Second Harmonic Generation Signal Research Articles

Micro-strain administered SHG intensity enhancement by heavy Ce doping in co-precipitated ZnO nanoparticles

The objective of the present study is to explore the non-linear behaviour of Ce-doped ZnO system. Undoped and Ce-doped ZnO nanoparticles of size ~23–70 nm were prepared by sol-gel co-precipitation approach. There was complete absence of Ce-containing phase until 3 at.% dopant. Band-gap decreased in a linear manner from 3.262 to 3.212 eV on doping with Ce. Emission peaks at 360 and 380 nm were observed in the PL spectra. However, at 7 at.% dopant, CeO2 and Ce2O3 were observed. Interestingly, a significant increase in second harmonic generation (SHG) signal intensity of ~3.5 times was observed when the dopant concentration was increased from 3 to 7 at.% in ZnO nanoparticles. The SHG signal in doped-ZnO was governed by micro-strain such that it increased with decrease of micro-strain and vice-versa. The dependence of ZnO morphology and SHG intensity on Ce-dopant concentration in ZnO has potential for applications in bio-imaging and bio-sensing.

Large few-layer hexagonal boron nitride flakes for nonlinear optics.

Hexagonal boron nitride (hBN) is a layered dielectric material with a wide range of applications in optics and photonics. In this work, we demonstrate a fabrication method for few-layer hBN flakes with areas up to 5000µm2. We show that hBN in this form can be integrated with photonic microstructures: as an example, we use a circular Bragg grating (CBG). The layer quality of the exfoliated hBN flake on and off a CBG is confirmed by Raman spectroscopy and second-harmonic generation (SHG) microscopy. We show that the SHG signal is uniform across the hBN sample outside the CBG and is amplified in the center of the CBG.

Backward Phase-Matched Second-Harmonic Generation from Stacked Metasurfaces.

We demonstrate phase-matched second-harmonic generation (SHG) from three-dimensional metamaterials consisting of stacked metasurfaces. To achieve phase matching, we utilize a novel mechanism based on phase engineering of the metasurfaces at the interacting wavelengths, facilitating phase-matched SHG in the unconventional backward direction. Stacking up to five metasurfaces,we obtain a phase-matched SHG signal, which scales superlinearly with the number of layers. Our results motivate further investigations to achieve higher conversion efficiencies also with more complex wave fronts.

Diagnosing Hirschsprung disease by detecting intestinal ganglion cells using label-free hyperspectral microscopy

Hirschsprung disease (HD) is a congenital disorder in the distal colon that is characterized by the absence of nerve ganglion cells in the diseased tissue. The primary treatment for HD is surgical intervention with resection of the aganglionic bowel. The accurate identification of the aganglionic segment depends on the histologic evaluation of multiple biopsies to determine the absence of ganglion cells in the tissue, which can be a time-consuming procedure. We investigate the feasibility of using a combination of label-free optical modalities, second harmonic generation (SHG); two-photon excitation autofluorescence (2PAF); and Raman spectroscopy (RS), to accurately locate and identify ganglion cells in murine intestinal tissue without the use of exogenous labels or dyes. We show that the image contrast provided by SHG and 2PAF signals allows for the visualization of the overall tissue morphology and localization of regions that may contain ganglion cells, while RS provides detailed multiplexed molecular information that can be used to accurately identify specific ganglion cells. Support vector machine, principal component analysis and linear discriminant analysis classification models were applied to the hyperspectral Raman data and showed that ganglion cells can be identified with a classification accuracy higher than 95%. Our findings suggest that a near real-time intraoperative histology method can be developed using these three optical modalities together that can aid pathologists and surgeons in rapid, accurate identification of ganglion cells to guide surgical decisions with minimal human intervention.

Investigating the optical clearing effects of 50% glycerol in ex vivo human skin by harmonic generation microscopy

Imaging depth and quality of optical microscopy can be enhanced by optical clearing. Here we investigate the optical clearing of the ex vivo human skin by 50% glycerol topical application, which is allowed for cosmetic usage. Harmonic generation microscopy, by combining second and third harmonic generation (THG) modalities, was utilized to examine the clearing effect. The THG image intensity is sensitive to the improved optical homogeneity after optical clearing, and the second harmonic generation (SHG) image intensity in the dermis could serve as a beacon to confirm the reduction of the scattering in the epidermis layer. As a result, our study supports the OC effect through 50% glycerol topical application. Our study further indicates the critical role of stratum corneum shrinkage for the observed SHG and THG signal recovery.

Assessment of the corneal collagen organization after chemical burn using second harmonic generation microscopy.

