**2. Examples of optical infrared antennas**

There are various types of optical antennas that have been known so far. To name some, a half-wave dipole optical antenna can concentrate infrared onto nanometer germanium photodiode. This optical antenna is polarization sensitive. It covers a bandwidth of 1.34 to 1.48 μm and it shows a factor of 20 enhancement in detector response [8]. Another example is a silicon plasmonic horn that has periodic grooves on its surface. It has a short length and gives a maximum coupling efficiency of 27% within the wavelength range of 1538 to 1562 nm [9]. A silver nano-array of coaxial rings shows extraordinary transmission with almost a factor of 4 enhancement in transmitted optical intensity [10]. A hybrid silicon-gold nano-particles optical antenna is also demonstrated with multipole resonance. It covers all the visible wavelength range and can enhance optical absorption by a factor of 2.5 [11]. A plasmonic spiral ring grating could be coupled to a vertical nano-optical antenna to enhance optical field intensity by up to 7 orders of magnitude. The simulated collection efficiency can reach up to 68% [12]. A dielectric silicon nanoantenna is demonstrated with an ultra-low optical power loss and heating conversion. It covers the near-infrared band and can enhance surface fluorescence by about 3 orders of magnitude [13]. A trench thin metal polarization-insensitive antenna is demonstrated. It is polarization insensitive with high responsivity at telecommunication infrared wavelengths [14]. A nanoantenna sandwiched between two graphene monolayers photo-detector is demonstrated. It covers both visible and infrared bands with 8 times enhancement in detection response and up to 20% internal quantum efficiency [15].

#### **Figure 1.**

*The checkerboard optical antenna detection device. (a) the SEM image of the fabricated checkerboard device sample (30 x 30 μm<sup>2</sup> ). (b) Magnified SEM image of four cross-oriented unit cells, L = 1049 nm, D = 530 nm, W = 510 nm. (c) a schematic cross-section in detection device, L = 1000 nm, W = 500 nm, D = 500 nm,T1 (gold) = 50 nm,T2 (titanium) = 10 nm,T3 (Si3N4) = 400 nm [16].*

In the following subsections, structures of some types of optical antennas are explored and examined in detail.

### **2.1 Checkerboard optical antenna**

The gold checkerboard antenna consists of nanoscale dipole antennas [16, 17]. **Figure 1a** and **b** show scanning electron microscope (SEM) images of such checkerboard structure. It consists of subwavelength gold stripes arranged in cross-oriented unit cells. The stripes are separated by subwavelength gaps. Each stripe acts like a dipole antenna that can receive long infrared radiation (8-12 μm). The gold stripes interact with incident light electromagnetic waves resulting in collective electrons oscillations (plasmons) in direction of light polarization (i.e. forming dipoles). The oscillating electrons accumulate at the stripes' edges resulting in a very high electric field with spatial nano-resolution [21]. These localized surface plasmons are coupled together within the gaps among stripes, thus creating what is called hot spots. These hot spots have very high concentrated infrared optical intensities. Thus, it can increase the optical absorption within the underneath absorbing layer of silicon

#### **Figure 2.**

*(a) The SNOM image of the scattered electric field (E), in arbitrary units, from the checkerboard sample (rotated) at a 10.19 μm wavelength. (b) the FDTD simulation of scattered electric field magnitude under the same SNOM measurement conditions in part (a). (c) a comparison of the SNOM scattered electric field magnitudes with/ without checkerboard on the same sample at 10.6 μm wavelength [16].*

### *Infrared Nano-Focusing by a Novel Plasmonic Bundt Optenna DOI: http://dx.doi.org/10.5772/intechopen.104695*

nitride (Si3N4) and in turn the detection efficiency. The detection device layers structure is shown in **Figure 1c**.

**Figure 2a** and **c** show images of scattered electric fields from the checkerboard antenna taken by scanning near-field optical microscope (SNOM) at a wavelength of 10.19 μm. **Figure 2b** shows a corresponding image to that of **Figure 2a**, simulated using a three-dimensional finite-difference time-domain (FDTD) method. As seen in **Figure 2c**, there is a dark area underneath the checkerboard structure. This darkness indicates enhanced absorption within the silicon nitride thin-film layer.

The tested checkerboard antenna shows broadband and polarization-independent average absorption enhancement of 63.2% over the wavelength range 8–12 μm. Also, it shows a maximum absorption enhancement of 107% at 8 μm wavelength and a minimum enhancement of 24.8% at 12 μm wavelength.

