Second Harmonic Generation Intensity Research Articles

三元AGa5S8 (A = K, Rb, Cs): 通过“一替多取代”策略制备的红外非线性光学材料

Commercially available infrared (IR) nonlinear optical (NLO) materials, such as diamond-like AgGaQ2 (Q = S, Se), display large NLO coefficients but relatively low laser-induced damage thresholds (LIDTs), which seriously hinder their widespread laser applications. Herein, the “one-for-multiple substitution” strategy, namely, [SZn46+ + 5Zn2+ ⇒ A+ + 5Ga3+] (A = K, Rb and Cs), is applied on the diamond-like zinc-blende ZnS, and affords three new polar ternary crystals AGa5S8 (A = K, Rb and Cs) through the solid-state method. These compounds inherit the diamond-like anionic backbone framework in ZnS where the NLO functional motifs GaS4 are arranged in a parallel manner. This characteristic accounts for the remarkable phase-matchable second-harmonic generation intensities (1.1–1.2 × AgGaS2). In addition, the inclusion of the highly electropositive A+ cations affords a band gap ranging from 3.10 to 3.37 eV, which facilitates the improvement of LIDT (9.3–12.4 × AgGaS2). To the best our knowledge, crystals AGa5S8 (A = K, Rb and Cs) are the first series of ternary A-inclusion chalcogenides with large second-harmonic generation responses (≥1.0 × AgGaS2) and wide band gaps (≥3.0 eV), fulfilling the rigorous requirements of outstanding IR NLO materials. In addition, the “one-for-multiple substitution” strategy presents the great significance of diamondlike structure evolution and provides a remarkable opportunity to achieve NLO materials.

Hybrid Germanium Bromide Perovskites with Tunable Second Harmonic Generation

AbstractGe‐based hybrid perovskite materials have demonstrated great potential for second harmonic generation (SHG) due to the geometry and lone‐pair induced non‐centrosymmetric structures. Here, we report a new family of hybrid 3D Ge‐based bromide perovskitesAGeBr3,A=CH3NH3(MA), CH(NH2)2(FA), Cs and FAGe0.5Sn0.5Br3, crystallizing in polar space groups. These compounds exhibit tunable SHG responses, where MAGeBr3shows the strongest SHG intensity (5×potassium dihydrogen phosphate, KDP). Structural and theoretical analysis indicate the high SHG efficiency is attributed to the displacement of Ge2+along [111] direction and the relatively strong interactions between lone pair electrons of Ge2+and polar MA cations along thec‐axis. This work provides new structural insights for designing and fine‐tuning the SHG properties in hybrid metal halide materials.

Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and second-harmonic generation.

Colloidal metal nanocrystals (NCs) show great potential in plasmon-enhanced spectroscopy owing to their attractive and structure-depended plasmonic properties. Herein, unique Au rod-cup NCs, where Au nanocups are embedded on the one or two ends of Au nanorods (NRs), are successfully prepared for the first time via a controllable wet-chemistry strategy. The Au rod-cup NCs possess multiple plasmon modes including transverse and longitudinal electric dipole (TED and LED), magnetic dipole (MD), and toroidal dipole (TD) modulated LED resonances, producing large extinction cross-section and huge near-field enhancements for plasmon-enhanced spectroscopy. Particularly, Au rod-cup NCs with two embedded cups show excellent surface-enhanced Raman spectroscopy (SERS) performance than Au NRs (75.6-fold enhancement excited at 633 nm) on detecting crystal violet owing to the strong electromagnetic hotspots synergistically induced by MD, LED, and TED-based plasmon coupling between Au cup and rod. Moreover, the strong TD-modulated dipole-dipole double-resonance and MD modes in Au rod-cup NCs bring a 37.3-fold enhancement of second-harmonic generation intensity compared with bare Au NRs, because they can efficiently harvest photoenergy at fundamental frequency and generate large near-field enhancements at second-harmonic wavelength. These findings provide a strategy for designing optical nanoantennas for plasmon-enhanced applications based on multiple plasmon modes.Electronic Supplementary MaterialSupplementary material (SEM image of Au rod-one-cup NCs; TEM image of Au/PbS hybrids; SEM image of Au rod-two-cup NCs; low-amplification SEM image of Au rod-two-cup NCs; experimental extinction and calculated electric field distributions of Au NR excited at different wavelengths; calculated absorption and scattering spectra of Au rod-one-cup NCs; schematic illustration of the cut plane and the corresponding magnetic field distribution under L3 excitation; Raman spectra of CV (10−6 M) adsorbed on Au rod-cup NCs with different cup sizes; calculated magnetic field distribution of Au rodcup NCs excited at 532 and 633 nm; calculated electric field distributions of Au rod-one-cup NC excited at 600 nm along TE and LE; the models of Au rod-cup NCs used in the simulations) is available in the online version of this article at 10.1007/s12274-022-4562-5.

