Free Electron Laser FELIX Research Articles

Picosecond intersubband dynamics in p-Si/SiGe quantum-well emitter structures

We report time-resolved (ps) studies of the dynamics of intersubband transitions in p-Si/SiGe multiquantum-well structures in the far-infrared (FIR) regime, ℏω<ℏωLO, utilizing the Dutch free electron laser, (entitled FELIX—free electron laser for infrared radiation). The calculated scattering rates for optic and acoustic phonon, and alloy scattering have been included in a rate equation model of the transient FIR intersubband absorption, and show excellent agreement with our degenerate pump-probe spectroscopy measurements where, after an initial rise time determined by the resolution of our measurement, we determine a decay time of ∼10 ps. This is found to be approximately constant in the temperature range from 4 to 100 K, in good agreement with the predictions of alloy scattering in the Si0.7Ge0.3 wells.

The South Hall Ring and the BLAST detector

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.

Vibrational spectroscopy of H− and D− ions in CaF2, SrF2 and BaF2

H− and D− ions are excellent spectroscopic probes of ionic sites in fluoride crystals. The H− or D− ions substitute for F− ions and produce sharp infrared absorption lines, whose spectroscopy reveals hydrogenic centres for co-doped R3+ or alkali-ion crystals. The results on the dynamics of the local modes for H− and D− ions in CaF2, SrF2 and BaF2 obtained at the Felix free-electron laser facility in Utrecht are reported and discussed.

Multi phonon emission in InAs quantum wells studied with a free electron laser and high pulsed magnetic fields

The short pulsed, tunable output from the Dutch free electron laser FELIX, coupled with a 45 T pulsed magnet has been used to measure the non-radiative decay rates between Landau levels in n-type InAs/GaSb quantum wells. The decay rate is 2 ps −1 when the cyclotron resonance energy, E CR= E LO=30 meV, and only slightly less, 0.8 ps −1 when E CR=2 E LO when the principal relaxation pathway is the emission of multiple longitudinal optical (LO) phonons.

Characterization of a Rydberg atom-based streak camera operating in synchroscan mode

A streak camera that operates in synchroscan mode has been developed with a spectral response throughout the infrared. A gas-phase sample of Rydberg atoms is used as a photocathode. This compact device possesses 5 ps time resolution and can be used with a total infrared energy of about 1 nJ, or 10 −7 of the total macropulse energy of the FELIX free electron laser. This combination of characteristics makes it not only an attractive device for use in a variety of infrared experiments, but also a powerful tool for the study of photo-induced electron emission in atomic systems. As an example, a Rydberg-atom based electron gun which produces about 20 pulses of electrons at a 70 GHz repetition frequency has been characterized using this synchroscan streak camera.

Sum-Frequency Generation Spectra of Thin Organic Films on Silver Enhanced Due to Surface Plasmon Excitation

We have studied vibrational spectra of thin films of copper phthalocyanine and fullerene of different thicknesses on silver obtained by a visible-infrared sum-frequency generation (SFG). The free electron laser FELIX was used as a source of broadband infrared and the pulsed Nd:YLF laser system as a source of visible radiation. Excitation of a surface plasmon on silver has allowed to enhance the efficiency of the SFG process. The SFG signal has appeared to be almost independent of the film thickness demonstrating the exclusive contribution of the interfacial layer to the SFG signal.

Sum-Frequency Generation Spectra of Thin Organic Films on Silver Enhanced Due to Surface Plasmon Excitation

(a) Institute of Spectroscopy, Russian Academy of Sciences, 142092 Troitsk,Moscow region, Russia(b) FOM-Institute for Plasma Physics, P.O. Box 1207, NL-3430 BE Nieuwegein,The Netherlands(c) Istituto di Metodologie Avanzate Inorganiche, CNR, C.P. 10, I-00016 Monterotondo Sc.,Italy(Received May 8, 1999)We have studied vibrational spectra of thin films of copper phthalocyanine and fullerene of differ-ent thicknesses on silver obtained by a visible-infrared sum-frequency generation (SFG). The freeelectron laser FELIX was used as a source of broadband infrared and the pulsed Nd:YLF lasersystem as a source of visible radiation. Excitation of a surface plasmon on silver has allowed toenhance the efficiency of the SFG process. The SFG signal has appeared to be almost independentof the film thickness demonstrating the exclusive contribution of the interfacial layer to the SFGsignal.

