Polarization-independent Absorber Research Articles

Broadband multilayer graphene metamaterial absorbers

Two types of broadband and polarization independent graphene metamaterial absorbers are designed. One is a composite four-ring structure with azimuthally symmetry, the other is a nested multi-ring structure. The nested structure is more compact and has a broader band of absorption. The bandwidth with over 90% absorption is 7.1 THz for the composite four-ring structure. The bandwidth increases to 8.7 THz and 11.9 THz for the nested dual-ring and three-ring absorber, respectively. The wide bandwidth and polarization independent absorption owe to the combination of symmetric multi-resonators with different sizes in unit cell. Physical mechanisms of the broadband absorbers are given by the impedance matching theory as well as the surface current distributions. Instead of one single layer graphene as papers reported before, multilayers of graphene are coated on the dielectric surface to form the metal resonant-multilayer graphene-dielectric-metal ground structure. It is found that both the absorbing width and magnitude increase greatly as the graphene layers increase while a large dip appears when the number of graphene layers is large enough. Moreover, by investigating the absorptions under different graphene permittivities, we find that the reduced graphene permittivity is better to simplify the calculation.

Polarization-Independent Narrowband Near-Perfect Absorption Based on One-Dimension Embedded Aluminum Grating

In the past, the polarization-insensitive absorption is realized mainly employing the two-dimensional periodic structure of fourfold rotational symmetry, which greatly increases the manufacturing complexity and cost. Here, we present the numerical design of a polarization-independent near-perfect absorber incorporating one-dimensional embedded aluminum grating. The absorption peaks near 99% at 520-nm wavelength for different polarization angles are achieved. The underlying mechanism associated with the resonance is attributed to the magnetic polariton resonance and the cavity-mode resonance for TM and TE polarization, respectively, and further explained by the inductor-capacitor circuit model and the eigen equation of cavity mode. Furthermore, the effects of geometrical parameters of the nano-cavity on the absorption performance are discussed. The proposed structure may provide a new way to achieve polarization-independent absorption with one-dimensional meta-surface, which has broad applications in plasmonic sensors, photo-detectors, and so on.

Active-Tuning and Polarization-Independent Absorber and Sensor in the Infrared Region Based on the Phase Change Material of Ge2Sb2Te5 (GST)

Phase-change materials (PCMs), possessing thermo-optic and thermo-electric properties, have constantly enabled the rewritable optical data storage and the commercialized phase-change memory devices. In particular, Ge2Sb2Te5 (GST) has been considered for configurable photonics applications, such as active dielectric metasurface. In this paper, we report an active absorber with metal-insulator-metal (MIM) scheme with GST in the infrared region. The absorber consists of Al disk and reflective Al film with a GST spacer layer. Extraordinary absorption with peaks of more than 90% can be achieved over a broad bandwidth, attributing to highly confined gap surface plasmon resonance. In addition, the absorption can be tuned via adjusting the proportion of GST crystallization, which is a unique advantage to design active device. Meanwhile, the absorption is polarization-independent owing to its structural symmetry. Furthermore, we introduce the designed absorber to the application of sensing. This nearly perfect absorbing strategy offers great potential in sensing applications due to its flexibility and polarization-independence.

Analysis of Biaxially Tensile Strained Ge/SiGe Multiple Quantum Wells for Electro-Absorption Modulators With Low Polarization Sensitivities

We analyze theoretically biaxially tensile strained Ge/Si0.18Ge0.82 multiple quantum wells (MQWs) for electro-absorption modulators with low polarization sensitivities. The difference between TE and TM polarized momentum matrix elements and absorption spectra are discussed. Our calculation indicates that polarization independent electroabsorption can be achieved by introducing 0.78% biaxial tensile strain in the buffer layer. The simulation results show that the maximum polarization independent absorption contrast ratio is 7.6 dB under 0 V/2 V operation at 1485 nm. The absorption contrast ratio is over 6 dB for both TE and TM polarizations with the wavelength ranging from 1467 to 1489 nm. The proposed scheme provides a promising approach to design highly efficient polarization independent Ge/SiGe MQWs electroabsorption modulators for on-chip optical transmission applications.

Broad-band and polarization-independent perfect absorption in graphene-gold cylinder arrays at visible and near-infrared wavelengths

A wavelength tunable perfect absorber with graphene-hexagonal gold (Au) cylinder array on a ground plate is investigated theoretically. The interactions between electromagnetic (EM) waves and monolayer graphene are analyzed through the field distributions and spectral responses in detail. The finite-difference-time-domain (FDTD) method is used to investigate the tunable properties of the absorber. It is demonstrated that in an optimized configuration, monolayer graphene can interact with light via critical coupling, and the absorptance can be greatly enhanced and reach to 100% for both transverse magnetic (TM) and transverse electronic (TE) polarizations. Furthermore, the influence of geometrical parameters of the structure on the response of the hybrid structure is studied. It is expected that the proposed graphene perfect absorbers can be applied for many applications in the visible (VIS) and the near-infrared (NIR) spectral ranges such as wavelength selective infrared photodetectors and plasmonic sensors.

