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Author(s) of the publication: Alexander SERGEYEV

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by Alexander SERGEYEV, RAS Corresponding Member, head of the Nonlinear Dynamics and Optics Division of the RAS Institute of Applied Physics (IAP), Deputy Director of IAP for Research

The Nonlinear Dynamics and Optics Division set up in the mid-1990s is the youngest department of the Institute of Applied Physics (IAP) involved with a variety of fields, such as femtosecond optics and quantum generators, high-precision measurements and spectroscopy, biophotonics and ultrasound medical diagnostics... Neurophysics, among other promising areas, is likewise the target of its research.

OPTICS OF GREAT POSSIBILITIES

This is surprising indeed: processes underlying, e. g. the generation of superstrong optical fields in lasers or data management by the brain can be described in one physical language and studied within similar models. It is an important factor for express problem solving by interactive research teams. This job involves eminent scientists whose names are known in Russia and elsewhere for their pioneering work: such as Drs. Viktor Bespalov and Gennady Freidman (high-power lasers and nonlinear optical interactions); Dr. Israel Bernstein (high-resolution optical measurements); Dr. Yakov Khanin (quantum radiophysics); Dr. Andrei Krupnov (high-precision molecular spectroscopy); Dr. Maria Grekhova (radiophysical methods in medicine); Dr. Mikhail Rabinovich (nonlinear dynamics of non-equilibrium systems)... The schools founded by these scientists (holders of doctorates in physics and mathematics) are active at our Nonlinear Dynamics and

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Optics Division and keep bringing in new blood from young talents.

Femtosecond optics is one go-ahead research line that has been making particularly good headway in the past ten years or so. Now why femtosecond optics? Well, we know that many processes in physical, chemical and biological systems at the molecular level take place within ultrashort time intervals, down to pico-, femto-, and even attoseconds (10-12, 10-13 and 10-18 s, respectively). Such blitz events can be diagnosed, studied and modeled only with the use of lasers.* Early in the 1960s the laser made it possible to obtain a pulse in the nanosecond range (1 ns = 10-9 s), and get into the femtosecond region late in the 1980s. At the turn of the present 21st century experimentalists succeeded in generating a 5 femtosecond (fs) pulse, which is comparable to a period of optical oscillation; thereby they approached the limit provided by quantum generators. This staggering progress could be made owing to fresh evidence on the mechanisms implicated in oscillation self-synchronization, and thanks to new crystals and optical elements for wideband frequency control of laser resonators.

Femtosecond optics opens up boundless opportunities in such areas as process control in physical, chemical and biological systems at the molecular level, in communication technologies with record-high bit density - over 1012 bits per second, in high-precision micro-machining and microprocessing, and in other uses.

Femtosecond optics is also a groundwork for yet another strategic area, the physics of superstrong fields and extreme states of matter they induce. These fields of immense intensity (over 1019 W/cm2) are generated by ultrashort laser pulses with giant peak power values from terawatt (1012) to petawatt (1015) levels. Today multiterawatt femtosecond laser-driven complexes are becoming an experimental base for certain new lines of research in nuclear physics, high-energy physics, and in nuclear and thermonuclear research. IAP has Russia's largest pool of such facilities. These include a petawatt complex on wide-aperture (large-diameter) nonlinear DKDP (deuterated potassium dihydrophosphate) crystals as well as terawatt, subterawatt and other laser-pulsed complexes and sources. Each one is geared to a specific research program. The whole job is supervised by Dr. Yefim Khazanov and by the author of the present article.

The petawatt laser complex-this country's largest and among the world's five largest setups of this kind-is our major achievement realized in collaboration with colleagues from the Russian Federal Nuclear Center-the Research Institute for Experimental Physics (RFNC-RIEP) based in the town of Sarov (Nizhni Novgorod Region). Compared with similar facilities in USA, Britain and Japan, ours is more compact, and it used parametric light amplification instead of laser-driven one. Now, in ordinary optical quantum generator or amplifier the active medium (substance) is first rendered into an excited state, and thereupon the energy thus built up is released. For


* Laser - abbreviation of (l)ight (a)mplification by (s)timulated (e)mission of (r)adiation - a device that produces an extremely powerful beam of light waves that are of the same wavelength and are in phase. The laser has many possible uses in medicine, communications, and warfare. The possibility of laser actualization was validated by Charles Towns jointly with Arthur Shavloff (both from the United States), Nikolai Basov and Alexander Prokhorov of the Soviet Union in 1958. - Ed.

