Unidirectional reflectionless propagation in a non-ideal parity-time metasurface based on far field coupling.

We propose a scheme to achieve a controllable unidirectional reflectionless propagation at exceptional point (EP) in a non-ideal parity-time metasurface consisting of two silver ring resonators. The unidirectional reflectionless propagation can be manipulated by simply adjusting the angle of incident wave and the distance s between two silver rings based on the far field coupling. In addition, the angle of incident wave in a wide range of ∼25° is available to achieve the unidirectional reflectionless propagation. Moreover, the unidirectional reflectionless propagation at EP is insensitive to the polarization of incident wave due to the two-ring structure.

Analysis of a systematic error appearing as a periodic fluctuation in the frequency-domain absorption spectra of metamaterial absorbers.

A metamaterial is a periodic structure which can be considered as an effective medium that exhibits an atypical response for a corresponding range of electromagnetic (EM) waves. For metamaterial absorbers (MMA) working in the GHz regime, the frequency-domain-measurement setup consisting of two horn antennas connected to a network analyzer is widely used. The absorption spectra obtained with this setup usually exhibits some periodic fluctuation (PF) across the entire range of measurement. Typically, this PF in the frequency-domain spectrum is ignored because it is distinct from major features and has a small amplitude. We have examined the PF through a finite-difference time-domain (FDTD) simulation in order to verify the validity of such assumption. We concluded that the PF is caused by a part of EM wave radiated by an antenna (source), and directly propagating to another (detector), resulting in an interference between this leaked EM wave and the one reflected from the sample. We have successfully reproduced the periodic fluctuation of the experimental spectra by FDTD simulation. We examined the angle of incidence dependence. The amplitude increases because the amount of leaked EM waves increases, while the periodicity increases because the distance between the two antennas increases, resulting in a shortened path difference. We conclude that the PF is a systematic error which can be safely ignored.

Optical and structural properties of Al-ZnO nanocomposites.

The optical and structural properties of aluminium-doped zinc oxide (AZO) films were investigated by photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. Pure zinc oxide (ZnO) and AZO composite films were deposited using vacuum evaporation method. The films exhibited different morphologies and crystallinity depending on the Al-doping. The SEM micrographs showed that a granular and compact structure could be seen for the ZnO film, while a nanoleaf structure with relatively porous nature was observed for the AZO composite film. The XRD patterns indicated that the crystalline growth orientation would be significantly affected by addition of Al. Compared with pure ZnO, the XRD peak intensity of the AZO composite was stronger and the line-width was narrower. Two-probe resistivity measurements showed that the AZO composites could be used as transparent conducting materials. The PL spectra revealed that the PL intensities of the AZO composites were stronger than that of the pure ZnO. The PL enhancement might be ascribed to the surface plasmon resonance of metal nanoclusters within the composite. Another possible reason of the PL enhancement would be the metal-induced crystallization caused by doping Al to ZnO matrix.

Optical and structural properties of Ag:Ta2O5 nanocomposites.

In this paper, we investigated the optical and structural properties of the Ag:Ta2O5 nanocomposites. The nanocomposite films were prepared by magnetron co-sputtering of Ta2O5 with Ag chips. The transmission electron microscopy images clearly revealed that Ag nanoparticles were successfully produced by post-thermal annealing of the deposited films. The reflection spectra exhibited the surface plasmon resonance behavior. The photoluminescence (PL) spectra indicated that the Ag:Ta2O5 nanocomposites exhibited larger PL intensity than pure Ag or pure Ta2O5. The PL enhancement is thought to be due to the surface plasmon resonance of Ag nanoparticles. From the spatial intensity distributions that were obtained by solving Maxwell equations using the three-dimensional, finite-difference time-domain method, we found that there was strong confinement of optical field near the Ag nanoparticle. This optical confinement effect makes it possible to exhibit the enhanced PL.

Optical properties of Ag hemisphere-like nanoparticles.

