Dentcho A. Genov

Three-dimensional optical metamaterial with a negative refractive index

Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded ‘fishnet’ structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.

Cloaking of matter waves.

Invariant transformation for quantum mechanical systems is proposed. A cloaking of matter wave can be realized at given energy by designing the potential and effective mass of the matter waves in the cloaking region. The general conditions required for such a cloaking are determined and confirmed by both the wave and particle (classical) approaches. We show that it may be possible to construct such a cloaking system for cold atoms using optical lattices.

Observation of Stimulated Emission of Surface Plasmon Polaritons

We report a direct experimental evidence of stimulated emission of surface plasmon polaritons (SPPs) at telecom wavelengths (1532 nm) with erbium doped glass as a gain medium. We observe an increase in the propagation length of signal surface plasmons when erbium ions are excited optically using pump SPP. The design, fabrication, and characterization of SPP waveguides, thin gold metal strips, embedded in erbium (Er) doped phosphate glass is presented. Such systems can be suitable as integrated devices coupling electronic and photonic data transmissions as well as SPP amplifiers and SPP lasers.

Superlens based on metal-dielectric composites

Pure noble metals are typically considered to be the materials of choice for a near-field superlens that allows subwavelength resolution by recovering both propagating and evanescent waves. However, a superlens based on bulk metal can operate only at a single frequency for a given dielectric host. In this paper, it is shown that a composite metal-dielectric film, with an appropriate metal filling factor, can operate at practically any desired wavelength in the visible and near-infrared ranges. Theoretical analysis and simulations verify the feasibility of the proposed lens.

Ray optics at a deep-subwavelength scale: a transformation optics approach.

We present a transformation optics approach for molding the light flow at the deep-subwavelength scale, using metamaterials with uniquely designed dispersion. By conformal transformation of the electromagnetic space, we develop a methodology for realizing subwavelength ray optics with curved ray trajectories. This enables deep-subwavelength-scale beams to flow through two- or three-dimensional spaces.

Anomalous spectral scaling of light emission rates in low-dimensional metallic nanostructures

The strength of light emission near metallic nanostructures can scale anomalously with frequency and dimensionality. We find that light-matter interactions in plasmonic systems confined in two dimensions (e.g., near metal nanowires) strengthen with decreasing frequency owing to strong mode confinement away from the surface-plasmon frequency. The anomalous scaling also applies to the modulation speed of plasmonic light sources, including lasers, with modulation bandwidths growing at lower carrier frequencies. This allows developing optical devices that exhibit simultaneously femtosecond response times at the nanometer scale, even at longer wavelengths into the mid-IR, limited only by nonlocal effects and reversible light-matter coupling.

Surface Plasmon Amplification in Planar Metal Films

The propagation and amplification of surface plasmon polaritons (SPPs) is studied at the interfaces between metals and active media. A permittivity renormalization technique is proposed and developed to obtain an explicit analytic expression for the critical gain required to achieve infinite SPP propagation length. A specific multiple quantum-well (MQW) system is identified as a prospective medium for demonstrating efficient SPP amplification at telecommunication frequencies. The proposed system may have a strong impact on a variety of photonic devices ranging from plasmonic nanocircuits, subwavelength transmission lines and plasmonic cavities to nanosized transducers.

Active Plasmonics: Surface Plasmon Interaction With Optical Emitters

The interaction between surface plasmons and optical emitters is fundamentally important for engineering applications, especially surface plasmon amplification and controlled spontaneous emission. We investigate these phenomena in an active planar metal-film system comprising InGaN/GaN quantum wells and a silver film. First, we present a detailed study of the propagation and amplification of surface plasmon polaritons (SPPs) at visible frequencies. In doing so, we propose a multiple quantum well structure and present quantum well gain coefficient calculations accounting for SPP polarization, line broadening due to exciton damping, and particularly, the effects of finite temperature. Second, we show that the emission of an optical emitter into various channels (surface plasmons, lossy surface waves, and free radiation) can be precisely controlled by strategically positioning the emitters. Together, these could provide a range of photonic devices (for example, surface plasmon amplifiers, nanolasers, nanoemitters, plasmonic cavities) and a foundation for the study of cavity quantum electrodynamics associated with surface plasmons.

METAL-DIELECTRIC COMPOSITE FILTERS WITH CONTROLLED SPECTRAL WINDOWS OF TRANSPARENCY

In this report we show the possibility of broadband low-pass filters with windows of transparency in pre-set spectral ranges. Those filters are based on the unique optical properties of metal-dielectric composites near the percolation threshold. In such composites, metal clusters of different sizes and shapes have different plasmon resonances and resonate nearly independently at different wavelengths. All together, the resonant plasmon modes cover a very broad spectral range. Applying the effect of spectrally selective photo-modification, we develop a procedure that creates mid-infrared windows of transparency within the broadband filters. We also investigate the possibilities of making low-pass filters composed of spheroids and conductive sticks with certain distributions of aspect ratios.

Metal coverage dependence of local optical properties of semicontinuous metallic films

Semicontinuous silver films on insulator substrates, which exhibit unique electrical and optical properties, were studied experimentally and theoretically. The percolation threshold of the films, which were synthesized by laser ablation, was determined from a combination of studies of the surface morphology by electron microscopy and the dc electrical resistance as a function of metal concentration. Local optical properties measured by near-field optical microscopy were compared with theoretical results obtained using the Block elimination method, with good agreement. Local field distributions were found to depend on the metal concentration and wavelength of illumination. The degree of localization was found to increase at metal concentrations above and below the percolation threshold.