Confocal nonlinear optical imaging on hexagonal boron nitride nanosheets

• Published in: PhotoniX 2023-12, Vol.4 (1), p.27-16, Article 27
• Main Authors: Lee, Gwanjin, Jyothsna, Konkada Manattayil, Park, Jonghoo, Lee, JaeDong, Raghunathan, Varun, Kim, Hyunmin
• Format: Article
• Language: English
• Subjects: 2D materials; Boron; Coherent anti-Stokes Raman spectroscopy; Engineering; Graphene; Hexagonal boron nitride nanosheets; Microscopy; Microwaves; RF and Optical Engineering; Spectrum analysis; Sub-diffraction-limited nonlinear optical microscopy; Ultrafast phonon dynamics
• ISSN: 2662-1991; 2662-1991
• DOI: 10.1186/s43074-023-00103-6

Optical microscopy with optimal axial resolution is critical for precise visualization of two-dimensional flat-top structures. Here, we present sub-diffraction-limited ultrafast imaging of hexagonal boron nitride (hBN) nanosheets using a confocal focus-engineered coherent anti-Stokes Raman scattering (cFE-CARS) microscopic system. By incorporating a pinhole with a diameter of approximately 30 μm, we effectively minimized the intensity of side lobes induced by circular partial pi-phase shift in the wavefront (diameter, d 0 ) of the probe beam, as well as nonresonant background CARS intensities. Using axial-resolution-improved cFE-CARS (acFE-CARS), the achieved axial resolution is 350 nm, exhibiting a 4.3-folded increase in the signal-to-noise ratio compared to the previous case with 0.58 d 0 phase mask. This improvement can be accomplished by using a phase mask of 0.24 d 0 . Additionally, we employed nondegenerate phase matching with three temporally separable incident beams, which facilitated cross-sectional visualization of highly-sample-specific and vibration-sensitive signals in a pump-probe fashion with subpicosecond time resolution. Our observations reveal time-dependent CARS dephasing in hBN nanosheets, induced by Raman-free induction decay (0.66 ps) in the 1373 cm −1 mode.