Millisecond-long suppression of spectroscopic optical signals using laser filamentation.

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Optimization of microwave emission from laser filamentation with a machine learning algorithm.

We demonstrate that is it possible to optimize the yield of microwave radiation from plasmas generated by laser filamentation in atmosphere through manipulation of the laser wavefront. A genetic algorithm controls a deformable mirror that reconfigures the wavefront using the microwave waveform amplitude as feedback. Optimization runs performed as a function of air pressure show that the genetic algorithm can double the microwave field strength relative to when the mirror surface is flat. An increase in the volume and brightness of the plasma fluorescence accompanies the increase in microwave radiation, implying an improvement in the laser beam intensity profile through the filamentation region due to the optimized wavefront.

Demonstration of 0.67-mJ and 10-ns high-energy pulses at 2.72 µm from large core Er:ZBLAN fiber amplifiers.

We explored generation of high-energy nanosecond short pulses in the mid-IR wavelength range using 30-70-µm-core Er:ZBLAN fiber amplifiers. The highest energies achieved were ∼0.7mJ at 2.72 µm in 11.5-ns-long pulses, with the corresponding peak power of 60.3 kW, obtained with a 70-µm-diameter core fiber amplifier pumped at 976 nm and seeded by a KTiOAsO4-based optical parametric oscillator/optical parametric amplifier system. To the best of our knowledge, these pulse energies are the highest achieved to date from mid-IR fiber lasers at longer than 2-µm wavelengths with nanosecond pulses. The achieved highest pulse energies were limited by the surface damage of unprotected fiber output facets.

Control of the configuration of multiple femtosecond filaments in air by adaptive wavefront manipulation.

We demonstrate the ability to position single and multiple filaments arbitrarily within the energy reservoir of a high power femtosecond laser pulse. A deformable mirror controlled by a genetic algorithm finds the optimal phase profile for producing filaments at user-defined locations within the energy reservoir to within a quarter of the nominal filament size, on average. This proof-of-principle experiment demonstrates a potential technique for fast control of the configuration of the filaments.

Coherent pulse stacking amplification using low-finesse Gires-Tournois interferometers.

We demonstrate a new technique of coherent pulse stacking (CPS) amplification to overcome limits on achievable pulse energies from optical amplifiers. CPS uses reflecting resonators without active cavity-dumpers to transform a sequence of phase- and amplitude-modulated optical pulses into a single output pulse. Experimental validation with a single reflecting resonator demonstrates a near-theoretical stacked peak-power enhancement factor of ~2.5 with 92% and 97.4% efficiency for amplified nanosecond and femtosecond pulses. We also show theoretically that large numbers of equal-amplitude pulses can be stacked using sequences of multiple reflecting resonators, thus providing a new path for generating very high-energy pulses from ultrashort pulse fiber amplifier systems.

Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii.

Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of [Formula: see text]35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

High-order harmonic generation from solid targets with 2 mJ pulses.

Harmonics up to the 18th order are generated from solid targets by focusing 2 mJ, 50 fs pulses at 800 nm to a spot size of 1.7 µm (FWHM). To our knowledge, this is the first demonstration of high-harmonic generation with a very short focal length paraboloid (f/1.4) and kilohertz laser system. The harmonics have a low divergence (<4°) compared to the driving beam and conversion efficiencies (>10(-7) per harmonic) comparable to gas harmonics. No contrast enhancement techniques are employed, and the system is capable of operating at 500 Hz.

Pair creation in QED-strong pulsed laser fields interacting with electron beams.

QED effects are known to occur in a strong laser pulse interaction with a counterpropagating electron beam, among these effects being electron-positron pair creation. We discuss the range of laser pulse intensities of J≥5×10(22) W/cm2 combined with electron beam energies of tens of GeV. In this regime multiple pairs may be generated from a single beam electron, some of the newborn particles being capable of further pair production. Radiation backreaction prevents avalanche development and limits pair creation. The system of integro-differential kinetic equations for electrons, positrons and γ photons is derived and solved numerically.

Towards isolated attosecond electron bunches using ultrashort-pulse laser-solid interactions.

We investigate MeV-level attosecond electron bunches from ultrashort-pulse laser-solid interactions through similarities between experimental and simulated electron energy spectra. We show measurements of the bunch duration and temporal structure from particle-in-cell simulations. The experimental observation of such bunches favors specular reflection direction when focusing the laser pulse onto a subwavelength boundary of thick overdense plasmas at grazing incidence. Particle-in-cell simulation further reveals that the attosecond duration is a result of ultra-thin ([Formula: see text]tenth of a micron) gaps of zero electromagnetic energy density in the modulated reflected radiation, while the bunching (locally peaked electron concentration) comes from the highly-directional electron angular distribution acquired by the electrons in a grazing incidence setup. To isolate a single electron bunch, we perform simulations using 1-cycle laser pulses and analyze the effect of carrier-envelop phase with particle tracking. The duration of the electron bunch can be further decreased by increasing the laser intensity and the focal spot size, while its direction can be changed by tuning the preplasma density gradient.

Quasimonoenergetic electron beams with relativistic energies and ultrashort duration from laser-solid interactions at 0.5 kHz.

