Browsing by Subject "Perfect absorber"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Open Access Bismuth-based metamaterials: From narrowband reflective color filter to extremely broadband near perfect absorber(De Gruyter, 2019) Ghobadi, Amir; Hajian, Hodjat; Gökbayrak, Murat; Bütün, Bayram; Özbay, EkmelIn recent years, sub-wavelength metamaterials-based light perfect absorbers have been the subject of many studies. The most frequently utilized absorber configuration is based on nanostructured plasmonic metals. However, two main drawbacks were raised for this design architecture. One is the fabrication complexity and large scale incompatibility of these nano units. The other one is the inherent limitation of these common metals which mostly operate in the visible frequency range. Recently, strong interference effects in lithography-free planar multilayer designs have been proposed as a solution for tackling these drawbacks. In this paper, we reveal the extraordinary potential of bismuth (Bi) metal in achieving light perfect absorption in a planar design through a broad wavelength regime. For this aim, we adopted a modeling approach based on the transfer matrix method (TMM) to find the ideal conditions for light perfect absorption. According to the findings of our modeling and numerical simulations, it was demonstrated that the use of Bi in the metal-insulator-metal-insulator (MIMI) configuration can simultaneously provide two distinct functionalities; a narrow near unity reflection response and an ultra-broadband near perfect absorption. The reflection behavior can be employed to realize additive color filters in the visible range, while the ultra-broadband absorption response of the design can fully harvest solar irradiation in the visible and near infrared (NIR) ranges. The findings of this paper demonstrate the extraordinary potential of Bi metal for the design of deep sub-wavelength optical devices.Item Open Access Correction to: Active tuning from narrowband to broadband absorbers using a sub-wavelength VO2 embedded layer(Springer, 2021-02-04) Osgouei, Ataollah Kalantari; Hajian, Hodjat; Khalichi, Bahram; Serebryannikov, Andriy E.; Ghobadi, Amir; Özbay, EkmelMetamaterial perfect absorbers (MPAs) with dynamic thermal tuning features are able to control the absorption performance of the resonances, providing diverse applications spanning from optical switches and filters to modulators. In this paper, we propose an MPA with diverse functionalities enabled by vanadium dioxide (VO2) embedded in a metal-dielectric plasmonic structure. For the initial design purpose, a silicon (Si) nanograting on a silver (Ag) mirror is proposed to have multiple resonant responses in the near infrared (NIR) region. Then, the insertion of a thin VO2 layer at the right position enables the design to act as an on/off switch and resonance tuner. In the insulator phase of VO2, in which the permittivity data of VO2 is similar to that of Si, a double strong resonant behavior is achieved within the NIR region. By increasing the temperature, the state of VO2 transforms from insulator to metallic so that the absorption bands turn into three distinct resonant peaks with close spectral positions. Upon this transformation, a new resonance emerges and the existing resonance features experience blue/red shifts in the spectral domain. The superposition of these peaks makes the overall absorption bandwidth broad. Although Si has a small thermo-optic coefficient, owing to strong light confinement in the ultrasmall gaps, a substantial tuning can be achieved within the Si nanogratings. Therefore, the proposed hybrid design can provide multi-resonance tunable features to cover a broad range and can be a promising strategy for the design of linearly thermal-tunable and broadband MPAs. Owing to the proposed double tuning feature, the resonance wavelengths exhibits great sensitivity to temperature, covering a broad wavelength range. Overall, the proposed design strategy demonstrates diverse functionalities enabled by the integration of a thin VO2 layer with plasmonic absorbers.Item Open Access Dielectric metasurfaces as passive radiative coolers, colorimetric refractive index sensors, color filters, and one-way perfect absorber/reflectors with transparent sidebands(Bilkent University, 2020-07) Yıldırım, Deniz UmutMetamaterials define the class of synthetic, man-made materials with exotic properties that cannot be observed with natural materials. Their sub-wavelength counterparts are called metasurfaces. In particular, dielectric metasurfaces are extensively studied due to the advantages they offer in comparison to metamaterials, which are mainly their reduced thickness and not suffering from ohmic losses that are present in metals. Here, we design and implement dielectric metasurfaces in four important application areas, namely 1. Passive radiative coolers for spacecraft, 2. Colorimetric refractive index sensors, and 3. Color filters based on monolayer graphene. 