We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on microscales using highly ordered Si microwire arrays. The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam. The self-generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration. DOI: 10.1103/PhysRevLett.116.085002 Laser-matter interactions at relativistic intensities have exhibited many interesting physical processes. These include the acceleration of electrons [1][2][3][4], protons, and heavy ions [5][6][7], the creation of electron-positron jets [8][9][10], and attosecond pulse generation [11,12]. The investigation of ultrashort pulse lasers interacting with initially soliddensity matter has been mainly focused on flat targets, with little or no control over the interaction. Recently the focus has shifted toward using advanced targets with the aim of increasing laser beam absorption and subsequent energy partition among various plasma species. Structured interfaces including nanoparticles [13], snowflakes [14], and nanospheres [15] have been reported to enhance laser absorption and proton acceleration, and the trapping of femtosecond laser pulses of relativistic intensity deep within ordered nanowires resulted in volumetric heating of dense matter into a new ultrahot plasma regime [16]. Another proposal addressed the potential for prescribing geometrical structures on the front of a target to greatly enhance the yield of high-energy electrons while simultaneously confining the emission to narrow angular cones [17].Microengineering laser plasma interactions, at intensities above the material damage threshold, has not been extensively explored. The main reason is that the amplified short pulses are inherently preceded by nanosecond-scale pedestals [18]. This departure from an ideal pulse can substantially modify or destroy any guiding features before the arrival of the intense portion of the pulse.Laser-pulse cleaning techniques are now being employed to significantly minimize unwanted prepulse and pedestals. For example, Ti:sapphire-based short-pulse high-intensity lasers routinely use a cross-polarized wave generation technique to achieve a contrast of at least 10 10 on the nanosecond time scale [19]. The manufacturing of advanced micro-and nanostructures has been the domain of specialized scientific disciplines such as nanoelectronics [20], microfluidics [21], and photovoltaics [22]. Microstructures with features as small as 200 nm can now be easily manufactured by nonexperts using commercially available 3D direct laser writing instruments [23]. Furthermore, 3D large-scale simulations with enough spatial and temporal resolution to capture the details of the interaction are now possible thanks to recent advances in massiv...

We present an experimental demonstration of the efficient acceleration of electrons beyond 60 MeV using micro-channel plasma targets. We employed a high-contrast, 2.5 J, 32 fs short pulse laser interacting with a 5 m inner diameter, 300 m long microchannel plasma target. The micro-channel was aligned to be collinear with the incident laser pulse, confining the majority of the laser energy within the channel. The measured electron spectrum showed a large increase of the cut-off energy and slope temperature when compared to that from a 2 m flat Copper target, with the cutoff energy enhanced by over 2.6 times and the total energy in electrons >5 MeV enhanced by over 10 times. Three-dimensional particle-in-cell simulations confirm efficient direct laser acceleration enabled by the novel structure as the dominant acceleration mechanism for the high energy electrons. The simulations further reveal the guiding effect of the channel that successfully explains preferential acceleration on the laser/channel axis observed in experiments. Finally, systematic simulations provide scalings for the energy and charge of the electron pulses. Our results show that the micro-channel plasma target is a promising electron source for applications such as ion acceleration, Bremsstrahlung X-ray radiation, and THZ generation.

We present an experimental study investigating laser-driven proton acceleration via target normal sheath acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant regime (∼1 μm) down to below the onset of relativistic laser-transparency (<40 nm). This is done with a single target material in the form of freely adjustable films of liquid crystals along with high contrast (via plasma mirror) laser interaction (∼2.65 J, 30 fs, I 1 10 21 >´W cm −2 ). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ∼40 nm indicate the influence of relativistic transparency on TNSA, observed via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed the fielding of numerous diagnostics to determine the interaction quality and details: ion energy and spatial distribution was measured along the laser axis and both front and rear target normal directions; these along with reflected and transmitted light measurements on-shot verify TNSA as dominant during high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-incell simulations qualitatively support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

Articles you may be interested inCooperative molecular field effect and induced orientational ordering effect in polar liquid crystalline films on metalsWe have developed a new type of target for intense laser-matter experiments that offers significant advantages over those currently in use. The targets consist of a liquid crystal film freely suspended within a metal frame. They can be formed rapidly on-demand with thicknesses ranging from nanometers to micrometers, where the particular value is determined by the liquid crystal temperature and initial volume as well as by the frame geometry. The liquid crystal used for this work, 8CB (4 0 -octyl-4-cyanobiphenyl), has a vapor pressure below 10 À6 Torr, so films made at atmospheric pressure maintain their initial thickness after pumping to high vacuum. Additionally, the volume per film is such that each target costs significantly less than one cent to produce. The mechanism of film formation and relevant physics of liquid crystals are described, as well as ion acceleration data from the first shots on liquid crystal film targets at the Ohio State University Scarlet laser facility. V C 2014 AIP Publishing LLC. [http://dx.

Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400-800 mJ/cm(2) range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.