Second Harmonic Generation Intensity Research Articles

The role of conduction band electrons in promoting O2 adsorption to silicon interfaces

Resonance enhanced Second Harmonic Generation (SHG) was employed to assess if conduction band electrons in silicon (Si) will promote molecular adsorption of ambient species and how such adsorption depends on temperature. Experiments were performed with three types of Si (n-doped or n-Si, p-doped or p-Si, and undoped Si) at temperatures between 18 and 260 °C and under atmospheres of dry N2 and dry (cylinder) air. All Si types were covered with a 2–4 nm thick native oxide layer. Under N2, all Si types behave similarly, with SHG intensity [I(2ω)] diminishing with increasing temperature. This effect was reversible and attributed to electron–phonon scattering. In the presence of O2, I(2ω) from n-Si at room temperature is enhanced significantly. Neither p-doped Silicon (p-Si) nor undoped Si show similar effects at room temperature, with I(2ω) being independent of gas phase composition. At temperatures ≥175 °C, all three Si types behaved similarly with no dependence on atmospheric O2 content. Varying the amount of O2 above n-Si at room temperature and measuring I(2ω) suggested that O2 adsorption to n-Si could be described with a Langmuir isotherm and an adsorption energy of −0.13 ± 0.05 eV. Increasing n-Si’s oxide thickness (to 600 nm) rendered the substrate insensitive to ambient gas phase composition. Taken together, these findings support a description of Si’s surface electronic structure that is controlled by n-Si conduction band electrons backbonding into the π* orbitals of adjacent O2 and imply that these conduction band electrons can affect adsorption despite the presence of a native oxide film.

Imaging with a twist: Three-dimensional insights of the chiral nematic phase of cellulose nanocrystals via SHG microscopy.

Cellulose nanocrystals (CNCs) are bio-based nanoparticles that, under the right conditions, self-align into chiral nematic liquid crystals with a helical pitch. In this work, we exploit the inherent confocal effect of second-harmonic generation (SHG) microscopy to acquire highly resolved three-dimensional (3D) images of the chiral nematic phase of CNCs in a label-free manner. An in-depth analysis revealed a direct link between the observed variations in SHG intensity and the pitch. The highly contrasted 3D images provided unprecedented detail into liquid crystal's native structure. Local alignment, morphology, as well as the presence of defects are readily revealed, and a provisional framework relating the SHG response to the orientational distribution of CNC nanorods within the liquid crystal structure is presented. This paper illustrates the numerous benefits of using SHG microscopy for visualizing CNC chiral nematic systems directly in the suspension-liquid phase and paves the road for using SHG microscopy to characterize other types of aligned CNC structures, in wet and dry states.

Optical evaluation of internal meat quality deterioration in a tuna fillet based on second-harmonic generation anisotropy measurement

Methods for evaluating fish freshness have transitioned toward non-invasive, non-destructive, and non-contact approaches. However, non-contact methods for texture assessment that usually require contact for measurement remain limited. This study proposes a new non-contact method to quantify muscle degradation, which is thought to reflect fish meat texture, using optical methods. Second-harmonic generation (SHG) is a non-linear light scattering process. The anisotropy of SHG light reflects the structural conditions in muscle myosin filament and collagen fibers that directly contribute to the texture of fish meat. This study assessed the feasibility of using SHG anisotropy measurements to evaluate muscle degradation in fish fillets during chilled storage. Analyses of the SHG intensity and anisotropy scattered from a tuna fillet successfully identified three degradation processes with turning points at 24 and 72 h after thawing. This study clearly demonstrates that SHG anisotropy measurements have potential as a standard for fish quality evaluation.

Two-Dimensional MXene Flakes with Large Second Harmonic Generation and Unique Surface Responses.

