Selective electro-magnetic absorbers based on metal-dielectric-metal thin-film cavities

Efficient absorption of light is required for a large number of applications such as thermo-photovoltaics,thermal imaging, bio-sensing, thermal emitters, astronomy, and stealth technology. Strong light absorbers found in nature with high intrinsic losses such as carbon black, metal-black, and carbon nano-tubes etc. are bulky, not design-tunable and are hard to pattern for micro- and nano- devices. We developed thin-film, high performance absorbers in the visible, near-, mid-, long-wave - and far-IR region based on a 3 layer metal-dielectric-metal (MDM) structure. We fabricated a 3-layerMDMabsorber with large band-widths in the visible and near IR spectral range without any lithographic patterning. This was the first demonstration in the optical range of the Salisbury Screen, which was originally invented for radar absorption. A Fabry-Perot cavity model depending on the thickness of the dielectric, but also the effective permittivity of the semi-transparent top metal gives calculated spectra that agree well with experiment. Secondly, we fabricated long-wave IR and far-IR MDM absorbers comprising surface patterns of periodic metal squares on the dielectric layer. Strong absorption in multiple bands were obtained, and these depended weakly on polarization and angle of incidence. Though such absorbers had been extensively studied by electrodynamic simulations and experiment in the visible to far- R regions, there existed no analytic model that could accurately predict the wavelengths of the multiple resonances. We developed a theoretical model for these absorbers based on standingwave resonances, which accurately predicts resonance wavelengths for experiment and simulation for the first time. Unlike metamaterial theories our model does not depend on the periodicity of the squares but only on their lateral dimension and the thickness of the dielectric. This feature is confirmed by synchrotron-based IR spectral imaging microscopy of single isolated squares

Optical Salisbury screen with design-tunable resonant absorption bands

A thin-film selective absorber at visible and near infra-red wavelengths is demonstrated. The structure consists of an optically thick layer of gold, a SiO2 dielectric spacer and a partially transparent gold film on top. The optical cavity so formed traps and absorbs light at a resonance wavelength determined by the film thicknesses. Observed fundamental-resonance absorption strengths are in the range 93%-97%. The absorption red-shifts and broadens as the thickness of the top gold layer is decreased with little change in absorption strength. Thus, strong absorption with design-tunable wavelength and width is achieved easily by unstructured blanket depositions. Observed angle-dependent spectra agree well with the recent three-layer analytical model of Shu et al. [Opt. Express 21, 25307 (2013)], if effective medium approximation is used to calculate the permittivity of the top gold film when it becomes discontinuous at the lowest thicknesses

Infrared surface polaritons on bismuth

Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 mu m. The bulk plasma frequency omega(p) and high-frequency limit to the permittivity epsilon(infinity) were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time tau and effective mass m*. This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity

Knowledge, Attitude, and Practices toward Plastic Pollution among Malaysians: Implications for Minimizing Plastic Use and Pollution

Excessive production, consumption, and indiscriminate disposal of plastic waste contribute to plastic pollution, which has a negative impact on the environment and human health. The KAP (knowledge, attitude, and practices) study is thought to be useful in mitigating plastic pollution because understanding the public’s knowledge, attitude, and practices toward plastic pollution can help identify problems and challenges, allowing appropriate policy decisions to be made to set up plans or implement interventions. This study assesses Malaysians’ level of knowledge, attitude, and practices toward plastic pollution, as well as the variation of plastic pollution related to KAP among various socio-demographic groups. For this study, an online survey received 294 valid responses. Descriptive statistics, KAP scoring, and response cross-tabulation were calculated. This study received 294 valid responses via an online survey. Descriptive statistics, KAP scoring, and response cross-tabulation were estimated. A one-way analysis of variance, paired t-test, and binary logistic regressions was performed. Respondents’ overall knowledge score (95% CI mean score: 3.88–10.94 on a scale of 11) and practice score (95% CI 1.09–6.53 on a scale of 6) were poor across socio-demographics when a cut point of 80% was used. Older (>46 years) respondents seemed to have more knowledge than younger (18–30 years) respondents (odds ratio, OR 4.304; p p p p < 0.01). To encourage good practices toward minimizing plastic use and pollution, conclusions are drawn about undertaking interventions such as raising environmental awareness, incorporating plastic pollution topics into formal and informal education, and providing recycling facilities in nearby communities

