The table-top generation of high average power coherent soft x-ray radiation in a compact set up is of high interest for numerous applications. We have demonstrated the generation of bright soft x-ray laser pulses at 100 Hz repetition rate with record-high average power from compact plasma amplifiers excited by an ultrafast diode-pumped solid state laser. Results of compact λ=18.9nm Ni-like Mo and λ=13.9nm Ni-like Ag lasers operating at 100 Hz repetition rate are discussed.
We discuss recent advances in the development of high repetition rate table-soft soft x-ray lasers resulting from
research conducted at Colorado State University. Advancing saturated table-top lasers to shorter wavelengths we report
the operation of gain-saturated sub-10 nm table-top lasers at 1 Hz repetition rate. We also present experimental results
that show that injection-seeding of solid-target soft x-ray plasma amplifiers reduces the far field divergence by an order
of magnitude and to allow for control of the far-field beam characteristics by tailoring the divergence of the seed. We
finally discuss progress towards the development of high repetition rate compact all-diode-pumped soft x-ray lasers. We
have operated the front end of the diode-pumped soft-ray laser driver at 100Hz repetition rate, obtaining sub-5 ps optical
laser pulses of 100 mJ energy.
We describe our recent progress in the investigation of the spectral properties of collisional XUV lasers, including both
experimental measurements and numerical calculations. Using a wavefront-division, variable path-difference
interferometer, we have characterized the temporal coherence and the spectral width of an injection-seeded transient
XUV laser emitted at 18.9 nm from a Ni-like Mo plasma. Our results show that the temporal coherence of the beam is
significantly increased by the injection-seeded operation, compared to the standard ASE mode, in agreement with
detailed numerical simulations. Using the PPP code we have calculated the intrinsic linewidth of the same lasing line
over a range of electron density and at temperatures that are relevant to transient collisional pumping. We discuss the
relative contributions of homogeneous and inhomogeneous broadening to the overall profile.
We describe recent advances in the demonstration of table-top full field microscopes that use soft x-ray lasers for illumination. We have achieved wavelength resolution and single shot exposure operation with a very compact 46.9 nm microscope based on a desk-top size capillary discharge laser. This λ=46.9 nm microscope has been used to captured full field images of a variety of nanostructure systems and surfaces. In a separate development we have demonstrated a zone plate microscope that uses λ=13.2 nm laser illumination to image absorption defects in a extreme ultraviolet lithography (EUVL) mask in the same geometry used in a 4x demagnification EUVL stepper. Characterization of the microscope's transfer function shows it can resolve 55 nm half period patterns. With these capabilities, the λ=13.2 nm microscope is well suited for evaluation of pattern and defect printability of EUVL masks for the 22 nm node.
We report new advances in the development of high repetition rate table-top soft x-ray lasers. We have extended the gain-saturated operation of these lasers to 10.9 nm demonstrating a 1 Hz repetition rate laser operation in Ni-like tellurium with an average power of 1 microwatt. In a separate development we have demonstrated the first all-diodepumped soft x-ray laser. Lasing was achieved in the 18.9 nm line of Ni-like molybdenum in a plasma heated by a compact all-diode-pumped Yb:YAG laser. The solid state pump laser produces 8.5 ps pulses with up to 1 J energy at 10 Hz repetition rate. This diode-pumped laser has the potential to greatly increase the repetition rate and average power of soft x-ray lasers on a significantly smaller footprint. These compact soft X-ray lasers offer new scientific opportunities in small laboratory environments.
We have recently demonstrated high repetition rate tabletop lasers operating at wavelengths as short as 10.9 nm
based on collisional transient excitation of ions in plasmas created by laser heating of solid targets. As a further step in
the development of these lasers into very high brightness and fully coherent soft x-ray sources, we have demonstrated
injection seeding of the amplifiers with high harmonic seed pulses. We report results of an experiment in which a 32.6
nm Ne-like Ti amplifier was used to amplify a seed pulse from the 25th harmonic of Ti:Sapphire into the gain saturation
regime. Simultaneous amplification of the 27th harmonic at 30.1 nm was also observed. The seeded soft x-ray laser beam
was measured to approach full spatial coherence. We have demonstrated that this scheme is scalable to shorter
wavelengths and that is capable of producing extremely bright soft x-ray laser pulse with essentially full coherence.
