Photonic integrated circuit with multiple waveguide layers for broadband high-efficient 3D OPA.

The traditional photonic integrated circuit (PIC) inherits the mature CMOS fabrication process from the electronic integrated circuit (IC) industry. However, this process also limits the PIC structure to a single-waveguide-layer configuration. In this work, we explore the possibility of the multi-waveguide-layer PIC by proposing and demonstrating a 3D optical phased array (OPA) device, with the light exiting from the edge of the device, based on multi-layer Si3N4/SiO2 stacks. This device is in a multi-waveguide-layer configuration at every single position of the device. This configuration offers the possibility of using edge couplers at both the input and the emitting ends to achieve broadband high efficiency, and its uniqueness provides the potential for a more extended detection range in the lidar application. The device has been studied by numerical simulation, and proof-of-concept samples have been fabricated and tested.

Phase-combining unit for aliasing suppression in an optical phased array.

Integrated optical phased array (OPA) devices have been widely studied as a solution for solid-state light detection and ranging technology in the autonomous driving application. In this work, a phase-combining unit (PCU) is proposed and studied. With a given number (N) of phase shifters, instead of the general N (phase shifters) to N (emitters) control, the PCU can enable an N to 2N-1 control, which efficiently suppresses the aliasing effect. The theoretical analysis, numerical simulation, and experimental proof-of-concept have been completed in this work. The results show that a maximum suppression of 92.54% can be achieved for the grating lobes in simulation, and an average 53.76% is tested for one grating lobe in the experiment. In conclusion, the PCU can be used as a universal aliasing suppression unit on many types of integrated OPA devices.

Optimization of the silicon-based aperiodic optical phased array antenna.

Beam engineering is one of the most important functionalities in light detection and ranging (LiDAR). In this work, a silicon optical phased array (OPA) is employed to control the beam profile. Machine-learning-based genetic algorithm optimization is utilized to suppress the sidelobes of the far field pattern assuming the random distribution of aperiodic arrays. The optimized mainlobe position versus wavelength relationship in two-dimensional aperiodic arrays is distinctly different from prior works. Analysis was performed to show the effect of fabrication error of arrays on the side mode suppression ratio. Our study provides an effective pathway to optimize the random distributed OPAs within a controllable time frame among the vast number of parameters.

Optimization of aperiodic 3D optical phased arrays based on multilayer Si3N4/SiO2 platforms.

Silicon-based optical phased arrays (OPAs) have been widely explored, while the design of the structure with high sidelobe level reduction, remains a big challenge. This work investigated the optimization of the optical path-modulated 3D OPAs with Si3N4 as the core layer and SiO2 as the cladding layer. We used the particle swarm optimization algorithm to optimize high-performance random distributed OPAs. Our study provides an effective pathway to optimize the random distributed OPAs within a controllable time frame among a vast number of parameters.

High-efficiency end-fire 3D optical phased array based on a multi-layer Si3N4/SiO2 platform.

Beam-steering devices such as optical phased arrays (OPAs) are key components in the applications of solid-state Lidar and wireless communication. The traditional single-layer OPA results in a significant energy loss due to substrate leakage caused by the downward coupling from the grating coupler structure. In this work, we have investigated a structure based on a multi-layer ${\rm Si}_{3}{\rm N}_{4}/{\rm SiO}_{2}$Si3N4/SiO2 platform that can form a 3D OPA to emit light from the edge of the device with high efficiency; a 2D converged out-coupling beam will be end-fired to the air. High efficiency and wide horizontal beam steering are demonstrated numerically, and the influence of vertical crosstalk, delay length, and number of waveguide layers are discussed, as well as the fabrication feasibility.

Compound period grating coupler for double beam generation and steering.

Grating couplers are one of the most basic integrated photonic structures. They have raised tremendous research interest due to their outstanding performance in compact nonmechanical beam steering. Here we propose a new compound period grating coupler formed by combining two grating structures with different periodicities. The new compound period grating coupler structure can couple the waveguide mode into two radiation modes with different angles. Therefore, the beam steering range is doubled due to the extra beam. We numerically demonstrate this idea, and a 26.20° steering range is observed within a wavelength tuning range of 1500 to 1600 nm. The compound period grating structure with a distributed Bragg reflector as the substrate is also numerically demonstrated, and its energy leakage to the substrate is highly suppressed. In addition, the investigation of fabrication tolerance shows that the new structure can be fabricated with the current CMOS technology.

