Chieh-Hsiung Kuan

Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings.

In this article, we demonstrate that a subwavelength metal slit surrounded by dielectric surface gratings possesses a directional beaming effect. We propose a surface plasmon diffraction scheme to explain the three kinds of beaming conditions. The numerical simulations of the illustrative structures undertaken used a Finite Difference Time Domain (FDTD) Method and a Rigorous Coupled Wave Analysis (RCWA) Method. Our simulations were found to be consistent and in agreement with the experimental results. In comparison with other metal structures, we find that dielectric metal structures offer better performance as well as the advantage of being able to be efficiently mass produced for large volume industrial applications.

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.

Enhanced luminescence of organic/metal nanostructure for grating coupler active long-range surface plasmonic device

This letter is intended to demonstrate the effect of coupled active long-range surface plasmon polaritons (SPPs) on the plasmonics response of a lamellar grating nanostructure with organic material on the surface. The phenomenon of nano-optics gives rise to a selective spectral response and a local field enhancement. The authors’ fabricated device consists of coupled organic/metal nanostructure with specific width and symmetric dielectric structure. The interaction between organic/metal interface SPPs can allow specific directional emission rather than isotropic emission. The authors present recent experimental results and discuss potential applications of such an active plasmonic biosensor with enhanced resonance energy emission due to interactions on the organic/metal nanograting.

Effects of low-temperature Si buffer layer thickness on the growth of SiGe by molecular beam epitaxy

The thickness of a low-temperature silicon (LT-Si) buffer layer has been found to affect the growth of a SiGe overlayer significantly. 300-nm-thick Si0.7Ge0.3 films were grown on 50- to 300-nm-thick LT-Si buffer layers at 450 °C by solid-source molecular beam epitaxy. The threading dislocation density was found to decrease with the thickness of the LT-Si buffer in the thickness range of 50–200 nm. The density remains at the same low level when the thickness was increased from 200 to 300 nm. A relatively dense misfit dislocation network was observed to form at the SiGe/Si interface in samples with the LT-Si buffer layer thickness exceeding 200 nm. It is suggested that the presence of more point defects in the thicker LT-Si buffer layer is more effective to block the propagation of threading dislocations.

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.

Enhancement and tunability of active plasmonic by multilayer grating coupled emission

The effect of coupled mode surface plasmon polaritons (SPPs) on the active emission of a nanostructure grating with organic semiconductor material, Alq(3), on the surface was investigated in this study. We report surface plasmon grating coupled emission (SPGCE) from excited organic layer on metal grating in both organic/metal (2-Layer) and organic/metal/organic/metal (4-Layer) structures. The dispersion relation was obtained from angle-resolved photoluminescence measurement. The resultant emission intensity can have up to 6 times enhancement on the 4- Layer device and the Full-Width Half-Maximum (FWHM) is less than 50 nm. The combination of SPPs on organic/metal interface allows specific directional emission and color appearance of Alq(3) fluorophores. Potential applications of such an active plasmonics with enhanced resonant energy emission due to interactions on the organic/metal nano-grating as biosensor were presented and discussed.

Room temperature unpassivated InAs p-i-n photodetectors grown by molecular beam epitaxy

An unpassivated InAs p-i-n photodetector with excellent performance at room temperature was demonstrated. The zero-bias resistance area products of the diode with 720-nm thick i-layer are 8.1 /spl Omega/-cm/sup 2/ at room temperature and as high as 1.3 M /spl Omega/-cm/sup 2/ at 77 K. At 77 K, the diode exhibits a breakdown voltage exceeding 13 V. When tested under a 500 K blackbody source, the measured detectivity limited by Johnson noise is 1.2/spl times/10/sup 10/ cm-Hz/sup 1/2 //W at room temperature and 8.1/spl times/10/sup 11/ cm-Hz/sup 1/2 //W at 77 K. To our knowledge, this is the best data for a room temperature infrared detector.

GaN Metalens for Pixel-Level Full-Color Routing at Visible Light

Metasurface-based components are known to be one of the promising candidates for developing flat optical systems. However, their low working efficiency highly limits the use of such flat components for feasible applications. Although the introduction of the metallic mirror has been demonstrated to successfully enhance the efficiency, it is still somehow limited for imaging and sensing applications because they are only available for devices operating in a reflection fashion. Here, we demonstrate three individual GaN-based metalenses working in a transmission window with extremely high operation efficiency at visible light (87%, 91.6%, and 50.6% for blue, green, and red light, respectively). For the proof of concept, a multiplex color router with dielectric metalens, which is capable of guiding individual primary colors into different spatial positions, is experimentally verified based on the design of out-of-plane focusing metalens. Our approach with low-cost, semiconductor fabrication compatibility and high working efficiency characteristics offers a way for establishing a complete set of flat optical components for a wide range of applications such as compact imaging sensors, optical spectroscopy, and high-resolution lithography, just named a few.

Multicolor infrared detection realized with two distinct superlattices separated by a blocking barrier

A multicolor infrared photodetector was realized with two superlattices separated by a blocking barrier. The photoresponse is switchable between 7.5–12 and 6–8.5 μm by the bias polarity, and is also tunable by the bias magnitude in each wavelength regime. In addition, our detector exhibits advantages including little temperature dependence of the spectral response and the same order of responsivity in the two wavelength regimes. The measured peak responsivities in the two regimes are 117 mA/W at 9.8 μm under 1 V and 129 mA/V at 7.4 μm under −0.8 V, respectively. Also, the detectivities are comparable with the conventional multistack detector. The zero background peak detectivities are 2.3×1010 cm Hz0.5/W at 50 K and 9.8 μm under 0.7 V, and 8.7×1010 cm Hz0.5/W at 70 K and 7.4 μm under −0.7 V.

Design and calibration of a noise measurement system

The setup principle and calibration method of a noise measurement system for frequencies from dc to 10 kHz are described. This system measures the current noise power spectral density of some device, and consists of a low-noise current preamplifier, a voltage preamplifier, and a dynamic signal analyzer which implements the fast Fourier transform (FFT). The noise aspects of the whole system can be modeled as a serial noise voltage generator and a parallel noise current generator at the input port, plus a system free of noise. The cross correlation of the two noise generators is an imaginary number because the system input stage is composed of some junction field effect transistors (JFETs). Via the thermal noise measurement of several resistors, we derive the magnitudes of the noise generators in addition to the input impedance and the total system gain. The imaginary cross correlation is obtained by the noise measurement of pure capacitance. With a well-calibrated procedure, we can measure the noise power down to 2/spl times/10/sup -27/ A/sup 2//Hz. Two systems with different input stages were calibrated with the same procedure and the noise measurement results of the various resistance values with the two systems all agree well with theoretical values. One of these with an input stage which has a much smaller noise current generator shows great improvement in the noise measurement of the high-impedance device.