Low-Pressure Deuterium Storage on Palladium-Coated Titanium Nanofilms: A Versatile Model System for Tritium-Based Betavoltaic Battery Applications.

Deuterium (D2(g)) storage of Pd-coated Ti ultra-thin films at relatively low pressures is fine-tuned by systematically controlling the thicknesses of the catalytic Pd overlayer, underlying Ti ultra-thin film domain, D2(g) pressure (PD2), duration of D2(g) exposure, and the thin film temperature. Structural properties of the Ti/Pd nanofilms are investigated via XRD, XPS, AFM, SEM, and TPD to explore new structure-functionality relationships. Ti/Pd thin film systems are deuterated to obtain a D/Ti ratio of up to 1.53 forming crystallographically ordered titanium deuteride (TiDx) phases with strong Tix+-Dy- electronic interactions and high thermal stability, where >90% of the stored D resides in the Ti component, thermally desorbing at >460 °C in the form of D2(g). Electronic interaction between Pd and D is weak, yielding metallic (Pd0) states where D storage occurs mostly on the Pd film surface (i.e., without forming ordered bulk PdDx phases) leading to the thermal desorption of primarily DOH(g) and D2O(g) at <265 °C. D-storage typically increases with increasing Ti film thickness, PD2, T, and t, whereas D-storage is found to be sensitive to the thickness and the surface roughness of the catalytic Pd overlayer. Optimum Pd film thickness is determined to be 10 nm providing sufficient surface coverage for adequate wetting of the underlying Ti film while offering an appropriate number of surface defects (roughness) for D immobilization and a relatively short transport pathlength for efficient D diffusion from Pd to Ti. The currently used D-storage optimization strategy is also extended to a realistic tritium-based betavoltaic battery (BVB) device producing promising ß-particle emission yields of 164 mCi/cm2, an open circuit potential (VOC) of 2.04 V, and a short circuit current (ISC) of 7.2 nA.

Bismuth plasmonics for extraordinary light absorption in deep sub-wavelength geometries.

In this Letter, we demonstrate an ultra-broadband metamaterial absorber of unrivaled bandwidth (BW) using extraordinary optical response of bismuth (Bi), which is the material selected through our novel analysis. Based on our theoretical model, we investigate the maximum metal-insulator-metal (MIM) cavity BW, achievable by any metal with known n-k data. We show that an ideal metal in such structures should have a positive real permittivity part in the near-infrared (NIR) regime. Contrary to noble and lossy metals utilized by most research groups in the field, this requirement is satisfied only by Bi, whose data greatly adhere to the ideal material properties predicted by our analysis. A Bi nanodisc-based MIM resonator with an absorption above 0.9 in an ultra-broadband range of 800 nm-2390 nm is designed, fabricated, and characterized. To the best of our knowledge, this is the broadest absorption BW reported for a MIM cavity in the NIR with its upper-to-lower absorption edge ratio exceeding best contenders by more than 150%. According to the findings in this Letter, the use of proper materials and dimensions will lead to realization of deep sub-wavelength efficient optical devices.

Lithography-free, manganese-based ultrabroadband absorption through annealing-based deformation of thin layers into metal-air composites.

Fabrication, characterization, and analysis of an ultrabroadband lithography-free absorber is presented. An over 94% average absorption is experimentally achieved in the wavelength range of 450-1400 nm. This ultrabroadband absorption is obtained by a simple annealed trilayer metal-insulator-metal (MIM) configuration. The metal used in the structure is manganese (Mn), which also makes the structure cost-effective. It is shown that the structure retains its high absorption for TM polarization, up to 70 deg, and, for TE polarization, up to 50 deg. Moreover, the physical mechanism behind this broadband absorption is explained. Being both lithography-free and cost-effective, the structure is a perfect candidate for large-area and mass production purposes.

Lithography-Free Planar Band-Pass Reflective Color Filter Using A Series Connection of Cavities.

