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

The\ndeflection of a submerged mirror due to light pressure was\nused to compare competing theories of light momentum inside a dielectric\nmedium. In this case, a significant bottleneck is to find a mirror\nthat reflects light with zero phase shift without requiring multiple\nsets of metamaterial mirrors, as conventional mirrors reflect light\nwith a 180° phase shift, demonstrating (formally, as we shall\nsee) the Minkowski momentum. Introducing a nanometric gap between\nthe mirror and the convex lens can vary the phase angle from 0 to\n180°, covering the momentum range between the Abraham and Minkowski\nvalues (2ℏω<sub>0</sub>/<i>nc</i>, 2<i>n</i>ℏω<sub>0</sub>/<i>c</i>). Our study\nused interferometry to measure the deflection of a submerged, partially\nmetallic-coated vertical cantilever caused by radiation pressure with\nnanometric precision. Our results showed that light momentum within\na dielectric follows Minkowski’s form (2<i>n</i>ℏω<sub>0</sub>/<i>c</i>) for conventional mirrors. However, with\na nanogap between the convex lens and a vertically suspended fiber,\nthe momentum transferred to a submerged mirror varied from 2ℏω<sub>0</sub>/<i>nc</i> to 2<i>n</i>ℏω<sub>0</sub>/<i>c</i>, depending on the mirror’s phase\nangle. This approach takes an intriguing step illustrating the rivaling\ntheory of light momentum in a medium: numerical simulations based\nupon the formula derived by [Mansuripur, M. Phys. Rev. A 2012, 85, 023807]agree\nwith our experimental results. These basic results imply promising\napplications in microfluidics and optofluidics.