Experimental demonstration of frequency-agile terahertz metamaterials

Metamaterials exhibit numerous novel effects and operate over a large portion of the electromagnetic spectrum. Metamaterial devices based on these effects include gradientindex lenses, modulators for terahertz radiation and compact waveguides. The resonant nature of metamaterials results in frequency dispersion and narrow bandwidth operation where the centre frequency is fixed by the geometry and dimensions of the elements comprising the metamaterial composite. The creation of frequency-agile metamaterials would extend the spectral range over which devices function and, further, enable the manufacture of new devices such as dynamically tunable notch filters. Here, we demonstrate such frequency-agile metamaterials operating in the far-infrared by incorporating semiconductors in critical regions of metallic split-ring resonators. For this first-generation device, external optical control results in tuning of the metamaterial resonance frequency by 20%. Our approach is integrable with current semiconductor technologies and can be implemented in other regions of the electromagnetic spectrum. Electromagnetic metamaterials are structured composites with patterned metallic subwavelength inclusions. These mesoscopic systems are built from the bottom up, at the unit cell level, to yield specific electromagnetic properties. Individual components respond resonantly to the electric, magnetic or both components of the electromagnetic field. In this way electromagnetic metamaterials can be designed to yield a desired response at frequencies from the microwave through to the near visible. Importantly, additional design flexibility is afforded by the judicious incorporation of naturally occurring materials within or as part of the metamaterial elements. Specifically, hybrid metamaterial composites result when the properties of a natural material, for example, semiconductors, strongly couple with the resonance of a metamaterial element. The resulting hybrid metamaterials will still exhibit ‘passive’ properties (such as a negative electric response, negative index or gradient index), as determined by the patterning of the metamaterial elements. However, the aforementioned coupling engenders control of the passive metamaterial response by means of an external stimulus of the natural material response (such as photoconductivity, nonlinearity, gain). For example, amplitude control has been demonstrated through carrier injection or depletion15 in a semiconductor substrate, both of which shunt the capacitive region of the metamaterial elements. However, this simple approach, where the entire capacitive region is continuously covered with semiconductor, is insufficient to tune the resonance frequency. To achieve frequency tunability, due consideration must be given to the incorporation of the semiconductor into the metamaterial elements. We describe hybrid metamaterials where precise patterning of semiconductors permits frequency tuning of the metamaterial resonance on photoexcitation. Our results represent an important initial step towards extending the spectral range over which a specific metamaterial device can operate, in addition to enabling the implementation of new device concepts. There have been some efforts to realize frequency-tunable metamaterials, most of which have been demonstrated in the microwave frequency region. However, it should be noted that these designs only control the resonant properties of one or a handful of individual metamaterial elements. Thus, these techniques are not suitable at terahertz (THz) and higher frequencies, where the entire metamaterial composite may have more than 1 10 unit cells and monolithic integration of the tuning elements is important. In Fig. 1a we show a scanning electron microscopy (SEM) image of the unit cell of a frequency-tunable THz metamaterial. It is a variant of previously demonstrated designs that exhibit a Lorentz-like resonant response described by an effective complex permittivity 1(v) 1⁄4 11 þ i12, where 11 and 12 are the real and imaginary parts, respectively. The split gap at the centre of the unit cell can be thought of as a capacitor, where charge accumulates on resonance. By constructing the capacitor plates from a semiconductor such as silicon, we can control the conductivity in response to the photoexcitation of free charge carriers, thereby altering the effective size of the capacitor and tuning the capacitance, C. As the resonance frequency is strongly dependent on the capacitance, that is, v0 (LC) – , where L is the effective inductance of the split-ring resonators (SRRs), then v0 should shift monotonically to lower frequencies as the photoexcitation fluence increases. The metamaterials were fabricated on a silicon-on-sapphire (SOS) wafer. The (100) silicon layer was 600 nm thick, with an intrinsic resistivity .100 V cm. The R-plane sapphire substrate was 530 mm thick. The planar metamaterial array was fabricated using standard photolithographic methods, consisting of electron-beam LETTERS