[Phys-seminars] 2017-01-26 Lasers Seminar

barmash barmash at bgu.ac.il
Thu Jan 19 13:57:47 IST 2017

Lasers Seminar

 DATE: 26-01-17

 TIME: 15:30

 PLACE: Physics building (#54) room 207

High Harmonic Generation Polarization States: Circular, Linear and Foucault-Fan

Avner Fleischer, Solid State Institute, Technion

High harmonic generation (HHG) has been providing unique experimental access to ultrafast processes. The polarization state of high-order harmonics (HHs) is generally linear in the vast majority of HHG studies and applications. The recent demonstration of efficient high harmonics with fully controlled polarization state (ranging from circular with right helicity, through linear, to circular with left helicity) pumped by a counter-rotating bi-chromatic laser field, using either a Mach-Zehnder configuration [1-2] or an inline setup [3] opened a route for new opportunities, e.g. investigating ultrafast chiral dynamics with unprecedented temporal and spatial resolution using table-top sources [4]. In the talk I will describe the schemes we developed in order to gain HHG polarization control. I will show how we used our source to explore the role of spin mixing in HHG [1,5]. Finally, I will demonstrate how we advanced high harmonic polarization control even further, by suggesting 
 a completely new concept in HHG: Foucault-fan (polarization-fan) HHG [6], which yield spectrum of harmonics where the polarization within the spectral bandwidth of each harmonic peak is not fixed but rather significantly frequency-dependent. For example, the interaction of counter-rotating circularly-polarized bichromatic drivers having close central frequencies with isotropic gas leads to the emission of Foucault-fan harmonics where each harmonic in the spectrum has the following property: it is nearly circularly-polarized in one tail of the harmonic peak, linear in the center of the peak and nearly circular with the opposite helicity in the opposite tail. Foucault-fan harmonics are obtained as a result of multiple (at least two) head-on recollisions of electrons with their parent ions occurring from different angles in a two-dimensional plane. The use of bichromatic drivers with close central frequencies largely preserves the single-cycle, single-atom and macroscopic physi
 cs of ‘ordinary’ high harmonic generation, where both th!
 e driver
 and high harmonics are linearly polarized. Thus, it should offer several attracting features, including (i) a direct route for extending the maximal photon energy of observed helical high harmonics to keV by using bichromatic drivers only in the mid-IR region and (ii) utilizing phase matching methods that were developed for high harmonic generation driven by quasi-monochromatic pulses (e.g. pressure tuning phase matching). These Foucault-fan harmonics may be utilized for exploring non-repetitive ultrafast chiral phenomena, e.g. dynamics of magnetic domains, in a single shot. References [1] A. Fleischer et al., \" Spin angular momentum and tunable polarization in high-harmonic generation\", Nat. Photon. 8, 543-549 (2014). [2] O. Kfir et al., \"Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics\", Nat. Photon. 9, 99-105 (2015). [3] O. Kfir et al., \" In-line production of a bi-circular field for generation of helically polarized high-ord
 er harmonics\", Appl. Phys. Lett. 108 211106 (2016). [4] O. Smirnova et al., \"Opportunities for chiral discrimination using high harmonic generation in tailored laser fields\", J. Phys. B 48 234005 (2015). [5] E. Pisanty et al., \"Spin conservation in high-order-harmonic generation using bicircular fields\", Phys. Rev. A 90, 043829 (2014). [6] A. Fleischer et al., \"High-order har

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