Discoverer Of picosecond Electron-optical chronography. Part II
Technical implementation of the physical principles of pico-femtosecond electron-optical photography prolonged to many years and the potential capabilities of increase of IIT time resolution by three orders of magnitude (from 10 ps to 10 fs) still stir up the interest of many researchers. Progress connected with the application of time analyzing IITs in physical experiments was simultaneously powerful stimulus for the improvement of IITs, and the need which occurred in the electron-optical diagnostic equipment led to the necessity of production of small batches of these devices [23-25].
After the fundamental experiments carried out in the middle of 50s by Y.K.Zavoisky and colleagues, the devices of UMI-95V series replaced the six-stage UMI-95 with the field intensity near the photocathode PIM-3 of ≤60 V/mm and scanning speed of 2 ∙ 109 cm/s in 1959. At the expense of cathode circular electrode the electric field intensity near the entrance photocathode was increased by more than an order of magnitude (up to 600–900 V/mm) (according to the data of measurements in electrostatic bath). For the scanning of photoelectronic images limited by point instead of standard condenser-type deflecting plates with top caps, S.D.Fanchenko with colleagues developed new deflecting system. It consisted of two open resonators which were tuned in the wave length of 10 cm located at the right angle toward each other with vacuum-tight coaxial inputs on the edges. Continuous elliptical scan with the semimajor axis of 10 mm was performed on UMI-95V. The maximum scanning speed of dot-element photoelectron images was increased by an order of magnitude and had the value of (1–2) 1010 cm/s which basically provided the technical time resolution up to 8∙10-13 s.
Conceptually new situation for testing and improvement of pico-femtosecond IITs occurred at the second half of 60s when IITs revealed their unique recording capabilities in laser experiments and experiments with laser controlled thermonuclear fusion (CTF). In these experiments the practical capability to implement the time resolution better than 10 ps was specifically in demand. After the occurrence of the first lasers with mode locking the real need in the cameras based on IIT with the time resolution of the order of one or even fractions of picosecond arose. And Y.K. Zavoisky with colleagues made the significant contribution into the solution of this task. In the creative cooperation with M.M. Butslov, B.M. Stepanov and other colleagues from the All-Union Research Institute of Opticophysical Measurements  Yevgeny Konstantinovich focused the efforts of his team on the significant modernization of UMI-95V using the resonance super-high frequency (SHF) scanning with continuous action operating on the wave length of 3 cm [28-29]. In this new time-analyzing IIT designed in 1970 and called "Picochrone" electrons got into the deflecting system through the inlet port with the diameter of 2.5 mm. Deflection was accomplished by the electrical SHF-field in the area of "slit" of two mutually perpendicular resonators. Both resonators having the type "slit-opening" were tuned in the same frequency equal to 10 GHz. For the accurate adjustment the plunger was provided in one of the resonators. Excitation of every resonator was accomplished through the coupling loop of coaxial input the outward end of which was the element of waveguide-to-coaxial adapter. Upon the scanning of the image of continuously luminous point it was possible to give the shape of circle, ellipse or straight line to the scan easily by changing the phase shift using the phase shifter.
One of the first experiments of the scientific team under the supervision of Y.K. Zavoisky using the Picochrone consisted in the scanning of pulses of many-electron component of dark emission of entrance silver-oxygen-cesium photocathode. It is known that upon the absence of photocathode illumination the electrons of "dark noise" go out of it; this dark noise consists of two components – single-electron and many-electron components. Single-electron component (102–103 electrons cm2/s) can be explained by the mechanism of thermionic emission from photocathode and described by the standard Richardson equation. Apparently, many-electron pulses (104–105 electrons cm2/s) occur as a result of entering of the ion (for example, cesium ion) accelerated by electric field to photocathode. Pulses have the considerable statistical dispersion by the number of electrons in individual pack (5 to 15). Under the conditions of continuous elliptical scan of pulses of many-electron dark emission on Picochrone  it was experimentally proved that the electrons of every group outgoing from the common photocathode point leave it with the time spread of not more than several picoseconds.
