A method is presented for predicting and precisely controlling the structure of photonic crystals fabricated using sacrificial‐layer atomic layer deposition. This technique provides a reliable method for fabrication of high‐quality non‐close‐packed inverse shell opals with large static tunability and precise structural control. By using a sacrificial layer during opal infiltration, the inverse‐opal pore size can be increased with sub‐nanometer resolution and without distorting the lattice to allow for a high degree of dielectric backfilling and increased optical tunability. For a 10 % sacrificial layer, static tunability of 80 % is predicted for the inverse opal. To illustrate this technique, SiO2 opal templates were infiltrated using atomic layer deposition of ZnS, Al2O3, and TiO2. Experimentally, a static tunability of over 600 nm, or 58 %, was achieved and is well described by both a geometrical model and a numerical‐simulation algorithm. When extended to materials of higher refractive index, this method will allow the facile fabrication of 3D photonic crystals with optimized photonic bandgaps.

Recent advances in the photonics and optics industries have produced great demand for ever more sophisticated optical devices, such as photonic crystals. However, photonic crystals are notoriously difficult to manufacture. Increasingly, therefore, researchers have turned towards naturally occurring photonic structures for inspiration and a wide variety of elaborate techniques have been attempted to copy and harness biological processes to manufacture artificial photonic structures. Here, we describe a simple, direct process for producing an artificial photonic device by using a naturally occurring structure from the wings of the butterfly Papilio blumei as a template and low-temperature atomic layer deposition of TiO2 to create a faithful cast of the structure. The optical properties of the organic-inorganic diffraction structures produced are assessed by normal-incidence specular reflectance and found to be well described by multilayer computation method using a two-dimensional photonic crystal model. Depending on the structural integrity of the initially sealed scale, it was found possible not only to replicate the outer but also the inner and more complex surfaces of the structure, each resulting in distinct multicolor optical behavior as revealed by experimental and theoretical data. In this paper, we also explore tailoring the process to design composite skeleton architectures with desired optical properties and integrated multifunctional (mechanical, thermal, optical, fluidic) properties.

For the first time, response of personal exposimeters (PEMs) is studied under diffuse field exposure in indoor environments. To this aim, both numerical simulations, using finite-difference timedomain method, and calibration measurements were performed in the range of 880-5875 MHz covering 10 frequency bands in Belgium. Two PEMs were mounted on the body of a human male subject and calibrated on-body in an anechoic chamber (non-diffuse) and a reverberation chamber (RC) (diffuse fields). This was motivated by the fact that electromagnetic waves in indoor environments have both specular and diffuse components. Both calibrations show that PEMs underestimate actual incident electromagnetic fields. This can be compensated by using an on-body response. Moreover, it is shown that these responses are different in anechoic chamber and RC. Therefore, it is advised to use an on-body calibration in an RC in future indoor PEM measurements where diffuse fields are present. Using the response averaged over two PEMs reduced measurement uncertainty compared to single PEMs. Following the calibration, measurements in a realistic indoor environment were done for wireless fidelity (WiFi-5G) band. Measured power density values are maximally 8.9 mW/m 2 and 165.8 mW/m 2 on average. These satisfy reference levels issued by the International Commission on Non-Ionizing Radiation Protection in 1998. Power density values obtained by applying on-body calibration in RC are higher than values obtained from no body calibration (only PEMs) and on-body calibration in anechoic room, by factors of 7.55 and 2.21, respectively. Bioelectromagnetics. 2016;9999:XX-XX.

Abstract-The World Radiocommunications Conference WRC15 identified a number of frequency bands between 24-86 GHz as candidate frequencies for future cellular networks. In this paper an extensive review of propagation characteristics and challenges related to the use of millimetre wave in future wireless systems is presented. Reference to existing path loss models including atmospheric and material attenuation in recommendations of the International Telecommunication Union is given and the need for new multidimensional models and measurements is identified. A description of state of the art mm wave channel sounders for single and multiple antenna measurements is followed by a discussion of the most recent deterministic, semi-deterministic and stochastic propagation and channel models. Finally, standardization issues are outlined with recommendations for future research.

