Demonstration of partially transparent thick metallic sodium in the vacuum ultraviolet spectral range.

We describe the direct measurement of actual transmittance of sodium samples with thickness of a 2 mm and 3 mm in a spectral range >115 nm, resulting in >50% transmittance of 3 mm thick solid and liquid sodium samples including transmission of a pair of the windows at the wavelength of 120 nm, giving an extinction coefficient of ~10(-6) to ~10(-7) which represents the sodium with a few cm thickness to be partially transparent for this wavelength. To confirm the measurement, we perform simple imaging experiments by the ultra-violet light passing through a 8 mm-thick sodium sample to illuminate a mesh as an object, resulting in obtaining a clear image.

Amplification properties of vacuum ultraviolet Ar*2 produced by infrared high-intensity laser.

We report optical amplification of Ar(2)* at 126 nm, pumped by optical-field-induced ionization (OFI) created by an infrared high-intensity laser. A gain-length product of 0.84 was obtained by using multipass amplification with a vacuum ultraviolet (VUV) cavity. The gain-length product was increased up to 4.3 through the use of single-pass amplification with a VUV reflector and a hollow 5.0 cm-long fiber. Similar small signal gain coefficients of 0.84 and 0.86 cm(-1) were obtained in two different experiments, in which OFI Ar plasma gain media were produced in free space filled with Ar and inside an Ar-filled hollow fiber.

Generation of 5 fs, 0.5 TW pulses focusable to relativistic intensities at 1 kHz.

We have demonstrated the generation of 5 fs, 0.5 TW pulses at 1 kHz repetition rate using a pulse compression technique in a hollow fiber with a pressure gradient. Owing to the excellent beam quality by passing through the hollow fiber, the beam after pulse compression could be focused to a nearly diffraction-limited spot size. We obtained for the first time a peak intensity as high as 5x10(18) W/cm(2) in the 2-cycle regime.

Generation of extreme ultraviolet continuum radiation driven by a sub-10-fs two-color field.

We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.

Observation of vacuum-ultraviolet Ar2* radiation gain at 126 nm produced by an ultrashort high-intensity laser pulse propagating in a hollow fiber.

We observed a small-signal gain of Ar2* emission at 126 nm by use of a hollow fiber to guide the high-intensity laser propagation in high-pressure Ar. The small-signal gain coefficient was measured to be 0.05 cm(-1) at 126 nm. Kinetic analysis revealed that the electrons produced by the high-intensity laser through an optical-field-induced ionization process initiated the Ar2* production processes. The increase in the emission intensity was measured to be exp(2.5), with an increase in the fiber length.