Pseudo-heterodyne (PsHet) interferometric detection in s-SNOM

Martin Schnell – CIC nanoGUNE BRTA, San Sebastián
key learning objectives:

Understanding SNOM and its Challenges: SNOM is likened to a confocal microscope where a tip is placed in the focus, but unlike fluorescence or Raman spectroscopy, SNOM operates with elastic light scattering, meaning the signal frequency is unchanged upon scattering. The unique challenges in SNOM include the need to demodulate the signal to isolate the weak near-field scattering from the much stronger background scattering and employing interferometric detection to account for phase shifts caused by sample absorption, which are not detectable by photodetectors that only sense intensity.

Image Processing and Phase Correction in SNOM: In SNOM, amplitude and phase images should be combined to form a complex-valued matrix, which helps in correcting phase jumps and gradients due to interferometer drift without altering the actual data. The phase, being a relative measure, can be adjusted to avoid discontinuities (phase jumps) that are artifacts of visualization and not actual sample properties. Advanced image processing techniques can also address background subtraction and the correction of asymmetric amplitude patterns, which are crucial for accurately interpreting near-field signals from weak scatterers and mapping antenna modes or polariton fields.

Reference and Calibration in SNOM Data Analysis: SNOM data analysis requires careful calibration and referencing to known sample points to interpret the amplitude and phase signals accurately. It is essential to normalize to a part of the sample with known properties (e.g., a non-absorbing substrate) for meaningful comparison and to use the phase as an indicator of sample absorption. The calibration includes setting the correct modulation depth and ensuring that all harmonics are synchronized to prevent artifacts such as inverted phase contrast, particularly when measuring weakly absorbing samples with significant topography.

In essence, a solid understanding of SNOM principles, meticulous image processing, and correct calibration are key to obtaining and interpreting accurate near-field optical data.

This video was recorded with the financial support of the Teaming for Excellence program (European Union Horizon 2020; GA 857543) during the Near-field Optical Nanoscopy Summer School (Donostia- San Sebastian; 6-9 June 2023) organized by neaspec/attocube AG, CIC nanoGUNE BRTA and the ENSEMBLE3 Centre of Excellence.