We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.

Abstract:We have demonstrated a new type of high repetition rate 46.9 nm capillary discharge laser that fits on top of a small desk and that it does not require a Marx generator for its excitation. The relatively low voltage required for its operation allows a reduction of nearly one order of magnitude in the size of the pulsed power unit relative to previous capillary discharge lasers. Laser pulses with an energy of ~ 13 µJ are generated at repetition rates up to 12 Hz. About (2-3) x 10 4 laser shots can be generated with a single capillary. This new type of portable laser is an easily accessible source of intense short wavelength laser light for applications. XUV optical constants by reflectometry using a high-repetition rate 46.9-nm laser," IEEE J. Sel. Top.

We demonstrate a significant extension of the high-order harmonic cutoff by using a fully-ionized capillary discharge plasma as the generation medium. The preionized plasma dramatically reduces ionization-induced defocusing and energy loss of the driving laser due to ionization. This allows for significantly higher photon energies, up to 150 eV, to be generated from xenon ions, compared with the 70 eV observed previously. We also demonstrate enhancement of the harmonic flux of nearly 2 orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a plasma as a medium for highorder harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths using ions. High-order harmonic generation (HHG) coherently upconverts laser light from the visible and infrared into the extreme-ultraviolet region of the spectrum. Over the past decade, HHG has been demonstrated as a useful light source for a wide range of applications, such as investigating surface dynamics [1], holographic imaging [2], and more recently for probing static molecular structure [3,4], or internal molecular dynamics. In HHG, the nonlinear interaction between a material, typically a gas [5,6], and an intense laser field produces high-order harmonics of the fundamental laser. The laser field first ionizes the atom or molecule, then accelerates the liberated electron away from the ion, finally generating high-order harmonic photons when the laser field reverses and the oscillating electron recollides with its parent ion. The highest photon energy that can be produced via this interaction is predicted [7,8] by the cutoff rule to be h max I p 3:17U p , where I p is the ionization potential of the atom and U p / I L 2 is the ponderomotive energy of the electron in the laser field. Here I L is the peak laser intensity and is the wavelength of the driving laser field.From the cutoff rule, the range of photon energies that are generated in HHG is determined by the laser intensity. However, in most experiments to date, the maximum observed HHG photon energy has been limited not by the available laser intensity, but by the intensity at which the target atoms are nearly completely ( 98%) ionized -or the ''saturation intensity'' I s (< I L ) of the medium. This is because at near full ionization in a medium, ionizationinduced refraction [9,10] of the laser beam reduces the effective laser intensity compared with what could be obtained in a vacuum. Moreover, reduced coherence lengths also limit the number of atoms contributing to the harmonic emission. To obtain the highest possible photon energy, I s can be increased either by using a shorter duration laser pulse, or by using atoms with a higherionization potential, both of which allow neutral atoms to survive to a higher laser intensity. Partial phase matching of the harmonic emission in a plasma can also be implemented to increase the harmonic output. Recently...

