Sub-nanometer height sensitivity by phase shifting interference microscopy under environmental fluctuations.

Area of Science:

• Optical Physics
• Metrology
• Biomedical Imaging

Background:

• Phase Shifting Interferometry (PSI) offers high sensitivity for phase measurement.
• Environmental instability and uncalibrated equipment (piezoelectric transducer - PZT) limit PSI accuracy.
• Accurate phase mapping requires stable environments and precisely calibrated PZTs.

Purpose of the Study:

• To develop an inverse approach for accurate phase map retrieval in PSI under environmental fluctuations.
• To overcome the limitations of environmental instability and uncalibrated PZTs in PSI.
• To demonstrate the robustness and potential of the proposed method for quantitative phase imaging.

Main Methods:

• Recording continuous, temporally phase-shifted interferograms.
• Calculating phase shifts between frames using a Fourier transform algorithm (accuracy ≤ 5.5 × 10⁻⁴ π rad).
• Implementing an inverse approach without requiring a calibrated PZT or highly stable environment.

Main Results:

• The inverse approach successfully retrieves accurate phase maps under environmental fluctuations (vibrations, air turbulence).
• Quantitative phase imaging of optical waveguides and red blood cells was achieved with high accuracy, comparable to calibrated PZT results.
• Demonstrated height sensitivity of 4 Å for a 2 nm shallow rib waveguide using a 12-bit CMOS camera without PZT.

Conclusions:

• The proposed inverse PSI method provides accurate phase measurements robust to environmental instability.
• This approach eliminates the need for precisely calibrated PZTs and stable environments, simplifying PSI implementation.
• The method holds significant potential for high-sensitivity quantitative phase imaging in various scientific and biomedical applications.