Optical antenna

The simplest and the most basic nanoantenna is the nanosphere. This structure actually behaves like a dipole. Its scattering properties can be calculated using the Mie theory [266]. Noble metal nanoparticles are often fabricated in spherical form. This makes them the simplest nanoantennas. 

Another very often met nanoantenna stmcture is the bowtie nanoantenna [313]. It consists of two triangles aligned along their symmetry axes. A feed gap is formed between their tips. Bowtie antennas have a broader bandwidth together and at the same time ensure large field localizations in the feed gap. A similar type of antenna is the diabolo-type nanoantenna [314], where the triangles overlap. 

There is a plasmonic analogy of the well-known Yagi-Uda antenna. It can be fabricated by placing a resonant nanorod antenna between a reflector nanorod and a group of director nanorods [315]. Similar to such antennas used in radiofrequent domain, it ensures a good directivity. 

Among the nanoantennas mentioned in the literature are spiral nanoantennas [316] and those with fractal geometries [317]. Many other shapes can be used. Basically, all particles used to fabricate metamaterials can also function as nanoantennas. This includes single and double split rings, crescent antennas, etc. 

An important group of nanoantennas are those based on the Babinet principle. A metal shape surrounded by dielectric and a dielectric-filled hole in metal with identical shape and size have identical diliiaction patterns. Thus bowtie shaped holes in metal can be used, two holes are a Babinet equivalent of a nanodimer, arrays of nanoholes are equivalent of arrays of nanoparticles, crossed arrays of nanoholes correspond to crossed arrays of nanoparticles, etc. [318]. 

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Kempa K, Rybczynski J, Huang ZP, Gregorczyk K, Vidan A, Kimball B, Carlson J, Benham G, Wang Y, Herczynski A, Ren ZF (2007) Carbon nanotubes as optical antennae. Advanced Materials 19 421. 

Fig. 4. Detection of single molecules by near-field techniques, (a), (b) SNOM images of single molecules embedded in a polymer film with random orientations, from [29]. (c) Sketch of the field distribution in the near field of the aperture, (d) Comparison of measured field distributions to calculated, once for both fundamental orientations, from [27]. (e) Effect of an optical antenna on the emission of a single emitter [36].

J. N. Farahani, D. W. Pohl, H.-J. Eisler and B. Hecht, Single quantum dot emitter coupled to a scanning optical antenna A tunable superemitter, Phys. Rev. Lett. 95, 017402 (2005). 

Experiments with shaped nanoholes (in contrast to circular holes) offer new possibilities of control over the spectroscopic response of adsorbed species. This is because anisotropic structures present polarization-dependent conditions for SP excitation [29,58-60]. Moreover, apexes in hole structure can further localize the light field, as illustrated in Fig. 10. The double-hole structure in Fig. 10 is formed by two nanoholes holes (150 nm diameter) separated by a controllable distance [29,61-65]. The SP resonance depends, then, not only on the same parameters as the circular holes (such as hole diameter and array periodicity), but also on the presence of the apex formed when the two holes touch each other, as shown in Fig. 10c. The apexes act as an optical antenna, focussing the electromagnetic field to a very small region, as shown in Fig. lOd, which is a FDTD calculation of the field distribution around a double-hole with overlapping holes. 

Chen G, Wu J, Lu QJ, Guitierrez HRH, Xiong Q, Pellen ME, Petko JS, Werner DH, Eklund PC (2008) Optical antenna effects in semiconducting nanowires. Nano Lett 8 1341-1346... 

L. Novotny, Effective wavelength scaling for optical antennas. Phys. Rev. Lett. 98, 266802 (2007)... 

For the future work, elastomer tunable optofluidic devices are expected to extend into the nano-optics or nanofluidic fields. Several tunable nano-optical antenna devices fabricated on a stretchable PDMS substrate have been demonstrated recently. Combining elastomer-based micro/nano-devices with nanoplasmonic elements can be interesting for molecule-level imaging and spectroscopy. A tunable elastic nanofluidic channel was demonstrated recently on a PDMS chip for nanoparticle separation and molecule trapping [12]. One of the challenges of PDMS-based tunable nano-devices is to realize the high accuracy in control. High-precision control of PDMS-based tunable structures could be realized by very fine pneumatic actuation or connection to a piezo-actuator. 

