Near-unity Absorption Research Articles

Hybrid Metasurface Based Tunable Near-Perfect Absorber and Plasmonic Sensor.

We propose a hybrid metasurface-based perfect absorber which shows the near-unity absorbance and facilities to work as a refractive index sensor. We have used the gold mirror to prevent the transmission and used the amorphous silicon (a-Si) nanodisk arrays on top of the gold mirror which helps to excite the surface plasmon by scattering light through it at the normal incident. We numerically investigated the guiding performance. The proposed absorber is polarization independent and shows a maximum absorption of 99.8% at a 932 nm wavelength in the air medium. Considering the real applications, by varying the environments refractive indices from 1.33 to 1.41, the proposed absorber can maintain absorption at more than 99.7%, with a red shift of the resonant wavelength. Due to impedance matching of the electric and magnetic dipoles, the proposed absorber shows near-unity absorbance over the refractive indices range of 1.33 to 1.41, with a zero-reflectance property at a certain wavelength. This feature could be utilized as a plasmonic sensor in detecting the refractive index of the surrounding medium. The proposed plasmonic sensor shows an average sensitivity of 325 nm/RIU and a maximum sensitivity of 350 nm/RIU over the sensing range of 1.33 to 1.41. The proposed metadevice possesses potential applications in solar photovoltaic and photodetectors, as well as in organic and bio-chemical detection.

Titanium resonators based ultra-broadband perfect light absorber

We propose a new approach for achieving ultra-broadband perfect absorber by using titanium (Ti) resonators. Near-unity absorption with a high average absorptivity of 91.4% is achieved through the whole spectral range from 0.4 μm to 2 μm for the double-period lattice of Ti resonators array on the top of a silica layer coated on the opaque Ti substrate. In contrast to the common metal-insulator-metal (MIM) absorbers only with tens of nanometers absorption bandwidth, the Ti based ultra-broadband perfect absorber (T-UPA) can show a perfect absorption window (i.e., the absorptivity exceeding 90%) with the spectral bandwidth over 1007 nm from visible to near-infrared region (i.e., from 0.485 μm to 1.492 μm). The cooperative effects of the propagating surface plasmons and the corresponding localized plasmonic resonances of Ti resonators contribute to the broadband absorption response. Moreover, the ultra-broadband perfect absorption is polarization-independent and angle-insensitive, which hold the proposed T-UPA with strong practicality and high tolerance in the complex electromagnetic environment (i.e. sun radiation). Furthermore, the absorption window for this T-UPA can be greatly broadened via using a thin anti-reflection coating film, which shows a perfect absorption with the bandwidth of 1555 nm in the visible and near-infrared range. The numerically demonstrated thin-film absorber configuration facilitates the scalability to optoelectronics applications such as thermal photovoltaics and hot-electron devices.

Multispectral perfect absorbers using plasmonically induced interference

Through numerical and theoretical investigations of plasmonically induced interference, we obtain multispectral metamaterial absorbers based on a periodic sub-wavelength array of nanogrooves side-coupled to nanorings. Finite-difference time-domain simulations indicate that a classical three-level system forms upon introducing a dark-mode nanoring into the narrow-band perfect absorber consisting of a periodic nanogroove array, thereby leading to dual-band near-unity absorption. Numerical results are consistent with the predictions of coupled-mode theory. Slow light occurs, accompanied by extensive destructive interference in the reflection window. In particular, multispectral absorption occurs only upon introducing multiple sub-dark modes, and the absorber exhibits outstanding absorption stability over a wide range of incident angles. Thus, plasmonically induced interference may offer a new way to obtain multiband absorbers. These results should be useful for multichannel optical filtering and slow-light devices.

Experimental realization of Mie-resonance terahertz absorber by self-assembly method.

Mie-resonance terahertz absorbers by self-assembly method are designed and demonstrated in experiments and simulations. A monolayer of zirconium dioxide (ZrO2) microspheres fixed on a copper film with designed grids that were manufactured by direct writing with a composite ink system composed of polydimethylsiloxane (PDMS). More importantly, different spacing and array configurations were created economically and efficiently, showing visual performance. Magnetic resonance leads to near-unity absorption at about 0.4 THz in the samples. This work demonstrates efficient terahertz absorbers and highlights a novel direct writing fabrication method that can be extended to produce other optical devices for applications.

