Zhongzhou Ren

Superluminal group velocity in an anisotropic metamaterial

Based on boundary condition and dispersion relation, the superluminal group velocity in an anisotropic metamaterial (AMM) is investigated. The superluminal propagation is induced by the hyperbolic dispersion relation associated with the AMM. It is shown that a modulated Gaussian beam exhibits a superluminal group velocity which depends on the choice of incident angles and optical axis angles. The superluminal propagation does not violate the theory of special relativity because the group velocity is the velocity of the peak of the localized wave packet which does not carry information. It is proposed that a triglycine sulfate (TGS) crystal can be designed and the superluminal group velocity can be measured experimentally.

Construct a polarizing beam splitter by an anisotropic metamaterial slab

The propagation of electromagnetic waves in the anisotropic medium with a single-sheeted hyperboloid dispersion relation is investigated. It is found that in such an anisotropic medium Eand H-polarized waves have the same dispersion relation, while E- and H-polarized waves exhibit opposite amphoteric refraction characteristics. E- (or H-) polarized waves are positively refracted whereas H- (or E-) polarized waves are negatively refracted at the interface associated with the anisotropic medium. By suitably using the properties of anomalous refraction in the anisotropic medium it is possible to realize a very simple and very efficient beam splitter to route the light. It is shown that the splitting angle and the splitting distance between E- and H- polarized beam is the function of anisotropic parameters, incident angle and slab thickness.

Focusing and phase compensation of paraxial beams by a left-handed material slab

On the basis of angular spectrum representation, a formalism describing paraxial beams propagating through an isotropic left-handed material (LHM) slab is presented. The treatment allows us to introduce the ideas of beam focusing and phase compensation by LHM slab. Because of the negative refractive index of LHM slab, the inverse Gouy phase shift and the negative Rayleigh length of paraxial Gaussian beam are proposed. It is shown that the phase difference caused by the Gouy phase shift in right-handed material (RHM) can be compensated by that caused by the inverse Gouy phase shift in LHM. If certain matching conditions are satisfied, the intensity and phase distributions at object plane can be completely reconstructed at the image plane.

Anomalous wave propagation in quasiisotropic media

Based on boundary conditions and dispersion relations, the anomalous propagation of waves incident from regular isotropic media into quasiisotropic media is investigated. It is found that the anomalous negative refraction, anomalous total reflection and oblique total transmission can occur in the interface associated with quasiisotropic media. The Brewster angles of E- and H-polarized waves in quasiisotropic media are also discussed. It is shown that the propagation properties of waves in quasiisotropic media are significantly different from those in isotropic and anisotropic media.

Polarization-sensitive propagation in an anisotropic metamaterial with double-sheeted hyperboloid dispersion relation

The polarization-sensitive propagation in the anisotropic metamaterial (AMM) with double-sheeted hyperboloid dispersion relation is investigated from a purely wave propagation point of view. We show that TE and TM polarized waves present significantly different characteristics which depend on the polarization. The omnidirectional total reflection and oblique total transmission can occur in the interface associated with the AMM. If appropriate conditions are satisfied, one polarized wave exhibits the total refraction, while the other presents the total reflection. We find that the opposite amphoteric refractions can be realized by rotating the principle axis of AMM, such that one polarized wave performs the negative refraction, while the other undergoes positive refraction. The polarization-sensitive characteristics allow us to construct two types of efficient polarizing beam splitters under certain achievable conditions.

Extinction theorem and propagation of electromagnetic waves between two anisotropic materials

Abstract.We systematically investigate the reflection and refraction of annelectromagnetic wave between two semi-infinite anisotropicnmagnetoelectric materials. Using the integral formulation of Hertznvectors and the principle of superposition, we generalize thenextinction theorem and derive the propagation characteristics ofnwave. Applying the results obtained, we find a general origin ofnBrewster effect. We also show that, through choosing appropriatenmaterial parameters, oblique or omnidirectional total transmissionncan occur to TE and TM waves. Compared to the traditional method,nthe method used here discloses the underlying mechanism of wavenpropagation between two arbitrary anisotropic materials and can benapplied to other problems of propagation. nn

Phonon elastic scattering on two-level systems in cryogenic piezoelectric crystals

We demonstrate that two-level systems (TLSs), which are caused by impurities or defects, lead to a kind of phonon elastic scattering in cryogenic piezoelectric crystals. We apply a model in which a TLS is coupled to the strain field in disordered crystals. Due to this coupling, we propose that phonons can be elastically scattered by TLSs when their frequency is much lower than the transition frequency of TLSs. Based on the second-order perturbation theory, we then obtain an attenuation formula. Consequently, we show how frequency and temperature influence the attenuation, which gives a theoretical explanation for the recent experiment (Goryachev M. et al., Phys. Rev. Lett., 111 (2013) 085502). This work provides some help for understanding the loss mechanism and designing low-loss cryogenic resonant cavities made of piezoelectric quartz.

Phonon-phonon interactions and phonon damping for the curvature modes in carbon nanotubes

We focus on the damping of the lowest-lying gapped modes with integer angular-momentum quantum number in carbon nanotubes (CNTs). These modes, called C modes simply, can be predicted within the framework of the continuum elasticity theory with the curvature term. Based on the phonon-phonon interactions due to the anharmonic effect, we obtain the three-phonon coupling coefficients of different damping processes of C modes. Applying perturbation theory, we calculate relaxation rates and upper bounds of quality factors for the long-wavelength C modes. In addition, we display the wave vector dependence of and show the importance of the C mode damping to thermal conductivity.