Sensitivity improvement by optically-absorbent plastics of electro-optical probes for high-intensity electromagnetic-fields generated by laser-matter interaction

The characterization of intense radiofrequency-microwave electromagnetic pulses (EMPs) generated by the interaction of powerful lasers with matter is a very hot topic of research, for both inertial-confinement-fusion and laser-plasma acceleration facilities spread around the world. These fields are often detrimental for the operation and even the survival of active diagnostics placed around the interaction region, but they might be also used as a plasma diagnostics or for application to studies on materials and devices in these extreme-field conditions. The reliable measurement of these intense fields by classical conductive probes is a well-known problem and for this reason fully electro-optical (EO) probes have been recently developed and proved to be a very interesting solution even at petawatt laser-pulse regimes. Indeed, in some experiments performed with the nanosecond ABC laser we found that part of the diffused main laser-pulse may occasionally be coupled with the optical path of the probing CW laser within the crystal of the electro-optical probe, and lead to important reductions of the detector sensitivity. To resolve this problem we investigated here, by two experimental campaigns, the use of thick dielectric shields for the crystal, to be highly absorbing for the main laser wavelength and in general for visible radiation, and at the same time highly transparent to the wide radiofrequency-microwave range of the EMP fields to be measured. As result of this study we found that Ertalon 6XAU is an excellent material for these purposes, and thus we carefully designed and realized a suitable shielding for the EO probes made of this material.