A Dual-Band Tunable Metamaterial Near-Unity Absorber Composed of Periodic Cross and Disk Graphene Arrays

A three-dimensional dual-band perfect absorber in the THz range (3–10 THz) is theoretically investigated. The absorber is composed of periodically placed cross- and disk-shaped graphene arrays on the top of a gold layer separated by a thick SiO $_{2}$ dielectric spacer. The simulated results show two absorption peaks with near-unity absorbance ( $99.923\%$ and $99.601\%$ ) at wavelengths 43.747 $\mu$ m and 69.94 $\mu$ m, respectively. Also, the absorber is insensitive to the polarization of the incident light and has a good tolerance to the incident angle whether it is TE or TM waves. Moreover, the peak wavelengths of the absorber can be flexibly modulated by varying the Fermi level $\mu _{C}$ of graphene while no need to refabricate the structure. This paper provides a new perspective for the design of graphene-based tunable multiband perfect THz absorbers, but not limited to THz absorbers, and may also be used in other THz graphene-based photonic devices.

Broadband and wide angle near-unity absorption in graphene-insulator-metal thin film stacks

Broadband unity absorption in graphene-insulator-metal (GIM) structures is demonstrated in the visible (VIS) and near-infrared (NIR) spectra. The spectral characteristics possess broadband absorption peaks, by simply choosing a stack of GIM, while no nanofabrication steps and patterning are required, and thus can be easily fabricated to cover a large area. The electromagnetic (EM) waves can be entirely trapped and the absorption can be greatly enhanced are verified with the finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA) methods. The position and the number of the absorption peak can be totally controlled by adjusting the thickness of the insulator layer. The proposed absorber maintains high absorption (above 90%) for both transverse electric (TE) and transverse magnetic (TM) polarizations, and for angles of incidence up to 80°. This work opens up a promising approach to realize perfect absorption (PA) with ultra-thin film, which could implicate many potential applications in optical detection and optoelectronic devices.

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.

Characterizations of an infrared polarization-insensitive metamaterial perfect absorber and its potential in sensing applications

An increasing interest has been paid for metamaterial perfect absorbers, which offer attractive platforms for electromagnetic radiation based sensing applications. In this paper, we systematically characterize an infrared polarization-insensitive metamaterial absorbers by means of finite integration simulations and spectroscopic experiments. The metamaterial absorber is composed of symmetric disk-type metal-dielectric-metal structure, which shows a near-unity absorption peak at about 70 THz. It is found that the absorption frequency can be well predicted using the magnetic resonance theory while the dielectric-thickness dependent absorptivity can only be explained by the destructive interference theory. Spectral analysis reveals the possibility of using the proposed absorber as a high-performance refractive-index and thickness sensor for novel sensitive IR inspection technologies.

Anti-Hermitian photodetector facilitating efficient subwavelength photon sorting

The ability to split an incident light beam into separate wavelength bands is central to a diverse set of optical applications, including imaging, biosensing, communication, photocatalysis, and photovoltaics. Entirely new opportunities are currently emerging with the recently demonstrated possibility to spectrally split light at a subwavelength scale with optical antennas. Unfortunately, such small structures offer limited spectral control and are hard to exploit in optoelectronic devices. Here, we overcome both challenges and demonstrate how within a single-layer metafilm one can laterally sort photons of different wavelengths below the free-space diffraction limit and extract a useful photocurrent. This chipscale demonstration of anti-Hermitian coupling between resonant photodetector elements also facilitates near-unity photon-sorting efficiencies, near-unity absorption, and a narrow spectral response (∼ 30 nm) for the different wavelength channels. This work opens up entirely new design paradigms for image sensors and energy harvesting systems in which the active elements both sort and detect photons.

Ultrathin Semiconductor Superabsorbers from the Visible to the Near-Infrared.

