Second Harmonic Generation Signal Research Articles

Highly Aligned Bacterial Nanocellulose Films Obtained During Static Biosynthesis in a Reproducible and Straightforward Approach.

Bacterial nanocellulose (BNC) is usually produced as randomly‐organized highly pure cellulose nanofibers films. Its high water‐holding capacity, porosity, mechanical strength, and biocompatibility make it unique. Ordered structures are found in nature and the properties appearing upon aligning polymers fibers inspire everyone to achieve highly aligned BNC (A‐BNC) films. This work takes advantage of natural bacteria biosynthesis in a reproducible and straightforward approach. Bacteria confined and statically incubated biosynthesized BNC nanofibers in a single direction without entanglement. The obtained film is highly oriented within the total volume confirmed by polarization‐resolved second‐harmonic generation signal and Small Angle X‐ray Scattering. The biosynthesis approach is improved by reusing the bacterial substrates to obtain A‐BNC reproducibly and repeatedly. The suitability of A‐BNC as cell carriers is confirmed by adhering to and growing fibroblasts in the substrate. Finally, the thermal conductivity is evaluated by two independent approaches, i.e., using the well‐known 3ω‐method and a recently developed contactless thermoreflectance approach, confirming a thermal conductivity of 1.63 W mK−1 in the direction of the aligned fibers versus 0.3 W mK−1 perpendicularly. The fivefold increase in thermal conductivity of BNC in the alignment direction forecasts the potential of BNC‐based devices outperforming some other natural polymer and synthetic materials.

Multiphoton microscopy assessment of the structure and variability changes of dermal connective tissue in vulvar lichen sclerosus: A pilot study.

In this article, we offer a novel classification of progressive changes in the connective tissue of dermis in vulvar lichen sclerosus (VLS) relying on quantitative assessment of the second harmonic generation (SHG) signal received from formalin fixed and deparaffinized tissue sections. We formulate criteria for distinguishing four degrees of VLS development: Initial-Mild-Moderate-Severe. Five quantitative characteristics (length and thickness type I Collagen fibers, Mean SHG signal intensity, Skewness and Coherence SHG signal) are used to describe the sequential degradation of connective tissue (changes in the structure, orientation, shape and density of collagen fibers) up to the formation of specific homogeneous masses. Each of the degrees has a characteristic set of quantitatively expressed features. We focus on the identification and description of early, initial changes of the dermis as the least specific. The results obtained by us and the proposed classification of the degrees of the disease can be used to objectify the dynamics of tissue changes during treatment.

(Digital Presentation) Characterization of Passivation Dielectrics on Silicon Through Second Harmonic Generation: Effect of Fixed Charge