Electron pulse shaping in the FELIX RF accelerator

The FELIX free-electron laser uses short pulses of relativistic electrons produced by an RF accelerator. The design target for the duration of these electron bunches was around 3 ps. In experiments we observed that the bunches emit coherently enhanced spontaneous emission (CSE) when they travel through an undulator. It was demonstrated that the power level of the CSE critically depends on the settings of the accelerator. In this article we seek to explain these observations by studying the length and shape of the electron bunches as a function of the settings of the accelerator. A particle-tracking model was used to simulate the acceleration and transport processes. These include bunch compression in a 14-cell travelling wave buncher cavity, acceleration in a travelling wave linear accelerator, and passage through a (dispersive) chicane structure. The effect of the phase setting of the RF accelerating field with respect to the arrival time of the electron bunch in each accelerator structure was studied. The parameter range of the simulations is related to that of an actual free-electron laser experiment using these bunches. We find that, for specific settings of the accelerating system, electron pulses with a length of 350 μm FWHM (1 ps) are produced. The charge in the bunch rises steeply within a distance of 25 μm. This bunch shape explains the high level of coherently enhanced spontaneous emission observed in the FELIX laser.

Sum-frequency generation from thiophenol on silver in the mid and far-IR

Sum-frequency (SF) spectra of a monolayer of thiophenol on silver are reported in the mid and far-IR (infrared). The free-electron laser FELIX was used to reach wavelengths up to 54 μm. Molecular vibrations of thiophenol are observed at wavelengths near 10 μm (three modes), 14 μm (1 mode), and 24 μm (1 mode). The appearance of the different vibrational modes in the spectra varies dramatically due to interference between the resonant sum-frequency signal and the nonresonant sum-frequency signal from silver. The standard model used to describe line shapes in SF spectra is shown to be insufficient to explain the different line shapes for the various vibrational modes of thiophenol on silver.

Coherent spontaneous emission and spontaneous phase locking in a free-electron laser

We present measurements that demonstrate the existence of spontaneous coherence between independently generated laser pulses in the FELIX free-electron laser. The experiments show that the interpulse coherence is caused by a high level of coherently enhanced spontaneous emission. We have been able to manipulate the level of interpulse coherence by changing the settings of the electron accelerating system. The shape of the electron bunches critically determines the amount of coherent emission. Results of simulations of the acceleration process are used to discuss the influence of the accelerator parameters on the electron bunch shape. The experimentally determined correlation function of the laser pulses is reproduced by a simple pulse-evolution model. Thereby, an estimate of the timing jitter of the RF accelerating system is obtained.

Intracavity filtering of the spectrum of a short-pulse FEL by inducing interpulse phase coherence

The number of active cavity modes in the short-pulse free-electron laser FELIX was reduced by a factor of 40 at a constant level of the saturated power. This was achieved by inducing phase-coherence between the 40 optical micropulses that are independently amplified at 1 GHz in the 25-MHz cavity. A 1-GHz Fox-Smith intracavity etalon was used to this aim. The resulting spectrum consists of a comb of frequencies that are spaced by 1 GHz. Based on a CW frequency analysis of the coupled cavities we predict that the individual frequencies have a linewidth of 170 kHz. The stability of the selected frequencies was analyzed. Mode hopping over 25 MHz was occasionally observed between macropulses of the laser, but not actually during the macropulse. Simulations by means of a simple pulse evolution model corroborated this behavior. We show that the comb of resonant frequencies can be scanned over a range of 1 GHz by scanning the length of the intracavity etalon. The work presented here gives the theoretical and experimental background of single-linewidth experiments that will be described in a separate paper. The latter experiments concern the selective transmission of a single cavity mode from the phase-locked signal by means of external etalons. This single line should be narrow, stable, and continuously tunable for high-power high-resolution experiments in the far-infrared region of the electromagnetic spectrum.

Experiments on semiconductor systems using a pulsed-field-magnet free-electron-laser combination

The FELIX free-electron laser is continuously tuneable over the wavelength range 5–100 μm and can provide great power and the possibility of time-resolved experiments in the ns to μs range. We describe a pulsed magnet specifically designed for semiconductor experiments using FELIX, which provides fields of up to 45 T, with a pulse-shape tailored to the macropulses of the FELIX source. The magnet consists of a conventional outer coil providing fields of up to 30 T with a pulse length of several ms, plus a single-layer inner coil with a rise-time of ∼10 μs, matched to duration of the FELIX macropulses. The magnet is to be extended to fields of up to 60 T.

