Ultrathin Lossy Research Articles

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.

Low-Frequency Low-Reflection Bidirectional Sound Insulation Tunnel with Ultrathin Lossy Metasurfaces

We report both numerical and experimental constructions of a tunnel structure with low-frequency low-reflection bidirectional sound insulation (BSI). The designed tunnel was constructed from a pair of lossy acoustic metasurfaces (AMs), which consists of six ultrathin coiled unit cells, attached on both sides. Based on the generalized Snell′s law and phase modulations for both AMs, the tunnel with the low-frequency BSI was constructed based on sound reflections and acoustic blind areas created by the AMs. The obtained transmittances were almost the same for sound incidences from both sides and were lower than −10 dB in the range 337–356 Hz. The simulated and measured results agreed well with each other. Additionally, we show that the low-reflection characteristic of the tunnel can be obtained simultaneously by thermoviscous energy loss in coiled channels of the unit cells. Finally, an interesting application of the designed tunnel in an open-window structure with low-frequency low-reflection BSI is further simulated in detail. The proposed tunnel based on the ultrathin lossy AMs has the advantages of ultrathin thickness (about λ/35), low-frequency low-reflection BSI, and high-performance ventilation, which may have potential applications in architectural acoustics and noise control.

Flexible, Large‐Area Covert Polarization Display Based on Ultrathin Lossy Nanocolumns on a Metal Film

AbstractCovert polarization displays provide a barrier to the inadvertent viewing of stored optical information. For security and anti‐counterfeiting purposes, access to concealed information without compromising packaging aesthetics is required in certain situations. However, optical conversion with polarized light typically requires sophisticated nanostructures only possible with limited materials, which are not appropriate for application to objects requiring flexibility or conformability, and color selectivity. A flexible, large‐area covert polarization display based on ultrathin lossy nanocolumns with wide color selectivity is presented. Self‐aligned porous nanocolumns (PNCs) fabricated by glancing angle deposition are a facile approach to polarization distinguishable structures. PNCs deposited on metal are designed to switch color in accordance with polarization by ultrathin resonance, which is modeled using the complex effective refractive index. Several combinations of material and thickness are presented to extend color selectivity with the standard red green blue gamut and color palette. As a demonstration, covert polarization display labels are attached to daily objects with curved and wrinkled surfaces, and hidden quick response codes are revealed by polarization adjustment in indoor and outdoor environments. Moreover, a multifunctional water contact detection covert polarization display is demonstrated based on the sensitivity of PNCs to the refractive index of the analyte.

Defining Deep‐Subwavelength‐Resolution, Wide‐Color‐Gamut, and Large‐Viewing‐Angle Flexible Subtractive Colors with an Ultrathin Asymmetric Fabry–Perot Lossy Cavity

AbstractAchieving structural colors with wide color gamut, large viewing angle, and high resolution remains practically challenging. Here, proposed is an asymmetric Fabry–Perot (F–P) lossy cavity to realize subtractive colors, simultaneously featuring wide gamut, angle insensitivity, high resolution, and good flexibility. The experimental results demonstrate that the realized structural colors are insensitive to the viewing angle up to ±60°, arising from the negligible light propagation phase shift in the ultrathin lossy cavity made of an amorphous silicon (a‐Si) layer; they also exhibit a wide color gamut covering the whole cyan, magenta, and yellow (CMY) color system, simply achieved through changing the thicknesses of the top metal layer and the middle lossy a‐Si layer. The ultrathin configuration enables printing such colors at a deep‐subwavelength resolution, and the ease of experimental fabrication enables their implementation on soft substrates. Accordingly, a 2D sketched image is printed using this cavity at a resolution of 150 000 dots per inch (dpi) in the geometrical space, corresponding to a minimum color pixel of 160 nm, well beyond the diffraction limit; a flexible structural color membrane with wide color gamut and large viewing angle is well adhered on an airplane model with a complex surface profile.

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.

Tunable reflective color filters based on asymmetric Fabry-Perot cavities employing ultrathin Ge2Sb2Te5 as a broadband absorber.

We demonstrated a tunable structural color filter based on an asymmetric Fabry-Perot cavity employing germanium antimony tellurium alloy Ge2Sb2Te5 (GST) as a switchable ultrathin lossy layer. The color tunability and switch mechanism of our designed structure were investigated by both simulation and analytical approaches. Both numerical simulations and analytical results show that the tunable reflective colors can be generated through the reversible phase transition of GST from amorphous to crystalline. Additionally, the generated colors possess high brightness, high saturation, and a wide gamut. Our designed structure will inspire phase-transition-based systems' potential applications in colorimetric sensing, smart windows, full-color printing and displays, anti-counterfeiting, and data encryption.

