A sub-50 femtosecond bunch arrival time monitor system for

Abstract A bunch arrival time monitor using the future laser based synchronization system at FLASH has been developed. The signal of a beam pick-up with several GHz bandwidth is sampled by a sub-ps laser pulse using a broadband electro-optical modulator. Bunch arrival time deviations are converted into amplitude modulations of the sampling laser pulses which are then detected by a photo-detector. A resolution of 30 fs could be reached with the capability towards sub-10 fs level. In this paper we describe the design of the optical system and present recent results

Exploring semantic verbal fluency patterns and their relationship to age and Alzheimer's disease in adults with Down syndrome

Introduction: Adults with Down syndrome (DS) are at ultra-high risk of developing Alzheimer's disease (AD), characterized by poor episodic memory and semantic fluency in the preclinical phase in the general population. We explored semantic fluency performance in DS and its relationship to age, AD, and blood biomarkers. Methods: A total of 302 adults with DS at baseline and 87 at follow-up from the London Down Syndrome Consortium cohort completed neuropsychological assessments. Blood biomarkers were measured with the single molecule array technique in a subset of 94 participants. Results: Poorer verbal fluency performance was observed as age increases. Number of correct words declined in those with AD compared to those without over 2 years and was negatively correlated with neurofilament light (r = –0.37, P =.001) and glial fibrillary acidic protein (r = –0.31, P =.012). Discussion: Semantic fluency may be useful as an early indicator of cognitive decline and provide additional information on AD-related change, showing associations with biomarkers in DS

Photo- and Electroproduction of Eta Mesons

Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominant $S_{11}$(1535) resonance.Comment: 24 pages RevTeX/LaTeX2.09, NFSS1, 13 figures (in separate file (tar,gzip and uue)), accepted for publication in Z. Phys.

STATUS OF THE FIBER LINK STABILIZATION UNITS AT FLASH

Abstract State-of-the-art X-ray photon science with modern freeelectron lasers (FEL) like FLASH (free-electron laser in Hamburg) and the upcoming European X-ray Free-Electron Laser Facility (XFEL) requires timing with femtosecond accuracy. For this purpose a sophisticated pulsed optical synchronization system distributes precise timing via lengthstabilized fiber links throughout the entire FEL. Stations to be synchronized comprise bunch arrival time monitors (BAM's), RF stations and optical cross-correlators (OXC) for external lasers. The different requirements of all those stations have to be met by one optical link stabilization unit (LSU) design, compensating drifts and jitter in the distribution system down to a fs-level. Five years of LSU operation at FLASH have led to numerous enhancements resulting in an elaborate system. This paper presents these enhancements, their impact on synchronization performance and the latest state of the LSUs

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

Optical Kerr nonlinearity (n2) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n2-value of 4.1 x 10-5 cm2/GW at 720-nm wavelength and an electron density of Nd = 5.8 x 1020 cm-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ~1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching.Comment: 15 pages, 1 table, 4 figure

DEVELOPMENT OF AN ALTERNATIVE, PHOTODIODE-BASED, FEMTOSECOND STABLE DETECTION PRINCIPLE FOR THE LINK STABILIZATION IN THE OPTICAL SYNCHRONIZATION SYSTEMS AT FLASH AND XFEL

Abstract The fs-stable timing information in the optical synchronization system at FLASH and the upcoming European XFEL is based on the distribution of laser pulses in optical bers. The optical length of the bers is continuously monitored and drifts in signal propagation time are actively compensated in order to provide a phase stable pulse train at the end of the ber link. At present, optical cross-correlation is used to measure the optical length changes. To overcome some of the disadvantages of the current scheme, a different approach for the detection of the optical ber link length variation was developed. This new scheme uses 10 GHz photodiodes to measure the amplitude modulation of harmonics created by overlapping two pulse trains. The long-term stability of the prototype of this detector over 33 h was demonstrated to be below 5 fs (peakto-peak) with a rms jitter of about 0.86 fs. The detection principle itself is practically insensitive to environmental in uences and needs only about 10 % of the optical power, compared to the optical cross-correlator