Boris A. Knyazev

Novosibirsk terahertz free electron laser: instrumentation development and experimental achievements

Nowadays, the Novosibirsk free electron laser (NovoFEL) is the most intense radiation source in the terahertz spectral range. It operates in the continuous mode with a pulse repetition rate of up to 11.2 MHz (5.6 MHz in the standard mode) and an average power of up to 500 W. The radiation wavelength can be precisely tuned from 120 to 240 mm with a relative line width of 0.3–1%, which corresponds to the Fourier transform limit for a micropulse length of 40–100 ps. The laser radiation is plane-polarized and completely spatially coherent. The radiation is transmitted to six user stations through a nitrogen-filled beamline. Characteristics of the NovoFEL radiation differ drastically from those of conventional low-power (and often broadband) terahertz sources, which enables obtaining results impossible with other sources, but necessitates the development of special experimental equipment and techniques. In this paper, we give a review of the instrumentation developed for control and detection of high-power terahertz radiation and for the study of interaction of the radiation with matter. Quasi-optic elements and systems, one-channel detectors, power meters, real-time imagers, spectroscopy devices and other equipment are described. Selected experimental results (continuous optical discharge, material and biology substance ablation, real-time imaging attenuated total reflection spectroscopy, speckle metrology, polarization rotation by an artificial chiral structure, terahertz radioscopy and imaging) are also presented in the paper. In the near future, after commissioning another four electron racetracks and two optical resonators, intense radiation in the range from 5 to 240 µm will be available for user experiments.

Novosibirsk Free Electron Laser—Facility Description and Recent Experiments

The design and operational characteristics of the Novosibirsk free electron laser facility are described. Selected experiments in the terahertz range carried out recently at the user stations are surveyed in brief.

Imaging with a 90frames∕s microbolometer focal plane array and high-power terahertz free electron laser

An uncooled microbolometer focal plane array (FPA) has been developed and used for imaging of objects illuminated by monochromatic coherent radiation of a free electron laser tunable in the range of 1.25–2.5THz. A sensitivity threshold of 1.3×10−3W∕cm2 was obtained for the FPA with a homemade absolute interferometric power meter. Videos up to 90frames∕s were recorded in both transmission and reflection/scattering modes. When objects were illuminated by laser radiation scattered by a rough metal surface, speckled images were observed. Good quality terahertz images were achieved through the fast rotation of the scatterer.

Generation of Terahertz Surface Plasmon Polaritons Using Nondiffractive Bessel Beams with Orbital Angular Momentum.

Bessel vortex beams with topological charges of l=±1 and l=±2 were produced in the terahertz spectral range from a free electron laser Gaussian beam (λ=141  μm) transformed using silicon binary diffractive optical elements. The spatial characteristics of the beams were obtained using a microbolometer array. A radius to path length ratio of 1:100 was achieved for nondiffractive beams with the average power of 30 W. Surface plasmon polaritons (SPPs) on gold-zinc-sulphide-air interfaces were generated due to diffraction of vortex beams on a sample edge. A new effect, a dependence of the efficiency of SPP generation on the direction of the azimuthal component of incident-radiation Poynting vector, was revealed.

Classical Holography in the Terahertz Range: Recording and Reconstruction Techniques

In-line holography systems have been designed and investigated using Novosibirsk free electron laser as a source of monochromatic tunable terahertz (THz) radiation. A thermograph, a thermal sensitive phosphor plate, and a microbolometer array were applied to recording of holograms in the spectral ranges of 128-160 μm and 68 μm, including recording in real time. Specific features of these devices are described in detail. Five methods of image reconstruction are analyzed and verified using recorded holograms. Prospects for the use of THz holography are discussed in brief.

Microbolometer detector arrays for the infrared and terahertz ranges

This paper presents the results of the development of photodetectors for the IR and terahertz ranges, based on uncooled microbolometer detector arrays fabricated by the sol-gel technology. Detectors with a 320×240-element format are demonstrated, having a temperature resolution better than 100mK and a threshold power of 160pW/pixel in the 8-14-μm range, with a response time of 12-16ms. The threshold power at a wavelength of 130μm was 33nW/pixel for a terahertz image visualized in a 160×120 format at a rate of 90frame/sec. A design was developed to match an antenna with an uncooled high-resistance integrated microbolometer, allowing the detector sensitivity to be significantly increased in the terahertz range.

Visualization of radiation from a high-power terahertz free electron laser with a thermosensitive interferometer

A thermosensitive interferometer based on a plane-parallel glass plate is used for visualization of a high-power terahertz radiation. The plane wavefront of visible radiation emitted by a semiconductor laser is reflected from the two surfaces of the plate and forms on a screen an interference pattern recorded by a digital video camera. Terahertz radiation being measured is incident on the outer surface of the plate and heats a thin surface layer, which causes a shift of interference fringes. For K8 glass, a shift by one fringe corresponds to an absorbed energy of 5.1 J/cm2. The problem of determining the sign of the phase shift was solved by comparing the interference patterns with the images obtained with an infrared imager sensitive to near IR radiation. The processing of interference patterns makes it possible to determine the power density distribution over the beam cross section of the Novosibirsk free electron laser. In these measurements, the absolute value of the beam power determined by integrating over the cross section was 65 ± 7 W for a 130-μm wavelength. Visualization of the complex image with a spatial resolution no worse than 1 mm and a frame repetition rate of 25 Hz is demonstrated.

Diffraction of a surface wave on a conducting rectangular wedge

Diffraction of a surface wave on a rectangular wedge with impedance faces is studied using the Sommerfeld-Malyuzhinets technique. An analog of Landaus bypass rule in the theory of plasma waves is introduced for selection of a correct branch of the Sommerfeld integral, and the exact solution is given in terms of imaginary error function. The formula derived is valid both in the near-field and far-wave zones. It is shown that a diffracted surface wave is completely scattered into freely propagating electromagnetic waves and neither reflected nor transmitted surface waves are generated in case of bare metals which have positive real part of surface impedance. The scattered waves propagate predominantly at a grazing angle along the direction of propagation of the incident surface wave and mainly in the upper hemisphere regarding the wedge face. The profile of radiated intensity is nonmonotonic and does not resemble the surface wave profile which exponentially evanesces with the distance from the wedge face. Comparison with experiments carried out in the terahertz spectral range at Novosibirsk free electron laser has shown a good agreement of the theory and the experiments.

Status of the Novosibirsk high power free electron laser

The first stage of Novosibirsk high power free electron laser (FEL) was commissioned in 2003. It is based on normal conducting CW energy recovery linac. Now the FEL provides electromagnetic radiation in the wavelength range 120-180 micron. The average power is 100 W. The measured linewidth is 0.3%, which is close to the Fourier-transform limit. The assembly of user beamline is in progress. Plans of future developments are discussed.

Classic holography, tomography and speckle metrology using a high-power terahertz free electron laser and real-time image detectors

Experiments with in-line and two-beam holography as well as speckle photography and speckle interferometry using highpower monochromatic radiation of free electron laser have been carried out. An experimental-and-theoretical approach to tomography with monochromatic terahertz sources is presented. All measurements were carried out using real-time image sensors: a microbolometer focal plane array and a luminescence-quenching thermal image plate with intensified CCD camera.