Lossy Film Research Articles

The Low-Attenuation Endfire Leaky-Wave State on an Optically Transparent Lossy Film

The development of high-performance optically transparent radio frequency (RF) radiators is limited by the intrinsic loss issue of transparent conductive films (TCFs). Instead of pursuing expensive endeavors to improve the TCFs’ electrical properties, this study introduces an innovative approach that leverages leaky-wave mode manipulation to mitigate the TCFs’ attenuating effect and maximize the RF radiation. Our finding reveals that the precise control of the mode confinement on glass-coated TCFs can create a low-attenuation window for leaky-wave propagation, where the total attenuation caused by TCF dissipation and wave leakage is effectively reduced. The observed low-attenuation leaky-wave state on lossy TCFs originates from the delicate balance between wave leakage and TCF dissipation, attained at a particular glass cladding thickness. By leveraging the substantially extended radiation aperture achieved under suppressed wave attenuation, this study develops an optically transparent antenna with an enhanced endfire realized gain exceeding 15 dBi and a radiation efficiency of 66%, which is validated to offer competitive transmission performance for advancing ubiquitous wireless communication and sensing applications.

High-purity and wide-angle reflective structural colors based on an all-dielectric Fabry-Pérot cavity structure.

High-purity structural colors with low fabrication cost are in demand for their commercial applications. Here, we demonstrate an all-dielectric Fabry-Pérot cavity structure consisting of four-layer lossy and lossless dielectric films alternately stacked for producing high-purity and angle-invariant reflective colors. Multiple cavity resonances function together to significantly suppress the undesired reflection with the enhanced optical absorption, leading to a distinct and saturated color with a high efficiency of ∼70%. Besides, due to the high refractive indices of constituent materials, the color appearance of the designed structure can be maintained well at ±50° incident angle for two polarization states. The excellent color performance of the proposed device together with cost-effective manufacturing convenience opens up new avenues for their large-area applications in various areas.

Scalable Superabsorbers and Color Filters Based on Earth-Abundant Materials.

Optical materials based on unconventional plasmonic metals (e.g., magnesium) have lately driven rising research interest for the quest of possibilities in nanophotonic applications. Several favorable attributes of Mg, such as earth abundancy, lightweight, biocompatibility/biodegradability, and its active reactions with water or hydrogen, have underpinned its emergence as an alternative nanophotonic material. Here, we experimentally demonstrate a thin film-based optical device composed exclusively of earth-abundant and complementary metal-oxide semiconductor (CMOS)-compatible materials (i.e., Mg, a-Si, and SiO2). The devices can exhibit a spectrally selective and tunable near-unity resonant absorption with an ultrathin a-Si absorbing layer due to the strong interference effect in this high-index and lossy film. Alternatively, they can generate diverse reflective colors by appropriate tuning of the a-Si and SiO2 layer thicknesses, including all the primary colors for RGB (red, green, blue) and CMY (cyan, magenta, yellow) color spaces. In addition, the reflective hues of the devices can be notably altered in a zero power-consumption fashion by immersing them in water due to the resulted dissolution of the Mg back-reflection layer. These compelling features in combination with the lithography-free and scalable fabrication steps may promise their adoption in various photonic applications including solar energy harvesting, optical information security, optical modulation, and filtering as well as structure reuse and recycling.

Fundamental limits for transmission modulation in VO2 metasurfaces

The interest in dynamic modulation of light by ultra-thin materials exhibiting insulator–metal phase transition, such as VO 2 , has rapidly grown due to the myriad industrial applications, including smart windows and optical limiters. However, for applications in the telecommunication spectral band, the light modulation through a thin VO 2 film is low due to the presence of strong material loss. Here, we demonstrate tailored nanostructuring of VO 2 to dramatically enhance its transmission modulation, reaching a value as high as 0.73, which is 2 times larger than the previous modulation achieved. The resulting designs, including free-topology optimization, demonstrate the fundamental limit in acquiring the desired optical performance, including achieving positive or negative transmission contrast. Our results on nanophotonic management of lossy nanostructured films open new opportunities for applications of VO 2 metasurfaces.

