Zhi-Yuan Li

Metal-Enhanced Near-Infrared Fluorescence by Micropatterned Gold Nanocages

In metal-enhanced fluorescence (MEF), the localized surface plasmon resonances of metallic nanostructures amplify the absorption of excitation light and assist in radiating the consequent fluorescence of nearby molecules to the far-field. This effect is at the base of various technologies that have strong impact on fields such as optics, medical diagnostics, and biotechnology. Among possible emission bands, those in the near-infrared (NIR) are particularly intriguing and widely used in proteomics and genomics due to its noninvasive character for biomolecules, living cells, and tissues, which greatly motivates the development of effective and, eventually, multifunctional NIR-MEF platforms. Here, we demonstrate NIR-MEF substrates based on Au nanocages micropatterned with a tight spatial control. The dependence of the fluorescence enhancement on the distance between the nanocage and the radiating dipoles is investigated experimentally and modeled by taking into account the local electric field enhancement and the modified radiation and absorption rates of the emitting molecules. At a distance around 80 nm, a maximum enhancement up to 2–7 times with respect to the emission from pristine dyes (in the region 660–740 nm) is estimated for films and electrospun nanofibers. Due to their chemical stability, finely tunable plasmon resonances, and large light absorption cross sections, Au nanocages are ideal NIR-MEF agents. When these properties are integrated with the hollow interior and controllable surface porosity, it is feasible to develop a nanoscale system for targeted drug delivery with the diagnostic information encoded in the fluorophore.

An Optically Driven Bistable Janus Rotor with Patterned Metal Coatings

Bistable rotation is realized for a gold-coated Janus colloidal particle in an infrared optical trap. The metal coating on the Janus particles are patterned by sputtering gold on a monolayer of closely packed polystyrene particles. The Janus particle is observed to stably rotate in an optical trap. Both the direction and the rate of rotation can be experimentally controlled. Numerical calculations reveal that the bistable rotation is the result of spontaneous symmetry breaking induced by the uneven curvature of the coating patterns on the Janus sphere. Our results thus provide a simple method to construct large quantities of fully functional rotary motors for nano- or microdevices.

Experimental realization of Bloch oscillations in a parity-time synthetic silicon photonic lattice

As an important electron transportation phenomenon, Bloch oscillations have been extensively studied in condensed matter. Due to the similarity in wave properties between electrons and other quantum particles, Bloch oscillations have been observed in atom lattices, photonic lattices, and so on. One of the many distinct advantages for choosing these systems over the regular electronic systems is the versatility in engineering artificial potentials. Here by utilizing dissipative elements in a CMOS-compatible photonic platform to create a periodic complex potential and by exploiting the emerging concept of parity-time synthetic photonics, we experimentally realize spatial Bloch oscillations in a non-Hermitian photonic system on a chip level. Our demonstration may have significant impact in the field of quantum simulation by following the recent trend of moving complicated table-top quantum optics experiments onto the fully integrated CMOS-compatible silicon platform.

Optics and photonics at nanoscale: Principles and perspectives

Nanophotonics is a multidisciplinary frontier of science that merges nanoscience and nanotechnology with conventional optics and photonics. We focus on two principal issues of nanophotonics: manipulation of optical field and light-matter interaction via various optical nanostructures. These two issues are behind all the efforts to explore, design, and build nanophotonic devices to accomplish the fundamental cause of large-scale optical integration for information processing, interconnection, and computing. We discuss various mechanisms of light-matter interaction enhancement to realize bright fluorescence, Raman, and nonlinear optical radiation, and explore methodologies and various devices for highly sensitive optical sensing and detecting, ultrahigh spatial resolution imaging, and high-efficiency energy conversion between light and electricity, heat, and other forms. All these concepts, insights, methodologies, and technologies in nanophotonics will set a solid platform to explore and achieve better future information and energy technologies that use light as powerful information and energy carriers and as prominent media to probe and manipulate the intrinsic properties of matters via light-matter interaction.

Direct laser writing of pyramidal plasmonic structures with apertures and asymmetric gratings towards efficient subwavelength light focusing

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.

The dynamic process and microscopic mechanism of extraordinary terahertz transmission through perforated superconducting films.

