Stimulated Excitation of an Optical Cavity by a Multibunch Electron Beam via Coherent-Diffraction-Radiation Process

With a low emittance and short-bunch electron beam at a high repetition rate realized by a superconducting linac, stimulated excitation of an optical cavity at the terahertz spectrum range is shown. The electron beam passes through small holes in the cavity mirrors without being destroyed. A sharp resonance structure which indicates wideband stimulated emission via coherent diffraction radiation is observed while scanning the round-trip length of the cavity.

Figure 1

Layout of the system. The beam passes through the center holes of the optical cavity.

Figure 2

Simulation of the cavity length scan. Excited power in the cavity is plotted as a function of cavity length variation with respect to the perfect synchronization condition. The wideband case refers to sum of the signals for all longitudinal modes. The narrow-band case refers to the weighted sum of the longitudinal modes, assuming the effect of a realistic bandpass filter with 0.5 THz center frequency and 10% bandwidth (full width at half maximum) at the detector.

Figure 3

The experimental layout. The radiation emitted toward the upstream direction was measured. A bolometer was placed at the focal point.

Figure 4

(Left panel) Signal strength of the loss monitor as a function of beam position at the cavity. Note that the loss monitor was saturated when the beam hit the cavity. (Right panel) A comparison of three cases was used to estimate the beam loss fraction. In this measurement, the gain of the loss monitor was reduced to avoid saturation.

Figure 5

Cavity length scan results. (Top panel) Wide range scans in $5\mu \mathrm{m}$ increments. Since the peak was not correctly sampled in the rough step, the actual peak heights are shown with arrows. In the narrow-band data, small bumps are seen at around $-0.3$ and $+0.3\mathrm{mm}$ of cavity length variation. (Bottom panel) Fine scans at the peaks in 50 nm increments.

Figure 6

Long-term stability measurement. The sharp resonance was maintained for 23 min.