Metasurface-based triple-band beam splitter with large spatial separation at visible wavelengths.

The dual-function of a wavelength beam splitter and a power beam splitter is desired in both classical optics and quantum optics. We propose a triple-band large-spatial-separation beam splitter at visible wavelengths using a phase-gradient metasurface in both the x- and y-directions. Under x-polarized normal incidence, the blue light is split in the y-direction into two equal-intensity beams owing to the resonance inside a single meta-atom, the green light is split in the x-direction into another two equal-intensity beams owing to the size variation between adjacent meta-atoms, while the red light passes directly without splitting. The size of the meta-atoms was optimized based on their phase response and transmittance. The simulated working efficiencies under normal incidence are 68.1%, 85.0%, and 81.9% at the wavelengths of 420 nm, 530 nm, and 730 nm, respectively. The sensitivities of the oblique incidence and polarization angle are also discussed.

Beam manipulation for quantum dot light-emitting diode with an Ag grating and a phase-gradient metasurface.

The quantum dot (QD) light-emitting diode (LED) is a robust scheme for single photon source. However, the spontaneous emission of a QD LED has arbitrary directions and polarizations, which is disadvantage for photon collection and manipulation. We propose a QD LED integrated with an Ag grating and a phase-gradient metasurface. The circular patterned Ag grating is adopted to collimate the emission beam with right phase and improve its spatial coherence, therefore a phase-gradient metasurface can work for beam manipulation. The 10°, 20°, and 30° angle deflection as well as doughnut-pattern generation are demonstrated by numerical simulation. A small metasurface with the width of 6 µm can provide a collection efficiency of 25.9% at the deflection angle of 10°. Furthermore, only one single QD can be selected from a QD assembly with a low-density.

Optimization of location, power allocation and orientation for lighting lamps in a visible light communication system using the firefly algorithm.

To optimize the uniformity of signal-to-noise ratio (SNR) distribution in a visible light communication (VLC) system, the firefly algorithm is improved for joint optimization of location, power allocation and orientation of a light-emitting diode (LED) lamp array. Taking 16 LED lamps as an example, optimizations with a different number of degrees-of-freedom (DOF) are investigated. The orientation-involved optimizations significantly decrease average SNR and average illuminance. However, if the average illuminance is restricted to a large value, the effects of the orientation DOF would be small. With the restriction of illuminance, the optimization with all the three DOFs gives an improvement of 4.18 times in SNR uniformity, compared to the typical square-circle layout. The optimizations are further studied by varying the number of LED lamps.

Low-loss two-dimensional silicon photonic grating coupler with a backside metal mirror.

We design and fabricate a low-loss silicon photonic two-dimensional grating coupler that serves to couple light between standard single-mode fibers and single-mode waveguides in the silicon-on-insulator platform and to split both orthogonal polarization states. The efficiency of the fabricated device is enhanced by a backside metal mirror and reaches a record value of -1.8 dB with a 1 dB bandwidth of 32 nm around 1550 nm. The demonstrated coupling efficiency is 2.2 dB better than the conventional two-dimensional grating coupler without a metal mirror.

Microscale hemisphere patterned backside mirror for GaN-based light-emitting diodes.

Three-dimensional backside mirrors patterned by microscale hemispheres were proposed to enhance the light extraction efficiency (LEE) of light-emitting diode (LED) chips. In a thorough investigation, various design parameters, including duty ratio, pattern materials, concave/convex pattern style, aspect ratio, types of mirrors, and chip configurations, were studied using the Monte Carlo ray tracing method. The square lattice of hemispheres with various radius and lattice constants was designed. It is shown that the close-packed hemisphere pattern is preferred. Several materials, including SiO2, polystyrene, sapphire, ZnO, GaN, and TiO2, were selected for the pattern layer. Generally, the optimized pattern material is the substrate material. Comparing the LEE induced by a concave pattern and a convex pattern, it is shown that the former benefits a higher LEE. Various aspect ratios of the spherical caps were also discussed. As the height of the cap increases, the LEE increases or decreases for a concave pattern or a convex pattern, respectively. Considering various mirrors, it is shown that the LEE increases with the increasing of the integral reflectivity of backside mirrors. Finally, the optimized structure parameters for backside mirrors were used in different chip configurations. In the case that of all lights extracted from LED chips were calculated, the LEE can be enhanced by 52%, 135%, and 173% for conventional, flip-chip, and thin-film LED chips, respectively. If only the lights that escaped from the top surface were calculated, the LEE can be enhanced by 65%, 209%, and 263% for conventional, flip-chip, and thin-film LED chips, respectively.

Light-output enhancement of GaN-based light-emitting diodes with three-dimensional backside reflectors patterned by microscale cone array.

Three-dimensional (3D) backside reflector, compared with flat reflectors, can improve the probability of finding the escape cone for reflecting lights and thus enhance the light-extraction efficiency (LEE) for GaN-based light-emitting diode (LED) chips. A triangle-lattice of microscale SiO2 cone array followed by a 16-pair Ti3O5/SiO2 distributed Bragg reflector (16-DBR) was proposed to be attached on the backside of sapphire substrate, and the light-output enhancement was demonstrated by numerical simulation and experiments. The LED chips with flat reflectors or 3D reflectors were simulated using Monte Carlo ray tracing method. It is shown that the LEE increases as the reflectivity of backside reflector increases, and the light-output can be significantly improved by 3D reflectors compared to flat counterparts. It can also be observed that the LEE decreases as the refractive index of the cone material increases. The 3D 16-DBR patterned by microscale SiO2 cone array benefits large enhancement of LEE. This microscale pattern was prepared by standard photolithography and wet-etching technique. Measurement results show that the 3D 16-DBR can provide 12.1% enhancement of wall-plug efficiency, which is consistent with the simulated value of 11.73% for the enhancement of LEE.

Design of temperature-independent arrayed waveguide gratings based on the combination of multiple types of waveguide.

We develop a design theory for a temperature-independent arrayed waveguide grating (TI-AWG) based on the combination of multiple types of waveguide. Each type of waveguide has a path-length difference between adjacent arrayed waveguides, and the path-length difference ratio is introduced as tuning parameter. A TI-AWG with Si wire and slot waveguides is given as an example. The thermal spectra shift of the TI-AWG can be tuned from redshift to blueshift in an ultralarge range, and the modified interference order can be reduced or enhanced. The device size is about one-fifth that of the narrow-wide-wire design that uses a combination of narrow and wide Si wire waveguides. The results are verified by the simulation of prototype devices via a two-dimensional finite-difference time-domain program.