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Headway made in creating parametrically amplified laser systems in comparison with petawatt lasers, based on a conventional pattern of pulse amplification.

active medium solid-body lasers employ ruby crystals as well as crystals of yttrium aluminum garnet, neodymium glass, and other materials. In parametric light amplification this role is assigned to nonlinear optical crystals grown for this purpose. They transform the pumped wave energy into amplified signal energy by splitting each high-frequency quantum into two quanta of lower frequency. This process is absent when a conventional amplification pattern is used.

This achievement prepares the ground for a hyper-high multipetawatt laser facility to be created in this country in the nearest future. We have already built an analogous 100 terawatt laser facility and turned it over to the Russian Federal Nuclear Center (RFNC-RIEP); this facility will be supplemented with a parametric amplification cascade on a unique wide-aperture DKDP crystal of 30x30 cm2. For its pumping it is planned to use emission from one of the channels of the LUCH will be capable of generating an output pulse of laser radiation of more than 100 J and less than 50 fs long. Focused in a vacuum chamber on a target, the pulse can produce a field of truly immense intensity - 1022 W/s.

Such fields are nearly 30-fold as intense as the "atomic" field retaining an electron in the first orbit of a hydrogen atom (according to the model suggested by Niels Bohr in 1913). Such kind of action on the substance (medium) results in its extreme state characterized by new properties, something that could be of interest to basic science (for instance, a modeling of processes in the interior of stars and planets or studying nonlinear characteristics of vacuum). On the other hand, superstrong optical fields offer most intriguing spin-offs from electromagnetic radiation quanta and charged particles of energy above 108 eV generated in laser targets. All that anticipates new sources of hard coherent radiation, a triggering of controlled nuclear fusion reaction and other uses.

The petawatt level is reached not at the expense of studies on the terawatt Ti: sapphire laser complex-such studies are still topical. One of the most productive cycles of experimental work deals with the phenomenon of filamentation* of femtosecond radiation in the atmospheric air. Using this effect, we can create "white light lidars"** or wideband sources located downstream the laser beam and applied for sounding the atmosphere for its chemical composition.

UNIQUE LASERS

Lasers can generate high-quality coherent beams of high-power light radiation remarkable for low beam divergence (spread) and capable of carrying concentrated energy fluxes over large distances. The sticking point here is that intensive pumping causes inhomogeneities of the light refraction index in the active medium, and that impairs the three-dimensionality of the beam. Methods of adaptive and nonlinear optics are applied for redressing such distortions - we at IAP have been in search of such techniques from the very start. Accordingly, our research scientists have devel-


* Filamentation - formation of a thin elongated channel (filament) of high-power density via propagation of high-intensity laser pulses in a medium. - Ed.

** Lidar - (l)aser (i)nfrared ra(dar) used for remote measurements of atmospheric characteristics. - Ed.

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oped essentially new high-power pulsed lasers of good radiation directivity. Such laser facilities have a future in high-precision micromachining, remote sounding and in other areas. Here are just two examples.

A laboratory headed by Nikolai Andreyev, M. Sc., has developed laser nonlinear optical receivers of spatially inhomogeneous light imaging signals. They combine a unique set of parameters: the narrowest reception band, a wide field of view (about 500x500 pixels, or 5 - 10°) and a general increase in the power of a picked signal up to 1012 - 1013. A level of sensitivity thus achieved-about 2 quanta per resolution element of the receiver-is close to a physical limit determined by the level of quantum noises. Also, a group led by Oleg Kulagin, M. Sc., has come up with compact picosecond solid-body facilities emitting radiation close to the refraction limit (the minimal possible size of a light spot) and, what is likewise important, operating in the vision-safe spectral region.

The ever wider sphere of laser uses calls for new spectral bands of generation. Much effort in many countries is being made in developing compact hardware operating in the medium infrared spectrum, which is indispensable in astrophysics, spectroscopy (including process monitoring in the chemical industry), optical communications, infrared (IR) imaging, environmental control and many other areas. Our IAP is likewise busy with such work. Jointly with Nizhni Novgorod State University and RFNC-RIEP group headed by Oleg Antipov, Cand. Sc. (Phys. & Match.), we have created a model of a solid-body laser system adjustable to wavelength frequencies of atmospheric transparency (3 to 5 Lim). Its weight is not above 40 kg, it can be carried on board a light aircraft and used for ecological monitoring and remote gas escape diagnostics in trunk pipelines.

