High-index Dielectric Resonators Research Articles

Low threshold lasing from silicon Mie resonators

The unique electromagnetic response of high-index dielectric resonators with Mie resonances facilitates the selective enhancement and confinement of light, holding great promise for nanoscale lasing. However, up to now, nanolasing exploiting Mie resonances of individual Si nanowires has been limited to theoretical proposals. Here, we demonstrate a strategy to realize nanolasing with Si nanowires in the SOI platform. It is found that, by carefully tailoring the sizes of the Si nanowires, the Mie resonances of Si nanowires enhance the pumping/emission fields, leading to continuous-wave lasing at room temperature with low threshold in the visible range. The lasing of a dye at the wavelength of 523 nm assisted by the Mie resonances of Si nanowire is demonstrated experimentally. It is shown that the lasing wavelength changes with the width of nanowire due to the different scattering peak from Mie theory. In addition, the quality factor Q of lasing modes can be affected by the thickness of silica shell which determines the interaction volume of the pumping light and dyes. The proposed strategy paves the way towards practical integrated nanolasers for single-photon sources and micro-display, which have potential applications of on-chip optoelectrical devices.

Enhanced tunneling distance of near field radiative energy with high-index dielectric resonators

By placing high-index dielectric resonators on surfaces supporting surface plasmons in the near field, strong magnetic resonance can be observed in the high-index dielectric resonators with appropriate heights around the surface plasmon resonance frequencies. The strong magnetic resonance allows strong thermal photon tunneling across a 1 μm gap, which is one order longer than the previous demonstrations of near field radiation with surface plasmons. The thermal photon tunneling happens when the horizontal wavenumber is kx∼4πw with w is the width of the high-index resonators. The height of the high-index dielectric resonators should provide enough retardation of the electric field between the top and bottom of the resonator to form a displacement current loop. Therefore, similar magnetic field resonance occurs in the resonator when we triple rather than double the height of the high-index dielectric resonators. The usage of dielectric resonators to amplify the thermal electric field in the near field domain can be a potential method to increase the quasi-monochromatic radiation distance of an emission domain by one order or more at the frequencies of the surface waves.

Subwavelength dielectric resonators for nonlinear nanophotonics.

Subwavelength optical resonators made of high-index dielectric materials provide efficient ways to manipulate light at the nanoscale through mode interferences and enhancement of both electric and magnetic fields. Such Mie-resonant dielectric structures have low absorption, and their functionalities are limited predominantly by radiative losses. We implement a new physical mechanism for suppressing radiative losses of individual nanoscale resonators to engineer special modes with high quality factors: optical bound states in the continuum (BICs). We demonstrate that an individual subwavelength dielectric resonator hosting a BIC mode can boost nonlinear effects increasing second-harmonic generation efficiency. Our work suggests a route to use subwavelength high-index dielectric resonators for a strong enhancement of light-matter interactions with applications to nonlinear optics, nanoscale lasers, quantum photonics, and sensors.

Silicon-based light absorbers with unique polarization-adjusting effects

We propose and demonstrate a silicon-based absorber platform, which can show three different kinds of polarization-adjusting behaviors under certain structural features. First, perfectly polarization-dependant absorption is observed when the rings are packed with a relatively large gap distance. The classical Malus law is reproduced by the absorption spectra via this asymmetrical system. Then, the polarization-adjusting response can be strongly weakened when the dimer rings are physically contacted with each other, suggesting a polarization-independent absorption. Third, partially matched polarization-manipulation absorption can be further achieved when the absorber is formed by closely packed rings dimer. These findings show remarkable different behaviors to that of the conventional metallic plasmons, introducing impressive insight on the polarization manipulation behaviors of the high-index dielectric resonators, metamaterials and metasurface.

Bound states in the continuum and Fano resonances in the strong mode coupling regime

The study of resonant dielectric nanostructures with high refractive index is a new research direction in nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique optically-induced electric and magnetic Mie resonances, high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications. Here we study strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator's aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference. We explain the physics of these states in terms of multipole decomposition and show that their appearance is accompanied by drastic change of the far-field radiation pattern. We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of a high-index cylindrical resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high quality factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices.

High-Q states and Strong mode coupling in high-index dielectric resonators.

We study strong coupling between eigenmodes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances by varying resonator’s aspect ratio. We demonstrate that the strong mode coupling is associated with the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference. We confirm our theoretical findings with microwave experiments by using a high-index cylindrical resonator with tunable aspect ratio.

High-Q Supercavity Modes in Subwavelength Dielectric Resonators.

Recent progress in nanoscale optical physics is associated with the development of a new branch of nanophotonics exploring strong Mie resonances in dielectric nanoparticles with a high refractive index. The high-index resonant dielectric nanostructures form building blocks for novel photonic metadevices with low losses and advanced functionalities. However, unlike extensively studied cavities in photonic crystals, such dielectric resonators demonstrate low quality factors (Q factors). Here, we uncover a novel mechanism for achieving giant Q factors of subwavelength nanoscale resonators by realizing the regime of bound states in the continuum. In contrast to the previously suggested multilayer structures with zero permittivity, we reveal strong mode coupling and Fano resonances in homogeneous high-index dielectric finite-length nanorods resulting in high-Q factors at the nanoscale. Thus, high-index dielectric resonators represent the simplest example of nanophotonic supercavities, expanding substantially the range of applications of all-dielectric resonant nanophotonics and meta-optics.

