Jia-Wern Chen

Versatile Polarization Generation with an Aluminum Plasmonic Metasurface

All forms of light manipulation rely on light-matter interaction, the primary mechanism of which is the modulation of its electromagnetic fields by the localized electromagnetic fields of atoms. One of the important factors that influence the strength of interaction is the polarization of the electromagnetic field. The generation and manipulation of light polarization have been traditionally accomplished with bulky optical components such as waveplates, polarizers, and polarization beam splitters that are optically thick. The miniaturization of these devices is highly desirable for the development of a new class of compact, flat, and broadband optical components that can be integrated together on a single photonics chip. Here we demonstrate, for the first time, a reflective metasurface polarization generator (MPG) capable of producing light beams of any polarizations all from a linearly polarized light source with a single optically thin chip. Six polarization light beams are achieved simultaneously including four linear polarizations along different directions and two circular polarizations, all conveniently separated into different reflection angles. With the Pancharatnam-Berry phase-modulation method, the MPG sample was fabricated with aluminum as the plasmonic metal instead of the conventional gold or silver, which allowed for its broadband operation covering the entire visible spectrum. The versatility and compactness of the MPG capable of transforming any incident wave into light beams of arbitrary polarizations over a broad spectral range are an important step forward in achieving a complete set of flat optics for integrated photonics with far-reaching 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.

Vertical split-ring resonator based anomalous beam steering with high extinction ratio

Metasurfaces created artificially with metal nanostructures that are patterned on surfaces of different media have shown to possess “unusual” abilities to manipulate light. Limited by nanofabrication difficulties, so far most reported works have been based on 2D metal structures. We have recently developed an advanced e-beam process that allowed for the deposition of 3D nanostructures, namely vertical split-ring resonators (VSRRs), which opens up another degree of freedom in the metasurface design. Here we explore the functionality of beam steering with phase modulation by tuning only the vertical dimension of the VSRRs and show that anomalous steering reflection of a wide range of angles can be accomplished with high extinction ratio using the finite-difference-time-domain simulation. We also demonstrate that metasurfaces made of 3D VSRRs can be made with roughly half of the footprint compared to that of 2D nano-rods, enabling high density integration of metal nanostructures.

Ultrathin Planar Cavity Metasurfaces

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a-Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two-level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization-independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.

Material-assisted metamaterial: a new dimension to create functional metamaterial

A high Q-value reflective type metasurface consisting of 1D Au nanorods, a SiO2 spacer and a Au back reflector is demonstrated. It is shown that the sideband of the resonant mode can be suppressed as the resonant wavelength close to the phonon absorption of SiO2. By combining both designed structured resonance and inherent property of the based materials, a low angle-dependent metasurface with a Q-value of 40 has been demonstrated. The proposed structure will be useful for high sensitivity sensing and narrow band thermal emitter.

Optical toroidal response in three-dimensional plasmonic metamaterial

Toroidal dipole moments, the third kind of fundamental dipole moment, have unusual electromagnetic properties different from the electric and magnetic multipoles. We fabricate a new type of 3D plasmonic toroidal metamaterial by using mutual coupling between dumbbell-shaped gold apertures with vertical split-ring resonators (VSRRs) at optical frequency. The radiated power of multipole moments are calculated and analyzed to improve the meta-system is dominated by the toroidal dipole moment. This result paves a way for practical application on metamaterial based devices, such as biosensor and lasing spaser.

