Soft X-ray Generation Research Articles

Nanohole-array size dependence of soft x-ray generation enhancement from femtosecond-laser-produced plasma

Nanostructured targets are very attractive for enhancing the intensity of x-ray pulses generated from laser-produced-plasma. In order to clarify the enhancement mechanism, the nanohole-array size dependence of the characteristics of soft x-ray pulse generation from femtosecond-laser-produced plasma was investigated in detail. We found that the highest x-ray intensity can be obtained and the x-ray pulse duration kept relatively short with a nanohole-array alumina target with a 500 nm hole interval and a 450 nm hole diameter. A 40-fold soft x-ray fluence enhancement and a nine-fold soft x-ray pulse peak intensity enhancement can be obtained. The relatively short x-ray pulse duration of 19 ps can be maintained because the target structure has high local density and nanometer-sized spaces. Similar enhancement effects can be expected by using a nanostructured target with wall thickness of less than 100 nm, space size of around a few 100 nm, and nanostructure depth larger than 20 μm.

High-brightness soft X-ray generation under optimized laser-cluster interaction

High-brightness soft X-ray emissions are obtained by optimizing the interaction between high-intensity ultrashort optical pulses and Ar clusters. With only 100-mJ pump energy, the yield at 13.8 nm with 0.1-nm bandwidth reaches 0.3 mJ/pulse, while the yield in the spectral range of 11–20 nm reaches 12 mJ/pulse in 4π solid angle. The relation between optimal pulse durations and cluster sizes are found to conform with the nano-plasma expansion model, while the optimal pump beam size is related to the pulse contrast by the condition that prepulse ionization is sufficiently weak to preserve the local plasma density.

Soft x-ray generation after arc extinction of a short vacuum gap operated with low-current, repetitive discharge

Soft x-ray emission from a short vacuum gap, operated with a repetitive low-current arc with a period of ∼2 µs and peak current of 200 A, was investigated. Electrons are accelerated by an electric field in the cathode sheath that is formed after arc extinction, and soft x-rays were emitted in the period between arc extinction and subsequent breakdown. The bulk of the radiation was detected in the 1–3 keV soft x-ray range when a tungsten anode was employed. The dimension of the point-like x-ray source in the radial direction was less than 170 µm. The current density at the time of x-ray emission was ∼107 A m−2.

Coherent soft x-ray generation in the water window with quasi-phase matching.

We demonstrate enhanced generation of coherent light in the "water window" region of the soft x-ray spectrum at 4.4 nanometers, using quasi-phase-matched frequency conversion of ultrafast laser pulses. By periodically modulating the diameter of a gas-filled hollow waveguide, the phase mismatch normally present between the laser light and the generated soft x-ray light can be partially compensated. This makes it possible to use neon gas as the nonlinear medium to coherently convert light up to the water window, illustrating that techniques of nonlinear optics can be applied effectively in the soft x-ray region of the spectrum. These results advance the prospects for compact coherent soft x-ray sources for applications in biomicroscopy and in chemical spectroscopy.

Coherent soft X-ray (λ=18.9 nm) generation in a transient gain scheme in molybdenum plasma

We investigate the threshold conditions for coherent soft x-ray generation using a compact hybrid (neodymium-doped glass-sapphire titanate) laser with an output energy of several hundred millijoules. Collisional excitation of molybdenum ions in a longitudinal-pumping scheme was used for x-ray (λ=18.9 nm) generation. The generated radiation exhibited a low divergence and a strong dependence on the delay between picosecond and femtosecond pumping radiations.

X-ray generation from fs laser heated Xe clusters

The property of soft X-ray generation from Xe clusters irradiated with intense lasers was studied. The wavelength, the pulse width, and the pump energy were widely varied. Through this survey, even with the comparatively low-density Xe jet of <5×10 18 cm −3 average atomic density, the highest conversion efficiency of over 10% from laser energy to soft X-ray (5–18 nm) was obtained with a femtosecond KrF laser pulse, where a 4π source was assumed.

