Thin Perfect Absorbers for Electromagnetic Waves: Theory, Design, and Realizations

Perfect light absorption in the visible and near-infrared (NIR) was demonstrated using metamaterials, plasmonic nanostructures, and thin films. Thin film absorbers offer a simple and low-cost design as they can be produced on large areas and without lithography. Light is strongly absorbed in thin film metal-dielectric-metal (MDM) cavities at their resonance frequencies. However, a major drawback of MDM absorbers is their strong resonance iridescence, i.e., angle dependence. Here, we solve the iridescence problem by achieving angle-insensitive narrowband perfect and near-perfect light absorption. In particular, we show analytically that using a high-index dielectric in MDM cavities is sufficient to achieve angle-insensitive cavity resonance. We demonstrate experimentally angle-insensitive perfect and near-perfect absorbers in the NIR and visible regimes up to ±60°. By overcoming the iridescence problem, we open the door for practical applications of MDM absorbers at optical frequencies.

Perfect light absorption (PLA) in nanophotonics has a wide range of applications from solar-thermal based applications to radiative cooling. However, most of the proposed platforms require intense lithography which makes them of minor practical relevance. On the other hand, thin-film light absorbers are lithographically free and can be deposited cheaply on large area based on matured technologies. However, thin-film light absorbers were thought to have major limitation and cannot be tailored compared to metamaterials. Here, we show how to design PLA using thin-films in terms of wavelength range, bandwidth, spatial profile of optical losses, directionality and iridescence. We also show that iridescent free, PLA can occur by simply heating metallic thin-films when the metal is of low reflectance and its oxide is of high refractive index. We theoretically and experimentally demonstrate Generalized Brewster angle effect in thin film light absorbers. In addition, we demonstrate hydrogen sensing using three different PLA strategies showing record sensitivity and figure of merit. Furthermore, we show various strategies to create ultra-pure structural colors. Finally, we demonstrate different solar-thermal applications for novel thin-film PLA designs.

We show theoretically that coherent perfect absorption (CPA) of light is possible by a graphene monolayer without doping. To achieve the CPA, undoped graphene is embedded inside a multilayer structure called mirror structure, which increases the effective impedance of the surrounding medium. The increased effective impedance allows the destructive interference of light outside the structure that induces an increased Joule heat generating perfect absorption of light by graphene. We find that the CPA is possible for any wavelength as long as the thickness of each layer of the structure is a quarter of the corresponding wavelength. By changing the incident wavelength, but keeping the thickness of each layer, we show that the optical absorption is sensitive to the change of the incident wavelength. Therefore, we can design a photo detector for a specific wavelength.

The efficient harvesting of electromagnetic (EM) waves by sub-wavelength 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. In this talk, we will explore 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 will provide a general theoretical formulation to find the ideal condition for achieving near unity light absorption. Later, these theoretical estimations will be coupled with findings of recent studies on light perfect 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 will 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 part of the talk, we will discuss the progress, challenges, and outlook of this field to outline its future direction.

Recently, the concepts of parity–time (PT) symmetry and band topology have inspired many novel ideas for light manipulation in their respective directions. Here we propose and demonstrate a perfect light absorber with a PT phase transition via coupled topological interface states (TISs), which combines the two concepts in a one-dimensional photonic crystal heterostructure. By fine tuning the coupling between TISs, the PT phase transition is revealed by the evolution of absorption spectra in both ideal and non-ideal PT symmetry cases. Especially, in the ideal case, a perfect light absorber at an exceptional point with unidirectional invisibility is numerically obtained. In the non-ideal case, a perfect light absorber in a broken phase is experimentally realized, which verifies the possibility of tailoring non-Hermiticity by engineering the coupling. Our work paves the way for novel effects and functional devices from the exceptional point of coupled TISs, such as a unidirectional light absorber and exceptional-point sensor.

The perfect absorption of light has been traditionally achieved by layered structures whose thickness is at least λ/4n. To achieve electromagnetically thin perfect absorbers, research has focused on metallic metasurfaces which require complex nanofabrication. MoS2 is a promising material for ultra-thin perfect absorbers due to the high oscillator strength of its excitonic transitions and layered nature. However previous efforts to increase MoS2 absorption involve intricate fabrication. Our work shows enhanced absorption in monolayers, perfect absorption and exciton-polaritons in MoS2 slabs. This work simultaneously presents a simple approach to achieve perfect absorption in ultra-thin non-metallic structures and to observe cavity-free exciton-polaritons.

A quasi-transverse electric (TE) surface wave mode exists at a metal surface coated with an ultrathin high-index dielectric layer. As the coating is in dielectric stripe arrays, nearly perfect absorption of TE-polarized incidence light is observed in simulations, due to resonances of the quasi-surface waves at each segment of the dielectric-coated metal surfaces. In analysis, the Fabry-Perot-like nature of the resonances is clarified, and effects of symmetry on different behaviors of the odd- and even-order resonance modes are discussed. While the absorption peak is tunable, perfect absorption appears near cut-off wavelength of the surface mode.

