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Dr Deborah Birx, the White House Coronavirus Task Force coordinator, told NBC News on "Meet the Press"that "[T]he U S needs a 'breakthrough' in coronavirus testing to help screen Americans and get a more accurate picture of the virus' spread "We have been involved with biopathogen detection since the 2001 anthrax attacks and were the first to detect anthrax in real-time A variation on the laser spectroscopic techniques we developed for the rapid detection of anthrax can be applied to detect the Severe Acute Respiratory Syndrome-Corona Virus-2 (SARS-CoV-2 virus) In addition to detecting a single virus, this technique allows us to read its surface protein structure In particular, we have been conducting research based on a variety of quantum optical approaches aimed at improving our ability to detect Corona Virus Disease-2019 (COVID-19) viral infection Indeed, the detection of a small concentration of antibodies, after an infection has passed, is a challenging problem Likewise, the early detection of disease, even before a detectible antibody population has been established, is very important Our team is researching both aspects of this problem The paper is written to stimulate the interest of both physical and biological scientists in this important problem It is thus written as a combination of tutorial (review) and future work (preview) We join Prof Federico Capasso and Editor Dennis Couwenberg in expressing our appreciation to all those working so heroically on all aspects of the COVID-19 problem And we thank Drs Capasso and Couwenberg for their invitation to write this paper © 2020 Navid Rajil et al , published by De Gruyter, Berlin/Boston 2020

Spectroscopy and microscopy in the so-called “water-window” is a holy grail of modern molecular biology. A pulsed source of coherent X-rays within this spectral window, falling between 2.3 nm and 4.4 nm, provides a unique tool for time-resolved imaging of bio-systems in their naturally water-rich state. Within this spectral range, water is mostly transparent, while proteins are mostly opaque. This results in a high-contrast image on the sub-cellular level. Here we present, for the first time, generation of a very high gain of G≈ 60/cm in He-like CV ions via transitions to the ground state at 4.03 nm in a table-top device.

We propose a new scheme for compression of ultrashort laser pulses enabled by Near-Forward Raman Amplification (NFRA) in plasma. In contrast to traditional forward Raman amplification in plasma, the seed pulse in NFRA propagates faster than the pump through manipulation of the spatiotemporal properties of the pump beam. A “π-pulse” solution similar to that of stimulated Raman backscattering amplification is obtained, indicating that NFRA can be used for compression of ultrashort laser pulses. NFRA is numerically demonstrated with a 2D simulation using a front-tilt near-forward pump pulse.

An efficient method to transform a radiation of X-ray plasma lasers into a train of sub-fs pulses, as well as to form sub-fs pulses directly in an active medium of plasma lasers is suggested. This method is based on temporal and spatial modulation of the parameters of a resonant transition in absorbing or amplifying medium by a moderately strong IR field propagating in the same direction as an X-ray radiation.

Coherent intense attosecond x-ray pulses could lead to a fast dynamical imaging of biological macromolecules and other material nanostructures with a unique combination of a record high temporal and spatial resolution. Plasma-based x-ray laser sources are capable of producing high-energy x-ray pulses but with relatively long picosecond duration. The sources based on high-harmonic generation (HHG) of a laser field allow one to produce much shorter pulses but of lower energy. We suggest two different paths towards intense subfemtosecond x-ray sources: (i) via efficient transformation of the picosecond radiation of the x-ray plasma lasers into the trains of subfemtosecond pulses in a resonantly absorbing medium, and (ii) via amplification of HHG radiation in the active medium of the x-ray plasma lasers. We show that essentially the same technique can be used for realization of both paths. This technique is a modulation of the parameters of the resonant transition (accordingly in absorbing or amplifying medium) produced under the action of a sufficiently strong infrared or optical field. We propose experimental realization of the suggested technique in the passive and/or active media of (i) Li iii ions modulated by the mid-IR laser field and (ii) C vi ions modulated by the optical laser radiation.