Efficient soft X-ray generation from femtosecond-laser-produced plasma and its application to time resolved spectroscopy

The experimental results on efficient soft x-ray generation from fs-laser-produced plasma and the application of short x-ray pulse to time-resolved spectroscopy are presented. Soft x-ray generation properties were evaluated for both flat Al targets and nanohole - alumina targets. The experimental results for flat metal targets revealed the fundamental properties of soft x-ray emission such as broadband continuum spectra and short pulse duration. By adopting nano-structured targets, a more than 30-hold enhancement of x-ray generation yield is achieved compared with that for flat targets of the same materials with a slight increase of pulse duration. The time-resolved measurement of the inner-shell absorption change of Si during the irradiation with a high-intensity fs laser pulse is achieved by using a short soft x-ray pulse as a probe pulse in pump-probe experiments for the first time A more than 10% increase in the absorption of Si membrane at the L II,III edge (around 100eV) was observed. The recovery time of the absorption change was measured to be about 20ps. This absorption change is assumed to be the bandgap renormarization of Si.

Particle dynamics of debris produced during laser–plasma soft x-ray generation

The dynamics of debris in a buffer gas after ejection from the solid target during laser–plasma soft x-ray generation is studied. Analytical formulas for the flight range of cylindrical and spherical debris are derived using the incompressible flow theory. The validity range of the formulas and the related error margins are discussed. Finally, the flight range formula for spherical particles is extended to two times the Mach number at the expenses of a somewhat higher error margin (up to 25%).

Novel Applications of Millimeter and Submillimeter Wave Gyro-Devices

The possible applications of high-power millimeter (mm) and submillimeter waves from gyro-devices span a wide range of technologies. The plasma physics community has already taken advantage of recent advances in applying high-power micro- and mm-waves generated by gyrotron oscillators in the areas of RF-plasma production, heating, noninductive current drive, plasma stabilization and active plasma diagnostics for magnetic confinement thermonuclear fusion research, such as lower hybrid current drive (8 GHz), electron cyclotron resonance heating (ECRH) (28–170 GHz), electron cyclotron current drive (ECCD), collective Thomson scattering and heat-wave propagation experiments. Other important applications of gyrotrons are electron cyclotron resonance (ECR) discharges for generation of multi-charged ions and soft X-rays, as well as industrial materials processing and plasma chemistry. Submillimeter wave gyrotrons are employed in high-frequency broadband electron paramagnetic resonance (EPR) spectroscopy. Future applications which await the development of novel high-power gyro-amplifiers include high resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators (supercolliders). The present paper reviews the state-of-the-art and future prospects of these recent applications of gyro-devices.

High harmonic generation of soft X-rays by carbon nanotubes

We extend the method for the formulation of selection rules for high harmonic generation spectra [Phys. Rev. Lett. 80, 3743 (1998)] beyond the dipole approximation and apply it to single-walled carbon nanotubes interacting with a circularly polarized laser field. Our results show that the carbon nanotubes can be excellent systems for a selective generation of high harmonics, up to the soft x-ray regime.

Stability of droplet-target laser-plasma soft x-ray sources

The spatial stability of microscopic target droplets used for laser-plasma soft x-ray generation in vacuum is investigated. A long-term drift in drop position is characterized with an ultrafast laser-diode imaging system. The drift is experimentally and theoretically shown to be due to a temperature-induced increase in target-liquid viscosity as a result of evaporation. Finally, the drift is compensated for and stable, long-term unattended operation of the source is demonstrated with an automatic phase-delay drop-to-laser synchronizing system. This is important for future compact lithography and microscopy systems.

Greatly enhanced soft x-ray generation from femtosecond-laser-produced plasma by using a nanohole-alumina target

By making an array of nanoholes on an alumina target, x-ray emission from laser-produced plasma can be greatly enhanced even in soft x-ray energy regions (&amp;lt;0.25 keV). X-ray fluence enhancement around 30 times was achieved in the 5–25 nm wavelength range. The enhancement increases as the ionization level of Al becomes higher and the x-ray wavelength becomes shorter. Over 50-fold enhancement was obtained at a soft x-ray wavelength around 6 nm, which corresponds to the emission from Al8+,9+ ions. X-ray pulse duration was 17 ps, which is much shorter than that obtained by using the prepulse technique.

Development of a high repetition rate Nd:YAG slab laser and soft X-ray generation by cryogenic target

High repetition (320 pps), high average power (100 W) pulse Nd:YAG slab laser has been developed for x-ray driver. A strong X-ray emission around 10.8 nm, which wavelength is attractive for x-ray projection lithography, was generated from cryogenic Xe targets pumped by this laser pulses. CO 2 and H 2O were also tested for cryogenic targets and we found that the emission intensity from Xe around this wavelength region was at least 10 times higher than that from H 2O, CO 2.

