Imaging the Nonlinear Susceptibility Tensor of Collagen by Nonlinear Optical Stokes Ellipsometry

Optical diffraction tomography of second-order nonlinear structures in weak scattering media: theoretical analysis and experimental consideration

Computed tomography (CT) allows for high lateral and axial resolution imaging of the endogenous structure of matter thanks to its large spatial frequency support and has been realized in X-ray and linear optical domain known as optical diffraction tomography (ODT). Here, we present the theoretical basis and experimental considerations for ODT of second-order nonlinear structures in weak scattering media. We have derived the relation between second harmonic wave and the anisotropic nonlinear tensor in spatial frequency domain under first-order Born approximation. Our results show that, under a plane wave illumination, the two dimensional (2D) spatial spectra of generated second harmonic complex field relates to the inverse lattice of nonlinear structure on Ewald sphere shells. The centers of the Ewald spheres are determined by 2 times wavevector of the incident fundamental wave and the radii are determined by the modulus of the second harmonic wavevector. More importantly, it shows that the 2D spatial spectra is a superposition of the Ewald spheres of different components of the anisotropic nonlinear tensor. We propose to solve the inverse problem by controlling the polarizations of the fundamental and second harmonic signal. We tested the feasibility of the proposed method using a numerical phantom and make some discussions on practical implementations, including angular scanning schemes, polarization detection and illumination profile for optimizing reconstruction region. Possessing high resolution, wide-field imaging and polarization-sensitive property, we believe that the proposed scheme would have important applications in nonlinear microscopy.

Diagnosing temporomandibular joint disorders using second harmonic imaging of collagen fibers

This proposed optical imaging method is a nondestructive, real-time and high-resolution approach to distinguish healthy and injured temporomandibular joint (TMJ) tissues. And the TMJ health index was invented. TMJ pathologies are commonly and reported frequently. It could be associated with the damage of collagen, cartilage and bone tissue. The second harmonic generation images could be obtained by a femtosecond laser pulses, so the aligned information of the collagen fibers in all directions for the TMJ disorders was collected. The disorder degree of collagen fibers was quantified and ranked using a fast Fourier transform (FFT) method. The TMJ health index can effectively present the TMJ healthy condition and the disorder degree of collagen fibers, a valuable objective tool for tissue characterization for TMJ healthy condition. Integrated with the staining methods, we can provide the scaling information at different injury degree.

Sum frequency generation as a proxy for ellipsometry: Not just a phase

Infrared refractive indices of organic materials are typically resolved through IR ellipsometry. This technique takes advantage of optical interference effects to solve the optical constants. These are the same effects that complicate the analysis of coherent spectroscopy experiments on thin films. Vibrational sum frequency generation is an interface-specific coherent spectroscopy that requires spectral modeling to account for optical interference effects to uncover interfacial molecular responses. Here, we explore the possibility of leveraging incident beam geometries and sample thicknesses to simultaneously obtain the molecular responses and refractive indices. Globally fitting a higher number of spectra with a single set of refractive indices increases the fidelity of the fitted parameters. Finally, we test our method on samples with a range of thicknesses and compare the results to those obtained by IR ellipsometry.

Phase-Sensitive Vibrationally Resonant Sum-Frequency Generation Microscopy in Multiplex Configuration at 80 MHz Repetition Rate

Vibrationally resonant sum-frequency generation (VR SFG) microscopy is an advanced imaging technique that can map out the intensity contrast of infrared and Raman active vibrational modes with micron to submicron lateral resolution. To broaden its applications and to obtain a molecular level of understanding, further technical advancement is needed to enable high-speed measurements of VR SFG microspectra at every pixel. In this study, we demonstrate a new VR SFG hyperspectral imaging platform combined with an ultrafast laser system operated at a repetition rate of 80 MHz. The multiplex configuration with broadband mid-infrared pulses makes it possible to measure a single microspectrum of CH/CH₂ stretching modes in biological samples, such as starch granules and type I collagen tissue, with an exposure time of hundreds of milliseconds. Switching from the homodyne- to heterodyne-detected VR SFG hyperspectral imaging can be achieved by inserting a pair of optics into the beam path for local oscillator generation and delay time adjustment, which enables self-phase-stabilized spectral interferometry. We investigate the relationship between phase images of several different C-H modes and the relative orientation of collagen triple-helix in fibril bundles. The results show that the new multiplex VR SFG microscope operated at a high repetition rate is a powerful approach to probe the structural features and spatial arrangements of biological systems in detail.

