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Nonlinear optics has generated a wide range of applications in the fields of optical communications, biomedicine, and materials science, with nonlinear conversion efficiency serving as a vital metric for its progress. However, the weak nonlinear response of materials, high optical loss, and inhomogeneous distribution of the light field hamper the improvement of the conversion efficiency. We present a composite grating waveguide structure integrated into a Bragg reflector platform. This design achieves high Q in the spectral range by exploiting the unique properties exhibited by the bound states in the Bloch surface wave-enhanced continuum, and efficient second-harmonic generation by close-field amplification with the optical field tightly localized in a nonlinear material. By manipulating the symmetry of the grating, a precise tune over the near field within a designated wavelength range can be achieved. Specifically, we select a photonic crystal configuration supporting surface waves, employing TE polarization conditions and an asymmetry factor of −0.1 between the composite gratings. This configuration resonates at a fundamental wavelength of 783.5 nm, exhibiting an impressive -factor of 10 . Notably, at an incident light intensity of 1.33 GW/cm , we achieve a normalized electric field strength of up to 940 at the fundamental frequency and a second-harmonic conversion efficiency of up to 6 × 10 , significantly amplifying the second-harmonic response. The proposed configuration in this investigation has the potential to be integrated into the field of nonlinear optics for sensing frequency conversion applications.

Identifying brain-tissue types holds significant research value in the biomedical field of non-contact brain-tissue measurement applications. In this paper, a layered metastructure is proposed, and the second harmonic generation (SHG) in a multilayer metastructure is derived using the transfer matrix method. With the SHG conversion efficiency (CE) as the measurement signal, the refractive index ranges that can be distinguished are 1.23~1.31 refractive index unit (RIU) and 1.38~1.44 RIU, with sensitivities of 0.8597 RIU−1 and 1.2967 RIU−1, respectively. It can distinguish various brain tissues, including gray matter, white matter, and low-grade glioma, achieving the function of a second harmonic mode sensor (SHMS). Furthermore, temperature has a significant impact on the SHG CE, which can be used to define the switch signal indicating whether the SHMS is functioning properly. When the temperature range is 291.4~307.9 Kelvin (K), the temperature switch is in the “open” state, and the optimal SHG CE is higher than 0.298%, indicating that the SHMS is in the working state. For other temperature ranges, the SHG CE will decrease significantly, indicating that the temperature switch is in the “off” state, and the SHMS is not working. By stimulating temperature and using the response of SHG CE, the temperature-switch function is achieved, providing a new approach for temperature-controlled second harmonic detection.

A new series of push-pull chromophores ( 1 – 6 ) with ferrocene and methoxyphenyl donors and π-acceptor groups were synthesized and characterized. The chromophores ( 1 – 3 ) [Fc-C = C(CN)-(C 6 H 4 )-(C 6 H 3 )-R {R = H ( 1 ), OCH 3 ( 2 ), CF 3 ( 3 )}] and D-D’-π-A ( 4 – 6 ), [Fc-(OCH 3 -C 6 H 4 )-C = CH=CN-(C 6 H 4 )-(C 6 H 3 )-R {R = H ( 4 ), OCH 3 ( 5 ), CF 3 ( 6 )}] were studied for their photophysical and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction studies confirmed the crystal structures of selected chromophores 1 , 3 , 4 , and 6 , showing various non-covalent interactions such as H-bonding and C-H⋅⋅⋅⋅⋅π interactions. The second-order nonlinear optical (NLO) response of chromophore 6 shows significantly enhanced second-harmonic generation (SHG) efficiency, approximately 2.9 times higher than that of standard potassium dihydrogen phosphate (KDP), due to the extended π-conjugation results in deviation of chromophores from planarity, which prevents antiparallel alignment in the bulk. To gain deeper insight into structure-property relationships, bond length alternation (BLA) values were evaluated based on BLA, correlating with NLO performance. Furthermore, density functional theory (DFT) calculations at the B3LYP/6–31 + G** level included both static and dynamic energy-dependent hyperpolarizabilities, showing good agreement with experimental optical and NLO results.

