Mechanically exfoliated graphene mounted on a SiO2/Si substrate was subjected to HF/H(2)O etching or irradiation by energetic protons. In both cases gas was released from the SiO2 and accumulated at the graphene/SiO2 interface resulting in the formation of "bubbles" in the graphene sheet. Formation of these "bubbles" demonstrates the robust nature of single layer graphene membranes, which are capable of containing mesoscopic volumes of gas. In addition, effective mass transport at the graphene/SiO2 interface has been observed.

We demonstrate that trains of subpicosecond electron microbunches, with subpicosecond spacing, can be produced by placing a mask in a region of the beam line where the beam transverse size is dominated by the correlated energy spread. We show that the number, length, and spacing of the microbunches can be controlled through the parameters of the beam and the mask. Such microbunch trains can be further compressed and accelerated and have applications to free electron lasers and plasma wakefield accelerators.

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and gamma-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO2 laser beam.

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Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO2 laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into approximately 3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

We report the observation of a strong wakefield induced energy modulation in an energy-chirped electron bunch passing through a dielectric-lined waveguide. This modulation can be effectively converted into a spatial modulation forming microbunches with a periodicity of 0.5-1 ps and, hence, capable of driving coherent terahertz radiation. The experimental results agree well with theoretical predictions.

A strong energy modulation in an electron bunch passing through a dielectric-lined waveguide was recently demonstrated in Antipov et al., Phys. Rev. Lett. 108, 144801 (2012). In this Letter, we demonstrate a successful conversion of this energy modulation into a beam density modulation, and the formation of a series of microbunches with a subpicosecond periodicity by means of magnetic optics (chicane). A strong coherent transition radiation signal produced by the microbunches is obtained and the tunability of its carrier frequency in the 0.68-0.9 THz range by regulating the energy chirp in the incoming electron bunch is demonstrated using infrared interferometry. A tabletop, compact, tunable, and narrowband source of intense THz radiation based on this technology is proposed.

7.6 3 10 6 x-ray photons per 3.5 ps pulse are detected within a 1.8-2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW CO 2 laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a s 32 mm spot. Our next plan is to demonstrate 10 10 x-ray photons per pulse using a CO 2 laser of ϳ1 TW peak power.