Gain-length Product Research Articles

Saturated amplification of a collisionally pumped optical-field-ionization soft X-ray laser at 41.8 nm.

We report the first saturated amplification of an optical-field-ionization soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 330-mJ, 35-fs, 10-Hz Ti:sapphire laser system in a few-mm cell filled with xenon. A gain of 67 cm(-1) on the 4d(9)5p-4d(9)5d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 x 10(9) photons per pulse. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

Optimization of nickel-like collisional soft-X-ray lasers at low pump power

We report on the experimental demonstration of saturated X-ray laser output from collisionally pumped Ni-like Pd, Ag, and Sn at wavelengths of 14.7, 14.0, and 12.0 nm, respectively. Using a 100 ps drive pulse irradiation and a `twin-0.25%' prepulse configuration on flat slab targets, gain-length products >16 were obtained for the three elements. The drive power required for saturation was only 250 GW in a 100 ps pulse (25 J), corresponding to an irradiance of 10 TW/cm 2.

Collisionally pumped hybrid soft X-ray laser in Ne-like sulphur

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3 p 1 S 0–3 s 1 P 1 transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.

Review of Ni-like ion X-ray laser research at Lawrence Livermore National Laboratory

We have demonstrated saturated output on several nickel-like ion X-ray lasers ranging from niobium to silver by using a psec, high-power, chirped-pulse-amplification, tabletop laser. These results have been achieved at wavelengths from 20.3 to 13.9 nm on the Ni-like 3 d 94 d 1 S 0→3 d 94 p 1 P 1 laser line using a total of 5 to 7 J of energy in a traveling wave excitation scheme. Strong amplification is also observed for Ni-like Sn at 11.9 nm. Gain of 41 cm −1, gain-length product of 18, and output energy of 12 μJ are measured for the Ni-like Pd line at 14.7 nm. For Ni-like Mo, experiments are done using multilayer mirrors to obtain two-dimensional images of the output aperture of the laser and to measure the total laser energy as a function of various parameters such as the delay between the short and long pulses and the energy of the two pulses. For Mo we measure an output energy of 2 μJ and a gain-length product of 16.6. To model the Mo experiments, the LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the XRASER code, which then does the kinetics calculations to determine the gain. The temporal and spatial evolution of the plasma is studied both with and without radiation transport included for the 4 f and 4 p→3 d Ni-like Mo resonance lines. High gains are predicted and observed for both the 3 d 94 d 1 S 0→3 d 94 p 1 P 1 laser line at 18.9 nm and the 3 d 94 f 1 P 1→3 d 94 d 1 P 1 photopumped line at 22.6 nm.

A self-terminating X-ray amplifying medium: Kinetics of K-shell photoionization and stimulated emission

A two-stage model for achieving X-ray amplifying action on photoionized K-transitions pumped by the soft X-ray broadband flux from laser-produced plasma is treated. A simple pumping criterion is considered by the analysis of the positive gain-length product for the two-stage model of innershell transitions (0) → (3) and (2) → (1). The incident photon flux integrated over the effective lifetime T21 of an innershell transition (2) → (1) is related to the photoionization coefficient μ03(ph), stimulated emission cross section σ21(stim), and the effective gainlength L(eff) ≃ 0.63/μ03(abs), where μ03(abs) is the absorption coefficient of a pump soft X-ray radiation. Numerical estimates for the feasibility of K-shell X-ray amplification are given for C, F, Na, Ti, and Sn. For the example of an exponential X-ray carrier pulse (XCP) propagating through a resonant medium, an amplifying effect depending on the ratio of the input pulse temporal bandwidth τband to the effective lifetime T21 is investigated. It is shown that for an XCP with a finite bandwidth τband the logarithm of an energy gain, ln[U(z)], falls below a linear relation with z and thus, from the viewpoint of maximum amplifying effect bandwidths τband with τband/T21 > 1 are preferable. The requirements for achieving X-ray K-shell photoionization amplification for C are discussed in more detail. PACS numbers: 32.80.Hd, 42.50.Md, 42.55.Vc, 42.65.Re, 52.25.Nr.

Experimental characterization of short-wavelength Ni-like soft-x-ray lasing toward the water window

We report experimental results of studies of Ni-like x-ray lasing at wavelengths as short as 4.5 nm. The performance of x-ray lasing pumped by various types of pulse train that were composed of 100-ps pulses was investigated with a double-slab target placed in series to double the gain length. Two opposing laser beams separated by an appropriate time difference irradiated the double targets for quasi-traveling-wave pumping. Well-collimated double-target amplification was successfully demonstrated for Yb and Hf lasing at 5.0 and 4.7 nm whose gain–length products were 11 and 6, respectively. The Ni-like lasing lines of Ta have also been observed at 4.5 nm.