Second harmonic generation (SHG) with a material of large transparency is an attractive way of generating coherent light sources at exotic wavelength range such as VUV, UV and visible light. It is of critical importance to improve nonlinear conversion efficiency in order to find practical applications in quantum light source and high resolution nonlinear microscopy, etc. Here an enhanced SHG with conversion efficiency up to 10 % at SH wavelength of 282.7 nm under 11 GW/cm pump intensity via the excitation of anapole in lithium niobite (LiNbO , or LN) nanodisk through the dominating nonlinear coefficient is investigated. The anapole has advantages of strongly suppressing far-field scattering and well-confined internal field which helps to boost the nonlinear conversion. Anapoles in LN nanodisk is facilitated by high index contrast between LN and substrate with properties of near-zero-index via hyperbolic metamaterial structure design. By tailoring the multi-layers structure of hyperbolic metamaterials, the anapole excitation wavelength can be tuned at different wavelengths. It indicates that an enhanced SHG can be achieved at a wide range of pump light wavelengths via different design of the epsilon-near-zero (ENZ) hyperbolic metamaterials substrates. The proposed nanostructure in this work might hold significances for the enhanced light–matter interactions at the nanoscale such as integrated optics.

Background Bronchopulmonary dysplasia (BPD) is a chronic lung disease of preterm neonates that carries significant long-term implications for respiratory health. Its pathogenesis is multifactorial, with oxygen toxicity and injury from mechanical ventilation being key contributing factors. While neonatal animal models exposed to hyperoxia are widely used to mimic human BPD, lung development is often assessed through classical histological analyses. Although these methods remain fundamental in preclinical study, they may require multiple staining techniques and sections to highlight key features of BPD. Here, we introduce a label-free, multi-modal imaging platform combining two-photon excitation fluorescence (TPEF), second harmonic generation (SHG), and fluorescence lifetime imaging microscopy (FLIM) to characterize the lung alterations induced by a 7-day hyperoxia (95% O 2 ) exposure in preterm rabbits. Methods Lung sections were obtained from preterm rabbit pups delivered at 28 gestational age (GA) (term 31 GA) either exposed to normoxia (21% O 2 ) or hyperoxia (95% O 2 ) for seven days. Lung sections were scanned with TPEF microscope at 780 nm and tissue intrinsic signals including autofluorescence intensity and lifetime, as well as SHG from collagen were simultaneously collected in the 470–570 nm, 420–460 nm and 380–410 nm ranges, respectively. BPD-relevant features were extracted from images, validated through traditional staining and immunolabelling and analyzed using an optimized pipeline. The tool reliability was tested by correlating label-free features with conventional Hematoxylin and eosin (H&E)-derived histomorphological parameters and lung functions measurements. Results This method simultaneously resolves key BPD-related features, such as tissue density (TD%), alveolar exudates, collagen deposition, arterial medial thickness (MT%) alterations, and alveolar simplification, capturing the differences between hyperoxia and normoxia samples without dyes or antibodies. Quantitative outputs from our label-free pipeline strongly correlate with conventional histology and lung function measurements, thus validating its robustness. Conclusion This approach holds promise as a powerful tool for preclinical research, enabling simultaneous imaging and quantification of multiple pathological features with a single acquisition. This tool is envisioned to be integrated with classical histology and immunolabelling for a more comprehensive and information rich assessment of tissue alterations.

Targeting the tumor-associated extracellular matrix (ECM) offers a promising strategy for breast cancer therapy. During cancer progression, collagen remodeling within the ECM exposes cryptic collagen epitope sites that antibodies can selectively recognize. Here, we investigate the therapeutic potential of targeting the D93 cryptic collagen epitope in 3D human metastatic breast cancer spheroids derived from MDA-MB-231 and MCF10CA1a (M4) cell lines embedded in collagen type I hydrogels. Treatment with monoclonal antibody (mAb) D93 reduced cellular migration into collagen type I hydrogels, an effect likely mediated by integrin α2β1. Two-photon microscopy further revealed that breast cancer cells drive the exposure of D93 sites and alter collagen architecture at both the fiber and fibril levels. Interestingly, collagen remodeling was altered more in the MDA-MB-231 spheroid models whereas the reduction in cellular migration was more pronounced in the M4 spheroid models, indicating a cell-line specific response to mAb D93. Together, these findings suggest that mAb D93 may inhibit integrin α2β1-dependent metastatic migration in breast cancer.

