## 5 Sensors
### Temperature Sensor  
The first sensor, in the strict sense of the definition, is the temperature one, i.e., a thermometer. To simulate this sensor, it is necessary to have experimental results or models for the dependence of the optical properties with the temperature.  
It was impossible to find air and quartz models for different temperatures. However, as these materials do not have an interband region in the simulated range, it is not necessary to have its models. On the other hand, the temperature has a small influence on the dielectric function real part, in comparison with the imaginary one. Thus, the influence of the metal will always be predominant and so therefore, structures as the ones presented in this work can be used as temperature sensors [21; 22; 23].  
A temperature sensor based on a gold nanoantenna is going to be simulated, for five different temperatures, T (23 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 250 \({}^{\circ}\)C and 300 \({}^{\circ}\)C), using models for each temperature [44]. The real part of the gold dielectric function is almost independent of the temperature until approximately a wavelength of 550 nm [44]. After it, the temperature  
There, it is observable that it is possible to differentiate each temperature only using the maximum gain value. Moreover, as expected, the conclusions taken from the complex dielectric function analysis are reflected on the nanostructure response.  
Furthermore, based on Figure 15, which is the combination of the responses shown in Figures 13 and 14, it is quite impressive how the spectrum retains the same shape and intensity values, in spite of the temperature variation, except in the work region (the peak values).  
These simulations allow us to conclude that each temperature will generate a different nanostructure response, which is well defined. Thus, measuring the output spectrum at wavelengths near the peak one, it is possible to determine what is the temperature around the nanostructure, since different temperatures produce different output electric field norms. The sensibilities of this specific structure (with this physical parameters) are presented in Table 1. There, it is possible to observe that the most sensible temperature range is the one above the critical value, in terms of gain variation. On the other hand, it is important to remember that this structure is not able to associate wavelength shifts to temperature variations, since its sensibility has a small value.  
Figure 13: Gold nanostructure response for different temperatures below the critical one.  
This structure is not optimized to have the highest sensitivity. Nonetheless, the aim of this article is accomplished since it is shown that this structure can be tuned and work as a sensor. Better sensibilities can be found using an application for multi-factorial optimization.  
Figure 14: Gold nanostructure response for different temperatures above the critical one.  
Figure 15: Gold nanostructure response for different temperatures.  
Moreover, if the critical temperature of gold is available, the determination of the temperature value only demands the measurement of the output signal for a given wavelength of the input signal. If the peak value and the uncertainty range are specified for each structure, it is possible to use only one wavelength measurement, since around the peak there are no equal values for different temperatures.  
Finally, the last conclusion that has to be clearly explained is the possibility of producing electrical energy. First, at around room temperature the nanostructure has optical gain; however, after a certain temperature, the gain will increase as the temperature increases. For that reason, it is possible to tune the energy production and actually generate energy using, for example, chemical reactions to produce heat and the environment radiation, which is the opposite of having the nanoantenna as a temperature sensor for chemical reactions.