Revisiting Newton's rings with a plasmonic optical flat for high-accuracy surface inspection

Area of Science:

• Plasmonics
• Nanophotonics
• Optical Metrology

Background:

• Interference of light between parallel optical surfaces creates colorful fringes, enabling observation of surface features.
• Traditional optical flats are used to detect subwavelength corrugations on reflective surfaces.
• Extending interference principles to plasmonic nanostructures offers new metrology possibilities.

Purpose of the Study:

• To adapt the optical flat principle for use with two-dimensional plasmonic nanostructures.
• To quantitatively measure optical couplings between gold nanodisk arrays and gold thin films.
• To demonstrate the potential of plasmonic nanostructures as optical flats for nanoscale imaging.

Main Methods:

• Utilized the classical Newton's rings method for interference pattern analysis.
• Employed a closed-loop nano-positioning system for precise distance control and measurement.
• Applied numerical fitting techniques to determine resonance wavelengths with high precision.

Main Results:

• Observed extremely high spectral sensitivity to inter-surface distance in the near-field coupling regime.
• Demonstrated that a 1-nm distance change altered the resonance wavelength by ~10 nm, over 40 times greater than without near-field coupling.
• Achieved a distance precision of 0.003 nm, derived from a resonance wavelength precision of 0.03 nm.

Conclusions:

• A plasmonic nanodisk array can function as a plasmonic optical flat.
• This plasmonic optical flat enables direct visualization of nanometer-deep grooves using a low-cost microscope.
• The study extends optical interference principles to nanostructure metrology with high precision.