Free electron lasers based on radio-frequency linear accelerators provide an important source of farinfrared radiation which allow exciting experiments that cannot be performed in any other way. Facilities such as FELIX (Nieuwegein, The Netherlands), JFEL (Newport News VA, USA), FELBE (Dresden, Germany), CLIO (Paris, France) and others provide mid-and far-infrared output in picoseconds pulses with micro-joules of energy. They give continuous, wide tuning in far-IR for resonant pumping of discrete transitions (with simultaneous coverage of mid-IR) from around 3 to 250 µm wavelength. This enables time-resolved spectroscopy, non-linear optics and spectroscopy of weak absorptions. They have been applied to a wide variety of problems in condensed matter physics, physical chemistry and biophysics. We review the physics applications of these sources.
Far-infrared Free-Electron Lasers and their output characteristicsThe technology of 4 th generation light sources, or free-electron lasers (FELs), pumped by radiofrequency linear accelerators (r.f. linacs) is mature enough to have enabled well established user facilities that provide thousands of hours of beam-time per year. The typical engineering constraints put the wavelength of these sources in the mid-to far-infrared wavelength, or THz frequency, range of the spectrum. This wavelength range is relatively poorly covered by other source types, and the combination of output characteristics is unique, and allows a range of experiment types that cannot be performed by other means. The output may give (as exemplified by the FELIX laser in Nieuwegein, the Netherlands):o Continuous and easy tuning over a wide range of wavelengths in a matter of seconds without beam steering o high pulse power for non-linear optics in the megawatt range; o short pulses for picosecond dynamics (of order 10-1000 optical cycles); o controllable repetition rate of the micropulse (from single pulses up to order 1 GHz); o coherent (band-width limited) pulses of controllable length/bandwidth (width typically from 1-10%). We explain briefly (and highly schematically) the operating principles of the r.f. linac pumped FEL, in order to explain how these characteristics come about, and how they are controlled. For a recent review of the physics and output characteristics see [1].In the FEL a relativistic beam of electrons produced by the accelerator is injected into an optical cavity, containing an undulator. The undulator is a periodic magnetic field perpendicular to the direction of the electron beam of alternating polarity, that causes a periodic deflection of the electrons as shown in Fig 1. The transverse motion in the inertial frame travelling down the undulator with the longitudinal speed of the electron bunch is analogous to the oscillatory motion of electrons in a stationary dipole antenna and hence results in the emission of radiation with a frequency equal to the oscillation frequency. In this inertial frame the undulator period is length contracted, and the emitted radiation when view...