Femtosecond Laser Pulses: Generation, Measurement and Propagation

In this contribution some basic properties of femtosecond laser pulse are summarized. In sections 2.1–2.5 the generation of femtosecond laser pulses via mode locking is described in simple physical terms. In section 2.6 we deal with measurement of ultrashort laser pulses. The characterization of ultrashort pulses with respect to amplitude and phase is therefore based on optical correlation techniques that make of the short pulse itself. In section 3 we start with the linear properties of ultrashort light pulses. However, due to the large bandwidth, the linear dispersion is responsible for dramatic effects. To describe and manage such dispersion effects a mathematical description of an ultrashort laser pulse is given first before we continue with methods how to change the temporal shape via the frequency domain. The chapter ends with a paragraph of the wavelet representation of an ultrashort laser pulse.

Optical current generation in graphene: CEP control vs. ω + 2ω control

The injection of directional currents in solids with strong optical fields has attracted tremendous attention as a route to realize ultrafast electronics based on the quantum-mechanical nature of electrons at femto- to attosecond timescales. Such currents are usually the result of an asymmetric population distribution imprinted by the temporal symmetry of the driving field. Here we compare two experimental schemes that allow control over the amplitude and direction of light-field-driven currents excited in graphene. Both schemes rely on shaping the incident laser field with one parameter only: either the carrier-envelope phase (CEP) of a single laser pulse or the relative phase between pulses oscillating at angular frequencies ω and 2ω, both for comparable laser parameters. We observe that the efficiency in generating a current via two-color-control exceeds that of CEP control by more than two orders of magnitude (7 nA vs. 18 pA), as the ω + 2ω field exhibits significantly more asymmetry in its temporal shape. We support this finding with numerical simulations that clearly show that two-color current control in graphene is superior, even down to single-cycle pulse durations. We expect our results to be relevant to experimentally access fundamental properties of any solid at ultrafast timescales, as well as for the emerging field of petahertz electronics.

Smooth pulse recovery based on hybrid wavelet threshold denoising and first derivative adaptive smoothing filter

Based on the pulse-shaping unit in the front end of high-power laser facilities, we propose a new hybrid scheme in a closed-loop control system including wavelet threshold denoising for pretreatment and a first derivative adaptive smoothing filter for smooth pulse recovery, so as to effectively restrain the influence of electrical noise and FM-to-AM modulation in the time–power curve, and enhance the calibration accuracy of the pulse shape in the feedback control system. The related simulation and experiment results show that the proposed scheme can obtain a better shaping effect on the high-contrast temporal shape in comparison with the cumulative average algorithm and orthogonal matching pursuit algorithm combined with a traditional smoothing filter. The implementation of the hybrid scheme mechanism increased the signal-to-noise ratio of the laser pulse from about 11 dB to 30 dB, and the filtered pulse is smooth without modulation, with smoothness of about 98.8%.

Are people happy when they smile? Affective assessments based on automatic smile genuineness identification

Smiles are one of the most ubiquitous facial expressions. They are often interpreted as a signalling cue of positive emotion. However, as any other facial expression, smiles can also be voluntarily fabricated, masked or inhibited with different communication goals. This review discusses automatic identification of smile genuineness. First, emotions and their bodily manifestation are introduced. Second, an overview of the literature on different types of smiles is provided. Afterwards, different techniques used to investigate smile production are described. These techniques range from human video-coding, bio-signal inspection, and novel sensors that, together with automated techniques using machine learning, aim to investigate facial expression characteristic’s beyond human perception. Next, a general summary of the spatio-temporal shape of a smile is provided. Finally, the remaining challenges regarding individual and cultural differences are discussed.

Neo-Confederates take their stand: Southern Agrarians and the Civil War

Ninety years ago, a group of twelve Southern intellectuals published I’ll Take My Stand, a manifesto dedicated to reviving Southern values and ideals in direct opposition to Northern industrialism and philosophy. Ever since 1930, the Southern Agrarians have been frequently presented as critics of modern life, but this kind of focus overshadows another way in which they were described in those early days: as neo-Confederates. The Agrarians’ ongoing and wide-ranging engagement with the Civil War ‐ especially in the work of Allen Tate and Donald Davidson ‐ was, I argue, hugely significant for the planning and writing of the manifesto. Examining the ways in which these writers used the war also shows how they sought to retard modernist progress, embrace failure as an element of Lost Cause ideology, and distort the temporal shape of Civil War memory. Furthermore, I show here how bound up in the manifesto and related writing by its contributors is a commitment to white supremacy and violence ‐ a kind of fanatical dedication that speaks to events in the United States today.