Second Harmonic Generation Intensity Research Articles

230-fold Enhancement of second-harmonic generation by coupled double resonances in a dolmen-type gold metasurface

Optical metasurfaces, artificial planar nanostructures composed of subwavelength meta-atoms, have attracted significant attention due to their ability to tailor optical nanoscale properties, making them a versatile platform for shaping light in both linear and nonlinear regimes. This paper reports on the realization of second harmonic generation (SHG) enhancement based on a dolmen-type gold metasurface containing two resonances. Nonlinear scattering theory is employed to numerically investigate the SHG enhancement phenomenon in the resonant metasurface. The periodic dolmen-type gold metasurface introduces a diffraction coupling effect between Fano resonance and surface lattice resonance (SLR), providing strong local-field enhancement and significantly enhancing the nonlinear effect. We analyze the influence of the coupling between Fano resonance and SLR on the SHG intensity and achieve a 230-fold enhancement in SHG intensity compared to the single resonance case by adjusting the periodicity of the metasurface. The SHG-enhanced gold metasurface may find applications in sensing, imaging, optical computing, and integrated nonlinear optics.

Controlled Second Harmonic Generation with Optically Trapped Lithium Niobate Nanoparticles.

We report the second harmonic generation (SHG) response from a single 34 nm diameter lithium niobate nanoparticle. The experimental setup involves a first beam devoted to the optical trapping of single nanoparticles, whereas a second arm involves a femtosecond laser source leading to the SHG emission from the trapped nanoparticles. SHG operation where one to three nanoparticles are present in the optical trap is first demonstrated, highlighting the transition between coherent and incoherent SHG, the latter known as hyper-Rayleigh scattering (HRS). With a spatial light modulator moving the optical trap in and out of the focus of the femtosecond beam, the SHG intensity is switched back and forth between a low and a high level. This controlled operation opens new avenues for nanoparticle characterization and applications in sensing or communication and information technologies and constitutes the first step in the design of active substrateless metasurfaces.

Crystallization of SrB4O7 in a glass tape and its electrical, dielectric, ferroelectric and nonlinear optical properties

This paper presents the results of an investigation of the electrical, dielectric, ferroelectric, and second-order nonlinear optical properties of SrB4O7 synthesized by methods of both solid-state reaction (SSR) and crystallization of a glass tape (GT). To obtain GTs of SrB4O7, the melt consisting of appropriate precursors was rapidly cooled using a twin-roller. The prepared GTs were crystallized at 730°C for different durations and analyzed by X-ray diffraction. The formation of “extended” crystals in the central part of GTs due to the application of the GT technology is discussed. The investigation of the current-voltage characteristic of the sample prepared by SSR showed that its breakdown voltage is higher than 1000 V. The measurements of the dielectric parameters in the frequency range of 1 KHz–1 MHz showed that the dielectric constant is about 7.5, and the dielectric loss varies from 0.002 to 0.0008. These parameters are sufficiently stable in the temperature range from room temperature to 150°C. The ferroelectric properties of the GT crystallized at 730°C for 3 h were also investigated. At 20 kV/cm, a remnant polarization of 0.18 μC/cm2 was measured. Optical measurements showed that the intensity of second-harmonic generation in the GT crystallized at 730°C for 3 h is 5.4 times higher than that of KH2PO4 (KDP).

Intense second-harmonic generation in two-dimensional PtSe2

Intense second-harmonic generation in two-dimensional PtSe₂

Broadband nonlinear conversion and random quasi-phase-matching in transparent glass composite

With the development of laser technology, nonlinear optics plays a crucial role in frequency conversion. However, the generation of second harmonics in nonlinear optical crystals is generally subject to rigorous phase-matching conditions that hinder the performance of broadband tunability. It is believed that introducing disorders in nonlinear optical materials is helpful to overcome this obstacle. In this work, we have prepared a nonlinear microcrystal-doped glass (NMG) composite material, allowing for tunable and polarization-independent nonlinear conversion from visible to near-infrared. The linear dependence of SHG intensity versus sample thickness indicated the facilitation of random quasi-phase matching by using the NMG. Our results provide a more stable and promising platform for disordered nonlinear photonic materials and suggest the possibility of more efficient nonlinear conversions using the NMG composite glass fibers in future.