The organization of the corneal stoma is modified due to different factors, including pathology, surgery or external damage. Here the changes in the organization of the corneal collagen fibers during natural healing after chemical burn are investigated using second harmonic generation (SHG) imaging. Moreover, the structure tensor (ST) was used as an objective tool for morphological analyses at different time points after burn (up to 6 months). Unlike control corneas that showed a regular distribution, the collagen pattern at 1 month of burn presented a non-organized arrangement. SHG signal levels noticeably decreased and individual fibers were hardly visible. Over time, the healing process led to a progressive re-organization of the fibers that could be quantified through the ST. At 6 months, the stroma distribution reached values similar to those of control eyes and a dominant direction of the fibers re-appeared. The present results show that SHG microscopy imaging combined with the ST method is able to objectively monitor the temporal regeneration of the corneal organization after chemical burn. Future implementations of this approach into clinically adapted devices would help to diagnose and quantify corneal changes, not only due to chemical damages, but also as a result of disease or surgical procedures.

Second harmonic generation in AB-type LaTiO3/SrTiO3 superlattices

Abstract We report the generation and control of second harmonic generation (SHG) from a series of LaTiO3/SrTiO3 superlattices. We demonstrate that the nonlinear optical susceptibility χ(2) and the SHG pattern can be tuned by tailoring the number of superlattice interfaces. Significantly, unlike the conventional ABC-type nanostructures, our AB-type nanostructures can also produce SHG signals. The χ(2) value (24.3 pm/V) is even one to two orders of magnitude greater than the conventional ones. Hence, we demonstrate that SHG is a local microscopic process—it does not rely on macroscopic broken inversion symmetry. Moreover, we show that temperature dependence or structural phase transition can further enhance the SHG intensity. Our results introduce a scheme for SHG, extending the generation and control of SHG in nano-photonics to nearly all the accessible centrosymmetric materials.

Inversion symmetry broken in 2H phase vanadium-doped molybdenum disulfide.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received much attention in nonlinear optical applications due to their unique crystal structures and second harmonic generation (SHG) efficiency. However, SHG signals in TMDs show a layer-dependent behavior, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer) of TMDs. Herein, we synthesized monolayer and bilayer 2H and 3R phase vanadium (V)-doped MoS2 crystal. Raman spectroscopy, XPS, and STEM were used to identify the chemical composition and crystalline structure of as-grown nanoflakes. SHG measurement was used to research the symmetry of V-doped MoS2 crystals with different stacking orders. Significantly, the SHG efficiency in bilayer 2H phase V-doped MoS2 is equivalent to the 3R phase, indicating an inversion symmetry broken lattice structure caused by the in situ V substitute for Mo sites. This study will be conducive to promote the development of promising nonlinear optical devices based on 2D material.

High-Pressure Synthesis of Rare-Earth Borate-Nitrate Crystals for Second Harmonic Generation.

The crystals of a novel family of rare-earth borate-nitrate compounds, Ln7(BO3)3(NO3)N3O (Ln = Pr, Nd), were grown at high-pressure in KAs flux and their crystal structure was determined. The new type of the crystalline structure consists of parallel chains of Ln6 octahedra connected by common faces and forming the channels with the NO3 triangular planar motifs in the center, and isolated OLn4 tetrahedra separated from each other by N3 triangular motifs. Each NO3 triangle is in fact a part of rather unusual (NB3O12) block consisting of 3 distorted BO4 tetrahedra around central nitrogen atom. Under near-infrared (NIR) (λex = 1064 nm) excitation, both compounds revealed a strong signal of second harmonic generation (SHG) at half the excitation wavelength (λem = 532 nm), which is in agreement with their noncentrosymmetric structure. In addition, a photon up-conversion (UC) emission at λem = 880 nm was observed for microcrystals of Nd7(BO3)3(NO3)N3O, which was assigned to the UC process occurring within the 4f electronic manifold of Nd3+ ions. The dual-emission (SHG/UC) properties of Nd7(BO3)3(NO3)N3O microcrystals, concomitant with the absence of photobleaching, makes them prospective candidates for microscopic probes in biological studies.

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr2IrO4

An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr$_2$IrO$_4$ and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a two-fold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr$_2$IrO$_4$, our results altogether show that the anomalous SHG signal in parent Sr$_2$IrO$_4$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.

A Nonlinear Optical Switchable Sulfate of Ultrawide Bandgap

Nonlinear optical (NLO) switchable materials have attracted intense attention because of their promising applications in optoelectronic devices. However, previous studies are mainly limited to mole...