Far-Infrared Absorber Based On Standing-Wave Resonances In Metal-Dielectric-Metal Cavity

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ∼53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers

Thin-Film, Wide-Angle, Design-Tunable, Selective Absorber From Near Uv To Far Infrared

We experimentally demonstrate a structured thin film that selectively absorbs incident electromagnetic waves in discrete bands, which by design occur in any chosen range from near UV to far infrared. The structure consists of conducting islands separated from a conducting plane by a dielectric layer. By changing dimensions and materials, we have achieved broad absorption resonances centered at 0.36, 1.1, 14, and 53 microns wavelength. Angle-dependent specular reflectivity spectra are measured using UV-visible or Fourier spectrometers. The peak absorption ranges from 85 to 98%. The absorption resonances are explained using the model of an LCR resonant circuit created by coupling between dipolar plasma resonance in the surface structures and their image dipoles in the ground plane. The resonance wavelength is proportional to the dielectric permittivity and to the linear dimension of the surface structures. These absorbers have application to thermal detectors of electromagnetic radiation. © 2013 SPIE

Mems Clocking-Cantilever Thermal Detector

We present performance calculations for a MEMS cantilever device for sensing heat input from convection or radiation. The cantilever deflects upwards under an electrostatic repulsive force from an applied periodic saw-tooth bias voltage, and returns to a null position as the bias decreases. Heat absorbed during the cycle causes the cantilever to deflect downwards, thus decreasing the time to return to the null position. In these calculations, the total deflection with respect to absorbed heat is determined and is described as a function of time. We present estimates of responsivity and noise. © 2013 SPIE

Cathodoluminescence Of Conducting Gratings And Implications For Electron-Beam Investigations Of Nano-Photonic Devices

Cathodoluminescence (CL) spectroscopy is performed on conducting 1- and 2-dimensional gratings of metals, semimetals and semi-conductors of varying periods from 0.5 to 20 microns for a range of grating amplitudes from 0.1 to 4.6 microns. The overall emission spectrum consists of a 400 nm wide band centered at ~600 nm which depends little on the grating period, grating amplitude, material, e-beam energy, or temperature. CL intensity increases and the center wavelength blue shifts with increasing excitation beam current. For the larger amplitude 1-dimensional gratings fringes appear in the emission spectrum, which is due to interference between emission from grating bars and grooves. Surface corrugation is necessary to the emission as none is observed from smooth surfaces. The same band appears weakly in CL of a Cu sample with random ~1 micron surface roughness, but this emission is strongly reduced when the same sample is highly polished. The CL signal appears even when the ~10 nm electron-beam is at least 2 mm away from the grating edge, suggesting electron-beam induced currents are important to the emission, whose precise mechanism remains unclear. Previously suggested mechanisms - electron collision with image charge, transition radiation, surface contamination, and inverse photoemission effect - all fail to explain the observed spectrum and its lack of beam-energy dependence. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)

Patterning Of Oxide-Hardened Gold Black By Photolithography And Metal Lift-Off

A method to pattern infrared-absorbing gold black by conventional photolithography and lift-off is described. A photo-resist pattern is developed on a substrate by standard photolithography. Gold black is deposited over the whole by thermal evaporation in an inert gas at ∼1 Torr. SiO2 is then deposited as a protection layer by electron beam evaporation. Lift-off proceeds by dissolving the photoresist in acetone. The resulting sub-millimeter size gold black patterns that remain on the substrate retain high infrared absorption out to ∼5 μm wavelength and exhibit good mechanical stability. This technique allows selective application of gold black coatings to the pixels of thermal infrared imaging array detectors. © 2013 Elsevier B.V. All rights reserved