Recent Progress in Optical Sensors for Biomedical Diagnostics

In recent years, several types of optical sensors have been probed for their aptitude in healthcare biosensing, making their applications in biomedical diagnostics a rapidly evolving subject. Optical sensors show versatility amongst different receptor types and even permit the integration of different detection mechanisms. Such conjugated sensing platforms facilitate the exploitation of their neoteric synergistic characteristics for sensor fabrication. This paper covers nearly 250 research articles since 2016 representing the emerging interest in rapid, reproducible and ultrasensitive assays in clinical analysis. Therefore, we present an elaborate review of biomedical diagnostics with the help of optical sensors working on varied principles such as surface plasmon resonance, localised surface plasmon resonance, evanescent wave fluorescence, bioluminescence and several others. These sensors are capable of investigating toxins, proteins, pathogens, disease biomarkers and whole cells in varied sensing media ranging from water to buffer to more complex environments such as serum, blood or urine. Hence, the recent trends discussed in this review hold enormous potential for the widespread use of optical sensors in early-stage disease prediction and point-of-care testing devices.

Stochastic Differential Scanning Calorimetry by Nonlinear Optical Microscopy

Stochastic phase transformations within individual crystalline particles were recorded by integration of second harmonic generation (SHG) imaging with differential scanning calorimetry (DSC). The SHG activity of a crystal is highly sensitive to the specific molecular packing arrangement within a noncentrosymmetric lattice, providing access to information otherwise unavailable by conventional imaging approaches. Consequently, lattice transformations associated with dehydration/desolvation events were readily observed by SHG imaging and directly correlated to the phase transformations detected by the DSC measurements. Following studies of a model system (urea), stochastic differential scanning calorimetry (SDSC) was performed on trehalose dihydrate, which has a more complex phase behavior. From these measurements, SDSC revealed a broad diversity of single-particle thermal trajectories and direct evidence of a "cold phase transformation" process not observable by the DSC measurements alone.

A Low-Cost Beam-Scanning Second Harmonic Generation Microscope with Application for Agrochemical Development and Testing

A low-cost second harmonic generation (SHG) microscope was constructed, and, for the first time, SHG microscopy was used for imaging agrochemical materials directly on the surface of common commercial crop leaves. The microscope uses a chromatically fixed (1560 nm) femtosecond fiber laser, a commercial 2D galvanometer mirror system, and a PCIe digital oscilloscope card, which together kept total instrument costs under $40 000 (USD), a significant decrease in cost and complexity from common systems (commercial and home-built) using tunable lasers and faster beam-scanning architectures. The figures of merit of the low-cost system still enabled a variety of measurements of agrochemical materials. Following confirmation of largely background-free SHG imaging of common crop leaves (soybean, maize, wheatgrass), SHG microscopy was used to image active ingredient crystallization after solution-phase deposition directly on the leaf surface, including at industrially relevant active ingredient concentrations (<0.05% w/w). Crystallization was also followed in real-time, with differences in crystallization time observed for different application procedures (spraying vs single droplet deposition). A strong dependency of active ingredient crystallization on the substrate was found, with an increased crystallization tendency observed on leaves vs on glass slides. Different crystal habits for the same active ingredient were also observed on different plant species. Finally, a model extended-release formulation was prepared, with a decrease in active ingredient crystallinity observed vs solution-phase deposition. These collective results demonstrate the need for making diagnostic measurements directly on the leaf surface and could help inform the next generation of pesticide products that ensure optimized agricultural output for a growing world population.

Mueller Tensor Nonlinear Optical Polarization Analysis in Turbid Media

A mathematical framework to treat partial polarization in second harmonic generation imaging of nonlinear optical susceptibility is described and applied to imaging tissue sections 5, 40, and 70 μm thick, sufficient to introduce significant depolarization of the incident field. Polarization analysis becomes complicated in turbid media, in which scattering can result in degradation of polarization purity. The simplest framework for describing the polarization of purely polarized light is the Jones framework, which has been applied to great effect in the polarization analysis of second harmonic generation. However, the Jones framework lacks the necessary generality to describe a partially polarized electric field, (i.e., ones positioned within the volume of the Poincaré sphere rather than on the surface). Recent work connecting the Jones framework to the Mueller-Stokes framework has enabled interpretation of results with the more intuitive Jones framework while maintaining generality of the Mueller-Stokes method. The magnitude and nature of linear interactions of the tissue with the incident infrared field are discussed. Despite substantial depolarization, the nonlinear optical susceptibility tensor elements of collagen was recoverable at each pixel images of thick tissue utilizing the described framework. For thick and thin tissues, values of the tensor element ratio ρ were recovered in good agreement with previous studies. Both hyperpolarizing and depolarizing effects of SHG were observed, and the mechanism of hyperpolarization was determined to rest upon the interplay of orientation and relative contribution of polarized and depolarized incident light to elicit SHG.

Changes in the crystallographic structures of cardiac myosin filaments detected by polarization-dependent second harmonic generation microscopy

Detecting the structural changes caused by volume and pressure overload is critical to comprehending the mechanisms of physiologic and pathologic hypertrophy. This study explores the structural changes at the crystallographic level in myosin filaments in volume- and pressure-overloaded myocardia through polarization-dependent second harmonic generation microscopy. Here, for the first time, we report that the ratio of nonlinear susceptibility tensor components d₃₃/d₁₅ increased significantly in volume- and pressure-overloaded myocardial tissues compared with the ratio in normal mouse myocardial tissues. Through cell stretch experiments, we demonstrated that mechanical tension plays an important role in the increase of d₃₃/d₁₅ in volume- and pressure-overloaded myocardial tissues.

