Nikolai S. Vorobiev

500-fs photoelectron gun for time-resolved electron diffraction experiments

A photoelectron gun as a source of a photoinduced, monoen- ergetic (energy spread ,0.5 eV), well-collimated (divergence ,0.5 deg), sharp (diameter ,0.7 mm at the 1/e level), and ultrashort (<500 fs) bunch of electrons to be used for time-resolved electron diffraction (TRED) experiments is computer designed, assembled, and tested. In single-shot mode, it generates up to 10 3 of 30 keV electrons, and the electron pulse can be either measured in streak mode or focused onto a solid state target chosen from a set of interchangeable targets. High temporal resolution enables measurement with femtosecond precision of the diffraction pattern perturbation after the exiting laser radiation drops onto a target. Demonstration experiments with a 300-A A1 target in transmission-type mode result in diffraction images of reasonable quality under accumulation of up to 4310 4 500-fs photoelectron pulses.

A PS-1/S1 Picosecond Streak Camera in Experimental Physics

The application of a PS-1/S1 picosecond streak camera (SC), which was developed at the General Physics Institute (Russian Academy of Sciences) for investigating fast processes in semiconductor physics, laser physics, and accelerator engineering, is described. It is shown that using the PS-1/S1 SC it is possible to record one-dimensional images (restricted by a narrow slit) of fast processes with a time resolution of no worse than 1 ps in a wide spectral range: from UV (115 nm) to near-IR (1.5 μm) radiation with a dynamic recording range of ≥10. The presented experimental results show the wide potential capabilities of applying the developed SC in various fields of experimental physics.

Set of versatile streak cameras

A set of versatile streak camera was developed in GPI Photoelectronics Department to meet various experimental requirements in laser and plasma physics, laser ranging, fiber optics communication and ecology. Among these camera there are milli-micro-nanosecond camera intended for explosion studies, nano-picosecond streak camera--for application in communication field and nano-subpicosecond streak camera--for laser and laser plasma research.

Designing and application of solid state lasers for streak cameras calibration

A family of specially developed solid-state lasers intended for dynamic calibration of various type streak cameras is presented. The developed lasers are able to generate either ultrasho,rt single pulses or harmonically modulated signals of nanosecond duration. 5±lps light pulses of 0.5% output energy instability were emitted by YAlO :Nd crystal laser with hybrid mode-locking and passive intracavity negative feedback produced by the action of a GaAs plate. After compression of single pulse inside the fiber optics compressor, the minimum available pulse duration approaches 300fs. 100 percent sinousoidally modulated nanosecond optical radiat ion with modulation period adjustable in the 0 .3ps - 6. 6ns range was generated by electro-optical positive feedback Nd:glass laser. Both ultrashort pulses and harmonically modulated radiation were utilized for streak cameras dynamic parameters evaluation. It is shown that for our streak camera equiped with PVOO1 tube its minimum temporal response function approaches to O.7ps, while its intrinsic triggering jitter does not exceed

Picosecond streak-cameras for bunch diagnostics in accelerators

Temporal parameters of synchrotron pulse radiation at damping ring (DP) installation of VEPP-5 type were measured with the help of PS-1/S1 picosecond streak camera having 1.5 ps time resolution. These measurements were proceeded within 400-900 nm spectral range. It has been shown that our streak camera may record either a train of electron bunches with ns-duration or internal structure inside a single bunch. We were able to record the distance ~ 1.5 ns between separate bunches as well as their amplitude, which depends on particle numbers inside a bunch. Depending on linear accelerator mode of operation it was possible to define a single bunch duration, which was deviated within the range of 20-100 ps. The temporal structure of a single bunch was measured with 1.5 ps time accuracy. As a result, the VEPP-5 damping ring parameters were optimized, and particles injection conditions were improved. In addition, we have measured the temporal parameters of Vavilov-Cherenkov radiation (VCR) emitted by electron beam of linear accelerator. Our results provided important information on electron bunches formation and their quality inside linear accelerator before electrons injection inside a damping ring. Another series of experiments were done at VEPP-4M electron-positron collider. The dependence of beam length of the beam current measured with streak-camera allowed us to compute the wide-band impedance of the accelerator. The same data were obtained at Siberia-2 synchrotron radiation source (NRC “Kurchatov Institute”, Moscow).

