-Wen Cheng

Wide-angle polarization independent infrared broadband absorbers based on metallic multi-sized disk arrays

Two-dimensional metallic broadband absorbers on a SiO(2)/Ag/Si substrate were experimentally studied. The absorptivity of such structure can be increased by tailoring the ratio of disk size to the unit cell area. The metallic disk exhibits a localized surface plasmon polariton (LSPP) mode for both TE and TM polarizations. A broadband thermal emitter can be realized because the LSPP mode is independent of the periodicities. By manipulating the ratios and disk sizes, a high-performance, wide-angle, polarization-independent dual band absorber was experimentally achieved. The results demonstrated a substantial flexibility in absorber designs for applications in thermal photovoltaics, sensors, and camouflage.

Angle and polarization independent narrow-band thermal emitter made of metallic disk on SiO2

It is shown that the metallic disk structure can be used as an efficient narrow-band thermal emitter in the IR region. The absorption spectra of such structure are investigated both theoretically and experimentally. Calculations of thermal radiation properties of the metallic disk show that the metallic disk is a perfect emitter at a specific wavelength, which can be tuned by varying the diameter of the disk. The metallic disk exhibits only one significant localized surface plasmon polariton (LSPP) mode for both TM and TE polarizations simultaneously. The LSPP mode can be tuned by either varying the disk diameter or the spacer (made of SiO2).

Characterization of the surface plasmon polariton band gap in an Ag/SiO 2 /Ag T-shaped periodical structure

In this study, the localized surface plasmon polariton (LSPP) band gap of an Ag/SiO(2)/Ag asymmetric T-shaped periodical structure is demonstrated and characterized. The Ag/SiO(2)/Ag asymmetric T-shaped periodical structure was designed and fabricated to exhibit the LSPP modes in an infrared wavelength regime, and its band gap can be manipulated through the structural geometry. The LSPP band gap was observed experimentally with the absorbance spectra and its angle dependence characterized with different incident angles. Such a T-shaped structure with a LSPP band gap can be widely exploited in various applications, such as emitters and sensors.

Hydrodynamic modeling of surface plasmon enhanced photon induced current in a gold grating

The current induced by incident photons on an gold grating slab is investigated numerically and experimentally. A semiclassical electrodynamic model is developed under the weak nonlinearity approximation. Electrons in the conduction band are treated as an electron gas in the presence of a self-consistent electromagnetic field. The model is solved by the finite element method and compared with measurements. The calculated current density as a function of incident angle and wavelength is found to be in qualitative agreement with the experimental measurements. The results show that increasing surface plasmon spatial variation enhances photon induced current.

Phase matching for surface plasmon enhanced second harmonic generation in a gold grating slab

Surface plasmon enhanced second harmonic generation in gold grating slabs was investigated. The efficiency is analyzed with respect to the phase matching at the fundamental and the second harmonic frequencies. A classical electromagnetic model was developed under the weak nonlinearity approximation and solved by the finite element method. The measured zeroth order transmitted second harmonic intensity was found to be in quantitative agreement with numerical results. It is shown experimentally and numerically that proper phase matching at both frequencies improves the second harmonic efficiency.

Epitaxial ferromagnetic Fe3Si on GaAs(111)A with atomically smooth surface and interface

Single crystal ferromagnetic Fe3Si(111) films were grown epitaxially on GaAs(111)A by molecular beam epitaxy. These hetero-structures possess extremely low surface roughness of 1.3 A and interfacial roughness of 1.9 A, measured by in-situ scanning tunneling microscope and X-ray reflectivity analyses, respectively, showing superior film quality, comparing to those attained on GaAs(001) in previous publications. The atomically smooth interface was revealed by the atomic-resolution Z (atomic number)-contrast scanning transmission electron microscopy (STEM) images using the correction of spherical aberration (Cs)-corrected electron probe. Excellent crystallinity and perfect lattice match were both confirmed by high resolution x-ray diffraction. Measurements of magnetic property for the Fe3Si/GaAs(111) yielded a saturation moment of 990 emu/cm3 with a small coercive field ≤1 Oe at room temperature.

A new stable, crystalline capping material for topological insulators

To preserve the high quality topological surface state after air exposure without degradation, it is crucial to identify an effective capping layer. In this study, we report an effective capping layer obtained by crystallizing Se. Upon extended exposure to ultrahigh vacuum or humid air, we show by using x-ray photoemission spectroscopy that the stability and resistance to oxidation of crystalline Se capping layers are superior to those of the amorphous Se capping layer, which has been commonly used by current communities. Furthermore, time-dependent Hall measurements showed that crystalline Se capping layers had a much stronger ability to sustain the intrinsic transport properties of Bi2Se3.To preserve the high quality topological surface state after air exposure without degradation, it is crucial to identify an effective capping layer. In this study, we report an effective capping layer obtained by crystallizing Se. Upon extended exposure to ultrahigh vacuum or humid air, we show by using x-ray photoemission spectroscopy that the stability and resistance to oxidation of crystalline Se capping layers are superior to those of the amorphous Se capping layer, which has been commonly used by current communities. Furthermore, time-dependent Hall measurements showed that crystalline Se capping layers had a much stronger ability to sustain the intrinsic transport properties of Bi2Se3.

Demonstration of large field effect in topological insulator films via a high-κ back gate

The spintronics applications long anticipated for topological insulators (TIs) has been hampered due to the presence of high density intrinsic defects in the bulk states. In this work we demonstrate the back-gating effect on TIs by integrating Bi2Se3 films 6–10 quintuple layer (QL) thick with amorphous high-κ oxides of Al2O3 and Y2O3. Large gating effect of tuning the Fermi level EF to very close to the band gap was observed, with an applied bias of an order of magnitude smaller than those of the SiO2 back gate, and the modulation of film resistance can reach as high as 1200%. The dependence of the gating effect on the TI film thickness was investigated, and ΔN2D/ΔVg varies with TI film thickness as ∼t−0.75. To enhance the gating effect, a Y2O3 layer thickness 4 nm was inserted into Al2O3 gate stack to increase the total κ value to 13.2. A 1.4 times stronger gating effect is observed, and the increment of induced carrier numbers is in good agreement with additional charges accumulated in the higher κ oxides. Moreover, we have reduced the intrinsic carrier concentration in the TI film by dopingTe to Bi2Se3 to form Bi2TexSe1−x. The observation of a mixed state of ambipolar field that both electrons and holes are present indicates that we have tuned the EF very close to the Dirac Point. These results have demonstrated that our capability of gating TIs with high-κ back gate to pave the way to spin devices of tunable EF for dissipationless spintronics based on well-established semiconductor technology.

Localized Surface Plasmon Polarition (LSPP) Based Sensor Made of a Round-Shaped Metal Array for Applications in Biosensing

Two-dimensional metallic round-shaped disk structure on a SiO2/Au/Si substrate was experimentally studied. By using a Fourier transform infrared spectrometer, an angle and polarization independent localized surface plasmon polariton mode caused by a Fabry-Perot-like resonance was observed. Such structure has the ability to sense changes in refractive index from 1 to 1.39. Because of its large incident angle tolerance and local sensing ability, the structure can potentially be utilized for applications in biomedical diagnostics, food safety monitoring, and environmental detection.