ABSTRACT
We evidence by numerical calculations that optically pumped graphene is suitable for compensating inherent loss in terahertz (THz) metamaterials. We calculate the complex conductivity of graphene under optical pumping and determine the proper conditions for terahertz amplification in single layer graphene. It is shown that amplification in graphene occurs up to room temperature for moderate pump intensities at telecommunication wavelength λ = 1.5 µm. Furthermore, we investigate the coupling between a plasmonic split ring resonator (SRR) metamaterial and optically pumped graphene at a temperature T = 77 K and a pump intensity I = 300 mW/mm(2). We find that the loss of a SRR metamaterial can be compensated by optically stimulated amplification in graphene. Moreover, we show that a hybrid material consisting of asymmetric split-ring resonators and optically pumped graphene can emit coherent THz radiation at minimum output power levels of 60 nW/mm(2).
ABSTRACT
We propose a device to break the valley degeneracy in graphene and produce fully valley-polarized currents that can be either split or collimated to a high degree in a experimentally controllable way. The proposal combines two recent seminal ideas: negative refraction and the concept of valleytronics in graphene. The key new ingredient lies in the use of the specular shape of the Fermi surface of the two valleys when a high electronic density is induced by a gate voltage (trigonal warping). By changing the gate voltage in a n-p-n junction of a graphene transistor, the device can be used as a valley beam splitter, where each of the beams belong to a different valley, or as a collimator. The result is demonstrated through an optical analogy with two-dimensional photonic crystals.
ABSTRACT
We observe, by means of finite element calculations, that some photonic crystals produce negative refraction with almost circular isofrequency lines of their band diagram, so that a slab of this structure presents a large degree of isoplanatism and thus can behave like an imaging system. However, it has aberrations on comparison with a model of ideal lossless left-handed material within an effective medium theory. Further, we see that it does not produce subwavelength focusing. We discuss the limitations and requirements for such photonic crystal slabs to yield superresolved images of extended objects.
ABSTRACT
By means of both finite elements and FDTD calculations, we demonstrate that a structure of photonic crystal, constituted by two dimensional arrays of dielectric cylinders in air, or viceversa, previously proposed as capable of producing negative refraction with superlensing properties and subsequently proved to lack this characteristic, do possesses however the property of giving rise to effects of total internal reflection that allow both waveguiding, bending and collimation with high intensity subwavelength concentration of wavefronts. This is a consequence of both the dominant propagation along the GammaM direction due to diffraction, and of intensity localization in the cylinder regions as a result of the operating frequency being in the lower part of the bandgap, namely, in the so-called dielectric band.
ABSTRACT
Within an effective medium theory, we numerically study by means of a finite element method the transmission properties of prisms and slabs of media with negative refractive index. The constitutive parameters employed are similar to those of recent experiments that confirmed the existence of negative refraction as well as the focusing property of flat slabs. In this way, we further analyze in detail the influence of diffraction and scattering due to the large wavelength of the radiation in use, and its suppression by employing waveguide configurations with absorbing walls. Also, we address the effects of different amounts of absorption on both the angle of refraction, (for which we derive a new refraction law in prisms), and on the position, resolution and isoplantism of the focus produced by flat slabs.
