Vibrational modes in an optically levitated droplet.

Levitation by optical tweezers provides a unique non-invasive tool for investigating a microscale object without external perturbations. Here we experimentally levitate a micrometer-sized water droplet in the air using an optical tweezer. Meanwhile, vibrational modes of a levitated water droplet are excited by modulating the trapping laser. From their backscattered light, vibrational modes with mode numbers are observed in the spectra. Additionally, their corresponding free spectral ranges are analyzed and compared with theory and numerical simulations. This Letter, establishing a non-invasive and all-optical detection technique of optomechanical properties of levitated droplets, paves the way for their practical applications in aerosol and biomedical science.