Optical-field-induced Ionization Research Articles

Amplification properties of vacuum ultraviolet Ar_2* produced by infrared high-intensity laser

We report optical amplification of Ar(2)* at 126 nm, pumped by optical-field-induced ionization (OFI) created by an infrared high-intensity laser. A gain-length product of 0.84 was obtained by using multipass amplification with a vacuum ultraviolet (VUV) cavity. The gain-length product was increased up to 4.3 through the use of single-pass amplification with a VUV reflector and a hollow 5.0 cm-long fiber. Similar small signal gain coefficients of 0.84 and 0.86 cm(-1) were obtained in two different experiments, in which OFI Ar plasma gain media were produced in free space filled with Ar and inside an Ar-filled hollow fiber.

Optical amplification of argon excimers in the vacuum ultraviolet spectral region, produced by ultrashort-pulse high-intensity-laser-produced electrons

Optical-field-induced ionization (OFI) electrons were produced in a 30 cm Ar-filled hollow fiber by shining a high-intensity laser into the fiber. By use of such a long plasma with the help of a multilayer dielectric mirror made for 126 nm emission, we observed significant optical amplification of the Ar2* emission at 126 nm. The optical amplification resulted in a maximum small signal gain coefficient of 0.16 cm-1 at 126 nm at 10 atm of Ar pressure.

Dissociative ionization of methane by chirped pulses of intense laser light.

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.

Vacuum ultraviolet argon excimer production by use of an ultrashort-pulse high-intensity laser

We report on the observation of ${\mathrm{Ar}}_{2}^{*}$ emission at 126 nm by use of a hollow fiber to guide the high-intensity laser propagation in high-pressure Ar. A small-signal gain coefficient of $0.05{\mathrm{cm}}^{\ensuremath{-}1}$ was measured at 126 nm by this method. The increase of the ${\mathrm{Ar}}_{2}^{*}$ emission intensity was measured to be $\mathrm{exp}(2.5),$ with an increase of the fiber length up to 50 cm. Kinetic analysis revealed that the ${\mathrm{Ar}}_{2}^{*}$ production processes were initiated by the electrons produced by the high-intensity laser through an optical-field-induced ionization (OFI) process. A rapid conductive cooling of the OFI electrons is favorable to initiate the ${\mathrm{Ar}}_{2}^{*}$ formation kinetics more efficiently. This high-density cold plasma production method should be applicable for other vacuum ultraviolet rare-gas excimer lasers.

Application of frequency-domain interferometry in the extreme-ultraviolet range by use of high-order harmonics

We apply the frequency-domain interferometry technique in the extreme-ultraviolet (XUV) range using high-order harmonics. The technique is first used to study the influence of ionization of the generating gas on the harmonic emission. We report a thorough study of the fringe contrast as a function of the generating parameters: gas pressure, laser intensity and focus position, harmonic order, delay, and relative intensity of the two laser pulses. We show that two phase-locked time-delayed harmonic pulses can be produced under optimal conditions. However, a strong distortion of the fringe pattern (low contrast and large asymmetry) can be induced by the deleterious effects of the medium ionization, i.e., depletion of the emitters, laser defocusing, and loss of mutual coherence that is due to free-electron dispersion. XUV frequency-domain interferometry is then applied to the measurement of the electron density of a plasma created by optical field-induced ionization of a high-pressure helium jet by use of an intense short-pulse laser. The measured temporal evolution of the electron density shows that the resolution of our experimental setup was of the order of 200 fs, which, to our knowledge, provides the first XUV interferometry measurement on a femtosecond time scale.

Time-Resolved Absorption Spectroscopy Using Femtosecond Laser-Produced Plasma X-rays.

High-density plasmas created near a solid surface by a femtosecond laser pulse emit ultrashort x-ray pulses that are synchronized with the laser pulse. We achieved more than 30-fold enhancement of the soft x-ray emission by fabricating an array of nanoholes on an alumina surface. We measured the duration of the soft x-ray emitted from the laser plasma by the cross-correlation method using an optical field-induced ionization process in Kr gas. Utilizing a 10-ps soft x-ray pulse, we measured the time-resolved absorption of optically excited silicon near its LII,III edge. We found that laser-pulse irradiation caused a more than 10 % increase in the soft x-ray absorption near the edge, which recovered within 20 ps. We assume the origin of this absorption change to be the bandgap renormalization of Si. We also employed picosecond soft x-rays to measure the ablated particles in Al plasma created by a 100-fs laser pulse.