Flexible metasurface black nickel with stepped nanopillars.

We report on a monolithic, all-metallic, and flexible metasurface perfect absorber [black nickel (Ni)] based on coupled Mie resonances originated from vertically stepped Ni nanopillars homoepitaxially grown on an Ni substrate. Coupled Mie resonances are generated from Ni nanopillars with different sizes such that Mie resonances of the stepped two sets of Ni nanopillars occur complementarily at different wavelengths to realize polarization-independent broadband absorption over the entire visible wavelength band (400-760nm) within an ultra-thin surface layer of only 162nm thick in total. Two-step double-beam interference lithography and electroplating are utilized to fabricate the proposed monolithic metasurface that can be arbitrarily bent and pressed. A black nickel metasurface is experimentally demonstrated in which an average polarization-independent absorption of 0.972 (0.961, experiment) in the entire visible band is achieved and remains 0.838 (0.815, experiment) when the incident angle increases to 70°.

Polarization-independent absorption enhancement in a graphene square array with a cascaded grating structure.

The polarization-independent enhanced absorption effect of graphene in the near-infrared range is investigated. This is achieved by placing a graphene square array on top of a dielectric square array backed by a two-dimensional multilayer grating. Total optical absorption in graphene can be attributed to critical coupling, which is achieved through the combined effect of guided-mode resonance with the dielectric square array and the photonic band gap with the two-dimensional multilayer grating. To reveal the physical origin of such a phenomenon, the electromagnetic field distributions for both polarizations are illustrated. The designed graphene absorber exhibits near-unity polarization-independent absorption at resonance with an ultra-narrow spectrum. Moreover, the polarization-independent absorption can be tuned simply by changing the geometric parameters. The results may have promising potential for the design of graphene-based optoelectronic devices.

A refractory metamaterial absorber for ultra-broadband, omnidirectional and polarization-independent absorption in the UV-NIR spectrum.

In this paper, efficient ultra-broadband absorption from ultraviolet (UV) to near infrared (NIR) is achieved using a metamaterial perfect absorber (MPA) with refractory constituents. Both simulated and experimental results indicate that this proposed MPA exhibits an average absorption over 95% at wavelengths ranging from 200 nm to 900 nm. Besides, owing to the ultrathin thickness and symmetrical topology of this device, it exhibits great angular tolerance up to 60° independent of the incident polarizations. Excellent thermal stability is also demonstrated at high operation temperatures. The physical origin of the ultra-broadband characteristics is mainly based on diffraction/interference engineering at short wavelengths and the anti-reflection effect at long wavelengths. We believe that such a device may find potential applications ranging from photodetection and photothermal energy conversion to ultraviolet protection and thermophotovoltaics.

Wide-angle absorption of visible light from simple bilayers.

Color-selective absorption of light is a very significant operation used in numerous applications, from photonic sensing and switching to optical signal modulation and energy harnessing. We demonstrate angle-insensitive and polarization-independent absorption by thin bilayers comprising ordinary bulk media: dielectrics, semiconductors, and metals. Several highly efficient designs for each color of the visible spectrum are reported, and their internal fields' distributions reveal the resonance mechanism of absorption. The proposed bilayer components are realizable, since various physical or chemical deposition methods can be used for their effective fabrication. The absorption process is found to exhibit endurance with respect to the longitudinal dimension of the planar structure, which means that the same designs could be successfully utilized in non-planar configurations composed of arbitrary shapes.

Polarization-independent, wide-incident-angle and dual-band perfect absorption, based on near-field coupling in a symmetric metamaterial

We numerically and experimentally investigated a dual-band metamaterial perfect absorber (MPA), utilizing the near-field coupling of double split-ring resonators (DSRRs). Owing to the near-field coupling between resonators, two arms in each DSRR resonate in different phases, leading to a dual-band perfect absorption. The proposed MPA also exhibits polarization-insensitive behavior and maintains the high absorption above 90% up to a wide range of incident angle more than 45°. Finally, to further consolidate our approach, a multi-band absorption is also studied by exploiting the near-field coupling among a larger number of DSRRs. Our work is expected to be applied to future broadband devices using MPA.

