台大光电所所讯

Broadband Achromatic Metalenses at visible light

Professor Chieh-Hsiung Kuan

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所管杰雄教授

Traditional bulky and heavy optical components usually use phase accumulation along optical path, spatial refractive-index distribution, or surface morphology to tailor the wavefront of electromagnetic waves. Novel metasurfaces have revolutionized the ways with which we design and engineer the wavefront shaping.With abrupt phase change at the surface, we can create the new wavefront with anomalous propagation by orderly arranging the nano-resonators on a metasurface that leads to functionalities that were impossible to realize in conventional devices are now made possible with such metasurfaces. Metalens is one application for the wavefront shaping of metasurfaces. In our E-beam lab, we have successfully developed a broadband achromatic metalens composed of GaN nano-rods that can eliminate chromatic aberration at visible light, showing a low-cost fabrication and high-efficiency platform, envisioning future optics.

Fig. 1. (a) Optical image of the fabricated achromatic metalens. Scale bar: 10 μm. The dashed squares indicate the position of the zoomed-in SEM images shown in (b) and (c). Scale bars: 500 nm.

Fig. 2. Experimental light intensity profiles for the achromatic metalens. The white dashed line indicates the position of the focal plane.

Mobility Enhancement of Back-Channel-Etch Amorphous InGaZnO TFT with Double Layers

Professor Chee-Wee Liu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所刘致为教授

The back-channel-etch amorphous InGaZnO (a-IGZO) double-layer thin-film transistor (DL-TFT) consists of an IGZO channel layer with no oxygen flow (NOF) and an IGZO barrier layer with oxygen flow (OF). The DL-TFT demonstrates the field-effect mobility of 19 cm²/ V-s, which is 1.6X of the NOF and the OF single-layer TFTs (SL-TFTs) at the overdrive voltage of 18V and the drain voltage of 0.1V. Several measurements are performed to obtain the band alignment between the NOF and the OF layer for further understandings of the mechanism of the improvement in the field-effect mobility. Tauc method, X-ray photoelectron spectroscopy (XPS), and Kelvin probe force microscopy (KPFM) measurements were used to extract the optical bandgaps, the Fermi level position, and the work function of the NOF and the OF, respectively. The conduction band difference of 0.28 eV between the NOF and the OF layer is obtained. The carriers in the DL-TFT are confined in the NOF layer by quantum confinement, where the OF layer serves as the barrier to reduce the Coulomb scattering between the channel electrons and oxide charge, and the surface roughness scattering from the IGZO/oxide interface. DL-TFT can achieve high mobility with manufacturing ability by the simple change of oxygen flow.


Fig. 1 Schematics of the cross-sectional diagrams (not scaled) of (a) SL-TFT and (b) DL-TFT.	Fig. 2 Transfer characteristics of the SL-TFT with either NOF or OF channel, and the DL-TFT by bottom gate operation with top gate ground. The inset is the linear scale of drain current versus overdrive voltage.


Fig. 3 The band alignment for NOF and OF a-IGZO.	Fig. 4 Simulation of the electron density distribution and the band diagram of the DL-TFT.