In applications such as image sensors or solar cells, the performance of the device can be improved thanks to the passivation of the silicon substrate using high-k dielectrics such as alumina, hafnium dioxide etc. To assess the quality of the dielectric/semiconductor interface, conventional electrical characterization techniques can be employed, but they require fabrication of dedicated test devices. Among the methods that can be directly used at wafer-level, COCOS [1] allows obtaining electrical information such as the interface states density, but the charging of the sample surface can be an issue. In this paper, we discuss an alternative sensitive and non-destructive optical technique to characterize the electrical properties of a passivation scheme: the second harmonic generation (SHG). In centrosymmetric materials, in the dipolar approximation, the bulk SHG response is zero [2], so the signal mainly contains interface-related information. In particular, when a static electric field (Edc) is present at the interface, the SHG is reinforced and a phenomenon called EFISH (electric field SHG) appears [3]. The electric field at the interface between a dielectric and a semiconductor is related to fixed charges (Qox) and/or interface states (Dit), so the SHG signal can provide these two parameters. Additionally the technique has a potential to probe small areas (within the micrometer size).In this work, the study is focused on the ability of the SHG to measure Qox. The samples used have a layer of Al2O3 of 13nm thickness on Si (100), deposited with different processes, followed or not by an annealing. The annealing step is known to activate negative charges in alumina layers, which promotes the field effect passivation and it can also modify the interface defects [4]. The samples were measured with the SHG tool from FemtoMetrix [5]. Complementary electrical characterizations were performed with the COCOS technique on the full wafer and with capacitance versus voltage (C-V) on dedicated MOS capacitors, in order to obtain Qox. The electric field at the interface can then be calculated through Gauss’ law.Figure 1a shows a typical experimental response of the SHG versus time, obtained on two different samples. The parameters Qtot and Dit were extracted through COCOS. The time-dependence of the SHG is due to the time-dependence of the electric field during the measurement because of charging/discharging effects by carriers generated under the incident laser of the SHG. The SHG signature is obviously stronger for a high Qtot, which indeed is associated with a higher electric field at the interface. Figure 1b shows (with symbols) the experimental results of SHG versus the incidence angle (AOI) for the two samples. The increase of SHG signal for the sample with higher Qtot is confirmed, regardless of the angle of incidence.A general calibration, only based on these types of measurements is not sufficient, since optical phenomena such as transmission & interferences depend on the thickness of the layers involved. In order to better anticipate the effect of these phenomena for a future calibration, we developed a home-made simulator, based on absorption in each layer, boundary conditions at each interface and which contains the “static” interface electric field between materials. Using the electric field calculated with the charge parameters extracted from the COCOS/C-V measurements, we traced the simulated SHG versus AOI (Figure 1b, lines). The tendency observed in experiments is reproduced, even though the simulations are not fully superimposed. This mismatch is probably due to the fact that some of the material parameters in the simulations (e.g. the non-linear susceptibility values) are not very well known.The full paper will show additional samples, corresponding C-V and SHG experimental and simulated data, with the aim of proposing a calibration method that would allow the direct extraction of Qox based only on the SHG measurement.AcknowledgementsThis work was supported by the French National Plan Nano2022, within the IPCEI Nanoelectronics for Europe program.[1] M. Wilson, J. Lagowski, L. Jastrzebski, A. Savtchouk, and V. Faifer, AIP Conference Proceedings, 2001, vol. 550, no. 1, pp. 220–225.[2] R. W. Boyd, Nonlinear optics. Academic press, 2020.[3] N. M. Terlinden, G. Dingemans, V. Vandalon, R. Bosch, and W. M. M. Kessels, J. Appl. Phys., vol. 115, no. 3, p. 033708, 2014.[4] D. K. Schroder. John Wiley & Sons, 2015.[5] http://femtometrix.com Figure 1

Nonlinear optical studies of the temperature evolution of local asymmetric regions in solid solutions (1-x) PbMg1/3Nb2/3O3-xPbSc1/2Nb1/2O3

The temperature dependencies of second harmonic generation (SHG) in (1-x)PMN–xPSN solid solutions (x = 1, 0.35, 0.23, 0.1, 0) are measured. Presence of SHG signal in centrosymmetric phase in all samples studied is demonstrated. It indicates asymmetric regions in the centrosymmetric phase. In PMN crystal (x = 0) in the centrosymmetric phase, SHG temperature dependence follows deviations of strain and refractive index from their high-temperature dependencies, which are determined by random electric fields in local polar regions. In the intermediate temperature range, where macroscopic spontaneous polarization disappears, SHG signal is caused by relatively large and long-lived polar asymmetric regions.

Second-harmonic generation in germanium-on-insulator from visible to telecom wavelengths

The second-order χ(2) process underpins many important nonlinear optical applications in the field of classical and quantum optics. Generally, the χ(2) process manifests itself only in a non-centrosymmetric dielectric medium via an anharmonic electron oscillation when driven by an intense optical field. Due to inversion symmetry, group-IV semiconductors, such as silicon (Si) and germanium (Ge), are traditionally not considered as ideal candidates for second-order nonlinear optics applications. Here, we report the experimental observation of the second-harmonic generation (SHG) in a Ge-on-insulator (GOI) sample under femtosecond optical pumping. We especially, report the measurement of the SHG signal from a GOI sample in the telecom S-band by pumping at ∼3000 nm.

Artificial Intelligence Improves Pathologist Agreement for Fibrosis Scores in Nonalcoholic Steatohepatitis Patients

Artificial Intelligence Improves Pathologist Agreement for Fibrosis Scores in Nonalcoholic Steatohepatitis Patients

Augmentation of surface plasmon-enhanced second harmonic generation from Au nanoprisms on SiO2/Si: interference contribution.