Design of broadband electromagnetic metamaterials absorption in double-layer resistance film

This paper designs a lossy double-layer resistance film absorption structure, which consists of two layers of resistance film and PFA medium alternately superimposed, the bottom of the Cu conductive film is used as the reflection bottom plate, and the metamaterials absorber thickness is 4.267mm. Simulation results show that an absorption rate greater than 90% is achieved in the 6.4-21.1GHz frequency interval, with a maximum absorption peak of 98.5%, a bandwidth reaching 14.6GHz, and a relative bandwidth of up to 106.2%. The wave absorption structure presents polarized insensitive absorption and has good wide-angle incident absorption characteristics. The single-layer resistance film realized periodic and multi-resonant peak narrow-band absorption, with a maximum absorption rate of 99.9%. Based on the software simulation results, the current density field distribution map and impedance characteristics, and the absorption mechanism were analysed.

Near-perfect (>99%) dual-band absorption in the visible using ultrathin semiconducting gratings.

Electromagnetic perfect absorption entails impedance-matching between two adjacent media, which is often achieved through the excitation of photonic/plasmonic resonances in structures such as metamaterials. Recently, super absorption was achieved using a simple bi-layer configuration consisting of ultrathin lossy films. These structures have drawn rising interest due to the structural simplicity and mechanical stability; however, the relatively broadband absorption and weak angular dependence can limit its versatility in many technologies. In this work, we describe an alternative structure based on an ultrathin semiconducting (Ge) grating that features a dual-band near-perfect resonant absorption (99.4%) in the visible regime. An angular-insensitive resonance is attributed to strong interference inside the ultrathin grating layer, akin to the resonance obtained with a single ultrathin planar film, while an angular-sensitive resonance shows a much narrower linewidth and results from the diffraction-induced surface mode coupling. With an appropriately designed grating period and thickness, strong coherent coupling between the two modes can give rise to an avoided-crossing in the absorption spectra. Further, the angular-insensitive resonance can be tuned separately from the angularly sensitive one, yielding a single narrow-banded absorption in the visible regime and a broadband absorption resonance that is pushed into the near-infrared (NIR). Our design creates new opportunities for ultra-thin and ultra-compact photonic devices for application in technologies including image sensing, structural color-filtering and coherent thermal light-emission.

Dependence of the coupling properties between a plasmonic antenna array and a sub-wavelength epsilon-near-zero film on structural and material parameters

The resonance properties of a plasmonic dipole antenna array depend on its geometry and the properties of its surrounding medium. The linear optical properties of an array of plasmonic dipole antennas can be modified with the inclusion of an epsilon-near-zero (ENZ) thin film. In this work, we numerically investigate the roles of the antenna dimensions, the ENZ film thickness and loss, and the separation between the antenna and the ENZ film in determining the linear optical response of the antenna–ENZ metasurface. The results show that for a sufficiently small separation, the linear optical properties of the antenna array are determined by the strong or ultrastrong coupling with the ENZ film and are only weakly dependent on the antenna geometry. We show that for metasurfaces with thick, lossy ENZ films, the lower polariton branch is not observable due to the high loss of ENZ films. Since the dependence of the upper polariton on antenna length is weak, this results in a single antenna-length-invariant resonance. However, in the presence of low-loss ENZ films, the lower polariton branch is also visible for antenna–ENZ metasurfaces with thicker ENZ films, indicating a strong coupling between the antenna array and the ENZ film. For a given antenna geometry, the coupling strength increases with increasing thickness of the ENZ film and can reach up to ∼50% of the zero-permittivity frequency of the ENZ film, indicating an ultrastrong coupling between the plasmonic antenna array and the ENZ film.

