Optical Traction Based On Photonic Crystal Structures

Optical force originates from the exchange of momentum in the interaction of light and matter,and the use of optical force can achieve non-contact manipulation of objects.Ashkin shared the 2018 Nobel Prize in Physics for his contribution in the Optical tweezers and promote the application of this technology in the field of biology.Optical traction is a research hotspot in the field of optical force and optical manipulation in recent years.Exploring this effect can provide a new dimension for optical manipulation and investigate the physical connotation of light and matter interaction.In the past,most of the research on optical traction was carried out in a uniform background medium,but this paper focus on the background of photonic crystal.The paper proposes two new mechanisms for realizing optical traction,and establishes three photonic crystal models for verification by using FDTD(Finite-Difference Time-Domain).Firstly,it is proposed to realize optical traction by using a background structure that supports concave dispersion.Establishing a photonic crystal structure that satisfies this special dispersion relationship,the momentum of the light are related to its propagation direction.Specifically,the momentum of the light propagating in the direction of the main axis is the smallest,and the momentum increases sharply when it deviates from the direction of the main axis.Based on this physical mechanism,a scatterer is placed on the path of the propagating beam along the main axis,so that the beam is scattered to both sides and the backward reflection is suppressed,as a result,the amount of scattered light momentum increases along the direction of light propagation.According to the the linear momentum conservation law,the object will be subjected to a force directed to the light source.Secondly,another method is utilizing non-diffractive propagation mode which supported by photonic crystal channel,including Self-collimation mode and Line defect waveguide mode here.Optical pulling force generated by this method due to the local light field gradient inside the dielectric object.The optical gradient force causes the object to move toward the strongest part of the light field,and the position of the strongest light field intensity induced by the object is always in front of the object,which creates continuous negative force.According to Bloch's theorem,it can be understood that this force is periodically.It is worth noting that the realization of both modes is independent of the incident light angle and the object will be subjected to the restoring force at the unbalanced position to drive the object back to the equilibrium position,which provides conditions for long-distance stable manipulation of the object.In summary,this work proposes two theories based on different physical mechanisms to achieve optical traction,and has been verified in three different photonic crystal structures,which provides new theory for achiecing optical pulling force.Finally,this work can help to study the momentum transformation relationship between light,object and background media,which provide a new degree of freedom for modulating optical force and explore the the physical connation of the interaction between light.