plasmonics beyond diffraction limit

nature photonics | VOL 4 | FEBRUARY 2010 | www.nature.com/ naturephotonics 83 review article published online: 29 january 2010 | doi: 10.1038/nphoton.2009.282 T he perormance, speed and ease-o-use o semiconductor devices, circuits and components is dependent on their miniaturization and integration into external devices. However, the integration o modern electronic devices or inormation processing and sensing is rapidly approaching its undamental speed and bandwidth limitations, which is an increasingly serious problem that impedes urther advances in many areas o modern science and technology . One o the most promising solution s is believed to be in replacing electronic signals (as inormation carriers) by light. However, a major problem with using electromagnetic waves as inormation carriers in optical signal-processing devices and integrated circuits is the low levels o integration and miniaturization available, which are ar poorer than those achievable in modern electronics. Tis problem is a consequence o the diraction limit o light in dielectric media, which does not allow the localization o electromagnetic waves into nanoscale regions much smaller than the wavelength o light in the material 1 . Te use o materials with negative dielectric permittivity is one o the most easible ways o circumventing the diraction limit and achieving localization o electromagnetic energy (at optical requencies) into nanoscale regions as small as a ew nanometres. Te most readily available materials or this purpose are metals below the plasma requency. Metal structures and interaces are known to guide surace plasmon–polariton (SPP) modes 2 , electromagnetic waves coupled to collective oscillations o electron plasma in the metal. As a result, plasmonics is an area o nanophotonics beyond the diraction limit that studies the propagation, localization and guidance o strongly localized SPP modes using metallic nanostructures. Te recent rapid development o plasmonic waveguides whose mode connement is not limited by the material parameters o the guiding structure has been primarily driven by the tantalizing prospect o combining the compactness o an electronic circuit with the bandwidth o a photonic network. r gg f m Te existence o surace plasmon–polariton waves, which are localized near and propagate along the interace o a plasma- like medium, has been known or decades 2 . However, the ‘second birth’ o SPPs and the recent rapid development o research in this area 3–5 occurred when scientists realized that SPP modes in metallic nanostructures may lead to the localization o guided light signals ar beyond the diraction limit or electromagnetic pm f m dm K. Gm v 1 * sg i. bzv 2 * Rnt yr v n rpd xpnn rr nt nnptn bd n r plmn–plrtn. T ltrmgnt wv prpgt lng mtl–dltr ntr nd n b gdd by mtll nntrtr bynd t drtn lmt. T rmrkbl pblty nq prpt r t dgn gly ntgrtd ptn gnl-pr ng ytm, nnrltn ptl mgng tnq nd nr. T Rvw mmrz t b prnpl nd mjr vmnt plmn gdng, nd dtl t rrnt tt--t-rt n bwvlngt plmn wvgd, pv nd tv nnplmn mpnnt r t gnrtn, mnpltn nd dttn rdtn, nd nfgrtn r t nnng lgt. Ptntl tr dvlpm nt nd ppltn nnptn dv nd rt r l dd , n ptl gnl prng, nnl ptl dv nd nr-fld mrpy wt nnl rltn. waves in dielectric media. Tis allows visible and inrared light to be concentrated into regions as small as a ew nanometres, limited only by the atomic structure o matter, dissipation and the spatial dispersion o light 6,7 . Te beginning o this major renewed interest in SPPs — particularly in SPP modes not subject to the diraction limit — is marked by two papers published in 1997. Te rst demonstrated the possibility o subdiraction guiding SPP modes in cylindrical metal nanowire or nanohole congurations 8 , and the second introduced the idea o nanoocusing (that is, concentrating light energy into nanoscale regions) using SPP modes in wedge- like metallic structures 9 . Tese papers laid the oundation or one o the major research areas o modern plasmonics — SPP-based waveguides with subwavelength localization. Tis research has applications in the development o a new generation o nanophotonics devices and circuits, and as a means o concentrating and delivering light energy to nanoscale regions. Plasmon nanoguiding. Various types o metallic nanostructure have been proposed or guiding SPP modes. Tese include thin metal lms 10,11 , chains o metal nanoparticles 12,13 , cylindrical metal nanorods and nanoholes in a metallic medium 8,14 , metal nanos- trips on a dielectric substrate 15–19 , nanogaps between metallic media 10,20–22 , slot waveguides in the orm o rectangular nanogaps in thin metal lms 23–26 , sharp metal wedges 27–31 , nanogrooves in metal substrates 30–35 and hybrid plasmonic waveguides ormed by dielectric nanowires coupled to a metal surace 36 . It is important to note that not all SPP modes guided by these structures can be used or achieving subwavelength localization o the guided sig nals. For example, metal lms 10,11 and strips 15–19 can guide either long- or short-range SPPs, and decreasing the thickness o the lm or strip results in poorer localization o the long-range mode. Tese long- range SPP modes experience only weak dissipation because only a small portion o the wave ’ s energy is carried in the dissipative metal. Hence, they can propagate relatively large distances (o the order o millimetres at telecommunications requencies), but do not exhibit subwavelength eld localization, making them impossible to use in highly integrated optical circuits. In contrast, short-range SPP modes increase their localization with decreasing lateral dimen- sions o the guiding strip 15–19 (similar to metal nanorods; Fig. B1b). Although such modes are characterized by relatively strong dissipation and signicantly smaller propagation distances than long- range SPP modes, their major advantage is the possibility o strong 1 Nanophotonics, GPO Box 786, Brisbane, Queensland 4035, Australia. 2 Institute o Sensors, Signals and Electrotechnics (SENSE), University o Southern Denmark, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark. *e-mail: [email protected] ; [email protected] © 20 Macmil lan Pu blishe rs Limit ed. All rights r eserve d 10

plasmonics beyond diffraction limit

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