Large‐Area Wide‐Incident‐Angle Metasurface Perfect Absorber in Total Visible Band Based on Coupled Mie Resonances

A large‐area, ultrathin metasurface perfect absorber (MSPA) based on coupled Mie resonances from integrated arrays of silicon (Si) and metallic pillars is reported. In contrast to previous structures and mechanisms, coupled Mie resonances are generated from two pillars of dissimilar materials (dielectric and metallic) within an ultrathin structure such that the Mie resonances of dielectric and metallic patterns occur complementarily at different wavelengths to realize broadband absorption over the entire visible wavelength band. Double‐beam interference lithography, reactive ion etching, and sputter coated depositions are utilized to fabricate the proposed MSPA. With an appropriate arrangement of the patterned geometry, an average polarization independent absorption of 0.975 (0.958, experiment) in the entire visible band (from 400 to 760 nm) is achieved and remains high (0.930, simulation, and 0.912, experiment) when the incident angle increases to 70°. The proposed MSPA is thus an effective way to realize a large‐area, ultrathin metasurface absorber with a relatively simple and easy to fabricate structure.

Near-Unity Unselective Absorption in Sparse InP Nanowire Arrays

We experimentally demonstrate near-unity, unselective absorption, broadband, angle-insensitive, and polarization-independent absorption, in sparse InP nanowire arrays, embedded in flexible polymer sheets via geometric control of waveguide modes in two wire motifs: (i) arrays of tapered wires and (ii) arrays of nanowires with varying radii. Sparse arrays of these structures exhibit enhanced absorption due to strong coupling into the first order azimuthal waveguide modes of individual nanowires; wire radius thus controls the spectral region of the absorption enhancement. Whereas arrays of cylindrical wires with uniform radius exhibit narrowband absorption, arrays of tapered wires and arrays with multiple wire radii expand this spectral region and achieve broadband absorption enhancement. Herein, we present an economic, top-down lithographic/etch fabrication method that enables fabrication of multiple InP nanowire arrays from a single InP wafer with deliberate control of nanowire radius and taper. Using this...

Utilization of monolayer MoS2 in Bragg stacks and metamaterial structures as broadband absorbers

We numerically study the possibility of using atomically thin transition metal dichalcogenides (TMDs) for applications requiring broadband absorption in the visible range of the electromagnetic spectrum. We demonstrate that when monolayer TMDs are positioned into a finite-period of multilayer Bragg stack geometry, they make broadband, wide-angle, almost polarization-independent absorbers. In our study, we consider molybdenum disulfide (MoS2) and silicon dioxide (SiO2) as semiconducting and dielectric thin film of alternate high- and low- index films, respectively. By optimizing the thickness of the SiO2 film, we find that monolayer MoS2 based Bragg stacks can absorb 94.7% of the incident energy in the visible (350–700nm). Similar structures can be engineered to make perfect reflectors for saturable absorption applications. We also demonstrate that bandwidth of metamaterial absorbers can be expanded using monolayer TMDs.

Two-dimensional QR-coded metamaterial absorber

In this paper, the design of metamaterial absorbers is proposed based on QR coding and topology optimization. Such absorbers look like QR codes and can be recognized by decoding softwares as well as mobile phones. To verify the design, two lightweight wideband absorbers are designed, which can achieve wideband absorption above 90 % in 6.68–19.30 and 7.00–19.70 GHz, respectively. More importantly, polarization-independent absorption over 90 % can be maintained under incident angle within 55°. The QR code absorber not only can achieve wideband absorption, but also can carry information such as texts and Web sites. They are of important values in applications such identification and electromagnetic protection.

Common Metal-Dielectric-Metal Nanocavities for Multispectral Narrowband Light Absorption

We present a method to theoretically achieve multispectral narrowband light absorption in common metal-dielectric-metal nanocavities. Polarization-independent and wide-angle multi-band light absorption with absorbance up to 99 % is achieved owing to the excitation of multiple localized plasmon cavity modes. Strong interactions between plasmon resonances and photonic modes are further introduced for achieving sharp absorption spectrum with sub-10-nm bandwidth via a high-index dielectric spacer with a thickness exceeding λ/2. These findings can offer new perspectives for multispectral nano-optics devices including perfect light absorbers and subtractive polychromatic filters.

