Enhancement Of Second Harmonic Generation Research Articles

Coherent control of enhanced second-harmonic generation in a plasmonic nanocircuit using a transition metal dichalcogenide monolayer

Nonlinear nanophotonic circuits, renowned for their compact form and integration capabilities, hold potential for advancing high-capacity optical signal processing. However, limited practicality arises from low nonlinear conversion efficiency. Transition metal dichalcogenides (TMDs) could present a promising avenue to address this challenge, given their superior optical nonlinear characteristics and compatibility with diverse device platforms. Nevertheless, this potential remains largely unexplored, with current endeavors predominantly focusing on the demonstration of TMDs’ coherent nonlinear signals via free-space excitation and collection. In this work, we perform direct integration of TMDs onto a plasmonic nanocircuitry. By controlling the polarization angle of the input laser, we show selective routing of second-harmonic generation (SHG) signals from a MoSe2 monolayer within the plasmonic circuit. Routing extinction ratios of 14.86 dB are achieved, demonstrating good coherence preservation in this hybrid nanocircuit. Additionally, our characterization indicates that the integration of TMDs leads to a 13.8-fold SHG enhancement, compared with the pristine nonlinear plasmonic nanocircuitry. These distinct features—efficient SHG generation, coupling, and controllable routing—suggest that our hybrid TMD-plasmonic nanocircuitry could find immediate applications including on-chip optical frequency conversion, selective routing, switching, logic operations, as well as quantum operations.

Tunable Enhanced Second-Harmonic Generation in InP-InAsP Quantum Well Nanomembranes.

Second-harmonic generation (SHG) offers a convenient approach for infrared-to-visible light conversion in tunable nanoscale light sources and optical communication. Semiconductor nanostructures offer rich possibilities to tailor their nonlinear optical properties. In this study, strong second-harmonic generation in InP nanomembranes with InAsP quantum well (QW) is demonstrated. Compared with bulk InP, up to 100 times enhancement of SHG is achieved in the short-wave infrared range. This enhancement is shown to be predominantly induced by the resonance-enhanced absorption and quantum confinement of fundamental wavelengths in the InAsP QW. The thin nanomembrane structure will also provide nanocavity enhancement for second-harmonic wavelengths. The enhanced SHG peak wavelengths can also be tuned by changing the QW composition. These findings provide an effective strategy for enhancing and manipulating the second-harmonic generation in semiconductor quantum-confined nanostructures for on-chip all-optical applications.

Enhanced second harmonic generation by an atomically thin MoS2 sheet attached to a resonant metasurface.

Two-dimensional transition metal dichalcogenides have shown large second-order nonlinear responses due to their broken crystal inversion symmetry. However, their nonlinear interaction with light is restricted to an atomically thin layer. Placing a sheet of transition metal dichalcogenides on a resonant metasurface enhances the field interacting with the nonlinear material thus compensating for this shortcoming. But, it remains a challenge to tune resonances such, that they coincide with fundamental and second harmonic frequencies simultaneously. Here we demonstrate two independent methods to achieve that goal and numerically illustrate our findings for a MoS2 layer combined with silicon nitride photonic crystals. We numerically demonstrate 20-fold and 170-fold enhancement of second-harmonic generation compared with a design based on a single resonant structure. Although we focus on that specific configuration our approach can likewise be applied to other dielectrics combined with highly nonlinear 2D materials.

Enhancement of second-harmonic generation in a 64° stacked WSe2/WS2 heterobilayer with local strain

Transition metal dichalcogenides (TMDs) are actively studied in various fields of optics and optoelectronics, including nonlinear optics of second-harmonic generation (SHG). By stacking two different TMD materials to form a heterobilyaer, unique optical properties emerge, with stronger SHG at a twist angle of 0° between TMDs and weaker SHG at a twist angle of 60°. In this work, we demonstrate the enhancement of SHG in a heterobilayer consisting of WSe2 and WS2 monolayers stacked at a twist angle of 64.1°, using a nanoparticle to induce local strain. The interatomic spacing of the heterobilayer is deformed by the nanoparticle, breaking the inversion symmetry, resulting in a substantial increase in the SHG of the heterobilayer at room temperature. The SHG increases depending on the polarization of the pump laser: 15-fold for linear polarization, 9-fold for right-circular polarization, and up to 100-fold for left-circular polarization. In addition, the SHG enhanced in the heterobilayer with local strain satisfies the same chiral selection rule as in the unstrained TMD region, demonstrating that the chiral selection rule of SHG is insensitive to local strain. Our findings will increase the applicability of TMD heterobilayers in nonlinear optoelectronics and valleytronics.

