Generating periodic terahertz structures in a relativistic electron beam through frequency down-conversion of optical lasers

We report generation of density modulation at terahertz (THz) frequencies in a relativistic electron beam through laser modulation of the beam longitudinal phase space. We show that by modulating the energy distribution of the beam with two lasers, density modulation at the difference frequency of the two lasers can be generated after the beam passes through a chicane. In this experiment, density modulation around 10 THz was generated by down-converting the frequencies of an 800 nm laser and a 1550 nm laser. The central frequency of the density modulation can be tuned by varying the laser wavelengths, beam energy chirp, or momentum compaction of the chicane. This technique can be applied to accelerator-based light sources for generation of coherent THz radiation and marks a significant advance toward tunable narrow band THz sources.

Figure 1

Schematic layout of the beam line for generation of THz density modulation in a relativistic electron beam through laser modulation at SLAC’s NLCTA [25, 26].

Figure 2

(a) Electron beam THz bunching for various laser energy modulations; (b) Simulated beam density distribution for $\Delta E_2=50\mathrm{keV}$ (solid blue) and $\Delta E_2=140\mathrm{keV}$ (dashed red) with $\Delta E_1=40\mathrm{keV}$. The initial beam distribution without laser modulations is shown in bold green.

Figure 3

Raw images of beam transverse profile measured at YAG1 with lasers off (a) and with 1550 nm laser on (b); (c) Projected beam energy distribution with lasers off (blue solid line) and with 1550 nm laser on (red dashed line).

Figure 4

Raw images of beam density distribution (only part of the beam distribution is shown for purpose of visualization) measured with TCAV with the lasers off (a) and lasers on (b); Projected beam current distribution (c) and the corresponding spectrum (d).

Figure 5

Central frequency of density modulation for various momentum compactions of chicane $C1$.