Superconductor is a compelling plasmonic medium at terahertz frequencies owing to its intrinsic low Ohmic loss and good tuning property. However, the microscopic physics of the interaction between terahertz wave and superconducting plasmonic structures is still unknown. In this paper, we conducted experiments of the enhanced terahertz transmission through a series of superconducting NbN subwavelength hole arrays, and employed microscopic hybrid wave model in theoretical analysis of the role of hybrid waves in the enhanced transmission. The theoretical calculation provided a good match of experimental data. In particular, we obtained the following results. When the width of the holes is far below wavelength, the enhanced transmission is mainly caused by localized resonance around individual holes. On the contrary, when the holes are large, hybrid waves scattered by the array of holes dominate the extraordinary transmission. The surface plasmon polaritions are proved to be launched on the surface of superconducting film and the excitation efficiency increases when the temperature approaches critical temperature and the working frequency goes near energy gap frequency. This work will enrich our knowledge on the microscopic physics of extraordinary optical transmission at terahertz frequencies and contribute to developing terahertz plasmonic devices.

Efficient manipulation of graphene absorption by a simple dielectric cylinder

We theoretically study the absorption property of graphene manipulated by a dielectric cylinder through an analytical method. The distinctive absorption properties of incident waves with different polarizations (TM and TE) are analyzed and they are strongly correlated with the structure resonance and material dispersion. Besides, the characteristics of graphene absorption tuned by the cylinder radius and refractive index as well as the chemical potential of graphene are systematically investigated. It is found that enhancement and continuous tunability of graphene absorption can be achieved by utilizing the whispering gallery mode produced in the dielectric cylinder and harnessing the graphene optical conductivity via tuning its chemical potential by exterior electrical grating. The theoretical studies open up a simple while efficient means to manipulate the absorption of graphene in a broad frequency range via the geometric and physical configuration of hybrid graphene-microstructures.

Observation of broadband unidirectional transmission by fusing the one-way edge states of gyromagnetic photonic crystals

We experimentally demonstrate a broadband one-way transmission by merging the operating bands of two types of one-way edge modes that are associated with Bragg scattering and magnetic surface plasmon (MSP) resonance, respectively. By tuning the configuration of gyromagnetic photonic crystals and applied bias magnetic field, the fused bandwidth of unidirectional propagation is up to 2 GHz in microwave frequency range, much larger than either of the individual one-way bandwidth associated with Bragg scattering or MSP resonance. Our scheme for broadband one-way transmission paves the way for the practical applications of one-way transmission.

Multiple shape light sources generated in LiNbO3 nonlinear photonic crystals with Sierpinski fractal superlattices

LiNbO3 nonlinear photonic crystals with two kinds of Sierpinski fractal superlattice structures, in which their domain walls are circle and square cylindrical surfaces for the second order, respectively, are successfully fabricated. Quasi-phase matching and nonlinear Cerenkov harmonics under pulse laser beams are measured. The spot- and line-shaped light sources are realized by quasi-phase matching harmonics. Ten kinds of spot-shaped outputs can be accomplished, in which the quasi-phase matching harmonic conversion efficiency for the fundamental wavelength 1.352 μm reaches 27%. A green line-shaped light source with expansion angle of about 3° is observed, which originates from the coexistence of imperfect collinear and non-collinear quasi-phase matching processes. In addition, the ring-shaped light sources of continuous wavebands from 425 nm to 675 nm are obtained by nonlinear Cerenkov harmonics. The optical properties of quasi-phase matching and Cerenkov harmonics in two kinds of crystals are highly similar to each other.

Optimization and maximum potential of optical antennae in near-field enhancement

We investigate four types of gold nanoantennae (the monopole, the dipole, the cone-shaped, and the cone-bowtie antenna), under a fixed working wavelength. The finite-difference time-domain (FDTD) simulations show that the near-field enhancement values do not increase monotonously when the antennae sizes decrease, and optimization conditions vary with the antenna shapes. We also propose a distributed dipole ring model to analytically calculate the near field. The size condition for the strongest enhancement is the compromising result of the total radiated energy and the near-field distribution factor. Assuming the cone-bowtie antenna is the best for high enhancement, the maximum potential in near-field enhancement is 2×10(5) for a linear signal or 4×10(10) for typical nonlinear signals.