What are plasmons?

In metals, light can couple to electrons to form a wave that is bound to the surface of the metal. This wave is called the surface plasmon. The surface plasmon mode is generally characterized by intense fields that decay quickly away from the interface between the metal and the surrounding environment. Surface plasmons display very important properties, including strongly enhanced local fields; tremendous sensitivity to changes in the local environment; and the ability to localize energy to tiny volumes not restricted by the wavelength of the exciting light.

Due to their unique properties, plasmons have found a broad range of applications in various areas of science. In chemistry and biology for example, the sensitivity of surface plasmons is used as the basis for powerful chemical and biochemical detectors that can monitor molecular binding events. In optics, the large field strengths of surface plasmons can dramatically enhance a variety of phenomena such as Raman scattering and light transmission through sub-wavelength apertures. In addition, the size of certain surface plasmonic configurations can be smaller than the wavelength of the exciting light, thus offering a path to scaling the sizes of optical components to below the diffraction limit.

Our group studies all aspects of surface plasmons and plasmon nanostructures, from integrated plasmonic waveguides to the enhancement of photodynamic processes. Surface plasmons are potentially critical enablers in many areas of nanophotonics, as their sensitivity can be leveraged for enhancing the performance of modulators, switches, chip-scale optical sensors and many other integrated optical components. The field enhancement associated with nanostructures can change the radiative environment of embedded fluorophores, quantum dots and other emitters, fundamentally changing the way such systems radiate. Spontaneous emission can be dramatically enhanced or suppressed according to the design of the plasmonic structure and the details of the field enhancement region, leading for example to ultrabright light emitting diodes. Yet another area where plasmonic components can have a critical impact is that of detectors and imaging devices. Plasmonic nanostructures can be used to form or enhance absorbing materials or pixels that can absorb light and minimize reflection with tremendous efficiency.

Plasmonics projects in our lab

Plasmonic Transmission Lines Plasmonic structures can be engineered into integrated plasmonic circuits. An integrated circuit contains, on a single compact substrate, a variety of functional components (filters, couplers, modulators) that are connected with integrated waveguides. One must be able to precisely calculate how the energy is distributed among the elements of the circuit, how to calculate the transitions between different elements of the circuit and how to minimize all the losses. Another specific concern is the reproducibility, as one should be able to fabricate circuits that behave in a predictable manner, aligning with numerical simulations and systematically yielding the same behavior no matter the sample.

Enhanced Fluorescence Radiating systems, such as fluorophores and quantum dots, see a drastically different optical environment when located within the high-field regions of nanoplasmonic structures. Light emission from a fluorophore or other system is a statistical process: Once excited into a higher electronic state, a molecule eventually relaxes to a lower state by either emitting a photon via spontaneous emission, or via a nonradiative transition. All relaxation paths are characterized by lifetimes that are inversely related to the probability of the transition. When there are more possible modes for the photon to occupy, the radiative rate is correspondingly larger. The enhanced region of a plasmonic system dramatically increases the number of modes into which a photon can be emitted, thus drastically decreasing the emission lifetime--one of the most fundamental characteristics of a radiator. Enhanced fluorescence is a fascinating quantum plasmonic phenomenon that enables us to directly engineer the photodynamic processes of an emitter using a nanoplasmonic structure. Enhanced spontaneous emission can be leveraged for ultrabright and highly efficient light sources across the optical spectrum.

Extreme Plasmonic Enhancement The degree of field enhancement in a plasmonic structure depends on its geometry. As an example, two nanospheres placed close to each other but separated by a small gap produce very large fields localized to the interstitial nanometer volume between the spheres. As the spheres are brought closer together, the enhancement increases to extreme values. A simple question that can then be asked is, how large can the optical field be in the gap? Since every important plasmonic phenomenon depends on the strength and details of the field enhancement, the upper bound of field enhancement will bound the strength of all plasmonic phenomena. The answer to this question lies in understanding the details of the electronic response of conductors at optical wavelengths. Taking into account the repulsion between electrons, or even the quantum mechanical nature of the electronic response, the ultimate limits of enhancement can be found. But, even though the field enhancement may be bounded, other interesting physics emerges, making extreme plasmonics one of the important frontiers of plasmonic research.

Colloidal Plasmonic Absorber Plasmon nanostructures scatter light with tremendous efficiency. That scattering efficiency can, in fact, be flipped around to make extraordinarily efficient absorbing materials--materials that can absorb incident light. An array of nanocubes over a metal film, for example, can form a surface capable of absorbing nearly 100% of the incident light over a band of wavelengths, and over a wide angular span. Even more interesting is that the array need not be periodic or precisely patterned; in fact, a randomly deposited collection of colloidal nanocubes can create an absorbing surface, tunable over the infrared and visible spectrums. The film-coupled nanocubes used to create the absorber can be viewed as the optical analogs of patch antennas common in communications applications; the same patch antenna theory, modified for the plasmonic response, can be used to describe optical patch antennas and thus the design of nanocube absorbing surfaces. One interesting application of nanocube absorbers is to modify the blackbody spectrum of a surface, imparting a designer emissivity spectrum. Absorbing surfaces thus have relevance to detectors, as well as thermophotovoltaic cells.

Nonlinear Plasmonics Because nonlinear processes depend on higher order powers of the field, such processes can be dramatically enhanced by leveraging the field enhancement associated with plasmonic nanostructures. Second and third harmonic generation and three- or four-wave mixing are just some of the types of processes that can undergo significant enhancement with plasmonic nanostructures. One path to achieving this enhancement is to embed an inherently nonlinear material within the gap region of interaction plasmonic nanoparticles, such that its nonlinear properties are magnified by the field enhancement. Alternatively, the metal itself comprising the plasmonic device can be used as the nonlinear medium, since metals have large nonlinear responses. In fact, the nonlinear mechanisms of plasmonic materials at optical wavelengths are quite complex, containing new physics that is only now being vigorously explored. Nonlinear plasmonic media have the potential to serve as components in next generation active photonic devices.