Second Harmonic Generation of Lamb Wave in Numerical Perspective

The influences of phase and group velocity matching on cumulative second harmonic generation of Lamb waves are investigated in numerical perspective. Finite element simulations of nonlinear Lamb wave propagation are performed for Lamb wave mode pairs with exact and approximate phase velocity matching, with and without group velocity matching, respectively. The evolution of time-domain second harmonic Lamb waves is analyzed with the propagation distance. The amplitudes of primary and second harmonic waves are calculated to characterize the acoustic nonlinearity. The results verify that phase velocity matching is necessary for generation of the cumulative second harmonic Lamb wave in numerical perspective, while group velocity matching is demonstrated to not be a necessary condition.

Photoinduced Second Harmonic Generation of Bi_2S_3 Microcrystallite Doped Silica Glass

Silica glasses doped with Bi2S3 microcystallite was prepared by the sol-gel process. Photoinduced second harmonic generation (SHG) was observed in the glass when it was irradiated with intense 1.06 mum and frequency doubled laser beams from a mode-locked Nd: YAG laser. It was found that the signal intensity increased with the irradiating time and approached a saturation gradually. The effect may be explained reasonably by the DC field model.

Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional(2D)materials has opened new possibilities for exhibiting second harmonic genera-tion(SHG)at the nanoscale,due to their remarkable optical response related to stable excitons at room temperature.However,the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides(TM-Ds)monolayers naturally weak.Here,we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum(BIC).The BIC slabs are designed to exhibit a pair of BICs,reson-ant with both the fundamental wave(FW)and the second harmonic wave(SHW).Firstly,the spatial mode matching can be fulfilled by tilting FW's incident angle.We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs,under a pump light intensity of 0.1 GW/cm^(2).Moreover,we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient,which leads to an extra three orders of magnitude enhancement of SHG efficiency.These results demonstrate remarkable pos-sibilities for enhancing SHG with nonlinear 2D materials,opening many opportunities for chip-based light sources,nano-lasers,imaging,and biochemical sensing.

MULTIPHOTON MICROSCOPIC IMAGING OF MOUSE INTESTINAL MUCOSA BASED ON TWO-PHOTON EXCITED FLUORESCENCE AND SECOND HARMONIC GENERATION

Multiphoton microscopy(MPM),based on two-photon excited fuorescence and second harmonic generation,enables direct noninvasive visualization of tissue architecture and cell morphology in live tissues without the administration of exogenous contrast agents.In this paper,we used MPM to image the microstructures of the mucosa in fresh,unfixed,and unstained intestinal tissue of mouse.The morphology and distribution of the main components in mucosa layer such as columnar cells,goblet cells,intestinal glands,and a little collagen fibers were clearly observed in MPM images,and then compared with standard H&:E images from paired specimens.Our results indicate that MPM combined with endoscopy and miniaturization probes has the potential application in the clinical diagnosis and in vivo monitoring of early intestinal cancer.

Orientation Angle of Molecules at Hexadecane-Water Interface Studied with Total Internal Reflection Second Harmonic Generation

The orientation angle is an important parameter that reflects the structure of molecules at interfaces. In order to obtain this parameter, second order nonlinear spectroscopic techniques including second harmonic generation （SHG） and sum frequency generation-vibrational spec- troscopy （SFG-VS） have been successfully applied through analysis of the nonlinear signal from various polarizations. In some SHG and SFG-VS experiments, total internal reflection （TIR） configuration has been adopted to get enhanced signals. However, the reports on the detailed procedure of the polarization analysis and the calculation of the orientation angle of interracial molecules under TIR configuration are still very few. In this paper, we mea- sured the orientation angles of two molecules at the hexadecane-water interface under TIR and Non-TIR experimental configurations. The results measured from polarization analysis in TIR configuration consist with those obtained from Non-TIR configuration. This work demonstrates the feasibility and accuracy of polarization analysis in the determination of the orientation angle of molecules at the interfaces under TIR-SHG configuration.