Chapter Four - Generation of Ultrashort Electron Bunches for Time-Resolved Electron Diffraction Experiments

Chapter 4 considers the results of the design of 500 fs photoelectron gun with electrostatic focusing to be used in time-resolved electron diffraction experiments. The photoelectron gun represents a source of photo-induced, well-collimated (divergence <0.5°), sharp (effective diameter < 0.7mm), ultrashort (duration about 500 fs) bunches of electrons. Demonstration experiments in transmission mode with an aluminum sample of 300 A thicknesses resulted in diffraction pattern of reasonable quality under accumulation of up to 4 × 104 electron bunches with about 1500 electrons per each single bunch. Theoretical and experimental aspects of temporal focusing of electron bunches in time-dependent electric fields are also considered. Main essence of this approach is based on the fact that a real temporal focusing of photoelectron bunch can be attained using a properly chosen electric ramp. This new technology allows a real breakthrough in time resolution of photoelectron guns and diffractometers designated for time-resolved electron diffraction experiments.

Chapter Three - Examples of Streak Image Tube Application

Abstract Chapter 3 acquaints the reader with various applications of streak image tubes in laser and laser physics research. Among them are light “beats” generation using phase-modulated laser radiation, formation of bimodal temporal distribution of photons ultrashort laser pulse passed through a turbid medium, temporal shaping of ultrashort laser pulses by volume Bragg gratings, high-resolution investigation of radiation from a Ti: sapphire laser, time-intensity profile measurements of stimulated radiation from GaAs, dynamics of femtosecond laser microplasma in gases, streak-camera diagnostic of femtosecond laser spark, studies on optical fiber destruction under intense laser radiation.

Some Problems of Streak Tube Design

Abstract Chapter 2 embraces some aspects of streak image tube design, including physics of photoemission for certain types of photocathodes, electron-optical properties of a fine-structure mesh playing simultaneously the role of a photocathode, advantages of quasi-spherical electron optics as applied to high-speed single-frame and streak imaging with large-format photocathode, peculiarities of electrodynamics and electron optics of slow-wave comb structures as dynamic deflectors in streak image tubes, design of streak image tubes with different time resolution.

The granulated gold-film-based semitransparent photocathodes in the visible spectrum range for femtosecond time-resolution experiments

Developed is the granulated, Au film-based, semitransparent photocathode consisting of spherical Au nanoparticles. The granulated Au films are activated by a thin layer of cesium and oxygen of about two monolayer thicknesses to decrease the work function down to about 1 eV and gain the photoemission effect in the visible spectrum range. The sensitivity maximum equal to about one mA/W is located in the green spectrum range. The nanoparticles formation and photocathode surface structure are studied with the use of the X-ray photoelectron spectroscopy technique. Those studies have shown that the photoemission effect in the wavelength range &lgr;> 450 nm is conditioned by excitation of the surface plasmons in quasi-spherical Au nanoparticles. This has allowed manufacturing of a streak tube with the introducible, Au nanoparticles-based photocathode, stability of which has been remaining invariant.

Application of a linear picosecond streak camera to the investigation of a 1.55 um mode-locked Er3+ fiber laser

A modified Imacon type streak camera with temporal resolution of 1.5 psec was used for the measurement of pulses generated by a mode locked Er fiber laser operating at 1.55 micron wavelength. The specially developed S1-PV001 image tube had an initial sensitivity of 26 mu A/W at 1.3 micron. To improve the signal to noise ratio for recording low level CW laser radiation additional shutter circuitry was added to the Imacon camera electronics. The camera was operated at 15 Hz repetition rate, triggered by a sample of the RF signal used for driving the acoustooptic mode locker. A very sensitive and simple recording system was used for recording the time dispersed pictures generated in the ICT output screen using a high gain PMT and a 150 micron slit mounted on a translation stage. The signal from the PMT was read with a box car averager and sent to a PC type computer which controlled the translation stage. The acoustooptically mode locked Er fiber laser can generate pulses of 80 psec in CW operation with a peak power of 0.6 W.