Optical magnetic field enhancement using ultrafast azimuthally polarized laser beams and tailored metallic nanoantennas

L.G.and S.M.also thank Valentina Shumakova,Elizaveta Gangrskaia,Alessandra Bellissimo,Audrius Pugzlys,and Andrius Baltu?ka for fruitful discussions and computational support by the Vienna Scientific Cluster(VSC 4).The authors thankfully acknowledge the computer resources at MareNostrum and SCAYLE,and the technical support provided by Barcelona Supercomputing Center(RES-FI-2022-3-0041).R.M.-H.,L.S.-T.,L.P.,E.C.J.,and C.H.-G.acknowledge funding from the European Research Council(ERC)under the European Union’s Horizon 2020 research and innovation program(Grant Agreement No.851201)and support from Ministerio de Ciencia e Innovación(PID2022-142340NB-I00,PID2019-106910GB-I00).This research was funded in whole or in part by the Austrian Science Fund(FWF)(10.55776/ZK91).For open access purposes,the author has applied a CC BY public copyright license to any author accepted manuscript version arising from this submission.

Fourier-Limited Attosecond Pulse from High Harmonic GenerationAssistedbyUlitrafast MagneticFields

One of the main constraints for reducing the temporal duration of attosecond pulses is the attochirp inherent to the process of high-order harmonic generation(HHG).Though the attochirp can be compensated in the extreme-ultraviolet using dispersive materials,this is unfeasible toward x-rays,where the shortest attosecond or even sub-attosecond pulses could be obtained.We theoretically demonstrate that HHG driven by a circularly polarized infrared pulse while assisted by an strong oscillating ultrafast intense magnetic field enables the generation of few-cycle Fourier-limited few attosecond pulses.In such a novel scenario,the magnetic field transversally confines the ionized electron during the HHG process,analogously to a nanowire trapping.Once the electron is ionized,the transverse electron dynamics is excited by the magnetic field,acting as a high-energy reservoir to be released in the form of phase-locked spectrally wide high-frequency harmonic radiation during the electron recollision with the parent ion.In addition,the transverse breathing dynamics of the electron wavepacket,introduced by the magnetic trapping,strongly modulates the recollision efficiency of the electronic trajectories,thus the attosecond pulse emissions.The aftermath is the possibility of producing high-frequency(hundreds of eV)attosecond isolated few-cycle pulses,almost Fourier limited.The isolated intense magnetic fields considered in our simulations,of tens of kT,can be produced in finite spatial volumes considering structured beams or stationary configurations of counter-propagating state-of-the-art multi-terawatt/petawatt lasers.