Enhancing the Purity of Reflective Structural Colors with Ultrathin Bilayer Media as Effective Ideal Absorbers

AbstractStructural colors of high purity and brightness are desired in various applications. This study presents a general strategy of selecting the appropriate material and thickness of each layer to create high‐purity reflective colors in a classic asymmetric Fabry–Pérot cavity structure based on a dielectric–absorber–dielectric–metal multilayered configuration. Guided by the derived complex refractive index of the ideal absorber layer, an effective absorbing bilayer medium consisting of two ultrathin lossy films is used to improve the color purity of reflective colors by suppressing the reflection of its complementary colors with the enhanced optical absorption. Highly pure red, green, and blue reflective colors are designed and experimentally demonstrated employing different effective bilayer absorbers. Due to the high refractive index of the dielectric material, the colored structures exhibit great angle‐robust appearance (blue and red colors are up to ±60°, and green color is up to ±45°). The generalized design principles and the proposed method of using effective bilayer absorbers open up new avenues for realizing high‐purity thin‐film structural colors with more materials selections.

Polarization-independent and high-efficiency broadband optical absorber in visible light based on nanostructured germanium arrays.

A broadband optical absorber based on the nanostructured germanium (Ge) film composed of single-sized circular nanodisk-nanohole arrays is proposed, which demonstrates high efficiency, strong polarization independence, and large viewing angle. Due to the electric and magnetic resonance absorption modes excited by the nanostructure arrays in highly lossy Ge film, the absorber obtains a high absorptivity, reaching above 90% over the full visible wavelength, and it can be maintained well at a large viewing angle over ±50°. Based on the geometrical symmetry, the absorber is proved to be polarization independent. Moreover, the simple single-sized nanostructure within a certain size tolerance decreases the design and fabrication complexity. The structural configuration with a slight Ge sidewall formed in the nanofabrication process could enhance the overall light absorption. These results indicate that the proposed broadband absorber has great potential in various applications such as anti-reflective coating and perfect cloaking.

A Rasorber-Like Waveguide Based on Thin Film

A novel rasorber-like property is generated in the new designed waveguide, which is constructed by placing an etched lossy thin film in the rectangular waveguide. Two individual transmission modes are observed in this structure in the working range of the rectangular waveguide, which provide the absorption band and passband, respectively. A sample based on indium tin oxide film is also fabricated and measured, and the results show good agreements compared with those of the 3-D full-wave simulation. This letter may extend the application of thin films in the wave propagation area, especially in the electromagnetic shielding of practical electronics.

Optical absorbers based on strong interference in ultra‐thin films (Laser Photonics Rev. 10(5)/2016)

Structures that enhance the absorption of electromagnetic waves find uses in a variety of applications, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, thermal light sources, and color coatings. Recent efforts have sought to reduce the footprint of optical absorbers by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large-scale optical and optoelectronic devices. The cover article reviews recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These absorbers have a small physical footprint and require no nanoscale patterning, making them ideal for large-area applications. On the cover is a photograph of color images lithographically printed on a glass slide, using ultra-thin layers of lossy amorphous germanium on top of gold films. The different colors are generated by germanium thickness variations of about four nanometers from one region to another, which dramatically change the absorption spectrum, and hence the color. Photograph courtesy of M. Kats and L. Liu. (Picture: Mikhail A. Kats and Federico Capasso, 10.1002/lpor.201600098 pp. 735–749, in this issue)

Optical absorbers based on strong interference in ultra‐thin films

AbstractOptical absorbers find uses in a wide array of applications across the electromagnetic spectrum, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, and thermal light sources. Recent efforts have sought to reduce the footprint of optical absorbers, conventionally based on graded structures or Fabry‐Perot‐type cavities, by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large‐scale optical and optoelectronic devices.In this article, we summarize recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These structures have a small footprint and require no nanoscale patterning. We outline the theoretical foundation of these absorbers based on “ultra‐thin‐film interference”, including the concepts of loss‐induced phase shifts and critical coupling, and then review several applications, including ultra‐thin color coatings, decorative photovoltaics, high‐efficiency photochemical cells, and infrared scene generators. image

