We present the design, numerical simulations and experimental measurements of THz metamaterial absorbers with a broad and flat absorption top both for transverse electric and transverse magnetic polarizations over a wide incidence angle range. The metamaterial absorber unit cell consists of two sets of structures resonating at different but close frequencies. The overall absorption spectrum is the superposition of individual components and becomes flat at the top over a significant bandwidth. The experimental results are in excellent agreement with numerical simulations. c 2018 Optical Society of America OCIS codes: 160.3918, 230.3990, 300.6495, 310.3915 Since the first demonstration by Landy et al.[1], metamaterial absorbers have attracted a great deal of interest worldwide during the past few years due to a host of potential applications including detectors, imaging, and sensing. They are typically comprised two structured metallic layers separated with a dielectric spacer, either free standing or supported by a suitable substrate, with a total thickness of functional layers much smaller than the operational wavelength. The originally proposed bottom structured metallic layer was a resonant cut-wire array [1,2], which has evolved to a simpler metal ground plane [3]. This makes the metamaterial absorbers more or less resemble Salisbury screens or circuit analog absorbers [4]. A variety of metamaterial structures have been employed and the operational frequency has covered the microwave [1] through terahertz (THz) [2, 3, 5] to optical [6, 7] ranges. The generally accepted idea was that by tuning the effective electric permittivity ǫ and magnetic permeability µ independently, it is possible to realize impedance matching to free-space [1] and minimize the reflection. However, it has been recently found that a Fabry-Perót resonance between the two metallic layers is responsible for the observed metamaterial absorption [8], where the tuned reflection/transmission amplitude and phase by the metamaterial layers satisfy the antireflection requirements similar to a quarter wave antireflection, i.e. another kind of impedance matching. We have verified this mechanism for many metamaterial absorbers proposed in literature [9]. Besides its polarization and incidence angle dependence, the bandwidth of a metamaterial absorber is one of the important aspects that may affect many applications. So far, most designs of metamaterial absorbers operate at a specific narrow frequency range. and triple-band [13, 14] metamaterial absorbers have been demonstrated with distinct narrow absorption frequencies; however, broadband metamaterial absorbers remain a challenge and there have been only a few very recent studies, mostly focusing on theoretical and numerical investigation [15][16][17][18][19][20]. In this Letter, we experimentally demonstrate metamaterial absorbers operating in the THz frequency range with a broad and flat high absorption top over a wide incidence angle range, which are in excellent agreement with numerical ...

We investigate the impact of resonator geometry and its coupling with ground plane on the performance of metamaterial perfect absorbers. Using a cross-resonator as an example structure, we find that the absorber thickness can be further reduced through modifying the geometric dimensions of the resonators. Numerical simulations and theoretical calculations reveal that destructive interference of multiple reflections is responsible for the near-unity absorption. The near-field coupling between the resonator array and ground plane can be significant. When this coupling is taken into account, the theoretical results calculated using the interference model are in excellent agreement with experiments and numerical simulations.

MicroRNAs (miRNAs) interact with 3′ untranslated region (UTR) elements of target genes to regulate mRNA stability or translation and thus play a role in regulating many different biological processes, including circadian rhythms. However, specific miRNAs mediating the regulation of essential clock genes remain largely unknown. Because vesicles containing membrane-bound miRNAs are present in the circulatory system, we examined miRNAs predicted to target the clock gene, Bmal1, for evidence of rhythmic fluctuations in circulating levels and modulatory effects on the 3′ UTR activity of Bmal1. A number of miRNAs with Bmal1 as a predicted target were expressed in the serum of mice exposed to LD 12∶12 and of these miRNAs, miR-152 and miR-494 but not miR-142-3p were marked by diurnal oscillations with bimodal peaks in expression occurring near the middle of the day and 8 or 12 hr later during the night. Co-transfection of pre-miR over-expression constructs for miR-494 and miR-142-3p in HEK293 cells had significant effects in repressing luciferase-reported Bmal1 3′ UTR activity by as much as 60%, suggesting that these miRNAs may function as post-transcriptional modulators of Bmal1. In conjunction with previous studies implicating miRNAs as extracellular regulatory signals, our results suggest that circulating miRNAs may play a role in the regulation of the molecular clockworks in peripheral circadian oscillators.

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