We present an augmentation of Surface Plasmon (SP)-enhanced second harmonic generation (SHG) due to interference field enhancement in Au nanoprisms (AuNPs) on SiO2-coated Si substrates. The SiO2 spacer contributed for the optical interference and increased the coupling efficiency of the pump light with the SP polarization as well as a decoupling efficiency of the SHG waves from nonlinear polarization. The intensity of the SP-enhanced SHG signals increased 4.5-fold with respect to the AuNPs on the bare SiO2 substrate by setting the SiO2 spacer layer to the appropriate thickness. The numerical analysis revealed that the optimal SHG conversion was determined by the balance between the degree of the optical interference at the fundamental and SHG wavelengths.

Dimethylaniline-Based Hybrid Compounds of Cadmium Diiodide: Synthesis, Crystal Structure, and Physical Properties

In this work, synthesis, crystal structure, and selected properties of two new organic–inorganic compounds based on cadmium iodide with 2,5- and 2,6-dimethylaniline: CdI2(2,5-DMA)2 1 and CdI2(2,6-DMA)2 2, respectively, were investigated. Both compounds were obtained using the same methodology, and their crystal structures were determined from powder (1) and single-crystal (2) X-ray diffraction experiments. The examined materials crystallize in the noncentrosymmetric space group Fdd2 and contain large polarizable iodine ions and electron-rich aromatic ligands. Second harmonic generation (SHG) tests performed with urea as a reference revealed high SHG efficiency of powdered samples 1 and 2. SHG signals measured for samples 1 and 2 are, respectively, around 10 and 8 times larger than the signal for urea. The experimental part was supplemented with theoretical calculations of the second-order electric susceptibility tensor components, χijk(2). Calculated values of the χ113(2) tensor component around 1.2–1.4 pm·V–1 suggest that both materials are relatively efficient second harmonic generators. Fibrous morphology of the materials was revealed by scanning electron microscopy (SEM), and the observed bending of the fibers suggested their significant flexibility. Samples 1 and 2 were also characterized by spectroscopic techniques (IR, RS, UV–Vis-DRS) and their thermal stability and decomposition rates were researched using XRPD and TG/DSC, revealing that both investigated materials are thermally stable up to around 100 °C.

Second harmonic generation microscopy of otoconia.

The origin of second harmonic generation (SHG) signal in otoconia was investigated. SHG signal intensity from otoconia was compared to pure calcite crystals, given calcite is the primary component of otoconia and is known to emit surface SHG. The SHG intensity from calcite was found to be ∼41× weaker than the SHG intensity from otoconia signifying that the SHG signal from otoconia is likely generated from the organic matrix. Furthermore, the SHG intensity from otoconia increased when treated with a chelating agent known to dissolve calcite which confirms that calcite is not the source of SHG. Additionally, polarization-resolved SHG microscopy imaging revealed that the arrangement of the SHG emitters is radial and can form highly ordered domains.

Computer-Aided Detection of Quantitative Signatures for Breast Fibroepithelial Tumors Using Label-Free Multi-Photon Imaging.

Fibroadenomas (FAs) and phyllodes tumors (PTs) are major benign breast tumors, pathologically classified as fibroepithelial tumors. Although the clinical management of PTs differs from FAs, distinction by core needle biopsy diagnoses is still challenging. Here, a combined technique of label-free imaging with multi-photon microscopy and artificial intelligence was applied to detect quantitative signatures that differentiate fibroepithelial lesions. Multi-photon excited autofluorescence and second harmonic generation (SHG) signals were detected in tissue sections. A pixel-wise semantic segmentation method using a deep learning framework was used to separate epithelial and stromal regions automatically. The epithelial to stromal area ratio and the collagen SHG signal strength were investigated for their ability to distinguish fibroepithelial lesions. An image segmentation analysis with a pixel-wise semantic segmentation framework using a deep convolutional neural network showed the accurate separation of epithelial and stromal regions. A further investigation, to determine if scoring the epithelial to stromal area ratio and the SHG signal strength within the stromal area could be a marker for differentiating fibroepithelial tumors, showed accurate classification. Therefore, molecular and morphological changes, detected through the assistance of computational and label-free multi-photon imaging techniques, enable us to propose quantitative signatures for epithelial and stromal alterations in breast tissues.

