Shiqiang Li

A simple expansion method for understanding toroidal multipoles and anapoles in light-matter interactions

Toroidal multipoles are a topic of increasing interest in the nanophotonics and metamaterials communities. In this work, we separate out the toroidal multipole components of multipole expansions in polar coordinates (two- and three-dimensional) by expanding the Bessel or spherical Bessel functions. We discuss the formation of magnetic anapoles from the interaction between the magnetic toroidal dipole and the magnetic dipole. Our method also reveals that there are higher order current configurations other than the toroidal electric multipole that have the same radiation characteristics as the pure electric dipole.

Generalized Method of Images and Reflective Color Generation from Ultrathin Multipole Resonators

The multipole expansion has found limited applicability for optical dielectric resonators in inhomogeneous environment, such as on the surface of substrates. Here, we generalize the method of images to multipole analysis for light scattering by dielectric nanoparticles on conductive substrates. We present examples illustrating the physical insight provided by our method, including selection rules governing the excitation of the multipoles. We propose and experimentally demonstrate a new mechanism to generate high resolution surface color. The dielectric resonators employed are very thin (less than 50 nm), i.e., similar in thickness to the plasmonic resonators that are currently being investigated for structural color. The generalized method of images opens up new prospects for design and analysis of metasurfaces and optical dielectric resonators.

Wavelength-multiplexed spectrometer based on silicon nanowire photodetector array

It was recently demonstrated that by adding a filter array to photodetector array, a very compact wavelength-multiplexed spectrometer can be achieved [1, 2]. These previous efforts have had both strengths and weaknesses, however. In [1], the filter array comprised etalons with different thicknesses fabricated by a multi-step process. In [2], the filter array comprised quantum dots spotted onto the device with a pipette. Here, we study a spectrometer comprising an array of silicon nanowire (Si NW) photodetectors, formed above an array of planar photodetectors. This device would have manufacturing advantages, being based on Si technology and with all NWs defined in a single lithography step. The NWs would act as photodetectors (converting absorbed light to photocurrent) but would also filter the light transmitted onto the planar photodetectors. After measurement of the photodetector currents, the spectrum of light impinging on the device would be reconstructed numerically.

Vividly colored silicon metasurface, with applications in sensing and lighting

Silicon has many favorable attributes from both the device physics and manufacturing standpoints, and is therefore the pre-eminent material for micro- and nanoelectronics. Furthermore, due to its transparency at infrared wavelengths, silicon-based integrated photonics play a key role in modern optical communications devices. Silicon-based nano-optical antennas have also attracted much interest recently, due to the high refractive index and moderate losses available with silicon. Metamaterials and metasurfaces based on silicon and other dielectric materials have also gained much attention, due to the fact that they offer much lower levels of loss, compared to metals. In this work, we experimentally demonstrate a silicon metasurface in the visible frequency range, with several potential applications. (1) We demonstrate the magnetic mirror behavior of a silicon metasurface with high efficiency in the visible spectral range through direct phase measurement. The phase discontinuity between the reflected and incident light is wavelength dependent and can be engineered by appropriate design. The metasurface also reveals vivid colors across the entire red-green-blue range. (2)Using our silicon metasurface, we demonstrate a colorimetric refractometer from which a liquids refractive index can be directly monitored with a color camera. (3) We experimentally demonstrate highly directional fluorescent emission using our silicon metasurface at visible wavelengths. Our device offers not only high efficiency, but also a high degree of tolerance in terms of the position, orientation and wavelength of the dipole emitter. We perform a multipole expansion of the fields from each element of the metasurface (a silicon nanorod). This reveals that the metasurface largely behaves as if it comprises an array of electric and magnetic dipoles.

Silicon Metasurfaces as Magnetic Mirrors at Visible Wavelengths

We experimentally demonstrate a magnetic mirror comprising a silicon nanorod array. This all-dielectric metasurface mirror exhibits relatively low loss, vivid color and a wavelength dependent phase discontinuity. Physical interpretation via multipole expansion analysis is provided.

Isolated and collective magnetic resonances in dielectric nanoparticles at optical frequencies

We employ multipole expansion to analyze radiation modification and light scattering by isolated nanoparticles and by nanoparticle ensembles, unveiling directional emission mediated by magnetic resonance and two types of purely magnetic resonances at optical frequencies.

Vividly-colored silicon metasurface based on collective electric and magnetic resonances

We fabricate a silicon nanorod-based metasurface that shows vivid colors. Each nanorod supports electric and magnetic dipole modes whose coupling leads to collective resonances. The reflected field is described by a classical coupled dipole model.