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Register a new userNear-infrared negative-index metamaterials consisting of multiple layers of perforated metal-dielectric stacks
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In this paper, we numerically demonstrate a near-infrared negative-index metamaterial (NIM) slab consisting of multiple layers of perforated metal-dielectric stacks and exhibiting low imaginary part of index over the wavelength of negative refraction. The effective index is obtained using two different numerical methods and found to be consistent. Backward phase propagation is verified by calculation of fields inside the metamaterial. These results point to a new design of low loss thick metamaterial at optical frequencies.
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Negative index metamaterials (NIMs) give rise to unusual and intriguing properties and phenomena, which may lead to important applications such as superlens, subwavelength cavity and slow light devices. However, the negative refractive index in metamaterials normally requires a stringent condition of simultaneously negative permittivity and negative permeability. A new class of negative index metamaterials - chiral NIMs, have been recently proposed. In contrast to the conventional NIMs, chiral NIMs do not require the above condition, thus presenting a very robust route toward negative refraction. Here we present the first experimental demonstration of a chiral metamaterial exhibiting negative refractive index down to n=-5 at terahertz frequencies, with only a single chiral resonance. The strong chirality present in the structure lifts the degeneracy for the two circularly polarized waves and relieves the double negativity requirement. Chiral NIM are predicted to possess intriguing electromagnetic properties that go beyond the traditional NIMs, such as opposite signs of refractive indices for the two circular polarizations and negative reflection. The realization of terahertz chiral NIMs offers new opportunities for investigations of their novel electromagnetic properties, as well as important terahertz device applications.
Epsilon-Near-Zero materials exhibit a transition in the real part of the dielectric permittivity from positive to negative value as a function of wavelength. Here we study metal-dielectric layered metamaterials in the homogenised regime (each layer has strongly subwavelength thickness) with zero real part of the permittivity in the near-infrared region. By optically pumping the metamaterial we experimentally show that close to the Epsilon-Equal-to-Zero (EEZ) wavelength the permittivity exhibits a marked transition from metallic (negative permittivity) to dielectric (positive permittivity) as a function of the optical power. Remarkably, this transition is linear as a function of pump power and occurs on time scales of the order of the 100 fs pump pulse that need not be tuned to a specific wavelength. The linearity of the permittivity increase allows us to express the response of the metamaterial in terms of a standard third order optical nonlinearity: this shows a clear inversion of the roles of the real and imaginary parts in crossing the EEZ wavelength, further supporting an optically induced change in the physical behaviour of the metamaterial.
A robust wedge setup is proposed to unambiguously demonstrate negative refraction for negative index metamaterials. We applied our setup to several optical metamaterials from the literature and distinctly observed the phenomena of negative refraction. This further consolidates the reported negative-index property. It is found there generally exists a lateral shift for the outgoing beam through the wedge. We derived a simple expression for calculating this beam shift and interestingly, it provides us a strategy to quantitatively estimate the loss of the wedge material (Im[n]). Addition- ally, we offered a design of metamaterials, compatible with nano-imprinting-lithography, showing negative refractive index in the visible regime (around yellow-light wavelengths). The multi-layer- system retrieval was utilized to extract the effective refractive index of the metamaterial. It was also intuitively characterized through our wedge setup to demonstrate corresponding phenomena of refraction.
A wedge-shaped structure made of split-ring resonators (SRR) and wires is numerically simulated to evaluate its refraction behavior. Four frequency bands, namely, the stop band, left-handed band, ultralow-index band, and positive-index band, are distinguished according to the refracted field distributions. Negative phase velocity inside the wedge is demonstrated in the left-handed band and the Snells law is conformed in terms of its refraction behaviors in different frequency bands. Our results confirmed that negative index of refraction indeed exists in such a composite metamaterial and also provided a convincing support to the results of previous Snells law experiments.
Optical modulation of the effective refractive properties of a fishnet metamaterial with a Ag/Si/Ag heterostructure is demonstrated in the near-IR range and the associated fast dynamics of negative refractive index is studied by pump-probe method. Photo excitation of the amorphous Si layer at visible wavelength and corresponding modification of its optical parameters is found to be responsible for the observed modulation of negative refractive index in near-IR.
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