An optical laser filament
A high intensity femtosecond laser pulse that is propagating in the atmosphere can form a filament due to dynamical balance between self-focusing and plasm defocusing. The time scales of the physical effects supporting the filaments are very short: self-focusing and multiphoton ionization are instantaneous, while plasma effects survive a few nanoseconds after the femtosecond pulse has passed. For this reason, all waveguiding phenomena associated with femtosecond laser filaments were believed to last no more than a nanosecond. In contrast to this general belief, we have recently discovered that atomospheric filaments induce long-lived waveguides . With the group of Mordechai Segev, we experimentally demonstrated waveguiding effects that live for a microsecond range, long after the plasma is gone. These waveguides are based on air density and acoustic waves effects.
mp4 file in th URL is: http://tx.technion.ac.il/~oren/images/research/Filament_waveguid_movie.mp4
- O. Lahav, L. Levi, I. Orr, R. Nemirovsky, Y. Nemirovsky, I. Kaminer, M. Segev, and O. Cohen, Long-lived waveguides and sound wave generation by laser filamentation, Phys. Rev. A 90, 021801(R) (2014)
Generation & application of circularly-polarized high-order harmonics and attosecond pulses
High harmonic generation (HHG) is an extreme nonlinear optical process where many visible or infrared photons are converted into a single high-energy photon in the extreme UV and x-ray spectral regions. Light sources based on HHG exhibit unique and very useful features, including pulses with attosecond to femtosecond duration, and broadly tunable bandwidths (femto and atto stand for 10-15 and 10 -18). Indeed, over the past several years scientists around the world learn to employ HHG for exploring new frontiers in optical, atomic, molecular and solids state physics.
Peculiarly, one fundamental property of HHG has defied control: the polarization. Circularly polarized HHG radiation is promising for exploring the structural, electronic and magnetic properties and dynamics of atoms, molecules and materials. While changing the polarization of visible light is as simple as rotating a waveplate, it is more complicated in the XUV and x-ray spectral regions where waveplates are very lossy and spectrally limited. Hence, it is desirable to control the polarization of HHG-based sources in the generation step.
We have recently developed a method for generating high harmonics with fully controllable polarization — from linear through elliptic to circular — without compromising the efficiency of the process . The approach is based on mixing of spins, the radiation angular momentum associated with polarization. For example, high-harmonics with left-circular polarization are produced when m left-circular-polarized driver photons, at frequency ω1, and m-1 right-circular-polarized driver photons, at frequency ω2, are converted into an HHG photon at frequency mω1+(m−1)ω2 (according to conservation of energy) and spin (according to conservation of spin angular momentum). By delicately controlling the polbarization of one of the drivers (by rotating a waveplate), we obtained full control over the ellipticity of the harmonics. In a subsequent work, with the group of Margaret Murnane and Henry Kapteyn, we demonstrated that this HHG-based source is bright and can be used for downstream experiments .
Science’s cover highlighting our demonstration of bright circularly-polarized high-harmonics in its special issue for 2015 international year of light. When an atom is shined by a strong rosette-shape laser pulse (purple), which corresponds to superposition of red and blue circularly-polarized lasers with opposite helicity, an electron (green) is ripped from, and recollides with the parent ion from three directions. As a result, circularly-polarized high-order harmonics are emitted. This long-sought-for bright and chiral probe at the extreme UV allows unraveling the physics of ultrafast chirality.
- A. Fleischer, O. Kfir, T. Diskin, P. Sidorenko, and O. Cohen, Spin angular momentum and tunable polarization in high harmonics generation, Nature Photonics 8, 543 (2014). See also News & Views by M. Ivanov and E. Pisanty, High harmonic generation: Taking control of polarization, Nature Photonics 8, 501 (2014)
- O. Kfir, P. Grychtol, E. Turgut, R. Knut, D. Zusin, D. Popmintchev, T. Popmintchev, H. Nembach, J. M. Shaw, A. Fleischer, H. Kapteyn, M. Murnane, O. Cohen, Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics, Nature Photonics 9, 99–105 (2015)
- A. Fleischer, P. Sidorenko, and O. Cohen, Generation of high-order harmonics with controllable elliptical polarization , Opt. Lett. 38, 223 (2013)
- A. Fleischer, O. Kfir, T. Diskin, P. Sidorenko, and O. Cohen, Bright high-order harmonics with controlled polarization, Optics and Photonics News, Special Issue: Optics in 2014
- C. A. Mancuso, D. D. Hickstein, P. Grychtol, R. Knut, O. Kfir, X.-M. Tong, F. Dollar, D. Zusin, M. Gopalakrishnan, C. Gentry, E. Turgut, J. L. Ellis, M.-C. Chen, A. Fleischer, O. Cohen, H. C. Kapteyn, and M. M. Murnane, Strong-field ionization with two-color circularly polarized laser fields, Phys. Rev. A 91, 031402(R) (2015)
- A. Fleischer, O. Kfir, P. Sidorenko, and O. Cohen, High-order harmonics with frequency-varying polarization within each harmonic, arXiv: 1410.6257
- T. Fan, P. Grychtol, R. Knut, C. Hernández-García, D. D. Hickstein, D. Zusin, C. Gentry, F. J. Dollar, C. A. Mancuso, C. W. Hogle, O. Kfir, D. Legut, K. Carva, J. L. Ellis, K. M. Dorney, C. Chen, O. G. Shpyrko, E. E. Fullerton, O. Cohen, P. M. Oppeneer, D. B. Milošević, A. Becker, A. A. Jaroń-Becker, T. Popmintchev, M. M. Murnane, and H. C. Kapteyn, Bright circularly polarized soft x-ray high harmonics for x-ray magnetic circular dichroism, Proceedings of the National Academy of Sciences of the United States of America 112, 14206 (2015)
- C. Chen, Z. Tao, C. Hernandez-Garcia, P. Matyba, A. Carr, R. Knut, O. Kfir, D. Zusin, C. Gentry, P. Grychtol, O. Cohen, L. Plaja, A. Becker, A. Jaron-Becker, H. Kapteyn and M. Murnane,Tomographic reconstruction of circularly polarized high-harmonic fields: 3D attosecond metrology, Science Advances, 2, e1501333 (2016).
- O. Kfir, P. Grychtol, E. Turgut, R. Knut, D. Zusin, A. Fleischer, E. Bordo, T. Fan, D. Popmintchev, T. Popmintchev, H. Kapteyn, M. Murnane and O. Cohen, Helicity-selective phase-matching and quasi-phase matching of circularly polarized high-order harmonics: towards chiral attosecond pulses, J. Phys. B, 49, 123501 (2016). Invited article.
- O. Kfir, E. Bordo, G. Ilan Haham, O. Lahav, A. Fleischer and O. Cohen, In-line production of a bi-circular field for generation of helically polarized high-order harmonics, Appl. Phys. Lett., 108, 211106 (2016).