Optical and structural investigation on InGaN/GaN multiple quantum well light-emitting diodes grown on sapphire by metalorganic chemical vapor deposition

InGaN/GaN multiple quantum well (MQW) light emitting diode (LED) structures with blue and green light emissions have been grown on sapphire substrates by metalorganic chemical vapor deposition. They are investigated by high-resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) and photoluminescence excitation (PLE). HR-XRD showed multiple satellite peaks up to 10th order due to the quantum well superlattice confinement effects. HR-TEM determined the MQW structures and parameters, indicating the high quality of layer interfaces of these LED samples. Excitation power-dependent PL predicates that both piezoelectric field-induced quantum-confined Stark effect and band filling effect influence the luminescent properties. Temperature-dependent PL shows that the QW PL emission peak exhibits a monotonic red-shift and that the full width at half maximum of the PL band shows a W-shaped temperature-dependent behavior with increasing temperature. From the PLE results, a large energy difference, so-called quantum confined Stokes shift, between the band-edge absorption and emission was observed. Penetrating TEM revealed the V-shape defects, and quantum dot-like structures within the InGaN well region, which leads to intense light emissions from these MQW LEDs.

Horizontal toroidal response in three-dimensional plasmonic(Conference Presentation)

The toroidal dipole moments of natural molecules are hard to be detected so the artificial toroidal materials made by metamaterial attract more attentions. Metamaterial, the sub-wavelength artificial structures, can modulate reflection or transmission of light. The toroidal metamaterial can not only amplify the toroidal moment but also repress the electric and magnetic dipole so it can be used to study the properties of toroidal dipole moment. However, there are many limitations for the experiments, such as the lateral light is necessary to excite the toroidal response. Most of the toroidal dipole moments oscillate perpendicularly to the substrate, therefore it is difficult to couple it with other dipole moments and could be only excited in the microwave region. In this paper, we design a toroidal metamaterial consisting of dumbbell-shaped aperture and vertical split ring resonator (VSRR) vertically. The toroidal dipole moment of our metamaterial is excited in the optical region. The arrangement of our nanostructures is vertical instead of planar annular arrangement to reduce the size of the unit cell and increase the density of the toroidal dipole moment. Moreover, the direction of toroidal dipole moment is parallel to the substrate which can be used for the study of the coupling effect with other kinds of dipolar moments.

Application of lidar in the observation of atmospheric particulate pollutants in Taipei

For the assessment of climatic impact of aerosols, the knowledge of both the temporal and spatial distributions of aerosol is essential. Laser radar, more popularly known as Lidar, has becoming one of the most powerful techniques for active detection of aerosols in the atmosphere. Lidar can provide vertically resolved of extinction and backscatter coefficients, and thereby the height of the planetary boundary layer or the nighttime residual layer. As the long-term changes in the structure and dynamics of the lower and middle troposphere is now becoming a priority, a pulsed Nd:YAG Lidar system is applied for measuring the vertical distribution of aerosol properties in the metropolitan Taipei. Two years (2004-2005) of aerosol optical depth (AOD) measured by Lidar, Cimel Sunphotometer and MODerate resolution Imaging Spectroradiometer (MODIS) were compared. The AOD shows strong seasonal variation with maximum values (AODLidar > 1, AODCimel > 1 and AODMODIS > 0.39) occurred in April. AODMODIS shows significant underestimation. AODLidar has good correlation with AODCimel, but the Lidar measurement is biased toward lower values as presented by the 0.725 slope in the linear regression. This bias is mostly caused by the Lidar blind distance at the lowest part of the atmosphere. The R-squared of AODCimel and surface PM2.5 concentration is about 0.44. This reflects the fact that the atmospheric boundary layer is often not well-mixed, so aerosols there cannot represent the total AOD value. Particles in the free troposphere also need to be concerned. Further comparison of our Lidar data with the CALIPSO measurements is intended.

Metalens for structure light

Here we demonstrated a GaN metalens array to project a light spots array which can be a light shape generator in the structure light applications. The advantages of this metadevice is light weight, small, ultrathin, durable and easy to compact with other device. The light spot size is a function with the distance of detector. A metalens array which arranged by the single metalens diameter is 20 μm projected a light spots array whose diameter of single light spot is 2.22 um in average at the distance is 150 cm far away and. Our design provides a new avenue for the structure light application such as distance sensing and 3D environmental construction.