Coherent Optics Laboratory: Milestones of History
Coherent Optics Laboratory was founded in 1976 by D.Sci. Alexander K. Popov, on the base of a research group that earlier had began their studies within the Emission Spectroscopy Laboratory. The main subject of the studies at that time and forth, to nowadays, is the resonant interaction of laser radiation with atoms and molecules. Speaking in the popular language, the peculiarity of this subject can be described as follows. Laser light is extremely well organized electromagnetic waves at high frequencies. This kind of radiation is often called Coherent radiation. Three fundamental phenomena that underlie the Optics are Resonance, Coherence and Interference. In a Resonance, that is, when the frequency of electromagnetic wave coincides the characteristic frequency of the system, relatively small external field can "sway" the system to the great extent. Other surprising phenomena are Coherence and Interference. If you superimpose two identical coherent light fluxes, than instead of expected doubling of the brightness you will see the interference pattern, namely, complete darkness in certain places and four-fold increase in brightness in other places. Interference concept is applicable to the vibrations of any kind, including mechanical, electromagnetic, and quantum mechanical ones. With the help of Resonance, Coherence and Interference one can selectively suppress certain physical processes and. on the contrary, sharply emphasize other processes on the quantum or atomic scale. For instance, when two laser radiations act onto the atoms of certain kind, one can suppress the absorption of these atoms (this effect is called as electromagnetically induced transparency), while other processes are not suppressed but can be more intensely uprising. Particularly, in such conditions the symmetry between absorption and emission can be broken, and radiation amplification without population inversion will be achieved, or efficiency of nonlinear resonant processes will be enhanced in the conditions of sharp drop of resonant absorption of interacting fields.
The golden age of the laboratory falls on the period between 1980 and 1990. This period is featured by the wide spread of experimental and theoretical studies on the laser frequency conversion to various spectral ranges from infrared to vacuum ultraviolet and ultrasoft X-ray, on the laser nonlinear spectroscopy, light-induced drift of atoms and molecules in the field of laser radiation, quantum interference effects. Optical and hydrodynamic processes accompanying the pulsed laser radiation interaction with the solid state surface and the laser plasma spread. Part of the studies were conducted on the contracts with the enterprises like "Astrophysics" (Moscow) and State Optical Institute (St.Petersburg).
Main results of the studies of that period can be found in monographs: "Laser induced nonlinear resonances in the Continuum " (Yu. I. Heller, А.К. Popov, 1981); "Introduction to nonlinear spectroscopy " (А.К. Popov, 1983); "Nonlinear conversion of light in gases " (V.G. Аrkhipkin, А.К. Popov, 1987); as well as in review articles: "Nonlinear optics and conversion of light in gases " (V.G. Аrkhipkin, А.К. Popov, Sov.Phys.Usp. 1987); "Nonlinear optics in gases: generation, upconversion and optical phase-conjugation " (V.G. Arkhipkin, A.K. Popov, Proc. of the International School "Laser and Applications", 1991, part 1, 98-123); and also in two textbooks on nonlinear resonant optics and spectroscopy: "Resonant four-photon parametric frequency conversion in gaseous media " (V.G. Аrkhipkin, А.К. Popov, V.P.Timofeev, 1982); "Harmonics generation of quasi-monochromatic radiation in isotropic nonlinear media" (А.К. Popov, V.P. Timofeev, 1982)
Main scientific results in the area of nonlinear spectroscopy.
- The possibility is shown of controlling the amplification-absorption spectral band shapes of microwave. far IR and optical transitions on the base of resonant nonlinear processes. The theory of intense Doppler-free nonlinear resonances is developed that utilizes the effect of Doppler broadening compensation with the Stark shift induced by optical radiation. Such resonance play important role in nonlinear optics, spectroscopy and in physics of selective excitation of atoms and molecules.
- The theory is developed, and the method is suggested of nonlinear optical three-level ultrahigh resolution spectroscopy based on Doppler broadening compensation of fordidden microwave and far IR transitions with the help of elastic collisions. This effect leads to the formation of sharply narrowed structures on the optical transitions, and the properties of forbidden low-frequency transitions can be extracted from those spectral structures.
- Four-photon parametric generation of radiation using low-power fundamental radiations from gas lasers is demonstrated. The method of nonlinear spectroscopy and collision-induced coherence transfer is suggested on the base of this process.
- The spectral analysis method is developed that uses the investigation of the spectra of recombining laser plasma plume. This method was implemented afterwards at Beloyarskaya Atomic Power Station.
- Several possibilities of selective action of light on the matter in order to modify the physical properties of the latter.
- The theory of light-induced drift of gases in the field of non-monochromatic radiation is developed. The advantages of employing the non-monochromatic radiation are shown, and the conditions are clarified when these advantages are the most apparent. The possibility is shown of obtaining supersonic flows of resonant component in the gas mixture, and the conditions of obtaining them are formulated.
