Optimized back-focal-plane interferometry directly measures forces of optically trapped particles.

Area of Science:

• Optical physics
• Nanotechnology
• Biophysics

Background:

• Back-focal-plane interferometry offers high-resolution displacement measurements in optical traps.
• Its application for direct optical force measurement was historically limited to specific experimental setups.
• A key limitation was the requirement for accurate light momentum change detection, often needing counter-propagating beams.

Purpose of the Study:

• To experimentally extend the use of back-focal-plane interferometry for optical force measurements in single-beam optical traps.
• To demonstrate that the calibration product (trap stiffness κ times position sensitivity 1/β) directly relates detector signals to momentum changes.
• To show the robustness of this method even in non-ideal, non-optimized optical collection systems.

Main Methods:

• Experimental investigation of single-beam optical traps.
• Analysis of back-focal-plane interference pattern displacements.
• Calibration of detector signals against known optical forces and trap properties.

Main Results:

• Established a direct link between detector signals and momentum changes in single-beam traps, determined by instrument construction.
• Demonstrated that optical force measurements are not strictly limited to linear regimes and can be extended.
• Confirmed the method's effectiveness even with suboptimal light collection, broadening its applicability.

Conclusions:

• Back-focal-plane interferometry can be generally applied to measure forces in single-beam optical traps.
• The findings simplify force calibration and extend the technique's utility beyond previously known limitations.
• This work facilitates more widespread use of interferometry for precise force measurements in optical trapping.