Role of a Quarter-Wave Plate in Confocal Microscopy: Signature of Spin-Orbit Interactions

TL;DR

This study demonstrates how a quarter-wave plate in a confocal microscopy setup can induce spin-orbit interactions that reshape the beam's transverse mode, enabling polarization-controlled spatial mode engineering.

Contribution

It reveals a novel spin-orbit interaction effect in confocal systems and introduces a minimal Jones matrix model to predict mode transformations.

Findings

Polarization extinction ratio increased by over two orders of magnitude.

Gaussian beam transformed into a Hermite-Gaussian-like two-lobe mode.

Pattern orientation tunable via wave plate rotation.

Abstract

Spin-orbit interactions of light couple polarization and spatial degrees of freedom, underpinning phenomena such as the spin Hall effect of light. Although widely explored at interfaces and in tightly focused beams, their impact in nominally paraxial confocal systems remains largely unexamined. Here we show that a single quarter-wave plate embedded in a simple confocal geometry between polarizers can strongly reshape the transverse structure of a Gaussian beam. We observe an enhancement of the polarization extinction ratio by more than two orders of magnitude, accompanied by a transformation of the Gaussian intensity profile into a first-order Hermite-Gaussian-like two-lobe mode. The orientation of this pattern is continuously tunable via rotation of the wave plate, evidencing polarization-controlled reorientation of the transverse field. To explain these observations, we introduce a minimal extension of Jones matrix formalism incorporating complex parameters that quantitatively reproduces the measurements. Our results uncover a previously overlooked form of spin-orbit-mediated mode control in standard confocal optics and establish a simple route to on-demand spatial mode engineering for applications in resonant spectroscopy, optical imaging and quantum optics.