(Digital Presentation) Characterization of Passivation Dielectrics on Silicon Through Second Harmonic Generation: Effect of Fixed Charge

This paper presents the characterization of alumina passivation layers using second harmonic generation (SHG). Based on nonlinear optics, the technique has the advantage of being rapid and non-destructive, especially for thin oxide layers. For the materials under study, the SHG is sensitive to the electronic and structural properties of the interface, particularly to the interfacial electric field (Edc), and thus allowing to characterize fixed oxide charges (Qox) in the oxide and interface traps density (Dit). Time-dependent SHG (TD-SHG) is firstly used here to probe samples with various Dit and a total oxide charges (Qtot), previously extracted using corona oxide characterization of semiconductor (COCOS). The measurement conditions are adjusted to focus on the impact of the fixed charges on the SHG signal. A more detailed analysis of the SHG versus input polarization is conducted in both experiments and simulations and it shows coherent evolution tendencies with the oxide charge.

Multiphoton microscopy for label-free multicolor imaging of peripheral nerve

.Significance: Means for quantitation of myelinated fibers in peripheral nerve may guide diagnosis and clinical decision making in management of peripheral nerve disorders. Multiphoton microscopy techniques such as the third-harmonic generation enable label-free in vivo imaging of peripheral nerves.Aim: Develop a multiphoton microscope based on a custom high-power infrared fiber laser for label-free imaging of peripheral nerve.Approach: A cost-effective multiphoton microscope employing a single fiber laser source at 1300 nm was designed and used for stain-free multicolor imaging of murine and human peripheral nerve.Results: Second-harmonic generation signal from collagen centered about 650-nm delineated neural connective tissue, whereas third-harmonic general signal centered about 433-nm delineated myelin and other lipids. In sciatic nerve from transgenic reporter mice expressing yellow fluorescent protein within peripheral neurons, three-photon-excitation with emission peak at 527-nm delineated axoplasm. The signal obtained from unlabeled axially sectioned samples was adequate for segmentation of myelinated fibers using commercial image processing software. In unlabeled whole mount specimens, imaging depths over were achieved.Conclusions: A multiphoton microscope powered by a fiber laser enables stain-free histomorphometry of mammalian peripheral nerve. The simplicity of the microscope design carries potential for clinical translation to inform decision making in peripheral nerve disorders.

Sum-Frequency Generation and Scanning Electron Microscope Studies on Second-Harmonic Generation Active Structures of Sacran Aggregates

Optical second-harmonic generation (SHG) in self-assembled ampholytic mega-molecule polysaccharide sacran is an interesting phenomenon but its origin has not been clarified. SHG-active sacran aggregates in dried cast films made from ultrasonicated 0.5 wt% sacran aqueous solutions were studied at microscale using the scanning electron microscope and the energy dispersive X-ray technology. Hexagonal morphology was observed frequently with the size of a few to tens of micrometers. The calcium density was higher in the area showing the SHG signals. In sum-frequency generation spectra of the SHG-active sacran aggregates, vibrational modes of functional groups such as CH3, CH2, and −COOH of the sacran molecules and OH of the water molecules were identified. The structure of the SHG-active sacran aggregates was modeled from the observed facts.

Proton-Rich POM-Type K12Mo8O20(HPO4)8(PO4)Cl with Ion-Exchange Capabilities.