Checkerboard Nanoplasmonic Gold Structure for Long-Wave Infrared Absorption Enhancement

A localized nanoplasmonic induced absorption enhancement in silicon nitride ðSi3N4Þ dielectric material using a nanoscale novel checkerboard gold (Au) structure is demonstrated. The checkerboard structure is fabricated on a Si3N4 layer using electron- beam lithography and sputter deposition techniques. The plasmonic electric field and opti- cal absorption enhancement are measured using scanning near-field optical microscopy. Finite-difference time-domain simulations are utilized to characterize the absorption spec- tral response enhancement together with its dependence on incidence angle and polariza- tion. The checkerboard shows a broadband average spectral absorption enhancement of 63.2% over the wavelength range 8-12 � m with a maximum enhancement of 107% at 8 � m and a minimum enhancement of 24.8% at 12 � m. The degradation of enhanced absorption with incidence angle variation (0 � -60 � ) is less than 1.6% at 10.6-� mw ave- length. The checkerboard device shows polarization-independent absorption enhance- ment with incidence angles.

Aluminum plasmonics for enhanced visible light absorption and high efficiency water splitting in core-multishell nanowire photoelectrodes with ultrathin hematite shells.

The poor internal quantum efficiency (IQE) arising from high recombination and insufficient absorption is one of the critical challenges toward achieving high efficiency water splitting in hematite (α-Fe2O3) photoelectrodes. By combining the nanowire (NW) geometry with the localized surface plasmon resonance (LSPR) in semiconductor-metal-metal oxide core-multishell (CMS) NWs, we theoretically demonstrate an effective route to strongly improve absorption within ultrathin (sub-50 nm) hematite layers. We show that Si-Al-Fe2O3 CMS NWs exhibit photocurrent densities comparable to Si-Ag-Fe2O3 CMS and outperform Fe2O3, Si-Fe2O3 CS and Si-Au-Fe2O3 CMS NWs. Specifically; Si-Al-Fe2O3 CMS NWs reach photocurrent densities of ∼ 11.81 mA/cm(2) within a 40 nm thick hematite shell which corresponding to a solar to hydrogen (STH) efficiency of 14.5%. This corresponds to about 93% of the theoretical maximum for bulk hematite. Therefore, we establish Al as an excellent alternative plasmonic material compared to precious metals in CMS structures. Further, the absorbed photon flux is close to the NW surface in the CMS NWs, which ensures the charges generated can reach the reaction site with minimal recombining. Although the NW geometry is anisotropic, the CMS NWs exhibit polarization independent absorption over a large range of incidence angles. Finally, we show that Si-Al-Fe2O3 CMS NWs demonstrate photocurrent densities greater than ∼ 8.2 mA/cm(2) (STH efficiency of 10%) for incidence angles as large as 45°. These theoretical results strongly establish the effectiveness of the Al-based CMS NWs for achieving scalable and cost-effective photoelectrodes with improved IQE, enabling a novel route toward high efficiency water splitting.

A continuous wave Yb:KGW laser with polarization-independent pump absorption

A polarization-independent pumping scheme for an end-pumped Yb:KGd(WO4)2 laser oscillator is proposed. It enables the use of compact, high power and high brightness fiber-coupled laser diode modules as pump sources and results in reduced thermal effects, good beam quality and improved output power stability. Based on this design, a continuous wave Yb:KGW laser with polarization-independent pump scheme is demonstrated. The laser delivered 2 W of average output power in TEM00 mode at 1040 nm. The induced thermal lens is reduced by a factor of 2 when compared to conventional diode pumping schemes, which clearly demonstrates the potential of the geometry of this crystal for power scaling.

Polarization-independent dual-band terahertz metamaterial absorbers based on gold/parylene-C/silicide structure

We design, fabricate, and characterize dual-band terahertz (THz) metamaterial absorbers with high absorption based on structures consisting of a cobalt silicide (Co-Si) ground plane, a parylene-C dielectric spacer, and a metal top layer. By combining two periodic metal resonators that couple separately within a single unit cell, a polarization-independent absorber with two distinct absorption peaks was obtained. By varying the thickness of the dielectric layer, we obtain absorptivity of 0.76 at 0.76THz and 0.97 at 2.30THz, which indicates the Co-Si ground plane absorbers present good performance.

An interference coating of metamaterial as an ultrathin light absorber in the violet-to-infrared regime

A metamaterial with brief and ultrathin structure performs high efficiency in light absorption. An upright aluminum nanorod array (Al NRA) is obliquely deposited, measured, and analyzed its optical property. The Al NRA performs high efficiency of light absorption and low reflectance simultaneously. Based on the measured refractive index and impedances, the wave propagation through the Al NRA is traced to demonstrate the destructive interference that leads to antireflection. According to the analysis of wave tracing, an Al semicontinuous film with thickness of 15nm is introduced under an Al NRA with thickness of only 245nm as a brief and thin two-layered structure. The broadband and polarization-independent light absorption is measured over the violet-to-infrared regime.