Optical second- and third-harmonic generations have attracted a lot of attention in the biomedical imaging research field recently due to their intrinsic sectioning ability and noninvasiveness. Combined with near-infrared excitation sources, their deep-penetration ability makes these imaging modalities suitable for tissue characterization. In this article, we demonstrate a polarization harmonics optical microscopy, or P-HOM, to study the nonlinear optical anisotropy of the nanometer-scaled myosin and actin filaments inside myofibrils. By using tight focusing we can avoid the phase-matching condition due to micron-scaled, high-order structures in skeletal muscle fibers, and obtain the submicron-scaled polarization dependencies of second/third-harmonic generation intensities on the inclination angle between the long axes of the filaments and the polarization direction of the linear polarized fundamental excitation laser light. From these dependencies, detailed information on the tensor elements of the second/third-order nonlinear susceptibilities contributed from the myosin/actin filaments inside myofibrils can thus be analyzed and obtained, reflecting the detailed arrangements and structures of the constructing biomolecules. By acquiring a whole, nonlinearly sectioned image with a submicron spatial resolution, we can also compare the polarization dependency and calculate the nonlinear susceptibilities over a large area of the tissue at the same time-which not only provides statistical information but will be especially useful with complex specimen geometry.

We demonstrate a novel multimodal nonlinear spectral microscopy based on a femtosecond Cr:forsterite laser at 1230 nm. By acquiring the whole nonlinear spectrum in the visible and near-NIR region, this novel technique allows a combination of different imaging modalities, including second-harmonic generation, third-harmonic generation, and multiple-photon fluorescence. Combined with the selected excitation wavelength, which is located in the IR transparency window, this microscopic technique can provide high penetration depth with reduced damage and is ideal for studying living cells.

We provide evidence that the chirality of collagen can give rise to strong second-harmonic generation circular dichroism (SHG-CD) responses in nonlinear microscopy. Although chirality is an intrinsic structural property of collagen, most of the previous studies ignore that property. We demonstrate chiral imaging of individual collagen fibers by using a laser scanning microscope and type-I collagen from pig ligaments. 100% contrast level of SHG-CD is achieved with sub-micrometer spatial resolution. As a new contrast mechanism for imaging chiral structures in bio-tissues, this technique provides information about collagen morphology and three-dimensional orientation of collagen molecules.

Abstract:We solve the Maxwell and heat equations selfconsistently for metal nanoparticles under intense continuous wave (CW) illumination. Unlike previous studies, we rely on experimentally-measured data for metal permittivity for increasing temperature and for the visible spectral range. We show that the thermal nonlinearity of the metal can lead to substantial deviations from the predictions of the linear model for the temperature and field distribution and, thus, can explain qualitatively the strong nonlinear scattering from such configurations observed experimentally. We also show that the incompleteness of existing data of the temperature dependence of the thermal properties of the system prevents reaching a quantitative agreement between the measured and calculated scattering data. This modeling approach is essential for the identification of the underlying physical mechanism responsible for the thermo-optical nonlinearity of the metal and should be adopted in all applications of hightemperature nonlinear plasmonics, especially for refractory metals, for both CW and pulsed illumination.

We show that scattering from a single gold nanoparticle is saturable for the first time. Wavelength-dependent study reveals that the saturation behavior is governed by depletion of surface plasmon resonance, not the thermal effect. We observed interesting flattening of the point spread function of scattering from a single nanoparticle due to saturation. By extracting the saturated part of scattering via temporal modulation, we achieve λ/8 point spread function in far-field imaging with unambiguous separation of adjacent particles.

Nonlinear optical interaction is crucial to alloptical signal processing. In metallic nanostructures, both linear and nonlinear optical interactions can be greatly enhanced by surface plasmon resonance (SPR). In the last few decades, saturation and reverse saturation of absorption in plasmonic materials have been unraveled. It is known that scattering is one of the fundamental light−matter interactions and is particularly strong in metallic nanoparticles due to SPR. However, previous methods measure response from ensemble of nanoparticles and did not characterize scattering on a single particle basis. Here we report that backscattering from an isolated gold nanoparticle exhibits not only saturation, but also reverse saturation. Wavelength-dependent and intensitydependent studies reveal that nonlinear scattering is dominated by SPR and shares a similar physical origin with nonlinear absorption. The reversibility and repeatability of saturable scattering (SS) and reverse saturable scattering (RSS) are validated via repetitive excitation on the same set of particles. Compared to fluorescence, our novel discovery of single-particle-based SS and RSS does not suffer from bleaching and can be used as a more robust contrast agent for optical microscopy. Under a reflection confocal microscope, interesting point-spread functions are observed, with full-width-of-half-maximum of central and side lobes reduced to λ/5 and λ/13, showing great potential for superresolution microscopy.

The problem of weak harmonic generation signal intensity limited by photodamage probability in optical microscopy and spectroscopy could be resolved by increasing the repetition rate of the excitation light source. Here we demonstrate the first photomultiplier-based real-time second-harmonic-generation microscopy taking advantage of the strongly enhanced nonlinear signal from a high-repetition-rate Ti:sapphire laser. We also demonstrate that the photodamage possibility in common biological tissues can be efficiently reduced with this high repetition rate laser at a much higher average power level compared to the commonly used ~80- MHz repetition rate lasers.

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm 2. The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.