MXenes are a family of two-dimensional (2D) materials with broad and varied applications in biology, materials science, photonics, and environmental remediation owing to their layered structure and high surface area-to-volume ratio. MXenes have exhibited significant nonlinear optical characteristics, which have been primarily explored in the context of photonics applications, yet the second-harmonic generation (SHG) behavior of MXenes remains an unexplored aspect of their optical properties. Herein, we demonstrate and quantify large second-order responses of 2D Ti3C2Tx MXenes both in aqueous solutions and on a silicon substrate for the first time. MXene flakes showed strong second-harmonic scattering (SHS) in a dilute suspension with a sensitivity of less than 0.1 μg/mL. Angle-dependent SHS experiments further found that the second-order responses originate from coherent 2D dipole radiation. Through confocal and atomic force microscopies, we found that the intense SHG signal from free-standing MXene flakes increases exponentially with decreasing thickness, while two-photon fluorescence increases linearly with thickness. The second-order susceptibility of the MXenes was determined to be 3.6 pm V-1 with a thickness of 10 nm, almost twice of that for an often-used SHG crystal, beta barium borate. We further explored surface properties of the MXene sheets by investigating the SHS responses upon addition of organic dye molecules to the system. It was found that the adsorption of crystal violet (CV) obeys a Langmuir adsorption model while the addition of malachite green (MG) resulted in almost no change in SHG intensity, even though the adsorption capacities for both CV (61.3 ± 1.7 mg/g) and MG (54.8 ± 2.8 mg/g) are similar. Such a stark difference in adsorption characteristics between cationic organic CV and MG dyes is likely due to their distinct orientational orderings on the MXene surfaces. This work opens many possibilities for the further employment of the family of 2D materials in photonics, optics, and surface catalysis applications.

Microscopic view on the polarization-resolved S-SHG intensity of the vapor/liquid interface of pure water.

Second harmonic generation (SHG) is a nonlinear optical phenomenon where two photons at the frequency ω combine to form a single photon at the second-harmonic frequency 2ω. Since that second-order process is very weak in bulk isotropic media, optical SHG responses of interfaces provide a powerful and versatile technique to probe the molecular structure and dynamics of liquid interfaces. Both local dipole contributions and non-local quadrupole contributions can be interesting to investigate different properties of the interface, such as the molecular orientation or the charge density. However, a major difficulty is to comprehend the link between the S-SHG intensity and molecular details. This article reports a numerical approach to model the polarization-resolved SHG intensities of a model vapor/liquid interface of pure water. The influence of the interfacial local environment on the hyperpolarizability is taken into account using quantum mechanical/molecular mechanics calculations. The numerical predictions are in very good agreement with experiments. We detail the hypotheses made during the modeling steps and discuss the impact of various factors on the modeled SHG intensities, including the description of the exciting field in the interfacial layer, the effect of neighboring molecules on the second-harmonic polarization, and the presence of an additional static electric field at the interface.

Structural design and characterization of a new type of organic nonlinear optical flavanone crystal

Two new crystals were designed and obtained: the chalcone derivative HMMP (1-(2‑hydroxy-4-methoxyphenyl)-3-(4-(methylthio)phenyl)prop‑2-en-1-one) and the flavanone derivative 7M4MC (7‑methoxy-2-(4-(methylthio)phenyl)chroman-4-one). Yellow bulk 7M4MC single crystal with dimensions of 3 × 2 × 1 mm3 and orange-yellow needle-like HMMP single crystal were grown by solution evaporation. 7M4MC crystal belongs to the P21 space group of the monoclinic structure, which has a non-centrosymmetric structure. HMMP crystal monoclinic, belongs to the P21/m space group with a centrosymmetric structure. In the 7M4MC molecule, the methoxy and methylthio groups act as electron donors (D) and the carbonyl group acts as an electron acceptor (A), forming a d-π-A-π-D structure with an electron gradient. 7M4MC exhibits a high degree of second-harmonic generation intensity (11 × KDP), a broad transmission range (600–1600 nm, >88 %), an elevated optical energy bandgap (Eg = 2.57 eV), high thermal stability (Tm = 105 °C) and excellent dielectric properties (εr ≈ 2.1). This work not only provides two new crystals but also offers a viable molecular design option by changing the degree of intramolecular distortion by altering the backbone structure.

Synergistic corrosion inhibition of 4-hydroxypyridine and halogen ions: Insights from interfacial nonlinear spectroscopy.

