**5. Conclusions**

In this chapter, we have explored some types of plasmonic optical antennas that can focus optical infrared radiation to very small spot areas beyond the light diffraction limit. These antennas are essential for infrared optical detection devices that have tiny active areas in the range of a few micrometers or even nanometers. In addition, we have focused on one innovative type of optical antenna called Bundt Optenna. We investigated its design and optimization. The Bundt Optenna bandwidth covers the near-infrared, short-wave, and mid-wave infrared bands. The Bundt has a wide

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

**Figure 8.**

*The Bundt Optenna absorption enhancement factor inside the thin-film layer indicated in linear scale on the left axis, and in decibels on the right axis: (a) design "1", (b) design "2", (c) design "3", (d) design "4" [25].*

field-of-view equal to 80°. It has an ultra-broadband optical response and fractional bandwidth of up to 42%. It can nano-focus electric and magnetic fields to a 50 nmwide area, and thus enhance the optical absorption efficiency of the thin-film detection layer. The fields' intensity gain can reach up to 12.4 dB. The power absorption enhancement can be 8 orders of magnitude (i.e. 80 dB). The average ohmic power loss in Bundt Optenna is as low as 3 dB. The Optenna is polarization-insensitive and has a relatively compact size. The Bundt Optenna can be used in different nanoscale detection devices such as photodetectors, solar cells, cameras, and microbolometers, with potential applications in optical communications, imaging, energy harvesting, optical sensors, and biomedical technology.