A proton-rich POM-type molybdenum phosphate K12Mo8O20(HPO4)8(PO4)Cl was successfully obtained. It crystallizes in a noncentrosymmetric tetragonal space group of P-4 (No. 81) with the unit cell parameters of a = 9.6580(4) Å, c = 14.2607(10) Å, and Z = 1. The occurrence and positions of the light element H in the structure are inferred from single-crystal X-ray diffraction and confirmed by DFT calculations. The hydrogen atoms are found to form hydroxyl bonds with O atoms from P(2)O4 and P(3)O4 constituting the [Mo4P4O26H4]4- layers but are only weakly bound to the isolated P(3)O4 group through hydrogen bonds. The title compound presents a POM-type framework of corrugated [Mo4P4O26H4]4- layers with four K+ ions and mixed ions (K4Cl3+ and isolated PO43-) orderly imbedding in the interlayer spaces with distances of 5.0396 (1) and 5.5966 (3) Å, respectively. The proton-rich nature and the structure feature were further verified by a series of experiments including 1H, 7Li, and 31P MAS NMR spectra, IR spectroscopy, and thermal analysis. Moreover, the weak bonding and large interlayer spaces make K+ and H+ ions susceptible to exchange with ions of Cs+, Ba2+, Zn2+, Pb2+, Cu2+, and Ni2+ commonly presented in chemical pollutants or nuclear wastes. In addition, the title compound shows a small second-harmonic generation signal, consistent with its noncentrosymmetric structure.

Second harmonic generation control in twisted bilayers of transition metal dichalcogenides

The twist angle in transition metal dichalcogenide (TMD) heterobilayers is a compelling degree of freedom that determines electron correlations and the period of lateral confinement of moir\'e excitons. Here we perform polarization-resolved second harmonic generation (SHG) spectroscopy of MoS2/WSe2 heterostructures. We demonstrate that by choosing suitable laser energies the twist angle between two monolayers can be measured directly on the assembled heterostructure. We show that the amplitude and polarization of the SHG signal from the heterostructure are determined by the twist angle between the layers and exciton resonances at the SH energy. For heterostructures with close to zero twist angle, we observe changes of exciton resonance energies and the appearance of new resonances in the linear and non-linear susceptibilities.

Microporous Multiresonant Plasmonic Meshes by Hierarchical Micro-Nanoimprinting for Bio-Interfaced SERS Imaging and Nonlinear Nano-Optics.

Microporous mesh plasmonic devices have the potential to combine the biocompatibility of microporous polymeric meshes with the capabilities of plasmonic nanostructures to enhance nanoscale light-matter interactions for bio-interfaced optical sensing and actuation. However, scalable integration of dense and uniformly structured plasmonic hotspot arrays with microporous polymeric meshes remains challenging due to the processing incompatibility of conventional nanofabrication methods with flexible microporous substrates. Here, scalable nanofabrication of microporous multiresonant plasmonic meshes (MMPMs) is achieved via a hierarchical micro-/nanoimprint lithography approach using dissolvable polymeric templates. It is demonstrated that MMPMs can serve as broadband nonlinear nanoplasmonic devices to generate second-harmonic generation, third-harmonic generation, and upconversion photoluminescence signals with multiresonant plasmonic enhancement under fs pulse excitation. Moreover, MMPMs are employed and explored as bio-interfaced surface-enhanced Raman spectroscopy mesh sensors to enable in situ spatiotemporal molecular profiling of bacterial biofilm activity. Microporous mesh plasmonic devices open exciting avenues for bio-interfaced optical sensing and actuation applications, such as inflammation-free epidermal sensors in conformal contact with skin, combined tissue-engineering and biosensing scaffolds for in vitro 3D cell culture models, and minimally invasive implantable probes for long-term disease diagnostics and therapeutics.

Upconversion emission and second harmonic generation of the YVO4: Tm3+, Er3+, Yb3+ inverse opals

The YVO4:Tm3+, Er3+, Yb3+ nanomaterials were fabricated with a simple chemical method. The X-ray diffraction, scanning electron microscopy and transmission electron microscopy characterizations confirm that the YVO4:Tm3+, Er3+, Yb3+ nanomaterials have good structure with the polycrystalline. With the excitation of the 975 nm laser, the white color emissions were observed in the YVO4:Tm3+, Er3+, Yb3+ inverse opals with the polystyrene (PS) size of 450 nm and the composition of Y:Tm:Er:Yb = 77.5:0.5:2:20, in which the blue emission is the second harmonic generation (SHG) signal of the incident laser. The inverse opals have great effect on the upconversion emission and the SHG signal. With the inverse opal structures the blue SHG signal increases significantly for the inverse opals with the PS size of 250 and 450 nm, in which the SHG signal of the inverse opal with the PS size of 450 nm is stronger and is more suitable for the white emission due to the narrow width of the SHG signal. The power dependencies confirm that the upconversion emissions have the slope factor of 2–3, which means that the upconversion processes are the three-photon processes.