In this article, a lithography-free multilayer based color filter is realized using a proper series connection of two cavities that shows relatively high efficiency, high color purity, and a wide view angle. The proposed structure is a metal-insulator-metal-insulator-semiconductor (MIMIS) design. To optimize the device performance, at the first step, transfer matrix method (TMM) modeling is utilized to find the right choices of materials for each layer. Simulations are carried out later on to optimize the geometries of the layers to obtain our desired colors. Finally, the optimized devices are fabricated and experimentally characterized to evaluate our modelling findings. The characterization results of the fabricated samples prove the successful formation of efficient and wide view angle color filters. Unlike previously reported FP based designs that act as a band-stop filter in reflection mode (absorbing a narrow frequency range and reflecting the rest of the spectrum), this design generates a specific color by reflecting a narrow spectral range and absorbing the rest of the spectrum. The findings of this work can be extended to other multilayer structures where an efficient connection of cavities in a tandem scheme can propose functionalities that cannot be realized with conventional FP resonators.

Tailoring far-infrared surface plasmon polaritons of a single-layer graphene using plasmon-phonon hybridization in graphene-LiF heterostructures.

Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit (FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5-9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found.

Tunable, omnidirectional, and nearly perfect resonant absorptions by a graphene-hBN-based hole array metamaterial.

In this paper, we propose an electrically tunable mid-infrared plasmonic-phononic absorber with omnidirectional and polarization insensitive nearly perfect resonant absorption characteristics. The absorber consists of a graphene/hexagonal boron nitride (hBN)/graphene multilayer on top of a gold bottom reflector separated by a dielectric spacer. The graphene/hBN/graphene multilayer is patterned as a hole array in square lattice. We analytically and numerically prove that, due to the support of hybrid plasmon-phonon-polaritons, nearly perfect multi-resonant absorption peaks with high quality factors are obtained both inside and outside of the Reststrahlen band of hBN. As a result of the hybridization of graphene plasmons with the hyperbolic phonon polaritons of hBN, the high quality resonant absorptions of the metamaterial are almost unaffected by decreasing the phenomenological electron relaxation time of graphene. Moreover, the obtained resonances can be effectively tuned in practice due to the continuity of the graphene layers in the hole array metamaterial. These features make the graphene-hBN metamaterial a skeptical design for practical purposes and mid-infrared multi-functional operations such as sensing.

Large-Area, Cost-Effective, Ultra-Broadband Perfect Absorber Utilizing Manganese in Metal-Insulator-Metal Structure.

Achieving broadband absorption has been a topic of intensive research over the last decade. However, the costly and time consuming stage of lithography has always been a barrier for the large-area and mass production of absorbers. In this work, we designed, fabricated, and characterized a lithography-free, large-area compatible, omni-directional, ultra-broadband absorber that consists of the simplest geometrical configuration for absorbers: Metal-Insulator-Metal (MIM). We introduced and utilized Manganese (Mn) for the first time as a very promising metal for broadband absorption applications. We optimized the structure step-by-step and compared Mn against the other best candidates introduced so far in broadband absorption structures and showed the better performance of Mn compared to them. It also has the advantage of being cheaper compared to metals like gold that has been utilized in many patterned broadband absorbers. We also presented the circuit model of the structure. We experimentally achieved over 94 percent average absorption in the range of 400-900 nm (visible and above) and we obtained absorption as high as 99.6 percent at the wavelength of 626.4 nm. We also experimentally demonstrated that this structure retains broadband absorption for large angles up to 70 degrees.

Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals.

In this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 µm to 7.3 µm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design.

Ultra-broadband Asymmetric Light Transmission and Absorption Through The Use of Metal Free Multilayer Capped Dielectric Microsphere Resonator.