Plasmon-enhanced second harmonic generation of metal nanostructures.

As the most common nonlinear optical process, second harmonic generation (SHG) has important application value in the field of nanophotonics. With the rapid development of metal nanomaterial processing and chemical preparation technology, various structures based on metal nanoparticles have been used to achieve the enhancement and modulation of SHG. In the field of nonlinear optics, plasmonic metal nanostructures have become potential candidates for nonlinear optoelectronic devices because of their highly adjustable physical characteristics. In this article, first, the basic optical principles of SHG and the source of surface symmetry breaking in metal nanoparticles are briefly introduced. Next, the related reports on SHG in metal nanostructures are reviewed from three aspects: the enhancement of SHG efficiency by double resonance structures, the SHG effect based on magnetic resonance and the harmonic energy transfer. Then, the applications of SHG in the sensing, imaging and in situ monitoring of metal nanostructures are summarized. Future opportunities for SHG in composite systems composed of metal nanostructures and two-dimensional materials are also proposed.

Enhanced second-harmonic generation in a photonic crystal waveguide-coupled nanocavity using a wavelength-selective reflector

Photonic crystal waveguide-coupled photonic nanocavities are promising to develop integrated nonlinear nanophotonic devices because of their strong nonlinear optical process in cavities with high quality (Q) factors and small modal volume, multiple-wavelength-channel operation, and efficient and highly dense integration with other optical components. However, the intrinsic features of the standing-wave mode in the photonic crystal resonant cavity cause some waveguided light to pass through the nanocavity without coupling, which remains a significant challenge in achieving high nonlinear optical efficiency in integrated nanophotonic devices. To feed back the uncoupled light into the nanocavity and enhance the nonlinear optical efficiency in a photonic crystal waveguide-coupled nanocavity, we designed and fabricated a wavelength-selective reflector based on a silicon carbide two-dimensional photonic crystal structure and experimentally demonstrated the significant enhancement of second harmonic generation (SHG) using the reflector. The findings suggest that the reflector increases the electric field intensity in the nanocavity and improves Q-matching between the nanocavity and the waveguide. These two effects of the reflector significantly enhance the SHG efficiency by 11.5 compared to that without a reflector. The experimental results agree well with the calculation results obtained using coupled-mode theory. Our study paves the way for developing efficient nonlinear optical devices for high-density integrated nanophotonics and quantum applications.

High-throughput investigation of second harmonic generation enhancement in indium tin oxide films: Effects of Sn doping

The doping effect of the Sn atom in indium tin oxide (ITO) plays a crucial role in influencing the epsilon-near-zero (ENZ) behaviors and the related nonlinear optical properties. A ternary (In1−xSnx)2O3 film is fabricated by high-throughput magnetron sputtering technique. The relationship between Sn doping and second harmonic generation (SHG) enhancement is systematically interpreted through comprehensive characterization of structural and optical properties. Significantly, clear microstructural changes associated with different levels of Sn doping are identified by x-ray diffraction and Raman spectroscopy. These changes directly contribute to shifts in the ENZ wavelength (λENZ). The results indicate that the optimal region for SHG enhancement lies within 7.3–10.6 at. % Sn doping. Moreover, it is demonstrated that λENZ can be adjusted by manipulating the formation of electrically inactive defect clusters via Sn substitution at the 24d sites. This study not only provides valuable insight into the mechanism linking Sn doping and SHG enhancement of ITO but also exemplifies the high-throughput exploration of optical functional materials.