Sum frequency generation spectroscopy of thin organic films on silver using visible surface plasmon generation

We present vibrational spectra of thin films of copper phthalocyanine and hexamethylenetetramine on silver obtained by a visible–infrared sum frequency generation (SFG). Excitation of a surface plasmon on the silver layer was used to enhance the efficiency of the SFG process. The free electron laser FELIX was used as a source of a broadband infrared and a pulsed Nd:YLF laser as a source of a visible radiation.

Demonstration of a far-infrared streak camera

An atomic infrared (IR) streak camera is demonstrated that operates in the mid- and far-infrared (/spl lambda/=5-85 /spl mu/m), well beyond the long wavelength cutoff of conventional streak cameras. The temporal and spectral characteristics of the streak camera are determined using the FELIX free-electron laser as the IR light source. The temporal resolution of the streak camera was found to be as short as 1.2 ps. The high sensitivity of the streak camera is demonstrated by single-shot characterization of the IR pulses of FELIX.

Vibrational ladder climbing in NO by ultrashort infrared laser pulses

The anharmonic vibrational ladder of nitric oxide (NO) is climbed by irradiating the molecule with femtosecond infrared (IR) pulses. The free-electron laser FELIX is tuned to λ = 5.3 μm with a bandwidth of Δλ = 0.3 μm, corresponding to a pulse duration of 150 femtoseconds. Transfer to the excited states of NO is monitored by 1 + 1 REMPI. Population is found up to ν″ = 5, the highest level within reach of the IR bandwidth. The average transfer per step of the ladder is found to be 17%.

The infrared spectrum of pyrazine in its metastable triplet state. A free-electron-laser study

Vibrational transitions of pyrazine in the lowest triplet state have been observed as transient changes in the intensity of the phosphorescence induced by the free-electron laser FELIX. All seven fundamental infrared-active modes in the range of 250 to 1600 cm−1 have been detected and all vibrational frequencies are found to be considerably lower than the corresponding ones in the ground state.

Sub-nanosecond pulse modulation of a triode electron gun at 25 MHz

Although the electron injection system of the infrared free electron laser FELIX permits a high electron bunch repetition frequency of 1 GHz, user experiments often require a lower frequency. At the same time, the pulses from the gun have to be sufficiently short to fit within the longitudinal acceptance of a 1-GHz prebuncher. The generation of 300 ps electron bunches with a 25 MHz repetition rate, produced by pulse modulation of a triode gun, is addressed in this paper. The modulating waveform was generated by superposition of a 1 GHz sinusoidal wave, a 25 MHz train of pulses with a duration of 2 ns and a DC bias voltage. The hardware for the adopted modulation method, which consists of two RF cavities and an adjustable delay line, is described and analysed. Measurements of the electron bunch length are presented.

IR spectroscopy with FELIX Part 1.—Phosphorescence detection of IR transitions

P. Kok, E. J. J. Groenen, L. van der Meer and W. van Amersfoort, J. Chem. Soc., Faraday Trans., 1997, 93, 1721 DOI: 10.1039/A607261E

High-voltage switching by means of a stack of thyristors

The use of a stack of 32 thyristors for production of a 20-kV pulse is reported. The pulse power is 50 MW and the pulse duration is 25 μs. The thyristor stack serves as a line swith for a pulse-forming network which provides the input power for a 20-MW klystron. The klystron provides the rf power for the infrared free-electron laser FELIX. The choice of thyristors is motivated by the required relative voltage-stability during the pulse of 0.08%. Also the pulse-to-pulse reproducibility is required to stay within 0.08%. Detailed design considerations and operational results are presented.

Formation of multiple subpulses in a free-electron laser operating in the limit-cycle mode.

The evolution of the longitudinal pulse shape and the spectrum of the short-pulse, far-infrared free-electron laser FELIX are investigated. Depending on the amount of cavity desynchronization applied, the laser is found to operate in the stable-focus mode or in the limit-cycle mode. In the latter case, autocorrelation measurements that are made with a setup that is based on second-harmonic generation in CdTe show the formation of a train of up to four subpulses. These are separated by the synchrotron length, and the corresponding sidebands in the spectrum are separated by the synchrotron frequency, in excellent agreement with theory. The measurements are made at a wavelength of 24.5 \ensuremath{\mu}m. \textcopyright{} 1996 The American Physical Society.