4-Hydroxypiridine (4-HPy) is a green chemistry corrosion inhibitor for low-carbon steel, valued for its environmental compatibility and low toxicity. Despite lower initial effectiveness than 4-mercapto/4-aminopyridine, 4-HPy's performance is markedly enhanced by halogen ions. By employing second harmonic generation (SHG) spectroscopy combined with electrochemical methods, Raman spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and in situ UV spectroscopy, this study elucidates the synergistic enhancement mechanism of 4-HPy with Cl-, Br-, and I- in 0.5 mol/L HCl solution. Time-dependent SHG measurements showed a two-step process of rapid adsorption and subsequent orientation change, with a proposed mechanism to interpret the temporal changes in SHG intensity. Deducing the adsorption kinetic equations and their application to the experimental data yields the adsorption rate (kad) and orientation change rate (Kre). Halogens reduce the orientation angle of 4-HPy, facilitating its adsorption on the substrate surface and effectively inhibiting corrosion via distinct mechanisms. Cl- and Br- ions primarily adsorb onto the metal surface, forming an adsorption film that not only enhances the subsequent adsorption of 4-HPy but also provides a protective effect for the metal surface. Conversely, I- forms mainly complexes with 4-HPy in solution, co-adsorbs onto the metal surface, and demonstrates a significant synergistic effect. This study revealed the synergistic efficacy hierarchy among halogen ions, with the order 4HPy + NaCl < 4HPy + NaBr < 4HPy + NaI. This study enhances our molecular-level understanding of the synergistic mechanism between halogen ions and corrosion inhibitors and provides valuable insights for designing and developing effective corrosion inhibitors.

Substrate Interference and Strain in the Second-Harmonic Generation from MoSe2 Monolayers.

Nonlinear optical materials of atomic thickness, such as non-centrosymmetric 2H transition metal dichalcogenide monolayers, have a second-order nonlinear susceptibility (χ(2)) whose intensity can be tuned by strain. However, whether χ(2) is enhanced or reduced by tensile strain is a subject of conflicting reports. Here, we grow high-quality MoSe2 monolayers under controlled biaxial strain created by two different substrates and study their linear and nonlinear optical responses with a combination of experimental and theoretical approaches. Up to a 15-fold overall enhancement in second-harmonic generation (SHG) intensity is observed from MoSe2 monolayers grown on SiO2 when compared to its value on a Si3N4 substrate. By considering an interference contribution from different dielectrics and their thicknesses, a factor of 2 enhancement of χ(2) was attributed to the biaxial strain: substrate interference and strain are independent handles to engineer the SHG strength of non-centrosymmetric 2D materials.

K4Cd3Ge4O13: A Congruent-Melting Germanate Nonlinear Optical Crystal with a Moderate Second-Harmonic Generation Intensity and Broad Transparent Window.

Mid-infrared (IR) nonlinear optical (NLO) materials have generated extensive research interest because of their crucial role in laser technology applications. Here, we report the synthesis of a novel cadmium germanate NLO crystal, K4Cd3Ge4O13, using spontaneous crystallization. K4Cd3Ge4O13 demonstrates a distinct three-dimensional structural framework characterized by twisted [Ge4O13] and [Cd3O10] clusters, composed of [GeO4], [CdO4], [CdO5], and [CdO6] basic building units, respectively, which represents an unprecedented structural feature. The title compound undergoes a desirable congruent melting behavior at about 727 °C. Notably, K4Cd3Ge4O13 demonstrates a short UV cutoff edge at 261 nm, coupled with a wide energy gap of 4.4 eV, and maintains an extended IR transparency window at around 6.0 μm. More importantly, it demonstrates a strong second-harmonic generation activity comparable to that of KH2PO4 (KDP) at 1064 nm. Theoretical analyses further elucidate that the remarkable optical performances of K4Cd3Ge4O13 are predominantly attributed to the cooperative effects of Ge-O and Cd-O bond-based motifs. These desired characteristics underscore the potential of K4Cd3Ge4O13 as a good candidate material for mid-IR NLO applications.