In this paper, we propose a simple design with an excellent performance to obtain high contrast in transmission asymmetry based on dielectric microspheres. Initially, we scrutinize the impact of the sphere radius on forward and backward transmissions. Afterward, by introducing a capping layer on top of the sphere, transmission response for the backward illuminated light will be blocked. In the next step, in order to replace the reflecting metal cap with a metal free absorbing design, we adopt a modeling approach based on the transfer matrix method (TMM) to explore an ideal material to achieve metal free perfect absorption in a multilayer configuration of material-insulator-material-insulator (MIMI). As a result of our investigations, it is found that Titanium Nitride (TiN) is an excellent alternative to replace metal in a MIMI multilayer stack. Setting this stack as the top capping coating, we obtain a high contrast between forward and backward light transmission where in an ultra-broadband range of 400 nm-1000 nm, forward transmission is above 0.85 while its backward response stays below 0.2. Moreover, due to the existence of multilayer stack, a high asymmetry is also observed for absorption profiles. This design has a relatively simple and large scale compatible fabrication route.

Ultra-Broadband, Lithography-Free, and Large-Scale Compatible Perfect Absorbers: The Optimum Choice of Metal layers in Metal-Insulator Multilayer Stacks.

We report ultra-broadband perfect absorbers for visible and near-infrared applications that are based on multilayers of metal-insulator (MI) stacks fabricated employing straightforward layer deposition techniques and are, therefore, lithography-free and large-scale compatible. We scrutinize the impact of different physical parameters of an MIMI absorber structure with analysis of each contributing metal layer. After obtaining the optimal design parameters (i.e. material selection and their thicknesses) with both simulation and numerical analysis (Transfer Matrix Method) methods, an experimental sample is fabricated and characterized. Our fabricated MIMI absorber consists of an optically thick tungsten (W) back reflector layer followed by 80 nm aluminum oxide (Al2O3), 10 nm titanium (Ti), and finally another 80 nm Al2O3. The experimental results demonstrate over 90 percent absorption between 400 nm and 1640 nm wavelengths that is optimized for ultra-broadband absorption in MIMI structures. Moreover, the impedance matching method with free-space is used to shed light on the metallic layer selection process.

Disordered Nanohole Patterns in Metal-Insulator Multilayer for Ultra-broadband Light Absorption: Atomic Layer Deposition for Lithography Free Highly repeatable Large Scale Multilayer Growth.

In this paper, we demonstrate a facile, lithography free, and large scale compatible fabrication route to synthesize an ultra-broadband wide angle perfect absorber based on metal-insulator-metal-insulator (MIMI) stack design. We first conduct a simulation and theoretical modeling approach to study the impact of different geometries in overall stack absorption. Then, a Pt-Al2O3 multilayer is fabricated using a single atomic layer deposition (ALD) step that offers high repeatability and simplicity in the fabrication step. In the best case, we get an absorption bandwidth (BW) of 600 nm covering a range of 400 nm-1000 nm. A substantial improvement in the absorption BW is attained by incorporating a plasmonic design into the middle Pt layer. Our characterization results demonstrate that the best configuration can have absorption over 0.9 covering a wavelength span of 400 nm-1490 nm with a BW that is 1.8 times broader compared to that of planar design. On the other side, the proposed structure retains its absorption high at angles as wide as 70°. The results presented here can serve as a beacon for future performance enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity.

Visible light nearly perfect absorber: an optimum unit cell arrangement for near absolute polarization insensitivity.

In 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.

97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold.

In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO2) nanowires. It is found that the randomly oriented dense TiO2 nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO2 random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.

Ultra-broadband, wide angle absorber utilizing metal insulator multilayers stack with a multi-thickness metal surface texture.