Second-harmonic generation in atomically thin 1T−TiSe2 and its possible origin from charge density wave transitions

Optical second-harmonic generation (SHG) can only occur in noncentrosymmetric crystals in the leading electric-dipole approximation. Transition metal dichalco genides with the $1T$ octahedral coordination is centrosymmetric, hence precluding SHG. Here we report the surprising observation of SHG in atomically thin $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$, a prototypical charge density wave (CDW) material. Its intensity peaks in the trilayer, reaching 2% of that in monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$, a two-dimensional crystal featuring pronounced nonlinear optical effect. The SHG signal exhibits a sixfold polarization dependence characteristic of a lattice with threefold rotational symmetry. It monotonically decreases with increasing temperature and persists at room temperature. Raman spectroscopy demonstrates that the CDW order is robust in atomically thin samples, with the transition temperature slightly lower than in the bulk. The SHG can be explained by the lattice distortion associated with the CDW as well as its fluctuation above the transition temperature. These results challenge the exciton insulator scenario and the chiral nature of the CDW, but support the band Jahn-Teller mechanism. Our work demonstrates SHG as a sensitive probe of the stacking order of the CDW in $1T\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{Se}}_{2}$ and enriches the material base for nonlinear optical effects.

Separating Non-linear Optical Signals of a Sample from High Harmonic Radiation in a Soft X-ray Free Electron Laser

Second harmonic generation (SHG) is a unique non-linear optical effect that can be used to investigate chemical states of molecules at surfaces/interfaces. By the use of soft X-rays, SHG gains element selectivity through the inner-core excitation resonance. However, it is challenging to observe SHG signals separately from the second-order light generated from the undulator. Here, we report a new ellipsometry method for soft X-ray SHG to suppress the contribution of second-harmonic radiation from the light source. Through measurements of a GaAs(100) crystal, we demonstrate that pure SHG signals can be obtained for the horizontally polarized component. The present method is generally applicable regardless of the incident photon energy and hence the absorption edge of the targeted materials. If combined with optical filters blocking the second-harmonic radiation and equipped with soft X-ray phase shifters, the method allows one to obtain further information from SHG signals such as tensor components of second-order non-linear susceptibility.

Structurally abnormal collagen fibrils in abdominal aortic aneurysm resist platelet adhesion

BackgroundPlatelet adhesion to the subendothelial collagen fibrils is one of the first steps in hemostasis. Understanding how structural perturbations in the collagen fibril affect platelet adhesion can provide novel insights into disruption of hemostasis in various diseases. We have recently identified the presence of abnormal collagen fibrils with compromised D‐periodic banding in the extracellular matrix remodeling present in abdominal aortic aneurysms (AAA). ObjectiveIn this study, we employed multimodal microscopy approaches to characterize how collagen fibril structure impacts platelet adhesion in clinical AAA tissues. MethodsUltrastructural atomic force microscopy (AFM) analysis was performed on tissue sections after staining with fluorescently labeled collagen hybridizing peptide (CHP) to recognize degraded collagen. Second harmonic generation (SHG) microscopy was used on CHP‐stained sections to identify regions of intact versus degraded collagen. Finally, platelet adhesion was identified via SHG and indirect immunofluorescence on the same tissue sections. ResultsOur results indicate that ultrastructural features characterizing collagen fibril abnormalities coincide with CHP staining. SHG signal was absent from CHP‐positive regions. Additionally, platelet binding was primarily localized to regions with SHG signal. Abnormal collagen fibrils present in AAA (in SHG negative regions) were thus found to inhibit platelet adhesion compared to normal fibrils. ConclusionsOur investigations reveal how the collagen fibril structure in the vessel wall can serve as another regulator of platelet–collagen adhesion. These results can be broadly applied to understand the role of collagen fibril structure in regulating thrombosis or bleeding disorders.