Terahertz Metamaterials

Richard Averitt, Mengkun Liu, Andrew Strikwerda, Jingdi Zhang

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The initial impetus driving metamaterials research was the realization and demonstration that a negative refractive index could be obtained by creating patterned subwavelength composites consisting of highly conducting metals such as gold or copper where the effective permittivity and effective permeability are independently specified. Additionally, metamaterials allow for tailoring the impedance in a manner not easily achieved with naturally occurring materials. This newfound approach to engineering the optical properties of materials offers unprecedented opportunities to realize novel electromagnetic responses from the microwave through the visible. This includes cloaks, concentrators, modulators, with many more examples certain to be discovered in the coming years.

Our work in this area is focused on creating novel metamaterial composites which are resonant at far-infrared, or terahertz (THz) frequencies. We are investigating metamaterials at THz frequencies for two main reasons.

(a) The length scale of THz radiation (1THz corresponds to a wavelength of 300 microns) is such that conventional lithography can be utilized to create subwavelength composites. This allows us to investigate a host of interesting electromagnetic phenomena in an efficient manner. For example, with our collaborators, we have demonstrated resonant metamaterial absorbers, voltage controlled modulators, and optically tunable notch filters – all operating at THz frequencies. We employ an approach using electromagnetic simulation, fabrication, and electromagnetic characterization using terahertz time-domain spectroscopy with a view of understanding the fundamental properties of these electromagnetic composites.

(b) The second reason we investigate metamaterials at THz frequencies is to explore the possibility of filling the so-called “THz gap” using devices such as those described above. There is considerable interest in developing this portion of the electromagnetic spectrum for applications given the unique characteristics of THz radiation which includes the ability to transmit through materials that are opaque at other frequencies. Thus, non-invasive imaging and spectroscopic identification of illicit or hazardous materials are potential applications. However, to realize this potential, substantial effort is required to create fieldable sources and detectors along with the development of component technologies that we take for granted in the microwave, infrared, and visible portions of the electromagnetic spectrum. Metamaterials will play an important role to fill this THz technology gap.

Our current work in this area includes the development of thermally-based metamaterial detectors, strongly resonant absorbers, and reconfigurable metamaterials. We have ongoing collaborations with Prof. Xin Zhang in the Dept. of Mechanical Engineering at BU, Prof. Willie Padilla in the Physics Dept. at Boston College, Dr. Antoinette Taylor at Los Alamos National Laboratory, and Prof. Art Gossard at UCSB.