Possible dose-rate effects in a plasmid DNA exposed to pulsed extreme ultraviolet (XUV) and soft x-ray (SXR) water window radiation from two different table-top plasma-based sources was studied. Dose delivered to the target molecule was controlled by attenuating the incident photon flux with aluminum thin foils as well as varying the DNA/buffer-salt ratio in the irradiated sample. Irradiated samples were analyzed using the agarose gel electrophoresis. Some additional bands were identified in gel electrophoretograms as results of a DNA cross-linking. They were inspected by atomic force microscopy (AFM). Yields of single- and double-strand breaks (Gy-1 Da-1) were determined as a function of incident dose rate. Both yields decreased with a dose rate increasing. The ratio of single- and double-strand breaks exhibited only a slight increase at elevated dose rates. In conclusion, complex and/or clustered damages do not seem to be initiated under these irradiation conditions.
Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks
(DSBs), abasic sites, modified sugar and bases. Most theoretical and experimental studies have been focused on DNA
strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a
molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. The complexity of
lesions produced in DNA by ionizing radiations is thought to depend on the amount of energy deposited at the site of
each lesion. We have studied the nature of DNA damage induced directly by the pulsed 46.9 nm radiation provided by a
capillary-discharge Ne-like Ar laser (CDL). Different surface doses were delivered with a repetition rate of a few Hz and
an average pulse energy ~ 1 μJ. A simple model DNA molecule, i.e., dried closed-circular plasmid DNA (pBR322), was
irradiated. The agarose gel electrophoresis method was used for determination of both SSB and DSB yields. Results are
compared with a previous study of plasmid DNA irradiated with a single sub-nanosecond 1-keV X-ray pulse produced
by a large-scale, double-stream gas puff target, illuminated by sub-kJ, near-infrared (NIR) focused laser pulses at the
PALS facility (Prague Asterix Laser System).
Two novel schemes for efficient x-ray laser generation from laser-produced plasma and
capillary discharge-driven plasmas are described. The combination of nano-structured targets with the
high energy ultrashort pulse lasers can result in the generation of laser-produced plasmas that could lead
to high brightness sources of incoherent multi-KeV radiation and x-ray lasers of short pulse duration at
shorter wavelengths. The generation of 0.5-1 keV x-ray laser radiation from a Ni-like U plasma created
using excitation from a Petawatt laser is analyzed. The efficient excitation of capillary discharge plasmas
in micro-capillary discharge channels is discussed.
The desktop capillary-discharge Ne-like Ar laser (CDL) providing 10-μJ nanosecond pulses of coherent 46.9-nm
radiation with a repetition rate up to 12 Hz was developed and built at the Colorado State University in Fort Collins and
then installed in Prague. The beam of the laser was focused by a spherical mirror covered with Si/Sc multilayer coating
onto the surface of poly(methyl methacrylate) - PMMA. Interaction parameters vary by changing the distance between
sample surface and beam focus. The samples were exposed to various numbers of shots. Analysis of damaged PMMA by
atomic force (AFM) and Nomarski (DIC - differential interference contrast) microscopes allows not only to determine
the key characteristics of the focused beam (e.g. Rayleigh's parameter, focal spot diameter, tight focus position, etc.) but
also to investigate mechanisms of the radiation-induced erosion processes.
Multilayer mirrors for the extreme ultraviolet (EUV) are key elements for numerous applications of coherent EUV
sources such as new tabletop lasers and free-electron lasers. However the field of applications is limited by the radiation
and thermal stability of the multilayers. Taking into account the growing power of EUV sources the stability of the optics
becomes crucial. To overcome this problem it is necessary to study the degradation of multilayers and try to increase
their temporal and thermal stability. In this paper we report the results of detailed study of structural changes in Sc/Si
multilayers when exposed to intense EUV laser pulses. Various types of surface damage such as melting, boiling, shock
wave creation and ablation were observed as irradiation fluencies increase. Cross-sectional TEM study revealed that the
layer structure was completely destroyed in the upper part of multilayer, but still survived below. The layers adjacent to
the substrate remained intact even through the multilayer surface melted down, though the structure of the layers beneath
the molten zone was noticeably changed. The layer structure in this thermally affected zone is similar to that of
isothermally annealed samples. All stages of scandium silicide formation such as interdiffusion, solid-state
amorphization, silicide crystallization etc., are present in the thermally affected zone. It indicates a thermal nature of the
damage mechanism. The tungsten diffusion barriers were applied to the scandium/silicon interfaces. It was shown that
the barriers inhibited interdiffusion and increased the thermal stability of Sc/Si mirrors.
We have demonstrated imaging at soft x-ray wavelengths in transmission and reflection modes using high repetition rate table-top soft x-ray lasers. Transmission mode imaging with a resolution better than 50 nm was demonstrated using the output from a 13.9 nm Ni-like Ag laser in combination with condenser and objective Fresnel zone plate optics. Reflection mode imaging of a microelectronic chip with a resolution of 120-150 nm was demonstrated using the illumination provided by the 46.9 nm output from a compact capillary-discharge Ne-like Ar laser. This microscope combines a Schwarzschild condenser and a zone plate objective. The results demonstrate the feasibility of practical nanometer-scale microscopy with compact soft-x-ray laser sources.