Irradiation Effect on Second Harmonic Generation of Dyes Doped KDP Crystals

Most irradiation studies in the hydrogen bonded ferroelectrics have been concentrated on the transient defects induced by ionising radiation, such as ultraviolet （UV） light, where the defects are closely related to the optical properties. But heavy ion beam irradiation effects have rarely been studied. The structural, optical, and non-linear optical properties of the doped crystals were analyzed with the characterization studies, such as powder XRD, UV-Visible and second harmonic generation （SHG） measurements, respectively. The results for doped KDP crystal were compared with the results of the pure KDP crystals. The experiment results showed that Li^3＋ irradiation leads to the development of a well-defined surface H peak in dye doped KDP crystals. The stability of KDP single crystal was improved by doping organic dyes. The nano-islands of dye in KDP were likely to be dissolved and enhance the non-linear optical properties of these materials.

Surface second harmonic generation of chiral molecules using three-coupled-oscillator model

Based on the three-coupled-oscillator molecular model we proposed, the relation between the second-order susceptibilities of a chiral film and the molecular hyperpolarizabilities is given. The effect of microscopic parameters on the second-order susceptibilities is simulated numerically and the difference between the efficiencies of s-polarized secondharmonic fields induced by the left- and the right-handed circularly-polarized fundamental beams is discussed. The theoretical basis for studying second-order nonlinear optical properties of the chiral molecular media with a tripod-like structure is provided in this paper.

Properties of periodic multicrystal configurations in walk-off-compensating second harmonic generation of ultrashort pulses

This work designs a four-platelet periodic multicrystal configuration in the second harmonic generation of ultra- short pulses as a new walk-off-compensating device. It theoretically investigates a proposed active and a typical passive compensating scheme with the undepleted-pump approximation. The result shows that the angular and spectral band- widths are proportional to the number of crystal pairs as expected, but the temperature tunability is basically unaltered owing to inter-plate pulse interference. At the same time, an analysis reveals that a misuse of the phase mismatch factor is responsible for a historic controversy about pulse interference. A real design of an ultraviolet second harmonic generation （262.5 nm） is considered in a passive periodic [3-Barium Borate-calcite configuration, where the inter-plate pulse interference is found to form an azimuthal tuning restriction and to lower plate length tolerance. A subsequent numerical simulation with pump depletion is in good accordance with theoretical prediction.

Study of narrow-band second harmonic generation from a broad-band fundamental pulse

This paper studies the type-I phase-matched second harmonic generation using 25-fs input laser pulses in a thick BBO crystal. The harmonic signal exhibits a narrow spectrum bandwidth, even though the input pulse has a broad bandwidth. The energy transfer efficiency and modulation of the fundamental spectrum are investigated.

Second harmonic generation in inhomogeneous MgO:LiNbO_3 waveguides

A fibre laser at 1111.6 nm is frequency doubled by two inhomogeneous MgO：LiNbO3 waveguides and the output powers of 85 mW and 49 mW at 555.8 nm have been generated with the conversion efficiencies of 47% and 27% respectively. By analysing the second harmonic generation temperature tuning curves, we investigate the influence of the optical inhomogeneities upon the conversion efficiency. The final result shows that the efficiency difference is mainly affected by the optical inhomogeneities in our case.

Second harmonic generation of metal nanoparticles under tightly focused illumination

The near-field and far-field second harmonic （SH） responses of a metal spherical nanoparticle placed in the focal region of a highly focused beam are investigated by using the calculation model based on three-dimensional finite-difference time-domain （FDTD） method. The results show that off-axis backward-propagating SH response can be reinforced by tightly focusing, due to the increase of the relative magnitude of the longitudinal field component and the phase shift along the propagation direction.

Temperature-dependent second harmonic generation process based on an MgO-doped periodically poled lithium niobate waveguide

The temperature dependency of a 5-mo1% MgO-doped periodically poled lithium niobate waveguide was investi- gated in this paper. We started with the temperature-dependent refractive index equation for the waveguide. Secondly, the temperature dependency of the second harmonic generation effect was experimentally researched under different temperatures and pump powers. The quasi-phase matched wavelengths, efficiency bandwidths and peak efficiencies of the waveguide were measured. The experimental results agreed with theoretical simulations, which are indispensable in the following all-optical sampling studies based on the cascaded second harmonic generation/difference-frequency generation process in the current device.

