Abstract-Unusual terahertz waveforms from a resonant medium controlled by diffractive optical elements

A. V. Pakhomov, R. M. Arkhipov, M. V. Arkhipov, A. Demircan, U. Morgner, N. N. Rosanov,  I. Babushkin,



https://www.nature.com/articles/s41598-019-43852-w

Up to now, full tunability of waveforms was possible only in electronics, up to radio-frequencies. Here we propose a new concept of producing few-cycle terahertz (THz) pulses with widely tunable waveforms. It is based on control of the phase delay between different parts of the THz wavefront using linear diffractive optical elements. Suitable subcycle THz wavefronts can be generated via coherent excitation of nonlinear low-frequency oscillators by few-cycle optical pulses. Using this approach it is possible to shape the electric field rather than the slow pulse envelope, obtaining, for instance, rectangular or triangular waveforms in the THz range. The method is upscalable to the optical range if the attosecond pump pulses are used.

Abstract-All-optical attoclock: accessing exahertz dynamics of optical tunnelling through terahertz emission

I. Babushkin, A. J. Galan, A. Husakou, F. Morales, A. Demircan, J. R. C. Andrade, U. Morgner, M. Ivanov

https://arxiv.org/abs/1803.04187

The hot debate regarding attosecond dynamics of optical tunneling has, so far, been focused on the presence, or absence, of time delays associated to the electron tunnelling through the classically forbidden region during atomic ionization in intense infrared laser fields. Strong theoretical and experimental arguments have been put forward to advocate the polar opposite points of view. The underlying dynamics are richer. Here we propose to use the nonlinear optical responses of the tunnelling electrons to tailored light fields to track these dynamics in full complexity. Using the combination of single-color and two-color fields, we resolve not only ionization delays, but also temporal re-shaping and spatial re-focusing of the tunnelling wavepacket as it emerges from the classically forbidden region. Access to these details of the dynamics is facilitated by the near-instantaneous nature of the nonlinear optical response driven by ionization, and by using tailored laser pulses to induce this response. Our work introduces a new type of attoclock for optical tunnelling, one that is based on measuring light rather than photo-electrons. Our conclusions suggest a possible middle ground between the two conflicting points of view.

Abstract-Polarization control of terahertz radiation from two-color femtosecond gas breakdown plasma

O. Kosareva, M. Esaulkov, N. Panov, V. Andreeva, D. Shipilo, P. Solyankin, A. Demircan, I. Babushkin, V. Makarov, U. Morgner, A. Shkurinov, and A. Savel’ev

http://www.etoponline.org/ol/abstract.cfm?uri=ol-43-1-90

We individually control polarizations of 800 and 400 nm beams, which form a two-color femtosecond plasma filament in air irradiating a linear-to-elliptical THz signal. We detected a threshold-like appearance of THz ellipticity at the angle of ∼85°$\sim 85°$ between the fundamental and second-harmonic field polarization directions. The simulations confirm the abrupt change of THz polarization and reveal that the weak ellipticity of the second harmonic is sufficient to generate essentially elliptical THz radiation.

© 2017 Optical Society of America

Abstract-Symmetry Breaking and Strong Persistent Plasma Currents via Resonant Destabilization of Atoms

C. Brée, M. Hofmann, A. Demircan, U. Morgner, O. Kosareva, A. Savel’ev, A. Husakou, M. Ivanov, and I. Babushkin

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.243202

The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so-called Freeman resonances). This process is most prominent in the multiphoton ionization regime, meaning that the ionization event takes many optical cycles. Nevertheless, here, we show that these resonances can lead to rapid subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycle-to-cycle symmetry of the ionization process, resulting in the generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure, forming a comb from terahertz to visible. This radiation contains fingerprints of the attosecond electron dynamics in Rydberg states during ionization.

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