We investigated the optical properties of Ag hemisphere-like nanoparticles. The nanoparticles were prepared by using magnetron sputtering and post-thermal annealing. The scanning-electron microscopy images reveal that hemisphere-like nanoparticles were successfully produced. The transmission spectra exhibit the surface plasmon resonance effect. From the spatial intensity distributions obtained by solving Maxwell equations using the three-dimensional, finite-difference time-domain method, we found that there is strong confinement of the optical field near the nanoparticle. This optical confinement makes it easy to exhibit the plasmon resonance effect.

Interpretation of Reflection and Colorimetry Characteristics of Indium-Particle Films by Means of Ellipsometric Modeling.

It is of great technological importance in the field of plasmonic color generation to establish and understand the relationship between optical responses and the reflectance of metallic nanoparticles. Previously, a series of indium nanoparticle ensembles were fabricated using electron beam evaporation and inspected using spectroscopic ellipsometry (SE). The multi-oscillator Lorentz-Drude model demonstrated the optical responses of indium nanoparticles with different sizes and size distributions. The reflectance spectra and colorimetry characteristics of indium nanoparticles with unimodal and bimodal size distributions were interpreted based on the SE analysis. The trends of reflectance spectra were explained by the transfer matrix method. The effects of optical constants n and k of indium on the reflectance were demonstrated by mapping the reflectance contour lines on the n-k plane. Using oscillator decomposition, the influence of different electron behaviors in various indium structures on the reflectance spectra was revealed intuitively. The contribution of each oscillator on the colorimetry characteristics, including hue, lightness and saturation, were determined and discussed from the reflectance spectral analysis.

Nano-Cr-film-based solar selective absorber with high photo-thermal conversion efficiency and good thermal stability.

Optical properties and thermal stability of the solar selective absorber based on the metal/dielectric four-layer film structure were investigated in the variable temperature region. Numerical calculations were performed to simulate the spectral properties of multilayer stacks with different metal materials and film thickness. The typical four-layer film structure using the transition metal Cr as the thin solar absorbing layer [SiO(2)(90nm)/Cr(10nm)/SiO(2)(80nm)/Al (≥100nm)] was fabricated on the Si or K9 glass substrate by using the magnetron sputtering method. The results indicate that the metal/dielectric film structure has a good spectral selective property suitable for solar thermal applications with solar absorption efficiency higher than 95% in the 400-1200nm wavelength range and a very low thermal emittance in the infrared region. The solar selective absorber with the thin Cr layer has shown a good thermal stability up to the temperature of 873K under vacuum atmosphere. The experimental results are in good agreement with the calculated spectral results.

A solar photovoltaic system with ideal efficiency close to the theoretical limit.

In order to overcome some physical limits, a solar system consisting of five single-junction photocells with four optical filters is studied. The four filters divide the solar spectrum into five spectral regions. Each single-junction photocell with the highest photovoltaic efficiency in a narrower spectral region is chosen to optimally fit into the bandwidth of that spectral region. Under the condition of solar radiation ranging from 2.4 SUN to 3.8 SUN (AM1.5G), the measured peak efficiency under 2.8 SUN radiation reaches about 35.6%, corresponding to an ideal efficiency of about 42.7%, achieved for the photocell system with a perfect diode structure. Based on the detailed-balance model, the calculated theoretical efficiency limit for the system consisting of 5 single-junction photocells can be about 52.9% under 2.8 SUN (AM1.5G) radiation, implying that the ratio of the highest photovoltaic conversion efficiency for the ideal photodiode structure to the theoretical efficiency limit can reach about 80.7%. The results of this work will provide a way to further enhance the photovoltaic conversion efficiency for solar cell systems in future applications.

Finite-difference time-domain numerical simulation study on the optical properties of silver nanocomposites.