The interfacial structure of super-concentration LiNO3 aqueous electrolyte studied by second harmonic generation.

The interfacial structure of a super-concentration LiNO3 aqueous electrolyte was studied using non-resonant second harmonic generation (SHG) and heterodyne-detected SHG spectra. First, we investigated the electric double layer structure at the air/LiNO3 interface. As the concentration of LiNO3 increased, the SHG intensity first increased and then remained unchanged, while the SHG phase changed by about 5°. These results reveal that there was only a small amount of NO3 - at the interface. The increase of the SHG intensity resulted from the thickening of the interfacial water molecular layer. In addition, we studied the broadening mechanism of the electrochemical stability window (ESW) for the super-concentrated LiNO3 aqueous electrolyte. During cyclic voltammetry scanning, the potential-dependent SHG curves of the Pt/LiNO3 interface verify that at the cathodic end of the ESW, as the concentration of LiNO3 increased, the orientation angle θ of Pt-H changed less and the number density Ns of Pt-H gradually decreased, which indicates the decrease of the number of adsorbed H atoms on the Pt electrode surface. Therefore, the decrease of the number of free water molecules on the Pt electrode surface resulted in an expanded ESW.

Chirp-dependent dual light emission in Na0.95Er0.05Nb0.9Ti0.1O3 perovskite

Polar Na0.95Er0.05Nb0.9Ti0.1O3 perovskite has been synthesized as a nanostructured material by microwave assisted hydrothermal method. The characterization indicates that erbium is a constituent of the crystal structure and is preferentially located in sodium positions. The compound combines the nonlinear optical properties of the host (NaNb0.9Ti0.1O2.95) and the fluorescent properties of the Er3+-dopant. Under excitation by a single femtosecond (< 10 fs) laser in the near-infrared region, simultaneous dual emission signals of second harmonic generation (SHG) and up-converting fluorescence (UCF) are observed and the nonlinear dependencies of the SHG and UCF intensities on the excitation intensity are measured. In addition, both emissions are shown to be sensitive to the chirp of the exciting pulses, and for UCF, it can be explained by means of a simple theoretical model based on the density matrix equations. These nano-structured particles with chirp-dependent dual behavior can be very advantageous when used in biological systems, since they can provide complementary information in different spectral ranges and tissues and are susceptible of coherent control, which can be both useful in optical microscopy and bioimaging.

Synthesis of a new compound in the SrO-TiO2-B2O3 system and its dielectric, ferroelectric, nonlinear optical and electrical properties

In this paper, we report on a new compound in the SrO-TiO2-B2O3 system. For the first time, the compound was synthesized by crystallization of glass tapes (GTs) obtained from a melt of 2SrO-2TiO2-B2O3. The crystallization was performed at 670 °C for 96 h. X-ray studies showed that the formed new phase belongs to the hexagonal system. The lattice parameters were calculated as well. It was suggested to attribute the new material to stoichiometric compound Sr3Ti3O6(BO3)2. The dielectric constant, tangent loss, ac and dc conductivities of the material were found to be almost constant in a wide range of temperatures. Investigation of varistor properties showed that the current-voltage nonlinearity coefficient is 2.05. The measurement of the hysteresis loop confirms the ferroelectricity of the material. It is found that the second-harmonic generation intensity of the new material exceeds that of KDP (by a factor of 3.2 for particles in the size range of 100 ÷ 150 μm).

Second harmonic generation microscopy of otoconia.

The origin of second harmonic generation (SHG) signal in otoconia was investigated. SHG signal intensity from otoconia was compared to pure calcite crystals, given calcite is the primary component of otoconia and is known to emit surface SHG. The SHG intensity from calcite was found to be ∼41× weaker than the SHG intensity from otoconia signifying that the SHG signal from otoconia is likely generated from the organic matrix. Furthermore, the SHG intensity from otoconia increased when treated with a chelating agent known to dissolve calcite which confirms that calcite is not the source of SHG. Additionally, polarization-resolved SHG microscopy imaging revealed that the arrangement of the SHG emitters is radial and can form highly ordered domains.