Giant enhancement of second harmonic generation from monolayer 2D materials placed on photonic moiré superlattice

Abstract We numerically investigate second harmonic generation (SHG) from a monolayer of 2D-material placed on photonic moiré superlattice fabricated by dielectric materials. The greatly enhanced local field at the resonance modes of moiré superlattice can dramatically boost the SHG response in 2D materials. Considering a typical 2D-material MoS2 monolayer placed on a photonic moiré superlattice of a twist angle 9.43°, the maximum SHG conversion efficiency reaches up to 10−1 at a relatively low intensity of fundamental light 1 kW/cm2, which is around 14 orders of magnitude larger than that from the monolayer placed on a flat dielectric slab without moiré superlattices. The SHG conversion efficiency from the monolayer can be further enhanced with the decrease of the twist angles of moiré superlattice due to the even more confinement of local field. The flat bands in the moiré superlattices formed by the small twist angles can particularly ensure the efficiency even under wide-angle illuminations. The results indicate that photonic moiré superlattice which can tightly confine light is a promising platform for efficient nonlinear optics.

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.

Resonant nonlinear nanostructured grating in an unstructured lithium niobate on insulator platform

Lithium niobate (LN) is a versatile material for applications in integrated nonlinear optics; however, due to its stable physical and chemical properties, efficient nonlinear conversion with nanostructured LN is not easy to implement. Here, we propose a resonant nanostructure composed of a SiO2 grating and a lithium niobate on insulator (LNOI) layer, which has been proven to support bound states in the continuum (BIC) modes through simulations. By experimentally measuring the angle-resolved transmittance spectra of the fabricated sample, we confirm that our proposed nanostructure exhibits quasi-BIC modes transitioning from BIC modes, resulting in strong localized field confinement. In the nonlinear experiment, our results show the significant potential of the nanostructure in enhancing second harmonic generation (SHG). By pumping at the resonant wavelength, we achieve a SHG enhancement factor of 6400, compared to a flat LNOI thin film under a fundamental wave (FW) peak intensity of 460 MW/cm2. The SHG conversion efficiency reaches up to 1.741 × 10−7, leading to a normalized conversion efficiency of 3.785 × 10−7 cm2/GW. Although the experimental results are two orders of magnitude lower than the theoretical maximum SHG conversion efficiency of approximately 10−5, we believe that our proposed etchless LNOI nanostructure highlights the tremendous potential for applications in nonlinear resonant nanophotonics.

Electrically and all-optically switchable nonlocal nonlinear metasurfaces.

Nonlocal effects on metasurfaces play an important role to achieve high-Q spectral selectivity, beneficial for development of multifunctional, multispectral integrated optics. In addition, they enhance the optical interaction and promote a variety of nonlinear effects, including frequency conversion and stimulated scattering. Active tuning of nonlocal nonlinearity is highly desirable for sensing and signal processing but was hardly explored until now. Here, we show drastic electric and all-optical tunability of nonlocal second-harmonic generation (SHG) from nonlinear metasurface, functionalized with a twisted nematic liquid-crystal (LC) layer. The addition of LC results in the emergence of strong nonlocal SHG, due to a surface lattice resonance of the system. We demonstrate a notable enhancement of SHG on resonance, more than 25 dB electrical switching amplitude, and all-optically induced phase transition imprinted on SHG. Our results on dynamic nonlocal effects introduce a very promising route for active nonlinear optical metadevices at the nanoscale.

Manipulating Peierls Distortion in van der Waals NbOX2 Maximizes Second-Harmonic Generation.