Understanding the deterioration processes in wooden artefacts is essential for accurately assessing their conservation status and developing effective preservation strategies. Advanced imaging techniques are currently being explored to study the impact of chemical changes on the structural and mechanical properties of wood. Nonlinear optical modalities, including second harmonic generation (SHG) and two-photon excited fluorescence (TPEF), combined with fluorescence lifetime imaging microscopy (FLIM), offer a promising non-destructive diagnostic method for evaluating lignocellulose-based materials. In this study, we employed a nonlinear multimodal approach to examine the effects of artificially induced delignification on samples of Norway spruce (Picea abies) and European beech (Fagus sylvatica) subjected to increasing treatment durations. The integration of SHG/TPEF imaging and multi-component fluorescence lifetime analysis enabled the detection of localized variations in nonlinear signals and τ-phase of key biopolymers within wood cell walls. This methodology provides a powerful tool for early detection of wood deterioration, facilitating proactive conservation efforts of wooden artefacts.

[Display omitted] We study irreversible collagen arrangement processes in ex-vivo human amnions subjected to inflation tests, which simulate the mechanical conditions prior to and during the initiation of labor uterine contractions. The investigation is focused on the center of the membrane where the stresses are maximal and equibiaxial. Second harmonic generation reveals an unexpected collagen rearrangement in the compact layer that is responsible for the structural integrity of the fetal membrane. The observed bundling and alignment of the collagen fibers indicate a deviation from the expected equibiaxial stress state. The statistical analysis of the fiber orientations provides information on two driving forces for collagen alignment: microscale flaws and macroscale deviation from the equibiaxial strain. As the pressure increases, the macroscale effect becomes dominant, and a high density of fibers that are aligned along a specific direction is observed. A model that explains these observations and relates them to the material properties is presented. The results of this study indicate that a temporal increase in intrauterine pressure or uterine cervix dilatation causes irreversible changes in collagen molecular connections that may lead to biological changes, such as the initiation of term and preterm labor.

We report a new approach to enhance the photonic response of thin-film topological insulator Bi2Se3 by significantly reducing twin domains and antiphase disorder. The strategy employs closely lattice-matched trigonal substrates combined with surface structuring to preferentially seed a single rotational domain before epitaxy. Characterization using optical second harmonic generation (SHG), nonlinear optical tensor analysis, X-ray diffraction, and atomic force microscopy confirms the near-single crystal Bi2Se3 heteroepitaxial layers. These results show a clear six-fold symmetric sin2(3ϕ) SHG pattern at normal incidence, and a vanishingly small 100-to-1 peak-height ratio from X-ray pole-scans showing negligible twinning. These results show that this approach can yield near perfect single crystal heteroepitaxial Bi2Se3 whose photonic properties converge to those of bulk-grown single crystals.

In this perspective, we discuss how the nonlinear optical technique of second-harmonic generation (SHG) microscopy has been used to greatly enhance our understanding of the tumor microenvironment (TME) of breast and ovarian cancer. Striking changes in collagen architecture are associated with these epithelial cancers, and SHG can image these changes with great sensitivity and specificity with submicrometer resolution. This information has not historically been exploited by pathologists but has the potential to enhance diagnostic and prognostic capabilities. We summarize the utility of image processing tools that analyze fiber morphology in SHG images of breast and ovarian cancer in human tissues and animal models. We also describe methods that exploit the SHG physical underpinnings that are effective in delineating normal and malignant tissues. First we describe the use of polarization-resolved SHG that yields metrics related to macromolecular and supramolecular structures. The coherence and corresponding phase-matching process of SHG results in emission directionality (forward to backward), which is related to sub-resolution fibrillar assembly. These analyses are more general and more broadly applicable than purely morphology-based analyses; however, they are more computationally intensive. Intravital imaging techniques are also emerging that incorporate all of these quantitative analyses. Now, all these techniques can be coupled with rapidly advancing miniaturization of imaging systems to afford their use in clinical situations including enhancing pathology analysis and also in assisting in real-time surgical determination of tumor margins.