From Cd(SCN)2(CH4N2S)2 to Cd(SCN)2(C4H6N2)2: Controlling Sulfur Content in Thiocyanate Systems Significantly Improves the Overall Performance of UV Nonlinear Optical Materials.

Combining a strong second-order nonlinear optical (NLO) effect (>1×KH2PO4 (KDP)), a large band gap (>4.2 eV), and a moderate birefringence in ultraviolet (UV) NLO crystals remains a formidable challenge. Herein, Cd(SCN)2(C4H6N2)2, the first example of a thiocyanate capable of realizing a phase-matched UV NLO crystal material, is obtained by reducing the sulfur (S) content in the centrosymmetric (CS) structure of Cd(SCN)2(CH4N2S)2. Compared to the "shoulder-to-shoulder" one-dimensional (1D) chain of Cd(SCN)2(CH4N2S)2, Cd(SCN)2(C4H6N2)2 has a different sawtooth 1D chain structure. Cd(SCN)2(CH4N2S)2 has second harmonic generation (SHG) inertia with a band gap of 3.90 eV and a UV cutoff edge of 342 nm, however, it possesses a large birefringence (0.35@546 nm). In contrast, the symmetry center breaking of Cd(SCN)2(C4H6N2)2 leads to remarkably strong SHG intensity (10 times that of KDP). Furthermore, it has a wide band gap (4.74 eV), short UV cutoff edge (234 nm), and moderate birefringence capable of phase matching (0.17@546 nm). This research indicates that thiocyanates are a promising class of UV NLO crystal materials, and that modulation of the sulfur content of CS thiocyanates is an effective strategy for the development of UV NLO crystals with excellent overall performances.

Nonlinear optical diode effect in a magnetic Weyl semimetal

Diode effects are of great interest for both fundamental physics and modern technologies. Electrical diode effects (nonreciprocal transport) have been observed in Weyl systems. Optical diode effects arising from the Weyl fermions have been theoretically considered but not probed experimentally. Here, we report the observation of a nonlinear optical diode effect (NODE) in the magnetic Weyl semimetal CeAlSi, where the magnetization introduces a pronounced directionality in the nonlinear optical second-harmonic generation (SHG). We demonstrate a six-fold change of the measured SHG intensity between opposite propagation directions over a bandwidth exceeding 250 meV. Supported by density-functional theory, we establish the linearly dispersive bands emerging from Weyl nodes as the origin of this broadband effect. We further demonstrate current-induced magnetization switching and thus electrical control of the NODE. Our results advance ongoing research to identify novel nonlinear optical/transport phenomena in magnetic topological materials and further opens new pathways for the unidirectional manipulation of light.

Anhydrous Aluminum Iodate: Strong Second Harmonic Generation Effect Contributed by Unbonded and Antibonding Orbitals.

Exploring materials that balance the second harmonic generation (SHG) effect and laser-induced damage threshold (LIDT) is the frontier of nonlinear optical (NLO) crystal research at present. In this work, the NLO property of anhydrous aluminum iodate is extensively explored and discussed first. It exhibits a strong SHG intensity of 18.3 × KH2PO4 (KDP) and a high-powder LIDT of 1.4 × KDP at 1064 nm. Combining experimental and theoretical studies at the atomic level and electronic levels, it is found that the cations in the structure are replaced by cations with small radius and high valence, enabling the production of materials with large SHG responses. Unbonded and antibonding orbitals play a crucial positive role in the SHG response of the structure, whereas bonding orbitals produce a large negative contribution. This provides a scarce example of materials in which bonding orbitals make significant negative contributions.

UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System.