Designer Perfect Light Absorption Using Ultrathin Lossless Dielectrics on Absorptive Substrates

AbstractOptical 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.

Colorimetric detection of ultrathin dielectrics on strong interference coatings.

Metal films covered with ultrathin lossy dielectrics can exhibit strong interference effects manifested as the broad absorption of the incident light resulting in distinct surface colors. Despite their simple bilayer structures, such surfaces have only recently been scrutinized and applied mainly to color printing. Here, we report the use of such surfaces for colorimetric detection of ultrathin dielectrics. Upon deposition of a nanometer-thick dielectric on the surface, the absorption peak red shifts, changing the surface color. The color contrast between the bare and dielectric-coated surfaces can be detected by the naked eye. The optical responses of the surfaces are characterized for nanometer-thick SiO2, Al2O3, and bovine serum albumin molecules. The results suggest that strong interference surfaces can be employed as biosensors.

Ultrathin phase-change coatings on metals for electrothermally tunable colors

Metal surfaces coated with ultrathin lossy dielectrics enable color generation through strong interferences in the visible spectrum. Using a phase-change thin film as the coating layer offers tuning the generated color by crystallization or re-amorphization. Here, we study the optical response of surfaces consisting of thin (5–40 nm) phase-changing Ge2Sb2Te5 (GST) films on metal, primarily Al, layers. A color scale ranging from yellow to red to blue that is obtained using different thicknesses of as-deposited amorphous GST layers turns dim gray upon annealing-induced crystallization of the GST. Moreover, when a relatively thick (>100 nm) and lossless dielectric film is introduced between the GST and Al layers, optical cavity modes are observed, offering a rich color gamut at the expense of the angle independent optical response. Finally, a color pixel structure is proposed for ultrahigh resolution (pixel size: 5 × 5 μm2), non-volatile displays, where the metal layer acting like a mirror is used as a heater element. The electrothermal simulations of such a pixel structure suggest that crystallization and re-amorphization of the GST layer using electrical pulses are possible for electrothermal color tuning.

Ultrathin continuous lossy metal film for broadband light absorption and hot electron collection

A broadband perfect light absorption scheme relying on planar layers of lossy metallic and semiconductor films without structure pattering for hot electron photodetection is proposed and demonstrated. The nontrivial reflection phase shift occurring in the ultrathin lossy metal film is responsible for the strong resonance effects of the metal–semiconductor–metal (M-S-M) structures. Broadband and perfect light absorption is demonstrated experimentally for the platinum (Pt)—titanium dioxide (TiO2)—aluminum (Al) absorber, in which the ultrathin (10 nm) Pt layer electrically serves as the hot electron emitter. Under the monochromatic light illumination, the proposed M-S-M diode shows a clear hot-electron-based photoresponse.

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.

Simple Closed-Form Solution to Predict Short-Range Surface Plasmons in Thin Films Bounded by Asymmetric Media and Its Application to Hole Arrays

A theoretical study of surface plasmon polaritons (SPPs) in ultrathin lossy metal films is presented. The dispersion relation of such films is well known and can be solved numerically to obtain a combination of long-range (LR-) and short-range (SR-) eigenmodes. In this contribution, a simple solution for the SR-SPPs is derived. An approximation for the LR- eigenmodes can be found elsewhere. To validate the approximation, a two dimensional (2D) periodic array of small holes is studied subsequently. The spectral response of the array is obtained by full-wave simulations and the results compared with those calculated analytically, showing an excellent agreement for small holes.

Omnidirectional, broadband light absorption using large-area, ultrathin lossy metallic film coatings.

Resonant absorbers based on nanostructured materials are promising for variety of applications including optical filters, thermophotovoltaics, thermal emitters, and hot-electron collection. One of the significant challenges for such micro/nanoscale featured medium or surface, however, is costly lithographic processes for structural patterning which restricted from industrial production of complex designs. Here, we demonstrate lithography-free, broadband, polarization-independent optical absorbers based on a three-layer ultrathin film composed of subwavelength chromium (Cr) and oxide film coatings. We have measured almost perfect absorption as high as 99.5% across the entire visible regime and beyond (400–800 nm). In addition to near-ideal absorption, our absorbers exhibit omnidirectional independence for incidence angle over ±60 degrees. Broadband absorbers introduced in this study perform better than nanostructured plasmonic absorber counterparts in terms of bandwidth, polarization and angle independence. Improvements of such “blackbody” samples based on uniform thin-film coatings is attributed to extremely low quality factor of asymmetric highly-lossy Fabry-Perot cavities. Such broadband absorber designs are ultrathin compared to carbon nanotube based black materials, and does not require lithographic processes. This demonstration redirects the broadband super absorber design to extreme simplicity, higher performance and cost effective manufacturing convenience for practical industrial production.

