Impedance Matching and Emission Properties of Nanoantennas in an Optical Nanocircuit

Abstract

An experimentally realizable prototype nanophotonic circuit consisting of a receiving and an emitting nano antenna connected by a two-wire optical transmission line is studied using finite-difference time-and frequency-domain simulations. To optimize the coupling between nanophotonic circuit elements we apply impedance matching concepts in analogy to radio frequency technology. We show that the degree of impedance matching, and in particular the impedance of the transmitting nano antenna, can be inferred from the experimentally accessible standing wave pattern on the transmission line. We demonstrate the possibility of matching the nano antenna impedance to the transmission line characteristic impedance by variations of the antenna length and width realizable by modern microfabrication techniques. The radiation efficiency of the transmitting antenna also depends on its geometry but is independent of the degree of impedance matching. Our systems approach to nanophotonics provides the basis for realizing general nanophotonic circuits and a large variety of derived novel devices. 3 IntroductionMiniaturization and packaging density of integrated optics based on dielectrics is limited by the wavelength scale modal profiles of guided modes. 1 In contrast, plasmonic modes on noble metal nanostructures offer strong subwavelength confinement and therefore promise the realization of nanometer-scale integrated optical circuitry. 2,3 A truly subwavelength integrated photonic circuit based on plasmonic nano structures will generally consist of (i) a set of optical antennas 4,5,6,7 to efficiently excite specific local modes by far-field radiation, (ii) a very small footprint network of optical transmission lines 8 (OTLs) to distribute and manipulate plasmonic excitations, 9,10,11,12,13,14,15,16 and (iii) another set of optical antennas to efficiently convert local modes into propagating photons. The properties of metal nanoparticle chains, 17,18 metal nanowires, 19,20 line defects in plasmonic photonic crystals, 21 as well as gaps 22,23,24, and v-shaped grooves 9,25,26 in extended metal films have been explored as subwavelength waveguides for light.Efficient launching of specific guided modes on such structures is difficult since it requires matching of both, the small mode extension and the k-vector. It has been shown recently that efficient coupling between far-field photons and subwavelength spatial domains can be achieved using resonant optical antennas. 5,7,27,28,29,30,31 However, so far optical antennas have mostly been studied as isolated elements. Here we consider optical antennas as integral parts of an experimentally realizable integrated nanophotonic circuit where they act as efficient interfacing elements between propagating photons and guided modes of a plasmonic two-wire transmission line. We show by simulations that the principles of classical transmission line theory, e.g. impedance matching, 32 between the two-wire OTL and dipole antennas are fully applicable at optical frequencies. We further suggest tha...

Wireless at the Nanoscale: Optical Interconnects using Matched Nanoantennas

“…It is seen that the presence of the small loading nanoparticle drastically improves the matching at frequency f 0 , transforming the field distribution along the stripline from an almost pure standing wave (red dashed line in the bottom panel of [11]) to an almost perfect straight line (black solid line), with much reduced reflection at the load. Even the improvement with respect to optimum matching for the unloaded case, which arises at 455 THz, consistent with the matching geometries reported in [6], is quite significant, thanks to the proper matching and loading design of Fig. 2.…”

“…One of the key issues in the performance of such optical wireless links is represented by the reflection coefficients À r and À t , which manifest the impedance mismatch between the nanoantenna and the plasmonic waveguide, as discussed in recent papers [6]. Here, following these efforts, we establish a proper methodology to match these nanodevices and, as a result, maximize the relative power received at the destination points, which is made far larger than what would have been if only subdiffractive plasmonic striplines had been used to connect the two points (Fig.…”

Experimental cross-polarization detection of coupling far-field light to highly confined plasmonic gap modes via nanoantennas

“…Nanoantennas are highly compact and were proposed as a kind of nanocoupler to couple far-field light to two-dimensional, highly confined gap modes. 8,9 Recently, the idea of optical wireless interconnects via nanoantennas was theoretically proposed by Alù and Engheta. 10 Thus loading the nanoantennas with plasmonic waveguides is the first step toward optical wireless communication analogous to the ubiquitous loading of radio antennas with coaxial cables in the regime of radio frequency.…”

Plasmonic circuits for manipulating optical information

“…Initial studies on optical antennas verified the field enhancement properties of plasmons [174] and the spectral dependence of the antenna near-field [175]. Optical antennas have been used to enhance optical coupling into materials [176], plasmonic waveguides, and transmission lines [177][178][179]. Examples of different optical antenna designs are the Yagi-Uda antenna [180,181], cross antennas [182], J-pole [183,184], V antennas [183], and bow-tie antennas [185].…”