Vin-Cent Su

Broadband achromatic optical metasurface devices

Among various flat optical devices, metasurfaces have presented their great ability in efficient manipulation of light fields and have been proposed for variety of devices with specific functionalities. However, due to the high phase dispersion of their building blocks, metasurfaces significantly suffer from large chromatic aberration. Here we propose a design principle to realize achromatic metasurface devices which successfully eliminate the chromatic aberration over a continuous wavelength region from 1200 to 1680 nm for circularly-polarized incidences in a reflection scheme. For this proof-of-concept, we demonstrate broadband achromatic metalenses (with the efficiency on the order of ∼12%) which are capable of focusing light with arbitrary wavelength at the same focal plane. A broadband achromatic gradient metasurface is also implemented, which is able to deflect wide-band light by the same angle. Through this approach, various flat achromatic devices that were previously impossible can be realized, which will allow innovation in full-color detection and imaging.Metasurfaces suffer from large chromatic aberration due to the high phase dispersion of their building blocks, limiting their applications. Here, Wang et al. design achromatic metasurface devices which eliminate the chromatic aberration over a continuous region from 1200 to 1680 nm in a reflection schleme.

A broadband achromatic metalens in the visible

Metalenses consist of an array of optical nanoantennas on a surface capable of manipulating the properties of an incoming light wavefront. Various flat optical components, such as polarizers, optical imaging encoders, tunable phase modulators and a retroreflector, have been demonstrated using a metalens design. An open issue, especially problematic for colour imaging and display applications, is the correction of chromatic aberration, an intrinsic effect originating from the specific resonance and limited working bandwidth of each nanoantenna. As a result, no metalens has demonstrated full-colour imaging in the visible wavelength. Here, we show a design and fabrication that consists of GaN-based integrated-resonant unit elements to achieve an achromatic metalens operating in the entire visible region in transmission mode. The focal length of our metalenses remains unchanged as the incident wavelength is varied from 400 to 660 nm, demonstrating complete elimination of chromatic aberration at about 49% bandwidth of the central working wavelength. The average efficiency of a metalens with a numerical aperture of 0.106 is about 40% over the whole visible spectrum. We also show some examples of full-colour imaging based on this design.Integrating the Pancharatnam–Berry phase with integrated resonant nanoantennas in a metalens design produces an achromatic device capable of full-colour imaging in the visible range in transmission mode.

Plasmonic multilayer nanoparticles enhanced photocurrent in thin film hydrogenated amorphous silicon solar cells

A plasmonic-structure incorporated double layer of Au nanoparticles embedded in the transparent conducting oxide at the back-reflector of the hydrogenated amorphous silicon (a-Si:H) solar cells is demonstrated. These devices exhibit an increase of energy conversion efficiency of 18.4% and short-circuit current density of 9.8% while improving fill-factor and without sacrificing open-circuit voltage. The increase in photocurrent is correlated with the enhanced optical absorption in the cell, with improved optical-path-length by a factor of 7 at the wavelength of 800 nm, due to enhanced diffuse scattering of light through resonant plasmon excitations within Au nanoparticles. In addition to enhanced scattering, applying high-work-function Au nanoparticles can improve the work function match at TCO/a-Si:H interface.

Suppressed quantum-confined Stark effect in InGaN-based LEDs with nano-sized patterned sapphire substrates.

This paper demonstrates that quantum-confined Stark effect (QCSE) within the multiple quantum wells (MQWs) can be suppressed by the growths of InGaN-based light-emitting diodes (LEDs) on the nano-sized patterned c-plane sapphire substrates (PCSSs) with reducing the space. The efficiency droop is also determined by QCSE. As verified by the experimentally measured data and the ray-tracing simulation results, the suppressed efficiency droop for the InGaN-based LED having the nano-sized PCSS with a smaller space of 200 nm can be acquired due to the weaker function of the QCSE within the MQWs as a result of the smaller polarization fields coming from the lower compressive strain in the corresponding epitaxial layers.

