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Optical higher harmonic generation, including second harmonic generation and third harmonic generation, leaves no energy deposition to its interacted matters due to an energy-conservation characteristic, providing the “noninvasiveness” nature desirable for biological studies. Combined with its nonlinearity, higher harmonic generation microscopy provides excellent three-dimensional (3D) sectioning capability, offering new insights into the studies of embryonic morphological changes and complex developmental processes. By choosing a laser working in the biological penetration window, here we present a noninvasive in vivo light microscopy with sub-micron 3D resolution and millimeter penetration, utilizing endogenous higher harmonic generation signals in live specimens. Noninvasive imaging was performed in live zebrafish (Danio rerio) embryos. The complex developmental processes within >1-mm-thick zebrafish embryos can be observed in vivo without any treatment. No optical damage was found even with high illumination after long-term observations and the examined embryos all developed normally at least to the larval stage. The excellent 3D resolution of the demonstrated technology allows us to capture the subtle developmental information on the cellular or sub-cellular levels occurring deep inside the live embryos and larvae. This technique can not only provide in vivo observation of the cytoarchitecture dynamics during embryogenesis with submicron resolution and millimeter penetration depth, but would also make strong impact in developmental and structural biology studies. [Copyright &y& Elsevier]

Second-harmonic-generation (SHG) has proved itself as an important contrast mechanism in microscopic applications. Its noninvasiveness, optical sectioning capability, and high-penetrability provide attractive features in observation of thick biological tissues. Fibrous proteins, such as myosin and collagen, are dominant SHG harmonophores in vertebrates. Due to their biophotonic crystal nature, SHGs from these proteins are known to exhibit specific polarization dependencies, reflecting local molecule arrangements. Here the authors demonstrate a scheme to distinguish SHG from myosin-based muscle fibers and intertwined collagenous perimysium through polarization selection, without complicated staining or sample/image processing required. [ABSTRACT FROM AUTHOR]

Ultra-high-speed photodetectors and printed-circuit antennas construct photonic transmitters. In this letter, we demonstrate a terahertz (THz) photonic transmitter: edge-coupled membrane photonic transmitters based on metal-semiconductor-metal traveling-wave photodetectors, which are fabricated with low-temperature-grown GaAs photoabsorption layers. With a membrane-based and edge-coupled structure, the demonstrated photonic transmitters can eliminate the requirement of Si lenses and attain an over 20 times higher optical-to-THz power conversion efficiency 2 × 10[sup -4] than vertical illuminated photonic transmitters with Si lenses at the same operation frequency. [ABSTRACT FROM AUTHOR]

Silicon nanophotonics has attracted significant attention because of its unique optical properties such as efficient light confinement and low non-radiative loss. For practical applications such as all-optical switch, optical nonlinearity is a prerequisite, but the nonlinearity of silicon is intrinsically weak. Recently, we discovered a giant nonlinearity of scattering from a single silicon nanostructure by combining Mie resonance enhanced photo-thermal and thermo-optic effects. Since scattering and absorption are closely linked in Mie theory, we expect that absorption, as well as heating, of the silicon nanostructure shall exhibit similar nonlinear behaviors. In this work, we experimentally measure the temperature rise of a silicon nanoblock by in situ Raman spectroscopy, explicitly demonstrating the connection between nonlinear scattering and nonlinear heating. The results agree well with finite-element simulation based on the photo-thermo-optic effect, manifesting that the nonlinear effect is the coupled consequence of the red shift between scattering and absorption spectra. Our work not only unravels the nonlinear absorption in a silicon Mie-resonator but also offers a quantitative analytic model to better understand the complete photo-thermo-optic properties of silicon nanostructures, providing a new perspective toward practical silicon photonics applications. [ABSTRACT FROM AUTHOR]

High‐index nanostructures are found to exhibit large nonlinearity of spectral responses under photoexcitation, which is applicable to all‐optical modulation. The nonlinearity stems from Mie‐type resonance shift due to photoinduced variation of refractive index. Here, amorphous silicon‐based Huygens' metasurface absorber and ultrafast excitation are used to demonstrate two orders of magnitude enhancement of modulation depth, both theoretically and experimentally, when electric dipole lattice resonance matches the magnetic dipole resonance. Within a few square micrometer spatial confinement and picosecond scale delay, over 100% photothermal modulation depth is experimentally achieved at matched resonance condition. This concept is not limited to photothermal modulation in the amorphous silicon but is applicable to dielectric materials with various mechanisms to achieve modulation of refractive index. This work opens the avenue toward all‐optical information processing via optimized modulation by careful spatiotemporal and spectral control over a metasurface. [ABSTRACT FROM AUTHOR]