Laser-induced Plasma X-ray Source and Its Applications. Development of Laser-plasma Soft X-ray Source for Projection Lithography.

Laser-plasma soft X-rays as a projection X-ray lithograph source are required to be used for more than 8 successive hours. And serious problems are soft X-ray energy stability, damages to optics by debris and successive supply targets. From the measurements of soft X-ray spectra and amount, size and scattering direction of debris from target irradiated with focused laser, we have developed a laser-plasma soft X-ray source system with debris-shield, debris free X-ray filter, tape-target driver and the low debris tape-target of the thin aluminum and polymer films. We demonstrated 8 successive hours generation of soft X-rays with the developed laser-plasma soft X-ray system as a projection X-ray lithograph source. The energy conversion efficiency of the soft X-rays at the wavelength of 13 nm in our system was estimated to be 0.29%.

Soft X-ray Generation in a Laboratory.

To promote the common use of soft X-rays, development of an easy method for soft X-ray generation is necessary. Laser-plasma soft X-rays can be generated with a pulse laser system and some simple optics in a laboratory. For observation of plasma images and soft X-ray spectra, a pin-hole camera and a soft X-ray spectrometer with a diffraction grating can be used. Smart uses of “Low-Technology”; will advance many useful applications of soft X-rays in future.

Cryogenic liquid-jet target for debris-free laser-plasma soft x-ray generation

A new target system based on a continuous cryogenic liquid jet for debris-free laser-plasma soft x-ray generation is described. The system is experimentally evaluated with liquid nitrogen as target material. With this target the photon flux is 4.5×1011 photons/(sr×pulse) from the λ=2.88 nm N VI line. Brightness and stability are also investigated for this table-top soft x-ray microscope source. The possibility to utilize other cryogenic liquids such as neon, argon, and xenon and, thus, making the system interesting for short-wavelength lithography applications, is also discussed.

Phase-matched generation of coherent soft X-rays

Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.

Strongly enhanced soft x-ray emission at 8 nm from plasma on a neodymium-doped glass surface heated by femtosecond laser pulses

We have demonstrated efficient soft x-ray generation from femtosecond laser plasma created on a neodymium-doped glass target. Nd ions doped in glass target enhanced soft x-ray emission near 8 nm without broadening the soft x-ray pulse duration when there is no prepulse. A weak prepulse caused strong enhancement in soft x-ray intensity at 8 nm. By introducing a prepulse at 50 ns before the main pulse, 1% energy conversion efficiency from laser pulse into soft x-ray at 8±0.5 nm was achieved by using a neodymium-doped glass target. This value is 1.2 times as high as that for Nd metal target case.

High-gain harmonic generation of soft X-rays with the “fresh bunch” technique

We report numerical simulations (using the TDA code) and analytical verification of the generation of 64 Å high-power soft X-rays from an exponential regime single-pass seeded FEL. The seed is generated in the FEL using the High-Gain Harmonic Generation (HGHG) technique combined with the “fresh bunch” technique. A seed pulse at 2944 Å is generated by conventional laser techniques. The seed pulse produces an intense energy modulation of the rear part of a 1 GeV, 1245 A electron beam in a “modulator” wiggler. In the “radiator” wiggler (resonant at 64 Å), the energy modulation creates beam density modulation followed by radiation of the 46th harmonic of the seed. We use a magnetic delay to position the 64 Å radiation at the undisturbed front of the bunch to serve as a seed for a single pass, exponential growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.

X-ray generation enhancement from a laser-produced plasma with a porous silicon target

X-ray generation enhancement from a laser-produced plasma with a porous Si target is reported. For a porous surface formed on a Si wafer, the self-reflectivity of a femtosecond pulse becomes considerably small. The observed energy penetration depth is 25–30 μm, which is much larger than the skin depth of solid density matter. Using a porous Si target, the threshold of the pre-pulse intensity required for soft x-ray emission enhancement can be reduced. It also contributes to enhance the pre-pulse effect, and soft x-ray generation enhancement ranging from 1.6 to 6.5 times is observed depending on the pre-pulse intensity.