Full 3D+1 modelling of the tilted-pulse-front setups for single-cycle terahertz generation

The tilted-pulse-front setup utilizing a diffraction grating is one of the most successful methods to generate single- to few-cycle terahertz pulses. However, the generated terahertz pulses have a large spatial inhomogeneity, due to the noncollinear phase matching condition and the asymmetry of the prism-shaped nonlinear crystal geometry, especially when pushing for high optical-to-terahertz conversion efficiency. A 3D+1 (x,y,z,t) numerical model is necessary in order to fully investigate the terahertz generation problem in the tilted-pulse-front scheme. We compare in detail the differences between 1D+1, 2D+1 and 3D+1 models. The simulations show that the size of the optical beam in the pulse-front-tilt plane sensitively affects the spatio-temporal properties of the terahertz electric field. The terahertz electric field is found to have a strong spatial dependence such that a few-cycle pulse is only generated near the apex of the prism. The part of the beam farther from the apex contains a large fraction of the energy but has a waveform that deviates from a few-cycle. This strong spatial dependence must be accounted for when using the terahertz pulses for strong-field physics and carrier-envelope-phase sensitive experiments such as terahertz acceleration, coherent control of antiferromagnetic spin waves and terahertz high-harmonic generation.Comment: a typo of the minus sign and the ratio of f1/f2 is correcte

Experimenting with UD Adaptation of an Unsupervised Rule-based Approach for Sentiment Analysis of Mexican Tourist Texts

This paper summarizes the results of experimenting with Universal Dependencies (UD) adaptation of an Unsupervised, Compositional and Recursive (UCR) rule-based approach for Sentiment Analysis (SA) submitted to the Shared Task at Rest-Mex 2023 (Team Olga/LyS-SALSA) (within the IberLEF 2023 conference). By using basic syntactic rules such as rules of modification and negation applied on words from sentiment dictionaries, our approach exploits some advantages of an unsupervised method for SA: (1) interpretability and explainability of SA, (2) robustness across datasets, languages and domains and (3) usability by non-experts in NLP. We compare our approach with other unsupervised approaches of SA that in contrast to our UCR rule-based approach use simple heuristic rules to deal with negation and modification. Our results show a considerable improvement over these approaches. We discuss future improvements of our results by using modality features as another shifting rule of polarity and word disambiguation techniques to identify the right sentiment words.Comment: Proceedings of IberLEF 2023, Ja\'en, Spain, 202

Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation

Highly-efficient optical generation of narrowband terahertz (THz) radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation (DFG) driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband DFG using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.Comment: 25 pages, 5 figures, updated to the state before review at Nature Communications (updated the affiliations, title, some content, methods, etc.

Recent Progress at LBNL on Characterization of Laser WakefieldAccelerated Electron Bunches using Coherent Transition Radiation

At LBNL, laser wakefield accelerators (LWFA) can now produce ultra-short electron bunches with energies up to 1 GeV [1]. As femtosecond electron bunches exit the plasma they radiate an intense burst in the terahertz range [2,3] via coherent transition radiation (CTR). Measuring the CTR properties allows non-invasive bunchlength diagnostics [4], a key to continuing rapid advance in LWFA technology. Experimental bunch length characterization for two different energy regimes through bolometric analysis and electro-optic (EO) sampling are presented. Measurements demonstrate both shot-to-shot stability of bunch parameters, and femtosecond synchronization between the bunch, the THz pulse, and the laser beam. In addition, this method of CTR generation provides THz pulses of very high peak power suitable for applications. Recent results reveal LWFA to be a promising intense ultrafast THz source

Development of high gradient laser wakefield accelerators towards nuclear detection applications at LBNL

Compact high-energy linacs are important to applications including monochromatic gamma sources for nuclear material security applications. Recent laser wakefield accelerator experiments at LBNL demonstrated narrow energy spread beams, now with energies of up to 1 GeV in 3 cm using a plasma channel at low density. This demonstrates the production of GeV beams from devices much smaller than conventional linacs, and confirms the anticipated scaling of laser driven accelerators to GeV energies. Stable performance at 0.5 GeV was demonstrated. Experiments and simulations are in progress to control injection of particles into the wake and hence to improve beam quality and stability. Using plasma density gradients to control injection, stable beams at 1 MeV over days of operation, and with an order of magnitude lower absolute momentum spread than previously observed, have been demonstrated. New experiments are post-accelerating the beams from controlled injection experiments to increase beam quality and stability. Thomson scattering from such beams is being developed to provide collimated multi-MeV monoenergetic gamma sources for security applications from compact devices. Such sources can reduce dose to target and increase accuracy for applications including photofission and nuclear resonance fluorescence