## **2.2 Dipole nanoantenna coupled to plasmonic slot waveguide**

Another example of optical antennas is the dipole coupled to a plasmonic slot waveguide [18]. The dipole antenna is made of gold and has a bottom and side reflectors that can increase its coupling efficiency up to 26% at a wavelength of 1550 nm. The coupling efficiency is defined as the ratio of power delivered to the waveguide to incident power on the dipole antenna. In this configuration the dipole antenna is irradiated by a vertical optical fiber terminated by a focusing lens, see **Figure 3a**.

#### **Figure 3.**

*(a) Antenna working in vertical coupling configuration. The beam from the fiber excites slot plasmons. Instant images of the simulated electric field (E) in arbitrary units at ?=1.55 μm when coupling to (b) waveguide only, (c) single antenna, (d) antenna with side and bottom reflectors, (e) serial, and (f) parallel antenna array. The color scale is equal for (b) through (f). Reprinted with permission from [ref 18] © the Optical Society.*

The antenna collects fiber radiation and launches it inside an impedance-matched slot waveguide. The dipole antenna length is adjusted to be an integer number of halfwavelengths. Also, the side and bottom reflectors are positioned such that it forms a maximum of standing wave reflections at the antenna position. **Figure 3** shows different configurations of the tested system together with images of simulated electric fields. As seen, the coupled electric field magnitudes increase from **Figure 3a–d** here the coupling efficiency becomes maximum. The system is also tested with serial and parallel dipole arrays as shown in **Figure 3e** and **f**, respectively. However, these two configurations give the same performance as a single dipole antenna with side and bottom reflectors.

This dipole nanoantenna shows 185 times enhancement in coupling efficiency when compared to a bare waveguide. It can be utilized as an interface coupler between an optical fiber and a plasmonic slot waveguide.

### **2.3 Horn optical nanoantenna**

An additional example of optical antennas is a two-dimensional plasmonic horn nanoantenna made of silver. This antenna is impedance matched to a plasmonic slot feeding transmission line [19]. It has a broad bandwidth (1260–1675 nm) for applications in wireless on-chip communications. It is investigated by FDTD simulations as shown in **Figure 4**. In **Figure 4a, d** and **g**, the horn length and its flare angle should be

#### **Figure 4.**

*Near-field profiles and far-field radiation patterns of a bare waveguide and horn nanoantennas with different flare angles. (a–c) Bare waveguide case. The amplitude (a), phase (b) of electric field distribution, and radiation pattern (c) of a bare waveguide. (d–f) a horn nanoantenna with a length (*La*) of 3.5μm and flare angle (θ) of 10°. The amplitude (d), phase (e) of electric field distribution, and radiation pattern (f). (g–i) a horn nanoantenna with* La *= 3.5 μm and θ = 25°. The amplitude (g), phase (h) of electric field distribution, and radiation pattern (i) [19].*

### *Infrared Nano-Focusing by a Novel Plasmonic Bundt Optenna DOI: http://dx.doi.org/10.5772/intechopen.104695*

carefully designed to minimize back reflections at the horn input. The corresponding wavefront phase distributions are shown in **Figure 4b, e,** and **h**. In **Figure 4c, f,** and **i**, the far-field radiation patterns are shown for each case. The best design is found to be the case in **Figure 4d–f**, where the back-reflected field magnitude is minimized, the phase front looks like a plane-wave, and thus it is matched to that of free-space radiation, in addition, the radiation pattern is highly directional. This optical antenna can give a superior performance with almost 100-fold enhancement in power transfer when compared to conventional dipole nanoantennas.

It is worth mentioning that an optical antenna design should take into consideration some points that allow it to have better performance characteristics when compared to the previous designs. For example, it should have a special shape with a large aperture to collect infrared energy from all free-space and focuses it on a nanoscale area. Also, back-reflections of antenna optical energy should be minimized by matching antenna input optical impedance to that of free-space. That of course imposes a lot of design constraints on the optical antenna dimensions. It is also better to have a polarization-insensitive antenna to collect all infrared energy at different polarizations. In addition, a plasmonic optical antenna should be small enough to minimize the surface plasmon polaritons' ohmic power losses. It is also desirable to have a broadband antenna optical response. Moreover, an optical antenna should have a wide field of view to collect optical energy from all over freespace angles.

In the following sections, we will explore in detail a novel optical antenna called Bundt Optenna that realizes these design constraints and thus possess several advantages.
