Gerald Ramian

The new UCSB free-electron lasers

Abstract Two of three new FELs and their associated electron beam switchyard are now complete and operating at the University of California, Santa Barbara (UCSB). These constitute the primary radiation source of the UCSB Center for Free-Electron Laser Studies, dedicated to scientific research in the far-infrared. Operating characteristics and salient features of these machines are discussed.

The UCSB electrostatic accelerator free electron laser: First operation

Abstract A far-infrared, free electron laser, driven by a 3 MeV electrostatic accelerator modified for electron beam recovery, is described. In its initial operation at 400 μm, it has produced 10 kW peak power in 50 μs pulses whose time and frequency structure show unexpected behavior.

Pulsed electron paramagnetic resonance spectroscopy powered by a free-electron laser

Electron paramagnetic resonance (EPR) spectroscopy interrogates unpaired electron spins in solids and liquids to reveal local structure and dynamics; for example, EPR has elucidated parts of the structure of protein complexes that other techniques in structural biology have not been able to reveal. EPR can also probe the interplay of light and electricity in organic solar cells and light-emitting diodes, and the origin of decoherence in condensed matter, which is of fundamental importance to the development of quantum information processors. Like nuclear magnetic resonance, EPR spectroscopy becomes more powerful at high magnetic fields and frequencies, and with excitation by coherent pulses rather than continuous waves. However, the difficulty of generating sequences of powerful pulses at frequencies above 100 gigahertz has, until now, confined high-power pulsed EPR to magnetic fields of 3.5 teslas and below. Here we demonstrate that one-kilowatt pulses from a free-electron laser can power a pulsed EPR spectrometer at 240 gigahertz (8.5 teslas), providing transformative enhancements over the alternative, a state-of-the-art ∼30-milliwatt solid-state source. Our spectrometer can rotate spin-1/2 electrons through π/2 in only 6 nanoseconds (compared to 300 nanoseconds with the solid-state source). Fourier-transform EPR on nitrogen impurities in diamond demonstrates excitation and detection of EPR lines separated by about 200 megahertz. We measured decoherence times as short as 63 nanoseconds, in a frozen solution of nitroxide free-radicals at temperatures as high as 190 kelvin. Both free-electron lasers and the quasi-optical technology developed for the spectrometer are scalable to frequencies well in excess of one terahertz, opening the way to high-power pulsed EPR spectroscopy up to the highest static magnetic fields currently available.

Terahertz circular dichroism spectroscopy of biomolecules

Biopolymers such as proteins, DNA and RNA fold into large, macromolecular chiral structures. As charged macromolecules, they absorb strongly in the terahertz due to large-scale collective vibrational modes; as chiral objects, this absorption should be coupled with significant circular dichroism. Terahertz circular dichroism (TCD) is potentially important as a biospecific sensor, unobscured by spectral features related to abiological material. We have constructed atomistic simulations and elastic continuum models of TCD. These models estimate the magnitude of the TCD and the relation between TCD spectroscopic signatures (zero crossings) and the structure, charge distribution and mechanical properties of biomaterials. A broad band TCD spectrometer based on a polarizing interferometer is developed to explore TCD in biomolecules in aqueous solution. Preliminary results on TCD in lysozyme in water at several terahertz frequencies is presented.

Far‐infrared cavity dump coupling of the UC Santa Barbara free‐electron laser

A Nd:YAG laser‐induced semiconductor switch for far‐infrared (FIR) radiation was mounted inside a free‐electron laser (FEL) wiggler cavity to act as an intracavity output coupler. By timing the FEL to the Q‐switched laser, the FIR switch could be turned on during the intracavity laser saturation period. In addition to demonstrating the generation of short, high peak power pulses, the cavity dump coupler (CDC) was used for the first time to image the intracavity mode structure of a FEL and to perform steady‐state saturation spectroscopy studies in the FIR.

Observation of power instability and multimode behavior in a far‐infrared free‐electron laser

Measurements of the time dependence and the frequency spectrum of the output power in the far‐infrared free‐electron laser at the University of California at Santa Barbara are reported. In typical light pulses of 20–50 μs we have observed unexpected oscillations of the laser output with a characteristic period of 5 μs. At the same time, the laser frequency swept over a discrete set of frequency modes, separated by 1.3 GHz. We also present measurements of the gain and loss of the optical mode and discuss the problem of the accelerator terminal voltage drop in a single electron beam pulse with relation to the light spectrum.

Submegahertz linewidth at 240GHz from an injection-locked free-electron laser

Radiation from an ultrastable 240GHz solid state source has been injected, through an isolator, into the cavity of the University of California, Santa Barbara millimeter-wave free-electron laser (FEL). High-power FEL emission, normally distributed among many of the cavity’s longitudinal modes, is concentrated into the single mode to which the solid state source has been tuned. The linewidth of the FEL emission is 0.5MHz, consistent with the Fourier transform limit for the 2μs pulses. This demonstration of frequency-stable, ultranarrow-band FEL emission is a critical milestone on the road to FEL-based pulsed electron paramagnetic resonance spectroscopy.

First lasing of the UCSB 30 μm free-electron laser

Abstract Third harmonic lasing has been achieved in a Free-Electron Laser specifically designed to operate in that mode. So far, 40 W at 41 μm wavelength and 5.16 MeV beam energy has been measured. One of the most difficult challenges has been suppression of lasing at the fundamental wavelength. This is currently achieved with an intracavity cesium iodide restrahlen filter, but eventually, the original grating design will be needed to provide full tunability. Operation of this FEL demands electron beam stability and control system accuracy at the limits of the present system. Further progress in the development of this FEL may require significant improvement in these areas.

The new UCSB compact far-infrared FEL☆

Abstract A 2 MV electrostatic accelerator is being acquired by the FEL development group at the University of California, Santa Barbara (UCSB). This machine is being designed to provide an exceptionally low emittance electron beam suitable for a wide range of FEL related experiments. Foremost among these will be development of a compact submillimeter FEL employing “next generation”electrostatic accelerator FEL concepts. It is expected that this machine will serve as prototype for a new generation of FELs all based on commercially available electrostatic accelerators ranging in voltage from 0.5 to 25 MV. Generation of radiation from millimeter to visible wavelengths at power levels from kilowatts to megawatts from relatively cheap and compact machines is anticipated. Components for the 2 MV accelerator have been ordered and high-voltage tests are expected to begin early in 1988.

The UCSB two-stage FEL experiment

Abstract A two stage FEL experiment is under development at the UCSB FEL facility. The experiment uses a high quality 20 A electron beam generated by an electrostatic accelerator with beam recovery. A new gun and collector were developed for this experiment. The first stage, with a helical undulator, will produce a saturation flux of the order of 107 W/cm2 of 703 μm radiation. This will be the pump for the second stage where light of 1.08 μm will be generated.