Broadband Angle Research Articles

Enhanced absorption in thin and ultrathin silicon films by 3D photonic band gap back reflectors

Since thin and ultrathin silicon films have limited optical absorption, we explore the effect of a nanostructured back reflector to recycle the unabsorbed light. As a back reflector, we investigate a three-dimensional (3D) photonic band gap crystal made from silicon that is readily integrated with the thin silicon films. We numerically obtain the optical properties by solving the 3D time-harmonic Maxwell equations using the finite-element method, and model silicon with experimentally determined optical constants. The absorption enhancement spectra and the photonic band gap generated current density are obtained by weighting the absorption spectra with the AM 1.5 standard solar spectrum. We study thin films in two different regimes, much thicker (L Si = 2400 nm) or much thinner (L Si = 80 nm) than the wavelength of light. For L Si = 2400 nm thin film, the 3D photonic band gap crystal enhances the spectrally averaged (λ = 680 nm to 880 nm) silicon absorption by 2.22 × (s −pol.) to 2.45 × (p −pol.), which exceeds the enhancement of a perfect metal back reflector (1.47 to 1.56 ×). The absorption is considerably enhanced by the (i) broadband angle and polarization-independent reflectivity in the 3D photonic band gap, and (ii) the excitation of many guided modes in the film by the crystal’s surface diffraction leading to greatly enhanced path lengths. For L Si = 80 nm ultrathin film, the photonic crystal back reflector yields a striking average absorption enhancement of 9.15 ×, much more than 0.83 × for a perfect metal. This enhancement is due to a remarkable guided mode that is confined within the combined thickness of the ultrathin film and the photonic crystal’s Bragg attenuation length. An important feature of the 3D photonic band gap is to have a broad bandwidth, which leads to the back reflector’s Bragg attenuation length being much shorter than the silicon absorption length. Consequently, light is confined inside the thin film and the remarkable absorption enhancements are not due to the additional thickness of the photonic crystal back reflector. We briefly discuss a number of high-tech devices that could profit from our results, including ultrathin film solar cells.

Tunable daytime passive radiative cooling based on a broadband angle selective low-pass filter.

Passive daytime cooling could contribute to the reduction of our global energy consumption. It is capable of cooling materials down to below ambient temperatures without the necessity of any additional input energy. Yet, current devices and concepts all lack the possibility to switch the cooling properties on and off. Here, we introduce dynamic control for passive radiative cooling during daytime. Using an angle-selective solar filter on top of a nocturnal passive radiator allows tuning the surface temperature of the latter in a wide range by just tilting the filter from normal incidence up to around 23°. This angle-selective filter is based on optically engineered, one-dimensional photonic crystal structures. We use numerical simulations to investigate the feasibility of a switchable low-pass filter/emitter device.

Indoor Localization With a Single Wi-Fi Access Point Based on OFDM-MIMO

In this paper, we propose S-Phaser, an indoor localization system that uses a single Wi-Fi access point (AP) to locate terminals. Compared to traditional indoor localization systems based on the fingerprint positioning technology, S-Phaser does not need to deploy a large number of APs and has better localization accuracy. S-Phaser utilizes channel state information (CSI) to compute the direct path length between a single AP and terminals, thereby efficiently improving the accuracy in the line-of-sight (LOS) scenario. S-Phaser is still able to get satisfied accuracy in the non-LOS scenario even not as good as in the LOS scenario. Using the feature of multiple channels in 802.11n, we design an interpolation-based algorithm, named as the interpolation elimination method to calibrate signal phases from CSI, and then we use two different algorithms, the Chinese residue theorem and broadband angle ranging, to compute the real distance of the direct path from the calibrated phases. Then, S-Phaser uses a geometric positioning algorithm to determine the user's location. To verify the practicability of the proposed S-Phaser, we set up a localization system in an actual environment. S-Phaser can improve the median localization error to 1.5 m with a single Wi-Fi AP.

Broad‐band angle insensitive frequency‐selective surface with multiple resonant modes

AbstractIn this work, a broadband frequency selective surface (FSS), which is insensitive to incident angle is designed. This FSS is composed of three metallic layers and two identical dielectric layers. The top and bottom layers are two identical metal patches, while the middle layer is a slot structure etched in a metal screen. This FSS achieves a broadband pass‐band response from 9.8 to 20.8 GHz by combining different resonant modes at 12.6, 15.2, and 19.2 GHz, respectively. Meanwhile, this FSS can still function properly at up to 60° oblique incidence for TE polarization, which is attributed to the independence of incident angle of the middle slot layer and miniaturization of the whole structure. In addition, two key parameters (that are of great importance in adjusting flatness of the pass‐band) of the FSS element have been studied. A prototype of the FSS was manufactured and measured. Good agreements between measurements and simulations are observed.

Anisotropic Broadband Photoresponse of Layered Type-II Weyl Semimetal MoTe2.

Photodetectors based on Weyl semimetal promise extreme performance in terms of highly sensitive, broadband and self-powered operation owing to its extraordinary material properties. Layered Type-II Weyl semimetal that break Lorentz invariance can be further integrated with other two-dimensional materials to form van der Waals heterostructures and realize multiple functionalities inheriting the advantages of other two-dimensional materials. Herein, we report the realization of a broadband self-powered photodetector based on Type-II Weyl semimetal Td -MoTe2 . The prototype metal-MoTe2 -metal photodetector exhibits a responsivity of 0.40 mA W-1 and specific directivity of 1.07 × 108 Jones with 43 μs response time at 532 nm. Broadband responses from 532 nm to 10.6 μm are experimentally tested with a potential detection range extendable to far-infrared and terahertz. Furthermore, we identify the response of the detector is polarization angle sensitive due to the anisotropic response of MoTe2 . The anisotropy is found to be wavelength dependent, and the degree of anisotropy increases as the excitation wavelength gets closer to the Weyl nodes. In addition, with power and temperature dependent photoresponse measurements, the photocurrent generation mechanisms are investigated. Our results suggest this emerging class of materials can be harnessed for broadband angle sensitive, self-powered photodetection with decent responsivities.

Broadband angle- and permittivity-insensitive nondispersive optical activity based on planar chiral metamaterials

Because of the strong inherent resonances, the giant optical activity obtained via chiral metamaterials generally suffers from high dispersion, which has been a big stumbling block to broadband applications. In this paper, we propose a type of planar chiral metamaterial consisting of interconnected metal helix slat structures with four-fold symmetry, which exhibits nonresonant Drude-like response and can therefore avoid the highly dispersive optical activity resulting from resonances. It shows that the well-designed chiral metamaterial can achieve nondispersive and pure optical activity with high transmittance in a broadband frequency range. And the optical activity of multi-layer chiral metamaterials is proportional to the layer numbers of single-layer chiral metamaterial. Most remarkably, the broadband behaviors of nondispersive optical activity and high transmission are insensitive to the incident angles of electromagnetic waves and permittivity of dielectric substrate, thereby enabling more flexibility in polarization manipulation.