We investigate the production of electron beams from the interaction of relativistically-intense laser pulses with a solid-density SiO(2) target in a regime where the laser pulse energy is approximately mJ and the repetition rate approximately kHz. The electron beam spatial distribution and spectrum were investigated as a function of the plasma scale length, which was varied by deliberately introducing a moderate-intensity prepulse. At the optimum scale length of lambda/2, the electrons are emitted in a collimated beam having a quasimonoenergetic distribution that peaked at approximately 0.8 MeV. A highly reproducible structure in the spatial distribution exhibits an evacuation of electrons along the laser specular direction and suggests that the electron beam duration is comparable to that of the laser pulse. Particle-in-cell simulations which are in good agreement with the experimental results offer insights on the acceleration mechanism by the laser field.

Vacuum-free x-ray source based on ultrashort laser irradiation of solids.

A vacuum-free ultrafast laser-based x-ray source is demonstrated. Hard x-rays up to 80KeV are generated from Cu, Mo, Ag, Sn, and Ge targets in a laminar helium flow surrounded by atmosphere using tightly focused 33fs, 3mJ laser pulses. X-ray spectra, conversion efficiencies, and source sizes are presented. Six-fold efficiency improvement is observed, over similar sources found in the literature [1]. Source sizes determined for Cu and Mo show distinct dependences on laser pulse energy. It is also shown that the Cu source size has no dependence on the presence of the spectral band around the 8KeV K-shell lines.

High power fiber laser driver for efficient EUV lithography source with tin-doped water droplet targets.

In this paper we report the development of nanosecond-pulsed fiber laser technology for the next generation EUV lithography sources. The demonstrated fiber laser system incorporates large core fibers and arbitrary optical waveform generation, which enables achieving optimum intensities and other critical beam characteristics on a laser-plasma target. Experiment demonstrates efficient EUV generation with conversion efficiency of up to 2.07% for in-band 13.5-nm radiation using mass-limited Sn-doped droplet targets. This result opens a new technological path towards fiber laser based high power EUV sources for high-throughput lithography steppers.

Generation of hard X-rays using an ultrafast fiber laser system.

We report the first hard X-ray source driven by a femtosecond fiber laser. The high energy fiber CPA system incorporated a 65mum LMA fiber amplifying stage which provided 300-fs recompressed pulses and diffraction limited beam quality with M(2) < 1.07. A deformable mirror was used to optimize the wavefront and the spot size was focused down to 2.3 mum with an f/1.2 paraboloidal mirror. 50muJ was deposited on the nickel target with 2x10(15)-W/cm(2) focal intensity and a distinctive Ni K(alpha)-line (7.48 keV) emission was measured with 5x10(-8) energy conversion efficiency.

Emission and its back-reaction accompanying electron motion in relativistically strong and QED-strong pulsed laser fields.

The emission from an electron in the field of a relativistically strong laser pulse is analyzed. At pulse intensities of J>or=2x10(22) W/cm(2) the emission from counterpropagating electrons is modified by the effects of quantum electrodynamics (QED), as long as the electron energy is sufficiently high: E>or=1 GeV . The radiation force experienced by an electron is for the first time derived from the QED principles and its applicability range is extended toward the QED-strong fields.

All-reflective high fringe contrast autocorrelator for measurement of ultrabroadband optical pulses.

We describe an all-reflective interferometric autocorrelator designed to measure ultrabroadband optical pulses in the UV through IR spectral regions. By carefully choosing the device geometry we are able to obtain approximations for the nonlinear autocorrelation functions that reduce computation times to values acceptable for use in iterative pulse reconstruction schemes. We describe the optical design, autocorrelation functions, and present proof-of-principle experimental results measuring 20.6 fs pulses with a transform limit of 9.6 fs.

Demonstration of fiber-laser-produced plasma source and application to efficient extreme UV light generation.

Efficient generation of extreme UV (EUV) light at lambda = 13.5 nm from a bulk Sn target has been demonstrated by using a fiber laser. The conversion efficiency from the 1064 nm IR to the EUV was measured to be around 0.9% into 2pi steradians within a 2% bandwidth. To the best of our knowledge, this is the first time an all-fiber system was used to generate EUV or soft x rays.

Directly diode-pumped Yb:KY(WO(4))(2) regenerative amplifiers.

Two Yb(3+) -doped KY(WO(4))(2) regenerative amplifiers, one end pumped by two 1.6-W single-stripe diodes at 940 nm and the other side pumped by one 20-W diode bar at 980 nm, are demonstrated. When the regenerative amplifiers are injected, 40-muJ , 400-fs and 65-muJ , 460-fs pulses at a 1-kHz repetition rate are obtained following compression from the end- and side-pumped amplifiers, respectively.

Isolated attosecond pulses generated by relativistic effects in a wavelength-cubed focal volume.

Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (lambda3) can create relativistic intensity with maximal gradients using minimal energy. With particle-in-cell simulations we found that single 200-as pulses could be produced efficiently in a lambda3 laser pulse reflection by means of deflection and phase compression caused by the coherent motion of the plasma electrons that emit these pulses. This novel technique is efficient (approximately 10%) and can produce single attosecond pulses from the millijoule to the joule level.