Novel applications of high intensity femtosecond lasers to particle acceleration and terahertz generation
York, Andrew Gregory
Milchberg, Howard M
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We have investigated new applications for high intensity femtosecond lasers theoretically and experimentally, including a novel method to accelerate electrons to relativistic energy and a new type of coherent lasing medium for amplification of few-cycle, high energy pulses of terahertz radiation. We report the development of corrugated `slow wave' plasma guiding structures with application to quasi-phase-matched direct laser acceleration of charged particles. These structures support guided propagation at intensities up to 2x10^17 W/cm^2, limited by our current laser energy and side leakage. Hydrogen, nitrogen, and argon plasma waveguides up to 1.5 cm in length with corrugation period as short as 35 microns are generated, with corrugation depth approaching 100%. These structures remove the limitations of diffraction, phase matching, and material damage thresholds and promise to allow high-field acceleration of electrons over many centimeters using relatively small femtosecond lasers. We present simulations that show a laser pulse power of 1.9 TW should allow an acceleration gradient larger than 80 MV/cm. A modest power of only 30 GW would still allow acceleration gradients in excess of 10 MV/ cm. Broadband chirped-pulse amplification (CPA) in Ti:sapphire revolutionized nonlinear optics in the 90's, bringing intense optical pulses out of large government facilities and into the hands of graduate students in small university labs. Intense terahertz pulses (>> 10 microjoules, <5 cycles), however, are still only produced at large accelerator facilities like Brookhaven National Labs. CPA is theoretically possible for terahertz frequencies, but no broadband lasing medium like Ti:sapphire has been demonstrated for terahertz. Dipolar molecular gases such as hydrogen cyanide (HCN), `aligned' by intense optical pulses, are a novel and promising medium for amplification of broadband few-cycle terahertz pulses. We present calculations that show rotationally excited molecules can amplify a few-cycle seed pulse of terahertz radiation: a sequence of short, intense optical pulses aligns a dipolar gas, driving the molecules into a broad superposition of excited rotational states. A broadband seed terahertz pulse following the optical pulses can then be amplified on many pure rotational transitions simultaneously. We also discuss plans and progress towards experimental realization of a few-cycle terahertz amplifier.