Nonlinear optics is an important area of photonics research for realizing active optical functionalities such as light emission, frequency conversion, and ultrafast optical switching for applications in optical communication, material processing, precision measurements, spectroscopic sensing and label-free biological imaging. An emerging topic in nonlinear optics research is to realize high efficiency optical functionalities in ultra-small, sub-wavelength length scale structures by leveraging interesting optical resonances in surface relief metasurfaces. Such artificial surfaces can be engineered to support high quality factor resonances for enhanced nonlinear optical interaction by leveraging interesting physical mechanisms. The aim of this review article is to give an overview of the emerging field of nonlinear optics in dielectric based sub-wavelength periodic structures to realize efficient harmonic generators, wavelength mixers, optical switches etc. Dielectric metasurfaces support the realization of high quality-factor resonances with electric field concentrated either inside or in the vicinity of the dielectric media, while at the same time operate at high optical intensities without damage. The periodic dielectric structures considered here are broadly classified into guided-mode resonant structures and resonant metasurfaces. The basic physical mechanisms behind guided-mode resonances, electromagnetically-induced transparency like resonances and bound-states in continuum resonances in periodic photonic structures are discussed. Various nonlinear optical processes studied in such structures with example implementations are also reviewed. Finally, some future directions of interest in terms of realizing large-area metasurfaces, techniques for enhancing the efficiency of the nonlinear processes, heterogenous integration, and extension to non-conventional wavelength ranges in the ultra-violet and infrared region are discussed.

We present the design, fabrication, and experimental characterization of silicon nitride based medium-index contrast gratings on glass substrate for fluorescence enhancement in the yellow to red spectral range with resonances for both incident excitation and fluorescence emission wavelengths under surface normal incidence. A comparison of the design space to realize resonant field enhancement in high-index contrast silicon and medium-index contrast silicon nitride grating structures is presented. The one-dimensional sub-wavelength grating structures studied here are designed with large duty cycle (∼80%) to account for the medium refractive index contrast (n ∼ 0.5) between silicon nitride and the glass substrate to ensure that the device operates in the two-mode regime. The resonant enhancement of fluorescence is experimentally verified using rhodamine-B isothiocyanate dye as the fluorophore of interest. A resonant enhancement of 10.8 times is demonstrated in this sample when compared to un-patterned film for transverse electric-transverse magnetic (TE-TM) polarization combination. We have also performed simulation study with plane wave excitation and incoherent dipole array emission to model the resonant excitation and emission processes, respectively. The simulations corroborate well with the best observed experimental results for the doubly resonant fluorescence configuration. Silicon nitride based medium contrast gratings are a promising platform to fabricate scalable structures for resonant enhancement of light-matter interaction with potential applications in high-sensitivity biological fluorescence assays and as a platform for polarization selective interrogation of light emission from nanoscale emitters attached to the grating.

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We report strong second-harmonic generation (SHG) from 2H polytype of multilayer Tin diselenide (SnSe2) for fundamental excitation close to the indirect band-edge in the absence of excitonic resonances. Comparison of SHG and Raman spectra from exfoliated SnSe2 flakes of different polytypes shows strong (negligible) SHG and Raman Eg mode at 109 cm−1 (119 cm−1), consistent with 2H (1T) polytypes. The difference between the A1g–Eg Raman peak positions is found to exhibit significant thickness dependent for the 1T form, which is found to be absent for the 2H form. The observed thickness dependence of SHG with rapid oscillations in signal strength for small changes in flake thickness are in good agreement with a nonlinear wave propagation model considering nonlinear polarization with alternating sign from each monolayer. The nonlinear optical susceptibility extracted from SHG signal comparison with standard quartz samples for 1040 nm excitation is found to be more than 4-times higher than that at 1550 nm. This enhanced nonlinear response at 1040 nm is attributed to the enhanced nonlinear optical response for fundamental excitation close to the indirect band-edge. We also study SHG from heterostructures of monolayer MoS2/multilayer SnSe2 which allows us to unambiguously compare the nonlinear optical response of SnSe2 with MoS2. We find the SHG signal and any interference effect in the overlap region to be dominated by the SnSe2 layer for the excitation wavelengths considered. The comparison of SHG from SnSe2 and MoS2 underscores that the choice of the 2D material for a particular nonlinear optical application is contextual on the wavelength range of interest and its optical properties at those wavelengths. The present works further highlights the usefulness of near band-edge enhancement of nonlinear processes in emerging 2D materials towards realizing useful nanophotonic devices.

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We demonstrate polarization-independent resonant-enhancement of second harmonic generation (SHG) from multilayer Gallium Selenide (GaSe) on a silicon-based resonant metasurface. Two-dimensional hexagonal photonic lattice with circularly symmetric silicon meta-atoms are designed to achieve resonant field enhancement at the fundamental wavelength independent of the incident polarization direction. Such structures are however found to exhibit strong resonant field depolarization effects at the fundamental excitation fields resulting in modified nonlinear polarization components when compared to the native GaSe layer. Furthermore, the sub-wavelength metasurface designed to have resonances at the fundamental wavelengths act as a higher order diffraction grating at the second harmonic wavelength. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. This work shows that it is not sufficient to use symmetric meta-atoms in such 2D material integrated resonant metasurfaces for achieving polarization independent nonlinear optical enhancement. Depolarization of the resonant fields and higher-order diffraction at the nonlinear signal wavelength need to be considered as well.