Crescent-shaped shadow of second harmonic generation in dielectric microsphere/TMD monolayer heterostructure

For heterointerfaces at micro/nano scales, extremely-space-confined second harmonic generation (SHG) of transition metal dichalcogenide (TMD) monolayers can facilitate the extraction of TMD monolayers’ 3D dielectric environment. Here, we construct SiO2 microsphere/MoS2 monolayer (3D/2D) heterostructures and spatially resolve their SHG distributions via pixel-to-pixel spatial mapping. Asymmetric 3D refractive index distribution of the microsphere is successfully projected to 2D plane of TMD monolayer and visualized by corresponding 2D image of SHG intensity distribution, which presents a crescent-shaped shadow. Out results open up new possibilities for 3D interfacial sensing and imaging with SHG of 2D monolayers.

SrZnGeS4: A Dual‐Waveband Nonlinear Optical Material with a Transparency Spanning UV/Vis and Far‐IR Spectral Regions

AbstractNon‐linear optical chalcogenides with a wide band gap (Eg) and excellent NLO properties are key materials for highly desirable multiwaveband tunable optical parametric oscillators (OPOs). We exploit the “electronic structure engineer bucket effect” to develop a novel dual‐waveband SrZnGeS4with an ultrawide transparency window. It exhibits an asymmetricFdd2structure that consists of layers formed by corner‐sharing [ZnGeS6] dimers. SrZnGeS4is transparent from 0.30 to 23.6 μm, spanning the UV‐, vis‐, mid‐ and far‐IR spectral regions and has the widestEg(3.63 eV) in the AeMIIMIVQ4family to date. It exhibits phase matching, high SHG intensities (e.g., 11.0×KDP and 17.5×AGS underλinc=1450 and 950 nm, respectively), and a very high laser‐induced damage threshold (35×AGS). These results not only suggest bright prospects for high‐power laser applications but may also enable applications of the multiwaveband OPO system from the UV‐visible to far‐IR regions.

SrZnGeS4 : A Dual-Waveband Nonlinear Optical Material with a Transparency Spanning UV/Vis and Far-IR Spectral Regions.

Non-linear optical chalcogenides with a wide band gap (Eg ) and excellent NLO properties are key materials for highly desirable multiwaveband tunable optical parametric oscillators (OPOs). We exploit the "electronic structure engineer bucket effect" to develop a novel dual-waveband SrZnGeS4 with an ultrawide transparency window. It exhibits an asymmetric Fdd2 structure that consists of layers formed by corner-sharing [ZnGeS6 ] dimers. SrZnGeS4 is transparent from 0.30 to 23.6 μm, spanning the UV-, vis-, mid- and far-IR spectral regions and has the widest Eg (3.63 eV) in the AeMII MIV Q4 family to date. It exhibits phase matching, high SHG intensities (e.g., 11.0×KDP and 17.5×AGS under λinc =1450 and 950 nm, respectively), and a very high laser-induced damage threshold (35×AGS). These results not only suggest bright prospects for high-power laser applications but may also enable applications of the multiwaveband OPO system from the UV-visible to far-IR regions.

Dramatically Enhanced Second Harmonic Generation in Janus Group‐III Chalcogenide Monolayers

Abstract2D materials are excellent platforms for nonlinear optical (NLO) response, especially second harmonic generation (SHG), due to its large surface to volume ratio and sub‐nanometer thickness. The SHG susceptibility strongly relies on the symmetry of materials. Constructing Janus structures can break the out‐of‐plane mirror symmetry, bringing about asymmetric charge distribution and leading to a built‐in electric field. Consequently, SHG response along the out‐of‐plane direction can be improved. In the current work, combining first‐principles calculations with independent particle approximation, the SHG response of nine Janus group‐III chalcogenide monolayers (M2XX', MM'X2) are systematically evaluated. Both extraordinary in‐plane and out‐of‐plane SHG response are revealed in all the Janus structures. Besides, cation MM'X2 Janus structures exhibit systemically higher SHG response than that of anion M2XX’ due to stronger dipole. Among them, GaInTe2 possesses extremely high out‐of‐plane SHG response with d31 up to 10490.4 pm V−1 at photon energy (PE) of 4.7 eV, enabling promising applications in ultraviolet NLO devices. The SHG intensity polar plots from Janus structures display unusual rotational symmetry at different PEs allowed by the out‐of‐plane SHG components. This work provides theoretical guidelines for further experimental explorations in 2D group‐III monochalcogenide Janus structures and paves the way for their utilization in NLO devices.

Nonlinear Optical Imaging of In‐Plane Anisotropy in Two‐Dimensional SnS (Advanced Optical Materials 10/2022)

This cover illustration represents a laser field interacting with an ultrathin SnS crystal and the production of a second harmonic generation (SHG) field. Two representative polar diagrams of the polarization-resolved SHG intensity from different SnS crystals, and the fitting with the nonlinear optics model, are also shown, enabling the extraction of quantitative information on the in-plane anisotropy in SnS. For further details see article number 2102776 by Emmanuel Stratakis and co-workers.