Observations of intracellular second-harmonic generation imaging in black phosphorus nanosheets

Functionalized black phosphorus (BP) nanosheets have been considered as promising nanoagents in cancer therapy due to their excellent photothermal conversion efficiency. However, it is still difficult to visually monitor the dynamic localization of BP nanoagents in cancer cells. In this paper, we systematically studied the second-harmonic generation (SHG) signals originating from exfoliated BP nanosheets. Interestingly, under the excitation of a high frequency pulsed laser at 950 nm, the SHG signals of BP nanosheets in vitro are almost undetectable because of their poor stability. However, the intracellular SHG signals from BP nanosheets could be measured by in vivo optical [Formula: see text]imaging due to the efficient enrichment of living HeLa cells. Moreover, the SHG signal intensity from BP nanosheets increases with the prolonged incubation time. It can be expected that the BP nanosheets could be a promising intracellular SHG nanoprobe employed for visually in vivo biomedical imaging in practical cancer photothermal therapy (PIT).

Characterization of pathological thyroid tissue using polarization-sensitive second harmonic generation microscopy

AbstractPolarization-sensitive second harmonic generation (SHG) microscopy is an established imaging technique able to provide information related to specific molecular structures including collagen. In this investigation, polarization-sensitive SHG microscopy was used to investigate changes in the collagen ultrastructure between histopathology slides of normal and diseased human thyroid tissues including follicular nodular disease, Grave's disease, follicular variant of papillary thyroid carcinoma, classical papillary thyroid carcinoma, insular or poorly differentiated carcinoma, and anaplastic or undifferentiated carcinoma ex vivo. The second-order nonlinear optical susceptibility tensor component ratios, χ(2)zzz′/χ(2)zxx′ and χ(2)xyz′/χ(2)zxx′, were obtained, where χ(2)zzz′/χ(2)zxx′ is a structural parameter and χ(2)xyz′/χ(2)zxx′ is a measure of the chirality of the collagen fibers. Furthermore, the degree of linear polarization (DOLP) of the SHG signal was measured. A statistically significant increase in χ(2)zzz′/χ(2)zxx′ values for all the diseased tissues except insular carcinoma and a statistically significant decrease in DOLP for all the diseased tissues were observed compared to normal thyroid. This finding indicates a higher ultrastructural disorder in diseased collagen and provides an innovative approach to discriminate between normal and diseased thyroid tissues that is complementary to standard histopathology.Polarization-second harmonic microscopy was utilized to investigate whether collagen ultrastructure in thyroid due to four carcinoma types and Graves' disease could be differentiated in human histopathology samples. Three parameters were extracted, revealing that the degree of linear polarization and χ(2)zzz/χ(2)zxx were effective in differentiating some diseases, while the parameter χ(2)xyz/χ(2)zxx was less effective.

Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces.

The interaction between molecular species and charged interfaces plays an indispensable role in a multitude of electrochemical devices. Yet, very little is understood about the nature of this interaction, in particular, the interfacial electric field. Second-order nonlinear spectroscopy such as second-harmonic generation (SHG) can provide chemical information on these interfacial interactions; however, the phase information has received limited attention in electrochemical SHG studies. Here, we demonstrate that the phase of the SHG is essential to the measurement of the electric field at the electrode-electrolyte interface. Our in situ SHG experiment provides strong evidence in support of the parabolic model with complex nonlinear susceptibilities. We conclude that if the absolute phase of the total SHG signal with both χ(2) and χ(3) contributions can be obtained, it would be possible to measure the potential of zero charge of any electrochemical material.

Second harmonic generation microscopy using pixel reassignment.

Second harmonic generation (SHG) microscopy is expected to be a powerful tool for observing the cellular-level functionality and morphology information of thick tissue owe to its unique imaging properties. However, the maximum attainable resolution obtainable by SHG microscopy is limited by the use of long-wavelength, near-infrared excitation. In this paper, we report the use of pixel reassignment to improve the spatial resolution of SHG microscopy. The SHG signal is imaged onto a position-sensitive camera, instead of a point detector typically used in conventional SHG microscope. The data processing is performed through pixel reassignment and subsequent deblurring operation. We present the basic principle and a rigorous theoretical model for SHG microscopy using pixel reassignment (SHG-PR). And for the first time, the optimal reassignment factor for SHG-PR is derived based on the coherent characteristics and the dependence of wavelength in SHG microscopy. To evaluate the spatial resolution improvement, images of nano-beads separated by different distances and of a microtubule array have been simulated. We gain about a 1.5-fold spatial resolution enhancement compared to conventional SHG microscopy. When a further deblurring operation is implemented, this method allows for a total spatial resolution enhancement of about 1.87. Additionally, we demonstrate the validity of SHG-PR for raw data with noise. LAY DESCRIPTION: Second harmonic generation (SHG) microscopy has emerged as a powerful imaging technique in clinical diagnostics and biological research. SHG microscopy is label-free and provides intrinsic optical sectioning for three-dimensional (3D) imaging. However, a near-infrared excitation wavelength results a restriction in the maximum attainable spatial resolution of SHG microscopy. In this paper, we present a simple resolution-enhanced SHG imaging method, SHG microscopy using pixel reassignment (SHG-PR). We demonstrate a rigorous theoretical model for SHG-PR and derive the optimal reassignment factor. The simulation result shows the clear improvement of the image resolution and contrast in the SHG-PR after deblurring operation. The FWHM value of single microtubule shows that SHG-PR enables a spatial resolution enhancement by a factor of 1.5, compared to conventional SHG microscopy. After a proper deblurring operation, this method allows for a total spatial resolution enhancement of about 1.87. The improvements of spatial resolution and contrast are still valid for raw data with noise. It is expected that this method can contribute towards new insights in unstained tissue morphology, interaction of cells, and diseases diagnosis.