Two-dimensional (2D) van der Waals (vdW) materials, featuring relaxed phase-matching conditions and highly tunable optical nonlinearity, endow them with potential applications in nanoscale nonlinear optical (NLO) devices. Despite significant progress, fundamental questions in 2D NLO materials remain, such as how structural distortion affects second-order NLO properties, which call for advanced regulation and in situ diagnostic tools. Here, by applying pressure to continuously tune the displacement of Nb atoms in 2D vdW NbOI2, we effectively modulate the polarization and achieve a 3-fold boost of the second-harmonic generation (SHG) at 2.5 GPa. By introducing a Peierls distortion parameter, λ, we establish a quantitative relationship between λ and SHG intensity. Importantly, we further demonstrate that the SHG enhancement can be achieved under ambient conditions by anionic substitution to tune the distortion in NbO(I1-xBrx)2 (x = 0-1) compounds, where the chemical tailoring simulates the pressure effects on the structural optimization. Consequently, NbO(I0.60Br0.40)2 with λ = 0.17 exhibits a giant SHG of over 2 orders of magnitude higher than that in monolayer WSe2, reaching the record-high value among reported 2D vdW NLO materials. This work unambiguously demonstrates the correlation between Peierls distortion and SHG property and, more broadly, opens new paths for the development of advanced NLO materials by manipulating the structure distortions.

Extraordinary second harmonic generation modulated by divergent strain field in pressurized monolayer domes

The most prominent form of nonlinear optical (NLO) frequency conversion is second harmonic generation (SHG), where incident light interacts with a nonlinear medium producing photons at double the input frequency, which has vast applications in material and biomedical science. Emerging two-dimensional nonlinear optical materials led by transition metal dichalcogenides (TMDs) have fascinating optical and mechanical properties and are highly anticipated to overcome the technical limitations imposed by traditional bulky NLO materials. However, the atomic scale interaction length and low conversion efficiency in TMD materials prevent their further implementation in NLO applications. While some uniaxial strain-engineering studies intensively investigated the anisotropic SHG response in TMDs, they did not realize giant SHG enhancement by exploiting the opto-mechanical characteristics. Herein, we employ proton (H+) irradiation to successfully fabricate large pressurized monolayer TMD domes (d ≥ 10 μm) and conduct a comprehensive investigation and characterization of their SHG performance enhancement. We show that the intensity of SHG is effectively enhanced by around two orders of magnitude at room temperature. Such giant enhancement arises from the distinct separation distance induced by capped pressurized gas and the hemi-spherical morphology, enabling constructive optical interference. Moreover, the unique divergent strain field in TMD domes promotes the first experimental study on the anisotropic nonlinear optical behavior based on biaxial strain conditions in terms of varying strain orientation and relative weights. Our work demonstrates a promising system with enhanced NLO performance and well-preserved biocompatibility, paving a way toward the future nano-scaled quantum optics design and biomedical applications.

Laser-Activated Second Harmonic Generation in Flexible Membrane with Si Nanowires.

Nonlinear silicon photonics has a high compatibility with CMOS technology and therefore is particularly attractive for various purposes and applications. Second harmonic generation (SHG) in silicon nanowires (NWs) is widely studied for its high sensitivity to structural changes, low-cost fabrication, and efficient tunability of photonic properties. In this study, we report a fabrication and SHG study of Si nanowire/siloxane flexible membranes. The proposed highly transparent flexible membranes revealed a strong nonlinear response, which was enhanced via activation by an infrared laser beam. The vertical arrays of several nanometer-thin Si NWs effectively generate the SH signal after being exposed to femtosecond infrared laser irradiation in the spectral range of 800-1020 nm. The stable enhancement of SHG induced by laser exposure can be attributed to the functional modifications of the Si NW surface, which can be used for the development of efficient nonlinear platforms based on silicon. This study delivers a valuable contribution to the advancement of optical devices based on silicon and presents novel design and fabrication methods for infrared converters.

Near-field enhancement of optical second harmonic generation in hybrid gold–lithium niobate nanostructures

Nanophotonics research has focused recently on the ability of nonlinear optical processes to mediate and transform optical signals in a myriad of novel devices, including optical modulators, transducers, color filters, photodetectors, photon sources, and ultrafast optical switches. The inherent weakness of optical nonlinearities at smaller scales has, however, hindered the realization of efficient miniaturized devices, and strategies for enhancing both device efficiencies and synthesis throughput via nanoengineering remain limited. Here, we demonstrate a novel mechanism by which second harmonic generation, a prototypical nonlinear optical phenomenon, from individual lithium niobate particles can be significantly enhanced through nonradiative coupling to the localized surface plasmon resonances of embedded gold nanoparticles. A joint experimental and theoretical investigation of single mesoporous lithium niobate particles coated with a dispersed layer of ~10 nm diameter gold nanoparticles shows that a ~32-fold enhancement of second harmonic generation can be achieved without introducing finely tailored radiative nanoantennas to mediate photon transfer to or from the nonlinear material. This work highlights the limitations of current strategies for enhancing nonlinear optical phenomena and proposes a route through which a new class of subwavelength nonlinear optical platforms can be designed to maximize nonlinear efficiencies through near-field energy exchange.