In these series of experiments the scientists of the Atomic Energy Institute (AEI) proved one more time that for the photo-emission images, characterized by the spread of initial photoelectron energy with the value which is smaller by an order in comparison with the images from "dark noise" photocathode, the limit physical time resolution of PIM-3, estimated by the chromatic aberrations of the first order only, should not be worse than 5-10 picoseconds (Е<60 V/mm; Е=2·109 cm/s) and for Picochrone (Е<900 V/mm; Е=2·1010 cm/s) it should be about one picosecond or better!
In the laser experiments carried out in 1972–1975 by S.D. Fanchenko in the laboratories of the Academician N.G.Basov with the participation of P.G.Kryukov and his colleagues [30-32], "Picochrone" operated in the mode of trochoidal scan: with the help of resonator SHF-deflecting system the photoelectron images continuously scanned over the ellipse and upon the laser ignition of switching tube the single linear deflection of images occurred with the help of additional pair of plates located in Picochrone. The maximum scan speed on IIT screen was equal to (4–6)∙1010 cm/s which basically provided the achievement of technical time resolution with the value better than 5∙10-13 s. With such high time resolution the complete scan cycle was 50-100 ns which made it possible to record several axial periods of radiation of neodymium laser with self-mode locking. Scan period was precisely attached to the frequency of 3 cm magnetron and the requirements to instabilities of response of pulse control circuits reduced to 10-8 s. The image occurred from one edge of the screen, drew the trochoid and left outside the field of vision from the opposite side.
The system of repeated (multi-channel) registration was based on the fact that laser radiation before entering the IIT photocathode passed through two parallel semi-reflecting mirrors the distance between which could change. The lens focused all beams into one point of Picochrone entrance photocathode giving the spot diameter of ~0.2 mm on the screen. The method of repeated registration ensured the record of native instrumental effects, allowed easier access to the linear area of dynamic registration range and upon change of the distance between mirrors made it possible to determine the scanning direction. Resuming the experiments on "Picochrone" use for the measurement of time structure of neodymium laser radiation on silicate glass under the conditions of self-mode locking, we can state that the minimal recorded duration of single laser spikes turned out to be equal to 1.7 ps.
It should be noted that using the IIT with accelerating grid near photocathode developed in the All-Union Research Institute of Opticophysical Measurements under the order of the Lebedev Physical Institute of the Academy of Sciences in the early 70s the electric field intensity was increased by more than two orders of magnitude in comparison with PIM-3. For the first time, sub-picosecond time resolution equal to 0.7 ps was achieved in 1976 by the group of researchers from the Lebedev Physical Institute of the Academy of Sciences under the supervision of A.M. Prokhorov using the tube UMI-93M developed and produced in the All-Russian Research Institute of Opticophysical Measurements by G.I.Bryukhnevich, B.M.Stepanov  under the conditions of linear (slit-type) scanning upon the registration of sinusoidally modulated radiation with the period of 1.4 ps and modulation depth of >10% at the wave length of 1060 nm. The speed of slit scanning reached 5.5.1010 cm/s and electric field intensity near photocathode was ≥ 3 kV/mm. The main element of camera was the laser spark gap which formed the rectangular pulse with sub-nanosecond fronts and amplitude up to 20 kV. Amplification coefficient by IIT brightness reliably ensured conditions for the registration of every photoelectron leaving the entrance photocathode. Oxygen-silver-cesium photocathode with the surface resistance of ≤10 Ω/□ and spectral sensitivity extending to 1.5 μm was used. The peculiarities of the experiments with time-analyzing IITs carried out in the Lebedev Physical Institute of the Academy of Sciences consisted in the scanning of the images restricted by the narrow slit along which (as opposed to "Picochrone") many tens of spatially-resolved channels could be inserted simultaneously.
Thus, in the total agreement with the forecasts of Y.K. Zavoisky the time resolution of IIT within the range from ten to fractions of picosecond was reliably mastered by the end of 70s in our country as well as abroad.
Modern State and Development Trends of Pico-Femto-Attosecond Photoelectronics
Priority of the national science in the area of pico-femtosecond electron-optical photography was proved in many Russian reports presented at all International Congresses on the formation of BPP images and photonics: Japan (2000), France (2002), USA (2004), China (2006), Australia (2008), Japan (2010), South Africa (2012). We will mention just some results obtained in the Prokhorov General Physics Institute of the Russian Academy of Sciences which are important from our point of view:
•development, production and testing of femtosecond IIT of the type PV-FS-M with pulse power supply of the period photocathode-grid;
•creation of pico-femtosecond photoelectron gun intended for the experiments on the study of substances at atomic-molecular level using the methods of electron diffraction;
•completion of the concept of construction of slit (streak) camera providing the maximum time resolution not worse than 200 fs.