In this paper, we propose a facile route to fabricate a metal insulator multilayer stack to obtain ultra-broadband, wide angle behavior from the structure. The absorber, which covers near infrared (NIR) and visible (Vis) ranges, consists of a metal-insulator-metal-insulator (MIMI) multilayer where the middle metal layer has a variant thickness. It is found that this non-uniform thickness of the metal provides us with an absorption that is much broader compared to planar architecture. In the non-uniform case, each thickness is responsible for a specific wavelength range where the overall absorption is the superposition of these resonant responses and consequently a broad, perfect light absorption is attained. We first numerically examine the impact of different geometries on the overall light absorption property of the multilayer design. Afterward, we fabricate the designs and characterize them to experimentally verify our numerical findings. Characterizations show a good agreement with numerical results where the optimum absorption bandwidth for planar design is found to be 620 nm (380 nm-1000 nm) and it is significantly boosted to an amount of 1060 nm (350 nm-1410 nm) for multi-thickness case.

Planar Indium Tin Oxide Heater for Improved Thermal Distribution for Metal Oxide Micromachined Gas Sensors.

Metal oxide gas sensors with integrated micro-hotplate structures are widely used in the industry and they are still being investigated and developed. Metal oxide gas sensors have the advantage of being sensitive to a wide range of organic and inorganic volatile compounds, although they lack selectivity. To introduce selectivity, the operating temperature of a single sensor is swept, and the measurements are fed to a discriminating algorithm. The efficiency of those data processing methods strongly depends on temperature uniformity across the active area of the sensor. To achieve this, hot plate structures with complex resistor geometries have been designed and additional heat-spreading structures have been introduced. In this work we designed and fabricated a metal oxide gas sensor integrated with a simple square planar indium tin oxide (ITO) heating element, by using conventional micromachining and thin-film deposition techniques. Power consumption-dependent surface temperature measurements were performed. A 420 °C working temperature was achieved at 120 mW power consumption. Temperature distribution uniformity was measured and a 17 °C difference between the hottest and the coldest points of the sensor at an operating temperature of 290 °C was achieved. Transient heat-up and cool-down cycle durations are measured as 40 ms and 20 ms, respectively.

Fano resonances in THz metamaterials composed of continuous metallic wires and split ring resonators.

We demonstrate theoretically and experimentally that Fano resonances can be obtained in terahertz metamaterials that are composed of periodic continuous metallic wires dressed with periodic split ring resonators. An asymmetric Fano lineshape has been found in a narrow frequency range of the transmission curve. By using a transmission line combined with lumped element model, we are able to not only fit the transmission spectra of Fano resonance which is attributed to the coupling and interference between the transmission continuum of continuous metallic wires and the bright resonant mode of split ring resonators, but also reveal the capacitance change of the split ring resonators induced frequency shift of the Fano resonance. Therefore, the proposed theoretical model shows more capabilities than conventional coupled oscillator model in the design of Fano structures. The effective parameters of group refractive index of the Fano structure are retrieved, and a large group index more than 800 is obtained at the Fano resonance, which could be used for slow light devices.

Spectral response modification of TiO2 MSM photodetector with an LSPR filter.

We fabricated UVB filtered TiO2 MSM photodetectors by the localized surface plasmon resonance effect. A plasmonic filter structure was designed using FDTD simulations. Final filter structure was fabricated with Al nano-cylinders with a 70 nm radius 180 nm period on 360 nm SiO2film. The spectral response of the TiO2 MSM photodetector was modified and the UVB response was reduced by approx. 60% with an LSPR structure, resulting in a peak responsivity shift of more than 40 nm. To our knowledge, this is the first published result for the spectral response modification of TiO2 photodetectors with LSPR technique.

A hybrid light source with integrated inorganic light-emitting diode and organic polymer distributed feedback grating.

We report a compact light source that incorporates a semiconductor light-emitting diode, nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugated polymer film. With this hybrid structure, we transferred electrically generated 390 nm ultraviolet light to an organic polymer via optical pumping and out-couple green luminescence to air through a second-order DFB grating. We demonstrate the feasibility of electrically driven, hybrid, compact light-emitting devices and lasers in the visible range.