From molecules to materials: SHG-CD microscopy of structured chiral films

The interplay between molecular and structural chirality as a function of local sample morphology determines the nonlinear optical properties of many organic and hybrid organic–inorganic thin films. Here, we used second harmonic generation circular dichroism (SHG-CD) microscopy of thin molecular films of 1,1′-bi-2-naphthol (R-BINOL) as a research model. Our results show that the SHG signal measured at frequencies close to the electronic transition of BINOL molecules is resonantly enhanced by more than an order of magnitude compared to the non-resonant case. The extracted resonant SHG-CD signal is dominated by the chiral response of the molecule. In contrast, structural chirality determines the non-resonant SHG-CD images. We see clear evidence that the interference of the SHG signals caused by the molecular and structural chirality can lead to a decrease in the overall SHG intensity when both SHG signals are out of phase. These findings highlight the intricate relationship between molecular and structural chirality on the one hand and structural morphology on the other hand and pave the way for novel applications by exploiting the chiroptic properties of thin films.

A contactless in situ EFISH method for measuring electrostatic potential profile of semiconductor/electrolyte junctions.

In photoelectrochemical cells, promising devices for directly converting solar energy into storable chemical fuels, the spatial variation of the electrostatic potential across the semiconductor-electrolyte junction is the key parameter that determines the cell performance. In principle, electric field induced second harmonic generation (EFISH) provides a contactless in situ spectroscopic tool to measure the spatial variation of electrostatic potential. However, the total second harmonic generation (SHG) signal contains the contributions of the EFISH signals of semiconductor space charge layer and the electric double layer, in addition to the SHG signal of the electrode surface. The interference of these complex quantities hinders their analysis. In this work, to understand and deconvolute their contributions to the total SHG signals, bias-dependent SHG measurements are performed on the rutile TiO2(100)-electrolyte junction as a function of light polarization and crystal azimuthal angle (angle of the incident plane relative to the crystal [001] axis). A quadratic response between SHG intensity and the applied potential is observed in both the accumulation and depletion regions of TiO2. The relative phase difference and amplitude ratio are extracted at selected azimuthal angles and light polarizations. At 0° azimuthal angle and s-in-p-out polarization, the SHG intensity minimum has the best match with the TiO2 flatband potential due to the orthogonal relative phase difference between bias-dependent and bias-independent SHG terms. We further measure the pH-dependent flatband potential and probe the photovoltage under open circuit conditions using the EFISH technique, demonstrating the capability of this contactless method for measuring electrostatic potential at semiconductor-electrolyte junctions.

Gold Nanoparticles at a Liquid Interface: Towards a Soft Nonlinear Metasurface

Optical second-harmonic generation (SHG) is achieved using adsorbed gold nanoparticles (AuNPs) with an average diameter of 16 nm at the aqueous solution–air interface in reflection. A detailed analysis of the depth profile of the SHG intensity detected shows that two contributions appear in the overall signal, one arising from the aqueous solution–air interface that is sensitive to the AuNP surface excess and one arising from the bulk aqueous phase. The latter is an incoherent signal also known as hyper-Rayleigh scattering (HRS). The results agree with those of an analysis involving Gaussian beam propagation optics and a Langmuir-like isotherm. Discrepancies are revealed for the largest AuNP concentrations used and indicate a new route for the design of soft metasurfaces.

Theory of the interference tunability of second harmonic generation for two-dimensional materials in layered structures.

We theoretically study how the intensity of second harmonic generation (SHG) for a sheet of two-dimensional (2D) material is controlled by an underlying layered structure. By utilizing the transfer matrix method with the inclusion of a nonlinear sheet current to describe the response of the 2D material, an explicit expression for the intensity of upward propagating second harmonic (SH) light is obtained, and the effects of the layered structure can be identified by a structure factor β, defined as the ratio of SH intensity from such a structure to that from a freely suspended 2D material. Our results show that the influence of a layered structure on the SHG intensity arises from interference effects of both the fundamental light and the SH light; the value of the structure factor is 0 ≤ β ≤ 64. Furthermore, when the incident light is pulsed, the interference effects are partially canceled due to the existence of many wave vectors and frequencies, and the cancellation becomes severe for thick films, small beam spots, and short pulses. For a specific structure of 2D material/dielectric film/substrate, the thickness of the dielectric film can effectively tune the value of β in an interval [βmin, βmax], and detailed discussions are performed for the thicknesses when these two extreme values can be obtained. When there is optical loss or the substrate is not perfectly reflective, the extreme value of βmax or βmin cannot reach 64 or 0. A large βmax requires two conditions to be fulfilled: (1) the substrate should be highly reflective, and (2) the refractive indices of the dielectric film at the fundamental and the SH frequencies should differ. Our results indicate how practical substrate structures can be used to achieve high SH signals, and the simple expression we give for the SH enhancement will be useful in characterizing the nonlinear susceptibility of 2D materials.

