The efficient harvesting of electromagnetic (EM) waves by subwavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. These metamaterial-based perfect light absorbers are of particular interest in many applications, including thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection applications. Although advances in nanofabrication have provided the opportunity to observe strong light−matter interaction in various optical nanostructures, the repeatability and upscaling of these nano units have remained a challenge for their use in large scale applications. Thus, in recent years, the concept of lithography-free planar light perfect absorbers has attracted much attention in different parts of the EM spectrum, owing to their ease of fabrication and high functionality. This Perspective explores the material and architecture requirements for the realization of light perfect absorption using these multilayer metamaterial designs from ultraviolet (UV) to far-infrared (FIR) wavelength regimes. We provide a general theoretical formulation to find the ideal condition for achieving near unity light absorption. Later, these theoretical estimations are coupled with findings of recent studies on perfect light absorbers to explore the physical phenomena and the limits of different materials and design architectures. These studies are categorized in three main class of materials; metals, semiconductors, and other types of materials. We show that, by the use of proper material and design configuration, it is possible to realize these lithography-free light perfect absorbers in every portion of the EM spectrum. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large scale dimensions. In the last section, we discuss the progress, challenges, and outlook of this field to outline its future direction.

In this paper, we propose a methodology to maximize the absorption bandwidth of a metal-insulator-metal (MIM) based absorber. The proposed structure is made of a Cr-Al 2 O 3-Cr multilayer design. At the initial step, the optimum MIM planar design is fabricated and optically characterized. The results show absorption above 0.9 from 400 nm to 850 nm. Afterward, the transfer matrix method is used to find the optimal condition for the perfect light absorption in an ultra-broadband frequency range. This modeling approach predicts that changing the filling fraction of the top Cr layer can extend light absorption toward longer wavelengths. We experimentally proved that the use of proper top Cr thickness and annealing temperature leads to a nearly perfect light absorption from 400 nm to 1150 nm, which is much broader than that of a planar design. Therefore, while keeping the overall process lithography-free, the absorption functionality of the design can be significantly improved. The results presented here can serve as a beacon for future performance-enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity.

Achieving broadband absorption has been a topic of intensive research over the last decade. However, the costly and time consuming stage of lithography has always been a barrier for the large-area and mass production of absorbers. In this work, we designed, fabricated, and characterized a lithography-free, large-area compatible, omni-directional, ultra-broadband absorber that consists of the simplest geometrical configuration for absorbers: Metal-Insulator-Metal (MIM). We introduced and utilized Manganese (Mn) for the first time as a very promising metal for broadband absorption applications. We optimized the structure step-by-step and compared Mn against the other best candidates introduced so far in broadband absorption structures and showed the better performance of Mn compared to them. It also has the advantage of being cheaper compared to metals like gold that has been utilized in many patterned broadband absorbers. We also presented the circuit model of the structure. We experimentally achieved over 94 percent average absorption in the range of 400–900 nm (visible and above) and we obtained absorption as high as 99.6 percent at the wavelength of 626.4 nm. We also experimentally demonstrated that this structure retains broadband absorption for large angles up to 70 degrees.

Cataloged from PDF version of article.We report on the fabrication and characterization of solution-processed, highly flexible, silicon nanowire network based metal-semiconductor-metal photodetectors. Both the active part of the device and the electrodes are made of nanowire networks that provide both flexibility and transparency. Fabricated photodetectors showed a fast dynamic response, 0.43 ms for the rise and 0.58 ms for the fall-time, with a decent on/off ratio of 20. The effect of nanowire-density on transmittance and light on/off behavior were both investigated. Flexible photodetectors, on the other hand, were fabricated on polyethyleneterephthalate substrates and showed similar photodetector characteristics upon bending down to a radius of 1 cm. © 2013 AIP Publishing LLC

Abstract:In this work, we propose an optimum unit cell arrangement to obtain near absolute polarization insensitivity in a metal-insulator-metal (MIM) based ultra-broadband perfect absorber. Our findings prove that upon utilizing this optimum arrangement, the response of the absorber is retained and unchanged over all arbitrary incidence light polarizations, regardless of the shape of the top metal patch. First, the impact of the geometry of the top nanopatch resonators on the absorption bandwidth of the overall structure is explored. Then, the response of the MIM design for different incidence polarizations and angles is scrutinized. Finally, the proposed design is fabricated and characterized.

We report high performance visible-blind GaN-based p-i-n photodetectors grown by metal-organic chemical vapor deposition on c -plane sapphire substrates. The dark current of the 200 μm diameter devices was measured to be lower than 20 pA for bias voltages up to 5 V. The breakdown voltages were higher than 120 V. The responsivity of the photodetectors was ∼0.23 AW at 356 nm under 5 V bias. The ultraviolet-visible rejection ratio was 6.7× 103 for wavelengths longer than 400 nm. © 2008 American Institute of Physics

We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding.

Colorimetric detection of target molecules with insensitivity to incident-light polarization has attracted considerable attention in recent years. This resulted from the ability to provide rapid output and reduced assay times as a result of color changes upon altering the environment that are easily distinguishable by the naked eye. In this paper, we propose a highly sensitive refractive-index sensor, utilizing the excitation of guided modes of a novel two-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guidedmode resonances with an ultranarrow linewidth (full width at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high figure of merit of 190. The fabricated devices with 30 nm aluminum oxide and zinc oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highly sensitive, almost lossless, and compact molecular diagnostics in the field of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring.