We present a quasi-cw laser in vacuum ultraviolet region at megahertz repetition rate. The narrowband pulses generated from an ytterbium-fiber laser system at 33 MHz repetition rate at the central wavelength of 1074 nm is frequency-converted by successive stages of LBO crystals and KBBF crystals. The generated radiation at 153 nm has the shortest wavelength achieved through phase-matched frequency conversion processes in nonlinear optical crystals to our knowledge.
An Yb-based 78-MHz repetition rate fiber-amplified frequency comb is used to investigate the power scaling
limitations of a standard-design bow tie high-finesse enhancement cavity for XUV generation. With a Xenon
gas jet in the 22-μm-radius focus, the 200-fs intra-cavity circulating pulse reaches a maximum of 20 kW of time-averaged
power. A novel cavity design is presented, conceived to overcome the observed enhancement limitations
and offering the prospect of few-nm high-power high-harmonic generation. Several applications which come into
reach for the first time are discussed.
Photon sources for multi-photon entanglement experiments are commonly based on the process of spontaneous
parametric down conversion. Due to the probabilistic photon production, such experiments suffer from low multiphoton
count rates. To increase this count rate, we present a novel SPDC pump source based on a femtosecond
UV enhancement cavity that increases the available pump power while maintaining a high repetition rate of
80MHz. We apply the cavity as photon source for realizing symmetric, multi-partite entangled Dicke states,
which are observed with a high rate and high fidelity. We characterize the observed Dicke states of up to six
photons using efficient tools exploiting the state's symmetries.
While being the most precise measurement tool in physics, high precision laser spectroscopy is still limited to
wavelengths in the range between the infrared and the near ultraviolet. The generation of XUV frequency combs might
be a route to extend optical frequency metrology into extreme ultraviolet (XUV) spectral region where many elements
have fundamental transitions. The method of choice for XUV frequency comb generation has been cavity-assisted high
harmonic generation, where an infrared frequency comb is converted into the XUV inside a femtosecond enhancement
cavity at the full repetition rate of the oscillator. Our recent efforts have been directed towards a significant improvement
of the average power of XUV combs. To this end, we experimentally investigated the process of non-collinear high
harmonic generation (NCHHG) and proved it to be useful as a combined method for efficient generation and outcoupling
of XUV radiation. Also, we developed a high repetition rate single-pass amplifier which has the potential to boost the
available power for intracavity HHG.
Silicon immersion grating have opened the new possibilities of building compact high-resolution cryogenic spectrometers
for the near-infrared (NIR) region from 1.4 to 5.5μm. We are developing a silicon immersion grating
for a next-generation NIR high-resolution spectrometer attached to the Subaru 8.2 m telescope. Since a long
time, the anisotropic wet etching technique using photolithography has used for the fabrication of silicon immersion
gratings (e.g., Wiedemann & Jennings1 and Keller et al.2). Here, we present an alternative technique using
electron-beam (EB) lithography, which does not employ either any photolithography masks or UV light source.
This technique uses "direct" EBs to expose the resist. The EBs are precisely controlled by using a closed-looped
system comprising a laser interferometer. As compared to photolithographic technique, this technique results
in more accurate groove pitches and lower surface roughness near the edge of the mask. We fabricate a sample
grating with a groove pitch of 30 µm and a blaze angle of 69° on a 10mm × 20mm × 2mm flat substrate by
EB lithography. Our detailed optical testing of the grating using visible laser shows good optical performances:
extremely low scattered light (< 0.9%), less production of ghost light (< 0.01%), and high relative diffraction
efficiency (~ 88%). We plan to fabricate the final immersion grating by fixing the etched grating substrate to
a silicon prism using the optical-contact method. We are in the middle of R&D of this process and found that
much tighter optical-contact than usual seems to be required because of the high incidence angle to the contact
surface. Our first sample immersion grating shows a significant reflection loss at the contact surface when it is
used in an immersion mode with a NIR laser beam. We are trying to improve the polishing process of the thin
silicon substrate for better contact.
Semitransparent Mo/Si multilayer films with a completely free- standing active area have been developed for use as optical elements in the soft x-ray region. They work not only as a beam splitter, but also as a transmissive polarizer and quarter-wave plate. To achieve a flat, smooth free-standing reflecting surface with a high reflectivity, the following fabrication problems were investigated: stress control of Mo/Si multilayer films, the surface roughness of the initial membrane, and the removal of the initial membrane. A flatness of 1 nm (rms) in the active area was obtained for a fabricated 10-mm-square semitransparent multilayer film. A multilayer film consisting of 50 free-standing pairs of semitransparent Mo/Si were fabricated for transmissive polarizer, and a soft x-ray ellipsometer was developed based on them. The fabricated multilayer polarizer was found to have good polarization performance. Placing two transmissive polarizers in the polarizer/compensator-sample-analyzer configuration enabled full control of the polarization of the probe beam. The modified polarization of light reflected from a sample was analyzed by the rotating-analyzer ellipsometry method. This system was used to measure a multilayer mirror. We verified that the soft X-ray ellipsometer is a very promising tool for the structural evaluation of multilayer films, providing a sensitivity in the sub-angstrom range.
EUVL employs a reflective mask consisting of a metallized pattern less than 100 nm thick on a state-of-the-art multilayer mirror deposited on a substrate. The key technologies needed for mask fabrication are coating equipment to make defect-free multilayer films and a low- damage mask fabrication process. Current repair and inspection technologies are advanced enough to handle a pattern size of 0.3 micrometers .
We have developed semi-transparent multilayer polarizers for polarimetry or ellipsometry in the soft x-ray region. We have studied the fabrication processes to achieve a flat and smooth freestanding reflection surface for high-performance semi-transparent multilayer beam-splitters, mainly the stress control of Mo/Si multilayer films and the surface roughness of the support membrane. In this work, we verified that these processes can be applicable to fabricate not only beam-splitters but also polarizers with high reproducibility. The flatness of 1.1 nm (rms) was obtained in the active area of a 11 mm square semi-transparent multilayer polarizer. The measured reflectivity and transmittance rates for s-polarization using synchrotron radiation were 45.3% and 5.6%, respectively, at a wavelength of 13.9 nm. We estimated the polarization characteristics by calculating its relative reflectivity, relative transmittance and phase shift for polarization experiments. As a result of the calculation, we verified that the fabricated semi-transparent multilayer polarizers can be adequately used as a polarizer with polarizance of 69.5% and high throughput of 28.9%, or as a phase shifter of 68 degrees with equal transmission ratio and high throughput of 4.9% for x-ray polarimetry or ellipsometry.