Between any two resonance lines for an atomic medium there exists a dispersionless wavelength where the ac Stark shift from the two resonances cancel and the real part of the resonant susceptibility is 0. We experimentally demonstrate the effects of this wavelength in a pump-probe experiment in high-density atomic vapor. A strong pump field excites the medium, in which we counterpropagate a broadband probe field. In a long cell, only at the dispersionless wavelength will scattering and self-focusing or defocusing not cause attenuation, allowing a spectrometer to read off the dispersionless wavelength. We perform a propagation experiment that shows the passband centered around the 790-nm dispersionless wavelength between the ${D}_{1}$ and the ${D}_{2}$ lines of rubidium.

The main objective of this project was to conduct experimental and theoretical research to find conditions leading to higher, than previously obtained efficiency η of transfer the pump energy into the short seed beam in plasma of Stimulated Raman Back-Scattering (SRBS). We have demonstrated very large amplification and compression in our earlier SRBS plasma. However, the efficiency η was much too low to reach very high intensity of the output beam in the focal spot. Recently, by solving a very difficult technical SRBS’ problem, namely, the creation of very reproducible and much larger diameter plasma channels than in our earlier research, we propose a new approach to obtain higher efficiency η. The crucial new result was a very reproducible, low noise amplified seed in the larger diameter of the plasma channel leading to the higher efficiency. Using this new setup and very encouraging results about increase efficiency continuing this approach in the future the efficiency is expect to reach the range of η ≈15 - 20 % required to develop practical SRBS plasma laser. Intellectual Merit: The model for the present project was created by our earlier SRBS experiments. The main objective of those experiments was to amplify and compressmore » the seed pulses in a plasma . The experiments demonstrated an unprecedented large pulse intensity amplification of 20,000 in system of 2-passes in ~2mm long plasma, and the seed pulse compression from 550fsec down to ~50fsec. The pump and seed beams in the present project have diameters of ~0.2–0.25mm each, propagating in ~0.35 - 0.45mm diameter and ~2-2.5mm long plasma channels (optimal length for our SRBS experiment) with input pump and seed intensities of 2x1014 and 3x1013 W/cm2, respectively. Such an SRBS system design was “prescribed” by computer simulations, which predict elimination of the SRBS “ saturation” for a such relatively short plasma channel. Plasma channels has been created by combining shorter (200psec) and longer (5nsec) laser pulses using an axicon lens for shorter pulse and spherical lens for longer pulse, a unique procedure we have developed for SRBS experiments.« less

It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such as B3+ (at 6.1 nm).

Experiments demonstrate the ~77× amplification of 0.5 to 3.5-ps pulses of seed light by interaction with Langmuir waves in a low density (1.2 × 1019 cm−3) plasma produced by a 1-ns, 230-J, 1054-nm pump beam with 1.2 × 1014 W/cm2 intensity. The waves are strongly damped (kλD = 0.38, Te = 244 eV) and grow over a ~ 1 mm length, similar to what is experienced by scattered light when it interacts with crossing beams as it exits an ignition target. The amplification reduces when the seed intensity increases above ~1 × 1011 W/cm2, indicating that saturation of the plasma waves on the electron kinetic time scale (

A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.

We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.We propose a new mechanism for the self-guiding of ultra-intense sub-picosecond laser pulses in gaseous media. It can be realized via optical field ionization by a laser pulse as it propagates inside an expanding cylindrical shock wave launched into ambient gas by a decayed plasma filament. In experiments, the filament was created in a hydrogen jet by a low energy femtosecond laser pre-pulse line focused with axicon lens. We demonstrated ionization-assisted guiding in structures with diameter as small as 14 μm and up to 3.5 mm long. The intensity reached 5 × 1017 W/cm2 in a single mode propagating for more than 100 Rayleigh lengths.

Experiments on Stimulated Raman Backscattering (SRBS) in plasma have demonstrated significantly higher energy conversion in a double-pass amplifier where the laser pulses go through the plasma twice compared with a single-pass amplifier with double the plasma length of a single pass. In this paper, the improvement in understanding recent experimental results is presented by considering quite in detail the effects of plasma heating on the modeling of SRBS. Our simulation results show that the low efficiency of single-pass amplifiers can be attributed to Landau damping and the frequency shift of Langmuir waves. In double-pass amplifiers, these issues can be avoided, to some degree, because pump-induced heating could be reduced, while the plasma cools down between the passes. Therefore, double-pass amplifiers yield considerably enhanced energy transfer from the pump to the seed, hence the output pulse intensity.

