Abstract-Tapered N-helical metamaterials with three-fold rotational symmetry as improved circular polarizers

Johannes Kaschke, Mark Blome, Sven Burger, and Martin Wegener  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-17-19936

Optics Express, Vol. 22, Issue 17, pp. 19936-19946 (2014)
http://dx.doi.org/10.1364/OE.22.019936

Chiral helix-based metamaterials can potentially serve as compact and broadband circular polarizers. We have recently shown that the physics of structures composed of multiple intertwined helices, so called N-helices with N being an integer multiple of 4, is distinct from that of structures made of single circular helices (N = 1). In particular, undesired circular polarization conversion is strictly eliminated for N = 4 helices arranged on a square lattice. However, the fabrication of such structures for infrared/visible operation wavelengths still poses very significant challenges. Thus, we here revisit the possibility of reducing N from 4 to 3, which would ease micro-fabrication considerably. We show analytically that N = 3 helices arranged on a hexagonal lattice exhibit strictly vanishing circular polarization conversion. N = 3 is the smallest option as N = 2 obviously leads to linear birefringence. To additionally improve the circular-polarizer operation bandwidth and the extinction ratio while maintaining high transmission for the wanted polarization and zero conversion, we also investigate by numerical calculations N = 3 helices with tapered diameter along the helix axis. We find operation bandwidths as large as 2.4 octaves.

© 2014 Optical Society of America

Abstract-Broadband terahertz generation from metamaterials

• Liang Luo,
• Ioannis Chatzakis,
• Jigang Wang,
• Fabian B. P. Niesler,
• Martin Wegener,
• Thomas Koschny
• & Costas M. Soukoulis

http://www.nature.com/ncomms/2014/140108/ncomms4055/full/ncomms4055.html

The terahertz spectral regime, ranging from about 0.1–15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1–4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 μm (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility ~10⁻¹⁶ m² V⁻¹ that far exceeds thin films and bulk non-centrosymmetric materials.