Investigation of ultrashort soft x-ray pulses. (Research plan)

Ultrashort pulses from the visible to the x-rays.

       Even small-size, table-top lasers of subpicosecond pulse durations can provide high intensities. The availability of table-top short pulse systems in the recent years gave new impetus for small-scale research in the field of x-ray generation and spectroscopy in laser-produced plasmas. Recent developments of high-power lasers of ~100 fs duration will lead to bright laser plasma XUV sources of extremely short duration, too. There is a great demand for short and coherent x-ray pulses. The possible applications of these new XUV sources are x-ray microscopy, lithography, holography, investigation of biological specimen etc. The commonly used lasers are solid state devices based on chirped pulse amplification (CPA). An alternative is gas discharge excimer lasers, which allow direct amplification therefore pedestal-free-short pulses. Here the use of short-pulsed high-power excimer laser systems for this purpose is presented. As a consequence of the high focusability the short wavelength of these pulses, intensities as high as 1020 W/cm2 can easily be obtained with modest laser systems.

       Recently laser plasma experiments have been carried out by our group of the Department of Plasma Physics of the KFKI-Research Institute for Particle and Nuclear Physics using the KrF laser system of the HILL Laboratory of the Department of Experimental Physics of the University of Szeged in a close collaboration. The high intensity hybrid excimer-dye laser system generates hot plasmas, and excites nonlinear interactions. Our aim is the generation of ultrashort EUV (10-150 nm) and soft x-rays (0.3-10nm), and also seeking their applications.

       Summarizing the possibilities of generating coherent ultrashort XUV pulses, high harmonics generation is one of the most straightforward way, because the requirements are not as stringent for x-ray lasers. High harmonics can be generated in gases using modest laser intensities and from laser plasma interactions at high intensities. We intend to follow both ways. Our main goal is to obtain high conversion efficiency harmonics. Harmonics from plasmas are generally more effective, than that from gases, reaching conversion up to 10-4. However using resonances of the gaseous atoms the latter can be even more effective, but the high conversion is limited to resonant structures.

       Ultrashort x-ray pulses from the K a radiation of different materials can be obtained using the nonlinear interaction which accelerate electrons to high speed. Fast electrons may penetrate deeply into the solid target matter knocking out K-electrons of the still cold matter. This vacancy results in the generation of Ka radiation of pulse durations even shorter than that of the laser pulse. Besides Ka generation hot electrons have a heating effect as well, they heat up the target matter isochorically, thus generating K- and L-shell radiation of the hot dense matter which have typically some ps duration. We are planning to determine the conditions of these two processes and to find the border line between these phenomena.

       In order to study these processes we can use the spectroscopical tools of our laboratory which cover the full spectral range from the visible radiation to the 2 keV x rays. The upgraded laser system of the University of Szeged is the main tool of the investigations. Technical assistance is availbale in the workshop of the university, and also in the workshop of the KFKI-Research Institute for Particle and Nuclear Physics. Besides the researchers participating in this proposal the collaboration is to be continued with the University of Debrecen (E. Takács, Cs. Szabó) on x-ray spectroscopy, David Salzmann (Soreq NRC, Israel) on simulations, with the WAT university Warsaw (R. Rakowski, H. Fiedorowicz) on gas jet targets and with G. Holland of the NRL, USA on the x-ray detectors.

       The three main topics of our investigations are summarized below.

Resonant generation of harmonics in gases

       One of the most exciting application of ulrashort laser pulses is the generation of high harmonics in gases. These experiments made it possible in different laboratories to generate coherent radiation up to the keV photon energy, of durations in the attosecond range. Harmonic generation in gases is known to produce odd-integer harmonics. It is also possible to generate harmonics by ultrashort UV pulses using ions as a nonlinear medium as well. One of the reasons is that ionized atoms can be generated by UV lasers by keeping the temperature low. In one of the first efforts KrF laser produced high-harmonics using preionized rare-gas-like alkaline ions[1]. In the case the power of the generated harmonics does not follow the classical perturbative exponential decrease with the harmonic orders any more, but instead above a certain harmonic order a plateau range appears, where the harmonic power remains almost constant. Thus the conversion to a certain harmonics is – in general - low. This however was drastically changed in a recent experiment [2], where 3-photon-absorption resonance in the degenerate 4-wave mixing scheme in Ar yielded a conversion efficiency of ~1% for the third harmonics of the KrF laser, corresponding to 100m J energy for the 82.8 nm VUV radiation.

       Our aim is to optimize this scheme. While maintaining this conversion efficiency we plan to obtain more energy when upgrading our laser system from the present 10-15 mJ/pulse to 50-60 mJ. This could lead to a coherent radiation source in the VUV of mJ energy. We try to find other resonances[3] to obtain similarly efficient VUV sources for even shorter wavelengths. The upgrade our laser system to this energy is planned to be realized during 2005. The last year a gas jet target has been developed for these experiment, which was characterized by x-ray shadowgraphy[4]. This work is done in the frame of the Marie-Curie Network of the EC (project leader in Hungary: S. Szatmári).

