The versatility of optical levitation

Abstract

The breadth of applications of optical tweezers in life sciences is so large, that it merited Arthur Ashkin a Nobel prize in 2018. Although optical levitation in air was invented first, it has not managed to be so widely adopted as optical tweezers. Here, I present 4 different applications to showcase the versatility of optical levitation. First, I show the forces acting on a particle through the construction of a fully manipulable damped driven harmonic oscillator. The transition from the over- to under-damped regimes is clearly recognizable and a big harmonic resonance can be observed by eye. Second, to measure the particle’s displacement in the previous experiment, we focused the scattered light with a lens. Here, I noticed an intriguing spider pattern that we fully explain by analyzing the spherical aberration of the lens. This resulted in a clear visualization and measurement of optical aberrations. Third, once we were free of aberrations, we were able to measure minute movements of the particle. This allowed us to create a modern version of the Millikan experiment where we add individual electrons to a single levitated droplet and observe the quantization of the electron by eye. Fourth, as the droplets evaporated we found a fascinating pattern in the scattering intensity. This turned out to be a directional Mie scattering spectrum with over 100 evolving Fano resonances arranged in a comb structure. We intuitively describe this spectrum through a quantum mechanical analogy to a spherical well potential. This converts our experiment into a “toy atom” where we see quantized angular momentum, ground and excited states, and tunneling. Through these 4 different applications, I provide an overview of optical levitation and its wide applicability