We report results of the exposure of poly͑tetrafluoroethylene͒ -͑PTFE͒, poly͑methyl methacrylate͒ -͑PMMA͒, and polyimide -͑PI͒ to intense 46.9-nm-laser pulses of 1.2-ns-duration at fluences ranging from ϳ0.1 to ϳ10 J/cm 2 . The ablation rates were found to be similar for all three materials, ϳ80-90 nm/ pulse at 1 J / cm 2 . The results suggest that the ablation of organic polymers induced by intense extreme ultraviolet laser radiation differs from that corresponding to irradiation with longer wavelengths. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1854741͔ Ablation of organic polymers by optical radiation, in particular from excimer lasers, has been extensively studied. 1-3 However, polymer ablation induced by extreme ultraviolet ͑XUV͒ radiation with wavelength shorter than 100 nm is discussed in only a small number of publications. Poly ͑butene-1 sulphone͒, 4 PMMA, 5,6 poly ͑ethylene terephthalate͒, 7 and PTFE 5,8 were ablated by incoherent, nonmonochromatic XUV emission from laser-produced plasma. A Z-pinch plasma was used as an XUV source for ablation of PMMA 5 and PTFE. 5,9 Very recently PMMA was efficiently ablated by sub-100-fs pulses of 86-nm-radiation provided by a free-electron laser. 7,10 A large number of studies on direct photoetching of the organic polymers induced by XUV synchrotron radiation were also reported. 11-13 However, according to Haglund's criterion 14 this photoinduced material removal is closer to laser desorption than to laser ablation because of a low peak power of the photon beams delivered by a synchrotron radiation source.The recent advances in compact high repetition rate XUV lasers 15 producing nanosecond pulses of monochromatic radiation with energy of several hundred micro-Joules opens the possibility to study materials ablation in a new regime. In this letter, we report on the ablation behavior of three common organic polymers: PTFE, PMMA, and PI, irradiated with an intense focused 46.9-nm-laser beam. The ablation processes induced by nanosecond pulses of 46.9-nm-laser radiation are compared with those occurring in the polymer materials irradiated with conventional longer wavelength laser sources.The samples studied here were 1-mm-thick sheets ͑Goodfellow͒ cut into 2.0ϫ 5.0 mm 2 chips. The PTFE and PI samples were polished, while the PMMA was used directly with no additional treatment. The samples were placed into vacuum chamber where they were exposed to 1.2 ns FWHM pulses of 46.9 nm radiation from a capillary discharge Nelike Ar laser. 15 Laser pulses with energy of ϳ130 J were focused onto the sample surfaces by a spherical Sc/ Si multilayer-coated mirror 16 with measured reflectivity of ϳ30%. A motorized positioning system was used to translate the samples along the beam optical axis, as well as in the directions perpendicular to the beam. 17 The former motion allowed us to vary the irradiation fluence by controlling the laser spot diameter on the sample surface, while the pulse energy and duration were kept constant. Figure 1 shows an optical micrograph of a PM...

Images with a spatial resolution of 120-150 nm were obtained with 46.9 nm light from a compact capillarydischarge laser by use of the combination of a Sc-Si multilayer-coated Schwarzschild condenser and a freestanding imaging zone plate. The results are relevant to the development of compact extreme-ultraviolet laser-based imaging tools for nanoscience and nanotechnology. © 2005 Optical Society of America OCIS codes: 180.7460, 110.7440, 140.7240. Rapid progress in nanotechnology and nanoscience creates the need for new practical imaging tools capable of resolving nanometer-sized features. Shortwavelength light provides an opportunity to develop optical imaging systems with the highest resolution. The best resolution so far, 20 nm, has been obtained in imaging with soft-x-ray synchrotron radiation at 2.07 nm wavelength. 1 Submicrometer resolution was obtained with a soft-x-ray recombination laser, 2 and 75 nm resolution was reported with a low-repetitionrate (several pulses per day) laboratory-sized soft-xray laser.3 There is, however, a need for the development of more compact and practical nanometerresolution imaging systems. Toward this goal extreme-ultraviolet (EUV) light from high-order harmonic sources was used to demonstrate imaging systems with a resolution of better than 1 m, 4,5 and soft-x-ray imaging with laser-plasma-based sources has been investigated. [6][7][8] In this Letter we report what is to our knowledge the first demonstration of nanometer-scale imaging with a compact capillary-discharge pumped highrepetition-rate EUV laser. Spatial resolution of the 46.9 nm wavelength system is estimated to be 120-150 nm. This is to our knowledge the highest resolution achieved with a compact high-repetitionrate coherent EUV illumination source. The high average power ͑ϳ1 mW͒ and multihertz repetition rate of the Ne-like Ar capillary discharge laser source that we used 9,10 allowed us to perform real-time imaging, for which the image is continuously updated on the computer screen at the rate of the laser pulses.The imaging system is schematically illustrated in Fig. 1. It consists of a compact capillary-discharge 46.9 nm laser, a Sc-Si multilayer-coated reflective condenser, a zone-plate objective, and a CCD detector. The condenser, the imaged sample, and the objective were mounted onto motorized translation stages that were assembled inside a vacuum chamber connected to the EUV laser source with standard vacuum fittings. The illumination source is a compact capillary-discharge Ne-like Ar laser emitting at a wavelength of 46.9 nm with a pulse duration of ϳ1.2 ns. Its short wavelength, narrow spectral bandwidth, high photon fluence, and beam directionality make this source well suited for microscopy. The spectral bandwidth of the laser is ⌬ / Ͻ10 −4 . 9 The laser's output pulse energy and degree of spatial coherence depend on the capillary discharge length. For this experiment the laser was equipped with an 18 cm capillary discharge tube that provided an average pulse energy of ϳ0.1 mJ. This choice of cap...