High harmonics from laser plasmas

       Similarly to gas-harmonics, in our laboratory KrF laser is also used to obtain harmonics in laser plasmas [5]. Our investigations on this type of harmonics generation have been supported by several OTKA contracts. In the frame of the last contract - to be finished in 2005 - we have shown harmonics generation up to 62 nm with the experimental demonstration of the appearance of critical surface rippling above 1016 W/cm2 intensity[6]. The increase of laser energy should lead to a similar increase of the intensity as well, thus harmonics up to at least 50 nm are expected according to the particle in cell (PIC) simulations.

       A further advantage of the increase of laser energy is the availability of larger number of photons. In our previous efforts the laser intensity was increased by stronger focusing. As the total energy remained constant, the number of VUV photons was not increased significantly either, leading to detection problems. With the increase of laser energy a significant improvement in the signal to noise ratio is expected for the same intensity.

Isochoric heating and Ka generation by hot electrons

       In laser plasma interactions the high intensity laser field initiates nonlinear interactions. The strong ponderomotive force in the plasma corona accelerates the electrons to high energies; a significant part of the energy is transferred to fast electrons. The laser deposits its energy within the skin layer of 5-10nm thickness. This very thin corona layer expands rapidly already during the duration of the laser pulse. The layer below the corona is heated by electron heat conduction similarly to the laser-plasma interactions using longer pulses. Hot electrons however penetrate deeper into the target and heat a region whose thickness is determined by their mean free path. For our conditions the typical energy of the electrons is 10 keV and the thickness of the heated layer exceeds several hundred nanometers. This thick layer remains in the dense state for 1-2 ps before significant expansion developes. This isochoric heating produces several 100 eV temperatures in the laboratory. Matter in this extreme state can be found in the interior of stars and is therefore of interest for astrophysics. In earth-based experiments such plasmas are achievable in the compressed core of inertial confinement fusion pellets, whose realization requires large, high energy laser facilities firing a few shots per day only. The attractiveness of using fs-lasers for generating such plasmas is connected to their high repetition rates (10Hz or more) and their low cost. This opens a unique possibility to systematically study the complex behavior of dense hot plasmas. Line radiation of this matter can be studied spectroscopically, investigating the line broadening and line shift of these pulses of picosecond duration[7]. It is expected that KrF lasers are appropriate for isochoric heating, as the short wavelength and several hundred femtosecond pulse duration can result in high temperatures in the depth of the target[7].

       On the other hand multilayer targets can be used to generate Ka radiation of short pulse duration from the depth of the still cold matter. The multilayer targets can be made of thin boron and aluminium layers deposited on a quartz substrate with the boron layer facing the laser beam. Fast electrons generated in the boron layer penetrate through the boron and the Al layers into the quartz substrate. Throughout their motion thay knock out K-electrons from the Al and Si atoms. These atoms with a resulting vacancy in their K-shell, emit Ka photons that emerge from the target and are measured using an x-ray spectrometer. Besides generating XUV pulses of shorter duration than the laser pulse, the ratio of the Al Ka line (1468 eV) and the Si Ka line (1720 eV) is a good measure of the temperature of the hot electrons. David Salzmann of Soreq NRC carried out a numerical simulation for our conditions giving a good basis for the planning of our experiments[8].

       It can clearly be seen that both effects use the same fast electrons, the difference between cold matter Ka generation and isochoric heating lies in the characteristic of the multilayer targets; in the depth of the matter to be investigated. Therefore we plan to investigate both effects. Varying target configuration we intend to determine the conditions where isochoric heating and Ka generation dominate.


[1] Y. Akiyama, K. Midorikawa, Y. Matsunawa, Y. Nagata, M. Obara, H. Tashiro,K. Toyoda; Phys. Rev. Lett. 69, 2176(1992)

[2] C. Dölle, C. Reinhardt, P. Simon, B. Wellegehausen, Appl. Phys. B 75, 629 (2002)

[3] B. Wellegehausen, private communication

[4] R. Rakowski, A. Bartnik, H. Fiedorowicz, R. Jarocki, J. Kostecki, J. Mikołajczyk, A. Szczurek, M. Szczurek, I.B. Földes and Zs.Tóth, Nucl Inst. Methods, submitted

[5] I. B. Földes, J.S. Bakos, Z. Bakonyi, T. Nagy, S. Szatmári, IEEE J. Sel. Topics in Quantum Electronics 2, 776 (1996)

[6] E. Rácz, I.B. Földes, G. Kocsis, G. Veres, K. Eidmann , S. Szatmári, Appl. Phys. B, submitted

[7] U. Andiel, K. Eidmann, P. Hakel, R.C. Mancini, G.C. Junkel-Vives, J. Abdallah, and K. Witte, Europhys. Letters 60 , 861 (2002)

[8] D. Salzmann: Atomic Physics in Hot Plasmas, Oxford University Press, 1998, and private communications