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Site Updated:
09/18/2008

Magnetic Response Metamaterials

Objective

To demonstrate magnetic responses from THz to near IR frequencies, and to study collective magnetic excitations.

Background

A large magnetic response, especially a negative magnetic permeability at high frequency region (larger than several hundred gigahertz) does not occur in naturally existing materials. However, metamaterial-based resonators can tackle the above problem, which will play a crucial role in terahertz devices, sensing and imaging applications, as well as left-handed materials (LHM) at optical region.

Fig. 1: The secondary ion image of sample D1 taken by ion beam microscope, where the corresponding gap between the inner and outer ring (G), the width of the metal line (W), the length of the outer ring (L), and the lattice parameter are 2, 4, 26 and 36 um, respectively.	Fig.2: (a) The reflectance ratio of S- and P- polarizations; (b) and (c) show the real parts and imaginary parts of the effective magnetic permeabilities of three samples, respectively.

Results

We successfully design and fabricate micrometer split-ring resonators (SRR) with the resonance frequency at THz region. On a quartz substrate, the copper SRR structures are constructed by the photo proliferated process. The samples are subwavelength scale (, where is the wavelength of the excited field at the resonance frequency); therefore the effective media theory can be applied to describe the properties of SRRs.

Using a Fourier transform infrared spectrometer (FTIR), we perform ellipsometry measurements on three samples. The peaks of reflectance ratio shown in Fig. 2(a) represent strong magnetic responses of SRRs when the magnetic field penetrates the rings (S-polariztion). The simulated effective magnetic permeability shows good agreement with the experiment result. In particularly, the real parts of the magnetic permeability for the three samples are all negative around the respective resonant frequencies. Such a structure with a negative magnetic response, when combined with plasmonic wires that exhibit negative electric permittivity, should produce a LHM in the THz region.

Very recently, we designed and fabricated sub-microscale L-shaped resonators (LSR) whose resonance frequencies can be up to 44.7THz (i.e., in mid-IR region). LSR has four-fold rotation symmetry, which can significantly suppress the bianisotropy of the conventional SSR structure and ease the burden of the orientation issue. The ellipsometric measurements in free-space by FTIR directly demonstrate the magnetic response in the mid-IR region. Further simulations confirm that the mid-IR magnetic resonances generated by LSRs possess broadband and scalable characteristics compared with natural materials.

Another remarkable property associated with the LSR is the large nonlinear enhancement due to the strong localization of electromagnetic energy. Fig. 3 shows the electric field distribution of LSR400 at its resonance frequency. Around the edges of the structure, the electric field is dramatically enhanced with a maximum factor of 47, promising the great potential applications of LSR in nonlinear optics and microquantitative chemical analysis.

Fig. 1: A secondary ion image of the sample LSR300, taken by focused ion beam microscopy. All three samples are fabricated by electron beam lithography with a layer gold of 400 nm thick. Here arm width a equals 300nm	Fig. 2: The ratio of the magnetic to electric response for three LSR structures. Expressed by LSR300 (red), LSR350 (blue) and LSR400 (green), all their scalable and broadband magnetic responses locate in mid-IR region

Fig. 3: The electric field distribution inside LSR at the resonance frequency. The maximum E-field enhancement can be up to 47, indicating LSR has potential applications in nonlinear optics.