SSCU Student Seminar
Name: Ms. Parineeta Gogoi
Title: Phase-Resolved wide-field Sum Frequency Generation Microscopy
Date & Time: Thursday, 27th August 2026 at 4.00 p.m.
Venue: Rajarshi Bhattacharyya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
Vibrational sum frequency generation (SFG) spectroscopy has emerged as a powerful label-free probe of molecular structure, owing to its intrinsic sensitivity to the local breaking of inversion symmetry, since second-order nonlinear processes vanish identically in centrosymmetric materials. Despite its established use as a point-spectroscopic technique, extending SFG into a fast, widefield microscope has remained technically challenging, primarily due to three obstacles: the need for oblique sample illumination to access the structurally informative out-of-plane component of the second-order susceptibility, the difficulty of acquiring a full, phase-resolved spectrum at every point in the field of view and the inherently weak signal arising from the low conversion efficiency of nonlinear optical processes.
In this seminar, I will discuss how these three challenges have been addressed to realize a working widefield, phase-resolved SFG microscope. The oblique-illumination problem is resolved using a custom-drilled reflective (Schwarzschild) objective, which permits free-space delivery of the tilted pump beam through an off-axis channel while keeping the objective itself normal to the sample – thereby eliminating the image distortion characteristic of conventional tilted-illumination geometries. The phase-resolution problem is addressed through a time-domain interferometric scheme, in which the local oscillator delay is scanned to generate an interferogram independently at every pixel, Fourier transformation of this dataset yields complex (amplitude and phase) SFG spectra across the full field of view. The low signal-to-noise problem is addressed through heterodyne detection with a local oscillator, combined with a paired-pixel balanced imaging scheme implemented using a half-wave plate and Wollaston prism, which suppresses local-oscillator intensity fluctuations on a pixel-by-pixel basis. I will present the underlying derivation of the balanced detection signals, the spatial noise-correlation analysis motivating the paired-pixel approach and the resulting improvement in signal-to-noise ratio. Finally, I will highlight two applications of this technique -crystallographic orientation imaging of monolayer hexagonal boron nitride and molecularly selective imaging of model membranes – that demonstrate the broader utility of this instrument for structurally resolved, label-free chemical imaging.
References:
Khan, T.; John, B.; Niemann, R.; Paarmann, A.; Wolf, M.; Thämer, M., “Compact oblique-incidence nonlinear widefield microscopy with paired-pixel balanced imaging.” Optics Express 2023, 31 (18), 28792–28804.
Mueller, N. S.; Fellows, A. P.; John, B.; Naclerio, A. E.; Carbogno, C.; Gharagozloo-Hubmann, K.; Baláž, D.; Kowalski, R. A.; Heenen, H. H.; Scheurer, C.; Reuter, K.; Caldwell, J. D.; Wolf, M.; Kidambi, P. R.; Thämer, M.; Paarmann, A., “Full Crystallographic Imaging of Hexagonal Boron Nitride Monolayers with Phonon-Enhanced Sum-Frequency Microscopy.” Advanced Materials 2025, 38, e10124.
Fellows, A. P.; John, B.; Wolf, M.; Thämer, M., “Spiral packing and chiral selectivity in model membranes probed by phase-resolved sum-frequency generation microscopy.” Nature Communications 2024, 15, 3161.