Observation of negative refraction and negative phase velocity in true left-handed metamaterials (Invited Paper)

Abstract

ABSTRACT We report a true left-handed (LH) behavior in a composite metamaterial consisting of periodically arranged split ring resonator (SRR) and wire structures. The magnetic resonance of the SRR structure is demonstrated by comparing the transmission spectra of SRRs with that of closed SRRs. We confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined left-handed transmission band with a peak value of -1.2 dB (–0.3 dB/cm) is obtained. We also report the transmission characteristics of a 2D composite metamaterial (CMM) structure in free space. At the frequencies where left-handed transmission takes place, we experimentally confirmed that the CMM structure has effective negative refractive index. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments and the phase measurements are in good agreement. The experimental results agree extremely well with the theoretical calculations. Keywords: Negative refraction, negative phase velocity, split ring resonator, left-handed medium, metamaterial Veselago predicted that a medium with negative permittivity, , and negative permeability, µ , will exhibit negative indices of refraction [1]. When both the permittivity and the permeability are negative the electric field, the magnetic field and the wavevector components form a left-handed coordinate system, hence the name left-handed material (LHM) is used for description. Such a medium is expected to exhibit unusual physical properties such as negative refraction, reversal of Doppler shift, backward Cherenkov radiation. A medium with < 0 can easily be realized, e.g. by periodically arranged metallic wires [2]. On the other hand, the µ ( ) < 0 was a challenge due to lack of magnetic charge . Pendry et. al suggested that a periodic array of metallic split ring resonator (SRR) structures exhibit µ ( ) < 0 close to magnetic resonance frequency p [3]. This proposal brought the possibility of observing left-handed medium in to reality. Recently, this idea is brought to experimental investigation by constructing a composite metamaterial (CMM) consisting of two components which have ( ) < 0, a periodically arranged wire medium, and µ ( ) < 0, a periodically arranged SRR medium, simultaneously over a certain frequency range, respectively [4,5]. Various studies employing different structure designs extended this investigation [4-8]. Negative refraction of electromagnetic waves at the interface of CMMs is also observed [9-11] which supported the existence of a LHM. The parameters and µ can be obtained by a retrieval procedure from the numerically calculated transmission and reflection data for finite size CMMs [12,13], under the assumption of a homogeneous medium. This procedure confirmed [14] that a medium composed of SRRs and wires can indeed be characterized by effective and µ . A medium transmits electromagnetic waves when both and µ have the same sign, i.e., both are negative or both are positive. If they have opposite signs the medium effectively reflects the incoming electromagnetic wave. The existence of a pass band for the CMM within the respective stop bands of SRR-only and wire-only mediums is intuitively considered as evidence for LH behavior. We experimentally demonstrate that the dielectric response of the CMM differs substantially from that of the wire-only medium, by measuring