Tunable negative index metamaterials and devices
October 3, 2008
Dr. Alexandre M Bratkovski HP Lab
Recently, metamaterials with negative index of refraction have been demonstrated for microwave electromagnetic (EM) radiation. It has been anticipated in 1950-60s by Pafomov and Veselago that negative refraction should occur in homogeneous media with simultaneously negative dielectric permittivity and magnetic permeability, meaning the medium supports backward waves. It has been speculated by Pendry (2000) that the ideal Veselago lens can produce and image beyond the limits of geometrical optics, and may open up new possibilities in integrated photonics, imaging, and sensors. We explore these possibilities theoretically and experimentally. Performance of NIM lenses is hindered by losses and imperfect surfaces, so that one has to demonstrate the feasibility of these devices working at optical frequencies, or find ways to mitigate them. We have designed the metamaterial by means of FDTD modeling, which is a stack of metallic films with periodic hole arrays separated by dielectric layers (so-called fishnet, FN) to work at IR wavelengths ?=1.5-1.7 ?m, and fabricated a few samples by nanoimprint lithography. The FN transmission and reflectance characteristics showed unambiguously that the FN supports the backward waves and have overall negative index of refraction at IR frequencies.
We have achieved very fast optical modulation of the effective refractive properties of a fishnet metamaterial with a Ag/Si/Ag heterostructure in the near-IR range and the associated fast dynamics was studied by pump-probe method. Photo excitation of the amorphous Silayer 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 (with the fast ~1ps relaxation time followed by ~50ps tail).
New very interesting possibility is opened up by exploring the binary mixtures of e.g. lossy metallic nanostructure with gain medium, like e.g. PbSe nanoparticles, that may help to overcome loss in a particular frequency window where the refractive index remains /negative/. We have showed that a binary mixture of quantum dots exhibiting gain with silver nanorods makes a lossless negative operation for realistic material structures and parameters feasible.
We also explore excellent possibilities provided by plasmonic nanostructures for sensing, in particular Surface Enhanced Raman Scattering (SERS), where we design plasmonic field enhancers as part of high-performance systems capable of one-molecule detection. Some still controversial aspects of SERS will be touched upon.