The first examples of alkali metal selenite sulfates, namely, Na8(SeO3)(SO4)3 (1), Na2(H2SeO3)(SO4) (2), and K4(H2SeO3)(HSO4)2(SO4) (3), were successfully synthesized by hydrothermal reactions. Their structures display three different zero-dimensional configurations composed of isolated sulfate tetrahedra and selenite groups separated by alkali metals. Na8(SeO3)(SO4)3 (1) features a noncentrosymmetric structure, while Na2(H2SeO3)(SO4) (2) and K4(H2SeO3)(HSO4)2(SO4) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na8(SeO3)(SO4)3 (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na8(SeO3)(SO4)3 (1) has a short UV cutoff edge and a large optical band gap, which makes it a possible UV nonlinear optical material. Theoretical calculations revealed that the birefringence of Na8(SeO3)(SO4)3 (1) is 0.041 at 532 nm, which is suitable for phase-matching condition. This work provides a good experimental foundation for the exploration of new UV nonlinear crystals in an alkali metal selenite sulfate system.

Time-Dependent Ultrafast Quadratic Nonlinearity in an Epsilon-Near-Zero Platform.

Ultrafast nonlinearity, which results in modulation of the linear optical response, is a basis for the development of time-varying media, in particular those operating in the epsilon-near-zero (ENZ) regime. Here, we demonstrate that the intraband excitation of hot electrons in the ENZ film results in a second-harmonic resonance shift of ∼10 THz (40 nm) and second-harmonic generation (SHG) intensity changes of >100% with only minor (<1%) changes in linear transmission. The modulation is 10-fold enhanced by a plasmonic metasurface coupled to a film, allowing for ultrafast modulation of circularly polarized SHG. The effect is described by the plasma frequency renormalization in the ENZ material and the modification of the electron damping, with a possible influence of the hot-electron dynamics on the quadratic susceptibility. The results elucidate the nature of the second-order nonlinearity in ENZ materials and pave the way to the rational engineering of active nonlinear metamaterials and metasurfaces for time-varying applications.

Large nonlinear optical magnetoelectric response in a noncentrosymmetric magnetic Weyl semimetal

Weyl semimetals resulting from either inversion (P) or time-reversal (T) symmetry breaking have been revealed to show the record-breaking large optical response due to intense Berry curvature of Weyl-node pairs. Different classes of Weyl semimetals with both P and T symmetry breaking potentially exhibit optical magnetoelectric (ME) responses, which are essentially distinct from the previously observed optical responses in conventional Weyl semimetals, leading to the versatile functions such as directional dependence for light propagation and gyrotropic effects. However, such optical ME phenomena of (semi)metallic systems have remained elusive so far. Here, we show the large nonlinear optical ME response in noncentrosymmetric magnetic Weyl semimetal PrAlGe, in which the polar structural asymmetry and ferromagnetic ordering break P and T symmetry. We observe the giant second harmonic generation (SHG) arising from the P symmetry breaking in the paramagnetic phase, being comparable to the largest SHG response reported in Weyl semimetal TaAs. In the ferromagnetically ordered phase, it is found that interference between this nonmagnetic SHG and the magnetically induced SHG emerging due to both P and T symmetry breaking results in the magnetic field switching of SHG intensity. Furthermore, such an interference effect critically depends on the light-propagating direction. The corresponding magnetically induced nonlinear susceptibility is significantly larger than the prototypical ME material, manifesting the existence of the strong nonlinear dynamical ME coupling. The present findings establish the unique optical functionality of P- and T-symmetry broken ME topological semimetals.

Mixed-anion square-pyramid [SbS3I2] units causing strong second-harmonic generation intensity and large birefringence

Mixed-anion square-pyramid [SbS3I2] units causing strong second-harmonic generation intensity and large birefringence

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S−3AMP)x(R−3AMP)1−xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 − 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Excited States Dynamics at Pentacene/Perfluoropentacene Interfaces: A Femtosecond Time-Resolved Second Harmonic Generation Study.

Understanding the dynamics of excited states after optical excitation at donor-acceptor (D/A) interfaces is of paramount importance for improving the efficiency and performance of optoelectronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) pentacene (PEN) and the electron acceptor (A) perfuoropentacene (PFP) as well as within the single compounds (PEN and PFP) using femtosecond (fs) time-resolved second harmonic generation (SHG). In the single compounds singlet fission is observed on a time scale of around 200 fs. In the bilayer systems a huge SHG intensity rise is observed due to the creation of charge transfer states at the interface and accordingly to formation of a local electric field within tens of picoseconds. The local electric field and therefore the SHG signal intensity from the interface of PEN/PFP bilayer is much more intense compared to the PFP/PEN system because the PFP and PEN intermixing at the PEN/PFP interface is higher. Accordingly a population of defect states on a time scale of 55±12 ps has been proposed for PEN/PFP. Our study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of pentacene-based organic compounds.

