Target-normal sheath acceleration (TNSA) of ions by >100-fs relativistic laser pulses irradiating a multichannel target consisting of a row of parallel long wires and a plane back foil is studied. Two-dimensional particle-in-cell simulations show that the laser light pulls out from the wires a large number of dense hot attosecond electron bunches, which are synergetically accelerated forward by the relativistic ponderomotive force of the laser as well as the longitudinal electric field of a transverse magnetic mode that is excited in the vacuum channels between the wires. These electrons are characterized by a distinct two-temperature energy spectrum, with the temperature of the more energetic electrons close to twice the ponderomotive potential energy. After penetrating through the foil, they induce behind its rear surface a sheath electric field that is both stronger and frontally more extended than that without the channels. As a result, the TNSA ions have much higher maximum energy and the laser-to-ion energy conversion efficiency is also much higher. It is found that a laser of intensity 1.37 × 1020 W/cm2, duration 165 fs, and energy 25.6 J can produce 85 MeV protons and 31 MeV/u carbon ions, at 30% laser-to-ion energy conversion efficiency. The effects of the channel size and laser polarization on the TNSA ions are also investigated.

Broadband absorption of sunlight is key for solar cell technologies, so metasurface-based structure has emerged as a promising technique for their efficiency improvement. [15][16][17] It is widely used in photothermal energy generation, [18,19] thermal emitters, [20] and detectors [21] to combine the metasurface-based PAs like a blackbody with the broadband absorption of sunlight. To realize the broadband absorption of sunlight, carbon-based surfaces, [22][23][24] Si-based surfaces, [25][26][27][28] and thin-film metallic structures [29,30] are demonstrated with low-surface absorptivity over the whole solar spectrum. For the carbon-based surfaces, they generally have broadband absorption, nonpolarized selection, and wide-angle insensitivity for solar spectrum. However, most carbon-based absorbers have large thickness of tens to hundreds of micrometers which is a challenge for device integration. Silicon-based surfaces have good performance photovoltaic characteristics. However, some Sibased light-trapping schemes [16,27,28] indicated that limitation of Lambertian expression [31] brings the absorption of silicon cells only in the range from 400 to 1100 nm. In addition, it still has challenge to use patterned metal materials that are unstable at the high temperature to achieve solar spectral absorption. It is promising to design the ultrathin absorbers that have broadband solar absorption while save enabling material saving and shorter deposition times. [32] Due to the high-temperature stability and metal-like optical properties in the visible and nearinfrared spectral regions, titanium nitride (TiN) is an ideal candidate for solar absorbing materials. [33,34] In some reports, [35][36][37] it has shown that Mie resonance by combining ultrathin TiN gratings with different refractive index materials achieves perfect absorption in the near-infrared spectral region. However, inverse design toward this kind of Mie resonance is rarely reported, which may provide novel approach to the study of TiN structure with multimode coupling for broadband absorption.In order to obtain a TiN structure with excellent absorption characteristics, it is necessary to introduce inverse design. As shown in Figure 1a, in the traditional design process, most researches obtain the initial structure through a priori method, including but not limited patterned structure (topological structure), multilayer structure, and stage structure, Broadband absorption of sunlight plays a crucial role in applying solar energy. However, despite being a decade-old technology, there are only a handful of simple metasurfaces designed by conventional methods. This work theoretically combines inverse design with broadband absorption of sunlight to optimize a metasurface that exhibits triple coupling mode resonance for maximizing solar spectral absorption. The metasurface consists of dual-layer titanium nitride (TiN) cylinder grating arrays, TiN dielectric layers, and silicon nitride layers. The simulation results reveal the high absorptivity of 93% in the range o...

In this paper, we propose a graphene-based metasurface that exhibits multifunctions including tunable filter and slow-light which result from surface plasmon polaritons (SPPs) of graphene and plasmon induced transparency (PIT), respectively. The proposed metasurface is composed by two pairs of graphene nano-rings and a graphene nanoribbon. Each group of graphene rings is separately placed on both sides of the graphene nanoribbon. Adjusting the working state of the nanoribbon can realize the functional conversion of the proposed multifunctional metasurface. After that, in the state of two narrow filters, we put forward the application concept of dual-channel optical switch. Using phase modulation of PIT and flexible Fermi level of graphene, we can achieve tunable slow light. In addition, the result shows that the graphene-based metasurface as a refractive index sensor can achieve a sensitivity of 13670 nm/RIU in terahertz range. These results enable the proposed device to be widely applied in tunable optical switches, slow light, and sensors.

radiative cooling requires a radiator with spectral selectivity, which refers to high absorptivity/emittance only in the atmospheric window (8-13 µm) and an extremely high solar reflectance (0.3-2.5 µm). [2] Traditional solutions combine a shielding layer and a radiative layer [3] or a radiative layer and a reflective metal layer. [2b,4] However, conventional materials and their combinations have a limited cooling effect because of their inability to satisfy the stringent requirements for selectivity. In 2014, Raman et al. [4b] reported an advanced 1D nanophotonic device that can reflect 97% of solar irradiance and allow selective radiation in atmospheric windows. It is the first device to achieve absolute cooling, that is, ≈4.9 °C sub-ambient temperature at a solar intensity (I solar ) of 850 W m −2 . Subsequently, many precision photonic devices have been used for PDRC, such as 1D photonic films [5] and metasurfaces, [6] as well as the product of their combination. [1a,7] These photonic devices have resulted in high efficiencies in PDRC applications. Nonetheless, large-scale applications of these devices remain to be a challenge owing to the demand for manufacturing processes.Recently, PDRC designs based on porous polymer structures have attracted considerable attention because of their high cooling performance, simplicity, applicability, and economic efficiency. [8] For example, Yang et al. [8a] reported a porous poly (vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) coating that exhibit an average solar reflectance (R solar , 0.24-2.5 µm) and long-wave IR (LWIR) emittance (ε LWIR , 8-13 µm) of ≈96% and ≈97.3%, respectively, allowing for a sub-ambient temperature drops of ≈6 °C at a I solar of 890 W m −2 . Wu et al. [9] reported a hierarchically porous array polymethyl methacrylate film with a porosity of 60% and a close-packed micropore array on the surface combined with abundant random nanopores inside, which demonstrated excellent R solar (95%) and ε LWIR (98%), with sub-ambient cooling of ≈5.5 to ≈8.9 °C. The excellent sunlight reflection originates from the bimodal pore size distribution with nanopores centered at hundreds of nanometers and micropores centered at several microns and/or high porosity. However, there is a certain distance away from perfect spectral selectivity, [9,10] especially the less reflectivity in the mid-wave

Multiple lasers interacting with a deuterated (D) pitcher-catcher target and neutron generation are investigated using two-dimensional hybrid particle-in-cell and Monte Carlo simulations. It is found that when multiple laser pulses are focused on the front surface of the pitcher layer, D + ion acceleration by target normal sheath acceleration (TNSA) is enhanced by the interfering overlapped light fields and the resulting periodic target-surface structure. With three laser pulses each of 4.5 × 10 19 W cm −2 intensity, 33 fs duration and ~160 mJ energy, focusing at suitable angles on the pitcher layer, one can obtain 15 MeV D + ions and ~25% laser-to-D + ions energy conversion efficiency. As the resulting high-energy-density D + ions bombard the catcher layer, D-D fusion reactions are triggered. About 3.6 × 10 7 neutrons can be produced, with the maximum neutron production rate as high as 3.1 × 10 36 m −3 s −1 , almost an order of magnitude higher than that from a single laser of the same total energy.