Nonlinear Optical Imaging Research Articles

Multi-plasmon resonances enhanced two-photon coherent anti-Stokes Raman scattering by nanorods.

The development of new structures allows two-photon coherent anti-Stokes Raman scattering (TPCARS) to be strongly enhanced by multiple surface plasmon resonances (MSPRs). In this paper, plasmonic structure consisting of two Ag nanorods is designed and the enhancement of TPCARS is investigated. By properly selecting designing structure parameters, strong MSPRs peaks at 1020nm and 505nm are obtained, which can enhance the TPCARS signal based on the frequency match of the fundamental frequency and frequency doubling. The enhancement factor of TPCARS can reach as high as 3.66×1028 with significant electric field enhancements under appropriate selection of system parameters. Furthermore, the two-photon process can be controlled at different optical frequencies by changing the geometric parameters of Ag nanorods. The new scheme advanced in this work can help to achieve single molecule level of CARS, and may have a potential to increase the intensity and resolution of nonlinear optical imaging.

Generation of second harmonic Bessel beams through hybrid meta-axicons.

Bessel beams are of great potential applications in many fields due to their non-diffraction and self-reconstruction. Here we firstly present a type of nonlinear meta-axicon to generate second harmonic Bessel beams. The nonlinear meta-axicons are based on Au/WS2 hybrid nanostructures. Zero-order and first-order Bessel beams of second harmonic are generated under exciting of 810 nm femtosecond laser. In addition, the performances of the nonlinear meta-axicons, such as the second harmonic generation (SHG) efficiency, non-diffracting distance and full width at half maximum (FWHM) are analyzed theoretically and experimentally. The experimental results are consistent with the predicted, which can enable miniaturized nonlinear optical devices related to generate nonlinear Bessel beams, having potential application in nonlinear optical manipulation, imaging and tractor beams.

Tissue imaging depth limit of stimulated Raman scattering microscopy.

Stimulated Raman scattering (SRS) microscopy is a promising technique for studying tissue structure, physiology, and function. Similar to other nonlinear optical imaging techniques, SRS is severely limited in imaging depth due to the turbidity and heterogeneity of tissue, regardless of whether imaging in the transmissive or epi mode. While this challenge is well known, important imaging parameters (namely maximum imaging depth and imaging signal to noise ratio) have rarely been reported in the literature. It is also important to compare epi mode and transmissive mode imaging to determine the best geometry for many tissue imaging applications. In this manuscript we report the achievable signal sizes and imaging depths using a simultaneous epi/transmissive imaging approach in four different murine tissues; brain, lung, kidney, and liver. For all four cases we report maximum signal sizes, scattering lengths, and achievable imaging depths as a function of tissue type and sample thickness. We report that for murine brain samples thinner than 2 mm transmissive imaging provides better results, while samples 2 mm and thicker are best imaged with epi imaging. We also demonstrate the use of a CNN-based denoising algorithm to yield a 40 µm (24%) increase in achievable imaging depth.

High-power 2 GHz fs pulsed all-fiber amplified laser system at 2.0 µm.

High-power femtosecond (fs)-pulsed all-fiber lasers operating at high repetition rates are highly demanded for various applications, including laser micromachining, nonlinear optical imaging, high-speed optical sampling, arbitrary waveform generation, and frequency metrology. However, their performance has long been limited by either the average power, repetition rate, pulse width, or compactness, which prevent practical applications. In this work, we report a high repetition rate fs-pulsed all-fiber laser at 2.0 µm that so far provides the best performance metrics, to the best of our knowledge, i.e., ∼2GHz fundamental repetition rate, 126 fs pulse width, ∼8W average power, and all-fiber configuration. We anticipate that this laser can be a promising fs-pulsed fiber laser source for applications requiring a GHz repetition rate.

White-Light Image Reconstruction via Seeded Modulation Instability

Nonlinear-optical image recovery is very important for underwater, atmospheric and biological imaging, but such techniques are held back in practice because dephasing of temporally and spatially incoherent white light spoils most nonlinear optical effects. The authors demonstrate the nonlinear method of $d\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ $r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e$, based on seeded modulation instability. This study shows that the signal modes of the entire temporal spectrum carry different gain characteristics, and collectively contribute to the resonance. The work boosts the application of nonlinear optics in white-light imaging with natural or artificial illumination.

Label-free multiphoton microscopy promises real-time optical molecular imaging of live tissues

A new label-free nonlinear optical imaging tool provides a comprehensive picture of the molecular, metabolic and structural composition of live tissues in real timeA new label-free nonlinear optical imaging tool provides a comprehensive picture of the molecular, metabolic and structural composition of live tissues in real time

Simultaneous label-free autofluorescence-multiharmonic microscopy and beyond.

