Chiral longitudinal forces in dielectric photonic waveguides

¹ Nanophotonics Technology Center, Universitat Politècnica de València, Camino de Vera, s/n Building 8F, 46022, Valencia, Spain
² Department of Physics, King's College London, Strand, WC2R 2LS, London, United Kingdom

^* Email: francisco.rodriguez_fortuno@kcl.ac.uk
^** Email: amartinez@ntc.upv.es

Published online: 31 October 2024

Abstract

Chiral optical forces exhibit opposite signs for the two enantiomeric versions of a chiral molecule or particle. If large enough, these forces might be able to separate enantiomers all-optically, which would find numerous applications in different fields, from pharmacology to chemistry. Longitudinal chiral forces are especially promising for tackling the challenging scenario of separating particles of realistically small chiralities. In this work, we study the longitudinal chiral forces arising in dielectric integrated waveguides when the quasi-TE and quasi-TM modes are combined, for enabling the separation of absorbing and non-absorbing chiral particles. We show that chiral gradient forces dominate in the scenario of beating of non-degenerate TE and TM modes when considering non-absorbing particles. For absorbing particles, the superposition of degenerate TE and TM modes can lead to radiation pressure chiral forces that are kept along the whole waveguide length. We accompany the calculations of the forces with particle tracking simulations for one specific radius and chirality parameter and show that longitudinal forces can separate chiral nanoparticles suspended in water even for relatively low values of the particle chirality.