MetaTHz-Terahertz metamaterials

http://opto1.artov.imm.cnr.it/metathz/index.php/research-bar/terahertz-metamaterials

MetaTHz is a 3-year scientific collaboration project (2013-2015), in the frame of a bilateral agreement for scientific cooperation between Italy and Poland. The project is funded by the Italian Ministry of Foreign Affairs, Directorate General for the Country Promotion, as a "Particular Relevance" bilateral project, and it is implemented in the Rome Unit of the Institute for Microelectronics and Microsystems of the Italian National Research Council, in collaboration with the Military University of Technology in Warsaw, Poland.

The project targets the following objectives:

• The design, fabrication, and characterization of the SLM proof-of-concept for phase-modulation and beam shaping/manipulation at ~1 THz. Each pixel of the SLM shall incorporate frequency selective surfaces, including passive dielectrics, metals, and voltage-controlled liquid-crystals, which collectively act as a tunable metamaterial. The control voltage will be applied independently on each pixel and reorientate LC dielectric tensor sensed by propagating THz polarized radiation. The pixel matrix will permit the spatial focusing, shaping, and manipulation of terahertz beams.

• The development of advanced functional liquid-crystalline organic materials for THz. Novel LC materials with low absorption losses and high birefringence will be synthesized, allowing for the reduction of the overall thickness of the LC layers necessary to provide the tuning range in THz. In addition, low viscosity materials will be targeted that guarantee faster response speeds, as well as dual-frequncy LCs, whose sign of the dielectric anisotropy depends on the frequency of the control voltage, typically in the interval 1-200 kHz. The synthesis of chiral materials (blue phases) shall also be considered, as these provide a response speed independent of the component thickness, which could provide the "video-rate" response demanded in security equipment for the detection of hazardous materials.

Terahertz Metamaterials

Metamaterials are man-made artifical structures that present electromagnetic properties not occurring in natural materials. Typically, their design is based on periodic patterns of metallic elements of sub-wavelength scale. The collective response of these structures in the presence of electromagnetic fields leads to exciting properties, such as a negative effective index of refraction. By tailoring the design, shape and geometry of the metamaterial constitutive elements, these properties can span over a huge spectrum, ranging from microwaves to optical frequencies. Terahertz technology can also benefit from metamaterials, as these can provide the building blocks for THz devices, and offer advanced transmission, absorption and phase control. Various designs have been demonstrated thus far as candidates for THz metamaterials, e.g. split-ring resonators [1], cut-slabs and fishnet structures [2,3]. 

 

The layout of such a fishnet THz metamaterial structure is shown in Figure 1(a), where a unit cell is replicated in order to compose a metamaterial slab. The dimensions of the unit cell are 150x150 μm, which is deeply subwavelength compared to the target frequency of 1 THz. The transmittance, reflectance, and absorbance of this structure are shown in Figure 1(b). Absorption is maximized near the metamaterial resonance and stems from losses in the metallic films and the dielectric spacer, which is considered to be benzo-cyclo-butene. By assuming an effective thickness of the metamaterial, the effective parameters of the fishnet metamaterial can be calculated for the case of normal incidence [4,5], as shown in Figure 1(c). A figure of merit (FOM) typically used in the assessment of metamaterial performance is the ratio -Re{n}/Im{n}, where n is the metamaterial effective index.

  

References

[1] W. J. Padilla, A. J. Taylor, C. Highstrete, M. Lee, and R. D. Averitt, "Dynamic electric and magnetic metamaterial response at terahertz frequencies," Phys. Rev. Lett., art. no. 107401, 2006.

[2] M. Kafesaki, I. Tsiapa, N. Katsarakis, Th. Koschny, C. M. Soukoulis, and E. N. Economou, "Dynamic electric and magnetic metamaterial response at terahertz frequencies," Phys. Rev. B, vol. 75, art. no. 235114, 2007.

[3] P. Ding, E. J. Liang, W. Q. Hu, L. Zhang, Q. Zhou, and Q. Z. Xue, "Numerical simulations of terahertz double-negative metamaterial with isotropic-like fishnet structure," Photon. Nanostruct. - Fund. Appl., vol. 7, pp. 92-100, 2009.

[4] D. R. Smith, S. Schultz, P. Markoš, and C. M. Soukoulis, "Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients," Phys. Rev. B, vol. 65, art. no. 195104, 2002.

[5] D. R. Smith, D. C. Vier, Th. Koschny, and C. M. Soukoulis, "Electromagnetic parameter retrieval from inhomogeneous metamaterials," Phys. Rev. E, vol. 71, art. no. 036617, 2005.