## 5 Sensors
### General Purposes  
A sensor is characterized by a parameter whose values change with the occurrence of something. The parameter change has to be measured. Nonetheless, simpler applications must be analyzed first in order not to break the reasoning, even though these applications are not defined as sensors. For this reason, some applications are going to be covered, using the lastly presented nanostructures and the responses. The characterization of a sensor demands the measurement of a parameter whose values change with the occurrence of something. However, in some cases, the optimization of the detector, and namely of the nanoantenna response, is enough to optimize the sensor.  
An example is a nanoantenna of gold used to improve the sensibility of infrared sensors, for example on medical applications, since some infrared wavelengths have high amplification and the others have huge attenuation. Then the output of the sensor presents an improved sensitivity. Another example is the oximeter, where the sensor measures the amount of oxygen on blood. There are different types, from portable to the hospital ones. In both cases, there are LEDs emitting two distinct signals; for example, on the spectral red and blue regions. The optical spectrum of blood varies with the amount of oxygen, because cells can absorb more or less radiation according to the oxygen they transport. Thus, for example using the oximeter on fingers and analyzing the output spectrum resulting from the photodetection of the red and blue light after the radiation propagates through cells, it is possible to determine the percentage of oxygen. In this case, nanoantennas can detect and amplify the detected radiation, in the two spectra intervals of interest, replacing photodetectors and increasing the device sensibility. In this case, it is suggested that a gold nanoantenna can be used to detect the red signals and a silver one to detect the blues.  
Figure 12: Optical response of an air–aluminum–quartz nanostructure characterized by \(a_{0}=0.3\,\mathrm{\SIUnitSymbolMicro m}\), \(d=120\,\mathrm{nm}\), \(t=50\,\mathrm{nm}\) and \(t_{sub}=20\,\mathrm{nm}\).  
The overall sensibility will increase, weak signals can be generated and detected and small devices can be built.  
The optical response of the nanostructure with a nanoantenna of gold can be used to improve LED and LASER sensibilities, used for example in optical communications, in a certain wavelength band. As seen in the previous simulations, changing the material of the nanoantenna, it is possible to place the peak along the frequency/wavelength range. In the case of the spectrum in Figure 11, a near-infrared region is amplified, especially around 1000 nm. Another example on communication applications is the use of these nanoantennas to convert free-space energy into guided energy and vice-versa, as it is done by a regular antenna. In this case, in addition to this conversion, the structure can be tuned to filter and amplify, using just one miniaturized device.  
Lastly, the aluminum nanostructure, whose spectrum is in Figure 12, can be used to amplify the visible and ultraviolet regions. An example of this application is on the concentration and amplification of the solar irradiance, which is predominant on the visible region, incident on the active area of a solar cell, resulting in more incident power and consequently, on more generation of output power. However, it is necessary to guarantee the chemical stability of aluminum for long-term applications.