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.

Reduced near-infrared absorption using ultra-thin lossy metals in Fabry-Perot cavities.

We show that a triple-layer metal-insulator-metal (MIM) structure has spectrally selective IR absorption, while an ultra-thin metal film has non-selective absorption in the near infrared wavelengths. Both sub-wavelength scale structures were implemented with an ultra-thin 6 nm Cr top layer. MIM structure was demonstrated to have near perfect absorption at λ = 1.2 μm and suppressed absorption at λ = 1.8 μm in which experimental and simulated absorptions of the thin Cr film are even higher than the MIM. Occurrence of absorption peaks and dips in the MIM were explained with the electric field intensity localization as functions of both the wavelength and the position. It has been shown that the power absorption in the lossy material is a strong function of the electric field intensity i.e. the more the electric field intensity, the more the absorption and vice versa. Therefore, it is possible to engineer IR emissive properties of these ultra-thin nanocavities by controlling the electric field localization with proper designs.

Studies on dielectric relaxation and defect generation for reliability assessments in ultrathin high- k gate dielectrics on Ge

In this work, we present the results of dielectric relaxation and defect generation kinetics towards reliability assessments for Zr-based high- k gate dielectrics on p-Ge (1 0 0). Zirconium tetratert butoxide (ZTB) was used as an organometallic source for the deposition of ultra thin (∼14 nm) ZrO 2 films on p-Ge (1 0 0) substrates. It is observed that the presence of an ultra thin lossy GeO x interfacial layer between the deposited high- k film and the substrate, results in frequency dependent capacitance–voltage ( C– V) characteristics and a high interface state density (∼10 12 cm −2 eV −1). Use of nitrogen engineering to convert the lossy GeO x interfacial layer to its oxynitride is found to improve the electrical properties. Magnetic resonance studies have been performed to study the chemical nature of electrically active defects responsible for trapping and reliability concerns in high- k/Ge systems. The effect of transient response and dielectric relaxation in nitridation processes has been investigated under high voltage pulse stressing. The stress-induced trap charge density and its spatial distribution are reported. Charge trapping/detrapping of stacked layers under dynamic current stresses was studied under different fluences (−10 mA cm −2 to −50 mA cm −2). Charge trapping characteristics of MIS structures (Al/ZrO 2/GeO x /Ge and Al/ZrO 2/GeO x N y /Ge) have been investigated by applying pulsed unipolar (peak value – 10 V) stress having 50% duty-cycle square voltage wave (1 Hz –10 kHz) to the gate electrode.

TiO2/GeOxNy stacked gate dielectrics for Ge-MOSFETs

In this work, we present the results of physical and electrical characterization of Ti-based high-k gate dielectrics on Ge substrates. Titanium tetrakis iso- propoxide (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO2 films on p-Ge (1 0 0) at low temperature (<200 °C) by plasma enhanced chemical vapor deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of ∼65 Pa. The presence of an ultra-thin lossy GeO2 interfacial layer between the deposited high-k film and the substrate, results in frequency-dependent capacitance–voltage (C–V) characteristics in strong accumulation and a high interface state density (∼1013 cm−2 eV−1). To improve the electrical properties, nitrogen engineering has been employed to convert the lossy GeO2 interfacial layer to its oxynitride, thus forming TiO2/GeOxNy/Ge stacked-gate structure with improved interface/electrical properties. Different N sources, such as NO, NH3 and NO/NH3, have been used for nitrogen engineering. XPS and Raman spectroscopy analyses have been used for surface morphological study. Electrical properties, such as gate leakage current density, interface state density, charge trapping, flatband voltage shift, etc, have been studied in detail for TiO2/GeOxNy/Ge MIS capacitors using the current–voltage (I–V), capacitance–voltage (C–V), conductance–voltage (G–V) and stress (both constant voltage and current) measurements. Although a significant improvement in electrical characteristics has been observed after nitridation in general, the formation of the interfacial GeOxNy layer, obtained from NO-plasma nitridation, is found to provide the maximum improvement among all the nitridation techniques used in this study. It is shown that the insertion of an ultra-thin oxynitride (GeOxNy) interfacial layer is advantageous for producing gate-quality TiO2 high-k dielectric stacks on Ge substrates.