In this review we concentrate our attention on X-ray laser research and development covering the so called soft X-ray region (down to ~ 10 nm) and the ``water window'' region (down to 2.3 – 4.4 nm). We present the development of soft X-ray lasers (SXLs) and their applications using primarily collisional and recombination schemes. We present the paths towards compact systems and major achievements to date. We discuss conditions for using both schemes to reach the ``water window'' region. In this discussion we also present another possible schemes based on photo-pumping, inner shell transitions in atoms and ions and ``Doppler Compression'' of IR laser radiation down to the X-ray region with its intrinsic tunability possibility. A significant part of the review is dedicated to the wide range of applications of SXLs with wavelengths at and above 10 nm. Also discussed is the potential of extending the range of biological applications for the ``water window'' region.

The output power of any laser system is limited by the power that any of its individual components can withstand without being destroyed. This increases the size, complexity and cost of next-generation lasers built using conventional optical-gain media. But by using a plasma, which is impervious to optical damage, as the gain medium, such limitations could be significantly surpassed. Here, we report the use of stimulated Raman backscattering in a millimetre-scale plasma to simultaneously amplify and compress an input pulse to increase its intensity by more than two orders of magnitude. Further amplification and compression of such a pulse was achieved by passing it twice through our system, an approach that could be readily extended in future with a cavity-like multipass design. Such an approach could enable the construction of a new generation of compact, low-cost ultrahigh-intensity laser systems.

This paper is about the recent experimental results on amplification of the CV line in the “water window” at 4.03 nm from resonance transition to the ground level of He-like ions in recombination scheme. The indication of the amplification of the CV line has been observed when an elongated narrow plasma channel was created, where high intensity 100 fs beams, optimal for creating CV ions in high density plasma, was propagated up to 0.5–0.6 mm. Without channeling the effective plasma length was much shorter and there was no indication of amplification.The large interest in gain generation in He-like ions in the transition to ground state is due to the possibility of applying a recently developed theory of Lasing Without Inversion (LWI) in XUV and X-ray regions to largely increase the gain for such transitions. The presented results of the indication of CV line amplifications are being discussed from the point of view of using LWI as a superradiance gain increase, hence to construct a very compact soft X-ray laser in the “water window”.The last part of the paper is related to the application of the ultra-intensive fs plasma laser, which is currently in the process of development by using stimulated Raman backscattering (SRBS) to create a plasma amplifier and compressor, as the pump for compact laser operating in the “water window” and also at shorter wavelengths.

A minimally invasive keratorefractive procedure using high-intensity, low-energy femtosecond laser pulses to perform intrastromal ablation is described. Because of the low pulse energy and the ultrashort duration, tissue in the corneal stroma can be ablated with almost no heat or shockwave generation. This technique obviates the need for the laser in situ keratomileusis (LASIK) flap but retains the advantages of the LASIK procedure. In the technique, a series of femtosecond laser pulses create temporary microchannels in the stroma, oriented perpendicular to the eye’s optical axis. After the microchannels are created, a second series of femtosecond pulses directly ablate the desired amount of stromal tissue in a controlled fashion. The ablated material is ejected from the microchannels so the surface layer above the ablated regions collapses, with a consequent change in the refractive power of the cornea. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

We present a highly temporally and spatially resolved direct measurement of the propagation of ionization and recombination fronts in bulk ZnSe. The fronts were created by two-photon absorption (TPA) of 10 ps, 800 nm 'pump' pulses incident on both sides of a 1 mm thick crystal. The measurement was achieved by the spectral analysis of the frequency shift of a 100 fs 'probe' pulse transversely propagating through the crystal at the time of excitation and recombination. The probe was frequency shifted due to the temporal change in the refractive index of the medium. The delay between the probe and the pumps was finely scanned for time-resolved measurements, while imaging of the probe onto a spectrometer slit provided spatial resolution across the crystal. The spatially and temporally resolved shift agrees well with a model that includes TPA and recombination. Our set-up provides a general technique for measurements of rapid ionization and recombination processes.

