Guohua Zhu

Bio: Guohua Zhu is an academic researcher from Norfolk State University. The author has contributed to research in topics: Surface plasmon & Random laser. The author has an hindex of 22, co-authored 88 publications receiving 4810 citations.

Demonstration of a spaser-based nanolaser

Abstract: Nanoplasmonics — the nanoscale manipulation of surface plasmons (fluctuations in the electron density at a metallic surface) — could revolutionize applications ranging from sensing and biomedicine to imaging and information technology. But first, we need a simple and efficient method for actively generating coherent plasmonic fields. This is in theory possible with the spaser, first proposed in 2003 as a device that generates and amplifies surface plasmons in the same way that a laser generates and amplifies photons. Now Noginov et al. present the first unambiguous experimental demonstration of spasing, using 44-nm diameter nanoparticles with a gold core and dye-doped silica shell. The system generates stimulated emission of surface plasmons in the same way as a laser generates stimulated emission of coherent photons, and has been used to implement the smallest nanolaser reported to date, and the first operating at visible wavelengths. Nanoplasmonics promises to revolutionize applications ranging from sensing and biomedicine to imaging and information technology, but its full development is hindered by the lack of devices that can generate coherent plasmonic fields. In theory, this is possible with a so-called 'spaser' — analogous to a laser — which would generate stimulated emission of surface plasmons. This is now realized experimentally, and should enable many new technological developments. 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 technology1,2. However, the full development of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed3 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 suggestion4,5,6 to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently7,8,9,10 and even allowed the amplification of propagating surface plasmons in open paths11. Still, these experiments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles12,13,14 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 nanolaser15,16,17,18, 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.

Controlling spontaneous emission with metamaterials

Abstract: We have observed, in metamaterial with hyperbolic dispersion (an array of silver nanowires in alumina membrane), a sixfold reduction of the emission lifetime of dye deposited onto the metamaterial’s surface. This serves as evidence of an anomalously high density of photonic states in hyperbolic metamaterials, demonstrates the feasibility of an earlier-predicted single-photon gun, and paves the road for the use of metamaterials in quantum optics.

Stimulated emission of surface plasmon polaritons

Abstract: We have observed stimulated emission of surface plasmon polaritons (SPPs) propagating at the interface between a silver film and a film of optically pumped polymethyl methacrylate (PMMA) doped with rhodamine 6G (R6G) dye.

The effect of gain and absorption on surface plasmons in metal nanoparticles

Abstract: The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption.

Compensation of loss in propagating surface plasmon polariton by gain in adjacent dielectric medium

Abstract: We report the suppression of loss of surface plasmon polariton propagating at the interface between silver film and optically pumped polymer with dye. The large magnitude of the effect enables a variety of applications of ‘active’ nanoplasmonics. The experimental study is accompanied by the analytical description of the phenomenon. In particular, we resolve the controversy regarding the direction of the wavevector of a wave with a strong evanescent component in an active medium.

Plasmonics for extreme light concentration and manipulation.

Abstract: The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

Plasmonics beyond the diffraction limit

Abstract: Recent years have seen a rapid expansion of research into nanophotonics based on surface plasmon–polaritons. These electromagnetic waves propagate along metal–dielectric interfaces and can be guided by metallic nanostructures beyond the diffraction limit. This remarkable capability has unique prospects for the design of highly integrated photonic signal-processing systems, nanoresolution optical imaging techniques and sensors. This Review summarizes the basic principles and major achievements of plasmon guiding, and details the current state-of-the-art in subwavelength plasmonic waveguides, passive and active nanoplasmonic components for the generation, manipulation and detection of radiation, and configurations for the nanofocusing of light. Potential future developments and applications of nanophotonic devices and circuits are also discussed, such as in optical signals processing, nanoscale optical devices and near-field microscopy with nanoscale resolution.

Optical negative-index metamaterials

Abstract: Artificially engineered metamaterials are now demonstrating unprecedented electromagnetic properties that cannot be obtained with naturally occurring materials. In particular, they provide a route to creating materials that possess a negative refractive index and offer exciting new prospects for manipulating light. This review describes the recent progress made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

Planar Photonics with Metasurfaces

Abstract: Metamaterials, or engineered materials with rationally designed, subwavelength-scale building blocks, allow us to control the behavior of physical fields in optical, microwave, radio, acoustic, heat transfer, and other applications with flexibility and performance that are unattainable with naturally available materials. In turn, metasurfaces-planar, ultrathin metamaterials-extend these capabilities even further. Optical metasurfaces offer the fascinating possibility of controlling light with surface-confined, flat components. In the planar photonics concept, it is the reduced dimensionality of the optical metasurfaces that enables new physics and, therefore, leads to functionalities and applications that are distinctly different from those achievable with bulk, multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that has occurred over the past few years with an eye toward the promising future directions in the field.