L. Stolcova

Evidence of resonant surface-wave excitation in the relativistic regime through measurements of proton acceleration from grating targets.

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

Laser ion acceleration: from present to intensities achievable at ELI-Beamlines

Simulation studies of laser-induced ion acceleration are extended from the present intensities up to ~1022 W/cm2 that will be achieved soon at the ELI-Beamlines facility in Prague. Numerical simulations of target normal sheath acceleration (TNSA) enhancement by micro-structures on the front and rear sides of thin foils will be extended to higher laser intensities together with a brief description of target preparation techniques. Computational study of the impact of laser polarization, laser incidence angle, foil thickness and material is presented for PW laser beam of intensity of the order 1022 W/cm2. Acceleration regime that combines TNSA with radiation pressure acceleration (RPA) is identified.

Enhanced TNSA acceleration with 0.1-1 PW lasers

The enhancement of laser-driven proton acceleration mechanism in TNSA regime has been demonstrated through the use of advanced nanostructured thin foils. The presence of a monolayer of polystyrene nanospheres on the target frontside has drastically enhanced the absorption of the incident laser beam, leading to a consequent increase in the maximum proton beam energy and total laser conversion efficiency. The experimental measurements have been carried out at the 100 TW and 1 PW laser systems available at the APRI-GIST facility. Experimental results and comparison with particle-in-cell numerical simulations are presented and discussed.

Opal-based photonic crystals: modeling and realization

Photonic crystals (PhC) are very interesting periodic material entities with many attractive physical properties. Moreover opaline-based PhC represent very simple way of realization of these PhC. In this contribution, we present our recent studies on both modeling and realization of these opaline based PhC (primarily based on SiO2). Presented opals were prepared by self-assembly method based on the modification of a simple sedimentation technique. This alternation represented the spatial restriction of space region where the opal was constructed. Even more interesting properties, including the presence of the band gap, can be revealed via infiltrated and / or inverse opals. We have chosen photoconductive poly(9-vinylcarbazole) (PVK) as the material for infiltration and final inversion procedure. We have produced highly regular opals and stable inverse opals. Numerical modeling of these opaline structures has been carried out with the use of both MPB (simulations of simple and infiltrated PhC) and Meep tools (simulations of PhC in inverse configuration with the dispersion of PVK taken in account).

Hollow target for efficient generation of fast ions by ultrashort laser pulses

The efficiency of ion acceleration driven by high-power femtosecond laser pulses strongly depends on the target thickness and on the absorption of laser pulse energy into the ionized solid target. Enhanced absorption has been demonstrated for targets with submicrometer structures deposited on their front surface. However, increasing the overall thickness of the target by adding the layer with structures is undesirable. Here, microstructured hollow targets are proposed to enhance the absorption of the laser pulse energy while keeping the target thickness to minimum. It is demonstrated by full 3D particle-in-cell simulations that the efficiency of proton acceleration from hollow targets substantially exceeds the efficiency of the acceleration from flat foils of the same thickness. The fabrication of an ultrathin hollow target (prototype) by focused ion beam milling is also described.

XUV generation from the interaction of pico- and nanosecond laser pulses with nanostructured targets

Laser-produced plasmas are intense sources of XUV radiation that can be suitable for different applications such as extreme ultraviolet lithography, beyond extreme ultraviolet lithography and water window imaging. In particular, much work has focused on the use of tin plasmas for extreme ultraviolet lithography at 13.5 nm. We have investigated the spectral behavior of the laser produced plasmas formed on closely packed polystyrene microspheres and porous alumina targets covered by a thin tin layer in the spectral region from 2.5 to 16 nm. Nd:YAG lasers delivering pulses of 170 ps (Ekspla SL312P )and 7 ns (Continuum Surelite) duration were focused onto the nanostructured targets coated with tin. The intensity dependence of the recorded spectra was studied; the conversion efficiency (CE) of laser energy into the emission in the 13.5 nm spectral region was estimated. We have observed an increase in CE using high intensity 170 ps Nd:YAG laser pulses as compared with a 7 ns pulse.

Enhanced photoemission from laser-excited plasmonic nano-objects in periodic arrays

The process of photoelectron emission from gold surfaces covered with nano-objects that are organized in the form of a periodic array is addressed in the short laser pulse regime ([Formula: see text] fs) at moderate intensities [Formula: see text] W cm(-2) and for various laser wavelengths. The emission spectrum from a gold single crystal measured under the same conditions is used for reference. The comparison of the photo-emission yield and the energy of the ejected electrons with their counterparts from the (more simple) reference system shows that the periodic conditions imposed on the target surface drastically enhance both quantities. In addition to the standard mechanism of Coulomb explosion, a second mechanism comes into play, driven by surface plasmon excitation. This can be clearly demonstrated by varying the laser wavelength. This interpretation of the experimental data is supported by predictions from model calculations that account both for the primary quantum electron emission and for the subsequent surface-plasmon-driven acceleration in the vacuum. Despite the fact that the incident laser intensity is as low as [Formula: see text] W cm(-2), such a structured target permits generating electrons with energies as high as 300 eV. Experiments with two incident laser beams of different wavelengths with an adjustable delay, have also been carried out. The results show that there exist various channels for the decay of the photo-emission signal, depending on the target type. These observations are shedding light on the various relaxation mechanisms that take place on different timescales.

Laser plasma proton acceleration experiments using foam-covered and grating targets

Experimental results are reported for two different configurations of laser driven ion acceleration using solid foils with a structured layer on the irradiated side, aiming to improve the laser-target coupling by exploiting engineered surfaces. Two experimental campaigns have been performed exploiting a 100TW 25fs Ti:Sa laser capable of maximum intensity of 4 • 1019 W/cm2. ”Grating” targets have been manufactured by engraving thin mylar foils (0.9, 20 and 40 μm) with a regular modulation having 1.6 μm period and 0.5 μm depth. The periodicity of the grating corresponds to a resonant incident angle of 30° for the excitation of surface waves. Considering a target of 20 μm and changing the angle of incidence from 10° to 45°, a broad maximum in the proton energy cut-off was observed around the resonant angle (about 5 MeV) which was more than a factor two higher than the case of planar target. ”Foam” targets have been manufactured by depositing a porous 10 μm nanostructured carbon film with an average density of 1-5 mg/cm3 on a 1 μm thick aluminium foil. At maximum focalization the foam targets gave a maximum proton energy similar to the case of bare aluminium target (about 6 MeV), while educing the intensity the presence of the foam enhanced the maximum proton energy, obtaining about 1.5MeV vs. 500KeV at an intensity of 5 • 1016 W/cm2. 2D Particle-In-Cell simulations have been used to support the intepretation of the experimental results.