Conductivity Induced by High-Field Terahertz Waves in Dielectric Material

An intense, subpicosecond, relativistic electron beam traversing a dielectric-lined waveguide generates very large amplitude electric fields at terahertz (THz) frequencies through the wakefield mechanism. In recent work employing this technique to accelerate charged particles, the generation of high-power, narrow-band THz radiation was demonstrated. The radiated waves contain fields with measured amplitude exceeding $2\mathrm{GV}/\mathrm{m}$, orders of magnitude greater than those available by other THz generation techniques at a narrow bandwidth. For fields approaching the $\mathrm{GV}/\mathrm{m}$ level, a strong damping has been observed in ${\mathrm{SiO}}_2$. This wave attenuation with an onset near $850\mathrm{MV}/\mathrm{m}$ is consistent with changes to the conductivity of the dielectric lining and is characterized by a distinctive latching mechanism that is reversible on longer timescales. We describe the detailed measurements that serve to clarify the underlying physical mechanisms leading to strong field-induced damping of THz radiation ($h\omega =1.59\mathrm{meV}$, $f=0.38\mathrm{THz}$) in ${\mathrm{SiO}}_2$, a bulk, wide band-gap (8.9 eV) dielectric.

Figure 1

An electron beam, shown in blue, traverses the dielectric-lined structure in the central vacuum region. The electron beam couples to supported modes in the structure, exciting a wakefield (shown in black). The THz wakefield is outcoupled to free space (shown in red), collimated by an off-axis parabolic mirror, and transported to a THz interferometer for spectral measurements.

Figure 2

(a) Autocorrelation data of the CCR signal normalized to its maximum value, demonstrating a loss of oscillation amplitude away from zero delay for a higher charge. (b) Spectra generated via fast-Fourier transform of data from (a) show a shift in the central frequency for a higher field. Dashed lines correspond to Lorentzian fits.

Figure 3

(a) The loss parameter $\alpha$ and (b) central frequency of the excited ${\mathrm{TM}}_{01}$ mode as a function of the field amplitude. Black dashed lines represent a polynomial fit to the data. Error bars represent 95% confidence interval.