G.F. Cairns

Using low and high prepulses to enhance the J=0−1 transition at 19.6 nm in the Ne-like germanium XUV laser

Abstract We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where ≈20 mm long germanium slab targets were irradiated at ≈1.6×10 13 W cm −2 using ≈0.7 ns (1.06 μm) pulses from the VULCAN glass laser. Prepulses were generated at fractional power levels of ≈2×10 −4 (low) and ≈2×10 −2 (high) and arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse. For both the low and high prepulses the output of the 3p-3s, J =0–1, line at 19.6 nm was enhanced such that the peak radiant density (J/st) for this line became greater than that for the normally stronger J =2−1 lines at 23.2 and 23.6 nm. The J =0–1 line, whose FWHM duration was reduced from ≈450 ps to ≈100 ps, delivered ≈6× more power (W) than the average for the combinned J =2−1 lines, whose FWHM duration was ≈500 ps for both levels of prepulse. The higher prepulse was more effective, yielding ≈2× more radiant density and ≈7× more power on both the J =0–1 and J =2−1 transitions compared to the low prepulse case. The most dramatic observation overall was the ≈40× increase of power in the J =0–1 line for the high prepulse (≈2%) case compared with the zero prepulse case. These observations, coupled with measurements of beam divergence and beam deviation through refractive bending, as well as general agreement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and (c) to relax the plasma density gradients, particularly in the J =0–1 gain zone.

Preliminary studies of radiation coupling between remote soft X-ray laser amplifiers

Coupling of a soft X-ray laser beam with a relaying concave mirror in a sequentially pumped amplifier geometry using the Ne-like Ge system has been studied experimentally. Preliminary observations indicate an increase in the spatial coherence of the amplified relayed beam. In addition, near-field imaging of one of the amplifier plasmas shows a double-lobed intensity pattern of the emergent beam indicating refractive guiding of the amplified beam with components both normal and tangential to the target surface.

Dependence of spatial coherence of 23.2-23.6-nm radiation on the geometry of a multielement germanium x-ray laser target

The spatial coherence of a nanosecond pulsed germanium collisionally excited x-ray laser is measured experimentally for three target configurations. The diagnostic is based on Young’s slit interference fringes with a dispersing element to resolve the 23.2- and 23.6-nm spectral lines. Target configurations include a double-slab target, known as the injector, and geometries in which the injector image is image relayed to seed either an additional single-slab target or a second double-slab target. A special feature of this study is the observation of the change in the apparent source size with angle of refraction across the diverging laser beam. Source sizes derived with a Gaussian source model decrease from 44 μm for the injector target by a variable factor of as much as 2, according to target configuration, for beams leaving the additional amplifiers after strong refraction in the plasma.

A time-resolved spectroscopy study of the resonance-line emission in the Ge XXIII XUV laser

A comparison is presented of the temporally resolved resonance-line emission from the Ne-like Ge XUV laser (pumped with nanosecond pulses) with the predictions for the same emission from the hydro-atomic code EHYBRID. The specific lines chosen were the two 3s - 2p Ne-like lines at 10.01 and 9.762 A, and the 3s - 2p F-like group of lines in the 9.4 - 9.6 A region. Modification of the code to include 112 excited levels of the F-like ion facilitated a direct comparison between experiment and model of (i) the temporal variation of the emissions and (ii) the variation of the peak intensity ratios of the F-like to Ne-like emissions with irradiance on target. Agreement between experiment and model was generally good, with the trends of the F/Ne ratio variation with irradiance being clearly reproduced. The results show that the model is realistically simulating the hydrodynamics and atomic kinetics of the plasma. This provides further support that the model can simultaneously calculate the and the (sub-keV) transition intensities through population dynamics which are acceptably accurate.

Characterization of optical properties of PtSi at 3.392 μm from 300 K to 85 K and the relation of morphological effects

PtSi/Si Schottky junctions, fabricated using a conventional technique of Pt deposition with a subsequent thermal anneal, are examined using X-ray diffraction, atomic force microscopy and a novel prism/gap/sample optical coupling system. With the aid of X-ray diffraction and atomic force microscopy it is shown that a post-anneal etch in aqua regia is essential for the removal of an unreacted, rough surface layer of Pt, to leave a much smoother PtSi film. The prism/gap/sample or Otto coupling ring is mounted in a small UHV chamber and has facilities for remote variation of the gap (by virtue of a piezo-actuator system) and variation of the temperature in the range of approximately 300 K - 85 K. The system is used to excite surface plasmon polaritons on the outer surface of the PtSi and thus produce sensitive optical characterization as a function of temperature. This is performed in order to yield an understanding of the temperature dependence of phonon and interface scattering of carriers in the PtSi.

