A high-dynamic-range reflectivity measurement instrument has been developed with a dynamic range in excess of 100 dB and sensitivity below 1 pW. It can measure the angle-dependent back-scattered reflectivity of static surfaces up to 80º incidence angle over any orientation without the need for physical contact with the part. It has been designed to operate at both 800 nm and 1550 nm. The system operates through lock-in detection of amplitude-modulated laser light in a bi-static probe architecture, specially designed to minimise the effects of unwanted scattering and stray light. This development has been designed for on-site characterisation of the reflectivity of components of the ITER nuclear fusion reactor whose size, radioactivity or toxicity are inappropriate for insertion in traditional BRDF measurement instrumentation. Measuring the reflectivity of these components is critical for the development of tools for in-service inspection of ITER’s reaction chamber, a key element for the safety of the machine. The reflectivity of beryllium blanket module components and tungsten divertor components has been measured to vary over a range of up to 50 dB from normal to 80º incidence angle, to values below 10-5 /sr. Strong anisotropy of the reflectivity is also observed. This data has been matched with the inspection system performance in a custom simulator to confirm that inspection is possible over < 95 % of the ITER reactor plasma facing components.
We present a technical review of a beamline built to perform pump-probe experiments with a temporal resolution of < 200 attosecond. This is designed specifically to use the technique of attosecond transient absorption spectroscopy (ATAS) to resolve ultra-fast electron dynamics in atoms and molecules. A non-collinear, interferometrically stable geometry is adopted to allow us to individually control the characteristics of each of the pump and probe arms independent from each other. With the use of an auxiliary interferometer to correct for long-term drifts between the pump and probe arms we measure better than 150as resolution for our time-corrected delay despite having separated beam paths of over 4m in length. In our first experiment we have focused on the time dependence of the electronic states of an atom in a strong laser field. An extreme ultra-violet (XUV) attosecond pulse train (APT) and a precisely synchronized 30fs IR pulse are used in this work. Delay-dependent absorption modulations are observed at even multiples (2 and 4) of the IR dressing field frequency as the pump-probe delay is scanned. We investigate the dependence of the 2ω order absorption modulation amplitude from the transient absorption of laser-dressed helium as the IR dressing field ellipticity is varied, and we discuss the issues in obtaining such results. We present qualitative data indicating a clear anisotropy in the response of the atom to an IR dressing field, and discuss how we will improve this measurement in future experiments.
We present technical and experimental advances for performing HHG experiments in a range of substituted benzene molecules
starting in the liquid phase. We demonstrate sub 3% fluctutaions in the harmonic signal generated in a benzene vapour backed
continuous flow gas jet using a mid-IR laser source. The longer drive wavelength is necessary as the target molecules have low
ionization potential and exhibit femtosecond timescale dynamics. We present the acquisition of stable and reproducible harmonic spectra from a range of substituted benzene molecules and the dependence of HHG upon the ellipticity of the laser beam was measured for a number of molecules.
The Artemis facility for ultrafast XUV science is constructed around a high average power carrier-envelope phasestabilised
system, which is used to generate tuneable pulses across a wavelength range spanning the UV to the far
infrared, few-cycle pulses at 800nm and short pulses of XUV radiation produced through high harmonic generation. The
XUV pulses can be delivered to interaction stations for materials science and atomic and molecular physics and
chemistry through two vacuum beamlines for broadband XUV or narrow-band tuneable XUV pulses. The novel XUV
monochromator provides bandwidth selection and tunability while preserving the pulse duration to within 10 fs.
Measurements of the XUV pulse duration using an XUV-pump IR-probe technique demonstrate that the XUV
pulselength is below 30 fs for a 28 fs drive laser pulse. The materials science station, which contains a hemispherical
electron analyser and five-axis manipulator cooled to 14K, is optimised for photoemission experiments with the XUV.
The end-station for atomic and molecular physics and chemistry includes a velocity-map imaging detector and molecular
beam source for gas-phase experiments. The facility is now fully operational and open to UK and European users for
twenty weeks per year. Some of the key new scientific results obtained on the facility include: the extension of HHG
imaging spectroscopy to the mid-infrared; a technique for enhancing the conversion efficiency of the XUV by combining
two laser fields with non-harmonically related wavelengths; and observation of D3+ photodissociation in intense laser