Optical manipulation of semiconductor quantum dots in superfluid helium

Abstract

We have experimentally demonstrated purely optical manipulation of wide-gap semiconductor CuCl quantum dots in superfluid helium. The superfluidity provides an ideal cryogenic frictionless environment for the manipulation. In order to introduce the quantum dots into liquid helium, small particles of CuCl with a broad size-distribution ranging from 10 nm to 10 mm in radius have been fabricated from a bulk sample by laser ablation in a helium cryostat. We irradiated these particles with laser light covering the excitonic resonance levels of the quantum dots smaller than 50 nm to push them by using resonant radiation force. As a result, we have found that many quantum dots of which sizes range from 10 to 50 nm were transported and sorted over a macroscopic distance, ~1 cm. Importantly, the excitonic resonance condition was crucial for this optical manipulation. The result means that the resonant radiation force for the quantum dots is much stronger than the gravitational force. Feasibility of size-selective manipulation is also discussed.