General : Our major scientific achievements in Raman Backscattering (RBS) amplification and compression of short laser pulses in plasma. The laser system based on RBS steps in where the current technology of chirped pulse amplification (CPA) (extremely successful in developing ultra-short and ultra-intense laser pulses in last 2 decades) becomes difficult and very expensive to apply. Good base for such RBS laser was created by our recent experiments, which were supported by GPS grants. The main objective of the present grant was: improvement efficiency of energy transfer from pump to seed. The results surpassed our expectations; we improved the efficiency of energy transfer from pump to seed by a factor of 6 compared to the best of our previous results and amplified seed pulse compressed down to about 50 fsec.

Periodic plasma density modulations have been observed in a laser spark created by focusing a 5–20 MW laser pulse into nitrogen at 1 to 2 atmospheres. The formation of such structures has been studied by measuring the period as a function of the wavelength λ and the incident angle γ of the laser pulse. The results show good agreement with a surface-instability model. The intensity of the scattered laser light has the same periodicity and is consistent with the measured plasma density profile.

The results of two different series of experiments on generation lasing action in the soft x-ray region are presented. In the first series lasing in hydrogen-like Li III ions at 13.5 nm (2-1 transition to ground state) in a 5 mm long LiF microcapillary at 2 Hz repetition rate is demonstrated. The initial plasma was created in a 0. 3 mm diameter microcapillary by a low power Nd:YAG laser (0. 1 J in 5 ns), whereas the plasma pumping was proceeded with a 50 mJ, 250 fs UV laser. In the second series lasing is demonstrated in hydrogen-like B V ions at 26.2 nm (3-2 transition) using a very small Nd:YAG pumping laser (0.45 J in 8 ns) at a repetition rate of I Hz. The preplasma in a B 2 O 3 microcapillary was created from wall ablation with a 0.2 J (20 ns) KrF laser, and the whole soft x-ray lasing system takes up less space than is available on a 1.2 m × 3 m optical table. In the last part of the paper the development of soft-x-ray lasers (SXLs) for application in lithography and soft x-ray microscopy is discussed.

The main goal of this paper is to present experimental results on comparison of lasing efficiency for Li III 13.5 nm (2-1 transition) line in LiF microcapillary plasmas using 0.25 and 1 /spl mu/m subpicosecond pumping lasers. A formula for soft X-ray laser efficiency calculation is presented and used for such comparisons. The results for discharge created preplasma in L=4 mm and L=14 mm microcapillaries are also presented and compared with the results for laser created preplasma.

Using a very small Nd/YAG pumping laser (0.45 J in 8 nsec), we have demonstrated high gain $(G\ensuremath{\approx}14--19\mathrm{cm}{}^{\ensuremath{-}1})$ and gain-length product $(\mathrm{GL}\ensuremath{\ge}5)$ in hydrogenlike B V ions at a wavelength of 26.2 nm (the 3-2 transition) at a repetition rate of 1 Hz. The preplasma in a ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ microcapillary was created from wall ablation with a 0.2-J (in 20 nsec) KrF laser. The gain was generated in the microcapillary plasma in a traveling wave regime by longitudinal pumping. This very efficient soft x-ray lasing system takes up less space than is available on a $4\mathrm{ft}\ifmmode\times\else\texttimes\fi{}10\mathrm{ft}$ optical table.

