A review of active metamaterials incorporating gain device / medium

In the last one and half decade, metamaterials intrigued many exciting applications, such as cloaking, lens, electrically small antennas, etc., ranging from microwave to optical spectrum. The intrinsic loss and narrow bandwidth associated with existing passive metamaterials are still two main problems preventing many interesting applications from becoming reality. In recent years, the discussions of incorporating active devices or media into conventional passive metamaterial structures have been attracting more and more interests for loss compensation as well as dispersion control both in microwave and optical regime. The added design degree of freedom may enable new and rich physical phenomena and insights may be discovered in active metamaterials.