publications

2022

1. Fano Resonant Optical coatings platform for Full Gamut and High Purity Structural Colors

   Mohamed ElKabbash, Nathaniel Hoffman, Andrew R Lininger, and 5 more authors

   2022

2021

1. Cluster Variation Method Analysis of Correlations and Entropy in BCC Solid Solutions

   Nathaniel Hoffman, and Michael Widom

   Metallurgical and Materials Transactions A, May 2021

   Abs

   Solid solutions occur when multiple chemical species share sites of a common crystal lattice. Although the single site occupation is random, chemical interaction preferences bias the occupation probabilities of neighboring sites, and this bias reduced the entropy of mixing below its ideal value. Sufficiently strong bias leads to symmetry-breaking phase transitions. We apply the cluster variation method to explore solid solutions on body centered cubic lattices in the context of two specific compounds that exhibit opposite ordering trends. Employing density functional theory to model the energetics, we show that CuZn exhibits an order-disorder transition to the CsCl prototype structure, while AlLi instead takes the NaTl prototype structure, and we evaluate their temperature-dependent order parameters, correlations and entropies.

2. Fano-resonant ultrathin film optical coatings

   Mohamed ElKabbash, Theodore Letsou, Sohail A. Jalil, and 8 more authors

   Nature Nanotechnology, Apr 2021

   Abs

   Optical coatings are integral components of virtually every optical instrument. However, despite being a century-old technology, there are only a handful of optical coating types. Here, we introduce a type of optical coatings that exhibit photonic Fano resonance, or a Fano-resonant optical coating (FROC). We expand the coupled mechanical oscillator description of Fano resonance to thin-film nanocavities. Using FROCs with thicknesses in the order of 300\thinspacenm, we experimentally obtained narrowband reflection akin to low-index-contrast dielectric Bragg mirrors and achieved control over the reflection iridescence. We observed that semi-transparent FROCs can transmit and reflect the same colour as a beam splitter filter, a property that cannot be realized through conventional optical coatings. Finally, FROCs can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, presenting a possible solution to the dispatchability problem in photovoltaics, that is, the inability to dispatch solar energy on demand. Our solar thermal device exhibited power generation of up to 50% and low photovoltaic cell temperatures (~30\thinspace\textdegreeC), which could lead to a six-fold increase in the photovoltaic cell lifetime.

2020

1. Ultrathin-film optical coating for angle-independent remote hydrogen sensing

   Mohamed ElKabbash, Kandammathe Valiyaveedu Sreekanth, Arwa Fraiwan, and 9 more authors

   Measurement Science and Technology, Aug 2020

   Abs

   We demonstrated an optically-active antireflection, light absorbing, optical coating as a hydrogen gas sensor. The optical coating consists of an ultrathin 20 nm thick palladium film on a 60 nm thick germanium layer. The ultrathin thickness of the Pd film (20 nm) mitigates mechanical deformation and leads to robust operation. The measurable quantities of the sensors are the shift in the reflection minimum and the change in the full width at half maximum of the reflection spectrum as a function of hydrogen gas concentration. At a hydrogen gas concentration of 4%, the reflection minimum shifted by ∼46 nm and the FWHM increased by ∼228 nm. The sensor showed excellent sensitivity, demonstrating a 6.5 nm wavelength shift for 0.7% hydrogen concentration, which is a significant improvement over other nanophotonic hydrogen sensing methods. Although the sensor’s response showed hysteresis after cycling hydrogen exposure, the sensor is robust and showed no deterioration in its optical response after hydrogen deintercalation.

2017

1. Tunable Black Gold: Controlling the Near-Field Coupling of Immobilized Au Nanoparticles Embedded in Mesoporous Silica Capsules

   Mohamed ElKabbash, Ana Sousa-Castillo, Quang Nguyen, and 4 more authors

   Advanced Optical Materials, Aug 2017

   Abs

   Abstract Efficient light-to-heat conversion is central for various applications such as thermo-photovoltaics and solar steam generation. Although metals can strongly absorb light and generate heat, their free electrons shield the electric field before any substantial penetration in the metal. Excitation of surface plasmons can suppress metal reflection and convert it into a black metal, for example, black gold. In this work, mesoporous silica capsules grafted with immobilized Au nanoparticles (NPs) with different sizes via controlled chemical synthesis are synthesized. It is shown that changing the size of immobilized NPs modifies the interparticle coupling strength, thus, modifying the NPs absorption. The broadness of the plasmon resonance is tuned across the visible, near-infrared, and short wavelength infrared regions. The ability to control the broadness of black gold absorption is not possible in other systems based on bottom-up synthesis. The proposed approach broadens the possibilities of utilizing black gold in many applications such as thermo-photovoltaics, and solar energy harvesting especially in hybrid solar converters.

2. Iridescence-free and narrowband perfect light absorption in critically coupled metal high-index dielectric cavities

   M. ElKabbash, E. Ilker, T. Letsou, and 4 more authors

   Opt. Lett., Sep 2017

   Abs

   Perfect light absorption in the visible and near-infrared (NIR) was demonstrated using metamaterials, plasmonic nanostructures, and thin films. Thin film absorbers offer a simple and low-cost design as they can be produced on large areas and without lithography. Light is strongly absorbed in thin film metal-dielectric-metal (MDM) cavities at their resonance frequencies. However, a major drawback of MDM absorbers is their strong resonance iridescence, i.e., angle dependence. Here, we solve the iridescence problem by achieving angle-insensitive narrowband perfect and near-perfect light absorption. In particular, we show analytically that using a high-index dielectric in MDM cavities is sufficient to achieve angle-insensitive cavity resonance. We demonstrate experimentally angle-insensitive perfect and near-perfect absorbers in the NIR and visible regimes up to ±60°. By overcoming the iridescence problem, we open the door for practical applications of MDM absorbers at optical frequencies.