Monitoring dynamic collagen reorganization during skin stretching with fast polarization-resolved second harmonic generation imaging

The mechanical properties of biological tissues are strongly correlated to the specific distribution of their collagen fibers. Monitoring the dynamic reorganization of the collagen network during mechanical stretching is however a technical challenge, because it requires mapping orientation of collagen fibers in a thick and deforming sample. In this work, a fast polarization-resolved second harmonic generation microscope is implemented to map collagen orientation during mechanical assays. This system is based on line-to-line switching of polarization using an electro-optical modulator and works in epi-detection geometry. After proper calibration, it successfully highlights the collagen dynamic alignment along the traction direction in ex vivo murine skin dermis. This microstructure reorganization is quantified by the entropy of the collagen orientation distribution as a function of the stretch ratio. It exhibits a linear behavior, whose slope is measured with a good accuracy. This approach can be generalized to probe a variety of dynamic processes in thick tissues.

Analyzing the feasibility of discriminating between collagen types I and II using polarization-resolved second harmonic generation

According to previous studies, the nonlinear susceptibility tensor ratio χ₃₃ /χ₃₁ obtained from polarization-resolved second harmonic generation (P-SHG) under the assumption of cylindrical symmetry can be used to distinguish between fibrillar collagen types. Discriminating between collagen fibrils of types I and II is important in tissue engineering of cartilage. However, cartilage has a random organization of collagen fibrils, and the assumption of cylindrical symmetry may be incorrect. In this study, we simulated the P-SHG response from different collagen organizations and demonstrated a possible method to exclude areas where cylindrical symmetry is not fulfilled and where fibrils are located in the imaging plane. The χ₃₃ /χ₃₁ -ratio for collagen type I in tendon and collagen type II in cartilage was estimated to be 1.33 and 1.36, respectively, using this method. These ratios are now much closer than what has been reported previously in the literature, and the larger reported differences between collagen types can be explained by variation in the structural organization.

Spatially encoded polarization-dependent nonlinear optics

A single fixed optic is combined with the sample translation capabilities inherent to most microscopes to achieve precise polarization-dependent second harmonic generation microscopy measurements of thin tissue sections. Although polarization measurements have enabled detailed structural analysis of collagen, challenges in integrating rotation stages or fast electro-optic/photoelastic modulation have complicated the retrofitting of existing systems for precise polarization analysis. Placing a static microretarder array in the rear conjugate plane resulted in spatially encoded polarization modulation across the field of view. A complete set of polarization rotation measurements was acquired at each pixel by sample translation, recovering local-frame tensors relating to structure within collagenous tissue.

Second Harmonic Generation of Unpolarized Light

A Mueller tensor mathematical framework was applied for predicting and interpreting the second harmonic generation (SHG) produced with an unpolarized fundamental beam. In deep tissue imaging through SHG and multiphoton fluorescence, partial or complete depolarization of the incident light complicates polarization analysis. The proposed framework has the distinct advantage of seamlessly merging the purely polarized theory based on the Jones or Cartesian susceptibility tensors with a more general Mueller tensor framework capable of handling partial depolarized fundamental and/or SHG produced. The predictions of the model are in excellent agreement with experimental measurements of z-cut quartz and mouse tail tendon obtained with polarized and depolarized incident light. The polarization-dependent SHG produced with unpolarized fundamental allowed determination of collagen fiber orientation in agreement with orthogonal methods based on image analysis. This method has the distinct advantage of being immune to birefringence or depolarization of the fundamental beam for structural analysis of tissues.

Three-Dimensional Geometry of Collagenous Tissues by Second Harmonic Polarimetry

Collagen is a biological macromolecule capable of second harmonic generation, allowing label-free detection in tissues; in addition, molecular orientation can be determined from the polarization dependence of the second harmonic signal. Previously we reported that in-plane orientation of collagen fibrils could be determined by modulating the polarization angle of the laser during scanning. We have now extended this method so that out-of-plane orientation angles can be determined at the same time, allowing visualization of the 3-dimensional structure of collagenous tissues. This approach offers advantages compared with other methods for determining out-of-plane orientation. First, the orientation angles are directly calculated from the polarimetry data obtained in a single scan, while other reported methods require data from multiple scans, use of iterative optimization methods, application of fitting algorithms, or extensive post-optical processing. Second, our method does not require highly specialized instrumentation, and thus can be adapted for use in almost any nonlinear optical microscopy setup. It is suitable for both basic and clinical applications. We present three-dimensional images of structurally complex collagenous tissues that illustrate the power of such 3-dimensional analyses to reveal the architecture of biological structures.