As the basis of a diverse set of photonic applications, such as hologram imaging, polarization, and wave front manipulation, the local phase control of electromagnetic waves is fundamental in photonic research. However, currently available bulky, passive, range-limited phase modulators pose an obstacle in photonic applications. Here, we propose a new mechanism to achieve a wide phase modulation range, with graphene used as a tunable loss to drive an underdamped to overdamped resonator transition. Based on this mechanism, we present widely tunable phase modulation in the terahertz regime, realized in gate-tuned ultrathin reflective graphene metasurfaces. A one-port resonator model, supported by full-wave simulations, explains the underlying physics of the discovered extreme phase modulation and indicates general strategies for designing tunable photonic devices. As an example, we demonstrate a gate-tunable terahertz (THz) polarization modulator with a graphene metasurface. Our findings establish the possibility for photonic applications based on active phase manipulation.
We present the design, characterization, and modeling of a specific optical metamaterial, and employ it to manipulate the light polarizations at optical frequencies. Experimental results reveal that the maximum polarization conversion efficiency, i.e., the energy portion converted from s to p polarization after reflection, can be as high as 96% at the wavelength of ϳ685 nm. Simulations and analytical results, which are in reasonable agreements with the experimental results, reveal that the underlying physics are governed by the particular electric and magnetic resonances in the optical metamaterial.Electromagnetic ͑EM͒ metamaterials are artificially designed media composed of subwavelength-engineered units, exhibiting unique EM properties that are unattainable with natural materials ͓1,2͔. Recently, metamaterials functioning at optical frequencies gradually appear ͓3-8͔. However, although many fascinating phenomena were theoretically proposed for metamaterials ͓9-13͔ and some of them were successfully realized at microwave frequencies ͓14-16͔, very few were experimentally confirmed at optical frequencies. This is due to the significantly enhanced challenges faced by both experiment and theory at optical frequencies.In this paper, we combine experimental and theoretical efforts to demonstrate an important application of optical metamaterials-the polarization control. We present a specifically designed optical metamaterial, and experimentally demonstrate that it can convert light polarization with an efficiency of 96% at = 685 nm. Although similar effects had been proven in microwave frequency regime previously ͓17͔, realizing such effects at optical frequencies is still highly nontrivial, considering the rareness of metamaterialbased phenomena demonstrated at optical frequencies. In addition, the physics considered here is more general, involving both magnetic and electric resonances of the designed optical metamaterial, in contrast to the previous mechanism employing solely magnetic resonances ͓17͔.To illuminate the basic ideas, let us consider the light reflections at an air/metamaterial interface. The reflection coefficients for normally incident lights polarized along x and y directions arewhere Z x = ͱ y / ͱ x and Z y = ͱ x / ͱ y are the impedance of the metamaterial for different polarizations. Suppose we shine a light with a polarization E ជ i = E 0 ͑x + ŷ͒ on the metamaterial, the reflected light would take a polarization E ជ r = E 0 ͑r x x + r y ŷ͒. Then, if the condition r x =−r y , or equivalently,is satisfied, the polarization direction of the reflected light is perpendicular to that of the incident one. Condition ͑1͒ implies that, to convert light polarizations, the two directions in a metamaterial must satisfy an EM reciprocal principle. Apparently, condition ͑1͒ can only be satisfied with a metamaterial since a conventional dielectric ͑with =1͒ has Z Ͻ 1 for two directions. A limiting solution of Eq. ͑1͒ is that Z x → 0, Z y → ϱ, indicating that the system behaves as a perfect electric conductor ͑PE...
Since the discovery of graphene, layered materials have attracted extensive interest owing to their unique electronic and optical characteristics. Among them, Dirac semimetals, one of the most appealing categories, have been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe 5 was found to host signatures of a Dirac semimetal. However, the low Fermi level in ZrTe 5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe 5 through magneto-optics and magneto-transport. By applying a magnetic field, we observe a ffiffiffiffi B p dependence of the inter-Landau-level resonance and Shubnikov-de Haas (SdH) oscillations with a nontrivial Berry phase, both of which are hallmarks of Dirac fermions. The angle-dependent SdH oscillations show a clear quasi-two-dimensional feature with a highly anisotropic Fermi surface and band topology, in stark contrast to the three-dimensional (3D) Dirac semimetal such as Cd 3 As 2 . This is further confirmed by the angle-dependent Berry phase measurements and the observation of bulk quantum Hall effect (QHE) plateaus. The unique band dispersion is theoretically understood: the system is at the critical point between a 3D Dirac semimetal and a topological insulator phase. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe 5 as an ideal platform for exploring the exotic physical phenomena of Dirac fermions. NPG Asia Materials (2016) 8, e325; doi:10.1038/am.2016.166; published online 11 November 2016 INTRODUCTION Layered materials, formed by stacking strongly bonded layers with weak interlayer coupling, 1-10 have drawn immense attention in fundamental studies and device applications owing to their tunability in band structures and Fermi energy. 3,4,[11][12][13] Unlike other layered materials such as MoS 2 and BN, graphene stands out as an appealing candidate, as it is featured with a linear energy dispersion and low-energy relativistic quasi-particles. 9,14,15 Many exotic phenomena, such as a half-integer quantum Hall effect (QHE) 1,2 and Klein tunneling, 16 have been realized in graphene. Along this line, extensive efforts were also devoted to exploring new Dirac semimetal states in other layered systems beyond graphene. 5,6 Pentatelluride ZrTe 5 with a layered orthorhombic structure has been widely studied since the 1980s for its resistivity anomaly [17][18][19] and large thermopower. 20,21 For a long time, ZrTe 5 was considered to be a semimetal or degenerated semiconductor with a parabolic energy dispersion. 10,22 However, a recent study 7 revealed a linear dispersion in ZrTe 5 bulk states along with a chiral magnetic effect, hosting the signatures of a Dirac semimetal. Nevertheless, owing to the relatively
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