Soft x-ray lasing with a gain length GL{approx_equal}5.5 was demonstrated in hydrogenlike LiIII at 13.5 nm (2-1 transition) in a 5 mm long LiF microcapillary using a 50 mJ, 250 fs UV laser beam at a 2 Hz repetition rate. The initial plasma was created in the 0.3 mm diameter microcapillary by a low power Nd/YAG laser. The strongly nonlinear increase of the 13.5 nm line intensity with increasing microcapillary length was compared with a linear increase of the LiIII 11.4 nm (3-1 transition) line intensity. {copyright} {ital 1996 The American Physical Society.}

Using femtosecond transient absorption spectroscopy on excited-state helium atoms in a plasma created through optical field ionization, we measured the decay of 2(3)S-2(3)P excitation with sub-ps temporal resolution. The population evolution shows that initial decay is significantly faster than the electron-atom collisions and three orders of magnitude faster than the single atom spontaneous decay rate. This indicates on superradiant coherent behavior of the atomic system inside the plasma.

Gain measurements have been performed in a recombining carbon plasma generated by a line focussed Nd-glass laser beam on a multi-fin target with the aim of developing a “table-top” soft X-ray laser. With a laser beam of only 4 J (2ns) and a 6 mm long target gain up to 7.1 cm−1 was measured for the C VI 18.2 nm line (gain-length ∼4.3). During these measurements high gain (6.5 cm−1) was also measured for the first time on the C VI 13.5 nm line (4−2 transition). These measurements were done in the process of preparation for two-target experiments. High gain was measured independently for two separate, identical 6 mm targets. Changes in the influx into the plasma of iron ions from “cooling” blades significantly affected the location of the gain region and can be used as a mechanism to control the gain region in two-target experiments.

The frequency upshifting of 0.8 μm picosecond laser pulses was demonstrated using the temporal change of the free carrier density in a ZnSe semiconductor crystal. The crystal was ionized by transverse propagating ps pulses. Shifts of up to 1.6 nm were observed, which agree within 25% with the theory.

This talk provides the summary of experimental research at Princeton on Raman Backscattering (RBS) amplification and compression in plasma [1]. The main subject of the talk is about obtaining high efficiency of such system [2,3].

In our presentation we overview progress we made in developing a new ultrashort and ultraintensive laser system based on Raman backscattering (RBS) amplifier /compressor from time of 10th XRL Conference in Berlin to present time of 11th XRL Conference in Belfast. One of the main objectives of RBS laser system development is to use it for pumping of recombination X-ray laser in transition to ground state of CVI ions at 3.4 nm. Using elaborate computer code the processes of Optical Field Ionization, electron energy distribution, and recombination were calculated. It was shown that in very earlier stage of recombination, when electron energy distribution is strongly non-Maxwellian, high gain in transition from the first excited level n=2 to ground level m=1 can be generated. Adding large amount of hydrogen gas into initial gas containing carbon atoms (e.g. methane, CH4) the calculated gain has reached values up to 150–200 cm−2 Taking into account this very encouraging result, we have proceed with arrangement of experimental setup. We will present the observation of plasma channels and measurements of electron density distribution required for generation of gain at 3.4 nm.

Our presentation consisted of two parts. In the first part we presented main theoretical results on the plasma and pumping conditions required to generate gain at 3.4 nm in H-like CVI ions in transition from the first excited state to ground state. Transient population inversion is generated during the recombination process. It was shown that high gain (up to G~200 cm-1) can be achieved using currently available compact lasers. In the second part we presented a new type of compact laser generating ultra-short and ultra-intensive pulses via Raman Backscattering (RBS) amplification and compression in plasma. We achieved large (up to 1000) “seed” amplification and its compression from~1 psec down to 150 fsec in only 2 mm long plasma pumped with~1014 W/cm2 pulses. We also presented very recent experiments on amplification in 2 passes setup. Such RBS amplifier and compressor is expected to provide in not too far future intensities~1020 W/cm2 at high repetition rate in compact (university type) system, which would be ideal pump not only for 3.4 nm laser but even for shorter wavelength lasers.

Details of experiments aimed at achieving low ionization state selectivity in molybdenum are presented. Targets are excited with a 10 J CO2 laser and the resultant vacuum ultraviolet spectrum (300–700 A) has been studied. Combinations of focal spot size, target depth, and target geometries are compared. Simple attenuation of energy is shown not to vary ionization stage composition significantly. Experiments conducted with grazing incidence targets result only in a hot plasma. Modular targets with cooling cylinders of various radii demonstrated good selectivity of the ionization states, but with low absolute signals. Finally, results from combinations of focal spot adjustment and radiative cooling illustrate increased control over desired plasma temperature and density for spectroscopic studies of molybdenum.

