Confocal nonlinear optical imaging on hexagonal boron nitride nanosheets

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 30μm,we effectively minimized the intensity of side lobes induced by circular partial pi-phase shift in the wavefront(diameter,d0)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 d0 phase mask.This improvement can be accomplished by using a phase mask of 0.24 d0.Additionally,we employed nonde-generate 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.

Mid-infrared quasi-BIC resonances with sub-wavelength slot mode profiles in germanium-based coupled guided-mode resonance structures

We experimentally demonstrate a novel quasi-bound state in the continuum(BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot mode within a low-refractive-index medium sandwiched between high-index layers.The structures studied here comprise coupled amorphous germanium guided-mode resonance(GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer.The slot-mode profile within the silicon nitride layer with mode field confinement >30% is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers.The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering a realistic excitation beam with finite angular spread.The measured transmission peak is found to redshift(remain almost unchanged)under classical(full-conical) mounting conditions.The highest quality factor of ~400 is experimentally extracted at normal incidence under a classical mounting condition with a resonance peak at 3.41 μm wavelength.Such slot-mode GMR structures with appropriately chosen low-index intermediate layers can find applications in resonantly enhanced sensing and active photonic devices.