Without sophisticated data inversion algorithms, nonlinear optical microscopy can acquire images at subcellular resolution and relatively large depth, with plausible endogenous contrasts indicative of authentic biological and pathological states. Although independent contrasts have been derived by sequentially imaging the same sample plane or volume under different and often optimized excitation conditions, new laser source engineering with inputs from key biomolecules surprisingly enable real-time simultaneous acquisition of multiple endogenous molecular contrasts to segment a rich set of cellular and extracellular components. Since this development allows simple single-beam single-shot excitation and simultaneous multicontrast epidirected signal detection, the resulting platform avoids perturbative sample pretreatments such as fluorescent labeling, mechanical sectioning, scarce or interdependent contrast generation, constraints to the sample or imaging geometry, and intraimaging motion artifacts that have limited in vivo nonlinear optical molecular imaging.

Site-specific impairment of perivascular adipose tissue on advanced atherosclerotic plaques using multimodal nonlinear optical imaging

Perivascular adipose tissue (PVAT), as a mechanical support, has been reported to systemically regulate vascular physiology by secreting adipokines and cytokines. How PVAT spatially and locally changes as atherosclerosis progresses is not known, however. We aimed to reveal the molecular changes in PVAT in advanced atherosclerosis based on multimodal nonlinear optical (MNLO) imaging. First, using an atherogenic apolipoprotein E knockout mouse model, we precisely assessed the browning level of thoracic PVAT via a correlative analysis between the size and number of lipid droplets (LDs) of label-free MNLO images. We also biochemically demonstrated the increased level of brown fat markers in the PVAT of atherosclerosis. In the initial stage of atherosclerosis, the PVAT showed a highly activated brown fat feature due to the increased energy expenditure; however, in the advanced stage, only the PVAT in the regions of the atherosclerotic plaques, not that in the nonplaque regions, showed site-specific changes. We found that p-smad2/3 and TGF-β signaling enhanced the increase in collagen to penetrate the PVAT and the agglomeration of LDs only at the sites of atherosclerotic plaques. Moreover, atherosclerotic thoracic PVAT (tPVAT) was an increased inflammatory response. Taken together, our findings show that PVAT changes differentially from the initial stages to advanced stages of atherosclerosis and undergoes spatial impairment focused on atherosclerotic plaques. Our study may provide insight into the local control of PVAT as a therapeutic target.

Nonlinear Vectorial Metasurface for Optical Encryption

In the manipulation of the propagation of light, nonlinear optical metasurfaces have enabled a plethora of applications. A key issue in this area is how to arbitrarily control the vectorial polarization profile of nonlinear optical waves. This study demonstrates an ultrathin nonlinear photonic metasurface that can generate a vectorial polarization profile of the second-harmonic-generation beam, and therefore encodes a high-resolution grayscale image. This rich polarization information has significant potential for nonlinear optical image encryption and other security applications, such as anticounterfeiting.

Nonlinear optical microscopy with achromatic lenses extending from the visible to the mid-infrared

With the advent of near-infrared broadband sources stretching into the mid-infrared (MIR) region, there is a growing demand for optical components with utility over an increasingly broad spectral range. For refractive lenses, color correction over such broad bandwidths can be a challenge. In this work, we discuss and demonstrate a two-element lens design with achromaticity spanning the visible to the mid-infrared. The air-spaced doublet designed from commercially available materials shows a significant reduction in spot size and chromatic shift compared to single lens alternatives. We have tested these new broad bandwidth achromats for the purpose of laser-scanning sum-frequency generation microscopy, confirming their improved performance for nonlinear optical imaging applications. The super broadband achromatic lenses represent an attractive alternative to reflective components in ultrabroadband applications, as they enable compact transmission-based optical designs and good focusing performance at off-axis field angles.

Nonlinear-optical stain-free stereoimaging of astrocytes and gliovascular interfaces.

Methods of nonlinear optics provide a vast arsenal of tools for label-free brain imaging, offering a unique combination of chemical specificity, the ability to detect fine morphological features, and an unprecedentedly high, subdiffraction spatial resolution. While these techniques provide a rapidly growing platform for the microscopy of neurons and fine intraneural structures, optical imaging of astroglia still largely relies on filament-protein-antibody staining, subject to limitations and difficulties especially severe in live-brain studies. Once viewed as an ancillary, inert brain scaffold, astroglia are being promoted, as a part of an ongoing paradigm shift in neurosciences, into the role of a key active agent of intercellular communication and information processing, playing a significant role in brain functioning under normal and pathological conditions. Here, we show that methods of nonlinear optics provide a unique resource to address long-standing challenges in label-free astroglia imaging. We demonstrate that, with a suitable beam-focusing geometry and careful driver-pulse compression, microscopy of second-harmonic generation (SHG) can enable a high-resolution label-free imaging of fibrillar structures of astrocytes, most notably astrocyte processes and their endfeet. SHG microscopy of astrocytes is integrated in our approach with nonlinear-optical imaging of red blood cells based on third-harmonic generation (THG) enhanced by a three-photon resonance with the Soret band of hemoglobin. With astroglia and red blood cells providing two physically distinct imaging contrasts in SHG and THG channels, a parallel detection of the second and third harmonics enables a high-contrast, high-resolution, stain-free stereoimaging of gliovascular interfaces in the central nervous system. Transverse scans of the second and third harmonics are shown to resolve an ultrafine texture of blood-vessel walls and astrocyte-process endfeet on gliovascular interfaces with a spatial resolution within 1 μm at focusing depths up to 20 μm inside a brain.