Demonstration of XUV amplification to the ground state in low-charged nitrogen ions

We report the first demonstration of the gain to the ground state in low-charged nitrogen ions by optical-field ionization. A small signal gain coefficient of 9.6 cm −1 and a gain–length product that it is in the range 3.8–3.9 for the NIII 3 s( 2 S)−2 p( 2 P) transition XUV laser at a wavelength of 45.2 nm were obtained by an only 25-mJ linearly-polarized 100-fs laser pulse.

Soft-x-ray lasing and saturation in nickellike silver at pump energies below 30 J

We demonstrate that careful optimization of main-pulse–prepulse time delay and target curvature permits lasing and saturation to be achieved in Ni-like collisionally excited ions at pump energies that are achievable with tabletop systems. Saturation of the 13.9-nm, J=0–1 line of Ag, which has a gain–length product of 15.3, was obtained by use of a single prepulse pumping scheme and a pump energy of less than 30 J in 100 ps.

Generation of millijoule-level soft-x-ray laser pulses at a 4-Hz repetition rate in a highly saturated tabletop capillary discharge amplifier

Laser pulses with energies of as much as 1 mJ were generated at a wavelength of 46.9 nm by single-pass amplification in a 34.5 cm-long Ne-like Ar capillary discharge plasma. The large gain-length product of this plasma column allows for soft-x-ray amplification in a highly saturated regime, resulting in efficient energy extraction. Average laser output pulse energy of 0.88 mJ and peak power of 0.6 MW were obtained at a repetition rate of 4 Hz. With an estimated peak spectral brightness of approximately 1x10(23)photons /(s mm(2)mrad> (2)0.01% bandwidth) this tabletop laser is one of the brightest soft-x-ray sources to date.

Travelling wave chirped pulse amplified transient pumping for collisional excitation lasers

A ∼3 ps travelling wave chirped pulse amplified pulse at 6×10 14 W cm −2 superimposed on ∼300 ps background pulses is shown to be an efficient method to pump transient collisional excitation X-ray lasers in both Ni-like and Ne-like ions. Measurements of X-ray laser output as a function of plasma length are fitted with results of an amplified spontaneous emission model of the laser output taking account of travelling wave pumping effects. A small signal gain coefficient ∼42 cm −1 and a effective gain length product of ∼18 are measured for the Ni-like Sn laser at 120 Å. Simulations from a hydrodynamic and atomic physics code (EHYBRID) coupled to a ray trace code show that a spatially averaged small signal gain ∼65 cm −1 can be obtained in Ne-like Ge provided the optimum pumping pulse arrangement is used.

Amplified spontaneous emission and maximum gain–length product revised for general line shapes

The theory of amplified spontaneous emission (ASE) and the gain–length product at saturation is revised. Formulas valid for both peak and integrated lines are given. In the latter case the solution for a general Voigt stimulated-emission line shape is presented, whereas in the two limiting cases of Gaussian and Lorentzian line shapes an approximation better than the one obtained by the steepest-descent method is successfully achieved. Various parameters that take into account the ratio of spontaneous to stimulated linewidths, the form factor depending on the linewidths of the inhomogeneous and homogeneous convoluted line shapes, and the population inversion fraction arise from the theory, giving the complete functional dependence for both the ASE photon flux and the gain–length product.

Saturation in a Ni-like Pd soft-x-ray laser at 14.7 nm

Here we report on what is to our knowledge, the first achievement of saturated output in a collisionally pumped, Ni-like Pd soft-x-ray laser at $14.7$ nm $(J=0\ensuremath{-}1$ line). Using a single prepulse scheme and a 100-ps pulse duration we were able to observe the onset of saturated output and to measure a gain-length product of $15.8$ (gain=8.3 ${\mathrm{cm}}^{\ensuremath{-}1})$ on flat slab targets. A systematic investigation and discussion on beam divergence, deflection angle, and space-resolved measurements is also presented. All these aspects lead to the evidence of the onset of saturation. The measured drive-irradiance threshold for the x-ray laser emission was only $6.6$ ${\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ (\ensuremath{\approx}165 GW) while the maximum used drive irradiance was only $12$ ${\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ (\ensuremath{\approx}300 GW). These values correspond, respectively, to a pump energy of 16.5 J and 30 J in 100 ps and demonstrate the realization of a real table-top soft-x-ray laser. A key role in the achievement of these results was played by the reduction in the roughness of the target surface, which greatly improves the performances of the Pd x-ray laser.