- The possibility is shown of laser inducing narrow transparency windows in the absorbing continuum using the lasers in the visible and IR, and of controlling the nonlinear susceptibility for the processes of radiation generation in the ultraviolet, vacuum ultraviolet and ultrasoft X-ray ranges. The methods of creation the radiation switches controlled by optical pulses and of slow polarized electrons generation are suggested on the base of these phenomena.
- The experiments were done, in which the phenomenon of nonlinear resonances inducing was observed. These resonances were found to be similar to the emergence of artificial autoionizing levels within the ionization continuum of cesium atoms.
- Inversionless amplification and the possibility of coherent short-wavelength radiation generation at the transitions to autoionizing and autoionizing-like states.
- The development of the foundations of resonant nonlinear optics of gaseous media was paid the great attention. These methods are peculiarly prospective for the extension the laser wavelengths range to the regions of hard ultraviolet, vacuum ultraviolet and ultrasoft X-ray regions, as well as for infrared radiation conversion.
- The theory of radiation frequency conversion via resonant nonlinear processes in gases and metal vapors is developed. Several schemes of IR radiation conversion into the visible and near ultraviolet regions of spectrum for the purposes of improvement the registration of weak pulsed fluxes of IR radiation.
- The possibilities of tripling the frequency of mid-IR range lasers (mainly CO2-laser) using resonant nonlinearities in molecules were investigated.
- Several schemes of quasi-resonant 3d and 5th harmonics generation of neodymium laser to the UV range were proposed.
- The schemes of coherent VUV radiation generation at the set of wavelengths down to l=89,6 and l=71,6 nm using sum frequency mixing and frequency multiplying of powerful neodymium laser radiation were proposed. The possibilities of obtaining short-wavelength radiation in high-order nonlinear processes.
- The experiments were done that implemented the conversion of weak radiations of all main types of IR lasers (l=10,6; l=3,39; l=1,15; l=1,06 µm) to the near UV in metal vapors.
- The third harmonic (l=3,2 µm) of СО2 laser (l=9,6 µm) is obtained in cryogenic molecular liquids.
- Quasi-resonant third (l=359 nm) and fifth (l=215,4 nm) harmonic generation of neodymium laser in cesium vapor is obtained. Employing two-photon resonance in mercury vapor the coherent radiation at the shortest wavelength (l=89,6 nm, transient region between VUV and soft X-ray range) in the USSR (at the moment of obtaining) was generated.
- Direct conversion of near IR radiation (l=1,06 мкм) to the VUV (153,6 нм, 134 нм), is obtained using nonlinearities of 7th, 8th and 9th order in mercury vapor.
- The conversion of infrared and visible laser radiation into the UV and VUV in the vapors of molecules with conjugated doubled bonds is suggested and experimentally implemented. Nonlinear susceptibilities of such molecules were found to be comparable with resonant nonlinear susceptibilities of alkali metal atoms.
One of the nonlinear optics founders, professor of Moscow University Sergey А. Akhmanov wrote in a foreword to the monograph by V.G. Arkhipkin and А.К. Popov "Nonlinear conversion of light in gases " (1987): "Coherent optics laboratory of L.V. Kirensky Institute of Physics, where the authors work, is one of the leading centers among those working in the area of resonant interactions of laser radiation with the matter. Their studies are based on the original ideas suggested and developed by them and their stuff." Really, a number of scientific studies conducted in the laboratory are universally recognized. Citation index of some articles is 50 - 100 and more. During that period the Laboratory organized and held the International School "Lasers and applications " (Sayanogorsk, 1989), two All-union seminars on resonant nonlinear optics (Krasnoyarsk, 1978; Divnogorsk, 1986). More than 20 Candidate Sci. dissertations were defended.
In the second half of 80th a new research trend emerged in the laboratory, that was connected with the study of optical and nonlinear optical properties of metallic aggregates, being considered from the point of view of fractal physics. These studies were the result of collaboration between L.V. Kirensky Institute of Physics and Automation and Electrometry Institute SB RAS initiated by А.К. Popov. Joint theoretical and experimental studies lead to the discovery of a number of interesting effects that allowed to establish a new sight onto the properties of noble metal colloids in the field of nano- and picosecond laser pulses. The observation and investigation of the fractal aggregates photomodification effect should be particularly mentioned. On this base a new approach was suggested to the ultradense optical information recording. Another interesting effect is the photostimulated aggregation of metal sol. This photoaggregation is apparent under the action of both laser sources and non-monochromatic lamp sources. The growth rate of fractal structures inside the metal hydrosol increases up to 108 times.
A new stage in the laboratory life began in 2000, when its founder and the first head А.К. Popov permanently departed for a work in the USA, and D. Sci. V.G. Arkhipkin became new Head of laboratory since 2002. During last 15 years the research was done in the following areas:
1. Electromagnetically induced transparency, inversionless amplification, nonlinear optics and laser radiation frequency conversion; recording, storage and read-out of light pulses the control of the shape of laser pulses.
