A simplified description of X-ray free-electron lasers

An elementary derivation of fundamental properties of X-ray free-electron lasers is presented, including gain and saturation. Because of its simplicity, this approach is particularly suitable for teaching at different levels and for presentations to non-specialized audiences

Uncovering the nature of transient and metastable non-equilibrium phases in 1TT-TaS2_2

Complex systems are characterized by strong coupling between different microscopic degrees of freedom. Photoexcitation of such materials can drive them into new transient and long-lived hidden phases that may not have any counterparts in equilibrium. By exploiting femtosecond time- and angle-resolved photoemission spectroscopy, we probe the photoinduced transient phase and the recovery dynamics of the ground state in a complex material: the charge density wave (CDW)-Mott insulator 1$T$-TaS$_2$. We reveal striking similarities between the band structures of the transient phase and the (equilibrium) structurally undistorted metallic phase, with evidence for the coexistence of the low-temperature Mott insulating phase and high-temperature metallic phase. Following the transient phase, we find that the restoration of the Mott and CDW order begins around the same time. This highlights that the Mott transition is tied to the CDW structural distortion, although earlier studies have shown that the collapse of Mott and CDW phases are decoupled from each other. Interestingly, as the suppressed order starts to recover, a long-lived metastable phase emerges before the material recovers to the ground state. Our results demonstrate that it is the CDW lattice order that drives the material into this metastable phase, which is indeed a commensurate CDW-Mott insulating phase but with a smaller CDW amplitude. Moreover, we find that the long-lived state emerges only under strong photoexcitation and has no evidence when the photoexcitation strength is weak

Femtosecond polarization shaping of free-electron laser pulses

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter

Can a Metal Surface Repel Electric Charges?

We show that the interaction between a surface and a charge packet moving parallel to it can become repulsive above a critical relativistic energy. We find that this is true for a lossless dielectric surface and also for a Drude metallic surface-in apparent contrast with such common notions as image charge. This counterintuitive phenomenon occurs for packets larger in the transverse than in the longitudinal ( parallel to the motion) direction. The repulsion does not occur for a point charge that is instead attracted at all energies. In addition to the above attractive or repulsive transverse force, there is a longitudinal decelerating force, which for a dielectric corresponds to the Cerenkov effect. Once again, the behavior of a line packet differs from that of a point charge: for a packet with infinite transverse size, the decelerating field decreases to zero as the relativistic factor gamma -> infinity, whereas, for a point charge, the asymptotic value is finite. These findings have a potential impact not only on fundamental electrodynamics but also on accelerator physics and electron spectroscopy

Status and prospects of x-ray free-electron lasers (X-FELs): a simple presentation

The first part of this topical review provides the reader with a conceptual background sufficient to understand the mechanism of an X-FEL without using any formalism. The discussion is thus accessible to non-specialized scientists from any discipline. Then, we review the present status of selected X-FEL projects throughout the world. Examples of actual experiments are used to illustrate the potential impact of these new, exciting sources at the forefront of photon technology

The physics behind free electron lasers (FELs) based on magnetostatic and optical undulators

By explicitly writing the equations of motion for highly relativistic electrons in an electromagnetic field we show the equivalence between the magnetostatic and optical (laser) undulator-based free electron lasers (FELs). In order to gain insight into the physics behind both FEL mechanisms we use a simplified approach developed in Ref. 1 to describe the optical amplification process. By using elementary physics we explain the phenomenon of microbunching, caused by the interaction of electrons with the emitted wave, which leads to coherent emission of radiation and is the essential ingredient in the lasing effect of a FEL. With our simplified model we derive the two main FEL parameters the gain and saturation lengths. Finally, we discuss the potential advantages of using an optical instead of a magnetostatic undulator in a FEL. With our approach the physics behind FELs becomes evident and accessible to researchers outside the specialized field

Method and device for sensing humidity with reversible molecular dimerization

The present invention relates to the use of reversible dimerization of methylene blue (MB) for sensing humidity. The invention preferably uses titanate nanowires coated with MB. The self-organizational properties of MB on the surface of this nanostructured material studied by spectroscopic means revealed that the light absorption properties of the MB molecules are humidity dependent. Based on the observed humidity dependent metachromasy, we fabricated a humidity sensor using optical fiber technology which is adapted for medical, industrial or environmental applications. The sensor operates with excellent linearity over the relative humidity (RH) levels ranging from 8 to 98%. The response and recovery time can be reduced to 0.5 s while the device exhibits excellent reproducibility with low hysteresis. These performances allow the implementation of the sensor in a breathing monitoring system. Furthermore, the metachromasy was observed for other dyes. This calls for a detailed study of molecular configuration on functional surfaces since it can substantially modify the sensitization efficacy of dyes, e.g. in light conversion

Dye metachromasy on titanate nanowires: sensing humidity with reversible molecular dimerization

We report on the reversible dimerization of methylene blue (MB) on titanate nanowires. The self-organizational properties of MB on the surface of this nanostructured material studied by spectroscopic means revealed that the light absorption properties of the MB molecules are humidity dependent. Based on the observed humidity dependent metachromasy, we fabricated a humidity sensor using optical fiber technology which is adapted for medical, industrial or environmental applications. The sensor operates with excellent linearity over the relative humidity (RH) levels ranging from 8 to 98%. The response and recovery time can be reduced to 0.5 s while the device exhibits excellent reproducibility with low hysteresis. These performances led to the implementation of the sensor in a breathing monitoring system. Furthermore, the metachromasy was observed for other dyes. This calls for a detailed study of molecular configuration on functional surfaces since it can substantially modify the sensitization efficacy of dyes, e.g. in light conversion