A stokes polarimetric light microscopy view of liquid crystal droplets

J Gou,P Bao,S D Evans,T H Shen,J L Ramos Angulo

doi:10.1038/s41598-021-95674-4

Abstract

The optical characteristics of materials, such as their magnetooptical effects, birefringence, optical activities, linear and circular dichroism, are probed via the polarisation states of light transmitted through or reflected from the specimens. As such, the measurements of the polarisation states play an important role in many research disciplines. Experimentally, Stokes parameters provide a full description of the polarisation states of light. We report the implementation of a dual- photoelastic modulator based polarimeter in a light microscope, enabling the determination of Stokes parameters at each pixel. As a case study, polarimetric images of liquid crystal droplets of different internal structures are obtained, showing their distinct polarisation characteristics. We demonstrate that the prototype Stokes polarimetric microscope allows the quantitative determination of the polarisation characteristics of light at the object plane and enables the access of the information of full polarisation states as compared to a conventional cross polariser microscope. This work shows that Stokes polarimetric microscopy may find potential applications in a wide range of research fields.

Highlights

The optical characteristics of materials, such as their magnetooptical effects, birefringence, optical activities, linear and circular dichroism, are probed via the polarisation states of light transmitted through or reflected from the specimens
Speaking, accessing to the polarisation information of light can be achieved through either a combination of a retardation plate and a polariser at various angular settings or with modulators. For the former, by introducing polarisers and retardation plates to a light microscope, such a polarised light microscope enables a wealth of applications in materials research, for instance in micrometry, crystal morphology and dispersion staining[6] For the latter, variable liquid crystal retarders have been deployed in Mueller matrix polarisation microscope[7] and in ‘PolScope’ which is usually calibrated for birefringence measurements[8]
Unlike the approach where specially masked CCD detector with three out of four rows covered for the demodulation of the s ignals[11,32], in this work we report a dual photoelastic modulator (PEM) polarimeter based prototype of Stokes polarimetric microscope, implemented using a commercially available digital camera in conjunction with a purpose-built demodulation timing unit, enabling the lock-in signal recovery at each pixel

Summary

Introduction

The optical characteristics of materials, such as their magnetooptical effects, birefringence, optical activities, linear and circular dichroism, are probed via the polarisation states of light transmitted through or reflected from the specimens. Speaking, accessing to the polarisation information of light can be achieved through either a combination of a retardation plate and a polariser at various angular settings or with modulators For the former, by introducing polarisers and retardation plates to a light microscope, such a polarised light microscope enables a wealth of applications in materials research, for instance in micrometry, crystal morphology and dispersion staining (a particle identification technique based on the difference between the refractive index of a particle and that of the liquid medium in which the particle is immersed)[6] For the latter, variable liquid crystal retarders have been deployed in Mueller matrix polarisation microscope[7] and in ‘PolScope’ which is usually calibrated for birefringence measurements[8]. Several methods have been proposed to measure the Stokes parameters, for instance, with a rotating retarder[21,22], by the division-of-amplitude[23] and, in Stokes polarimetric imaging, with the division-of-aperture using Wollaston prism a rray[24], the division-of-focal-plane with an array of groups of micro polarisers and retarders in front of the digital sensor[25,26,27] and with matrix Fourier optics[28]

Methods

Results

Conclusion