Cross-correlation measurement of ultrashort soft x-ray pulse emitted from femtosecond laser-produced plasma using optical field-induced ionization

We propose a cross-correlation technique for measuring ultrashort soft x-ray pulse shapes using the rapid increase in Kr+ ion density caused by optical field-induced ionization, which operates as an ultrafast x-ray absorption switch. Using this technique, we measured the shape of a soft x-ray pulse near 15.6 nm emitted from W plasma produced by a 100 fs laser pulse, and found the duration to be about 4 ps assuming a Gaussian pulse. This result was in good agreement with the duration measured with an x-ray streak camera thus confirming the feasibility of our technique. The temporal resolution of this technique has the potential to overcome the limitation of the ionizing pulse duration.

Vacuum ultraviolet Ar excimer emission initiated by high intensity laser produced electrons

We have observed Ar 2* emission using a tabletop femtosecond high intensity laser as an excitation source. High intensity laser produced electrons via an optical field induced ionization (OFI) process initiated the Ar 2* production kinetics. Excimer production kinetic analysis suggests that ground state neutral dimers in a supersonic pulsed gas jet played a role in the production of Ar 2*. This production method differs from the three-body association process found in electron beam initiated high pressure Ar gas. A fast conductive cooling of the OFI plasma was found to be appropriate to initiate the excimer formation kinetics more efficiently.

Observation of X-ray spectra from nitrogen clusters irradiated with high-intensity ultrashort laser pulses

X-ray spectra from nitrogen clusters irradiated with ultrashort laser pulses are reported. The line spectra of H-like and He-like nitrogen ions have been observed by irradiation with 100 fs, 800 nm pulses at 7×1017 W/cm2 irradiance. The generation of highly charged ions of N6+ and N7+ is explained by the optical field-induced ionization and the subsequent collisional ionizations in the clusters. The He-δline has anomalously high brightness compared to other He-like lines. It is ascribed to charge exchange, which preferentially populates the n=5 level of N5+.

Collisional Effects on Population Inversion in Optical-Field-Ionized Plasmas

The population inversion between n =2 and 1 in hydrogen-like helium is calculated with a non-Maxwellian electron energy distribution in a plasma produced by optical field-induced ionization. Collisional relaxation of the electron energy distribution after passage of the laser pulse is followed by using a non-linear Fokker-Planck equation. In the low density regime the relaxation process plays an important role in limiting the population inversion, because the cold component of the electron distribution is reduced. A significant gain is obtained with a pulse length of less than 30 fs.

An optical field-induced ionization X-ray laser using a preformed plasma scheme

Soft X-ray amplification by optical field-induced ionization (OFI) of a preformed plasma is investigated. A small-signal gain of 20 cm/sup -1/ and a gain-length product of 4 at 13.5 nm on the Lyman-/spl alpha/ transition in hydrogen-like lithium have been obtained using the modified OFI scheme. Experimental and numerical results ensure that we have produced a plasma with a significantly lower electron temperature than what is expected by an above-threshold ionization model. To explain the results, a two component plasma model in which relatively high temperature electrons are produced by OFI in a cold electron bath of the preformed plasma is presented. The model indicates that the average electron temperature of the OFI plasma rapidly decreases since a high-temperature part of the electrons escapes from the focal volume without interaction. The initial electrons produced prior to the field ionization, which survive after the OEI, also significantly contribute to the rapid three-body recombination. Based on the ionization-induced refractive index change, the pulse propagation of a high-intensity pump laser during the OFI is also discussed. >

Tunable Solid-state Laser. Generation and Applications of High-Intensity Femtosecond Laser Pulses.

Recent developments of laser technology have made it possible to produce light intensities of 1015-1018 W/cm2, where the high-frequency radiation field becomes comparable to or in excess of the typical atomic Coulomb field. This paper describes the generation of high- intensity femtosecond laser pulses based on tunable solid-state lasers and reviews typical highfield phenomena such as optical-field induced ionization (OFI) of atoms, high-order harmonic generation in atomic gases, and x-ray lasers using OFI processes.