We report the demonstration of a high repetition rate desktop-size capillary discharge laser emitting at (= 46.9 nm (26.5 eV) used for experiments in photochemistry. Laser pulses with energy ~ 13 uJ were generated at 12 Hz repetition rate by single pass amplification in a 21 cm long Ne-like Ar capillary discharge plasma column. The capillary lifetime is 2-3 104 shots. This new type of portable laser is of interest for numerous applications requiring a compact intense source of short wavelength laser light. One such application that we are currently pursuing is the study of small molecules using time of flight mass spectroscopy. Molecules include ammonia, NO, oxygen, hydrogen bonded nanoclusters, and metal oxide nanoclusters. Through single photon ionization the reactivity and catalytic behavior of these molecules is studied.
For conventional wavelength (UV-vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (<100 nm) lasers emitting pulses with durations ranging from ~10 fs to ~1 ns have recently been put into routine operation. This makes it possible to investigate how ablation characteristics depend on pulse duration in the XUV spectral region. Four sources of intense short-wavelength radiation available in the authors' laboratories, including XUV and soft x-ray lasers, are used for the ablation experiments. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for nanosecond pulses with those measured for shorter pulses, we can study the influence of pulse duration on XUV ablation efficiency. The results of the experiments also show that the ablation rate increases while the wavelength decreases from the XUV spectral region toward x-rays, mainly due to increase of attenuation lengths at short wavelengths.
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.
For conventional wavelength (UV-Vis-IR) lasers delivering radiation energy to the surface of materials, ablation thresholds, ablation (etch) rates, and the quality of ablated structures often differ dramatically between short (typically nanosecond) and ultrashort (typically femtosecond) pulses. Various short-wavelength (l < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes it possible to investigate how the ablation characteristics depend on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser (Colorado State University, Fort Collins), focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiments, the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with those measured for ultrashort pulses, we can study the influence of pulse duration on XUV ablation efficiency. Ablation efficiencies measured with short pulses at various wavelengths (i.e. 86/46.9/21.2 nm from the above-mentioned lasers and ~ 1 nm from the double stream gas-puff Xe plasma source driven by PALS) show that the wavelength influences the etch rate mainly through the different attenuation lengths.
We demonstrate the use of a tabletop capillary-discharge Ne-like Ar laser emitting nanosecond duration pulses at a wavelength of 46.9 nm for investigation of radiation damage mechanism and damage threshold in Sc/Si extreme ultraviolet multilayer mirrors. To vary the emission load at the surface of the mirror under test the intense 0.13 mJ laser pulses were focused using a spherical Sc/Si multilayer mirror to obtain fluences ranging from ~ 0.01 to >10 J/cm2. Single spots and large area patterns (2x2 mm2) were irradiated depending on the type of surface analysis technique employed. Damage threshold fluences of ~ 0.08 J/cm2 were measured for Sc/Si coatings deposited on both borosilicate glass and Si substrates, compared to the 0.7 J/cm2 found necessary to damage a bare Si substrate. The use of scanning and transmission electron microscopy, and small-angle X-ray diffraction techniques revealed the thermal nature of the damage mechanism. These results are relevant to the use of the Sc/Si mirrors in combination with newly developed high power EUV sources, and provide a benchmark for their further improvement.
We present the first results of nano-patterning in poly-methyl methacrylate (PMMA) photo-resist using a 46.9 nm tabletop extreme ultraviolet (EUV) laser. As a proof of principle, we recorded a Fresnel diffraction pattern of a copper mesh with 19 μm square holes. Results of ongoing interference experiments will also be presented.
Ablation thresholds, etch rates, and quality of ablated structures often differ dramatically if a conventional, UV-Vis-IR laser delivers radiation energy onto a material surface in a short (nanosecond) or ultra-short (picosecond/femtosecond) pulses. Various short-wavelength (λ < 100 nm) lasers emitting pulses with durations ranging from ~ 10 fs to ~ 1 ns have recently been put into a routine operation. This makes possible to investigate how the ablation characteristics depends on the pulse duration in the XUV spectral region. 1.2-ns pulses of 46.9-nm radiation delivered from a capillary-discharge Ne-like Ar laser, focused by a spherical Sc/Si multilayer-coated mirror were used for an ablation of organic polymers and silicon. Various materials were irradiated with an ellipsoidal-mirror-focused XUV radiation (λ = 86 nm, τ = 30-100 fs) generated by the free-electron laser (FEL) operated at the TESLA Test Facility (TTF1 FEL) in Hamburg. The beam of the Ne-like Zn XUV laser (λ = 21.2 nm, τ < 100 ps) driven by the Prague Asterix Laser System (PALS) was also successfully focused by a spherical Si/Mo multilayer-coated mirror to ablate various materials. Based on the results of the experiment the etch rates for three different pulse durations are compared using the XUV-ABLATOR code to compensate for the wavelength difference. Comparing the values of etch rates calculated for short pulses with the measured ones for ultrashort pulses we may study the influence of pulse duration on the XUV ablation 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.
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