Differences in Lipid Order and Dynamics in Plasma Membranes Assessed by Nonlinear Optical Microscopy.

When amphiphilic polar dyes were added to the cells, they intercalated predominantly in the outer leaf of the plasma membrane, making them active for second harmonic generation (SHG). The fluorescence of the dye enabled simultaneous 3D imaging of SHG and two-photon excited fluorescence (TPF). Because SHG intensity is sensitive to the alignment of the dyes, which reflects lipid ordering in the plasma membrane, we assessed the difference in lipid ordering by comparing the SHG intensity normalized to the TPF intensity. Together with an enzyme release assay that detects pore formation in the plasma membrane, our SHG assay revealed how polycations affect lipid ordering at low concentrations, where membrane damage has not yet been examined. By scaling the results of the assays with the charge concentration of the two polycations, polyethylenimine (PEI) and poly-l-lysine (PLL), we found that PEI reduced the lipid order more than PLL, and PLL formed more pores than PEI. A comparison of the SHG and TPF images of the wounded cells revealed that one of the lipid dynamics (flip-flop) was significantly enhanced in the bleb membrane. Moreover, the SHG assay indicated that the biocompatible polymer, poly(N-(2-hydroxypropyl)methacrylamide), did not affect the lipid order. Thus, our technique allows the assessment of the plasma membrane structure at the molecular level.

Functional Assessment of Human Articular Cartilage Using Second Harmonic Generation (SHG) Imaging: A Feasibility Study.

Many arthroscopic tools developed for knee joint assessment are contact-based, which is challenging for in vivo application in narrow joint spaces. Second harmonic generation (SHG) laser imaging is a non-invasive and non-contact method, thus presenting an attractive alternative. However, the association between SHG-based measures and cartilage quality has not been established systematically. Here, we investigated the feasibility of using image-based measures derived from SHG microscopy for objective evaluation of cartilage quality as assessed by mechanical testing. Human tibial plateaus harvested from nine patients were used. Cartilage mechanical properties were determined using indentation stiffness (Einst) and streaming potential-based quantitative parameters (QP). The correspondence of the cartilage electromechanical properties (Einst and QP) and the image-based measures derived from SHG imaging, tissue thickness and cell viability were evaluated using correlation and logistic regression analyses. The SHG-related parameters included the newly developed volumetric fraction of organised collagenous network (Φcol) and the coefficient of variation of the SHG intensity (CVSHG). We found that Φcol correlated strongly with Einst and QP (ρ = 0.97 and - 0.89, respectively). CVSHG also correlated, albeit weakly, with QP and Einst, (|ρ| = 0.52-0.58). Einst and Φcol were the most sensitive predictors of cartilage quality whereas CVSHG only showed moderate sensitivity. Cell viability and tissue thickness, often used as measures of cartilage health, predicted the cartilage quality poorly. We present a simple, objective, yet effective image-based approach for assessment of cartilage quality. Φcol correlated strongly with electromechanical properties of cartilage and could fuel the continuous development of SHG-based arthroscopy.

K(NH4)Zn2(PO4)2: a Beryllium-Free Sr2Be2B2O7 Derivative with Enhanced Interlayer Connectivity.

A novel zinc phosphate derivative of Sr2Be2B2O7 (SBBO), K(NH4)Zn2(PO4)2 (KNZP), featuring [Zn2P2O8]∞2- double layers akin to the [Be2B2O7]∞4- layers in SBBO, was successfully synthesized via a moderate hydrothermal method. Through the substitution of BeO4 and BO3 with ZnO4 and PO4, the issue of toxicity has been effectively resolved, while the enhanced interlayer interactions facilitated by covalent and hydrogen bonding in KNZP overcome the inherent structural instability. Notably, KNZP exhibits a wide transparent window and a moderate second-harmonic generation (SHG) intensity, reaching 0.7 times that of KH2PO4 (KDP), rendering it type-I phase-matchable, indicating that it is a promising UV nonlinear optical (NLO) material.