Localized Surface Plasmon‐Enhanced Ultrathin Film Broadband Nanoporous Absorbers

Ultrathin lossy films have attracted much attention due to their strong interference persisting inside the lossy dielectric film on a reflective substrate. Here, a plasmon‐enhanced ultrathin film broadband absorber is proposed by combining the ultrathin film absorber with localized surface plasmon resonances. This concept can be realized by patterning nanoholes on an absorber comprised of an absorptive ultrathin Ge film and a reflective Au layer, where the localized surface plasmon mode is activated by metallic pore‐shaped holes. The plasmonic enhancement is resulting from the pore‐shape localized resonance mode, which increases the optical path length through scattering and concentrates the incident light field near the interface of Ge/Au. The experimental characterization results of a nanoporous ultrathin film absorber, which is fabricated with a scalable laser interference lithography approach, demonstrate its superior light absorption performance. Several materials, such as Ag, Al, and Cu, are proposed as an alternative to Au, and they can also provide plasmonic enhancement to ultrathin films. Furthermore, through an efficient way to optimize the structural dimensions of the nanoporous ultrathin film absorber, a trilayer system of TiO2/Ge/Au achieves the total solar absorptance over 89.3% with a wavelength range of 400–1100 nm.

A large scale perfect absorber and optical switch based on phase change material (Ge2Sb2Te5) thin film

In the conventional optical coating, the minimum thickness required to achieve anti-reflection should be quarter wavelength (λ/4n, where n is the refractive index). However, it was demonstrated that a lossy thin film with the thickness below quarter wavelength can also sustain the interference effect leading to a high absorption close to unity. In this paper we show that by depositing a phase change material Ge2Sb2Te5 (GST) thin film (amorphous) on a metal reflector, unity absorption is attainable. We attribute the high absorption to the interference effect within the GST thin film even though its thickness is less than a quarter wavelength. The wavelength of the absorption peak can be tuned by adjusting the GST film thickness. In addition, the performance of this perfect absorber is insensitive to the incidence angle variation. Relying on the fact that GST is a phase change material, the absorption band can be tuned by inducing the phase transition of GST. The huge difference of the reflectivity between amorphous and crystalline phase leads to a high optical contrast ratio as high as 400 at the specific wavelength, suggesting the potential in the application of optical switch and rewritable data storage.

Printable Planar Dielectric Waveguides Based on High-Permittivity Films

Planar microwave components are very desirable for many applications and are mainly realized by metallic structures. In this work, a planar dielectric waveguide is proposed. A high-permittivity micrometer-size thick film is patterned on a low-loss microwave substrate and is considered as a nearly perfect magnetic wall to realize the waveguide structure. The permittivity contrast between the film and substrate is kept high (e.g., > 30) in order to minimize the leakage power, and consequently, the insertion loss. The waveguide structure is theoretically analyzed using an approximation method and the results are confirmed by a simulation-based numerical method. The impact of dielectric loss of the film in waveguide performance is much less than that of the substrate, allowing waveguides with films having a medium or high loss (e.g., ε <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> = 3) result in a low insertion loss (e.g., up to 0.04 dB/mm at 40 GHz). Several prototypes are fabricated by screen-printing of barium-strontium-titanate (BST) pastes and characterized at 40 GHz. Using the extremely lossy BST film (tanδ = 0.2, ε <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">''</sup> = 70), the insertion loss of the waveguide is measured to be 0.18 dB/mm. Based on the current technologies, the printable planar dielectric waveguide can achieve 0.08 dB/mm in insertion loss. Further improvements are expected in the future as the materials and fabrication technologies progress.

Tuning Optical Absorption in an Ultrathin Lossy Film by Use of a Metallic Metamaterial Mirror

We present a way to tune the spectral light absorption properties of an ultrathin planar superabsorbing semiconductor film placed on a metallic metamaterial mirror. It is shown that we can change the wavelength of maximum absorption in a lossy film with a fixed thickness by tailoring the filling ratio of the metallic metamaterial. This remarkable ability of tuning originates from the control of the reflection phase pickup of light impinging from the lossy film upon the metallic metamaterial mirror. Full-field electromagnetic simulations show good agreement with the theoretical predictions. The proposed approach allows for a convenient and highly controllable way to achieve spatially variant tuning of absorption in a semiconductor film.