Optical source model for the 23.2-23.6 nm radiation from the multielement germanium soft X-ray laser

Abstract Distributions of source intensity in two dimensions (designated the source model), averaged over a single laser pulse, based on experimental measurements of spatial coherence, are considered for radiation from the unresolved 23.2/23.6 nm spectral lines from the germanium collisional X-ray laser.The model derives from measurements of the visibility of Young slit interference fringes determined by a method based on the Wiener–Khinchin theorem. Output from amplifiers comprising three and four target elements have similar coherence properties in directions within the horizontal plane corresponding to strong plasma refraction effects and fitting the coherence data shows source dimensions (FWHM) are ∼26 μm (horizontal), significantly smaller than expected by direct imaging, and ∼125 μm (vertical: equivalent to the height of the driver excitation).

Modeling the optical response of grating-profiled PtSi/Si infrared detectors

Modeling the optical response of grating profiled PtSi/Si structures is examined to demonstrate the potential of microstructuring in optimizing the absorption of infrared detectors. Coupling to angularly broad surface plasmon polariton resonances near normal incidence is, in fact, achieved at both Si/PtSi and SiO2/PtSi interfaces for the same grating parameters in the wavelength ranges 3.0 - 4.4 micrometer and 1.3 - 1.9 micrometer respectively. These ranges correspond to two infrared, atmospheric transmission windows, and demonstrate the potential for a single device geometry to operate optimally in two different spectral bands. It is also shown that, throughout these spectral bands, it is possible to attain reflectance significantly lower than that of the planar structure counterparts in the angle range 0 degrees to plus or minus 20 degrees (corresponding to the use of F1.4 optics), along with containment of low reflectance to that angle range. Absorption mediated by the PtSi/Si surface plasmon polariton mode may be of particular interest in these Schottky barrier structures, since there would be considerable enhancement in the generation of hot carriers in the near barrier region where they have a better chance of direct or indirect (via elastic scattering) promotion over the barrier to give rise to a detectable charge.

Measurement of the photo-pump strength of the 3d-5f transitions in the automatically line matched Ni-like Sm photo-pumped X-ray laser

The photo-pump strengths of both the ((3d4)0(3d6)0)0 - (((3d3)3/2(3d6)0)3/2(5f)5/2)1 and the ((3d4)0(3d6)0)0 - (((3d4)0(3d5)5/2)5/2(5f)7/2)1 transitions in Ni-like Sm34+ have been measured as 2.4*10(-4) and 2.4*10(-4) photons/mode respectively. The implications of the measurement are briefly discussed in a comparison of the merits of automatically line matched photo-pump scheme to those of the collisional excitation Ni-like Sm(+34) scheme.

A linearly polarized soft x‐ray laser

We report results of polarization experiments on the collisionally excited Ne‐like Ge soft x‐ray laser where we have used an injector‐amplifier multistage geometry. The polarization state of the x‐ray beam was analyzed by two crossed 45° angle of incidence multilayer mirrors which act as linear polarizers. Results were evaluated by comparing intensities of time‐integrated beam patterns behind each polarizer. The polarization state of the 23.2 and 23.6 nm ASE output of the injector plasma was systematically studied and, as expected, revealed no macroscopic degree of polarization observable within the precision of the experiment. When the injector output beam was linearly polarized and coupled into the amplifier plasma, the degree of polarization of the amplifier output was ∼0.98, the total gain‐length product attained for the polarized beam was ≊ 12 and the beam energy was ≊ 20 nJ.

Study of beam aberrations in a germanium XXIII XUV laser amplifier

Abstract A beam of amplified spontaneous emission at 23.2 23.6 nm from a GeXXIII XUV laser has been injected into a separate amplifier plasma and the astigmatic aberrations introduced by plasma density gradients in the amplifier have been estimated from analysis of images of the amplified beam.