Nonlinear silicon nano‐photonics has recently attracted significant attention due to the plethora of electric and magnetic Mie resonances that offer substantial enhancement of optical nonlinearities. Conventionally, the characterization of nonlinearity and its transient nature rely on intensity‐scan methods (z‐scan) in the spatial domain and pump‐probe techniques in the temporal domain. However, most studied ultrafast nonlinear effects are instantaneous, that is, strongest at zero pump‐probe delay, and have a solitary nonlinear power dependency (square, cubic, etc.). Here the authors found that when relaxation lifetime is dependent on pump fluence, transient nonlinearity appears. The effect is exemplified via Auger‐based nonlinear carrier dynamics of a nano‐silicon Mie‐resonator. The Auger‐induced transient nonlinearity not only locates at the time delay of several tens of picoseconds, but also displays diverse nonlinearities, including sub‐linear, super‐linear, and surprisingly full saturation, which features a "crossing point" where the probe scattering is pump‐fluence independent. The crossing point exists when the relaxation lifetime is inversely dependent to second power of carrier density. Combining confocal intensity‐scan (x‐scan) and pump‐probe temporal scan, the authors demonstrate that sub‐linearity and super‐linearity lead to swelled and reduced full‐width‐at‐half‐maximum (FWHM) of single‐nanostructure images, further confirming the nonlinearity as well as the potential of sub‐diffraction microscopy. The results open up a new avenue in nonlinear silicon nano‐photonics by adding new degrees of freedom in temporally tuning the types of transient nonlinearities, which are valuable in all‐optical signal processing and nano‐imaging. [ABSTRACT FROM AUTHOR]

Understanding how the brain functions is one of the grand challenges in modern scientific research. Similar to a computer, a functional brain is composed of hardware and software. The major bottleneck lies in the difficulty to directly observe the brain 'software', i.e. the rule and operating information used by the brain that might emerge from pan-neuron/synapse connectome. A recognized strategy for probing the functional connectome is to perform volumetric imaging in brains with high spatiotemporal resolution and deep brain penetration. Among various imaging technologies, optical imaging offers appealing combinations including spatial resolution of sub-micrometer to nanometer, temporal resolution of second to millisecond, penetration depth of millimeter or deeper, and molecular contrast based on the abundant choices of fluorescent indicators. Thus, it is ideal for enabling three-dimensional functional brain mapping of small animal models. In this review, we focus on recent technological advances in optical volumetric imaging, with an emphasis on the tools and methods for enhancing imaging speed, depth, and resolution. The review could serve as a quantitative reference for physicists and biologists to choose the techniques better suited for specific applications, as well as to stimulate novel technical developments to advance brain research. [ABSTRACT FROM AUTHOR]

Hole capture time in p-type GaN was measured by using a femtosecond pump-probe technique. The capture time constant that holes are trapped by Mg-related states was found to be shorter than 10 ps. The hole capture time increases with decreased hole excess energy. By comparing two samples with different doping concentrations, it was found that the hole capture time also decreases with increased doping concentrations. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Confocal pump-probe backscattering was used to study phonon dynamics of single nanoparticles, which can extend to confocal pump-probe microscopy for investigating carrier dynamics of single nanomaterials. Compared to previous pump-probe microscopy studies, in which nonlinear optical generation was widely utilized, spectral filters were employed in this work to separate the spectra from the optical pulses for pump/probe beams. This not only reduces the complexity of the experimental setup but can also circumvent the aberration issue. Phonon dynamics, including acoustic vibrations and heat diffusion, of a single gold nanoparticle were measured with a temporal delay of 8 ns, and a 0.5 K rise in the temperature of the surrounding media was resolved. The relative optical changes induced by the photoexcitation of the pump pulses in confocal reflection pump-probe traces were orders of magnitude larger than those in typical transmission-type pump probe traces. [ABSTRACT FROM AUTHOR]

In article number 2300102, Hao-Yu Cheng, Kung-Hsuan Lin, Kuo-Ping Chen, and co-workers demonstrate both theoretically and experimentally that modulation depth is enhanced by two orders of magnitude in Huygens' metasurface absorber where electric dipole lattice resonance matches the magnetic dipole resonance. Kerker effects, Mie resonances, photothermal effects, silicon nanostructures Keywords: Kerker effects; Mie resonances; photothermal effects; silicon nanostructures EN Kerker effects Mie resonances photothermal effects silicon nanostructures 1 1 1 05/24/23 20230519 NES 230519 Mie-type optical resonators are applicable to all-optical modulation. [Extracted from the article]