4. A metasurface with a resonant one-way absorption/reflection with transmissive sidebands functionality In the first work, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design an optical solar reflector, which constitutes the physical interface between the spacecraft and space. Our proposed metasurface is based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal-radiation to broad plasmonic resonances, and consequently an experimental emissivity of 0.968, in a very wide range from 2.5 µm to 25 µm. In the solar spectrum, low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. This design with high-throughput, robustness, low-cost and high-performance, therefore, shows great promise not only for space missions but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling. In the second work, we propose a highly-sensitive refractive index sensor, utilizing the excitation of guided-modes of a novel, 2-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guided-mode resonances with an ultra-narrow linewidth (fullwidth 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, highlysensitive, almost lossless, and compact molecular diagnostics platform in the fields of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring. In the third work, we numerically propose a graphene perfect absorber that can be utilized as a color filter, utilizing the excitation of guided-modes of a dielectric slab waveguide by a novel sub-wavelength dielectric grating structure. When the guided-mode resonance is critically coupled to the graphene, we obtain perfect absorption with an ultra-narrow bandwidth (full-width at half-maximum) of 0.8 nm. The proposed design not only preserves the spectral position of the resonance, but also maintains > 98% absorption at all polarization angles. The spectral position of the resonance can be tuned as much as 400 nm in visible and near-infrared regimes by tailoring geometrical parameters. The proposed device has great potential in efficient, tunable, ultra-sensitive, compact and easyto-fabricate advanced photodetectors and color selective notch filters. In the fourth and final work, we numerically propose the one-way perfect absorption of near-infrared (NIR) radiation in a tunable spectral range with high transmission in the neighboring spectral ranges. This functionality is obtained by using a 2-dimensional, guided-mode resonance based grating-waveguide metasurface that acts as a frequency-selective reflector, a spacer dielectric, and an absorbing oxide layer. Within the bandwidth of the excited guided-mode resonance excited at 1.82µm with a full-width at half-maximum of 19 nm), we confirmed perfect absorption when light is incident from one of the two opposite directions, whereas in the other direction, perfect reflection is observed. The forward-to-backward absorption ratio reached as high as 60, while the thickness of the entire structure is in the order of the operating wavelength. In addition to the spectral tunability of the excited resonances and their bandwidths, our proposed device supports transparency windows with 65% transmission in the adjacent frequency bands. Our 2D grating is also verified to enable near-absolute insensitivity to the polarization state of incident light. Geometrical parameter modification also gives our design great tunability, as we also designed a device with 300 nm absorption/reflection linewidth.Item Open Access Highly efficient semiconductor-based metasurface for photoelectrochemical water splitting: broadband light perfect absorption with dimensions smaller than the diffusion length(Springer, 2020) Ghobadi, Amir; Ulusoy-Ghobadi, Türkan Gamze; Karadaş, Ferdi; Özbay, EkmelIn this paper, we demonstrate a highly efficient light trapping design that is made of a metal-oxide-semiconductor-semiconductor (nanograting/nanopatch) (MOSSg/p) four-layer design to absorb light in a broad wavelength regime in dimensions smaller than the hole diffusion length of the active layer. For this aim, we first adopt a modeling approach based on the transfer matrix method (TMM) to find out the absorption bandwidth (BW) limits of a simple hematite (α-Fe2O3)-based metal-oxide-semiconductor (MOS) cavity design. Our modeling findings show that this design architecture can provide near-perfect absorption in shorter wavelengths. To extend the absorption toward longer wavelengths, a nanostructured semiconductor is placed on top of this MOS design. This nanostructure supports the Mie resonance and adds a new resonance in longer wavelengths without disrupting the lower wavelength absorption capability of MOS cavity. By this way, a polarization-insensitive absorption above 0.8 can be acquired up to λ=565 nm. Moreover, to have a better qualitative comparison, the water-splitting photocurrent of this design has been estimated. Our calculations show that a photocurrent as high as 10.6 mA cm−2 can be achieved with this design that is quite close to the theoretical limit of 12.5 mA cm−2 for hematite-based water-splitting photoanode. This paper proposes a design approach in which the superposition of cavity modes and Mie resonances can lead to a broadband, polarization-insensitive, and omnidirectional near-perfect light absorption in dimensions smaller than the carrier’s diffusion length. This can be considered as a winning strategy to design highly efficient and ultrathin optoelectronic designs in a variety of applications including photoelectrochemical water splitting and photovoltaics.Item Open Access Strong light-matter interaction in lithography-free planar metamaterial perfect absorbers(American Chemical Society, 2018) Ghobadi, Amir; Hajian, Hodjat; Bütün, Bayram; Özbay, EkmelThe 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.Item Open Access Tunable van der Waals-based metasurfaces for perfect absorption, sensing, radiative heat transfer, beam splitting and 5G/beyond applications(Bilkent University, 2022-09) Erçağlar, VeyselMetasurfaces are thin, subwavelength structures that have extraordinary properties that cannot be found naturally. Tunable metasurfaces drew attention not only for their lightweight designs but also with the tunning option, having multiple responses without complex fabrication steps for each desired response. Besides tuning the structures by their intrinsic properties, the addition of van der Waals materials which are a specific type of 2D materials, can expand their tuning flexibilities and offer a wide range of applications. Here, we propose and investigate tunable metasurfaces in the following areas: Perfect Absorption, Sensing, Radiative Heat Transfer, Beam Splitting and 5G/Beyond Applications as: 1. All-Dielectric Metamirror for thermally tunable spectrally selective absorber, 2. Metasurface Design for Phonon-Induced Transparency and Nearly Perfect Resonant Absorption, 3. Near-Field Radiative Heat Transfer in Parallel-Plate Structures, 4. Gradient Metasurfaces for Beam Splitting and Light Absorption, 5. 