Photonic crystals (PCs) offer far greater control over the generation and propagation of light than any other material structures. [1,2] Their defining characteristics, omnidirectional and directional photonic bandgaps (PBGs), can be manipulated to enable effects such as low losses in optical circuits and control of spontaneous emission. PCs are also creating much excitement because of their ability to form flat bands, yielding phenomena such as slow light and negative refraction. [3,4] The infiltration and inversion of synthetic-opal templates has been established as a promising method for obtaining the periodic structure and refractive-index contrast required in PCs. [5][6][7][8][9] In addition, topological tuning by shifting the distribution and filling fraction of a high-dielectric material within PCs offers a way to significantly change their photonic-band properties. For example, increased tunability, functionality, and bandgap properties can be accomplished through advanced architectures such as multilayered inverse opals and non-close-packed (NCP) inverse opals. [10][11][12][13] NCP geometries differ significantly from inverse opals by the formation of extended "air cylinders" between neighboring air spheres, as shown in Figure 1a, whereas, in the more-limited inverse opal, the air spheres are connected only by narrow "sinter necks." As reported by Doosje et al., silicon NCP architectures are predicted to yield a 100 % increase in the width of the omnidirectional PBG between the eighth and ninth photonic bands. [12] In this paper, we show that modification of PC-template topology by both heat treatment and multiple conformal infiltrations facilitates precise control and optimization of photonic-band properties. Static tuning is possible by precisely controlled backfilling of inverse opals to increase the filling fraction of dielectric material.[14] However, inverse opals have inherently limited filling-fraction tunability because the narrow sinter necks quickly close with backfilling. To solve this limitation, we report the implementation of a new, two-step atomic layer deposition (ALD) infiltration process to form a TiO 2 NCP inverse opal. Significantly, this new process allowed a static tunability of ∼ 400 nm in the position of the directional bandgap. In the first step, the void space available for infiltration was reduced to ∼ 25 % of the original volume by controllably collapsing the opal template by sintering followed by ALD infiltration, a method that is capable of high-finesse deposition of dense films within a nanoporous template. This facilitated the formation of an ultralow-filling-fraction inverse opal (5.8 %) with 193 nm diameter sinter necks. In the second step, backfilling of the low-volume-fraction inverse opal resulted in large tuning of the photonic-band properties and, ultimately, the formation of an NCP structure. Because of its many applications in biosensing, solar cells, catalysis, and environmental cleanup, optically active and tunable TiO 2 structures are of high interest. In this...

Abstract-This work presents an analysis of Dense MultipathComponents (DMC) in an industrial workshop. Radio channel sounding was performed with a vector network analyzer and virtual antenna arrays. The specular and dense multipath components were estimated with the RiMAX algorithm. The DMC covariance structure of the RiMAX data model was validated. Two DMC parameters were studied: the distribution of radio channel power between specular and dense multipath, and the DMC reverberation time. The DMC power accounted for 23 to 70% of the total channel power. A significant difference between DMC powers in line-of-sight and non-line-of-sight was observed, which can be largely attributed to the power of the line-of-sight multipath component. In agreement with room electromagnetics theory, the DMC reverberation time was found to be nearly constant. Overall, DMC in the industrial workshop is more important than in office environments: it occupies a fraction of the total channel power that is 4 to 13% larger. The industrial environment absorbs on average 29% of the electromagnetic energy compared to 45-51% for office environments in literature: this results in a larger reverberation time in the former environment. These findings are explained by the highly cluttered and metallic nature of the workshop.

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The photonic bands of two-dimensional (2D) triangular lattice photonic crystal Si slab waveguides were statically tuned using low temperature atomic layer deposition (ALD) of TiO2. Angular dependent reflectance measurements of bare and coated devices were well fitted by three-dimensional finite-difference time-domain calculations. The technique not only allows the physics of photonic band effects in 2D photonic crystals to be systematically studied but also demonstrates large static tuning and precise fine-scale control over band frequency and dispersion, with a frequency tuning range of 12% and precision of 0.005% per ALD cycle. Band tuning to achieve zero group velocity is demonstrated.