Giant Second Harmonic Generation in Supertwisted WS2 Spirals Grown in Step-Edge Particle-Induced Non-Euclidean Surfaces.

In moiré crystals resulting from the stacking of twisted two-dimensional (2D) layered materials, a subtle adjustment in the twist angle surprisingly gives rise to a wide range of correlated optical and electrical properties. Herein, we report the synthesis of supertwisted WS2 spirals and the observation of giant second harmonic generation (SHG) in these spirals. Supertwisted WS2 spirals featuring different twist angles are synthesized on a Euclidean or step-edge particle-induced non-Euclidean surface using carefully designed water-assisted chemical vapor deposition. We observed an oscillatory dependence of SHG intensity on layer number, attributed to atomically phase-matched nonlinear dipoles within layers of supertwisted spiral crystals where inversion symmetry is restored. Through an investigation into the twist angle evolution of SHG intensity, we discovered that the stacking model between layers plays a crucial role in determining the nonlinearity, and the SHG signals in supertwisted spirals exhibit enhancements by a factor of 2 to 136 when compared with the SHG of the single-layer structure. These findings provide helpful perspectives on the rational growth of 2D twisted structures and the implementation of twist angle adjustable endowing them great potential for exploring strong coupling correlation physics and applications in the field of twistronics.

Enhanced UV Nonlinear Optical Properties in Layered Germanous Phosphites through Functional Group Sequential Construction

AbstractThis study pioneers a novel strategy for synthesizing solar‐blind ultraviolet (UV) nonlinear optical (NLO) crystals through functional groups sequential construction, effectively addressing the inherent trade‐offs among broad transmittance, enhanced second‐harmonic generation (SHG), and optimal birefringence. We have developed two innovative van der Waals layered germanous phosphites: GeHPO3, the first Ge(II)‐based oxide NLO crystal which exhibits a black phosphorus‐like structure, and K(GeHPO3)2Br, distinguished by its exceptional birefringence and graphene‐like structure. Significantly, GeHPO3 exhibits a remarkable array of NLO properties, including the highest SHG coefficient recorded among all NLO crystals for phase‐matching and generating 266 nm coherent light via quadruple frequency conversion. It delivers a potent SHG intensity, surpassing KH2PO4 (KDP) by 10.3 times at 1064 nm and β‐BaB2O4 by 1.3 times at 532 nm, complemented by a distinct UV absorption edge at 211 nm and moderate birefringence of 0.062 at 546 nm. Comprehensive theoretical analysis links these exceptional characteristics to the unique NLO‐active GeO34− units and the distinctive, highly ordered layered structures. Our findings deliver essential experimental insights into the development of Ge(II)‐based optoelectronic materials and present a strategic blueprint for engineering structure‐driven functional materials with customized properties.

Enhanced UV Nonlinear Optical Properties in Layered Germanous Phosphites through Functional Group Sequential Construction.

This study pioneers a novel strategy for synthesizing solar-blind ultraviolet (UV) nonlinear optical (NLO) crystals through functional groups sequential construction, effectively addressing the inherent trade-offs among broad transmittance, enhanced second-harmonic generation (SHG), and optimal birefringence. We have developed two innovative van der Waals layered germanous phosphites: GeHPO3, the first Ge(II)-based oxide NLO crystal which exhibits a black phosphorus-like structure, and K(GeHPO3)2Br, distinguished by its exceptional birefringence and graphene-like structure. Significantly, GeHPO3 exhibits a remarkable array of NLO properties, including the highest SHG coefficient recorded among all NLO crystals for phase-matching and generating 266 nm coherent light via quadruple frequency conversion. It delivers a potent SHG intensity, surpassing KH2PO4 (KDP) by 10.3 times at 1064 nm and β-BaB2O4 by 1.3 times at 532 nm, complemented by a distinct UV absorption edge at 211 nm and moderate birefringence of 0.062 at 546 nm. Comprehensive theoretical analysis links these exceptional characteristics to the unique NLO-active GeO3 4- units and the distinctive, highly ordered layered structures. Our findings deliver essential experimental insights into the development of Ge(II)-based optoelectronic materials and present a strategic blueprint for engineering structure-driven functional materials with customized properties.

