69 research outputs found
Terahertz epsilon-near-zero graded-index lens
An epsilon-near-zero graded-index converging lens with planar
faces is proposed and analyzed. Each perfectly-electric conducting (PEC)
waveguide comprising the lens operates slightly above its cut-off frequency
and has the same length but different cross-sectional dimensions. This
allows controlling individually the propagation constant and the normalized
characteristic impedance of each waveguide for the desired phase front at
the lens output while Fresnel reflection losses are minimized. A complete
theoretical analysis based on the waveguide theory and Fermatâs principle
is provided. This is complemented with numerical simulation results of
two-dimensional and three-dimensional lenses, made of PEC and
aluminum, respectively, and working in the terahertz regime, which show
good agreement with the analytical work.Effort sponsored by Spanish Government under contracts Consolider âEngineering
Metamaterialsâ CSD2008-00066 and TEC2011-28664-C02-01. P.R.-U. is sponsored by the
Government of Navarra under funding program âFormaciĂłn de tecnĂłlogosâ 055/01/11. M.N.-
C. is supported by the Imperial College Junior Research Fellowship. M. B. acknowledges
funding by the Spanish Government under the research contract program Ramon y Cajal
RYC-2011-08221. N.E. acknowledges the support from the US Office of Naval Research
(ONR) Multidisciplinary University Research Initiatives (MURI) grant number N00014-10-1-
0942
Droplet impact on doubly re-entrant structures
Doubly re-entrant pillars have been demonstrated to possess superior static and dynamic liquid repellency against highly wettable liquids compared to straight or re-entrant pillars. Nevertheless, there has been little insight into how the key structural parameters of doubly re-entrant pillars influence the hydrodynamics of impacting droplets. In this work, we carried out numerical simulations and experimental studies to portray the fundamental physical phenomena that can explain the alteration of the surface wettability from adjusting the design parameters of the doubly re-entrant pillars. On the one hand, three-dimensional multiphase flow simulations of droplet impact were conducted to probe the predominance of the overhang structure in dynamic liquid repellency. On the other hand, the numerical results of droplet impact behaviours are agreed by the experimental results for different pitch sizes and contact angles. Furthermore, the dimensions of the doubly re-entrant pillars, including the height, diameter, overhang length and overhang thickness, were altered to establish their effect on droplet repellency. These findings present the opportunity for manipulations of droplet behaviours by means of improving the critical dimensional parameters of doubly re-entrant structures
Widely tuneable scattering-type scanning near-field optical microscopy using pulsed quantum cascade lasers
Hedgehog subwavelength hole arrays: control over the THz enhanced transmission
UPNa. Departamento de IngenierĂa ElĂ©ctrica y ElectrĂłnica. Laboratorio de Ondas MilimĂ©tricas y TeraherciosBy backing or sandwiching a holey metal layer with or between isotropic dielectric slabs, additional peaks of transmission within the long-wavelength regime arise as a result of the induced transverse magnetic (TM) or transverse electric (TE) grounded dielectric modes. A similar control of the complex surface wave modes, and thus of the extraordinary transmission (ET) peaks, is demonstrated here via anisotropic slabs in the form of a fakir's bed of nails. However, it is shown that those ET peaks formed from TE modes are suppressed because of the inherent dispersion characteristics of the free-standing grounded pins. This allows the red-shifting of the ET for the polarization parallel to the larger in-plane period of the hole array, but unlike the dielectric isotropic slab configuration, the orthogonal polarization remains inhibited.This work was sponsored by the Spanish Government and European Union funds under contracts Consolider âEngineering Metamaterialsâ CSD2008-
00066 and TEC2011-28664-C02-01. MN-C is supported by Imperial College through a Junior Research Fellowship. PR-U is sponsored by the Government of Navarre via the program âFormaciĂłn de TecnĂłlogosâ 055/01/11. MB is sponsored by the Spanish Government via RYC- 2011â08221
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