The adequate performance of lasers largely depends on the parameters of their optics. These problems are the domain of a department (head, Alexei Babin, Dr. Sc.) concerned with high-rate crystal growth, namely the growth of water-soluble wide-aperture crystals of up to 40x40 cm on the basis of technologies, developed at IAP under guidance of Vladimir Ershov, Vladimir Katsman (staff members of the Institute) and Viktor Bespalov, Dr. Sc. (Phys. & Match.). Such large crystals are needed for systems of laser-driven nuclear fusion now at the gestation stage in Russia, USA, France and Japan. Each setup like that needs as many as 10 elements as a basis for electrooptic shutters and laser radiation frequency multipliers. With conventional techniques it takes more than a year to grow this type of crystals. Our experts, however, have managed to speed up this process dozens of times over, and bring the growth rate to 2 cm (about an inch) per day. Our technology has been adopted by the Russian program for laser-controlled nuclear fusion as well as at laboratories of the United States, China and the Chech Republic.

стр. 56


A - optical coherent tomographer; B - images of an intact and C - carcinoma-affected stretch of the intestine. Wavelength of scanning, 1,300 nm; image dimensions, 1.5 mm x 1.5 mm.

HIGH-PRECISION MEASUREMENTS

Present-day metrology is largely based on the optical methods of measurement, and many standard procedures owe their existence to laser facilities. Hence it is natural that we attach much attention to this matter.

We have achieved best results by using optical interferometry* in combination with quality laser emission. Back in the mid-1990s Valentin Gelikonov, Dr. Sc. (Phys. & Math.), succeeded in measuring record low shifts of an optical mirror, down to 10-17 m/Hz1/2. Such kind of sensitivity was achieved only in 2000 at the giant 4-kilometer LIGO interferometer (USA) designated for detection of gravitational waves emanated by different sources of our galaxy or neighboring stellar systems. Our IAP, a member of the international scientific consortium LSC (LIGO Scientific Collaboration) is taking part in its work and related problem solving. One problem concerns control over the quality of wide-aperture (up to 30 cm across) optics and remote control over its longtime (over many months) performance under high radiation exposure. Our group has made a major contribution to the LIGO program by designing a wideband interferometer for measuring the inhomogeneities and optical thickness of samples from 250 mm to several microns across-the measurement accuracy was down to angstrom units. For linear surfaces of up to 100 mm measurements can be made at a distance of 3 meters, a procedure making it possible to monitor dynamic phenomena occurring within optical elements during their work. So far this is the world's only device with such characteristics.

Atomic and molecular spectroscopy goes along with metrology. A team under Dr. Andrei Krupnov and Mikhail Krupnov, M. Sc, is working on related facilities. Using such techniques it has become possible to conduct high-precision measurements of transition frequencies of the principal oscillatory state of a carbon oxysulfide molecule at frequencies ranging from 48 GHz to 1.1 THz, and calculate the rotation spectrum of this molecule to an accuracy dozens of times as high as that achieved previously. Thanks to high precision the spectrum can be adopted as a calibration standard in astrophysical, atmospheric and laboratory studies.

BIOPHONICS AND ULTRASOUND DIAGNOSTICS

Innovative laser radiation facilities and high-precision methods of optical diagnostics have given birth to a new R&D line known as "optical tomography of biological objects" (or "optical bioimaging"). This trend has come to the fore in our work over these ten years. Emissions on the longwave band of the visible or near-infrared spectrum allow to penetrate into living (biological) tissue as deep as a few centimeters; this noninvasive technique is absolutely safe, since optical quantum energy and radiation intensity are very low. The inner structure of biological objects on such wavelengths is characterized by great diversity of light absorption and diffusion factors; hence the imaging contrast is very high. Yet optically the biotissue is a turbid medium, and irradiated light diffusion within it is very high. Where is a way out? The clue was found in... subwater detection and ranging of objects: such techniques in turbid media with the use of nanosecond radiation were suggested by the Nizhni Novgorod radiophysics school as far back as the 1960s.

A research team under Dr. Valentin Gelikonov and yours truly has developed a lab "biotissue lidar" model at the femtosecond level. This broke ground for a new diagnostic method - that of optical coherent tomography. Our technique can be applied for the early diagnostication of tumors, and for intra- and post-surgical control. Joining hands with colleagues at Nizhni Novgorod State Medical Academy, we have developed a family of optical tomographs tested in this country and abroad, and now being adopted for


* Optical interferometry - measuring the wavelengths of spectral lines, the refraction indices of translucent media, angular dimensions of stars, velocity of light and other parameters. The device employed for the purpose is called an interferometer. - Ed.

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A mammary tumor imaged IN VIVO by an optical diffuse tomographer. Top, distribution of the detected amplitude of signals (scanning region, 70x70 mm; layer thickness, 60 mm). Bottom, restored distributions of biotissue components.

clinical uses. For our contribution we have merited an RF State Prize in science and engineering (1999).

