Unified theory of whispering gallery multilayer microspheres with single dipole or active layer sources

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.

Hollow aluminum microspheres with high mass extinction coefficients in the long wave infrared

Previous electromagnetic computations of multilayered dielectric/metallic spheres identified the ideal dimensions and composition for achieving optimized mass extinction coefficients (m²/g). A hollow metallic sphere, with a thin metallic shell, is one such example of a spherical structure that can theoretically achieve high mass extinction coefficients in the long wave infrared (LWIR) region (8-12 µm). To this end, we endeavored to demonstrate a cost-effective and scalable manufacturing approach for synthesizing and experimentally validating the mass extinction coefficients of hollow metallic spheres. Specifically, we detail a novel approach for fabricating hollow aluminum spheres using radio frequency (RF) magnetron sputter deposition. Sacrificial high-density polyethylene polymer microspheres were used as substrates for the deposition of thin layers of aluminum. The core shell structures were subsequently thermally processed to form the hollow micron sized aluminum shells. The mass extinction coefficients of the hollow aluminum spheres were subsequently measured and compared to computational results. A strong agreement between experimental and theoretical predictions was observed. Finally, the LWIR mass extinction coefficients of the hollow spheres were compared to high aspect ratio brass flakes, a common pigment used for LWIR attenuation, and other materials and geometries that are used for LWIR filtering applications. This comparison of both performance and availability revealed that the fabricated hollow aluminum spheres exhibited competitive LWIR properties using a more scalable and cost-effective manufacturing approach.

Radiated and guided optical waves of a magnetic dipole-nanofiber system

Nanophotonics-photonic structures with subwavelength features-allow accessing high intensity and localized electromagnetic field and hence is an ideal platform for investigating and exploiting strong lightmatter interaction. In particular, such a strong light-matter interaction requires investigating the interaction of a magnetic dipole with the electromagnetic field- a less-explored topic, which has usually been ignored within the framework of electric dipole approximation. Motivated by recent advances in the emerging field of multipolar nanophotonics, here we develop an analytical model that provides a new insight into analyzing a magnetic dipole and a nanofiber. This method enables us to examine the effect of second term in the multipolar expansion of light-matter interaction, magnetic dipole approximation, with individual guided and radiation modes of the nanofiber. This is a critical key in developing nanophotonic integrated devices based on magnetic nature of light for super-imaging, biosensing, and optical computing.

Theoretical and experimental investigation of broadband dispersion tailoring of high-order mode in the hybrid microsphere cavity

The large normal dispersion of the fundamental mode (TE_n=1 mode) in the whispering gallery modes (WGM) microsphere is detrimental to the visible comb generation. Herein, we demonstrate that this fundamental limitation can be removed by considering the high-order radial modes (TE_n=2 mode) of the hybrid microsphere cavity (HMC). The studied HMC consists of a high-refractive-index coating (TiO₂ or HfO₂) and silica microsphere. The simulated electric field energy distribution and measured Q value in our experiment show that optical confinement of the coating effectively excites the TE_n=2 mode and reduces the free spectral range (FSR) and modal dispersion. In addition, the observed redshift of WGM and decreased trend of FSR are in accordance with simulations. The zero-dispersion wavelength can be linearly shifted to a shorter wavelength or even into the visible region with the reduction of coating thickness or refractive index and larger microcavity, which advances the visible comb generation.

Dispersion engineering of a microsphere via multi-layer coating

Controlling dispersion of a whispering gallery mode resonator is of critical importance for many nonlinear applications, such as frequency comb generation, parametric oscillators, Raman lasers, stimulated Brillouin lasers, and ultrafast optics. Here, we show by numerical and theoretical modeling that dispersion can be strongly engineered in a three-layer-coated microsphere of high, low, and high refractive indices (RIs). We investigate the impact of the coating thickness, the gap between the two high-RI layers, the surrounding medium, and the coating materials on the group-velocity dispersion and discover that the dispersion is controllable over a broad range in both normal and anomalous dispersion regimes. Our approach provides dispersion engineering flexibility in any axisymmetric resonator with a three-layer-coating structure.