Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

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Plasmonics: Fundamentals and Applications

Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

Electronic circuits provide us with the ability to control the transport and storage of electrons. However, the performance of electronic circuits is now becoming rather limited when digital information needs to be sent from one point to another. Photonics offers an effective solution to this problem by implementing optical communication systems based on optical fibers and photonic circuits. Unfortunately, the micrometer-scale bulky components of photonics have limited the integration of these components into electronic chips, which are now measured in nanometers. Surface plasmon-based circuits, which merge electronics and photonics at the nanoscale, may offer a solution to this size-compatibility problem. Here we review the current status and future prospects of plasmonics in various applications including plasmonic chips, light generation, and nanolithography.

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Plasmonics: merging photonics and electronics at nanoscale dimensions.

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.

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Plasmonics for extreme light concentration and manipulation.

The presence of tiny holes in an opaque metal film, with sizes smaller than the wavelength of incident light, leads to a wide variety of unexpected optical properties such as strongly enhanced transmission of light through the holes and wavelength filtering. These intriguing effects are now known to be due to the interaction of the light with electronic resonances in the surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes. This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics.

Light in tiny holes

We present a scheme for exploiting the nonresonant second‐order nonlinearities in electro‐optic media to extend the bandwidth of coherent spectroscopy in the far‐infrared using ultrafast laser pulses. Using optical rectification and electro‐optic sampling in 〈110〉 ZnTe for the generation and coherent detection of freely propagating THz radiation, respectively, we have demonstrated spectral sensitivity beyond 3 THz. This was accomplished by achieving phase matching for both optical rectification and electro‐optic sampling over a broad range of THz frequencies.

A wideband coherent terahertz spectroscopy system using optical rectification and electro‐optic sampling

The optical characteristics and applications of the quasicrystal, a special form of aperiodic engineered structure, are explored in this Review article.

Optics of photonic quasicrystals

A Nature paper in 1998 reported the transmission of resonantly enhanced light through 'plasmonic lattices', which are metal films punctured with arrays of holes smaller than the wavelength of the light. The phenomenon generated significant interest, first as it was a surprise, and second because it has potential applications in near-field optical microscopy, photolithography, displays and elsewhere. The periodicity of the holes was thought crucial, but new experiments show that this is not so. Though randomly distributed holes arrays with quasicrystal or approximate quasicrystal structure do. As an added bonus, transmission resonances for the lattices used in this experiment are in the terahertz range, for which there is a shortage of optoelectronic materials. In contrast to the conventional view, this paper reports transmission resonances in the terahertz frequency range for aperiodic aperture arrays, with quasicrystal or approximate quasicrystal structure. Resonantly enhanced light transmission through periodic subwavelength aperture arrays perforated in metallic films1 has generated significant interest because of potential applications in near-field microscopy, photolithography, displays, and thermal emission2. The enhanced transmission was originally explained by a mechanism where surface plasmon polaritons (collective electronic excitations in the metal surface) mediate light transmission through the grating1,3. In this picture, structural periodicity is perceived to be crucial in forming the transmission resonances. Here we demonstrate experimentally that, in contrast to the conventional view, sharp transmission resonances can be obtained from aperiodic aperture arrays. Terahertz transmission resonances are observed from several arrays in metallic films that exhibit unusual local n-fold rotational symmetries, where n = 10, 12, 18, 40 and 120. This is accomplished by using quasicrystals with long-range order, as well as a new type of ‘quasicrystal approximates’ in which the long-range order is somewhat relaxed. We find that strong transmission resonances also form in these aperiodic structures, at frequencies that closely match the discrete Fourier transform vectors in the aperture array structure factor. The shape of these resonances arises from Fano interference4 of the discrete resonances and the non-resonant transmission band continuum related to the individual holes5. Our approach expands potential design parameters for aperture arrays that are aperiodic but contain discrete Fourier transform vectors, and opens new avenues for optoelectronic devices.

Transmission resonances through aperiodic arrays of subwavelength apertures

We report the demonstration of an electro‐optic sampling technique that allows for the detection of freely propagating terahertz radiation. Coherent sampling is performed in a poled polymer device that is physically separated from the emitter. The poling electrodes in the sampling element are found to have an integrating effect on the incident terahertz field. The shot noise limited minimum detectable field in the polymer is 100 (mV/cm)/√Hz. We discuss methods by which the sensitivity may be significantly enhanced.

Coherent detection of freely propagating terahertz radiation by electro‐optic sampling

We demonstrate an ∼104 increase in conversion efficiency for optical second-harmonic generation (SHG) from a periodically nanostructured metal structure consisting of a single subwavelength aperture in a thin silver film surrounded by a set of concentric surface grooves. The forward-transmitted second-harmonic radiation from this structure is measured relative to that from an identical aperture with no surrounding surface periodicity. We explain the observed SHG enhancement quantitatively in terms of a measured 120× increase in the strength of the fundamental radiation in the vicinity of the aperture resulting from the periodic nanostructuring.

Ajay Nahata

Papers

A wideband coherent terahertz spectroscopy system using optical rectification and electro‐optic sampling

Optics of photonic quasicrystals

Transmission resonances through aperiodic arrays of subwavelength apertures

Coherent detection of freely propagating terahertz radiation by electro‐optic sampling

Enhanced nonlinear optical conversion from a periodically nanostructured metal film.