrences in hydrogen and deuterium are measured in a single shot for the first time, in a room temperature gas cell. Single-shot Supercontinuum Spectral Interferometry (SSSI) is employed, with measurements also performed in room temperature samples of nitrogen, oxygen, and nitrous oxide. Unlike previous optical techniques for probing molecular alignment in gases or liquids, SSSI quantitatively and directly measures the degree of molecular alignment without reliance on model fits, and it can do so with spatial resolution transverse to the pump beam. In addition, wavepacket collisional dephasing rates can be directly measured in gas samples at useful densities.

Measurement of transient nonlinear refractive index in gases using xenon supercontinuum single-shot spectral interferometry

A highly stable version of spectral interferometry is demonstrated, allowing single shot measurement of ultrafast high field processes using modest energy lasers, with pump and probe pulses totaling less than 1 mJ. The technique makes possible reconstruction of ultrafast refractive index transients with one-dimensional spatial resolution, limited only by the bandwidth of the supercontinuum pulse (~100 nm) and instrument resolution. The ultrafast nonlinear Kerr effect in glass, and in Ar, N(2), and N(2)O gases is measured, along with plasma generation in Ar. The inertial contribution to the nonlinear index from N(2) and N(2)O molecular rotation is also observed.

Direct measurements of the nonlinear index of refraction of water at 815 and 407 nm using single-shot supercontinuum spectral interferometry

Single-shot supercontinuum spectral interferometry was used to measure the nonlinear index of refraction due to the optical Kerr effect in water at both 815 and 407 nm, with pump pulse lengths of ∼90 and ∼250 fs, respectively. Knowledge of the nonlinear index at 407 nm allows pulse tailoring to achieve remote underwater pulse compression and self-focusing.

The extreme nonlinear optics of gases and femtosecond optical filamentationa)

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas ...

Single-shot, space- and time-resolved measurement of rotational wavepacket revivals in H 2 and D 2

Femtosecond laser induced alignment and periodic recurrences of room temperature hydrogen and deuterium molecules are measured in a single shot for the first time, using single-shot supercontinuum spectral interferometry (SSSI). The measurements are also performed in oxygen and some other common linear molecular gases. Employing time-dependent first order perturbation theory, an analytical model of rotational wavepacket revivals is derived, which shows good agreement with the experiment.

Measurements of the High Field Optical Nonlinearity and Electron Density in Gases: Application to Filamentation Experiments

We discuss recent experiments and calculations of the high-intensity optical nonlinearity in gases. Spectral interferometry measurements of the nonlinear optical response of air constituents to laser intensities near the ionization threshold are performed. A calculation of the phase shift caused by a plasma grating created by interference between the pump and probe beams in a transient birefringence measurement suggests that experimental techniques measuring cross phase modulation of a probe pulse by a strong pump pulse are unreliable for studying the optical nonlinearity when the pump and probe pulses are of the same wavelength. An interferometric measurement of the electron density in a filament is also performed. The peak elec-tron density measured is consistent with a model that includes plasma defocusing, but not higher-order Kerr terms. These tech-niques promise to improve the quantitative understanding of nonlinear optics near the ionization threshold and filamentation.

Quantum molecular lensing of femtosecond laser optical/plasma filaments

The long-range filamentary propagation of intense femtosecond pulses in atmosphere has been observed for the first time to be strongly effected by quantum rotational wave packets. A two-pulse experiment shows that a filamenting probe pulse can be steered and trapped in or destroyed by the rotational alignment wake following a pump filament.

Periodic index-modulated plasma waveguide

We demonstrate a wire-obstructed cluster flow technique for making periodically modulated plasma waveguides in hydrogen, nitrogen, and argon with sharp, stable voids as short as 50 µm with a period as small as 200 µm.

THZ generation by ultra-short laser pulses propagating in nonuniform plasma chanels

Summary form only given. Electromagnetic THz radiation spans the range of wave lengths from millimeters to infrared. It has many potential applications in biological imaging, spectroscopy of solids and liquids, and remote sensing. Coherent pulses of THz are especially of interest for time domain spectroscopy. Conventional sources of THz using short pulse lasers rely on pulse generation in a solid and are generally limited to mJ/pulse. Higher energies per pulse can be generated at accelerator facilities with intense bunched electron beams via synchrotron or transition radiation. Recently, intense THz pulses with energies in excess of 100 mJ/pulse have been generated as transition radiation by a laser generated and accelerated electron beam passing from plasma to vacuum. We propose a scheme for THz generation that involves the creation of miniature corrugated plasma channels (period ~40 mm) that act as slow wave structures. THz can be generated in these channels by pulses of current driven by a laser pulse or injected from an accelerator. We consider the excitation of THz electromagnetic waves in a plasma by the ponderomotive force of an ultra-short laser pulse. For a uniform plasma such excitation is weak because electromagnetic waves have no density perturbation and do not couple to the ponderomotively driven plasma current. EM waves in a nonuniform plasma channel can be excited. Further, if the channel is axially modulated the EM waves can be slowed down and phase matched to the ponderomotive wave. We calculate the excitation of these waves by both fixed shape laser pulses and by parametric decay. Our calculations show that in the case of a laser driver, a large fraction of laser pulse energy can be down converted to THz. Thus, this scheme offers the possibility of producing 100 mJ pulses of duration of hundreds of femtoseconds. Experimental techniques for generating modulated channels are also explored