Terahertz emission enters narrowband regime

Unfortunately, it has proved difficult to generate intense radiation in this regime because terahertz waves are too long for direct optical techniques but too short for electronic devices. Various indirect methods are now in use, such as firing a powerful laser beam at a nonlinear optical material, but these offer limited output powers.

Researchers have also experimented with producing terahertz radiation from synchrotron sources, where the nonlinear behaviour of relativistic electrons has been shown to generate coherent terahertz emission with an average power level of several tens of watts. These experiments have so far only produced broadband emission, but the French/Japanese team have now demonstrated that a laser-based technique can produce narrowband emission that can be tuned over a range of frequencies (Nature Physics doi:10.1038/nphys916).

"The set-up of spectroscopy studies is simpler with narrowband sources than with broadband emission, while the combination of narrowband, tunable and high-power emission offers some important advantages." Serge Bielawski of the Lille University for Science and Technology told optics.org. "When it is used a pump light, one can selectively excite a particular molecular excitation mode. When it is used as a probe light, there is no need for filters and so there is no degradation in intensity."

Synchrotron scheme
In the new scheme, a pulse from an 800 nm Ti:Sapphire laser is first sinusoidally modulated with a period in the picosecond range. This modulated pulse is then made to interact with an electron bunch produced by the UVSOR-II storage ring in Okazaki, Japan, which operates at 600 MeV and has a circumference of 53 m.

The effect of the laser pulse is to create a modulation in the energy distribution of the electrons. In the first stage of the experiment, the laser–electron interaction occurs in a periodic magnetic field that is tuned to the laser wavelength, which modulates the energy distribution of the electrons on the optical scale. Passing the electrons through a bending magnet then induces the charge-density modulation that is necessary for obtaining coherent terahertz emission.

"To achieve narrowband terahertz emission, the key point is that the emitted spectrum is the Fourier transform of the longitudinal charge density modulation," explained Bielawski. "That's why we decided to create laser pulses with a longitudinally sinusoidal modulation."

The frequency of emission can be varied by simply adjusting the delay in a Michelson interferometer. In this experiment, the emission could be tuned between about 0.2 and 1.0 THz, the the team plan to redo the experiment at higher frequencies by using another beamline and different beam parameters. According to Bielawski, implementing the scheme on more advanced synchrotron sources would extend the tunability range to the multi-THz regime.

Route to hugher energies
The experiments revealed strong tunable terahertz emission when the laser pulse was modulated with a period of 1-2 ps. Terahertz pulses were produced with energies of a few nanojoules and a brilliance in the nJ/cm range – which is comparable to the performance of commercial terahertz sources – but Bielawski says that straightforward improvements to the optics would yield much better figures.

"The present feasibility study will be followed by a research programme to optimize the terahertz emission," he said. "Increases by orders of magnitude are expected to be obtained by elementary optimizations of overlap, incident power and current density in the storage ring."

One simple improvement will be to increase the energy of the incident laser pulses from about 150 µJ in the current work to 10–20 mJ. Experiments in more powerful synchrotron sources are also planned. According to Bielawski, the technique could also be used to probe the instabilities that emerge in electron bunches, which often leads to spontaneous bursts of terahertz emission. "We are interested in trying to 'seed' these instabilities using modulated pulses, which would increase the amount of terahertz produced," he said.