Periodic structures with subwavelength features are instrumental in the versatile and effective control of electromagnetic waves from radio frequencies up to optics. In this paper, we theoretically evaluate the potential applications and performance of electromagnetic metasurfaces made of periodically patterned graphene. Several graphene metasurfaces are presented, thereby demonstrating that such ultrathin surfaces can be used to dynamically control the electromagnetic wave reflection, absorption, or polarization. Indeed, owing to the graphene properties, the structure performance in terms of resonance frequencies and bandwidths changes with the variation of electrostatic bias fields. To demonstrate the applicability of the concept at different frequency ranges, the examples provided range from microwave to infrared, corresponding to graphene features with length scales of a few millimeters down to about a micrometer, respectively. The results are obtained using a full-vector semianalytical numerical technique developed to accurately model the graphene-based multilayer periodic structures under study.

We numerically investigate the acceleration and bunch compression capabilities of 20 mJ, 0.6 THz-centered coherent terahertz pulses in optimized metallic dielectric-loaded cylindrical waveguides. In particular, we theoretically demonstrate the acceleration of 1.6 pC and 16 pC electron bunches from 1 MeV to 10 MeV over an interaction distance of 20mm, the compression of a 1.6 pC 1 MeV bunch from 100 fs to 2 fs (50 times compression) over an interaction distance of about 18mm, and the compression of a 1.6 pC 10 MeV bunch from 100 fs to 1.61 fs (62 times) over an interaction distance of 42 cm. The obtained results show the promise of coherent THz pulses in realizing compact electron acceleration and bunch compression schemes.

A procedure for the optimal design of thin wideband radar absorbers is presented. The resulting absorbers are implemented by printing a frequency selective surface on a lossy perforated substrate. A binary hill climbing optimization scheme with random restart is used to find optimal solutions. The method of moments in conjunction with the transmission line method is used to calculate the fitness of structures designed by the optimizer. The optimization procedure is done in two steps in order to find a structure with best performance in terms of both operation bandwidth and angular stability. Three types of lossy substrate are considered: 1) unmodified radar absorbing substrate, 2) substrate with one circular hole per unit cell, and 3) substrate with several circular holes per unit cell. It is shown that by drilling only one hole - with optimized radius - in each unit cell, a %100 improvement in terms of operation bandwidth may be obtained. For the case with several holes per unit cell the observed performance was not as good, which might be due to a too rough optimization.

Ultrashort electron beams with a narrow energy spread, high charge, and low jitter are essential for resolving phase transitions in metals, semiconductors, and molecular crystals. These accelerated beams, produced by phototriggered electron guns, are also injected into accelerators for x-ray light sources. The achievable resolution of these time-resolved electron diffraction or x-ray experiments has been hindered by surface field and timing jitter limitations in conventional RF guns, which thus far are $<200\mathrm{MV}/\mathrm{m}$ and $>96\mathrm{fs}$, respectively. A gun driven by optically generated single-cycle terahertz (THz) pulses provides a practical solution for enabling not only GV/m surface fields but also absolute timing stability, since the pulses are generated by the same laser as the phototrigger. Here, we demonstrate an all-optical THz gun yielding a peak electron energy approaching 1 keV, accelerated by $>300\mathrm{MV}/\mathrm{m}$ THz fields in a micrometer-scale waveguide structure. We also achieve quasi-monoenergetic, sub-kiloelectron volt bunches with 32 fC of charge, which can already be used for time-resolved low-energy electron diffraction. Such ultracompact, easy-to-implement guns—driven by intrinsically synchronized THz pulses that are pumped by an amplified arm of the already-present photoinjector laser—provide a new tool with the potential to transform accelerator-based science.