The design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong-but narrowband-or broader-but moderate-light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay between Brewster modes, sustained by judiciously nanostructured thin semiconductors on metal films, and photonic crystal modes. Broadband near-unity absorption in Ge ultrathin films is demonstrated, which extends from the visible to the Ge bandgap in the near-infrared and is robust against angle of incidence variation. The strategy follows an easy and scalable fabrication route enabled by soft nanoimprinting lithography, a technique that allows seamless integration in many optoelectronic fabrication procedures.

Titanium nitride nano-disk arrays-based metasurface as a perfect absorber in the visible range

An ultrathin metasurface-based absorber consisting of titanium nitride (TiN) nano-disk arrays–dielectric layer-TiN substrate is proposed in this paper. The absorber exhibits near-unity absorption in the whole visible range of 380–780 nm. Our results demonstrate that the proposed metasurface-based absorber is not only independent of light polarization, but also exhibits angle-independent absorption behavior for oblique incidence up to 70[Formula: see text]. The high absorption performance of the TiN nano-disk arrays-based absorber can attribute to two different loss mechanisms associated with the intrinsic loss and plasmonic resonance.

Approaching total absorption of graphene strips using a c-Si subwavelength periodic membrane

Approaching total absorption of graphene strips at near infrared using a crystalline-silicon (c-Si) subwavelength periodic membrane (SPM) is presented. The absorption in graphene strips in a c-Si SPM is enhanced by a resonant tip, which is resulted from the coupling between the guided mode and the radiation mode through symmetry breaking of the structure at near-normal incidence. The enhancement of the electric field intensity is increased 1939 times and the group velocity of light is decreased to 3.55×10−4c at resonance, and 99.3% absorption in graphene strips can be achieved by critical coupling at the incident angle of 2°. High absorption of the graphene strips can be maintained as the etching thickness, the strip width, and the period are altered. When this type of c-Si SPM with graphene strips is used in refractive index sensors, it shows excellent sensing properties due to its stable near-unity absorption.

Efficient Broadband Terahertz Radiation Detectors Based on Bolometers with a Thin Metal Absorber

The matrix method has been used to calculate the coefficients of absorption of terahertz radiation in conventional (with radiation incident from vacuum adjacent to the bolometer) and inverted (with radiation incident from the substrate on which the bolometer was fabricated) bolometric structures. Near-unity absorption coefficients were obtained when an additional cavity in the form of a gap between the bolometer and the input or output window was introduced. Conventional bolometers then became narrowband, while inverted-type devices remained broadband.

Mid-infrared superabsorbers based on quasi-periodic moiré metasurfaces

We report on mid-infrared superabsorbers based on quasi-periodic moire metasurfaces in metal-insulator-metal form. By varying the spacer thickness, moire rotation angle, and filling factor of the superabsorbers, we can tune narrowband or broadband absorption in a systematic way. With their high tunability of near-unity absorption and simple fabrication, in combination with decoupled mode theory for an efficient design, moire superabsorbers are well-suited for a wide range of applications in sensing, imaging, and communication.

Metal Alloys for Superabsorption: Lithography‐Free, Omnidirectional, CMOS‐Compatible AlCu Alloys for Thin‐Film Superabsorbers (Advanced Optical Materials 2/2018)

Complementary metal–oxide–semiconductor (CMOS)-compatible metal alloys formed by earth-abundant materials can enable near-unity absorption (>99%) in the visible to near-infrared spectral region. In article number 1700830, Marina S. Leite and co-workers demonstrate that Si/AlCu produces multi-wavelength, omnidirectional superabsorption in a thin-film configuration, outperforming its pure metal constituents and other metals, such as Ag and Au. Metal alloys can be potentially implemented as active elements in photonic devices.

Triple-band tunable perfect terahertz metamaterial absorber with liquid crystal

We report a compact triple-band tunable perfect terahertz metamaterial absorber (TMA) at the subwavelength scale of thickness, which is composed of a planar metallic disk resonator array above a conductive ground plane separated with liquid crystal (LC) mixture. The calculations of terahertz absorption spectra demonstrate triple near-unity absorption bands in the gap plasmonic resonance coupling regime. Three resonance frequencies of the absorber exhibit continuous linear-tunability as changing the refractive index of LC. Remarkably, each peak absorbance of the triple bands maintains at a level of beyond 99% in the whole tuning operation, and the absorbance can remain more than 90% over a wide range of incident angles. Our work suggests that the LC tunable absorber scheme has the potential to overcome the basic difficulty to perform simultaneously multiband spectral tuning and near-unity absorbance with wide angle of incidence and weak polarization dependence. The proposed LC-tunable multiband perfect TMA is promising in the application of biomolecular spectra-selective terahertz imaging and sensing.