The new development trend of pico-femtosecond photoelectronics was indicated by the works on the formation of electron beams with femto- and even attosecond duration in quasi-stationary focusing fields (see Fig.). These works carried out in the Prokhorov General Physics Institute of the Russian Academy of Sciences were represented at XXV International Congress on High-Speed Photography and Photonics in France in 2002. The considerable contribution into the idea of use of quasi-stationary focusing fields was made by M.A.Monastyrsky . In the Department of Photoelectronics of the Prokhorov General Physics Institute of the Russian Academy of Sciences the experimental samples of pico-femtosecond electron gun were developed and tested. Representative experiment carried out on the prototype of photoelectron gun confirmed the capability of 25-fold compression of initial photoelectron beam of 7 ps with the duration up to 285 fms.
Thus, by the efforts of the school of the Academician Y.K. Zavoisky more than sixty years ago the fundament for the new chapter of technical physics – pico-femto-attosecond photoelectronics was laid. This school has been existing up to the present day; it is actively developing, first of all due to the efforts of the Russian academic science, making valuable contribution into the treasury of human knowledge in physics and bringing closer to the knowledge of fast processes occurring within progressively shorter time intervals .
Once, A.M. Prokhorov, answering the question about which Russian scientists he considered to be worthy of nomination for Nobel Prize, gave clear and unambiguous answer: "Academician Y.K. Zavoisky for IIT". Over the last years the physical principles of pico-femtosecond chronography formulated by Y.K. Zavoisky have remained unshakeable and received the further development and widespread application. On their basis important results were obtained in the fundamental studies: experimental evidence of self-focusing phenomenon by the observation of moving focuses in nonlinear optical media using IIT); in industry (diagnostics of internal combustion engines); in ecology (laser lidars), medicine and biology (femtosecond tomographs); in the areas providing the national security and control of terrorism (diagnostics of nuclear reactors, studies in the area of aero-hydrodynamics and explosion theory, gas analysis of narcotic and explosive substances), space research (precision measurements of artificial objects – "beacons", "stars" etc.).
In 2007 on the occasion of 25th anniversary of the establishment of the Prokhorov General Physics Institute the contest of scientific papers was carried out. The paper called "Design of Femtosecond Photoelectron Gun with Non-Stationary Focusing Field (Theory, Practical Implementation, Experiment)" received the most of votes. The main point of this paper was that additional focusing lens ensuring the dynamic compression of photoelectron beams under the action of non-stationary electric fields was introduced into the traditional time-analyzing IIT. The experimental prototype of such gun was estimated, simulated and designed; its initial 7 ps photoelectron beam was compressed by almost 25 times. In this paper it was shown that the theoretical limit of time resolution of electron-optical chronography estimated at about 10 femtoseconds can be surpassed by one-three orders of magnitude (up to hundreds and even tens of attoseconds) at the expense of time focusing of photoelectron beams in the special selected non-stationary electromagnetic fields. And these attoseconds do not contradict but prove again the views of the school of Y.K. Zavoisky on the physical time resolution of data path: de Broglie wave length of electrons with the energy of 20-30 keV is less than 10-2 nm which ensures the information transmission with sub-attosecond time resolution (10-19–10-20 s) at the extreme.
In conclusion one more time we would like to emphasize the fact that Yevgeny Konstantinovich was not only distinguished scientist but talented teacher of truly infatuated and committed scientists and specialists as well. Even today with enviable constancy, overcoming the technical and economical obstacles they make every effort in order to bring closer the time when the photography of fast processes with pico-femtosecond (and subsequently attosecond) time resolution will become ordinary and inexpensive procedure in experimental practice.
Author expresses heartfelt gratitude to his fellows and colleagues for the useful discussions, commentaries and assistance when preparing the manuscript for printing: N.Y. Zavoiskaya, G.I. Bryukhnevich, N.S. Vorobyev, V.V. Korobkin, Y.A. Kuzmenko, A.A. Manenkov, V.A. Skoryupin, A.V. Smirnov, V.K. Chukbar.