S. P. Le Blanc

Production And Characterization Of A Fully-Ionized He Plasma Channel

We report guiding of intense (I=1.3±0.7×1017 W/cm2) 80 fs laser pulses with negligible spectral distortion through 1.5-cm-long preformed helium plasma channels. Channels were formed by axicon-focused laser pulses of either 0.3 J energy, 100 ps duration, after preionizing a 200–700 Torr backfill of He gas to ne∼1016 cm−3 with a pulsed electrical discharge; or 0.6–1.1 J energy, 400 ps duration, which required neither preionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully ionized on the channel axis in both cases. Identical femtosecond pulses suffered substantial ionization-induced blueshifts after propagating through Ar and Ne channels of similar dimensions.

Femtosecond single-shot phase-sensitive autocorrelator for the ultraviolet

A single-shot phase-sensitive autocorrelator for the investigation of high-intensity ultraviolet laser pulses is developed and tested. The fluorescence of the two-photon-excited self-trapped excitons in BaF(2) is used as a nonlinear detector. Simultaneous measurements of subpicosecond KrF laser are reported.

Single-shot measurement of temporal phase shifts by frequency-domain holography

Frequency-domain holography is used to measure ultrafast phase shifts induced either by the nonlinear susceptibility ?(3) of fused silica or by ionization fronts in air over a temporal region of 1 ps with 70-fs resolution in a single shot. The use of an imaging spectrometer adds one-dimensional spatial resolution to the single-shot temporal measurements.

Spectral, temporal, and spatial characteristics of plasma-induced spectral blue shifting and its application to femtosecond pulse measurement

The spectral, temporal, and spatial characteristics of plasma-induced spectral blue shifting in atmospheric-density rare gases were investigated by a two-color pump–probe technique. When the degree of ionization is less than 1, experiments and theoretical results show that field ionization occurs over a time approximately equal to half the pump-pulse width and that ionization-induced defocusing causes spectral blue shifting to be spatially dependent. The pulse width of a 147-nm pulse generated by four-wave mixing of a femtosecond KrF laser pulse in xenon was determined to be 400 fs by a cross-correlation technique based on plasma defocusing.

Reflectivity of laser-produced plasmas generated by a high intensity ultrashort pulse

Solid state density aluminum and carbon plasmas were generated by 350 fs KrF laser pulses at intensities of 1017 W cm−2. Reflectivity, x‐ray emission, and spectra of the backscattered radiation were measured as a function of the laser polarization, angle of incidence, and intensity (I). For p‐polarized light, the absorption (A) is almost intensity independent for 1015 W cm−2<I<2×1017 W cm−2, and can reach values up to A=0.8. For constant laser intensity, p‐polarized light is up to a factor of 15 more efficient in generating x rays than s‐polarized light. A polarization and angle of incidence‐dependent absorption mechanism, such as resonance absorption or vacuum heating, is consequently important. The spectral line shape and positions of the reflected radiation from the carbon plasma showed a substantial intensity dependence. It is demonstrated that the backscattered spectrum is influenced by the plasma expansion (Doppler effect) and the temporal development of the electron density.

FEMTOSECOND VACUUM-ULTRAVIOLET PULSE MEASUREMENT BY FIELD-IONIZATION DYNAMICS

Three techniques are demonstrated for measuring femtosecond pulses from the deep vacuum ultraviolet to the visible based on the ionization dynamics of optical-field-ionized plasmas. A probe pulse propagates through a plasma generated by an intense femtosecond KrF excimer laser pump pulse. Ionization-induced defocussing or collisional absorption yield a cross-correlation measurement of the probe pulse. Ionization-induced spectral blue shifting of the probe can give an upper limit for the probe pulse duration.

Ultrashort pulse characterization using plasma-induced cross-phase modulation

Abstract Theoretical examples and experimental results are presented for ultrashort pulse characterization based on plasma-induced cross-phase modulation. Since this technique works without a nonlinear detector, the pump-probe measurements can be performed over a wide spectral range.

Plasma channels in doubly-ionized helium

Summary form only given. Plasma channels can be used to guide high intensity laser pulses over distances much longer than the diffraction limit, which is necessary for applications such as laser wakefield accelerators. Previous plasma channels have been formed only in incompletely ionized gases, in which a sufficiently intense guided pulse suffers ionization-induced phase distortion. In the paper we demonstrate formation of a plasma channel in almost fully doubly-ionized helium. The key enabling technique is use of an electrical discharge to pre-ionize the helium, thus lowering the threshold for ionization heating and channel formation by a line focused laser pulse by an order of magnitude. In our experiment, a 100 ps, 120 mJ Nd:YAG laser pulse is focused to a line by an axicon to intensities of /spl sim/10/sup 13/W/cm/sup 2/ into 300-600 torr Helium gas.

Guiding characteristics of an acoustic standing wave in a piezoelectric tube

Propagation of an He–Ne laser beam through a gas filled piezoelectric tube is used to characterize the guiding properties of a radially driven acoustic standing wave. Impedance matched driving at 1 MHz of the 5-cm-long piezotube yields radial density perturbations of 0.005 at 40 V driving voltage. The frequency spectrum of the cavity resonances is used to measure the damping of the standing wave due to shear viscosity in Ar.

Single shot measurement of optical phase by ultrafast frequency domain holography

Summary form only given. Femtosecond time resolved measurement of optical phase is an important technique for characterizing ultrashort laser pulses and probing intense laser-matter interactions. For the latter, probe pulse phase shifts caused by laser induced plasma wakefields or the expansion of the critical density surface of solid density plasmas have been diagnosed by frequency domain interferometry (FDI) using multiple delay time steps between pump and probe. For such high intensity laser plasma interactions, the low repetition rate of multiterawatt class lasers and the sensitivity of such interactions to small laser fluctuations motivates the development of single shot phase diagnostic techniques. We present a new single shot ultrafast phase measurement technique based on an extension of FDI to the regime of frequency domain holography (FDH).