Physical mechanisms and characteristics of X-ray generation in laser plasmas are reviewed including atomic number dependence of X-ray conversion efficiency. The critical issues for laser plasma X-rays to be of practical use are debris and continuous supply of target materials. Cryogenic target and gas puff target are discussed and their future prospects are also discussed.
A photoelectron micro-spectroscopy system with a laser-plasma x-ray source is being developed to achieve a submicron spatial resolution in an in-house system. Although a laser-plasma source has an ultra-high peak brilliance, timeaveraged power is relatively low because its emitting duration is very short and the duty factor is low even at a high-rep rate operation. However, two spectroscopical ideas in our system are making the “source brilliance limited”; spatial resolution of the system very high. One idea is the use of a filter to select a single line from the x-ray source, and the other is the time-of-flight method for the electron energy analysis. Experimental results obtained so far are presented and advantages of our method and future prospects are discussed.
We constructed a soft X-ray imaging microscope on a laboratory scale by using a laser produced plasma source. Two different soft X-rays (wavelength: 3.2 nm and 5.2 nm) were obtained by using a Nd-YAG laser with a power of 1.2 J and a pulse width of 8 ns. Wolter mirrors were used as a condenser and an objective. A back-illuminated CCD camera was introduced as an X-ray imaging detector. Spatial resolution was estimated to be about 60 nm with a transmission grating object. An X-ray micrograph of a stoma at epidermis of vincia faba L. was obtained. A human red blood cell in a solution was also observed and it showed that the soft X-ray microscope is available for living specimens in a wet state.
Generation properties of the soft X-ray from femtosecond laser produced plasma are reported. Table-top-tera watt Ti: Al2O3 laser system with a pulse width of 100 fs and a repetition rate of 10 Hz is used in the experiment. Spectra and pulse duration properties of the soft X-ray obtained from fs laser produced various metal plasmas are discussed. The typical soft X-ray spectra for various metal targets with different atomic numbers are experimentally obtained. The minimum pulse duration of 6 ps is measured by X-ray streak camera in the single pulse excitation to the target. As this value is limited by the instrument resolution, the further short pulse duration may be expected. The efficient soft X-ray generation is also demonstrated by adopting the double pulse irradiation to the target. Especially, by using this scheme, Nd doped glass targets generate the large photon number of 1011 photon/(Å· Sr) at 8±0.05 nm. Application of the X-ray produced by femtosecond laser pulse to material science is also reviewed. Preliminary experiment of double resonance reflection spectrum change at near Si L2, 3 absorption edge is demonstrated.
Laser plasma X-ray sources are currently used for various applications as a high-brightness source. The main problem, however, is the debris production from the source that causes degradation of the X-ray optics. To overcome this problem we pick up a high pressure gas-puff target created by pulsed injection of high-density gas through a nozzle. Measurements of X-ray spectra from the plasmas produced by using a gas puff target irradiated with a Nd: YAG laser are described. The obtained results would also allow to develop an efficient and debrisless laser-produced X-ray source to be used for the applications in various fields.
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%.
The time-and spatially-resolved properties of laser ablated carbon, boron and silicon particles were measured by LAPXAS (Laser Plasma Soft X-ray Absorption Spectroscopy). The maximum speed of positively charged ions is higher than those of neutral atoms and negatively charged ions. The spatial distributions of the laser-ablated particles in the localized rare gas environment were measured. In helium gas environment, by the helium cloud generated on the top of ablation plume depressed the ablation plume. There is no formation of silicon clusters till 15μs after laser ablation in the argon gas environment.
Hard X-ray generation by irradiating the femtosecond laser, produced by using a table-top tera-watt laser system, onto metal targets is reported. The X-rays generated from a Cu target has an energy of around 8 keV and a pulse width of 6 ps. X-ray diffraction has been made on a Si(111) surface using the X-ray. A preliminary experiment of picosecond time-resolved X-ray diffraction on the lattice expansion of a Si crystal by 300 ps laser irradiation is demonstrated.
Self-Organizing Maps (SOM) method which was developed by Kohonen is introduced. Here, the SOM was applied to the Round Robin CoNi spectra data. We obtained quite successful results, where we were able to determine the composition from the spectra not used for constructing SOM. And also, by using SOM, it seems to be possible to classify the instruments where their data were used for SOM.
The effects of surface acoustic waves (SAWs) and resonance oscillations (ROs) on the activity and selectivity of heterogeneous catalysts were reviewed. In ethanol oxidation over a Pd thin film deposited on the SAW propagation path, the SAW effects were greater for thicker Pd films and for oxidized surfaces than for metallic ones. Lattice displacement measurements and SAW propagation loss showed that the SAW effects of the oxidized catalyst surface are attributed to the interactions between electrons in the oxide layers and SAW. In RO of a poled ferroelectric Sr-doped lead zirconium titanate, the effects of a thickness extensional mode on ethanol oxidation over a Ag catalyst were associated with changes in surface potential and work functions of the Ag surface with RO. RO using a ferroelectric LiNbO3 single crystal caused a remarkably higher activity enhancement for ethanol oxidation on Pd and a selectivity change leading to higher ethylene production in the dehydration and dehydrogenation of ethanol on WO3. The SAWs and ROs are concluded to be useful for the design of a heterogeneous catalyst which has artificially controllable functions for chemical reactions.
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.