**1. Introduction**

Infrared detectors have many types, which can be classified based on their applications and physical response to infrared radiation. Some of them are sensitive to temperature changes due to the thermal effects of infrared radiation, others absorb infrared photons and generate free carriers. Examples of infrared detectors are microbolometers, pyroelectric detectors, cameras, photodetectors, and solar cells [1].

Infrared detectors are becoming ultra-compact in size with tiny active areas in the range of a few micrometers or nanometers. That enhances as well as yields novel

detection characteristics and properties [2]. The tiny size has several advantages. It allows for efficient cooling of the detector, thus reducing thermal noise and allowing good performance at high temperatures. It reduces device capacitance and thus allows for ultrafast optical switching and manipulation of high bitrates data. Also, it permits the collection of almost all photogenerated carriers due to their short path to electrodes, and thus high quantum efficiency. In cameras and sensors, tiny size means aggregating a lot of small-size pixels in a small area, and thus high spatial resolution imaging and the possibility of ultra-dense integration.

Downsizing the detection devices area comes at the expense of the smaller aperture area, and in turn inefficient collection of infrared radiation energy. The solution to such a problem is to utilize infrared plasmonic optical antennas in front of the detectors. The optical antennas can collect optical energy efficiently from free space to focus it on small size devices [3–8]. The plasmonic focusing of electric fields into sub-wavelength nano hot spots increases the atoms absorption cross-section areas within detection material (e.g. thin-film) and in turn the materials absorption coefficient [8–19].

The optical antenna design should have some characteristics for getting the best performance. In other words, it should have a special shape, different stages, and optimum dimensions. The shape should be selected to have a large aperture that collects infrared energy from all over the free space and focuses it down to a subwavelength nanoscale area. One aspect of that is to minimize the back-reflections of optical energy at the antenna input. That can be done by matching the antenna input optical impedance to that of free-space, in addition to optical impedances among different stages of the antenna. That of course imposes a lot of constraints on antenna dimensions. Talking about optical impedance, we mean the wave nature of light that allows us to treat light as an electromagnetic wave with ultra-high frequency traveling in a medium with impedance.

The antenna should also have a special shape that allows it to be polarizationinsensitive. In that way, it can collect infrared energy from different incident polarizations, which is useful, especially for solar cells and energy harvesting applications. However, it is worth mentioning that in some other applications like infrared cameras, for example, polarization-dependent (i.e. polarimetric) detection can be useful to distinguish between different features in a scene [20].

In addition, the plasmonic antenna should have an overall small size to minimize ohmic power losses associated with surface plasmon polaritons (SPP) traveling waves on antenna metal surfaces [21]. Moreover, the optical antenna should have a broad bandwidth. Thus, it can collect as much energy as possible within a specific wavelength band, and in turn, its average response over such particular bandwidth is maximized. Also, the optical antenna should have a wide field of view to collect as much radiation as possible from all angles of view.

It is worth mentioning that optical antennas are not solely useful in infrared detection applications. Optical nanoantennas can be also useful in detection applications within the visible range. For example, it can be used in energy harvesting and solar cells [22–24].

In the following sections, some examples of optical infrared antennas will be presented. In Section 2, a brief overview of some optical antenna examples will be discussed. In Section 3, a novel design of a plasmonic Bundt optical antenna will be presented and discussed. In Section 4, the performance of the Bundt optical antenna will be evaluated. Finally, a conclusion section will summarize the important points discussed in this chapter.