A simple design of triple-band perfect light absorber (PLA) based on hybrid metasurface in visible region has been presented in this work, which turns out to be applicable for refractive index (RI) sensing. Distinct from previous designs, the proposed hybrid metasurface for visible PLA is only consisted of periodic silicon cross nanostructure arrays and gold substrate. The periodic silicon cross arrays deposited on the gold substrate contribute to excite the guided modes under the normal incident light illumination. According to the simulation results, it can be found that three perfect absorption peaks of 98.1%, 98.7%, and 99.6% which are located at 402.5 THz, 429.5 THz, and 471.5 THz, respectively, have been clearly observed in PLA. This triple-band perfect absorption effect could be attributed to the intrinsic loss of silicon material originated from the guided mode excitations caused by the standing waves of different orders. It has been confirmed that the perfect absorption properties of the PLA can be easily regulated by changing the geometric parameters of the unit-cell nanostructure. Furthermore, the designed PLA served as a RI sensor can achieve sensitivity of about 25.3, 41.3, and 31.9 THz /refractive index unit (RIU). It can be believed that the proposed design of PLA for RI sensing would provide great potential applications in sensing, detecting, the enhanced visible spectroscopy, etc.

We experimentally demonstrate a wideband near-perfect light absorber in the midwave IR region using a multiplexed plasmonic metal structure. The wideband near-perfect light absorber is made of two different size gold metal squares multiplexed on a thin dielectric spacing layer on top of a thick metal layer in each unit cell. We also fabricate regular nonmultiplexed structure perfect light absorbers. The multiplexed structure IR absorber absorbs more than 98% of the incident light over a much wider spectral band than regular nonmultiplexed structure perfect light absorbers in the midwave IR region.

We demonstrated perfect light absorption inultra-thin optical nanocavity made of a percolation aluminum film and a thin silicon film deposited on aluminum metal surface. Critical coupling to the optical nanocavity results in complete light absorption at specific wavelength, determined by the silicon film thickness. Due to light penetration into aluminum metal on top and bottom, the optical nanocavity silicon layer thickness can be one order of magnitude less than the perfect light absorption wavelength.

Optical absorbers comprised of an ultrathin lossy dielectric film on an opaque metallic substrate are an attractive alternative to lithographically intense metamaterial and nanoplasmonic optical absorbers as they allow for large‐scale, cost‐effective fabrication. However, requiring that the dielectric is lossy and the metallic substrate is highly reflective but not a perfect electric conductor (PEC) limits the wavelength range and materials that can be used to realize strong to perfect light absorption. In this work, we theoretically and experimentally investigate light absorption using ultrathin lossless dielectric films. By choosing proper lossless ultrathin dielectrics and substrates, iridescence free, perfect light absorption is possible over the visible, near infrared (NIR), and short‐wave infrared (SWIR) wavelength ranges with designer absorption properties. The proposed class of ultrathin film absorbers relaxes many constraints on the type of materials used to realize perfect light absorption. The flexibility of our design makes it relevant for many applications specifically in structural coloring, selective thermal emission, thermo‐photovoltaics, photo‐thermoelectric generation, and gas sensing.

Dual-band perfect light absorber in visible region based on cylinder silicon resonator

Understanding the fundamental physics in light absorption and perfect light absorption is vital for device applications in detector, sensor, solar energy harvesting and imaging. In this research study, a large area fabrication of Al-doped ZnO/Al2O3/graphene/Al2O3/gold/silicon device was enabled by a spin-processable hydrophilic mono-layer graphene oxide. In contrast to the optical properties of noble metals, which cannot be tuned or changed, the permittivity of transparent metal oxides, such as Al-doped ZnO and indium tin oxide, are tunable. Their optical properties can be adjusted via doping or tuned electrically through carrier accumulation and depletion, providing great advantages for designing tunable photonic devices or realizing perfect absorption. A significant shift of Raman frequency up to 360 cm-1 was observed from graphene in the fabricated device reported in this work. The absorption from the device was tunable with a negative voltage applied on the Al-doped ZnO side. The generated absorption change was sustainable when the voltage was off and erasable when a positive voltage was applied. The reflection change was explained by the Fermi level change in graphene. The sustainability of tuned optical property in graphene can lead to a design of device with less power consumption.

The need for an easy to fabricate perfect and narrowband light absorber in the visible range of electromagnetic (EM) spectrum has always been in demand for many scientific and device applications. Here, we propose a metal-dielectric-metal (MDM) 1-D grating plasmonic structure as a perfect narrow band light absorber in the visible and its application in glucose detection. The proposed structure consists of a 1- D grating of gold on the top of a dielectric layer on a gold film. Optimization for dielectric grating index (n), grating thickness (t), grating width (W), and grating period (P) has been done to improve the performance of plasmonic structure by calculating its quality factor and figure-of-merit (FOM). The optimized plasmonic structure behaves as a perfect narrowband light absorber. The flexibility to work at a specific wavelength is also offered by the proposed structure through an appropriate selection of the geometrical parameters and refractive index of the dielectric grating. The equivalent RC model is used to understand different components of the proposed structure on the optical response. The absorption response of the structure is invariant to the incident angle. Moreover, the calculated absorbance of the proposed plasmonic structure is ~ 100% with a narrow full-width half maxima (FWHM) of ~ 2.8 nm. We have numerically demonstrated a potential application of the proposed MDM absorber as a plasmonic glucose sensor in the visible range with detection sensitivity in the range of 140 to 195 nm/RIU.

We present the perfect light absorption of monolayer molybdenum disulfide (MoS2) in a dielectric multilayer system with two different Bragg mirrors. The results show that the strong absorption of visible light in monolayer MoS2 is attributed to the formation of optical Tamm states (OTSs) between two Bragg mirrors. The MoS2 absorption spectrum is dependent on the layer thickness of Bragg mirrors, incident angle of light, and the period numbers of Bragg mirrors. Especially, the nearly perfect light absorption (99.4%) of monolayer MoS2 can be achieved by choosing proper period numbers, which is well analyzed by the temporal coupled-mode theory.