Frequency Domain Holography of laser wakefields

textIn this dissertation, single-shot measurement of longitudinal and transverse structure of resonantly driven laser wakefields by Frequency Domain Holography (FDH) is demonstrated. Visualization of laser wakefield structure is crucial for understanding the complex dynamics of the mutual interaction between the driving pulse, wake- field density oscillations and accelerated electron bunch. This diagnostic therefor provides a necessary tool for researchers to learn how to control and fine tune the interaction to produce electron beams with desirable properties. Wakefields were measured in a 2 mm Helium gas jet, for electron densities between 1 x 1018 and 6 x 1018 cm−3 , and laser pulses of 30 fs duration and peak powers between 10 TW and 30 TW. Detailed structural features of the wakefields, including radial extent of the wake, longitudinal wake development, and wavefront curvature, as well as features of the accompanying ionization front and plasma column are resolved. Results are compared with PIC simulations, and show a high degree of correspondence. Weaknesses of the current implementation of the technique and extensions to other parameter regimes are discussed.Physic

Geolocation of multiple sociolinguistic markers in Buenos Aires

Analysis of language geography on scales varying from countries down toneighborhoods is increasingly being used for studying spatial patterns of socialdynamics. This trend is fueled by social media platforms such as Twitter which provideaccess to large amounts of natural language data combined with geolocation and usermetadata enabling reconstruction of detailed spatial patterns of language use. Moststudies are performed on large spatial scales associated with countries and regions,where language dynamics are often dominated by the effects of geographic andadministrative borders. Extending to smaller, urban scales, however, offers thepossibility of visualizing spatial patterns of language use determined by socialdynamics within the city, providing valuable information for a range of social topics fromdemographic studies to urban planning. So far, few studies have been made in thisdomain, due, in part, to the challenges in developing algorithms that accurately classifylinguistic features. These studies include analysis of topics and foreign languageswhich are now identifiable using machine learning techniques but which provide limitedinformation about many socially relevant issues. Here we investigate the possibility ofextending urban-scale geographical studies of language use beyond lexical elementsto include other sociolinguistic markers that identify language style, dialect and socialgroups. Some features, which have not been explored with social-media data on theurban scale, can be used to target a range of social phenomena. Our study focuses onTwitter use in Buenos Aires, and our approach classifies tweets based on the presenceof specific tokens manually selected to target precise linguistic features. In order toincrease sensitivity to small variations and eliminate the dominant effect of populationdistributions, our approach is based on comparison of contrasting sets of tokens.Finally, we attempt to make direct connections between the observed patterns oflanguage use and spatial patterns associated with specific social contexts and socialgroups within the city. Identifying these connections is a key aspect of the socialdynamicsanalysis which has so far received insufficient attention

High-Energy Single-Cycle THz Sources for Compact Particle Accelerators and Manipulators

Terahertz-driven (THz) accelerators and manipulators promise to yield short femtosecond electron bunches of high brightness with intrinsic synchronization to the driv-ing laser at a compact and economic footprint. However,development of practical devices requires THz sources thatreliably provide pulse energies in the sub-mJ to mJ regime,which in turn require state-of-the-art pump laser systemsand carefully designed optical transport lines. Here, we in-vestigate both by experiments and simulations on how spa-tio-temporal coupling of pump pulse parameters in tilted-pulse-front based terahertz setups can be used to controlthe position of the “temporal focus”, which is where mini-mum pump pulse duration is reached. This concept opensa pathway to pump tilted-pulse-front setups with arbitrarilystretched pulses which significantly simplifies transportlines for lasers with high peak intensity. This concept is ex-perimentally demonstrated by efficiently pumping a tilted-pulse-front THz source with pulses stretched to 10 ps andextraction of a THz energy of 0.4 mJ while operating well-below damage threshold. Our findings are not just relevantfor THz based particle acceleration and strong-field phys-ics but any application that requires control over the tem-poral focus of beams with a tilted-pulse-front such as othernovel laser-based particle accelerator schemes