Field strength scaling in quasi-phase-matching of high-order harmonic generation by low-intensity assisting fields

High-order harmonic generation in gas targets is a widespread scheme used to produce extreme ultraviolet radiation, however, it has a limited microscopic efficiency. Macroscopic enhancement of the produced radiation relies on phase-matching, often only achievable in quasi-phase-matching arrangements. In the present work we numerically study quasi-phase-matching induced by low-intensity assisting fields. We investigate the required assisting field strength dependence on the wavelength and intensity of the driving field, harmonic order, trajectory class and period of the assisting field. We comment on the optimal spatial beam profile of the assisting field

Attoszekundumos impulzusok karakterizálása = Characterisation of attosecond pulses

Nagy intenzitású, ultrarövid lézerimpulzust vákuumban elhelyezett gáz targetre fókuszálva a lézer terjedési irányában páratlan rendű felharmonikusok keltődnek, a gázatomok legkülső elektronjai és a lézerimpulzus elektromágneses tere közötti nem-lineáris kölcsönhatás révén. A harmonikus sugárzást vizsgálva az elektron erős térbeli viselkedését tanulmányozhatjuk. A koherens harmonikus sugárzás akár több száz elektronvoltot átfogó sávszélessége lehetővé teszi attoszekundumos impulzusok keltését. Munkám célja az attoszekundumos impulzusok keltésének és karakterizálásának még teljesebb megismerése. Kötött elektronok mozgása egy gerjesztett atomban a 10-1000 attoszekundumos időskálán történik, a születőben levő 'attofizika' atomok és molekulák elektron-állapotait is vizsgálja. A projekt kereteiben több területen dolgoztam: gázokban keltett attoszekundumos impulzus sorozat teljes karakterizálását, egy XUV tartományban alkalmazható attoszekundumos kompresszor alkalmazhatóságát, plazma harmonikusok polarizációtól való függését, és THz-es tér jelenlétében való harmonikus-keltést vizsgáltam. A leendő európai szuperlézer (ELI) attoszekundumos pillére Szegeden épül. Olyan témák kapcsolódnak ide, mint molekulák elektron-eloszlásának vizsgálata, plazmafizika, nanoplazmonika, lézer-indukált elektron diffrakció vagy akár a vákuumbeli párkeltés vizsgálata. Az attoszekundumos impulzusok terén végzett kutatásaimnak köszönhetően én is részt veszek ELI előkészítésének munkájában. | When intense light from a short-pulse laser interacts with atoms or ions, it generates new frequencies of radiation that are multiples of the fundamental frequency. This is harmonic generation. Harmonics can be generated up to very high orders, corresponding to photon energies higher than 500 eV. The broad bandwidth of harmonic radiation supports attosecond pulse production. The goal of my research project was to gain more knowledge about the generation and characterization of attosecond pulses. 'Attophysics' is a new field of research where attosecond pulses can be used to probe dynamical processes in matter with an unprecedented time resolution. The duration of attosecond pulses is of the same order of magnitude as the classical orbit time for a ground-state electron in a hydrogen atom. Scientific achievements made during this project include complete characterisation of an attosecond pulse train generated in a rare gas medium; study of an XUV attosecond compressor for shaping these pulses; determining the polarization dependence of plasma harmonic generation; and examining the possibility of generating harmonics in the presence of a strong THz field. High order harmonic generation and attophysics is one of the main areas connected to the ELI (Extreme Light Infrastructure) project, the attosecond light source pillar of which is to be built in Szeged. I contribute to the work of the international group of scientists planning the European 'superlaser'

The effect of plasma-core induced self-guiding on phase matching of high-order harmonic generation in gases

In this work we numerically study a self-guiding process in which ionization plays a dominant role and analyze its effect on high-order harmonic generation (HHG) in gases. Although this type of self-guiding --- termed as plasma-core induced self-guiding in previous works --- limits the achievable cut-off by regulating the intensity of the laser beam, it provides favorable conditions for phase matching, which is indispensable for high-flux gas high-harmonic sources. To underline the role of self-guiding in efficient HHG, we investigate the time-dependent phase matching conditions in the guided beam and show how the spatio-temporally constant fundamental intensity contributes to the constructive build-up of the harmonic field in a broad photon-energy range up to the provided cut-off.Comment: 14 pages, 7 figure

Extended model for optimizing high-order harmonic generation in absorbing gases

We report on an extended version of the one-dimensional model proposed by Constant et al (1999 Phys. Rev. Lett. 82 1668) to study phase matching of high-order harmonic generation in absorbing and dispersive medium. The model-expanded from zeroth to first order-can be used with media having a pressure profile varying linearly with propagation length. Based on the new formulas, the importance of having a generation medium that ends abruptly with a steep pressure gradient for achieving high flux is highlighted. In addition to further rule-of-thumb guidelines for harmonic-flux optimization, it is shown that having a steep increase of pressure in the beginning of the medium increases harmonic flux, while it also decreases the required medium length to reach the absorption-limited maximum

Peer-to-Peer Networks: A Language Theoretic Approach

In this article a modification of a grammar systems theoretic construction, the so-called network of parallel language processors, is proposed to describe the behaviour of peer-to-peer (P2P) systems. In our model, the language processors form teams, send and receive information through collective and individual filters. The paper deals with the dynamics of string collections. The connection between the growth function of a developmental system and the growth function of networks of parallel multiset string processors with teams of collective and individual filtering is also established