Nanogap-Induced Phase Control Reveals the Momentum of Light Inside the Dielectric Medium

The deflection of a submerged mirror due to light pressure was used to compare competing theories of light momentum inside a dielectric medium. In this case, a significant bottleneck is to find a mirror that reflects light with zero phase shift without requiring multiple sets of metamaterial mirrors, as conventional mirrors reflect light with a 180° phase shift, demonstrating (formally, as we shall see) the Minkowski momentum. Introducing a nanometric gap between the mirror and the convex lens can vary the phase angle from 0 to 180°, covering the momentum range between the Abraham and Minkowski values (2ℏω₀/nc, 2nℏω₀/c). Our study used interferometry to measure the deflection of a submerged, partially metallic-coated vertical cantilever caused by radiation pressure with nanometric precision. Our results showed that light momentum within a dielectric follows Minkowski’s form (2nℏω₀/c) for conventional mirrors. However, with a nanogap between the convex lens and a vertically suspended fiber, the momentum transferred to a submerged mirror varied from 2ℏω₀/nc to 2nℏω₀/c, depending on the mirror’s phase angle. This approach takes an intriguing step illustrating the rivaling theory of light momentum in a medium: numerical simulations based upon the formula derived by [Mansuripur, M. Phys. Rev. A 2012, 85, 023807]agree with our experimental results. These basic results imply promising applications in microfluidics and optofluidics.