A high power UV laser has been developed as a pump source for short wavelength (down to 1 nm) X-ray lasers. Various schemes are considered and theoretical analysis is discussed. Spectroscopic studies of laser-target interaction have been performed and, in particular, the effect of a prepulse on plasma generation has been investigated. Analysis of the observed spectra indicates that reduction of the prepulse energy results in a higher temperature plasma. Investigation of the interaction using thin layered targets is also presented. These data provide evidence for initially hot plasma conditions generated from target layers ≦150 A. Discussions of proposed laser schemes at 1–5 nm are presented.

To generate plasmas with low average electron energy, the optical field ionized plasmas with helium and hydrogen gas mixture was studied. To measure the average electron energy, the line intensity ratio was measured experimentally and calculated using an atomic kinetic model. The measured line intensity ratio indicates that the average electron energy is 3 eV for the case of 5 % helium and 95 % hydrogen gas mixture.

Compressing laser pulses to extremely high intensities through backward Raman amplification might be accomplished in a plasma medium. While the theory is relatively straightforward for homogeneous fully ionized plasma, a number of important effects enter when the plasma is not fully ionized. In particular, when a mixture of gases is employed to accomplish the coupling, there can be several thresholds for incremental ionization. The refraction of both the pump and the seed is then strongly affected by the plasma ionization. Moreover, in the case of Raman backscattering in partially ionized plasma, the degree of plasma ionization is particularly sensitive to the counterpropagating geometry. This idea is examined in light of data for a recent experiment on a Raman amplifier.

The formation of plasma channels with nonuniformity of about +- 3.5% has been demonstrated. The channels had a density of 1.2x10^19 cm-3 with a radius of 15 um and with length >= 2.5 mm. The channels were formed by 0.3 J, 100 ps laser pulses in a nonflowing gas, contained in a cylindrical chamber. The laser beam passed through the chamber along its axis via pinholes in the chamber walls. A plasma channel with an electron density on the order of 10^18 - 10^19 cm-3 was formed in pure He, N2, Ar, and Xe. A uniform channel forms at proper time delays and in optimal pressure ranges, which depend on the sort of gas. The influence of the interaction of the laser beam with the gas leaking out of the chamber through the pinholes was found insignificant. However, the formation of an ablative plasma on the walls of the pinholes by the wings of the radial profile of the laser beam plays an important role in the plasma channel formation and its uniformity. A low current glow discharge initiated in the chamber slightly improves the uniformity of the plasma channel, while a high current arc discharge leads to the formation of overdense plasma near the front pinhole and further refraction of the laser beam. The obtained results show the feasibility of creating uniform plasma channels in non-flowing gas targets., 15 pages, 7 figures, submitted to Physics of Plasmas

We present a numerical investigation of the parameters characterizing the recombination gain in the LiIII 2→1 transition (13.5nm). The numerical model includes the initial Optical Field Ionization (OFI) of the plasma by an intense 100fs laser pulse, taking into account ATI heating, particle collisions and spatial effects. Gain is then calculated during the process of recombination as the plasma expands and cools down. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions. In addition we present the latest experimental results on Raman amplification of ultrashort pulses in the pursuit for a better and more compact scheme to enable a ``table-top'' x-ray laser.

Raman amplification of subpicosecond laser pulses up to 95 times is demonstrated at corresponding frequencies in a gas-jet plasma. The larger amplification is accompanied by a broader bandwidth and shorter pulse duration. Theoretical simulations show a qualitative agreement with the measurements, and the effects of the plasma conditions and laser intensities are discussed.