Fine details of nanostructures are carried by the high spatial frequency evanescent waves localized in the optical near fields, which exponentially decay from the source. In NSOM, a tiny local probe (aperture) was brought into the optical near fields that interacts with the evanesced waves. The probe functions as an optical antenna that converts localized energy into radiating waves which propagates into the far-field. 

FIGURE 13.64 Optical antenna feed systems (a) lens, and (b) reflector. 

RD Grober, RJ Schoelkopf, DE Prober. Optical antenna Towards a unity efficiency near-field optical probe. Appl Phys Lett 70 1354-1356, 1997. 

Knight MW, Sobhani H, Nordlander P, Halas NJ (2011) Photodetection with active optical antennas. Science 332 702-704... 

One of the approaches to the use of nanoantennas in photodetection is to use a Schottky metal-semiconductor junction. An optical antenna forms flie metal part of the metal-dielectric contact at the semiconductor detector surface [320]. Photoexcitation generates hot electron-hole pairs by plasmon decay and the electrons are injected over the Schottky barrier, thus directly generating photocurrent. A problem with this approach is its low efficiency. 

Y. Alaverdyan, B. Seplveda, L. Eurenius, E. Olsson, M. Kail, Optical antennas based on coupled nanoholes in thin metal films. Nat. Phys. 3(12), 884-889 (2007)... 

In Fig. 9.1 we sketch the fundamental configuration of the optical antenna used in near-field optical microscopy, i.e. a sharp noble-metal tip approaching the sample surface down to a few... 

Figure 9.1 Sketch of an optical antenna formed of a metal tip in close proximity to a metallic sample surface. The gap width is on the order of some nanometers, i.e. much smaller than the optical wavelength. The tip is polarized by the electric field of the incident electromagnetic radiation, leading to a surface charge at the tip apex and its mirror on the sample surface. The surface charges establish a strongly enhanced electric field in the gap.

Fleischer, M., Stanciu, C., Stade, E, Stadler, J., Braun, K., Heeren, A., Haffner, M., Kern, D. P., and Meixner, A. J. (2008) Three-dimensional optical antennas Nanocones in an apertureless scanning near-field microscope, App/. Phys. Lett, 93,111114/1-3. 

An impressive quantity of articles dealing with other applications of porphyrins and phthalocyanines can be found in the literature. For example, the utilization as optical antennas for plasmonic enhancement of photochemical reactions, still a challenge for scientists. In theory, the concept is very simple but its practical realization have demonstrated to be tricky showing that there are many still unknown aspects to be unveiled. Dceda et al. [245] compared the photocurrent generation efficiency with and without optical antennas on analogous porphyrin-based molecular monolayers, where they found out that the overall reaction efficiency is strongly dependent on the electrochemical properties of the optical antennas. 

Ikeda K, Sato S, Takahashi K, Masuda T, Murakoshi K, Uosaki K (2013) Surface optimization of optical antennas for plasmonic enhancement of photoelectrochemical reactions. Electrochim Acta 112 864—868... 

A nanovoid—a complementary structure of nanoparticles—also works as an optical antenna. DUV-SERS using an aluminum nanovoid structure has been reported [29]. The resonance wavelength of a nanovoid is determined by the diameter of the rim of the nanovoid. 

J.N. Farahani, H.J. Eisler, D.W. Pohl, M. Pavius, P. Fliickiger, P. Gasser, B. Hecht, Bow-tie optical antenna probes for single-emitter scanning near-field optical microscopy. Nanotechnology 18, 125506 (2007)... 

R.D. Grober, R.J. Schoelkopf, D.E. Prober, Optical antenna towards a unity efficiency nearfield optical probe. Appl. Phys. Lett. 70, 1354 (1997)... 

X. Jiao, S. Blair, Optical antenna design for fluorescence enhancement in the ultraviolet. Opt Express 20, 29909 (2012)... 

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