Effect of Li concentration on second harmonic generation from widegap ZnMgO thin films

Widegap ZnMgO thin films hold great potential for applications to the high-efficiency generation of deep UV laser via frequency conversion, because of low light propagation loss and reversible polarization. In this work, we studied the effect of Li concentration on the second harmonic generation (SHG) from a-axis oriented ZnMgO thin films with an optical bandgap of about 3.96 eV. The SHG intensity per unit thickness from the ZnMgO thin film with a small Li concentration is approximately 10.7 times larger than the one without Li incorporation. The Li incorporation obviously enlarges interplanar spacing of ( 112̅0 ) plane and reduces the leakage current by three orders of magnitude. X-ray photoelectron spectroscopy measurements suggest the Li incorporation enhances SHG via enhancing electronic polarization. These findings uncover defect complexes based on Li interstitials in oxygen octahedron play a very important role in enhancing the nonlinear polarization of widegap ZnMgO thin films under optical frequency electric field.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Efficient Second‐Harmonic Generation with Weak Polarization Sensitivity in Gallium Nitride Metasurfaces via Bound States in the Continuum

AbstractBound states in the continuum (BICs) are widely exploited in all‐dielectric metasurfaces to significantly enhance the Q‐factor and second harmonic generation (SHG). However, a higher SHG conversion efficiency is overshadowed by the narrow transparency window of all‐dielectric materials and strict requirement of polarization‐matching. Herein, an augmented SHG assisted is demonstrated by symmetry‐protected quasi‐BICs from z‐cut wurtzite GaN metasurfaces, whose wide transparent range suppresses self‐absorption of second harmonic. Strong resonances emerge under both X‐ and Y‐polarized excitations, where the multipole characters of each eigenmode are quantitatively identified via decomposition simulations. The local field enhancement enabled by the extreme energy confinement of quasi‐BICs contributes to an efficient SHG, recording a conversion efficiency up to 2 × 10−7 at moderately low input peak intensity (≈1 GW cm−2), orders of magnitude larger than bulk GaN. With proper design, the overlapping X‐ and Y‐polarized resonances can support SHG from excitation with arbitrary polarizations. The alleviation of polarization‐matching remarkably enhances the SHG power by 66.2% under unpolarized illumination condition. The results of intense SHG reveal GaN as a promising candidate for high‐performance nanoscale nonlinear photonic devices. The weak polarization sensitivity of the designed structure will chart a novel course for the advancement of next‐generation efficient all‐dielectric metasurfaces.

Symmetry-breaking-induced off-resonance second-harmonic generation enhancement in asymmetric plasmonic nanoparticle dimers.

The linear and nonlinear optical properties of metallic nanoparticles have attracted considerable experimental and theoretical research interest. To date, most researchers have focused primarily on exploiting their plasmon excitation enhanced near-field and far-field responses and related applications in sensing, imaging, energy harvesting, conversion, and storage. Among numerous plasmonic structures, nanoparticle dimers, being a structurally simple and easy-to-prepare system, hold significant importance in the field of nanoplasmonics. In highly symmetric plasmonic nanostructures, although the odd-order optical nonlinearity of the near-surface region will be improved because of the enhanced near-fields, even-order nonlinear processes such as second-harmonic generation (SHG) will still be quenched and thus optically forbidden. Under this premise, it is imperative to introduce structural symmetry breaking to realize plasmon-enhanced even-order optical nonlinearity. Here, we fabricate a series of nanoparticle dimers each composed of two gold nanospheres with different diameters and subsequently investigate their structural asymmetry dependent linear and nonlinear optical properties. We find that the SHG intensities of gold nanosphere dimers are significantly enhanced by structural asymmetry under off-resonance excitation while the plasmonic near-field enhancement mainly affects SHG under on-resonance excitation. Our results reveal that symmetry breaking will play an indispensable role when designing novel coupled plasmonic nanostructures with enhanced nonlinear optical properties.