We have acquired images with sub-38 nm spatial resolution using a tabletop extreme ultraviolet (EUV) imaging system operating at a wavelength of 13.2 nm, which is within the bandwidth of Mo/Si lithography mirrors This zone plate-based, full-field microscope has the power to render images in only several seconds with up to a 10,000 μm2 field of view. The ability to acquire such high-resolution images using a compact EUV plasma laser source opens many possibilities for nanotechnology, including in-house actinic inspection of EUV lithography mask blanks.
We have demonstrated gain-saturated operation of high repetition rate table-top soft x-ray lasers producing microwatt average powers at wavelengths ranging from 13.2 to 32.6 nm in transitions of Ni-like and Ne-like ions. Lasing was also observed for shorter wavelength transitions of the 4d1S0→4p1P1 Ni-like isoelectronic sequence, with amplification approaching gain saturation in the 11.9 nm line of Ni-like Sn and progressively reduced gain for wavelengths as low as 10.9 nm in Ni-like Te. The results were obtained heating a pre-created plasma with a picosecond optical laser pulse with an energy of only 1 J impinging at optimized grazing angles of incidence for maximum pump energy deposition efficiency. This pumping geometry takes advantage of the refraction of the pump beam to increase the energy deposition efficiency of the pump beam into the gain region, making it possible to operate soft x-ray lasers in this wavelength range at significantly increased repetition rates. The results demonstrate the feasibility of producing high average powers of coherent radiation in the 100 eV spectral region for applications using a table-top source.
We report high resolution imaging results obtained utilizing small-scale extreme ultraviolet laser sources. A compact capillary-discharge pumped Ne-like Ar laser emitting at a wavelength of 46.9 nm was used to demonstrate imaging with nanometer-scale resolution in transmission and reflection modes. We exploited the large photon fluence of this short wavelength laser to obtain high-resolution images with exposure times as short as 1-10 seconds. Images with a spatial resolution better than 140 nm were obtained using the combination of a Sc/Si multilayer coated Schwarzschild condenser and free-standing objective zone plate. Preliminary results of imaging with a 13.9 nm extreme ultraviolet laser light are also discussed.
In this work we report on the damage threshold of ion beam deposited oxide films designed for high peak power short pulse laser systems. Single layers of ZrO2, SiO2, Al2O3, TiO2, and Ta2O5 and multilayers of Al2O
3/TiO2, SiO2/Ta2O5, and SiO2/ZrO2 were grown on polished borosilicate glass substrates using ion beam sputter deposition. Deposition conditions were optimized to yield fully oxidized films as determined from x-ray photoelectron spectroscopy (XPS). Damage threshold testing was performed using an amplified Ti:Sapphire laser producing a train of 120 picosecond pulses at a wavelength of 800 nm. The laser output was focused with a lens to generate fluences ranging from 0.1 to 24 J/cm2. The highest damage threshold of 15.4 J/cm2 was measured for a single layer film of SiO2. The damage threshold of high reflectance and anti-reflection multilayer coatings fabricated for 800 nm applications was evaluated using the same procedure as for the single layer films. Highest damage thresholds of 2.5 and 3.5 J/cm2 were measured for a 6-pair ZrO2/SiO2 high reflectance coating and a 5 layer anti-reflection coating of the same materials.