Condition for unity absorption in an ultrathin and highly lossy film in a Gires–Tournois interferometer configuration

We present a condition for unity absorption for a Gires-Tournois interferometer configuration constructed from an ultrathin and highly lossy film on top of metallic mirror. From the impedance matching condition in the transmission line theory, we identify a solution space for the required complex refractive index of the lossy film in various film thickness and dielectric constants of the metallic mirror. It is shown that strong absorption requires the imaginary part of the refractive index of the ultrathin lossy film be larger than 0.64, and the physical origin of this condition is elucidated. The proposed method is useful in identifying candidate semiconductor materials that can be used as the lossy film in a unity-absorption Gires-Tournois interferometer configuration and designing the thickness of this film to maximize absorption.

Optimal Design of Broadband Radar Absorbing Sandwich Structure with Circuit Analog Absorber Core

Circuit analog (CA) absorbers have advantages of broadband radar absorption and feasibility in realizing the design blueprint. In this paper, a kind of radar absorbing sandwich structure (RASS) with rigid CA absorber core is proposed, which is capable of broadband absorption as well as load bearing. The numerical approach to predict the broadband reflectivity of CA absorber is introduced into an adaptive genetic algorithm (AGA) so as to derive the optimal RASS design within a certain frequency range. Furthermore, mechanical constraints including facesheet yielding, core shearing, and facesheet wrinkling, are incorporated into the optimization procedure to derive the optimal design of a one-side clamped RASS panel subjected to uniformly distributed loads. The results indicate that, when the layer number of lossy films is fixed, the radar absorbing performance (RAP) is not improved monotonically with the structure thickness, and by the present optimization procedure, an optimal structure thickness together with position, sheet resistance and geometry of the lossy films can be expected for such kind of RASS.

Wideband Omnidirectional Polarization‐Insensitive Light Absorbers Made with 1D Silicon Gratings

Photonic resonance in lossy films with 1D periodicity is generally associated with narrow spectral, angular, and polarization‐selective absorption. Contrarily, in this paper a 1D amorphous Si grating structure is demonstrated that efficiently absorbs fully hemispherical unpolarized light in the entire visible spectral domain. The absorber provides a broad spectral continuum of densely populated resonant photonic states as well as a cooperating wide‐angular antireflection effect, resulting in the observed broadband, omnidirectional, and polarization‐insensitive light absorption. The absorber performance is experimentally verified with precise fabrication and conical input beam spectral analysis. The absorber's fundamental mechanisms, explained in detail, are applicable to other semiconducting materials including GaAs, Ge, and SiC. These materials provide the absorber design with potential impact on practical device applications including near/mid/long infrared detectors and stray background rejection surfaces for imaging and spectroscopy.

Trapping mid-infrared rays in a lossy film with the Berreman mode, epsilon near zero mode, and magnetic polaritons

Triple mechanisms were employed to trap mid-infrared (mid-IR) rays within a semi-transparent SiO(2) film sandwiched between gold gratings and a gold substrate. Dimensions of four absorbers were explicitly determined using an LC (inductor-capacitor) circuit model considering the role transition of SiO(2) film. The film behaves as a capacitance and an inductance when the real part of relative electric permittivity for SiO(2) is positive and negative, respectively. At the normal incidence of transverse magnetic waves, every absorptance spectrum of absorbers showed a peak at wavelength λ = 10 μm due to the first mode excitation of magnetic polaritons (MP). At oblique incidence, the Berreman mode led to another peak at λ = 8 μm while its bandwidth was expanded with epsilon near zero mode excited by diffracted waves. The full-width-at-half-maximum of both peaks exceeded 0.6 μm thanks to the SiO(2) loss. Other minor absorptance peaks in the mid-IR were caused by variants of the same MP mode.