Broadband and efficient second harmonic generation in LiNbO_(3)-LiTaO_(3) composite ridge waveguides at telecom-band

Broadband nonlinear frequency conversions of optical waves are widely employed in multiple areas of optics and photonics. However, the broadening of conversion bandwidth is often at a cost of reduction in efficiency, which may induce a limitation on practical applications. Here we theoretically propose a novel design of LiNbO_(3) ridge waveguides on LiTaO_(3) substrates which can be used for efficient and broadband second harmonic generation. Through group velocity engineering of the ridge waveguides, acceptance bandwidth over 20 nm with a high conversion efficiency of > 25%W^(-1)·cm^(-2) is achieved at telecom-band.

Second harmonic generation from precise diamond blade diced ridge waveguides

Carbon ion irradiation and precise diamond blade dicing are applied to fabricate Nd∶GdCOB ridge waveguides.The propagation properties of the fabricated Nd∶GdCOB waveguides are investigated through experiments and theoret-ical analysis.Micro-Raman analysis reveals that the Nd∶GdCOB crystal lattice expands during the irradiation process.Micro-second harmonic spectroscopic analysis suggests that the original nonlinear properties of the Nd∶GdCOB crystal are greatly enhanced within the waveguide volume.Under pulsed 1064 nm laser pumping,second harmonic generation(SHG)at 532 nm has been achieved in the fabricated waveguides.The maximum SHG conversion efficiencies are determined to be~8.32%·W^(-1) and~22.36%·W^(-1) for planar and ridge waveguides,respectively.

Si(111)Electrode/Electrolyte Interfacial Studied by in-situ Second Harmonic Generation

Si(111)electrode has been widely used in electrochemical and photoelectrochemical studies.The potential dependent measurements of the second harmonic generation(SHG)were performed to study Si(111)electrode interface.At different azimuthal angles of the Si(111)and under different polarization combinations,the curve of the intensity of SHG with extern potential has a different form of line or parabola.Quantitative analysis showed that these differences in the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si(111)electrode.The change in the isotropic and anisotropic contribution of the Si(111)electrode may be attributed to the increase in the doping concentration of Si(111)electrodes.

QM/MM Study of the Second Harmonic Generation and Two-photon Absorption Properties of a Fluorescent Proteins-Dreiklang

A new reversibly switchable fluorescent protein（RSFP）, namely Dreiklang, exhibits prominent feature that the wavelengths for switching and fluorescence are decoupled due to its great different structures between bright and dark states. This feature might also induce some nonlinear optic（NLO） properties changing as switching between two states, which might promote new method of biological science. We employ the QM/MM method to simulate the structures of different states, and study their second harmonic generation（SHG） and two-photon absorption（TPA） properties. And we found different states of Dreiklang have different SHG and TPA responses. The SHG and TPA properties of Dreiklang are correlated to particularly geometrical structures of different states, especially the centrosymmetric or nocentrosymmetric π-stacking structures which are formed by chromophore and beside residue Tyr203, so the SHG and TPA responses could be changed as the light induces switching among different states of Dreiklang. This work would prospectively guide the application of Dreiklang on the NLO technology, and help the development of new RSFP with special NLO function.

An Investigation into the Role of Capping on Second Harmonic Generation from Nonlinear Organic Polymer and Guest-Host Thin Films by In-Situ Poling

In attempts to fabricate thermally stable second-order nonlinear polymer thin films, we have investigated the second harmonic generation (SHG) from both nonlinear polymer and guest-host thin films. We have also investigated the role of capping on the SHG, temporal stability and relaxation of dipole alignment. Corona poling techniques were employed to orient the dopants into the noncentrosymmetric structure required to obtain the SHG. The effect of capping with a polymeric encapsulant below the glass transition temperature of the polymers on the unpoled and corona poled thin films was studied. Capping of the nonlinear polymer and guest host thin films have resulted in high SHG with good temporal stability. SHG signal falls drastically during the first 8 days after poling while no further significant decay in SHG signal was observed after about 33 days. Our investigations have identified the characteristics required for a good encapsulant on a non-con-ductive surface.

