Narrow Bandwidth Gamma Comb from Nonlinear Compton Scattering Using the Polarization Gating Technique

Nonlinear Compton scattering is a promising source of bright gamma rays. Using readily available intense laser pulses to scatter off the energetic electrons, on the one hand, allows us to significantly increase the total photon yield, but on the other hand, leads to a dramatic spectral broadening of the fundamental emission line as well as its harmonics due to the laser pulse shape induced ponderomotive effects. In this Letter we propose to avoid ponderomotive broadening in harmonics by using the polarization gating technique—a well-known method to construct a laser pulse with temporally varying polarization. We show that by restricting harmonic emission only to the region near the peak of the pulse, where the polarization is linear, it is possible to generate a bright narrow bandwidth comb in the gamma region.

Figure 1

(Top) Vector potential and ellipticity of a polarization gated pulse for the optimal delay $\delta =\tau$. (Bottom) Corresponding backscattered spectrum. Vector potential and intensity are normalized. Laser pulse parameters: $a_0=3$, $\tau =8\pi$.

Figure 2

Radiation angular spectrum in the Lab frame for the optimal delay parameter $\delta =\tau$, $\gamma =529$. (Main) From one electron, $a_0=3$, $\tau =8\pi$. (Corner) From a realistic electron beam, $a_0=2$, $\tau =30\pi$.

Figure 3

Normalized backscattered harmonics spectrum of an optimal polarization gated pulse for different pulse lengths: $6\pi$, $12\pi$; and strengths $a_0=1.5$, 3. Red arrow shows the distance between the 31st and 33rd harmonic.

Figure 4

The differential number of photons in the gamma region scattered from a realistic electron beam (${10}^8$ electrons represented by 4800 macroparticles, $\gamma =529$). Laser pulse parameters: $a_0=2$, $\tau =30\pi$.