Trapping force and optical lifting under focused evanescent wave illumination

Area of Science:

• Optics and Photonics
• Microparticle Manipulation
• Nanotechnology

Background:

• Optical trapping commonly utilizes far-field illumination.
• Evanescent waves, generated by total internal reflection, offer unique near-field interaction possibilities.
• Understanding forces in evanescent fields is crucial for advanced microparticle manipulation.

Purpose of the Study:

• To develop a physical model for calculating trapping forces on dielectric micro-particles under focused evanescent wave illumination.
• To validate the model against experimental measurements.
• To explore the relationship between particle size and trapping force in evanescent fields.

Main Methods:

• Development of a vectorial diffraction model for high numerical aperture objectives under total internal reflection.
• Calculation of trapping forces for both plane wave (TEM00) and doughnut beam (TEM*01) illumination.
• Comparison of calculated trapping forces with experimental data.

Main Results:

• The physical model accurately calculates trapping forces in a focused evanescent spot, showing agreement with measured results.
• Optical axial trapping force in an evanescent focal spot increases linearly with the trapped particle's size.
• The model predicts the possibility of overcoming gravity to lift polystyrene particles up to 800 nm in radius using 10 μW laser power.

Conclusions:

• The presented physical model provides a robust framework for understanding optical trapping forces in evanescent fields.
• The linear dependence of axial trapping force on particle size in evanescent fields is a key finding.
• This research demonstrates the potential of evanescent wave optical trapping for manipulating micro-particles against gravity.