Multi-band metamaterial absorber with arbitrary polarization and wide-incident angle

We propose a multi-band metamaterial perfect absorber with a deep-subwavelength thickness and near-unity absorption coefficient at microwave frequencies. This planar metamaterial is composed of periodically arranged concentric metallic rings and a continuous metallic ground plate separated by one or two lossy dielectric layers. The origin of the induced multi-band perfect absorption is attributed to the multiple impedance-matching to free space and can be achieved through tuning the effective constitutive relations by interactions of several electromagnetic resonances. Numerical simulations reveal the electric field and surface current distributions at each absorption peak, and further analysis indicates that the absorption mainly arises from the dielectric loss. Moreover, the absorption could be enhanced and broaden by overlapping the modes of different rings. The absorption is identical for arbitrary polarizations due to the rotational symmetry of the subwavelength unit. It also remains over 90% energy absorption for a wide angle of incidence up to 45 $$^\circ$$ , which is confirmed by the experiment. The designed multi-band absorber is also easy to manufacture by the mature printed circuit board technology, which is favorable for a wide variety of applications.

Multi-mode absorption in multi-cavity photonic crystal with two graphene monolayers

Multi-mode absorption is achieved theoretically at terahertz frequencies by using the one-dimensional photonic crystal (PC) incorporating multiple Fabry-Perot resonance cavities. To simulate this multilayered structure, the method of rigorous coupled-wave analysis is adopted. Results show that for single-cavity PC with two graphene monolayers in this cavity, there are two near-unity absorption modes with low Q-factors. By cascading one empty cavity without graphene behind, an extra high-Q mode is induced in the middle. The number of modes is controlled directly by changing the positions of graphene sheets. Also, these modes are allowed to tune separately and be shifted by many manners. After another empty cavity is cascaded, two modes with higher Q-factors replace the middle one and then four near-unity absorption modes arise. Thus, the multi-cavity PC with a small number of graphene monolayers has the ability to gain multiple strong absorption modes.

Wide-angle infrared metamaterial absorber with near-unity absorbance

A wide-angle infrared perfect metamaterial absorber is experimentally verified. The perfect metamaterial absorber shows polarization-independent at normal incidence and displays high absorption rate at a large angle of incidence. The absorption property of the proposed metamaterial absorber is sensitive to the change of refractive index of environmental mediums. An absorption sensor scheme is proposed by combining the concept of the perfect metamaterial absorber and the variation of the refractive index of the environmental medium. Measured results indicate that the proposed absorption sensor scheme achieves high FOM values with different surrounding mediums (air, water, and glucose solution).

Hybrid metal-semiconductor cavities for multi-band perfect light absorbers and excellent electric conducting interfaces

It is desirable for optoelectronic devices to have the capability to simultaneously achieve excellent optical and electric features. Nevertheless, most investigations were performed separately for photon and electron management. In this work, we numerically propose and demonstrate a hybrid metal-semiconductor interface, which offers multi-band perfect light absorption and simultaneously retains the naturally perfect electrical conductivity of a flat metal film. Multi-band anti-reflection and near-unity light absorption is observed in this hybrid metal-semiconductor cavity based absorber (HMSA). Our results show that, the maximal absorption above 97% and the naturally perfect electric conductivity are realizable, suggesting the capability of providing both excellent optical and electric properties. Optical Mie-like resonances in the semiconductor cavities and the hybrid coupling with plasmonic resonances by the metal resonators cooperatively support strong optical field confinement effects, which eventually create the light trapped in the HMSA. These features indicate a platform wherein excellent electrical conducting and multispectral light absorption are designed for potential optoelectronic applications.