Vladimir P. Drachev

Loss-free and active optical negative-index metamaterials

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 years. 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 ranges. 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 designs. However, whether NIMs with low loss can be achieved has been the subject of theoretical debate. 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.

Metamagnetics with rainbow colors.

A family of coupled nanostrips with varying dimensions is demonstrated exhibiting optical magnetic responses across the whole visible spectrum, from red to blue. We refer to such a phenomenon as rainbow magnetism. The experimental and analytical studies of such structures provide us with a universal building block and a general recipe for producing controllable optical magnetism for various practical implementations.

The Ag dielectric function in plasmonic metamaterials

Ag permittivity (dielectric function) in coupled strips is different from bulk and has been studied for strips of various dimensions and surface roughness. Arrays of such paired strips exhibit the properties of metamagnetics. The surface roughness does not affect the Ag dielectric function, although it does increase the loss at the plasmon resonances of the coupled strips. The size effect in the imaginary part of the dielectric function is significant for both polarizations of light, parallel and perpendicular to the strips with relatively large A-parameter.

Dual-band negative index metamaterial: double negative at 813 nm and single negative at 772 nm

This work is concerned with the experimental demonstration of a dual-band negative index metamaterial. The sample is double negative (showing both a negative effective permeability and a negative effective permittivity) for linearly polarized light with a wavelength between 799 and 818 nm, and the real part of its refractive index is approximately -1.0 at 813 nm. The ratio of -Re(n)/Im(n) is close to 1.3 at 813 nm. For an orthogonal polarization, the same sample also exhibits a negative refractive index in the visible (at 772 nm). The spectroscopic measurements of the material are in good agreement with the results obtained from a finite-element electromagnetic solver for the actual geometry of the fabricated sample at both polarizations.

Enhancement of surface plasmons in an Ag aggregate by optical gain in a dielectric medium

We have observed the compensation of loss in a metal by a gain in a dielectric medium in the mixture of an Ag aggregate and a Rhodamine 6G dye. The demonstrated sixfold enhancement of the Rayleigh scattering is the evidence of the enhancement of the surface-plasmon resonance. The reported experimental observation facilitates many applications of nanoplasmonics.

Enhanced localized fluorescence in plasmonic nanoantennae

Pairs of gold elliptical nanoparticles form antennae, resonant in the visible. A dye, embedded in a dielectric host, coats the antennae; its emission excites plasmon resonances in the antennae and is enhanced. Far-field excitation of the dye-nanoantenna system shows a wavelength-dependent increase in fluorescence that reaches 100 times enhancement. Near-field excitation shows enhanced fluorescence from a single nanoantenna localized in a subwavelength area of ∼0.15μm2. The polarization of enhanced emission is along the main antenna axis. These observed experimental results are important for increasing light extraction from emitters localized around antennae and for potential development of a subwavelength sized laser.

Yellow Light Negative-Index Metamaterials

A well-established, silver fishnet design has been further miniaturized to function as a negative-index material at the shortest wavelength to date (to our knowledge). By studying the transmittance, reflectance, and corresponding numerical simulations of the sample, we report in this Letter a negative refractive index of -0.25 at the yellow-light wavelength of 580 nm.

Negative refractive index in optics of metal-dielectric composites

Specially designed metal-dielectric composites can have a negative refractive index in the optical range. Specifically, it is shown that arrays of single and paired nanorods can provide such negative refraction. For pairs of metal rods, a negative refractive index has been observed at 1.5 µm. The inverted structure of paired voids in metal films can also exhibit a negative refractive index. A similar effect can be accomplished with metal strips in which the refractive index can reach −2. The refractive index retrieval procedure and the critical role of light phases in determining the refractive index are discussed.

Tunable magnetic response of metamaterials

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

Hyperbolic metamaterials: new physics behind a classical problem

Hyperbolic materials enable numerous surprising applications that include far-field subwavelength imaging, nanolithography, and emission engineering. The wavevector of a plane wave in these media follows the surface of a hyperboloid in contrast to an ellipsoid for conventional anisotropic dielectric. The consequences of hyperbolic dispersion were first studied in the 50s pertaining to the problems of electromagnetic wave propagation in the Earths ionosphere and in the stratified artificial materials of transmission lines. Recent years have brought explosive growth in optics and photonics of hyperbolic media based on metamaterials across the optical spectrum. Here we summarize earlier theories in the Clemmows prescription for transformation of the electromagnetic field in hyperbolic media and provide a review of recent developments in this active research area.