Research collaboration between the X. Zhang group at UC Berkeley and OSU ECEN researchers, Dr. Weili Zhang and his graduate student Xinchao Lu, has led to a negative index of refraction in a chiral metamaterial first demonstrated in the terahertz regime. This work was published in Physical Review Letters [Vol. 102, 023901 (2009)] and was selected as a Viewpoint in Physics [Vol. 2, 3 (2009)]. Recently, this work was also highlighted as a News & Views in Nature Photonics [Vol. 3, 133 (2009)].

Scanning Electron Microscope image of the chiral metamaterial composed of 4.5 µm high and 40 µm × 40 µm sized unit cells made from gold. Upper inset: schematic of terahertz time-domain spectroscopy measurement. Lower inset: experimentally retrieved complex index of refraction of the left-handed circularly polarized terahertz wave, nL = nLr + inLi. The real part index of refraction nLr reaches negative values at frequencies of 1.06-1.27 THz.

Metamaterials are a new class of artificially structured matter, also referred to as left-handed materials or negative index materials. The concept of metamaterials was first proposed by V. G. Veselago, a Russian physicist, forty years ago and was first experimentally demonstrated at microwave frequencies earlier this decade. The fascination of metamaterials is that they have the ability to tailor the flow of electromagnetic waves in a way beyond what is possible with naturally existing materials. In the Veselago metamaterials, two essential parameters that determine the electromagnetic properties of matter, permittivity and permeability, are simultaneously negative. This in turn enables the index of refraction of metamaterials to possess negative values, thereby leading to unusually reversed propagation phenomena of electromagnetic waves, such as inverse Doppler shift and reverse Cherenkov radiation. Metamaterials have found a wide range of promising applications, including perfect lenses, perfect a bsorbers, cloak of invisibility, and anti-reflection structures, these would have significant impacts on the fields of communications, medicine, semiconductor fabrication, spectroscopy, and imaging.

In the collaborative work, Dr. Shuang Zhang and co-workers at UC Berkeley and OSU have demonstrated a unique metamaterial that exhibits strong chirality at terahertz frequencies. Composed of more than a hundred thousands 40 µm × 40 µm sized gold unit cells, the chiral chip has revealed a negative index in the frequency range of 1.06-1.27 THz. Different from the Veselago metamaterials, the chirality, as opposed to mirror symmetry, has enabled the creation of negative index of refraction, while did not require the permittivity and permeability to be simultaneously negative. In such metamaterial, when chirality was introduced, the left-handed and right-handed circularly polarized waves experienced different phase velocities. Consequently, negative index of refraction would occur if strong chirality was presented. The realization of terahertz chiral negative index metamaterials offers opportunities for investigation of their novel electromagnetic properties, such as negative refraction and negative reflection, as well as important terahertz device applications.

Terahertz metamaterials would extensively impact broad areas, including civilian and military radar systems, local covert communications, and terahertz imaging, as they can be designed to respond to terahertz frequencies with large values of positive or negative permeability, broad bandwidth and tunability. Terahertz metamaterials are a timely new field that offers promises in the development of terahertz technology. In the last two decades, remarkable progress has been achieved in terahertz generation and detection. However, there is a great demand of basic components necessary for the effective manipulation of terahertz waves. Terahertz metamaterials can be used to develop unique devices and compact and efficient circuit elements, such as terahertz modulators, switches, converters, and so forth.

Dr. Zhang’s research team
From left: Dr. Weili Zhang, Jianqiang Gu, Yuan-Cheng (Jason) Huang, Yongyao Chen, Ranjan Singh, Brady Whisenhunt, Dr. Mingwei Wang, Dr. Mingxia He, Dongwei Xu, Xinchao Lu, and Zhen Tian

The W. Zhang group at OSU initiated their research in metamaterials in late 2003 and has pioneered the use of terahertz time-domain spectroscopy in characterizing the electromagnetic properties of such unique composite structures [Azad, et al.; Opt. Lett. Vol. 31, 634 (2006)]. Recently, Dr. Weili Zhang and his graduate students Ranjan Singh and Jianqiang Gu have carried out systematic studies in planar terahertz metamaterials and have achieved a series of promising results, these include sub-skin depth metamaterials, metamaterial antennas, the behaviors of dark states, the impacts of metal permittivity, gap orientations of unit cells, and oblique incident terahertz field on the lower-order eigen modes of metamaterials, and the promising sensing potential with thin and high-aspect terahertz metamaterials. Besides UC Berkeley, the W. Zhang group at OSU has established long-term research collaborations with Los Alamos National Laboratory, Friedrich-Schiller-Universität at Jena, Germany, Tianjin University, China, National University of Singapore, and Sandia National Laboratories.

The ATRC cleanroom facilities situated on campus of OSU Stillwater have demonstrated exceptional capabilities in design and lithographic fabrication of terahertz metamaterials having a minimum feature size close to a micron. The metamaterials research in the W. Zhang group is financially supported by the National Science Foundation (under grant Nos. ECCS-0601574, ECCS-0725764, ECCS-0732417, and IIP-0832304).