Jeetendra Gour

Monolayers of transition metal dichacogenides (1L‐TMDs) show strong second‐order nonlinearity and symmetry‐driven selection rules from their threefold lattice symmetry. This process resembles the valley‐contrasting selection rules for photoluminescence (PL) in these materials. However, the underlying physical mechanisms fundamentally differ since second harmonic generation (SHG) is a coherent process, whereas PL is incoherent, leading to distinct interactions with photonic nanoresonators. In this study, the far‐field circular polarization properties of SHG from MoS ₂ monolayers resonantly interacting with spherical gold nanoparticles were investigated. The results indicate that the coherence of the second harmonic allows its polarization to be mostly preserved, unlike in an incoherent process, where the polarization is scrambled. These findings provide important insights for future applications in valleytronics and quantum nanooptics, where both coherent and incoherent processes can be probed in such hybrid systems without altering sample geometry or operational wavelength.

We investigate second harmonic generation from vertically stacked metal–insulator–metal nanobars arranged in a two-dimensional rectangular array. At specific angles of incidence, the nonlinear emission is maximized due to the near-field enhancement associated with the surface lattice resonance from the periodic arrangement, regardless of the number of insulator layers embedded in each nanoantenna. Intuitively, stacking should increase second harmonic generation with the number of metal layers. In fact, the maximum emission was reached at the highest stacking, but experimental results also indicate a far more complex relation between the individual layers. Due to the small vertical distance between the metallic layers, the fields in each nanogap are strongly coupled, leading to the generation of gap-plasmons and magnetic modes.

Gold gratings were measured by spectroscopic ellipsometry in reflection mode and modeled by the finite element method to investigate the capabilities of optical dimensional metrology for plasmonic diffractive structures. The gratings were prepared by electron beam lithography using parameters determined by finite element simulations for significant variations of the amplitude ratio and phase shift of the polarized reflection coefficients to achieve high sensitivity for both the measurement of the grating dimensions and the sensing capabilities. The sensitivity largely depends on the values in the five-dimensional parameter space including the grating parameters such as the critical dimension, the period, and the thickness of the grating, as well as the measurement parameters comprising the wavelength and the angle of incidence. The best limit of detection values are in the picometer range for the critical dimension and the thickness of the overlayer, and ≈10^-5 for the refractive index.

In the rapidly evolving field of plasmonic metasurfaces, achieving homogeneous, reliable, and reproducible fabrication of sub‐5 nm dielectric nanogaps is a significant challenge. This article presents an advanced fabrication technology that addresses this issue, capable of realizing uniform and reliable vertical nanogap metasurfaces on a whole wafer of 100 mm diameter. By leveraging fast patterning techniques, such as variable‐shaped and character projection electron beam lithography (EBL), along with atomic layer deposition (ALD) for defining a few nanometer gaps with sub‐nanometer precision, we have developed a flexible nanofabrication technology to achieve gaps as narrow as 2 nm in plasmonic nanoantennas. The quality of our structures is experimentally demonstrated by the observation of resonant localized and collective modes corresponding to the lattice, with Q‐factors reaching up to 165. Our technological process opens up new and exciting opportunities to fabricate macroscopic devices harnessing the strong enhancement of light–matter interaction at the single nanometer scale.

We investigate experimentally third harmonic generation (THG) from plasmonic metasurfaces consisting of two-dimensional rectangular lattices of centrosymmetric gold nanobars. By varying the incidence angle and the lattice period, we show how surface lattice resonances (SLRs) at the involved wavelengths are the major contributors in determining the magnitude of the nonlinear effects. A further boost on THG is observed when we excite together more than one SLR, either at the same or at different frequency. When such multiple resonances take place, interesting phenomena are observed, such as maximum THG enhancement for counter-propagating surface waves along the metasurface, and cascading effect emulating a third-order nonlinearity.

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation. Excitation of the surface lattice resonance either at the fundamental or second harmonic wavelength, achieved by varying the incidence angle, enhance the conversion efficiency. As a special case, we demonstrate that both the wavelengths can be simultaneously in resonance for a specific period of the lattice. In this double resonant case, maximum second harmonic power is achieved.