P.W. van Amersfoort

The Free-Electron-Laser user facility FELIX

Abstract The Free Electron Laser for Infrared eXperiments FELIX presents to its users a versatile source of radiation in the infrared and far-infrared spectral regions. Presently, the wavelength range of operation extends from 5 to 110 μm (2000−90 cm−1). The wavelength is continuously tunable over an octave in a few minutes. The output normally consists of macropulses of 5–10 μs duration, formed by a train of micropulses of a few ps length. Average power in the macropulses is of order 10 kW, peak power in the micropulses is in the MW range. The temporal and spectral characteristics of the micropulses can be controlled by varying the synchronism between the electron pulses and the optical pulses circulating in the laser cavity. Transform-limited pulse lengths in the range 2–20 ps can be generated. Long-range coherence has been induced by phase-locking successive micropulses, and narrow-band, essentially single-mode, radiation has been selected from the output.

Broadband tunability of a far‐infrared free‐electron laser

A unique property of the free‐electron laser (FEL) is its capability to be tuned continuously over a wide spectral range. This is a major difference with all other high‐power lasers. However, the tunability of first‐generation FELs used to be quite poor (typically 10% or less), due to constraints imposed by the accelerator and the undulator. The free electron laser for infrared experiments (FELIX) uses an undulator with an adjustable gap, which permits wavelength scans over an octave in typically 2 min without the need for any readjustment of the electron beam. Results obtained in operation of the long‐wavelength FEL of the FELIX facility are presented. These involve measurements of the spectral range covered (16–110 μm), the output power, and the influence of the cavity desynchronism. The results are compared with numerical simulations.

Free-electron laser operation and self-amplified spontaneous emission using a step-tapered undulator

Abstract We present an experimental and theoretical evaluation of a new method of enhancing the efficiency and gain of the free-electron laser (FEL) and observations of self-amplified spontaneous emission at start-up of the step-tapered FEL. The stepped undulator is divided into two uniform sections of different deflection strengths, the upstream K 1 and the downstream K 2 , and a step of Δ K = K 2 − K 1 ≈ 0.03 with K 1 K 2 for mid-infrared operation.

Generation of frequency-chirped pulses in the far-infrared by means of a sub-picosecond free-electron laser and an external pulse shaper

The generation of frequency-chirped optical pulses in the far-infrared is reported. The pulses are produced by the free-electron laser FELIX. The chirp is induced by means of an external shaping device consisting of a grating and a telescope. The shaper is based on reflective optics to permit operation in a wide spectral range. The present experiments were made at 8.2 μm wavelength. The fwhm duration of the incident pulse was 0.50 ps, which corresponds to a bandwidth of 2.2%. It has been checked that a linear chirp is produced, for the case that the frequency increases from the leading edge of the pulse to the trailing edge, as well as for the reverse case. This is accompanied by an increase of the fwhm pulse duration which ranges up to 16.5 ps.

Enhancement of FEL efficiency by using short electron pulses

In this paper we show that for an intermediate gain free electron laser oscillator, the efficiency is dramatically enhanced by operating with electron pulses shorter than the slippage distance. We discuss both the consequences of operating at longer wavelengths where lethargy effects are important and the implications of working with short electron pulses when inhomogeneous broadening is important. The discussion is supported by a one-dimensional multiparticle simulation that takes into account both saturation and short pulse effects.

Complete bleaching of the intersubband absorption in GaAs/AlGaAs quantum wells using a far‐infrared free‐electron laser

The intensity‐dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far‐infrared free‐electron laser. Complete bleaching of the absorption is observed at I=200 kW/cm2. Fitting the data with a two‐level system yields a characteristic time constant of 1–2 ps. Possible interpretations, considering the finite pulse width of the laser, are discussed.

Studies of interfacial regions by sum-frequency generation with a free-electron laser

The use of a Free-Electron Laser (FEL) allows the study of (non)linear optical properties of materials over unsurpassed large spectral intervals. As an example, we report on the use of a FEL as the infrared source in spectroscopic infrared-visible Sum-Frequency Generation (SFG). Employing the extremely wide tunability of the Free-Electron Laser for Infrared eXperiments (FELIX) at Rijnhuizen, we have studied the frequency dependence of the nonlinear susceptibility for sumfrequency generation in gallium phosphide between 20 and 32 μm in great detail. We have developed a shortpulse visible laser system that is highly synchronous with FELIX thereby creating a two-color setup that can be broadly applied. Resonantly enhanced SFG in alphaquartz has been used to study the relative timing stability of FELIX and the synchronized picosecond-laser system.

Hole coupling in free electron lasers

Numerical simulations of the performance of a far-infrared free electron laser with an aperture in the upstream cavity mirror are presented. Two different applications of mirror apertures are studied: broadband extraction of radiation and injection of the electron beam. The emphasis is on the effect of the aperture on the amplitude of the higher-order transverse modes. It is shown that the mode distribution at saturation can be greatly influenced by variation of the mirror radius of curvature. A simple formula which is quite useful for predicting the dominant higher-order modes is derived. This permits optimization of the cavity with respect to the application in question. >

Free electron laser induced two‐photon absorption in Hg1−xCdxTe

The powerful output of FELIX, the recently built Rijnhuizen free‐electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1−xCdxTe in the far‐infrared spectral region. Two‐photon interband absorption has been investigated as a function of power and wavelength from 20 to 40 μm. This nonlinear optical absorption is used to perform autocorrelation experiments yielding the length of the micropulses of the free‐electron laser on a femtosecond time scale. The predicted dependence of the micropulse length on the synchronization between optical and electron pulses in the laser cavity is observed.

Efficient frequency doubling of ps-pulses from a free-electron laser in ZnGeP2☆

Abstract We report second-harmonic generation in ZnGeP 2 in the mid-IR using a high power, short pulse, free-electron laser. For fundamental wavelengths between 5.5 to 9.0 μm external efficiencies for second-harmonic generation up to 49% have been reached, which is close to the theoretical limit. At longer wavelengths the efficiency is reduced as a result of absorption in the crystal.