Electronic system for modulating THz radiation of the Novosibirsk free electron laser: technical aspects and possible applications

The Novosibirsk free electron laser (FEL) is able to generate high-power tunable laser radiation in quasi-continuous mode in the terahertz (THz) frequency range. The ability to preserve the peak power of this radiation together with the possibility to control the radiation time is of great importance in a number of experiments. In this work an approach was developed and implemented that allows THz macropulses to be created at the Novosibirsk FEL with any repetition rate and almost any duration. The proposed electronic modulation system is based on a periodic shift of the phase of electron bunch injection. Such a shift suppresses lasing and forms macropulses. The system is embedded into the existing electronic infrastructure of the Novosibirsk FEL and can be triggered directly at the user endstations.

To characterise the system, macropulses of different duration in the range from 10 to 400 μs were measured for the three available frequency ranges of the FEL (mid-IR, far-IR, THz). Using the rising and falling edges of typical macropulses, the gain and total losses were calculated for the three optical resonators of the FEL. As an example of the use of short macropulses with a duration of 10–100 μs, experiments were carried out on the heating of a copper(II) complex single crystal by absorbed THz radiation and its subsequent thermal relaxation. The experiments were performed at the electron paramagnetic resonance spectroscopy endstation of the Novosibirsk FEL.

This work was supported by the Russian Science Foundation 17-13-01412.