Efficient enhancement of photoluminescence and second-harmonic generation of few-layer InSe coupled with surface-plasmonic Ag prism array

Efficient enhancement of photoluminescence and second-harmonic generation of few-layer InSe coupled with surface-plasmonic Ag prism array

Three orders of magnitude enhancement of second and third harmonic generation in the visible and ultraviolet ranges from plasmonic gold nanogratings

We report experimental observations and numerical simulations of second and third harmonic generation from a gold nanograting, which exhibits a plasmonic resonance in the near infrared. The resonance is tunable, with a spectral position that depends on the angle of incidence. All things being equal, the enhancement of nonlinear optical processes produced by the field localization in the nanograting when compared with a flat gold mirror manifests itself dramatically from the ultraviolet to the visible range: second harmonic generation conversion efficiencies increase by more than three orders of magnitude, while we report a third harmonic generation conversion efficiency enhancement factor of 3200, both in excellent agreement with our theoretical predictions. The clear inferences one may draw from our results are that our model describes the dynamics with unprecedented accuracy and that much remains to be revealed in the development of nonlinear optics of metals at the nanoscale.

Magnetic Fano resonance enhanced second-harmonic generation in chiral hybrid bismuth halides

Magnetic Fano resonance provides a potential opportunity to control both linearity and nonlinearity of light for their low radiation loss and near-field enhancement. Previous investigations have demonstrated its significant enhancement of nonlinearity in a plasmonic structure and the 2D materials coupled to it. In this work, the enhancement of second harmonic generation (SHG) of hybrid bismuth halides at the important communication wavelength of 1550 nm with a magnetic Fano dip is theoretically studied. To this end, a hybrid system composed of two asymmetric silver square split rings (SSRs) and this chiral perovskite film is designed. The simulation results show that magnetic Fano-like resonance is induced at the destructive interface of two magnetic modes in the SSR dimer, which can be inherited to hybrid bismuth halides, thereby leading to the increase of four orders of magnitude in its SH near-field enhancement factor. With a peak intensity of 0.16 GW cm−2, the composite structure features a high SHG conversion efficiency of up to 1.6 × 10−3 at the Fano resonance position. By rotating the polarization angle of fundamental optical excitation, the emitted SHG signal is switched on–off. Our research provides a valuable thought for enhancing the nonlinear optical process of the perovskite films by coupling the magnetic modes.

Giant enhancement of second-harmonic generation of indium selenide on planar Au.

Two-dimensional (2D) van der Waals layered γ-type indium selenide (γ-InSe) holds great promise for the development of ultrathin and low-energy-consumption nonlinear optical devices due to its broken inversion symmetry regardless of layer number. Nevertheless, the 2D InSe thin flakes still exhibit short light-matter interaction lengths, thus resulting in low efficiencies of nonlinear optical processes. In this work, we provide a facile 2D semiconductor-metal structure consisting of InSe thin flakes (thickness: 11-54 nm) on planar Au film, which exhibits great second-harmonic generation (SHG) enhancement by a factor of up to 1182. The SHG enhancement is attributed to the interference effect-induced strong electric field in highly absorbing InSe; meanwhile, the increase in reflectivity by Au film also plays an important role. Furthermore, the InSe thickness and excitation wavelength dependences of enhancement factors are revealed. This work provides a convenient approach to developing high-efficiency 2D nonlinear optical devices with ultrathin form.

Giant Enhancement of Optical Second Harmonic Generation in Hollow-Core Fiber Integrated with GaSe Nanoflakes

Giant Enhancement of Optical Second Harmonic Generation in Hollow-Core Fiber Integrated with GaSe Nanoflakes