First Electro-Optical Detection of Charged Particles

We have made the first observation of a charged particle beam by means of its electro-optical effect on the polarization of laser light in a birefringent crystal. The modulation of the laser light during the passage of a pulsed electron beam was observed using a fast photodiode and a digital oscilloscope. The fastest rise time measured in a single shot, 120 ps, was limited by the bandwidth of the oscilloscope and the associated electronics. This technology holds promise for detectors of greatly improved spatial and temporal resolution for single relativistic charged particles as well as particle beams. An effort has been initiated to develop an ultra-fast charged particle detector based on the birefringence, and hence the phase difference induced between orthogonal components of polarization (ellipticity), in an optical medium by the electric field of a relativistic charged particle [1]. The electro-optical effect in amorphous optical media is known as the Kerr effect [2] and is quadratic in the electric field E. In uniaxial crystals the induced ellipticity is linear in the E-field and is known as the Pockels effect [3]. The induced phase delay can then be given by φ = π(V/Vπ) with V the applied voltage and Vπ the voltage required for producing a phase shift between orthogonal components of polarization equal to π radians or an ellipticity of π/ √ 2 since the maximum ellipticity is induced when the polarization is at 45◦ with respect to the applied electric field direction. The passage of a tightly bunched relativistic electron beam within several millimeters of a commercially available LiNbO3 crystal [4] coupled to polarization maintaining fibers of 4 microns diameter propagating polarized infrared light (λ = 1.32μm) was observed by the effect of a microbunched electron beam on the polarization of the light in the crystal. This was determined by means of a λ/4 plate which converted the † Supported in part by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886. induced ellipticity to a rotation of the initial linear polarization which had previously been nearly extinguished by the analyzer (crossed polarizer). The resulting modulation of the transmitted laser light was detected by a photodiode of 45 GHz bandwidth and pre-amplifier. The output signals were digitized in a 7 GHz sampling oscilloscope and stored in memory. The optics setup is indicated in Fig.1 A bunched charged particle beam creates an electric field, E, at a distance r that is well approximated by Coulomb’s Law multiplied by γ, the relativistic Lorentz factor, andNe the number of particles in the beam bunch for a minimum distance r0 ≫ than the dimensions of the beam bunch. The LiNbO3 crystal had Vπ = 5 V with an electrode separation of 15μm and a length of l = 1.5 cm. The integral ∫