Vladimir M. Shalaev

Demonstration of a spaser-based nanolaser

One of the most rapidly growing areas of physics and nanotechnology focuses on plasmonic effects on the nanometre scale, with possible applications ranging from sensing and biomedicine to imaging and information technology. However, the full development of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed that in the same way as a laser generates stimulated emission of coherent photons, a ‘spaser’ could generate stimulated emission of surface plasmons (oscillations of free electrons in metallic nanostructures) in resonating metallic nanostructures adjacent to a gain medium. But attempts to realize a spaser face the challenge of absorption loss in metal, which is particularly strong at optical frequencies. The suggestion to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently and even allowed the amplification of propagating surface plasmons in open paths. Still, these experiments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles lack the feedback mechanism present in a spaser. Here we show that 44-nm-diameter nanoparticles with a gold core and dye-doped silica shell allow us to completely overcome the loss of localized surface plasmons by gain and realize a spaser. And in accord with the notion that only surface plasmon resonances are capable of squeezing optical frequency oscillations into a nanoscopic cavity to enable a true nanolaser, we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531 nm makes our system the smallest nanolaser reported to date—and to our knowledge the first operating at visible wavelengths. We anticipate that now it has been realized experimentally, the spaser will advance our fundamental understanding of nanoplasmonics and the development of practical applications.

Optical cloaking with metamaterials

Artificially structured metamaterials have enabled unprecedented flexibility in manipulating electromagnetic waves and producing new functionalities, including the cloak of invisibility based on coordinate transformation1,2,3. Unlike other cloaking approaches4,5,6, which are typically limited to subwavelength objects, the transformation method allows the design of cloaking devices that render a macroscopic object invisible. In addition, the design is not sensitive to the object that is being cloaked. The first experimental demonstration of such a cloak at microwave frequencies was recently reported7. We note, however, that that design7 cannot be implemented for an optical cloak, which is certainly of particular interest because optical frequencies are where the word ‘invisibility’ is conventionally defined. Here we present the design of a non-magnetic cloak operating at optical frequencies. The principle and structure of the proposed cylindrical cloak are analysed, and the general recipe for the implementation of such a device is provided.

Broadband Light Bending with Plasmonic Nanoantennas

A plasmonic antenna array is used to control the propagation of a light beam across an interface. The precise manipulation of a propagating wave using phase control is a fundamental building block of optical systems. The wavefront of a light beam propagating across an interface can be modified arbitrarily by introducing abrupt phase changes. We experimentally demonstrated unparalleled wavefront control in a broadband optical wavelength range from 1.0 to 1.9 micrometers. This is accomplished by using an extremely thin plasmonic layer (~λ/50) consisting of an optical nanoantenna array that provides subwavelength phase manipulation on light propagating across the interface. Anomalous light-bending phenomena, including negative angles of refraction and reflection, are observed in the operational wavelength range.

Alternative Plasmonic Materials: Beyond Gold and Silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

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.

The Case for Plasmonics

Light-induced surface excitations may offer a route to faster, smaller, and more efficient electronics as well as new technology opportunities. Just over a decade ago, the term “plasmonics” was coined for a promising new device technology that aims to exploit the unique optical properties of metallic nanostructures to enable routing and active manipulation of light at the nanoscale (1). At the same time, it was already well established that tiny metallic particles have a number of valuable optical properties that are derived from their ability to support collective light-induced electronic excitations, known as surface plasmons. Most notably, nanostructured metals dramatically alter the way light scatters from molecules, and this later led to the development of an important optical spectroscopy technique called surface-enhanced Raman spectroscopy (2–4).

Electromagnetic properties of small-particle composites

Abstract Recent advances in the electromagnetics of composite materials are reviewed. In particular, linear and non-linear optical properties of small-particle aggregates are considered. The effects of fractal morphology, such as a localization of dipolar eigenmodes and large fluctuations of local fields, are analyzed.

Nonmagnetic cloak with minimized scattering

In an electromagnetic cloak based on a transformation approach, reduced sets of material properties are generally favored due to their easier implementation in reality, although a seemingly inevitable drawback of undesired scattering exists in such cloaks. Here, the authors suggest the use of high-order transformations to create smooth moduli at the outer boundary of the cloak, therefore completely eliminating the detrimental scattering within the limit of geometric optics. The authors apply this scheme to a nonmagnetic cylindrical cloak and demonstrate that the scattered field is reduced substantially in a cloak with optimal quadratic transformation as compared to its linear counterpart.

Engineering photonic density of states using metamaterials

The photonic density of states (PDOS), like its electronic counterpart, is one of the key physical quantities governing a variety of phenomena and hence PDOS manipulation is the route to new photonic devices. The PDOS is conventionally altered by exploiting the resonance within a device such as a microcavity or a bandgap structure like a photonic crystal. Here we show that nanostructured metamaterials with hyperbolic dispersion can dramatically enhance the photonic density of states paving the way for metamaterial-based PDOS engineering.

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.