We consider plasmonic nanoantennas immersed in an active host medium. Specifically shaped metal nanoantennas can exhibit strong magnetic properties in the optical spectral range due to excitation of the magnetic plasmon resonance. A case in which a metamaterial comprising such nanoantennas can demonstrate both left handedness and negative permeability in the optical range is discussed. We show that high losses predicted for optical left-handed materials can be compensated in the gain medium. Gain allows one to achieve local generation in magnetically active metamaterials. We propose a plasmonic nanolaser where the metal nanoantenna operates in a fashion similar to a resonator. The size of the proposed plasmonic laser is much smaller than the light wavelength. Therefore, it can serve as a very compact source of coherent electromagnetic radiation and can be incorporated in future plasmonic devices.
Extending the range of electromagnetic properties of naturally occurring materials motivates the development of artificial metamaterials. For example, metamaterials with artificial microwave magnetism were known since the beginning of the 1950s. It has been demonstrated recently that metamaterials may exhibit such exotic properties as negative dielectric permittivity, negative magnetic permeability, and even both. The double-negative case of Re Îµ < 0 and Re Î¼ < 0 is often referred to as a left-handed material (LHM). Situations in which a negative refractive index can be realized in practice are particularly interesting because of the possibility of a âperfectâ lens with subwavelength spatial resolution. In addition to the superresolution not being limited by classical diffraction, many unusual and sometimes counterintuitive properties of negative refraction index materials (NIMs) make them very promising for applications in resonators, waveguides, and other microwave and optical elements. Negative refraction and subwavelength imaging have been demonstrated in the microwave and RF regimes.
For microwave NIMs, artificial magnetic elements providing Re Î¼ < 0 are the resonators of the split-ring type or helix type. In the microwave spectral range, metals can be considered as almost perfect conductors because the skin depth is much smaller than the metallic feature size. The strong magnetic response is achieved by operating in the vicinity of the LC resonance of the split ring. The same technique for obtaining Re Î¼ < 0 using split rings was extended to the mid-IR by scaling down the dimensions of the split rings. Therefore, the frequencies of the LC resonances are determined entirely by the split-ring geometry and size, not by the electromagnetic properties of the metal. In accordance with this statement, the ring response is resonantly enhanced at some particular ratio of the radiation wavelength and the structure size. Thus, we refer to the LC resonances of perfectly conducting metallic structures as geometric LC (GLC) resonances.