A large, UV preionized excimer krypton-fluoride (KrF) laser amplifier was designed and fabricated. This laser amplifier produced an on-target laser power density of 2*10/sup 18/ W/cm/sup 2/. Maximum output energy was determined to be 5 J in a 20-ns-long pulse, and maximum instantaneous power was achieved in a 250-350-mJ, 300-fs pulse with an injection (seed) pulse of approximately=3-4 mJ in a triple pass configuration. The output wavelength is 248 nm. The amplifier has a volume of approximately 5 liters. UV preionization was accomplished with a traveling spark board. Fast, low inductance switching of the high voltage was accomplished through the use of an electrically triggered rail-gap. Some of the design and tradeoffs of the electrical pulse discharge circuitry, considerations related to the materials used, the optics, gas handling, timing requirements, and some experimental results are discussed. >

Experimental evidences of Raman amplification of ultrashort pulses in microcapillary plasmas are presented. The amplification of 100\char21{}500 fs pulses was investigated in microcapillaries with different lengths. The experimental data, together with simulation results, indicate that the resonance condition for Raman amplification in high-density plasma, ${n}_{e}\ensuremath{\sim}1\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{20}{\mathrm{cm}}^{\ensuremath{-}3},$ existed only in a very short plasma column. Such an assumption makes it possible to reconcile the experimental results and theoretical predictions. Investigations in very short microcapillaries (0.2\char21{}0.5 mm) with a broadband seed pulse further support this hypothesis and the amplification factor is in agreement with the linear growth rate.

We present numerical calculation of recombination gain in LiIII transition to ground state (2 → 1). The model includes the initial ionization of the plasma by an intense fs laser pulse, and continues through the expansion and cooling of the plasma simultaneously with the recombination process. We show that although initial estimations of the energy absorption by the plasma from the ionization laser does not allow for recombination gain, the expansion and cooling processes that take place immediately after ionization, in addition to the non‐Maxwellian distribution of electrons in the plasma, give rise to high gain under feasible experimental conditions.

We present results related to the search for optimum conditions for lasing to ground state of H‐like LiIII ions at 13.5 nm. These conditions are being considered from the point of view of the development of a prototype of a very compact 13.5 nm laser for metrology of soft x‐ray (EUV) lithography. Theoretical simulations are discussed in relation to experimental data. Experiments on channeling of ultrashort high intensity pumping laser beam in microcapillary plasma are presented for conditions appropriate for lasing at 13.5 nm. We are also discussing Raman amplification of ultrashort pulses in microcapillary plasma as a possibility for future use in X‐ray lasers.

The optimum conditions for gain generation for the transition to the ground state at 13.5 nm in Liiii ions were investigated in discharge created pre‐plasma. The spatial and temporal evolution of plasma density was measured and a hollow density profile was observed. Waveguiding of high power laser pulses has been demonstrated in discharge created plasma channels, which can be helpful in increasing the interaction length, thus improving the gain‐length product.

Increasing the power of ultra-high-intensity lasers requires crystal amplifiers and metre-scale optical compression gratings that are ever more difficult to build. Simulations suggest that Raman amplification in a plasma could permit the generation of laser intensities many orders of magnitude higher than currently possible.

The experimental results on generation of high gain-length product for 13.5 nm radiation from 2 - 1 transition in hydrogen-like Li III ions are presented for 1 micrometers subpicosecond pumping laser. The comparison with earlier results, obtained with 0.25 micrometers subpicosecond pumping laser, is discussed in terms of gain generation efficiency. The results for discharge created pre-plasma in L equals 4 mm and L equals 14 mm microcapillaries are also presented.

The aim of this work was to investigate the conditions for waveguiding of nanosecond laser pulses in plasmas in microcapillary and gas jet for the development of a compact soft x-ray laser (SXL). A detailed study of He- and H-like carbon and nitrogen ion spectra, as a function of gas (plasma) densities, and plasma length was performed. The majority of the presented data were obtained in plasmas created in nitrogen injected into the vacuum chamber by pulsed electromagnetic valve. The waveguide conditions for the pump laser pulse (1.054 micrometer, 2 - 7 J, 1.5 ns) were achieved by line optical breakdown (up to 6 mm long) produced in gas by focusing a pre-pulse laser beam with an axicon lens. Intensive strong lines of H- and He-like ions were observed and the possibilities of obtaining the lasing conditions for SXL are discussed.