Excitation polarization-independent photo-induced restoration of inversion symmetry in Td-WTe2

Td-WTe2 is a topologically nontrivial material and exhibits a variety of physical properties, such as giant unsaturated magnetoresistance and the unconventional thermoelectric effect, due to its topological nature. It is also known to exhibit ultrafast topological phase transitions that restore its inversion symmetry by intense terahertz and mid-infrared pulses, and these properties demonstrate the possibility of ultrafast control of devices based on topological properties. Recently, a novel photo-induced topological phase transition by using polarization-controlled infrared excitation has been proposed, which is expected to control the material topology by rearranging the atomic orbitals near the Weyl point. To examine this topological phase transition, we experimentally studied the excitation-polarization dependence of the infrared-induced phase dynamics in a thin-layer of Td-WTe2. Time-resolved second harmonic generation (SHG) measurements showed that SHG intensity decreases after the infrared pump regardless of the polarization. Polarization-resolved infrared pump–probe measurements indicated that the polarization-selected excited state relaxes quite rapidly (i.e., within 10–40 fs). Considering these experimental results, we conclude that it is difficult to control the photo-induced phase transition through orbital-selective excitation owing to the rapid loss of carrier distribution created by polarization-selective excitation in thin-layer Td-WTe2 under our experimental condition. These results indicate that the suppression of the electron scattering process is crucial for experimentally realizing the photo-induced phase transition based on the polarization selection rule of the materials.

Label-Free Imaging of Membrane Potentials by Intramembrane Field Modulation, Assessed by Second Harmonic Generation Microscopy.

Optical interrogation of cellular electrical activity has proven itself essential for understanding cellular function and communication in complex networks. Voltage-sensitive dyes are important tools for assessing excitability but these highly lipophilic sensors may affect cellular function. Label-free techniques offer a major advantage as they eliminate the need for these external probes. In this work, it is shown that endogenous second-harmonic generation (SHG) from live cells is highly sensitive to changes in transmembrane potential (TMP). Simultaneous electrophysiological control of a living human embryonic kidney (HEK293T) cell, through a whole-cell voltage-clamp reveals a linear relation between the SHG intensity and membrane voltage. The results suggest that due to the high ionic strengths and fast optical response of biofluids, membrane hydration is not the main contributor to the observed field sensitivity. A conceptual framework is further provided that indicates that the SHG voltage sensitivity reflects the electric field within the biological asymmetric lipid bilayer owing to a nonzero tensor. Changing the TMP without surface modifications such as electrolyte screening offers high optical sensitivity to membrane voltage (≈40% per 100mV), indicating the power of SHG for label-free read-out. These results hold promise for the design of a non-invasive label-free read-out tool for electrogenic cells.

Temperature-dependent second-harmonic generation from color centers in diamond.

Under infrared ultrashort pulse laser stimulation, we investigate temperature-dependent second-harmonic generation (SHG) from nitrogen-vacancy (NV)-introduced bulk diamond. The SHG intensity decreases in the temperature range of 20-300°C, due to phase mismatching caused by refractive index modification. We discover that optical phonon scattering outperforms acoustic phonon scattering in NV diamond by fitting the temperature dependence of the SHG intensity using a model based on the bandgap change via the deformation potential interaction. This study presents an efficient and viable way for creating diamond-based nonlinear optical temperature sensing.

Nonlinear terahertz pulse induced polarization dynamics in ferroelectric Ba0.8Sr0.2TiO3 thin film

The ultrafast ferroelectric polarization dynamics excited by intense THz pulses (with an electric field up to 23 MV/cm) in a multidomain Ba0.8Sr0.2TiO3 thin film has been experimentally demonstrated using optical second harmonic generation (SHG) probe. The obtained nonlinear dependence of the SHG intensity on the electric field of the THz pulse can be explained in terms of Landau-Khalatnikov equation by a transient ferroelectric polarization reversal. The best agreement between the experimental and simulated data was obtained when taking into account the multidomain structure of the film and carrying out convolution of the simulated polarization dynamics and the optical probe pulse.

