Solving Maxwell’s equations in cylindrical coordinates yields states in quantum theory with definite values of energy ℏω, longitudinal momentum ℏ kz , and total angular momentum projection ℏ m on the z -axis (where ℏ is the Planck constant). Experimentally, values of up to ℏ m Ȉ 104 ℏ have been obtained for the last quantity. The wave front of such states is like a meat grinder screw, with the lines of force of the Poynting vector representing the screw line. Such states differ from plane waves by the nonzero orbital angular momentum projection on the direction of motion, and from spherical waves, by the definite direction of motion. For brevity, these states are referred to as ‘twisted photons’. In this paper, recent experimental and theoretical results on twisted photons are reviewed, to which the present authors actively contributed. Detailed discussion is given to recent experiments on the production of high-intensity beams of terahertz (wavelength: 140 μm) twisted photons performed on the Novosibirsk free electron laser at the Budker Institute of Nuclear Physics, SB RAS. Recent theoretical work on the interaction of twisted photons with atoms is summarized. Due to their extra degree of freedom — the projection of the total angular momentum on the direction of motion — twisted photons represent a novel research tool of potentially wide application in physics.

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.

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