Focal Field Engineered Infrared-sensitive Third-order Sum Frequency Generation Microscopy

In this work we present experimental demonstration of focal-field engineering in infrared-sensitive third-order sum frequency generation (TSFG) microscopy by utilizing beam-shaping technique. Two photons of the input mid-infrared (MIR) beam at 3000 nm are upconverted to 615 nm in the presence of a single photon at 1040 nm through the TSFG process. The focal-field engineering scheme studied here improves optical resolution and contrast of the TSFG imaging. We observe best improvement of ~43 % in the central-lobe full-width half diameter with ~35% side-lobe strength of that of the central-lobe with the use of optimum phase-mask using isolated amorphous silicon (a-Si) nano disks as the sample. We compare the contrast enhancement between the experiments and simulations as a function of varying grating pitch and find good overall agreement between the two. In addition to annular phase masks, we also demonstrate edge contrast enhancement by imaging gratings with higher-order Hermite-Gaussian beams profile generated using horizontally partitioned 0- phase profile. © 2022 SPIE

Focus-engineered sub-diffraction imaging in infrared-sensitive third-order sum frequency generation microscope

We experimentally demonstrate sub-diffraction imaging in infrared-sensitive third-order sum frequency generation (TSFG) microscope using focal-field engineering technique. The TSFG interaction studied here makes use of two mid infrared photons and a single 1040 nm pump photon to generate up-converted visible photons. Focal field engineering scheme is implemented using a Toraldo-style single annular phase mask imprinted on the 1040 nm beam using a spatial light modulator. The effect of focal field engineered excitation beam on the non-resonant-TSFG process is studied by imaging isolated silicon sub-micron disks and periodic grating structures. Maximum reduction in the measured TSFG central-lobe size by ∼43% with energy in the central lobe of 35% is observed in the presence of phase mask. Maximum contrast improvement of 30% is observed for periodic grating structures. Furthermore, to validate the infrared sensitivity of the focus engineered TSFG microscope, we demonstrate imaging of amorphous Germanium-based guided-mode resonance structures, and polystyrene latex beads probed near the O-H vibrational region. We also demonstrate the utility of the focus engineered TSFG microscope for high resolution imaging of two-dimensional layered material. Focus-engineered TSFG process is a promising imaging modality that combines infrared selectivity with improved resolution and contrast, making it suitable for nanostructure and surface layer imaging. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing AgreementFALS

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 350nm, 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 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 (0.66ps) in the 1373cm−1 mode. © 2023, Chinese Society for Optical Engineering.TRU

Sub 100 nm resolution confocal focus-engineered coherent anti-Stokes Raman scattering microscopy under non-degenerate pumping condition

For the development of microscopic tools that can resolve non-fluorescent samples beyond the diffraction limit, we propose focus-engineered non-degenerate pumped coherent anti-Stokes Raman scattering (CARS) using spa-tial light modulator (SLM)-based phase shaping, liquid lens focus control, and confocal detection. Non-degenerate pumped CARS (ND-CARS) with frequency-doubled probe pulses resulted in approximately 75% improvement in resolution compared to that of degenerate CARS. Focal adjustment using the liquid lens facilitated the accurate overlapping of three beams. The circular pi-phase modulation at the center of the probe-beam wavefront demar-cated the net CARS focal volume into a sub 100 nm-scale core and surrounding side lobes. The confocal geometry detection setup successfully removed the side lobes, allowing optical imaging of 81 nm-sized zinc oxide particles at 87 nm, and edge-to-edge resolution was determined to be 103 nm.FALS

Non-Destructive Classification of Diversely Stained Capsicum annuum Seed Specimens of Different Cultivars Using Near-Infrared Imaging Based Optical Intensity Detection

The non-destructive classification of plant materials using optical inspection techniques has been gaining much recent attention in the field of agriculture research. Among them, a near-infrared (NIR) imaging method called optical coherence tomography (OCT) has become a well-known agricultural inspection tool since the last decade. Here we investigated the non-destructive identification capability of OCT to classify diversely stained (with various staining agents) Capsicum annuum seed specimens of different cultivars. A swept source (SS-OCT) system with a spectral band of 1310 nm was used to image unstained control C. annuum seeds along with diversely stained Capsicum seeds, belonging to different cultivar varieties, such as C. annuum cv. PR Ppareum, C. annuum cv. PR Yeol, and C. annuum cv. Asia Jeombo. The obtained cross-sectional images were further analyzed for the changes in the intensity of back-scattered light (resulting due to dye pigment material and internal morphological variations) using a depth scan profiling technique to identify the difference among each seed category. The graphically acquired depth scan profiling results revealed that the control specimens exhibit less back-scattered light intensity in depth scan profiles when compared to the stained seed specimens. Furthermore, a significant back-scattered light intensity difference among each different cultivar group can be identified as well. Thus, the potential capability of OCT based depth scan profiling technique for non-destructive classification of diversely stained C. annum seed specimens of different cultivars can be sufficiently confirmed through the proposed scheme. Hence, when compared to conventional seed sorting techniques, OCT can offer multipurpose advantages by performing sorting of seeds in respective to the dye staining and provides internal structural images non-destructively