Loss-free and active optical negative-index metamaterials

Much research activity is being devoted to the design and fabrication of metamaterials, artificially tailored composites with the counter-intuitive optical property of a negative refraction index. There is an exciting and wide range of possible applications that could be developed with such negative-index materials, including invisibility cloaks and 'perfect' lenses, but a major hurdle is that the performance is severely limited by absorption losses. Vladimir Shalaev and colleagues now demonstrate an approach that could lead to a breakthrough in this area; they have incorporated an optical gain medium within the metamaterial as a way to compensate the intrinsic loss, and show that optical pumping leads to a significantly improved negative refraction index and performance-related figure-of-merit at visible wavelengths. The study confirms that it is possible to design an optical metamaterial that is not limited by the intrinsic loss of its metal constituent. Metamaterials have the counterintuitive optical property of negative refraction index. They have a wide range of possible applications, including 'invisibility cloaks' and perfect lenses, but their performance is severely limited by absorption losses. These authors have incorporated an optical gain medium within a metamaterial as a way to compensate the intrinsic loss, and show that optical pumping leads to a significantly improved negative refraction index and figure of merit within the 722–738-nm visible wavelength range. The recently emerged fields of metamaterials and transformation optics promise a family of exciting applications such as invisibility, optical imaging with deeply subwavelength resolution and nanophotonics with the potential for much faster information processing. The possibility of creating optical negative-index metamaterials (NIMs) using nanostructured metal–dielectric composites has triggered intense basic and applied research over the past several years1,2,3,4,5,6,7,8,9,10. However, the performance of all NIM applications is significantly limited by the inherent and strong energy dissipation in metals, especially in the near-infrared and visible wavelength ranges11,12. Generally the losses are orders of magnitude too large for the proposed applications, and the reduction of losses with optimized designs seems to be out of reach. One way of addressing this issue is to incorporate gain media into NIM designs13,14,15,16. However, whether NIMs with low loss can be achieved has been the subject of theoretical debate17,18. Here we experimentally demonstrate that the incorporation of gain material in the high-local-field areas of a metamaterial makes it possible to fabricate an extremely low-loss and active optical NIM. The original loss-limited negative refractive index and the figure of merit (FOM) of the device have been drastically improved with loss compensation in the visible wavelength range between 722 and 738 nm. In this range, the NIM becomes active such that the sum of the light intensities in transmission and reflection exceeds the intensity of the incident beam. At a wavelength of 737 nm, the negative refractive index improves from −0.66 to −1.017 and the FOM increases from 1 to 26. At 738 nm, the FOM is expected to become macroscopically large, of the order of 106. This study demonstrates the possibility of fabricating an optical negative-index metamaterial that is not limited by the inherent loss in its metal constituent.