We report the generation and characterization of plasma waveguides in a highly ionized Ar plasma created by a fast capillary discharge. Their results are of interest for the efficient longitudinal laser excitation of collisional soft x-ray lasers. A discharge-driven hydrodynamic compression guides progressively lower order modes through a plasma with increasing density and degree of ionization. The mode structure and guiding properties were investigated using near and far field imaging, and transmission measurements. The lowest order mode (FWHM diameter ~ 50 um) is guided with an energy transmission of ~ 75% over a 5.5 cm long plasma a fraction of a ns before the discharge plasma column reaches the conditions for lasing in Ne-like Ar. The rapid expansion of the highly ionized plasma column after the pinch forms a significantly more leaky and absorbent waveguide.
During recent months we have continued investigations of many different aspects of x-ray lasers to characterize and improve the source and applications. This work has included temporal characterization of existing laser-heated x-ray lasers under a wide range of pumping conditions. We have also looked into more details at different applications of x-ray lasers among which was the interferometry of laser-produced and capillary discharge plasmas in several irradiation conditions for different target Z materials. The reduction of pump energy remains the most important for the generation of new compact x-ray lasers. Numerical studies show that there are some ways to improve several of the key parameters of x-ray lasers specifically repetition rates and efficiency.
We report results of the development of capillary discharge driven metal-vapor plasma waveguides for the development of efficient laser-pumped soft x-ray lasers; and of the use of a previously developed capillary discharge Ne-like Ar 46.9 nm laser in study of the interaction of intense soft x-ray laser with materials. The guiding of a laser beam in a dense capillary discharge plasma channel containing a large density of Ag ions is reported. In term of applications we have conducted studies of materials modification and ablation with focalized 46.9 nm laser radiation at fluences between
0.1 and 100 J cm-2. The experiments demonstrated that the combined high repetition rate and high energy per pulse of the capillary discharge laser allows for the first time the processing of large surface areas with intense soft x-ray laser radiation. The damage threshold and damage mechanism of extreme ultraviolet Sc/Si multilayer mirror coatings was studied . Damage threshold fluences of ~ 0.08 J/cm2 were determined for coatings deposited on both borosilicate glass and Si substrates. Scanning and transmission electron microscopy, and small-angle X-ray diffraction techniques revealed the thermal nature of the damage mechanism. These results provide a benchmark for the use of Sc/Si multilayer mirrors in high fluence applications, and for the development of higher damage threshold mirrors. Soft x-ray laser ablation studies were also conducted for silicon and several plastic materials, including PMMA, Polyamide and PTFE.
This paper gives an overview of recent soft x-ray laser research at Colorado State University. Progress related to capillary discharge source development includes the observation of emission from the 13.2-nm laser line of Nickel-like Cd in a plasma column generated by a high power capillary discharge. This result suggests it might be possible to extend capillary discharge lasers to significantly shorter wavelengths. In another approach to the generation of coherent soft x-ray radiation we analyzed the possibility of amplifying high order harmonic pulses in a discharge-pumped amplifier. The study of the already well- characterized 46.9-nm Ne-like Ar laser was extended with new spatial coherence and laser wavefront measurements, in work conducted in collaboration with U. California Berkeley and U. of Paris-Sud groups. In the field of applications, we have extended our previous results of plasma interferometry with a tabletop laser to plasma densities up to 0.9 x 1021 cm-3. Sequences of soft x-ray laser interferograms of plasmas generated by a Nd-YAG laser at intensities between 1 x 1011 W cm-2 and 7 x 1012 W cm-2 show the development and evolution of a concave electron density profile. The detailed mapping of this phenomenon with soft x-ray interferometry exemplifies the usefulness of compact soft x-ray lasers in increasing the understanding of high density plasmas.
We have measured the absorption spectrum of the laser dye IR125 (also known as indocyanine green) at the water/air interface using resonant enhanced surface second harmonic generation. The spectra of the dye molecules at the interface are extremely sensitive to the bulk concentration. All the surface spectra reflect aggregation of the dye at the interface, even at the smallest concentrations detectable, below 1 (mu) M. Resonant enhanced second harmonic generation appears to be a good technique for measuring spectra at liquid interfaces.
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