Characterizing Vocal Fold Injury Recovery in a Rabbit Model With Three-Dimensional Virtual Histology.

In animal studies of vocal fold scarring and treatment, imaging-based evaluation is most often conducted by tissue slicing and histological staining. Given variation in anatomy, injury type, severity, and sacrifice timepoints, planar histological sections provide limited spatiotemporal details of tissue repair. Three-dimensional (3D) virtual histology may provide additional contextual spatial information, enhancing objective interpretation. The study's aim was to evaluate the suitability of magnetic resonance imaging (MRI), microscale computed tomography (CT), and nonlinear laser-scanning microscopy (NM) as virtual histology approaches for rabbit studies of vocal fold scarring. A unilateral injury was created using microcup forceps in the left vocal fold of three New Zealand White rabbits. Animals were sacrificed at 3, 10, and 39 days postinjury. ex vivo imaging of excised larynges was performed with MRI, CT, and NM modalities. The MRI modality allowed visualization of injury location and morphological internal features with 100-μm spatial resolution. The CT modality provided a view of the injury defect surface with 12-μm spatial resolution. The NM modality with optical clearing resolved second-harmonic generation signal of collagen fibers and two-photon autofluorescence in vocal fold lamina propria, muscle, and surrounding cartilage structures at submicrometer spatial scales. Features of vocal fold injury and wound healing were observed with MRI, CT, and NM. The MRI and CT modalities provided contextual spatial information and dissection guidance, whereas NM resolved extracellular matrix structure. The results serve as a proof of concept to motivate incorporation of 3D virtual histology techniques in future vocal fold injury animal studies. NA Laryngoscope, 131:1578-1587, 2021.

The effect of the sodium content on the structure and the optical properties of thermally poled sodium and niobium borophosphate glasses

Six niobium rich glasses presenting different sodium contents were synthetized. On the basis of complementary infrared and Raman analysis, the influence of the sodium content and its role on the structure of the glasses prior to and following poling was examined. Correlative second harmonic generation (SHG)/Raman microscopy on the poled glasses cross section has shown a co-localization of the SHG signal and the structural variations. It also evidenced similar structural rearrangements whether sodium is removed through poling (230–300 °C) or through the alkali content defined by the starting glass compositions (melted at 1300–1500 °C). The effect of the sodium content on the optical properties, prior to and after thermal poling, is also discussed. It was found that refractive index variations induced by poling (ranging between 10−3 and 3 × 10−2) are mainly the result of a density decrease in the poled region rather than compositional and structural changes. The electro-optic origin of the poling-induced second order nonlinear response is confirmed by the Maker Fringe SHG analysis, and the evolution of χ(2) (2–2.5 pm/V) with the sodium content is discussed based on a selection of parameters influencing either the third order susceptibility [χ(3)] or the internal electric field strength of the poled layer.

Label-free characterization of collagen fibers in cancerous esophagus tissues using ratiometric nonlinear optical microscopy.

The issue of classifying esophageal cancer at various developmental stages is crucial for determining the optimized treatment protocol for the patients, as well as the prognosis. Precision improvement in staging esophageal cancer keeps seeking quantitative and analytical imaging methods that could augment histopathological techniques. In this work, we used nonlinear optical microscopy for ratiometric analysis on the intrinsic signal of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) from single collagen fibers only in submucosa of esophageal squamous cell carcinoma (ESCC). The blind tests of TPEF/SHG and forward (F)/backward (B) SHG were demonstrated to compare with the histology conclusion. The discussion of sensitivity and specificity was provided via statistical comparison between the four stages of esophageal cancer. To the best of our knowledge, this is the first study of using these two ratios in combination for staging ESCC.

Near-Unity Polarization of Valley-Dependent Second-Harmonic Generation in Stacked TMDC Layers and Heterostructures at Room Temperature.

With unique valley-dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second-harmonic generation (SHG) in 2D TMDCs monolayers has shown valley-dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion-symmetry-breaking (3R-like phase) TMDCs (WSe2 , WS2 , MoS2 ) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular-polarization-resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.