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Electro-optic detection of the Coulomb field of a relativistic electron bunch combined with single-shot cross correlation of optical pulses is used to enable single-shot measurements of the shape and length of femtosecond electron bunches. This method overcomes a fundamental time-resolution limit of previous single-shot electro-optic measurements, which arises from the inseparability of time and frequency properties of the probing optical pulse. Using this new technique we have made real-time measurements of a 50 MeV electron bunch, observing the profile of 650 fs FWHM ( approximately 275 fs rms) long bunches.

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Plasma waves excited by intense laser beams can be harnessed to produce femtosecond duration bunches of electrons with relativistic energies. The very large electrostatic forces of plasma density wakes trailing behind an intense laser pulse provide field potentials capable of accelerating charged particles to high energies over very short distances, as high as 1 GeV in a few millimetres. The short length scale of plasma waves provides a means of developing very compact high-energy accelerators, which could form the basis of compact next-generation light sources with unique properties. Tuneable X-ray radiation and particle pulses with durations of the order of or less than 5 fs should be possible and would be useful for probing matter on unprecedented time and spatial scales. If developed to fruition this revolutionary technology could reduce the size and cost of light sources by three orders of magnitude and, therefore, provide powerful new tools to a large scientific community. We will discuss how a laser-driven plasma wakefield accelerator can be used to produce radiation with unique characteristics over a very large spectral range.

Highly collimated, quasimonoenergetic multi-MeV electron bunches were generated by the interaction of tightly focused, 80-fs laser pulses in a high-pressure gas jet. These monoenergetic bunches are characteristic of wakefield acceleration in the highly nonlinear wave breaking regime, which was previously thought to be accessible only by much shorter laser pulses in thinner plasmas. In our experiment, the initially long laser pulse was modified in underdense plasma to match the necessary conditions. This picture is confirmed by semianalytical scaling laws and 3D particle-in-cell simulations. Our results show that laser-plasma interaction can drive itself towards this type of laser wakefield acceleration even if the initial laser and plasma parameters are outside the required regime.

Waveguide propagation of sub-ps terahertz pulses in single-crystal sapphire fibers is reported. An incident THz pulse of approximately 0.6 ps duration undergoes considerable reshaping due to the absorptive and dispersive waveguide propagation, resulting in transmitted chirped pulse durations of 10-30 ps. Good agreement between theory and experiment is obtained by analyzing the propagation in terms of the single HE 11 waveguide mode. The dominance of the single HE 11 mode, despite the fiber dimensions allowing for multimode propagation, is attributed to the free-space to waveguide coupling. © 2000 American Institute of Physics. ͓S0003-6951͑00͒02215-4͔The technical capability to optoelectronically transmit and detect single cycle pulses of freely propagating THz electromagnetic radiation has generated much interest in the guided wave propagation of such pulses. At the present time, the highest guided-wave performance has been obtained with a 240-m-diam stainless steel waveguide over the frequency range from 0.8 to 3.5 THz and with a power absorption coefficient of less than 1 cm Ϫ1 . 1 This performance far exceeds that of coplanar and microstrip transmission lines. 1 An alternative approach would be to use dielectric waveguides. Such waveguides would not have the sharp lowfrequency cutoff of metal waveguides and would thereby extend the low frequency limit of the waveguide. Given a suitable low loss dielectric, such as high-resistivity silicon with a power absorption coefficient of less than 0.05 cm Ϫ1 over our frequency range, 2 dielectric waveguides could have much less absorption than metal waveguides. In addition, due to boundary considerations it should be possible to more cleanly couple, and with a much higher coupling efficiency, linearly polarized THz radiation into the single dominant mode for dielectric waveguides than for metal circular waveguides. Such dielectric single-mode THz waveguides would have the promise of an extremely low-loss, flexible interconnect and communications channel, with advantages similar to that of single-mode optical fiber.Here, we report demonstrations of single-mode propagation of sub-ps THz pulses in dielectric waveguides ͑fibers͒. These demonstrations prove the efficient quasioptical input and output coupling to and from such fibers and show the viability of the single-mode THz fiber interconnect. The fact that the diameters ͑325, 250, and 150 m͒ of the THz fibers are similar to those of optical fibers, including the core and cladding, gives the THz fibers similar flexibility and handling properties. Because this work was performed with single-crystal unclad fibers, the waveguide propagation characteristics vary significantly over the extensive frequency spectrum of the nearly single-cycle input pulses, giving rise to considerable absorptive and dispersive reshaping.The waveguides used are unclad single-crystal sapphire fibers, with the c axis directed along the length of the fiber, supplied by Saphikon Inc. In our analysis we assume a circular fiber cross-section, although ...