Effect of out of plane orientation on polarization second harmonic generation of single collagen fibrils.

Second harmonic generation (SHG) microscopy has emerged as a powerful technique for visualizing collagen organization within tissues. Amongst the many advantages of SHG is its sensitivity to collagen nanoscale organization, and its presumed sensitivity to the relative out of plane polarity of fibrils. Recent results have shown that circular dichroism SHG (CD-SHG), a technique that has been commonly assumed to reveal the relative out of plane polarity of collagen fibrils, is actually insensitive to changes in fibril polarity. However, results from another research group seem to contradict this conclusion. Both previous results have been based on SHG imaging of collagen fibrils within tissues, therefore, to gain a definitive understanding of the sensitivity of SHG to relative out of plane polarity, the results from individual fibrils are desirable. Here we present polarization resolved SHG microscopy (PSHG) data from individual collagen fibrils oriented out of the image plane by buckling on an elastic substrate. We show through correlation with atomic force microscopy measurements that SHG intensity can be used to estimate the out of plane angle of individual fibrils. We then compare the sensitivity of two PSHG techniques, CD-SHG and polarization-in, polarization-out SHG (PIPO-SHG), to the relative out of plane polarity of individual fibrils. We find that for single fibrils CD-SHG is insensitive to relative out of polarity and we also demonstrate the first direct experimental confirmation that PIPO-SHG reveals the relative out of plane polarity of individual collagen fibrils.

Investigating the Photovoltaic Performance in ABO3 Structures via the Nonlinear Bond Model for an Arbitrary Incoming Light Polarization.

ABO3 structures commonly known as perovskite are of high importance in advanced material science due to their interesting optical properties. Applications range from tunable band gaps, high absorption coefficients, and versatile electronic properties, making them ideal for solar cells to light-emitting diodes and even photodetectors. In this work, we present, for the first time, a nonlinear phenomenological bond model analysis of second harmonic generation (SHG) in tetragonal ABO3 with arbitrary input light polarization. We study the material symmetry and explore the strength of the nonlinear generalized third-rank tensorial elements, which can be exploited to produce a high SHG response if the incoming light polarization is correctly selected. We found that the calculated SHG intensity profile aligns well with existing experimental data. Additionally, as the incoming light polarization varies, we observed a smooth shift in the SHG intensity peak along with changes in the number of peaks. These observations confirm the results from existing rotational anisotropy SHG experiments. In addition, we show how spatial dispersion can contribute to the total SHG intensity. Our work highlights the possibility of studying relatively complex structures, such as ABO3, with minimal fitting parameters due to the power of the effective bond vector structure, enabling the introduction of an effective SHG hyperpolarizability rather than a full evaluation of the irreducible SHG tensor by group theoretical analysis. Such a simplification may well lead to a better understanding of the nonlinear properties in these classes of material and, in turn, can improve our understanding of the photovoltaic performance in ABO3 structures.

Exciton-Sensitized Second-Harmonic Generation in 2D Heterostructures.

The efficient optical second-harmonic generation (SHG) of two-dimensional (2D) crystals, coupled with their atomic thickness, which circumvents the phase-match problem, has garnered considerable attention. While various 2D heterostructures have shown promising applications in photodetectors, switching electronics, and photovoltaics, the modulation of nonlinear optical properties in such heterosystems remains unexplored. In this study, we investigate exciton-sensitized SHG in heterobilayers of transition metal dichalcogenides (TMDs), where photoexcitation of one donor layer enhances the SHG response of the other as an acceptor. We utilize polarization-resolved interferometry to detect the SHG intensity and phase of each individual layer, revealing the energetic match between the excitonic resonances of donors and the SHG enhancement of acceptors for four TMD combinations. Our results also uncover the dynamic nature of interlayer coupling, as made evident by the dependence of sensitization on interlayer gap spacing and the average power of the fundamental beam. This work provides insights into how the interlayer coupling of two different layers can modify nonlinear optical phenomena in 2D heterostructures.