Optical trapping of mesoscale particles and atoms in hollow-core optical fibers: principle and applications.

Hollow-core fiber (HCF) is a special optical waveguide type that can guide light in the air or liquid core surrounded by properly designed cladding structures. The guiding modes of the fiber can generate sufficient optical gradient forces to balance the gravity of the particles or confine the atom clouds, forming a stable optical trap in the hollow core. The levitated objects can be propelled over the fiber length along the beam axis through an imbalance of the optical scattering forces or by forming an optical lattice by the counter-propagating beams. The ability to overcome the diffraction of the laser beam in HCF can significantly increase the range of the optical manipulation compared with standard free-space optical tweezers, opening up vast ranges of applications that require long-distance optical control. Since the first demonstration of optical trapping in HCF, hollow-core-fiber-based optical trap (HCF-OT) has become an essential branch of optical tweezer that draws intense research interests. Fast progress on the fundamental principle and applied aspects of HCF-OT has been visible over the past two decades. In recent years, significant milestones in reducing the propagation loss of HCF have been achieved, making HCF an attractive topic in the field of optics and photonics. This further promotes the research and applications of HCF-OT. This review starts from the mechanism of light guidance of HCF, mainly focusing on the issues related to the optical trap in the hollow core. The basic principles and key features of HCF-OT, from optical levitation to manipulation and the detection of macroscopic particles and atoms, are summarized in detail. The key applications of HCF-OT, the challenges and future directions of the technique are also discussed.