Effect of rare-earth doping on the structural and optical properties of the Ag3AsS3 crystals

Non-linear optical (NLO) materials allow the production of the coherent laser beam in the difficult frequency ranges of the electromagnetic spectrum. Aiming to explore new classes of the NLO materials with high optical performance in the infrared region, in this work, we investigated the effect of the rare earth doping (Pr, Eu, Yb) on the crystal structure and optical properties of the Ag3AsS3 crystals. The performed analysis of the XRD patterns indicates that the rare earth elements are located in the Ag sites of the crystal lattice. As a result, the second harmonic generation intensity, which determines the effectiveness of the NLO materials, increases with the increase of rare earth dopant content up to 1.0%. Using the absorption analysis and Raman spectroscopy, we show that the increase in the SHG intensity can be related to the slight decrease of the bandgap, as well as with the increased electron–phonon interaction in rare-earth-doped Ag3AsS3 crystals. Considering the discovered enhancement of the SHG intensity, accompanied by the low melting temperature, this work offers rare-earth-doped Ag3AsS3 crystals as potential candidates for the non-linear optical applications for the infrared frequency range.

Influence of Acetaminophen on Molecular Adsorption and Transport Properties at Colloidal Liposome Surfaces Studied by Second Harmonic Generation Spectroscopy.

Time-resolved second harmonic generation (SHG) spectroscopy is used to investigate acetaminophen (APAP)-induced changes in the adsorption and transport properties of malachite green isothiocyanate (MGITC) dye to the surface of unilamellar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes in an aqueous colloidal suspension. The adsorption of MGITC to DOPC liposome nanoparticles in water is driven by electrostatic and dipole–dipole interactions between the positively charged MGITC molecules and the zwitterionic phospholipid membranes. The SHG intensity increases as the added MGITC dye concentration is increased, reaching a maximum as the MGITC adsorbate at the DOPC bilayer interface approaches a saturation value. The experimental adsorption isotherms are fit using the modified Langmuir model to obtain the adsorption free energies, adsorption equilibrium constants, and the adsorbate site densities to the DOPC liposomes both with and without APAP. The addition of APAP is shown to increase MGITC adsorption to the liposome interface, resulting in a larger adsorption equilibrium constant and a higher adsorption site density. The MGITC transport times are also measured, showing that APAP decreases the transport rate across the DOPC liposome bilayer, especially at higher MGITC concentrations. Studying molecular interactions at the colloidal liposome interface using SHG spectroscopy provides a detailed foundation for developing potential liposome-based drug-delivery systems.

Adsorption of CTAB on Sapphire-c at High pH: Surface and Zeta Potential Measurements Combined with Sum-Frequency and Second-Harmonic Generation.

The adsorption of cetyltrimethylammonium bromide (CTA+Br-) on sapphire-c surfaces was studied at pH 10 below the surfactants' critical micelle concentration. The evolution of interfacial potentials as a function of CTAB concentration was characterized by surface and zeta potential measurements and complemented by molecular dynamic (MD) simulations as well as by second-harmonic (SHG) and vibrational sum-frequency generation (SFG) spectroscopy. The changes in interfacial potentials suggest that the negative interfacial charge due to deprotonated surface aluminols groups is neutralized and can be even overcompensated by the presence of CTA+ cations at the interface. However, SFG intensities from strongly hydrogen-bonded interfacial water molecules as well as SHG intensities decrease with both increasing CTAB concentration and the magnitude of the surface potential. They do not suggest a charge reversal at the interface, while the change in zeta potential is actually consistent with an apparent charge inversion. This can be qualitatively explained by results from MD simulation, which reveal adsorbed CTA+ cations outside a first strongly bound hydration layer of water molecules, where they can locally distort the structural order and replace some of the interfacial water molecules adjacent to the first layer. This is proposed to be the origin for the significant loss in SFG and SHG intensities with increasing CTAB concentration. Moreover, we propose that CTA+ can act as a counterion and enhance the occurrence of deprotonated surface aluminols that is consistent with the decrease in surface potential.

Nonlinear Exciton‐Mie Coupling in Transition Metal Dichalcogenide Nanoresonators

AbstractThanks to a high refractive index, giant optical anisotropy, and pronounced excitonic response, bulk transition metal dichalcogenides (TMDCs) have recently been discovered to be an ideal foundation for post‐silicon photonics. The inversion symmetry of bulk TMDCs, on the other hand, prevents their use in nonlinear‐optical processes such as second‐harmonic generation (SHG). To overcome this obstacle and broaden the application scope of TMDCs, MoS2 nanodisks are engineered to couple Mie resonances with C‐excitons. As a result, their alliance produces 23‐fold enhancement of SHG intensity with respect to the resonant SHG from a high‐quality exfoliated MoS2 monolayer under C‐exciton excitation. Furthermore, SHG demonstrates a strongly anisotropic response typical of a MoS2 monolayer due to the single‐crystal structure of the fabricated nanodisks, providing a polarization degree of freedom to manipulate SHG. Hence, these results significantly improve the potential of bulk TMDCs enabling an avenue for next‐generation nonlinear photonics.