The effects of the nanoparticle geometry and the host matrix on the optical properties of silver (Ag) nanocomposites were investigated. The spatial intensity distribution and absorption spectra were obtained by solving Maxwell equations using the finite-difference time-domain method. Local enhancement of the optical field was produced near the surface of the Ag nanoparticle. As the nanoparticle size increased, the plasmon-induced absorption increased and the surface plasmon resonance (SPR) wavelength of the Ag nanocomposite was redshifted. As the nanoparticle geometry was transformed from a sphere to an ellipsoid, two plasmon peaks appeared and their spectral spacing became larger with increasing the aspect ratio. The effects of the nanoparticle size and the anisotropic geometry on the optical properties of the Ag nanocomposites can be described by the Maxwell-Garnett theory and the Drude model. From the absorption spectra of the Ag nanocomposites with five different host matrices (SiO2, Al2O3, ZnO, ZrO2, and TiO2), it was found that the SPR wavelength of the Ag nanocomposite was redshifted with increasing the refractive index of the host matrix.

A coma-free super-high resolution optical spectrometer using 44 high dispersion sub-gratings.

Unlike the single grating Czerny-Turner configuration spectrometers, a super-high spectral resolution optical spectrometer with zero coma aberration is first experimentally demonstrated by using a compound integrated diffraction grating module consisting of 44 high dispersion sub-gratings and a two-dimensional backside-illuminated charge-coupled device array photodetector. The demonstrated super-high resolution spectrometer gives 0.005 nm (5 pm) spectral resolution in ultra-violet range and 0.01 nm spectral resolution in the visible range, as well as a uniform efficiency of diffraction in a broad 200 nm to 1000 nm wavelength region. Our new zero-off-axis spectrometer configuration has the unique merit that enables it to be used for a wide range of spectral sensing and measurement applications.

Active manipulation of plasmonic electromagnetically-induced transparency based on magnetic plasmon resonance.

Plasmonic electromagnetically-induced transparency (EIT) can be excited by a single optical field unlike EIT in atom system, since the coupling between the bright and the dark modes is inherently induced through the near-field interaction in metamaterials. As a result, the complexity of the experimental realization can be reduced significantly, while the tunability is lost inevitably.We suggest a scheme that the plasmonic EIT is possible to be actively manipulated even by the single optical field. The bright and the dark modes are selective to be either coupled or uncoupled, depending on the angle of incidence. Even though the mechanical control has the disadvantage for high-speed applications, it paves the way for active manipulation of plasmonic EIT and benefits the clarification of its origin.

Strong optical absorption of a metallic film to induce a lensing effect in the visible region.

In this work, the two-dimensional profile of the light transmission through a prism-like metallic film sample of Au was measured at a wavelength of 632.8 nm in the visible intraband transition region to verify that, beyond the possible mechanisms of overcoming the diffraction limit, a strongly nonuniform optical absorption path length of the light traveling in the metal could induce a lensing effect, thereby narrowing the image of an object. A set of prism-like Au samples with different angles was prepared and experimentally investigated. Due to the nonuniform paths of the light traveling in the Au samples, lens-effect-like phenomena were clearly observed that reduced the imaged size of the beam spot with decreasing light intensity. The experimental measurements presented in the work may provide new insight to better understand the light propagation behavior at a metal/dielectric interface.

Ultrahigh-resolution spectrometer based on 19 integrated gratings.

Optical spectrometers play a key role in acquiring rich photonic information in both scientific research and a wide variety of applications. In this work, we present a new spectrometer with an ultrahigh resolution of better than 0.012 nm/pixel in the 170-600 nm spectral region using a grating-integrated module that consists of 19 subgratings without any moving parts. By using two-dimensional (2D) backsideilluminated complementary metal-oxide-semiconductor (BSI-CMOS) array detector technology with 2048 × 2048 pixels, a high data acquisition speed of approximately 25 spectra per second is achieved. The physical photon-sensing size of the detector along the one-dimensional wavelength direction is enhanced by a factor of 19 to approximately 428 mm, or 38912 pixels, to satisfy the requirement of seamless connection between two neighboring subspectral regions without any missing wavelengths throughout the entire spectral region. As tested with a mercury lamp, the system has advanced performance capabilities characterized by the highest k parameter reported to date, being approximately 3.58 × 104, where k = (working wavelength region)/(pixel resolution). Data calibration and analysis as well as a method of reducing background noise more efficiently are also discussed. The results presented in this work will stimulate further research on precision spectrometers based on advanced BSI-CMOS array detectors in the future.

