Relativistic laser nano-plasmonics for effective fast particle production

Abstract

Particle acceleration and X-ray generation in different nano-structured targets irradiated by high intensity laser pulses of high contrast have been studied. It is found that maximal energy of fast particles and its directionality can be significantly enhanced, by choosing nano-structured targets. Generation and propagation of fast electrons in laser targets consisting of nano-wires are studied. Such targets exhibit a large conversion of laser energy into electron kinetic energy. An electron bunch can propagate a long distance and can be focused by bringing wires together. The results of theory and simulations were compared with the experimental data and have shown a reasonable consistency. IntroductionLaser driven nano-plasmonics deals with optical processes in plasmas at relatively low intensities and on nanoscale, i. e. on the order of or smaller than the wavelength of the laser radiation [1]. The underlying physical process is connected with a laser field enhancement due to proper nano-structuring of a target. Laser-matter interaction including nanoscale confinement of radiation and its transformation provides attractive opportunities for both, fundamental research and technological applications. Material processing uses femtosecond laser pulses exceeding the material ablation threshold to drill micro holes, to realize micro cutting or to selectively remove a particular substance. Such an extreme precision processing adds high value [2]. Due to the ultrashort pulse durations the results benefit by very clean and defined processing areas with negligible structural changes around. This is important in surgery (e. g. ophthalmology, dermatology, dentistry), for instance, when the ultrashort laser pulse irradiation enables clean surgery without damaging the side areas of the processed tissue [3].Laser nano-plasmons produced by structured surfaces have recently gained growing attention in laser plasma ion acceleration physics, as it can significantly enhance the acceleration mechanism. Today's research on laser driven particle acceleration (ions and electrons) promises future fast ion sources for different applications. Here, a laser at relativistic intensity interacts with thin solid foils, which can be well characterized and easily handled [4]. First, the laser causes an acceleration of the ionized electron distribution, which subsequently accelerates ions up to high kinetic energies. The low emittance of such ion beams is a striking feature [5]. Nevertheless, up to now the achieved acceleration efficiency is low (around a few to ten percent of the laser energy), even if the target thickness is optimized with respect to the laser parameters, thus current research is focusing on its optimization [6].

Energy partitioning and electron momentum distributions in intense laser-solid interactions

“…As previous studies have not considered this separation it is difficult to make a straightforward comparison of the key results of this paper. However it can still be noted that, based on the total absorption also presented in Figure 5, our findings agree with previous studies of similar setups on, e.g., an optimal structure size of around d/λ = 1/2 and a total absorption of above 50% [42,43].…”

“…Most notably, the energy source is the laser pulse, and there is of course a limit on how much energy one can transfer from the laser to the target (and, therefore, to the ions). Theoretical and experimental studies show that the energy absorption can be significantly increased by structures on the surface [32,[36][37][38][39][40][41], and the absorption can potentially be close to 100% [42,43]. As a natural continuation of these studies, we consider here how the structures affect the partitioning of the absorbed energy between the low and high energy electrons as well as between their normal and transverse motion.…”

Table-top laser-based proton acceleration in nanostructured targets

“…Different approaches such as varying the target thickness [8], nanostructuring the back surface of the target [9,10] or growing a layer of low density foam [11][12][13] have already been studied. Several publications have reported that adding periodic nanostructures on the target front surface enhances drastically the laser energy absorption [13][14][15][16][17][18][19][20][21]. This generates ions with much higher energies than the ones obtained when targets with a flat surface are used [13,[20][21][22][23][24][25][26][27][28][29][30].…”

“…Several publications have reported that adding periodic nanostructures on the target front surface enhances drastically the laser energy absorption [13][14][15][16][17][18][19][20][21]. This generates ions with much higher energies than the ones obtained when targets with a flat surface are used [13,[20][21][22][23][24][25][26][27][28][29][30]. The nature of this enhancement is still a matter of discussion, however it is known that it is strongly dependent on the shape of the structures, as well as on the angle of incidence of the impinging laser [13-15, 17, 18, 21, 22, 26, 27].…”

“…As there are several laser absorption mechanisms in overdense plasmas, such as the generation of surface plasma waves (SPW) [13,15,16,29,[31][32][33][34][35], resonant absorption, vacuum heating or J×B heating [36][37][38], one can use nanostructures with properties especially suited to enhance a particular absorption mechanism. Our aim is to optimize the acceleration of electrons in the vacuum gaps of a periodic structure, so-called vacuum heating [17,21,27].…”