5G and Beyond applications of mentioned works and future outlook. In the first work, we numerically propose a temperature-tunable, ultranarrowband one-way perfect near-infrared radiation absorber with high transmission in the longer wavelength neighboring spectral range. We obtained this functionality by using a guided-mode resonance-based grating-waveguide metamirror that is comprised of silicon, a spacer dielectric, an absorbing semiconductor, and germanium. Within the ultra-narrow bandwidth of the guided-mode resonance excited at 1.16 µm with a full width at half-maximum of 3.3 nm, we confirmed perfect absorption when light is incident from one of the two opposite directions. Excitation from the opposite direction resulted in perfect reflection. The thickness of the entire structure is limited to about one third the operating wavelength. Furthermore, due to the temperature tunability of silicon and germanium the thermo-optical sensitivity was found to be approximately 0.068 nm/K. In addition to this spectral tunability, our proposed device supports transparency windows with 80% transmission in the higher wavelength ranges. Our device is highly promising in the applications of thermo-tunable modulators and obtaining single frequency near-infrared signals from broadband sources. In the second work, A bi-tunable hexagonal boron nitride (hBN)-based metasurface with bi-functional phonon-induced transparency (PIT) and nearly perfect resonant absorption features in the mid-infrared (MIR) range is proposed. The metasurface, that is composed of axially symmetric hBN rings, is separated from a uniform thin vanadium dioxide (V O2) film with a SiO2 spacing layer and is integrated with a top graphene sheet. For the insulating phase of V O2 (i-V O2), PIT with an 80% transmission contrast ratio is observed inside the reststrahlen (RS) band of hBN due to the support of hyperbolic phonon polaritons. A considerably large group delay of 9.5 ps and up to 1.8 THz RIU −1 frequency shift per refractive index unit is also achieved for the i-V O2 case. On the other hand, it is found that for the metallic phase of V O2 (m-V O2), light transmission is prohibited and nearly perfect resonant absorption peaks are appeared inside the RS band of hBN. Finally, by integrating the hBN-based metasurface into the graphene sheet on the top, a tunable PIT-like effect and nearly perfect light absorption is achieved duo to the hybridization of graphene plasmons and hBN phonons. This leads to a modulation depth as high as 87% in the transmission (i-V O2) and 62% in the absorption (m-V O2) responses. Our findings offer a tunable and bi-functional device that is practical for MIR slow-light, sensing, and thermal emission applications. In the third work, we comprehensively analyze the near-field radiative heat transfer (NFRHT) between a pair of parallel non-rotated BP flakes that occurs due to the tunneling of the coupled anisotropic surface plasmon polaritons (SPPs) supported by the flakes. It is demonstrated that the covering of the BP flakes with (hBN) films leads to the hybridization of the BP’s SPPs with the hBN’s hyperbolic phonon polaritons and to the significant enhancement of the NFRHT at the hBN’s epsilon-near-zero frequencies. It is also shown that the NFRHT in the BP/hBN parallel-plate structure can be actively switched between the ON and OFF states by changing the chemical potential of the BPs and that the NFRHT can be modified by altering the number of the BP layers. Finally, we replace hBN with α − MoO3 and explore how the NFRHT is spectrally and strongly modified in the BP/α − MoO3 parallel-plate structure. We believe that the proposed BP/polar-vdW-material parallel-plate structures can prove useful in the thermal management of optoelectronic devices. In the fourth work, we propose multifunctional gradient metasurfaces that are composed of a periodic array of binary Si microcylinders integrated with V O2 and graphene. The metasurfaces act as transmittive (reflective) beamsplitters for the dielectric (metallic) phase of V O2 with a switchable characteristic. Moreover, by integrating the metasurfaces with graphene and modifying its chemical potential, one can tune the intensity of the split beam as well as obtain nearly perfect resonant absorptions. Consequently, the proposed metasurfaces can find potential applications in THz interferometers, multiplexers, and light absorbers. Finally, the works mentioned above, as well as diverse works in the literature will be explained and discussed upon emerging technologies in the area of communication and applications for 5G and mostly beyond and future outlook.Item Open Access Visible light nearly perfect absorber: an optimum unit cell arrangement for near absolute polarization insensitivity(OSA - The Optical Society, 2017) Ghobadi, Amir; Hajian, Hodjat; Gökbayrak, Murat; Dereshgi, Sina Abedini; Toprak, Ahmet; Butun, Bayram; Özbay, EkmelIn 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. © 2017 Optical Society of America.