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The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase‐matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm‐scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm‐scale interaction lengths. This work harnesses modal phase matching for third‐harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3‐hexylthiophene‐2,5‐diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase‐matched modal dispersion results in THG efficiency as high as ≈ 10–3 W‐2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on‐chip applications). Plasmonic modal dispersion is exploited to fulfill the required phase matching condition for third‐harmonic generation in short (≈ 4.3 µm long) waveguides. In a two‐wire transmission line configuration, antisymmetric plasmonic gap modes mediate the nonlinear parametric process and the incident laser polarization dictates the third harmonic power, with excellent agreement between experiment and theory.

Optically resonant dielectric and semiconductor metasurfaces are an emerging and promising area of nanophotonics and light–matter interaction at the nanoscale. Recently, active tuning of the linear response and nonlinear effects of these components has received an increasing amount of interest. However, so far these research directions have remained separated with only few sporadic works that study their combination beginning to appear in the literature. The evolution of nonlinear metasurfaces based on dielectric and semiconductor materials toward reconfigurable and dynamic components could potentially answer the demand of integrated on-chip components that realize essential functionalities such as frequency conversion, active switching, optical isolation, and all-optical routing. This review provides an overview of recent investigations in this field, reviews the main physical phenomena enabling the dynamic control of the nonlinear response and compares the temporal dynamics of the diverse approaches that have been explored so far. Finally, future directions of dynamic nonlinear metasurfaces are outlined.

Nonlinear nanophotonic devices have shown great potential for on‐chip information processing, quantum source, 3D microfabrication, greatly promoting the developments of integrated optics, quantum science, nanoscience and technologies, etc. To promote the applications of nonlinear nanodevices, improving the nonlinear efficiency, expanding the spectra region of nonlinear response and reducing device thickness are three key issues. Herein, this study focuses on the nonlinear effect of third‐harmonic generation (THG), and present a thin Si meta‐sructure to improve the THG efficiency in the ultraviolet (UV) region. The measured THG efficiency is up to 10−5 at an emission wavelength of 309 nm. Also, the THG nanosystem is only 100 nm in thickness, which is two–five times thinner than previous all‐dielectric nanosystems applied in THG studies. These findings not only present a powerful thin meta‐structure with highly efficient THG emission in UV region, but also provide a constructive avenue for further understanding the light–matter interactions at subwavelength scales, guiding the design and fabricating of advanced photonic devices in future. By utilizing the confined hybrid anapole mode, the ultra‐thin silicon‐based meta‐structure can not only achieve ultraviolet third harmonic generation (THG), but also greatly enhance the conversion efficiency to as high as 10−5, which is a new record to date of the ultraviolet‐region THG in the all‐dielectric nanosystems.

Polarization is a fundamental property of light that can be engineered and controlled efficiently with optical metasurfaces. Here, we employ with monoclinic lattice geometry and achiral meta-atoms for resonant engineering of polarization states of light. We demonstrate, both theoretically and experimentally, that a monoclinic metasurface can convert linearly polarized light into elliptically polarized light not only in the linear regime but also in the nonlinear regime with the resonant generation of the third-harmonic field. We reveal that the ellipticity of the fundamental and higher-harmonic fields depends critically on the angle of the input linear polarization, and the effective chiral response of a monoclinic lattice plays a significant role in the polarization conversion.

We investigate resonant third-harmonic generation in near-zero index thin films driven out-of-equilibrium by intense optical excitation. Adopting the Landau weak coupling formalism to incorporate electron–electron and electron–phonon scattering processes, we derive a novel set of hydrodynamic equations accounting for collision-driven nonlinear dynamics in sodium. By perturbatively solving hydrodynamic equations, we model third-harmonic generation by a thin sodium film, finding that such a nonlinear process is resonant at the near-zero index resonance of the third-harmonic signal. Thanks to the reduced absorption of sodium, we observe that third-harmonic resonance can be tuned by the impinging pump radiation angle, efficiently modulating the third-harmonic generation process. Furthermore, owing to the metallic sodium response at the pump optical wavelength, we find that the third-harmonic conversion efficiency is maximised at a peculiar thin film thickness where evanescent back-reflection provides increased field intensity within the thin film. Our results are relevant for the development of future ultraviolet light sources, with potential impact for innovative integrated spectroscopy schemes.

