Optical microscopy and optical forces

Introduction

Light carries not only energy, but momentum and angular momentum. That is, it is able to push and twirl objects. For example, light reflecting off a solar sail exerts a small but consistent force on it, allowing tiny payloads to be accelerated by incident solar or laser radiation. Unfortunately, the forces associated with light are extremely small, requiring either large intensities or short distances in order for one to see a significant effect. In this research section, we perform precision measurements of the properties and forces exerted by light using tools that operate at the edge of what is physically possible. 

 

Scanning near-field optical microscopy (SNOM)

Conventional optical microscopy cannot overcome the diffraction limit, meaning that it is not possible to distinguish features smaller than a fraction of the wavelength. But by probing the near fields close to the sample surface using SNOM it is possible to beat this limit and explore phenomena that cannot be studied otherwise. Here we study the propagation of plasmons and guided waves in 2D materials to characterize their anisotropic optical properties.

SNOM

hBn resonator

Measurement of femtonewton-scale optical forces

Optical forces

It was predicted in recent years that a non-chiral particle in an evanescent optical field (such as one generated by total internal reflection) is able to acquire linear momentum perpendicular to the plane of incidence from the spin component of the incoming light.  When the helicity of the light is flipped, this momentum changes direction.  This particular kind of spin-momentum coupling has attracted keen interest in the physics community, due to a fundamental connection with the Belinfante–Rosenfeld energy-momentum tensor which emerged in quantum field theory in 1940. Many theoretical papers have been published exploring the interaction between this momentum with matter, yet only a qualitative confirmation of the existence of the resulting force has been thus far experimentally reported in the lab.

A true test of the theory is an experimental challenge as the spin-momentum force does not act in isolation.  Rather, it is merely a weak component of a 3D force vector whose magnitude is typically more than one hundred times larger in the other two directions.  More generally, all three components of this force vector are predicted to change as the incident light’s pure polarization state is altered.  Thus, measuring this spin-momentum force in context, that is, measuring both the magnitude and the direction of the composite vector force, using a probe whose geometry can be analytically modeled, is the only way to quantitatively determine the magnitude of the spin momentum force and unambiguously test the theory.

We have built a floating-probe force microscope using a micron-sized polystyrene sphere held in an optical tweezer.  The instrument is capable of femtonewton force sensitivity and piconewton range with simultaneous, time-synchronized position readouts in the three orthogonal directions.  This work represents a significant advancement in the systematic study of exotic optical forces.  The volume-scanning capabilities of the force microscope can enable, among other applications, the 3D mapping of a vector force field with high resolution and dynamic range.  As such, our results can be of interest to researchers in other fields studying microscopic interactions.