Denoising of stimulated Raman scattering microscopy images via deep learning.

Stimulated Raman scattering (SRS) microscopy is a label-free quantitative chemical imaging technique that has demonstrated great utility in biomedical imaging applications ranging from real-time stain-free histopathology to live animal imaging. However, similar to many other nonlinear optical imaging techniques, SRS images often suffer from low signal to noise ratio (SNR) due to absorption and scattering of light in tissue as well as the limitation in applicable power to minimize photodamage. We present the use of a deep learning algorithm to significantly improve the SNR of SRS images. Our algorithm is based on a U-Net convolutional neural network (CNN) and significantly outperforms existing denoising algorithms. More importantly, we demonstrate that the trained denoising algorithm is applicable to images acquired at different zoom, imaging power, imaging depth, and imaging geometries that are not included in the training. Our results identify deep learning as a powerful denoising tool for biomedical imaging at large, with potential towards in vivo applications, where imaging parameters are often variable and ground-truth images are not available to create a fully supervised learning training set.

Enhancement of Nonlinear Optical Scattering by Gold Nanoparticles through Aggregation‐Induced Plasmon Coupling in the Near‐Infrared

Gold nanoparticles (AuNPs) are regarded as promising building blocks in functional nanomaterials for sensing, drug delivery and catalysis. One remarkable property of these particles is the localized surface plasmon resonance (LSPR), which gives rise to augmented optical properties through local field enhancement. LSPR also influences the nonlinear optical properties of metal NPs (MNPs) making them potentially interesting candidates for fast, high resolution nonlinear optical imaging. In this work we characterize and discuss the wavelength dependence of the hyper-Rayleigh scattering (HRS) behavior of spherical gold nanoparticles (GNP) and gold nanorods (GNR) in solution, from 850 nm up to 1300 nm, covering the near-infrared (NIR) window relevant for deep tissue imaging. The high-resolution spectral data allows discriminating between HRS and two photon photoluminescence contributions. Upon particle aggregation, we measured very large enhancements (ca. 104 ) of the HRS intensity in the NIR, which is explained by considering aggregation-induced plasmon coupling effects and local field enhancement. These results indicate that purposely designed coupled nanostructures could prove advantageous for nonlinear optical imaging and biosensing applications.

In Vitro Quantification of Single Red Blood Cell Oxygen Saturation by Femtosecond Transient Absorption Microscopy.

Hemoglobin, the oxygen carrying protein, ferries nearly all bodily oxygen from the lungs to cells and tissues in need. Blood oxygen saturation (sO2) thus plays an important role in maintaining energy homeostasis throughout the body. Clinical and research tools have been developed to monitor sO2 at a wide range of temporal and spatial scales. However, real-time quantification of sO2 at single red blood cell (RBC) resolution remains challenging. Such capability is critically important to study energy metabolism in heterogeneous tissues including brain and tumor tissue. In this work, we develop a ratiometric transient absorption microscopy technique to image hemoglobin sO2. By exploiting differences in transient lifetime kinetics betweenoxyhemoglobinanddeoxyhemoglobin, we directly quantified the sO2 of single RBCs in real-time without the need for injection of exogenous agents. This simple and high-speed nonlinear optical imaging technique is well suited for in vitro and in vivo quantification of sO2.

Depth resolved label-free multimodal optical imaging platform to study morpho-molecular composition of tissue

Multimodal imaging platforms offer a vast array of tissue information in a single image acquisition by combining complementary imaging techniques. By merging different systems, better tissue characterization can be achieved than is possible by the constituent imaging modalities alone. The combination of optical coherence tomography (OCT) with non-linear optical imaging (NLOI) techniques such as two-photon excited fluorescence (TPEF), second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) provides access to detailed information of tissue structure and molecular composition in a fast, label-free and non-invasive manner. We introduce a multimodal label-free approach for morphomolecular imaging and spectroscopy and validate the system in mouse skin demonstrating the potential of the system for colocalized acquisition of OCT and NLOI signals.

