Linear to Circular Polarisation Conversion using Birefringent Properties of Aligned Crystals for Multi-GeV Photons

TL;DR

This paper demonstrates experimentally that birefringent aligned silicon crystals can convert high-energy linearly polarized photons into circularly polarized photons, confirming theoretical predictions and opening new avenues for photon polarization control.

Contribution

First experimental demonstration of using a thick aligned silicon crystal as a quarter wave plate to induce circular polarization in multi-GeV photon beams.

Findings

Measured circular polarization consistent with theoretical predictions.

Confirmed birefringence effect in high-energy photon beams.

Demonstrated polarization conversion using aligned silicon crystals.

Abstract

We present the first experimental results on the use of a thick aligned Si crystal acting as a quarter wave plate to induce a degree of circular polarisation in a high energy linearly polarised photon beam. The linearly polarised photon beam is produced from coherent bremsstrahlung radiation by 178 GeV unpolarised electrons incident on an aligned Si crystal, acting as a radiator. The linear polarisation of the photon beam is characterised by measuring the asymmetry in electron-positron pair production in a Ge crystal, for different crystal orientations. The Ge crystal therefore acts as an analyser. The birefringence phenomenon, which converts the linear polarisation to circular polarisation, is observed by letting the linearly polarised photons beam pass through a thick Si quarter wave plate crystal, and then measuring the asymmetry in electron-positron pair production again for a selection of relative angles between the crystallographic planes of the radiator, analyser and quarter wave plate. The systematics of the difference between the measured asymmetries with and without the quarter wave plate are predicted by theory to reveal an evolution in the Stokes parameters from which the appearance of a circularly polarised component in the photon beam can be demonstrated. The measured magnitude of the circularly polarised component was consistent with the theoretical predictions, and therefore is in indication of the existence of the birefringence effect.