Tunable, narrowband THz​

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In addition to high harmonics, the EEHG beamline can also be used to generate narrowband THz density modulations in the electron beam. By ​first modulating the beam energy distribution with two optical lasers with different wavelengths, the final chicane converts the resulting beat wave into a density modulation at the difference frequency of the laser harmonics. This scheme is analogous to the well-known difference frequency generation technique which has been widely used in the laser community for generation of narrow-band THz radiation. Here, however, the relativistic electron beam acts as the nonlinear medium. 

At NLCTA we recently demonstrated this concept​, observing a THz modulation in the beam by using the transverse cavity to streak the beam in time. Furthermore, we show that the central frequency of the density modulation can be easily tuned by varying the laser frequencies, energy chirp of the electron beam, or momentum compaction of the chicanes. We anticipate that this demonstrated technique will have wide applications in many accelerator-based light sources.