2. Optical, nonlinear optical and photophysical properties of metal sols and nanostructures based on them. The results of these studies were summarized in a monograph by S.V.Karpov and V.V. Slabko “Optical and photophysical properties of fractal structured metal sols” (2003).
3. Optical and spectral properties of photonic crystals with a defect containing media with electromagnetically induced transparency, Raman active media and liquid crystals.
4. The studies of nonlinear photonic crystals based on strontium tetraborate.
5. Nonlinear optical processes in metamaterials with negative refractive index.
6. Optical properties of new crystals, glasses and nanophosphors.
Among results obtained in this period, let us mention the following:
- Several schemes of inversionless amplification were suggested and analyzed.
- Several efficient schemes of resonant frequency conversion in the conditions of electromagnetically induced transparency were suggested. The peculiarities of nonlinear interference processes in quasi- and exactly resonant Doppler broadened optically dense media. The possibility of switching the medium from completely opaque to completely transparent state (and of backward switching), and even to the state with large gain, with the help of minor changes of the control and probe fields parameters.
- The peculiarities of short laser pulse propagation in three-level atoms under the electromagnetically induced transparency conditions are studied. The way of recording, storage and read-out of the data on the short pulses is suggested on this base. The way of read-out the recorded pulse with the time inversion is suggested.
- The method of laser pulse envelop control is suggested on the base of electromagnetically induced transparency, that allows compressing the pulses.
- The possibility is shown of simultaneous obtaining short pulses of the same shape but on the different carrier frequencies (concordant pulses). The use of electromagnetically induced transparency and Raman interaction is shown to allow obtaining controllable narrow resonances in the transmission of photonic crystal.
- The dependencies of polarized components in the PC transmission spectra on the temperature are investigated for normal incidence of light. The peculiarities are studied of photonic crystal defect modes behavior on the change of light incidence angle for the light polarization parallel or perpendicular to the director axis of the liquid crystal, in cases when the director axis was oriented whether in the incidence plain or perpendicularly to it. The magnetic and electric field tuning the transmission spectra of photonic crystal with liquid crystal defect are studied.
- Nonlinear optical evidence for existence of the domain structures in non-ferroelectric strontium tetraborate (SBO) was obtained for the first time (in collaboration with Crystallophysics Laboratory). Nonlinear optical studies of these structures and of nonlinear optical processes in them were done. Second and third harmonics of nanosecond pulses from neodymium laser was obtained in the nonlinear diffraction regime, the reciprocal superlattice vectors spectrum being measured and the shape and orientation of domains being determined. Domain structures in SBO are classified as 1D nonlinear photonic crystals (NPC). Nonlinear diffraction of femtosecond pulses is obtained with the efficiency of 1.9% being the record value for this type of phase matching. Random quasi-phase matching (RQPM) is demonstrated in the processes of frequency doubling the second harmonic of ns neodymium laser and fs Ti:S laser. In the latter case, spectral and angular super-noncritical behavior of the frequency converters based on NPC SBO is demonstrated. The concept of the NPC band structure is suggested, and the rotational red shift of the band structure was demonstrated. The autocorrelators using whether RQPM or nonlinear diffraction from virtual beam in NPC SBO were demonstrated.
- The way to compensate the losses in metamaterials with negative refractive index is suggested on the base of parametric amplification of signal wave. The cases are considered of three- and four-wave mixing, parametric light generation and nonlinear reflection on a metamirror.
- New concept is developed according to which the most important and universal characteristic of fractal aggregates formed by plasmon-resonant nanoparticles, lying in the base of their unique electromagnetic properties, is the local anisotropy or the particles environment. The ability of fractal aggregates to enhance local electromagnetic fields is shown to be only due to local anisotropy, while macroscopic characteristics of aggregates do not influence on the electromagnetic interactions with the external field. Experimental data confirming this concept are obtained.
- The laws are established of variation of spectra of silver sols and of their evolution during aggregation process on the base of account of factor of electrodynamic interaction between particles.
- The physical mechanisms are formulated and classified of photoaggregation of different types of metal sols, that account for the way of sol stabilization and the type of dispersive medium. It was shown that photoeffect is the base underlying the mechanism of aggregation acceleration of metal sols.
- The explanation is given for giant amplification of local electromagnetic fields in metal nanoparticle aggregates on the base of account of the anisotropy of their local structure. It is proved that fractal aggregates possess the ability to enhance local electromagnetic fields only due to their inherent local anisotropy. The experimental confirmations of this idea are obtained.
During the years of existence of the laboratory more than 30 Cand. Sci. dissertations were defended, and three employees (V.V. Slabko, V.G. Arkhipkin and S.V. Karpov) became D. Sci.