Rigorous approach on diffracted magneto-optical effects from polar and longitudinal gyrotropic gratings.

The rigorous coupled-wave analysis with Airy-like internal-reflection series and Fourier-factorization for the calculation of the diffracted magneto-optical (MO) effects from polar and longitudinally magnetized gyrotropic gratings are fully described. For both gratings the numerical and experimental results are in good agreement, and the enhancement of Kerr rotation in higher orders compared to that of the 0th order diffraction is calculated as a function of grating depth. At last, this numerical method can be applied to many other applications such as extraordinary optical transmission from metallic gratings either through surface plasmon or cavity mode, and MO hysteresis loops.

A High-Performance Spectrometer with Two Spectral Channels Sharing the Same BSI-CMOS Detector.

Optical spectrometers play an important role in modern scientific research. In this work, we present a two-channel spectrometer with a pixel resolution of better than 0.1 nm/pixel in the wavelength range of 200 to 950 nm and an acquisition speed of approximately 25 spectra per second. The spectrometer reaches a high k factor which characterizes the spectral performance of the spectrometer as k = (working wavelength region)/(pixel resolution) = 7500. Instead of using mechanical moving parts in traditional designs, the spectrometer consists of 8 integrated sub-gratings for diffracting and imaging two sets of 4-folded spectra on the upper and lower parts, respectively, of the focal plane of a two-dimensional backside-illuminated complementary metal-oxide-semiconductor (BSI-CMOS) array detector, which shows a high peak quantum efficiency of approximately 90% at 400 nm. In addition to the advantage of being cost-effective, the compact design of the spectrometer makes it advantageous for applications in which it is desirable to use the same two-dimensional array detector to simultaneously measure multiple spectra under precisely the same working conditions to reduce environmental effects. The performance of the finished spectrometer is tested and confirmed with an Hg-Ar lamp.

High solar absorption of a multilayered thin film structure.

We report a structure with 4 thin film layers composed of pure metal and dielectric materials and prepared by sputtering. The reflectance and transmittance are lower than 5% with the absorption to be achieved higher than 95% in the 400-1000nm wavelength region as match to the solar radiance spectrum. The thermal emittance of the structure is in the range of 0.063-0.10 through data analysis. The good reproducibility and stability of spectral data associated with the deposition process imply the advantage of the solar energy absorber which is cost-effective in application.

High photon-to-heat conversion efficiency in the wavelength region of 250-1200 nm based on a thermoelectric Bi2Te3 film structure.

In this work, 4-layered SiO2/Bi2Te3/SiO2/Cu film structures were designed and fabricated and the optical properties investigated in the wavelength region of 250-1200 nm for their promising applications for direct solar-thermal-electric conversion. A typical 4-layered film sample with the structure SiO2 (66.6 nm)/Bi2Te3 (7.0 nm)/SiO2 (67.0 nm)/Cu (>100.0 nm) was deposited on a Si or K9-glass substrate by magnetron sputtering. The experimental results agree well with the simulated ones showing an average optical absorption of 96.5%, except in the shorter wavelength region, 250-500 nm, which demonstrates the superior absorption property of the 4-layered film due to the randomly rough surface of the Cu layer resulting from the higher deposition power. The high reflectance of the film structure in the long wavelength region of 2-20 µm will result in a low thermal emittance, 0.064 at 600 K. The simpler 4-layered structure with the thermoelectric Bi2Te3 used as the absorption layer may provide a straightforward way to obtain solar-thermal-electric conversion more efficiently through future study.