Orthogonal and parallel lattice plasmon resonance in core-shell SiO(2)/Au nanocylinder arrays.

Height induced coupling behavior between the plasmonic modes and diffraction orders were studied in the core-shell SiO(2)/Au nanocylinder arrays (NCAs) using finite difference time domain (FDTD) simulations. New lattice plasmon modes (LPMs) are observed in the structures with high aspect ratio. Specifically, parallel coupling between the plasmonic modes and diffraction orders is obtained here, which shows different coupling behavior from orthogonal LPMs. Electromagnetic (EM) field distributions indicate that horizontal propagation of the magnetic or electric field component is responsible for the generation of these orthogonal and parallel LPMs, respectively. Radiative loss could be effectively suppressed when the height increases. This is important for the applications of fluorescence enhancement and nano laser. Further studies confirm that the LPMs associated with the superstrate diffraction orders could be well maintained even when the Au coating is imperfect. The interference from the substrate associated LPMs could be eliminated by cutting off the corresponding diffraction waves by inducing a Si(3)N(4) substrate. This study of coupling behavior in the core-shell NCAs enables a novel route to design and optimize the LPMs for applications of bio-sensing and nano laser.

Lattice plasmon resonance in core-shell SiO2/Au nanocylinder arrays.

Core-shell SiO2/Au nanocylinder arrays (NCAs) are studied using finite-difference time-domain simulations. The increase of height induces new surface plasmon resonances along the nanocylinders, i.e., dipole and quadrupole modes. Orthogonal coupling between superstrate diffraction order and the height-induced dipole mode is observed, which could achieve a well-defined lattice plasmon mode even for smaller NCAs in asymmetric environments. Electromagnetic field distribution has been employed to determine the coupling origin. Radiative loss could also be effectively suppressed in these core-shell NCAs, indicating the possibility of future applications in fluorescence enhancement and nanolasers.

Position-independent normal-mode splitting in cavities filled with zero-index metamaterials.

We study the normal-mode splitting when an oscillator is placed in a two-dimensional photonic crystal microcavity embedded with an impedance-matched or an impedance-mismatched zero-index medium (ZIM). Because of the (nearly) uniform localized fields in the ZIM, the normal-mode splitting remains (almost) invariant no matter where the oscillator is. When a split ring resonator is coupled to a transmission-line- based effective ZIM at various locations, nearly position-independent mode splitting is observed.

Giant light extraction enhancement of medical imaging scintillation materials using biologically inspired integrated nanostructures.

We have utilized biologically inspired (bio-inspired), moth-eye nanostructures and further improved this biomimetic structure to enhance the scintillator materials external quantum efficiency significantly. As a proof of concept, we have demonstrated very high light output efficiency enhancement for Lu(2)SiO(5):Ce(3+) (LSO:Ce) film in large area, the X-ray mammographic instrument was employed to demonstrate the light output enhancement of the Lu(2)SiO(5):Ce thin film with biologically inspired (bio-inspired) moth-eye-like nanophotonic structures. Our work could be extended to other thin film scintillator materials and is promising to achieve lower patient dose, higher resolution images of human organs and even smaller scale medical imaging.

Defect-induced whispering-gallery-mode resonances in optical microdisk resonators.

In this Letter we present results of theoretical and experimental studies of whispering-gallery modes in optical microdisk resonators interacting with subwavelength dielectric particles. We predict theoretically and confirm by direct observations that, contrary to the generally accepted models, both peaks of the particle-induced doublet of resonances are redshifted with respect to the position of the initial resonance.

Light Trapping in Single Elliptical Silicon Nanowires.

