NONLINEAR OPTICS OF IONIZED MEDIA

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Traditional nonlinear-optical effects, underlying many contemporary laser devices òÀÓ parametrical amplifiers, Raman- and CARS spectrometers, THG-microscopes òÀÓ are caused by nonlinear response of bound electrons and nuclei. Emergence of powerful laser systems, being able to deliver powerful ultrashort light pulses, allows one to investigate and exploit new class of nonlinear-optical phenomena, based on the media ionization, when the electrons are released by strong electromagnetic field directly or by collision of an atom with another electron accelerated in the field.

A number of nonlinear-optical ionization phenomena have fundamental importance for solid-state physics, atomic physics, quantum chemistry. Namely, electromagnetic field of a powerful ultrashort pulse may set the electron free on a very short time scale - of an attosecond order (1 attosecond, or as = 10-18 s). The released electron carries information about the state of the atom. This gives a unique opportunity to evaluate atomic and crystal dynamics with a temporal pace smaller than classical period of electron rotation around the nucleus. [1-3]

Nonlinear ionization optics is very perspective from the application point as well. The gas of released electrons, or plasma, changes refractive index of the medium, thus allowing to create optical elements òÀÓ mirrors, gratings, lenses, waveguides with high temporal resolution. Contemporary laser systems provide pulses of petawatt power (1 petawatt, or PW = 1015 W). Driving such radiation is a complex task 

since conventional optical elements degrade rapidly in the intense field. Plasma optical elements in gas overcome this disadvantage and thus may be suited for powerful radiation steering. On the other hand, plasma structures with high refractive index contrast and temporal resolution of the optical field cycle order provide a unique tool for temporal and spectral transformation of laser pulses. Our laboratory runs
theoretical and experimental investigation of nonlinear-optical ionization phenomena in gaseous and condensed media. The targets of study include ionization processes and plasma properties on femtosecond time scale [2-4] as well as transformations of powerful laser pulses in media during ionization processes [5-6]. Our group has done pioneering works on pulse transformations during ionization in hollow photonic-crystal fibers. Combination of ionization nonlinearity and guiding properties substantially expands potential of fiber-optical ultrashort-pulse transformers.

Our laboratory also explores properties of plasma structures in transparent solid dielectrics. Solid-state plasma is prospective source of optical nonlinearity for optical integrated circuits, allowing several-percent change of medium refractive index at femtosecond time scale without permanent damage[7]. Ionization processes play an important role in other direction being researched by our group òÀÓ in microspectroscopy of biological tissues. Free-electron generation destroys most of biological objects, limiting maximum intensity of the field exploited for nonlinear-optical object inspection. The analysis of free carrier generation and evolution is thus important for development of non-invasive bioimaging systems.

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[1] E. Goulielmakis, Zhi-Heng Loh, A. Wirth, R. Santra, N. Rohringer, V. S. Yakovlev, S. Zherebtsov, T. Pfeifer, A. M. Azzeer, M. F. Kling, S. R. Leone, F. Krausz. Real-time observation of valence electron motion
Nature, 466, 739-743 (2010)

[2] A.J. Verhoef, A.V. Mitrofanov, E.E. Serebryannikov, D.V. Kartashov, A.M. Zheltikov, A. Baltuska. Optical detection of tunneling ionization, Phys. Rev. Lett, 104, 163904 (2010).

[3] A. Mitrofanov, A. J. Verhoef, E. E. Serebryannikov, J. Lumeau, L. B. Glebov, A. M. Zheltikov, and A. BaltuŠáka. Optical Detection of Attosecond Ionization Dynamics in Transparent Solids in International Conference on Ultrafast Phenomena, OSA Technical Digest (CD) (Optical Society of America, 2010), paper FB2.

[4] I. V. Fedotov, P. A. Zhokhov, A. B. Fedotov, A. M. Zheltikov. Probing the ultrafast nonlinear-optical response of ionized atmospheric air by polarization-resolved four-wave mixing Phys. Rev. A, 80, 015802
(2009)

[5] A.B. Fedotov, E.E. Serebryannikov, A.M. Zheltikov. Ionization-induced blueshift of high-peak-power guided wave ultrashort laser pulses in hollow-core photonic crystal fibers. Phys.Rev. A 76, 053811 (2007)

[6] I.V. Fedotov, A.B. Fedotov, P.A. Zhokhov, A.A. Lanin, A.D. Savvin, A.M. Zheltikov. Parametric transformation and spectral shaping of supercontinuum by high-intensity femtosecond laser pulses. JETP Letters, 88, 157-159 (2008)

[7] A.M. Zheltikov. Ultrafast optical switching of an ionized medium by interfering ultrashort laser pulses. JETP Letters, 90, 90-95 (2009)

[8] A.A. Voronin, A.M. Zheltikov. Ionization penalty in bioimaging Phys. Rev. E, 81, 051918 (2010)