This paper proposed angle measurement methods based on direct third harmonic generation (THG) in centrosymmetric crystals. The principles of the intensity-dependent and the wavelength-dependent angle measurement methods were illustrated. In this study, three prospective centrosymmetric crystals and two different phase-matching types were investigated in a wavelength range from 900 nm to 2500 nm. For the intensity-dependent method, a dispersion-less wavelength range was found from 1700 nm to 2000 nm for α-BBO and calcite. Compared with rutile, α-BBO and calcite had relatively better measurement performance based on the angle measurement sensitivity calculation. The wavelength-dependent method was considered in a dispersive range of around 1560 nm. The results suggested that α-BBO and calcite were also suitable for wavelength-dependent measurement. In addition, the effects of focusing parameters were considered in the simulation, and the optimized focal length (f = 100 mm) and the focused position (in the center of the crystal) were determined.

Plasmonic nanoantennas have been extensively explored to boost nonlinear optical processes due to their capabilities to confine optical fields on the nanoscale. In harmonic generation, nanoantenna array architectures are often employed to increase the number of emitters in order to efficiently enhance the harmonic emission. A small laser focus spot on the nanoantenna array maximizes the harmonic yield since it scales nonlinearly with the incident laser intensity. However, the nonlinear yield of the nanoantennas lying at the boundary of a focused beam may exhibit significant deviations in comparison to those at the center of the beam due to the Gaussian intensity distribution of the beam. This spatial beam inhomogeneity can cause power instability of the emitted harmonics when the lateral beam position is not stable which we observed in plasmon-enhanced third-harmonic generation (THG). Hence, we propose a method for deterministically designing the density of a nanoantenna array to decrease the instability of the beam position-dependent THG yield. This method is based on reducing the ratio between the number of ambiguous nanoantennas located at the beam boundary and the total number of nanoantennas within the beam diameter to increase the plasmon-enhanced THG stability, which we term as the ratio of ambiguity ( ). We find that the coefficient of variation of the measured plasmonic THG yield enhancement decreases with the . Thus, our method is beneficial for designing reliable sensors or nonlinear optical devices consisting of nanoantenna arrays for enhancing output signals.

Nonlinear nanophotonic devices have brought about great advances in the fields of nano-optics, quantum science, biomedical engineering, etc. However, in order to push these nanophotonic devices out of laboratory, it is still highly necessary to improve their efficiency. Since obtaining novel nanomaterials with large nonlinearity is of crucial importance for improving the efficiency of nonlinear nanodevices, we propose the two-dimensional (2D) perovskites. Different from most previous studies which focused on the 2D perovskites in large scale (such as the bulk materials or the thick flakes), herein we studied the 2D perovskites nanosheets with thickness of ∼50 nm. The high-order nonlinear processes including multi-photon photoluminescence and third-harmonic generation (THG) have been systematically investigated, and it is found the THG process can have a high conversion efficiency up to ∼8 × 10 . Also, it is observed that the nonlinear responses of 2D perovskites have large optical anisotropy, i.e., the polarization ratio for the incident polarization dependence of nonlinear response can be as high as ∼0.99, which is an impressive record in the perovskite systems. Our findings reveal the properties of high efficiency and huge optical anisotropy in the nonlinear processes of 2D perovskite nanosheets, shedding light on the design of advanced integrated nonlinear nanodevices in future.

The particle-on-mirror nanocavity, supporting multiple plasmonic resonances, provides an ideal platform to efficiently boost the nonlinear optical processes at the nanoscale. Here, we report on the enhancement of the second (SHG) and third-harmonic generations (THG) from the monolayer MoS using a multi-resonant Au nanosphere dimer-on-mirror nanocavity (DoMN). The strong plasmon hybridization between the dimer and underlying Au substrate leads to the emergence of two distinct cavity modes, which are intentionally aligned with the SH and TH frequencies, rendering a 15- and 68-fold enhancement for the SHG and THG of the monolayer MoS , respectively. Further theoretical analysis yields that these remarkable nonlinearity enhancements are also ascribed to the amplification of nonlinear source because of the excellent spatial mode overlap and the high directivity of nonlinear emission enabled by the cavity modes. Our results pave the way for the implementation of low-cost, and highly efficient nonlinear photonics devices integrated with plasmonic nanocavities.

We perform a perturbative calculation of the third order optical conductivities of doped graphene, using approximations valid around the Dirac points and neglecting effects due to scattering and electron-electron interactions. In this limit analytic formulas can be constructed for the conductivities. We discuss in detail the results for third harmonic generation, the Kerr effect and two-photon carrier injection, parametric frequency conversion, and two-color coherent current injection. We find a complicated dependence on the chemical potential and photon energies. The linear dispersion causes resonances over a wide range of photon energies, and it is possible to obtain large optical nonlinearities by tuning the chemical potential.