Recent progress in the nanofabrication technologies inspired the demonstration and understanding of negative index materials (NIMs) first in microwave and then in optical ranges. In past few years, this kind of materials has attracted a significant amount of research attention following the prediction of superlensing and cloaking.

In this work, we demonstrate our recent progress in design, fabrication, and characterization of optical NIMs (ONIMs). We are focusing on several critical issues in order to push the optical NIMs into real applications.

First, we improved the nanofabrication recipes and demonstrated the ONIMs in optical wavelength range, i.e. at 710 nm and 580 nm.

Then, we experimentally demonstrated a tunable magnetic response in visible optical range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned by changing the refractive index of LC via phase transitions caused by the control of the ambient temperature. With the increasing of ambient temperature from 20°C to 50°C, the wavelength of magnetic response shifts from 650 nm to 632 nm. Numerical simulations confirm our tests and match the experimental observations well.

At last, we present our research on the loss, which is inherent in a plasmonic metal structure and is one of the most critical problems of ONIMs. Our idea is using gain materials as host to compensate the losses caused by the metallic structures. We experimentally demonstrated that the incorporation of gain material in the high-local-field areas of a metamaterial could generate extremely low-loss or active optical NIMs. The original loss-limited negative refractive index and the figure of merit (FOM) of the device have been dramatically improved with loss compensation in the visible wavelength range. This study demonstrates conclusively and for the first time an optical metamaterial that is not limited by the inherent loss of its metal constituent. We believe our studies will be very interesting and promising for real applications of NIMs in near future.