Evolution of template-assisted two-dimensional porphyrin chiral grating structure by directed self-assembly using chiral second harmonic generation microscopy

Directed self-assembly has been used to create micro-nano scale patterns,including chiral periodic structures of organic molecules,for potential applications in optics,photonics,metamaterials,and medical and sensing technologies.This study presents a straightforward approach for fabricating large-scale chiral grating porphyrin assemblies through template-assisted techniques.The solution of tetrakis(4-sulfonatophenyl)porphyrin(TPPS)was induced by chiral amino acids(L/D-arginine and L/D-serine)to selfassemble into highly ordered chiral grating structures with the assistance of sodium dodecyl sulfate(SDS).The structures show precise line widths(5.5μm)and gaps(18μm).Using in situ optical microscopy and second harmonic generation(SHG)microscopy,the chiral characteristics and dynamic evolution of the template-assisted self-assembly are investigated.It is found that the chirality of amino acids induced TPPS self-assembled into chiral structures and the liquid contraction interface significantly enhanced the chirality of the assemblies.This study is significant for understanding the mechanism of chiral evolution and designing novel micro-nano materials with predetermined chiral properties.

Porous organic cages as a novel platform for second harmonic generation

Second harmonic generation(SHG) is one of the most extensively applied nonlinear optical phenomena. Compared to traditional inorganic SHG materials, organic SHG materials, including non-centrosymmetric small molecular crystals and organic polymers, have been extensively explored for SHG optics due to their advantageous properties, including high light-induced damage threshold, ultrafast response speed, high polarization ratio, wide transparency range, and elevated dielectric constant. However,in organic SHG materials, most small molecular crystals often show poor thermal stability(<200 ℃) due to their low molecular weight, which is an important obstacle for practical application under high temperatures. On the other hand, the difficulty in the growth of large-size crystals as well as the disorderly array of chromospheres on polymer chains associated with organic polymers have hindered their widespread application in SHG due to the requirements of processability and homogeneity.Therefore, it is significantly necessary to develop novel organic SHG material with high thermal stability, large-size crystals,excellent processability, high homogeneity, and a well SHG effect. In the contribution, we developed herein porous organic cages(POCs) as a novel platform for SHG optics. Thanks to its inherent macromolecular composition and zero-dimensional discrete structures, POCs can not only solve the instability problem of small molecule crystals but also overcome the drawbacks in growing large-size crystals for organic polymer. The obtained two POCs with chiral asymmetric centers(CPOC-R-5 and CPOCS-7) display advantageous features for SHG optics including well SHG performance, large-size crystal(5 mm×3 mm×1 mm),high thermal stability(≥250 ℃), wide transparency window(750–2,000 nm) and high polarization ratio(up to 94.5%). This work thus develops POCs as a new platform toward the construction of organic NLO materials with the potential application in sorts of SHG optics.

Chip-scale spontaneous quasi-phase matched second harmonic generation in a micro-racetrack resonator

In this paper,we demonstrate efficient spontaneous quasi-phase matched(SQPM)second harmonic generation(SHG)in a microracetrack resonator on X-cut thin film lithium niobate.Our approach does not involve poling,but exploits the anisotropy of the crystals to allow the phase-matching condition to be fulfilled spontaneously as the TE-polarized light circulates in a specifically designed racetrack resonator.In experiment,normalized on-chip conversion efficiencies of 1.01×10-4/W and 0.43×10-4/W are achieved by 37th-order and 111th-order SQPM,respectively.The configurable SQPM will benefit the application of nonlinear frequency conversion and quantum source generation in chip-scale integrated photonics compatible with standard CMOS fabrication processes.