Beamline description

Seed Laser for Users (SLU)

The pulses dedicated to user experiments (referred to as SLU) are transported to the experimental stations via low-vacuum laser transport beamlines terminated by a quartz window and used for pump–probe experiments. A dedicated beam pointing feedback system secures the stability of the pointing of the laser beam to a few-μrad level. The laser pulse energy is up to 3 mJ and it can be decreased to a sub-μJ level by a two-stage variable polarization-based attenuator. The fundamental pulse duration is about 55 fs in FWHM at the maximum compression and can be lengthened by adding either positive or negative chirp in a grating-based compressor/stretcher.  The SLU system is on a dedicated optical table attached to the beamline chambers where pulse diagnostics, manipulations, and controls are performed. These include polarization state adjustment, harmonic conversion, pulse compression, pulse duration measurement, beam steering, pointing stabilization, and focusing.

SLU layout

A setup for non-linear frequency conversion can be used to generate the second [SHG (second harmonic generation) = 400 nm] and third [THG (third harmonic generation) = 266 nm] harmonics of the fundamental wavelength, allowing us to optically pump a large variety of metals, semiconductors, and rare-earth/transition metals oxides. This system uses a common-path type scheme and β-BaB2O4 (BBO) crystals for SHG and THG generation. Using different combinations of crystal thicknesses allows for optimizing the conversion efficiency or pulse duration depending on the needs of the measurements. The time delays between the fundamental and the SHG/THG pulses are compensated. The polarization of all harmonics can be varied ad-hoc by wave plates. Depending on the required wavelength, dielectric coated mirror sets are positioned after the SHG/THG stage for beam steering and removal of undesired lower harmonics. A compact fourth harmonic generation setup, delivering 200 nm light, is also available. If a tunable optical wavelength is required, the fundamental. SLU pulse can be, instead, used to pump a parametric amplifier TOPAS-C (Light Conversion). For this system, up-and-downconversion units are available, allowing to provide 60–150 fs range pulse spanning from UV to mid-IR, with energy per pulse from 10 to about 200 μJ. In this case, the optical setup for transporting and focusing the desired wavelength needs to be decided well in advance before the beamtime and may be limited by the availability of optimized optical components. The laser-FEL pulse-to-pulse timing at the sample can be set in the 1 fs to 1 ns range. The focused laser beam enters the high vacuum chambers of the MagneDyn end-station via a viewport flange equipped with a broadband anti-reflection coated optical quality window. A hollow-fiber-based pulse compression setup (HFC) has been recently added as an option to deliver sub-10 fs range IR pulses. The timing structure of the optical laser with 10 or 50 pulses per second reproduces the timing pattern of the FEL.

SLU intensities

Typical SLU pulse parameters at the sample. A lower repetition rate of the SLU can be customized with respect to the standard frequency (10 and 50 Hz). The abbreviation NM stands for “not measured.”

SLU temporal overlap and time zero

(left) relative change in the transient reflectivity (ΔR/R) of a single crystal of silicon measured at different probing wavelengths as a function of the delay time between the FEL pump and the SLU probe pulses. (right) Experimental SLU and FEL parameters for fine time zero measurements. SLU is used as a probe with almost comparable pulse energy (0.5 μJ) for all wavelengths.

 

 

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Ultima modifica il Domenica, 30 Aprile 2023 13:50