N. H. Matlis

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.

Propagation of intense laser pulses through inhomogeneous ionizing gas profiles

By simultaneously and self-consistently solving Maxwells equations, the Ammosov-Delone-Krainov (ADK) field ionization equation, and the relativistic cold plasma equations, we have investigated the propagation of intense, ultrashort laser pulses through spatially inhomogeneous longitudinal gas gradients. Along with highly accurate calculations of the spatial and temporal beam profiles of the pulse at the end of various gradients, we have also determined simple scaling rules for the location of the vacuum-gas interface in order to minimize the pulse distortion at the focus. We show the benefits of using either preionized or low-Z gases, and we discuss the implications of this work for plasma-channel laser wakefield acceleration.

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.

Analysis of sinusoidally modulated chirped laser pulses by temporally encoded spectral shifting

We present an analytical formalism elucidating how information is stored in chirped optical probes by describing the effects of sinusoidal temporal modulations on the electric field. We show that the modulations produce spectral sidebands which can be interpreted as temporal sidebands due to the time-wavelength mapping, an effect we call temporally encoded spectral shifting (TESS). A derivation is presented for the case of chirped-pulse spectral interferometry showing how to recover both the amplitude and the periodicity of the modulation from a Fourier transform of the interferogram. The TESS effect, which provides an intuitive picture for interpreting pump-probe experiments with chirped pulses, is illustrated for probing wakefields from a laser-plasma accelerator.

Snapshots of Laser‐Generated Wakefields

We report the first single‐shot holographic measurements of transverse and longitudinal structure of laser‐generated wakefields. Real‐time, non‐averaged measurements of resonant wakes reveal detailed temporal and spatial features that depend on pulse energy and electron density. Simulations with the PIC code WAKE show excellent agreement with the morphology of the wakefields.

Distortion-free guiding of 0.2 /spl times/ 10/sup 18/ W/cm/sup 2/ pulses through a fully-ionized 1.5 cm He plasma channel

Summary from only given. Plasma waveguides capable of guiding relativistically intense fs laser pulses without optical distortion are essential to developing GeV-scale Laser Wakefield (LWF) Accelerators and coherent short-wavelength sources. We report the highest intensity-length product (0.2/spl times/10/sup 18/ W/cm/sup 2/ /spl times/ 1.5 cm) to date for distortion-free guiding. We generated our channels in a backfill of He gas (300-700 Torr) using a modification of a method developed by Milchberg, who line-focused /spl sim/ 100 ps, /spl sim/ 0.3 J Nd:YAG laser pulses to intensities of /spl sim/ 10/sup 13/ W/cm/sup 2/ into a gas using an axicon. This intensity readily field-ionizes high-Z gases (e.g. Ar, N/sub 2/), thus initiating the channel-forming process, but forms an incompletely-ionized channel, in which relativistically intense pulses suffer ionization-induced phase distortion during guiding and ionization-induced defocusing when entering the channel. Indeed we observe distorted exit mode and spectrum and only 5% throughput when we attempt to guide a pulse at /spl ges/10/sup 17/ W/cm/sup 2/ through a nitrogen channel.

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 Nd:YAG laser pulses of either 0.3 J energy, 100 ps duration, after pre-ionizing 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 pre-ionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully-ionized on the channel axis in both cases. Identical fs pulses suffered substantial ionization-induced blueshifts after propagating through Ar and Ne channels of similar dimensions.