Demonstration of transient gain x-ray lasers near 20??nm for nickellike yttrium, zirconium, niobium, and molybdenum

We demonstrate strong lasing on the Ni-like 4d(1)S(0)?4p(1)P(1) transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17-26cm (-1) are determined on the 4d?4p transition, with overall gain-length products gL of 11-12. Lasing is observed and gain is measured on the 4f(1)P(1)?4d(1)P(1) transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time.

Transient inversion soft X-ray lasers

The collisionally excited transient inversion scheme is shown to produce exceptionally high gain coefficients and gain-length products. Data are presented for the Ne-like titanium and germanium and Ni-like silver X-ray lasers (XRL's) pumped using a combination of nanosecond and picosecond duration laser pulses. This method leads to a dramatic reduction of the required pump energy and makes down-sizing of XRL's possible, an important prerequisite if they are to become commonly used tools in the long-term.

Saturation in neon- and nickel-like collisional soft-X-ray lasers at low pump energy

We report on the experimental demonstration of saturated X-ray laser output from collisionally pumped Ne-like Fe at 25.5 nm as well as Ni-like Ag and Pd at 14.0 nm and 14.7 nm, respectively, using a 100-ps drive pulse irradiation. A double-prepulse scheme and a 3-m radius-of-curvature target resulted in a gain-length product of gL=16.5 in the case of Fe, With a single prepulse and flat slab targets, gain-length products of 15.3 and 15.8 were obtained for Ag and Pd, respectively. Saturation was also confirmed by the observed reduction in beam divergence with increasing target length. The required drive energy used was only 30 J in a 100-ps pulse, corresponding to an irradiance of 12 TW/cm/sup 2/. A key role in the achievement of these results was played by the reduction in the roughness of the target surface in the case of the Pd X-ray laser.

VUV laser in the Lyman band of molecular hydrogen pumped by fs titanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60/spl deg/ onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm/sup -1/ and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5/spl times/10/sup 15/ cm/sup -3/. A simple model of the H/sub 2/ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping.

Gain saturation of the Ni-like X-ray lasers

Driven by a double 75 ps pulse with 2.2 ns separation, saturated operation of nickel-like Ag, In, Sn, and Sm X-ray lasers have been demonstrated with only 75 J drive energy on target. The variation of X-ray laser output with target length is found to fit well to a simple model for an amplified spontaneous emission (ASE) laser including saturation. Small signal gains of ∼10 cm −1, effective gain length products of ∼18, and saturation irradiance of (1–5)×10 10 W/cm 2 are measured for these lasers using a fitting procedure.

Optical gain for the Ne VIII 4-3 transition by capillary discharge pumping

In order to investigate the potential of a capillary discharge as a pumping source for soft x-ray lasers the transitions in lithium-like neon were investigated. The discharge with a maximum current of 100 kA through 0.4-0.6 cm diameter gas-filled alumina tubes leads to two fluorescence maxima. The first is due to the collapse of the shock wave, whereas the second, appearing about 60 ns later, is ascribed to the recombination of ions and is enhanced by stimulated emission of radiation. By measuring at different tube lengths, the maximum gain-length product for the 29.2 nm transition was determined to be . A semi-empirical model for the discharge indicates that only part of the current drives a shock wave into the centre, while the other part flows through the surface region of the insulator. This plays an important role insofar as (i) it limits the maximum obtainable plasma temperature and (ii) it supplies sufficient ablated matter for a rapid cooling of the plasma necessary to obtain an inversion in the excited states of the ions. Gain appears after the electron temperature in the centre of the plasma has dropped below about 15 eV. This is reached during the falling edge of the recombination phase.

XUV-Amplification in an argon z-pinch plasma

Amplification of XUV radiation of the 3p–3s transitions at 46.9 and 67.1 nm with a gain-length product of 2.4 and 2.6 has been observed in an argon z-pinch plasma. The amplification was demonstrated by varying the discharge length between 40 and 120 mm. A temporal resolved measurement of the gain coefficient is presented here that shows a gain of >0.15 cm −1 for a duration of more than 40 ns with a peak value of 0.2 cm −1 for the 46.9 nm line. The plasma was generated in a fast z-pinch discharge driven by a 200 kV three-stage Marx-generator. The discharge current in the alumina insulator tube with a diameter of 7 mm was 40 kA.

Soft X-Ray Amplification at 26.2 nm with 1-Hz Repetition Rate in a Table-Top System

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.