Ultrashort pulse Kagome hollow-core photonic crystal fiber delivery for nonlinear optical imaging.

We report ultrashort pulse delivery through a hypocycloid-core inhibited-coupling Kagome hollow-core photonic crystal fiber (HC-PCF). Undistorted 10 fs and 6.6 nJ pulses were launched through 1 m long fiber without fiber dispersion pre-compensation and 80% efficiency. The performance of this technology for biomedical imaging is demonstrated on a biological sample by incorporating the fiber into a two-photon excited fluorescence (TPEF) laser scanning microscope (LSM) achieving a pulse width of 15 fs at the sample location. To the best of our knowledge, this is the first report on undistorted TPEF imaging in a LSM with 15 fs pulses delivered through a 1 m long Kagome HC-PCF with high throughput.

Optical Second-harmonic Observation of Stimulated Sacran Aggregates

Second-order nonlinear optical images of aggregates of the ampholytic megamolecular polysaccharide sacran under various stimuli were observed by optical second-harmonic generation (SHG) microscopy. SHG intensity microspots of several tens of micrometers in size are seen for sacran cotton-like lumps and fibers and they have very clear incident polarization dependence. In these microspots, the sacran molecules are oriented in concentric multilayers. We also observed SHG images of sacran prepared in a needle-ring electrode with applied voltage, where SHG was enhanced near the negatively biased needle electrode. We also observed SHG signals near the cast film edges of sacran. The appearance of the SHG image suggested a phase-separation structure in sacran aggregates. These results show that sacran molecules aggregate in several different ways.

Label-Free Assessment of Premalignant Gastric Lesions Using Multimodal Nonlinear Optical Microscopy

In this paper, a nonlinear optical microscopy employing two-photon excited fluorescence and second-harmonic generation was used for the detection of premalignant gastric lesions. It was found that gland morphology and collagen structure in mucosa will change with the progression of gastric diseases from normal to intestinal metaplasia, to low-grade intraepithelial neoplasia, and to high-grade intraepithelial neoplasia, and this microscopy was able to directly distinguish these warning symptoms. Furthermore, two features were quantified from nonlinear optical images to demonstrate the changes of gland size and collagen content during the development process of preneoplastic lesions. These results clearly show that nonlinear optical microscopy can effectively differentiate normal and precancerous gastric tissues without contrast agents, which would be helpful for early diagnosis and treatment of gastric diseases. This study may provide the groundwork for further application of nonlinear optical microscopy in clinical practice.

Coupling configurations between extended surface electromagnetic waves and localized surface plasmons for ultrahigh field enhancement

Abstract Local enhancement of electromagnetic (EM) fields near dielectric and metallic surfaces is usually associated with the existence of a confined EM wave at least in one direction. This phenomenon finds applications in enhancing optical spectroscopic signals, optical emission, nonlinear optical processes, biosensing, imaging contrast and superresolution, photovoltaics response, local heating, photocatalysis, and enhanced efficiency of optoelectronic devices. A well-known example is when the surface electromagnetic wave (SEW) is excited at the interface of two media, the field gets enhanced normally to that interface. This article reviews the different configurations revealing enhanced EM fields, particularly those giving ultrahigh enhancement, such as when a localized SEW is excited not from free space but via an extended SEW. Of particular interest are surface plasmon waves (SPWs) excited at the surface of metal-dielectric and particularly when exciting localized SPWs using extended ones. The latter case so far gave the highest local field enhancement; however, configurations involving Bloch SEWs, guided mode resonances, and cavity resonances have also been shown to give significant enhancement when used to excite localized surface plasmons. With this strategy, field enhancement by more than an order of magnitude can be attained. Using this ultrahigh enhancement, the strong coupling experiments between molecules and the intense optical field will be possible and new devices may emerge from those new methodologies for ultrahigh sensitive sensing for environmental and medical applications, as well as for improved optoelectronic devices.

Size Modification of Optically Active Contamination‐Free Silicon Nanoparticles with Paramagnetic Defects by Their Fast Synthesis and Dissolution

Contamination‐free silicon‐based nanoparticles (NPs) with several modalities are developed in this work. They are formed by non‐toxic laser‐assisted decomposition of silicon microgranules homogeneously dispersed in deionized water. Precise control of numerous experimental parameters allows repeatable fine tuning of nanoparticle size solving significant lacks of a direct laser ablation routine. Such a method provokes a huge amount of paramagnetic defect states of optically active silicon NPs that can serve as a contrast agent for magnetic resonance and nonlinear optical imaging. Perspectives of their bioapplication are driven by detected their fast size degradation in a NaCl‐based medium.