Light trapping in single nanowires (NWs) is of vital importance for photovoltaic applications. However, circular NWs (CNWs) can limit their light-trapping ability due to high geometrical symmetry. In this work, we present a detailed study of light trapping in single silicon NWs with an elliptical cross-section (ENWs). We demonstrate that the ENWs exhibit significantly enhanced light trapping compared with the CNWs, which can be ascribed to the symmetry-broken structure that can orthogonalize the direction of light illumination and the leaky mode resonances (LMRs). That is, the elliptical cross-section can simultaneously increase the light path length by increasing the vertical axis and reshape the LMR modes by decreasing the horizontal axis. We found that the light absorption can be engineered via tuning the horizontal and vertical axes, the photocurrent is significantly enhanced by 374.0% (150.3%, 74.1%) or 146.1% (61.0%, 35.3%) in comparison with that of the CNWs with the same diameter as the horizontal axis of 100 (200, 400) nm or the vertical axis of 1000 nm, respectively. This work advances our understanding of how to improve light trapping based on the symmetry breaking from the CNWs to ENWs and provides a rational way for designing high-efficiency single NW photovoltaic devices.

Off-Resonant Absorption Enhancement in Single Nanowires via Graded Dual-Shell Design.

Single nanowires (NWs) are of great importance for optoelectronic applications, especially solar cells serving as powering nanoscale devices. However, weak off-resonant absorption can limit its light-harvesting capability. Here, we propose a single NW coated with the graded-index dual shells (DSNW). We demonstrate that, with appropriate thickness and refractive index of the inner shell, the DSNW exhibits significantly enhanced light trapping compared with the bare NW (BNW) and the NW only coated with the outer shell (OSNW) and the inner shell (ISNW), which can be attributed to the optimal off-resonant absorption mode profiles due to the improved coupling between the reemitted light of the transition modes of the leak mode resonances of the Si core and the nanofocusing light from the dual shells with the graded refractive index. We found that the light absorption can be engineered via tuning the thickness and the refractive index of the inner shell, the photocurrent density is significantly enhanced by 134% (56%, 12%) in comparison with that of the BNW (OSNW, ISNW). This work advances our understanding of how to improve off-resonant absorption by applying graded dual-shell design and provides a new choice for designing high-efficiency single NW photovoltaic devices.

Controllable Biosynthesis and Properties of Gold Nanoplates Using Yeast Extract.

ABSTRACT: Biosynthesis of gold nanostructures has drawn increasing concerns because of its green and sustainable synthetic process. However, biosynthesis of gold nanoplates is still a challenge because of the expensive source and difficulties of controllable formation of morphology and size. Herein, one-pot biosynthesis of gold nanoplates is proposed, in which cheap yeast was extracted as a green precursor. The morphologies and sizes of the gold nanostructures can be controlled via varying the pH value of the biomedium. In acid condition, gold nanoplates with side length from 1300 ± 200 to 300 ± 100 nm and height from 18 to 15 nm were obtained by increasing the pH value. Whereas, in neutral or basic condition, only gold nanoflowers and nanoparticles were obtained. It was determined that organic molecules, such as succinic acid, lactic acid, malic acid, and glutathione, which are generated in metabolism process, played important role in the reduction of gold ions. Besides, it was found that the gold nanoplates exhibited plasmonic property with prominent dipole infrared resonance in near-infrared region, indicating their potential in surface plasmon-enhanced applications, such as bioimaging and photothermal therapy.

On-chip Si-based Bragg cladding waveguide with high index contrast bilayers.

A new silicon based waveguide with full CMOS compatibility is developed to fabricate an on-chip Bragg cladding waveguide that has an oxide core surrounded by a high index contrast cladding layers. The cladding consists of several dielectric bilayers, where each bilayer consists of a high index-contrast pair of layers of Si and Si3N4. This new waveguide guides light based on omnidirectional reflection, reflecting light at any angle or polarization back into the core. Its fabrication is fully compatible with current microelectronics processes. In principle, a core of any low-index material can be realized with our novel structure, including air. Potential applications include tight turning radii, high power transmission, and dispersion compensation.

Metallic nanoparticle on micro ring resonator for bio optical detection and sensing.

We have numerically investigated the unique effects of metallic nanoparticle on the ring resonator, especially multiple Au nanoparticles on the micro ring resonator with the 4-port configuration on chip. For the Au nanoparticle, because it has smaller real refractive index than air and large absorption refractive index, we found that there is a blue shift for the ring resonance wavelength, instead of red shift normally observed for dielectric nanoparticles. The drop port intensity is strongly dependent on both number and size of nanoparticles, while relatively independent on position of nanoparticles. The correlation between the penetration depth of Au and the resonance mode evanescent tail is also discussed to reveal the unique properties of Au nanoparticle to be used for detection, sensing and nano medicine.