Influence of patterned sapphire substrates with different symmetry on the light output power of InGaN-based LEDs

This paper aims to investigate the light output power (LOP) of InGaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) with different symmetry. The GaN epitaxial layers grown on the hexagonal lattice arrangement PSS (HLAPSS) have a lower compressive strain than the ones grown on the square lattice arrangement PSS (SLAPSS). The quantum-confined Stark effect (QCSE) is also affected by the residual compressive strain. Based on the experimentally measured data and the ray tracing simulation results, the InGaN-based LED with the HLAPSS has a higher LOP than the one with the SLAPSS due to the weaker QCSE within multiple-quantum wells (MQWs).

Influence of the absorber layer thickness and rod length on the performance of three-dimensional nanorods thin film hydrogenated amorphous silicon solar cells

Performance of substrate-configured hydrogenated amorphous silicon solar cells based on ZnO nanorod arrays prepared by hydrothermal method has been investigated. The light harvest ability of three-dimensional nanorods solar cells is a compromise between the absorber layer thickness and the nanorods geometry. By optimizing the intrinsic a-Si:H absorber layer thickness from 75 to 250 nm and varying the length of the nanorods from 600 to 1800 nm, the highest energy conversion efficiency of 6.07% is obtained for the nanorods solar cell having thin absorber layer thickness of 200 nm with the rod length of 600 nm. This represents up to 28% enhanced efficiency compared to the conventional flat reference cell with similar absorber layer thickness.

Utilizing Two-Dimensional Photonic Crystals in Different Arrangement to Investigate the Correlation Between the Air Duty Cycle and the Light Extraction Enhancement of InGaN-Based Light-Emitting Diodes

This paper demonstrates that the optimal light extraction enhancement of the 2-D photonic crystals (PhCs) light-emitting diodes (LEDs) among different air duty cycles (ADADCsCs) is independent of the geometry and the shape of the 2-D PhCs. Moreover, it also discusses that the side-directional emission property of the 2-D PhCs LEDs in hexagonal lattice arrangement (HLA) is better than that in square lattice arrangement (SLA). Finally, the light output power of the 2-D PhCs LEDs in SLA and in HLA with the ADC of 51% are significantly improved by 60.4% and 81.9%, respectively, as compared with the reference LED at an injection current of 350 mA.

Advances in optical metasurfaces: fabrication and applications [Invited]

The research and development of optical metasurfaces has been primarily driven by the curiosity for novel optical phenomena that are unattainable from materials that exist in nature and by the desire for miniaturization of optical devices. Metasurfaces constructed of artificial patterns of subwavelength depth make it possible to achieve flat, ultrathin optical devices of high performance. A wide variety of fabrication techniques have been developed to explore their unconventional functionalities which in many ways have revolutionized the means with which we control and manipulate electromagnetic waves. The relevant research community could benefit from an overview on recent progress in the fabrication and applications of the metasurfaces. This review article is intended to serve that purpose by reviewing the state-of-the-art fabrication methods and surveying their cutting-edge applications.

Performance enhancement in quantum well infrared photodetector utilizing the grating structure

We fabricate four different size grating structure on the top of the quantum well infrared photodetectors, then etch them into four different etching depth. With suitable design, the peak responsivity of the QWIPs is about thirteen times larger than that of the conventional QWIP.

Improved Device Performance of GaN/AlGaN High-Electron-Mobility Transistor Using PdO Gate Interlayer

We demonstrate significant improvements of GaN/AlGaN high-electron-mobility transistors (HEMTs) by employing a PdO gate interlayer, which exhibit device performance superior to that of Pd Schottky gate HEMTs. The PdO gate interlayer effectively reduces the gate leakage current by four orders of magnitude, and it also increases the ION/IOFF ratio to four orders of magnitude. The improved AlGaN/GaN/PdO HEMT shows a nearly ideal subthreshold slope of 66 mV/dec. The flicker noise characteristic is also observed to be lower in PdO-gate HEMTs than in Pd-Gate HEMTs. The high-work-function PdO layer and associated barrier height enhancement are the origins of the improved device performance.