Diffuse tomography (fluorescent tomography including) is another kind of optical bioimaging. It makes use of the strongly scattered (diffuse) component of radiation entering living tissues to a depth of several centimeters. The laboratory of Vladislav Kamensky, Mr. Sc, has devised an optical diffuse tomographer for mammary gland diagnostics - it scans the gland at three wavelengths simultaneously and processes the images for tissue analysis.

Biomedical acoustics, yet another ground-breaking line of research, is the subject-matter of a department headed by Anatoly Mansfeld, M. Sc. Pulsed-laser optoacoustic detection (diagnostics) makes it possible to spot pathological inhomogeneities in a tissue different from sound tissues only in the optical absorbtion factor. This is still another method for the early diagnostics of mammary tumors.

Now a few words about methods for visualization of body organs by means of a device registering the body's self-radiation. Another method is that of acoustic thermography (acoustothermography) for studying the distribution of temperatures inside a biological object. A new focus-antenna scanning acoustothermograph, which upgrades the space resolution and sensitivity of thermometry, has already been tested.

NONLINEAR DYNAMICS PLUS NEUROPHYSICS

Now what concerns the nonlinear dynamics of space-and-time resolution systems and processes, Russian science has always been in the lead. We at IAP are carrying on the line conceptualized by Acad. Gaponov-Grekhov and coworkers. This trend harks back to the school founded by two eminent physicists, Acads. Leonid Mandelstam and Alexander Andronov, in the 1930s and 1940s.

A department headed by Dr. Vladimir Shalfeyev concentrates on problems related to the spatial and temporal dynamics of discrete media formed by large assemblies of vigorous interactive elements. Dr. Vladimir Nekorkin's laboratory, for its part, is studying the dynamic mechanisms implicated in the formation of oscillatory coherent structures in active multistable systems. It has discovered the effect of dynamic imaging and transformation of the space-and-time structures of activity, and has demonstrated the possibility of pulsed controls over periodic and chaotic oscillations for forming phasic clusters of a pre-assigned configuration. Dr. Nekorkin and colleagues have proved the existence of steady running waves in the form of wave fronts, excitation spikes and the like. Such studies are important as a basic component for an oscillation-wave theory of complex multielement self-excited (self-sustained) oscillation systems. On the other hand, they offer certain spin-offs, e. g. in systems of parallel data evaluation and management in compliance with the principles on nonlinear wave physics.

A research collective headed by Dr. Alexander Yezersky is working out new models for space-and-time chaos, while the laboratory of Dr. Vladimir Yakhno is studying the dynamics of models describing the nonlocal distribution of interelemental interactions with the aim of synthesizing algorithms of par-

стр. 58


Fractal structure of activity in an assembly of excitable elements formed via instability of a large number of waves.

allel processing of composite videoimages. The laboratory has built demonstration data identification systems adjusted to biometric parameters.

Special mention should be made of a new research trend, and that is neurodynamics and optical neuroimaging connected with the application of methods and approaches of nonlinear dynamics and optics for elucidating the structure and functions of cerebral systems. Such data are retrieved thanks to progress made in laser physics and microelectronics as well as in cellular and molecular technologies. It becomes possible to construct mathematical and physical models of most complex neuron systems and, consequently, to describe, study and predict mechanisms implicated in their performance. The practical aspect of this work is related to a search for methods of diagnostics and treatment of pathologies, and to designing neurosimulating information technologies and gadgets on the basis of fundamental know-how. Unlike contemporary electronic computers, such facilities will operate on principles involved in the functioning of brain cells (cellular networks), much different from conventional logic.

This trend at IAP is developing in cooperation with the Basic Department of Neurodynamics and Neurobiology of Nizhni Novgorod State University (with Dr. Viktor Kazantsev as department head). They are working on a dynamic model of the olivocerebellar system of locomotion control and coordination responsible for locomotor stimuli realizing a set of muscular contractions (cloni). Also under study is a model of short-term memory in the form of an assembly of active elements interacting via feedback. The key characteristic of this system is that it can store several information images in the shape of periodic excitation spikes jogging neuron activity. Optical neuroimaging methods are used in studying mechanisms responsible for synaptic signal transmission amongst neurons. To obtain high space resolution fluorophors are excited by two photons in an IR femtosecond laser. Registered signals are also induced by conventional electric stimulation with the aid of extracellular electrodes, or by cell stimulation via photoexcitation of biologically active molecules.


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