Nonlinear metasurfaces: a paradigm shift in nonlinear optics

Frequency conversion processes, such as second- and third-harmonic generation, are commonly realized in nonlinear optics, offering opportunities for applications in photonics, chemistry, material science and biosensing. Given the inherently weak nonlinear response of natural materials, optically large samples and complex phase-matching techniques are typically required to realize significant nonlinear responses. To produce similar effects in much smaller volumes, current research has been devoted to the quest of synthesizing novel materials with enhanced optical nonlinearities at moderate input intensities. In particular, several approaches to engineer the nonlinear properties of artificial materials, metamaterials and metasurfaces have been introduced. Here, we review the current state of the art in the field of small-scale nonlinear optics, with special emphasis on high-harmonic generation from ultrathin metasurfaces based on plasmonic and high-index dielectric resonators, as well as semiconductor-loaded plasmonic metasurfaces. In this context, we also discuss recent advances in controlling the optical wavefront of generated nonlinear waves using metasurfaces. Finally, we compare viable approaches to enhance nonlinearities in ultrathin metasurfaces, and we offer an outlook on the future development of this exciting field of research.

Multi-Band Ultra-Sharp Transmission Response in All-Dielectric Resonant Structures Containing Kerr Nonlinear Media

All-dielectric resonant structure (ADRS) consisting of high-index nonlinear dielectrics has been theoretically and numerically demonstrated with multi-band ultra-sharp transmission response in this work. Bandwidth down to sub-nanometer and spectral Q-factor up to 920 are achieved in this ADRS-based metamaterial-like platform. Strong resonant electric field distributions by the high-index dielectric resonators and efficient coupling between the layered dielectric particles and the cavity mainly contribute to the multiple narrowband light transmission filtering. By using a Kerr nonlinear medium as the resonant dielectric, the positions of the transmission dips in the spectrum can be actively tuned by the incident light intensity. Due to the ultra-narrow spectral feature and the strong electric field distribution by the resonators, an efficient all-optical switching behavior with high spectral difference intensity and contrast ratio is obtained. Further study presents the observed multi-band transmission with high scalability by tuning the structural parameters. These optical features hold the predicted ADRS be potentially applied to constructing dielectric metamaterial-based all-optical switching or active subtractive transmission filtering with low power threshold at sub-diffraction scale.

Multi-band light perfect absorption by a metal layer-coupled dielectric metamaterial.

Multispectral light perfect absorption is desired for many applications. Herein, we propose and demonstrate a novel multi-band light perfect absorber (MLPA) scheme based on a triple-layer dielectric meta-material structure coupled with a metal substrate. Four absorption bands with the maximal absorbance up to 98.9% and the narrow bandwidth down to 2 nm are achieved in the visible range. Optical cavity resonances and the plasmon-like dipolar resonance of the high-index dielectric resonators and their hybridization effects contribute to the observed absorption behaviors. Moreover, the obtained MLPA is with high scalability in the frequency range by tuning the structural parameters. These features pave a new and feasible way for multispectral light absorption and hold applications in the optoelectronic detection, filtering and imaging.

Reconfigurable Semiconductor Phased-Array Metasurfaces

Phased-array metamaterial systems are enabling new classes of refractive and diffractive optical elements through spatial-phase engineering. In this article, we develop design principles for reconfigurable optical antennas and metasurfaces. We theoretically demonstrate the tunability of infrared scattering phase and radiation patterns in low-loss, high-index dielectric resonators using free carrier refraction. We demonstrate reconfigurable endfire antennas based on interference between multiple elements. Within single resonators, we demonstrate reconfigurable broadside antenna radiation lobes arising from interfering electric and magnetic dipole resonances. Extending this concept to infinite arrays, we design ideal Huygens metasurfaces with spectrally overlapping electric and magnetic dipole resonances. By introducing free charge carriers into these metasurfaces, we demonstrate continuously tunable transmission phase between 0 and 2π with less than 3 dB loss in intensity. Such tunable metasurfaces may for...

Designing dielectric resonators on substrates: Combining magnetic and electric resonances

High-performance integrated optics, solar cells, and sensors require nanoscale optical components at the surface of the device, in order to manipulate, redirect and concentrate light. High-index dielectric resonators provide the possibility to do this efficiently with low absorption losses. The resonances supported by dielectric resonators are both magnetic and electric in nature. Combined scattering from these two can be used for directional scattering. Most applications require strong coupling between the particles and the substrate in order to enhance the absorption in the substrate. However, the coupling with the substrate strongly influences the resonant behavior of the particles. Here, we systematically study the influence of particle geometry and dielectric environment on the resonant behavior of dielectric resonators in the visible to near-IR spectral range. We show the key role of retardation in the excitation of the magnetic dipole (MD) mode, as well as the limit where no MD mode is supported. Furthermore, we study the influence of particle diameter, shape and substrate index on the spectral position, width and overlap of the electric dipole (ED) and MD modes. Also, we show that the ED and MD mode can selectively be enhanced or suppressed using multi-layer substrates. And, by comparing dipole excitation and plane wave excitation, we study the influence of driving field on the scattering properties. Finally, we show that the directional radiation profiles of the ED and MD modes in resonators on a substrate are similar to those of point-dipoles close to a substrate. Altogether, this work is a guideline how to tune magnetic and electric resonances for specific applications.