Quantitative assessment of radiation force effect at the dielectric air-liquid interface OPEN (original) (raw)

We induce nanometer-scale surface deformation by exploiting momentum conservation of the interaction between laser light and dielectric liquids. The effect of radiation force at the air-liquid interface is quantitatively assessed for fluids with different density, viscosity and surface tension. The imparted pressure on the liquids by continuous or pulsed laser light excitation is fully described by the Helmholtz electromagnetic force density. The correct form of the momentum of light within dielectric materials and the effects caused by the radiation forces when light passes through adjacent media have been extensively debated for over a century 1–9. Although the radiation pressure effects were predicted in 1871 10 , and experimentally observed in 1900 11 , a dilemma was created by controversial interpretations of the theories proposed by Minkowski in 1908 12 and Abraham in 1909 13 to explain the energy-momentum tensor of light. The history of the Abraham-Minkowski dilemma is intimately linked to the difficulties in sensing and interpreting the effects produced by radiation forces, which led to erroneous interpretations favoring one of the theories. This is based on the fact that both momentum descriptions have simple forms when light is incident from free space on a transparent and non-dispersive dielectric medium; Minkowski predicts a momentum in the medium proportional to its refractive index (n) and the photon momentum in the vacuum () p 0 as = p np M 0 , while Abraham predictions is in the form = / p p n A 0. = / p U c 0 , U is the energy of light and c is the speed of light. The Minkowski-Abraham controversy has theoretically been resolved by the correct division of momentum between field and medium 1. Early experimental investigations pursued answers to the dilemma, and continued to shed light on to this controversy. A number of reviews discuss these early experiments in details 1–9 , although the conclusions derived favor either theory. For instance, Jones and coauthors 14 showed that a mirror submerged in a medium experiences a force consistent with each photon having the Minkowski momentum. Ashkin and Dziedzic 15 demonstrated that focused laser pulses created deformations of the water-air interface; the surface of the liquid experienced a net force outward from the water as predicted by Minkowski. Although, it was later assessed that the bulging of the liquid was also influenced by radial electrostriction forces 8,16. Walker and coauthors 17 measured the torque exerted on a disk suspended on a torsion pendulum. The experiments provide evidence in favor of the Abraham form. Zhang and coauthors 18 performed experiments based on Ashkin and Dziedzic 15 scheme. They show the interplay between Minkowski and Abraham forces illuminating water or mineral oil. On initial inspection, experimental results may appear to be in favor of one of the formulations. However, detailed analysis demonstrates explicitly and directly the equivalence of a number of different energy momentum tensors, provided the accompanying material tensor is taken into account 1,6. Yet there has been so far only limited qualitative experimental tests of our understanding of radiative transfer between electromagnetic radiation and dielectric media. Quantitative measurements of the effects of radiation forces on dielectric media have attracted large interest with the advent of optical manipulation of micro-particles in fluid media and its potential application in biological systems. Recently, Astrath and coauthors 19 measured surface deformation at the interface air-water generated by continuous and pulsed laser excitations using the photomechanical mirror (PM) method. The displacement caused by radiation forces was quantitatively described by the theory using the Helmholtz force density. The former