Auger recombination, confocal microscopy, Mie resonance, nonlinear lifetime dependency, photothermal and thermo-optical effect, pump-probe spectroscopy Keywords: Auger recombination; confocal microscopy; Mie resonance; nonlinear lifetime dependency; photothermal and thermo-optical effect; pump-probe spectroscopy EN Auger recombination confocal microscopy Mie resonance nonlinear lifetime dependency photothermal and thermo-optical effect pump-probe spectroscopy 1 1 1 03/11/22 20220301 NES 220301 In article number 2101711, Kung-Hsuan Lin, Shi-Wei Chu, and co-workers demonstrate a temporally tunable transient nonlinearity based on fluence-dependent carrier dynamics. By combining spatial confocal x-scan and temporal pump-probe techniques, the fluence dependence is analyzed at different pump-probe delays and various transient nonlinearities are found, including sub-linear, super-linear, and surprisingly full saturation behaviors. [Extracted from the article]

Abstract: We present a new method, second harmonic generation (SHG) imaging for the study of starch structure. SHG imaging can provide the structural organization and molecular orientation information of bio-tissues without centrosymmetry. In recent years, SHG has proven its capability in the study of crystallized bio-molecules such as collagen and myosin. Starch, the most important food source and a promising future energy candidate, has, for a decade, been shown to exhibit strong SHG response. By comparing SHG intensity from different starch species, we first identified that the SHG-active molecule is amylopectin, which accounts for the crystallinity in starch granules. With the aid of SHG polarization anisotropy, we extracted the complete χ (2) tensor of amylopectin, which reflects the underlying molecular details. Through χ (2) tensor analysis, three-dimensional orientation and packing symmetry of amylopectin are determined. The helical angle of the double-helix in amylopectin is also deduced from the tensor, and the value corresponds well to previous X-ray studies, further verifying amylopectin as SHG source. It is noteworthy that the nm-sized structure of amylopectin inside a starch granule can be determined by this far-field optical method with 1-μm excitation wavelength. Since SHG is a relatively new tool for plant research, a detailed understanding of SHG in starch structure will be useful for future high-resolution imaging and quantitative analyses for food/energy applications. [Copyright &y& Elsevier]

We studied third harmonic generation (THG) of melanin solution with concentrations similar to melanocytes in human skin. In contrast to conventional observation of THG at interface, bulk THG was detected inside the solution due to the formation of melanin hydrocolloids. A linear relationship between melanin concentration and THG intensity was found, suggesting THG originated from high-order hyper-Rayleigh scattering. By fitting this linear relationship, third-order hyperpolarizability of melanin hydrocolloids was determined to be three orders larger than that of water. Our result will be useful for interpretation of THG signals in skin and other tissues containing colloidal particles. [ABSTRACT FROM AUTHOR]

By coinciding the plasmon frequency with the third-harmonic frequency of the excitation light, the authors determined the plasmon-resonance enhanced optical third-harmonic-generation (THG) susceptibility of a polyvinylpyrrolidone-coated Ag nanoparticle with a 5–7 nm diameter. With dispersed Ag nanoparticles on a quartz surface and through measuring the frequency dependent THG intensities, interface THG showed evident enhancement when the third harmonic of excitation matched the Ag-nanoparticle’s plasmon-resonant frequency. According to the effective medium theory and by analyzing the interface THG under focused Gaussian beams, the ensemble-averaged χ(3)(3ω:ω,ω,ω) of a Ag nanoparticle can be estimated to be on the order of 2×10-11 esu. [ABSTRACT FROM AUTHOR]

Nonlinear optical properties of ZnO nanorods (NRs) are strongly influenced by its dimension and aspect ratio. Size-dependent second harmonic generation (SHG) in ZnO NRs has been investigated with polarized excitations recently. However, detailed description to the SHG dependency with NR dimensions has not yet been given. In this paper, the relationship between rod diameter/length and corresponding χ(2) values based on Lorentz local field is established, both theoretically and experimentally, for the first time. Theoretically, Lorentz local field induced spectral red shift and the consequent dielectric constant modification is used to elucidate the size effect for χ(2) under the condition that both excitation and SHG wavelengths are far from the band gap. Experimentally, χ(2) of ZnO nanorods with various sizes is measured via Maker fringe technique, and the results fit well to our theory. [ABSTRACT FROM AUTHOR]