Glass Superstrate Nanoantennas for Infrared Energy Harvesting Applications

Several nanoantennas for infrared energy harvesting applications at 30 THz are studied. Contrary to usual antenna designs, we implemented glass superstrate as opposed to glass substrate for better antenna performance. We defined a figure of merit (FOM) which includes antenna fractional bandwidth, peak gain, and half-power beamwidth of the antenna under consideration. Three different antenna structures with glass superstrate and one of themwith glass substrate are studied in detail. According to our FOM definition, the Archimedean balanced spiral antenna exhibited superior performance among other structures with less sensitivity to the incoming polarization of the electromagnetic wave.


Introduction
With the increasing applications of nanoantennas that can focus electromagnetic energy into small localized area [1][2][3][4][5][6][7][8][9], energy harvesting gained more attention and considerable research has been devoted to it [10][11][12][13][14].Energy harvesting at visible spectrum of electromagnetic waves has been mostly achieved using monocrystalline silicon which is an expensive process.On the other hand, thermal radiation which is between 7 and 16 m falls into infrared (IR) region of the spectrum and provides opportunity for most of the available ambient energy at room temperature.Despite the fact that the amount of energy at IR is smaller than that of the visible spectrum, energy conversion at IR can be more promising due to relatively less expensive and complicated processes involved in the conversion devices.The diode structures utilized in the photon-electron conversion process are usually either metal-oxide-metal or metal-insulator-metal type diodes and can be incorporated within the feed gap of the antenna [15][16][17][18].
Antennas for IR applications follow quite similar guidelines with those at RF region [19][20][21][22][23][24][25][26][27][28][29][30].The radiation efficiency, directivity, and bandwidth play an important role for the overall antenna performance.One important aspect of these antennas is that they must be polarization insensitive simply because incoming electromagnetic signal has no preferred polarization; it is rather arbitrary.Similar to those antennas used in indoor communications, the antennas can be made circularly polarized and they can pick up almost any polarized signal except those with crosspolarization.However, circular polarization is difficult to achieve with external components, and rather the antenna must be inherently built with such polarization.Examples of such antenna structures are abundant in RF applications.Nevertheless, simple structures that are easy to manufacture are primary choices for designers at nanoscale lengths.In that respect, we also considered crossed dipole, bowtie, Archimedean spiral, and balanced crossed dipole geometries for IR detection.Our analysis is based on radiation properties of these antenna structures, but their reception properties are identical to their transmission ones according to Reciprocity theory.
In our antenna study, we propose two different structures: one that utilizes glass as a superstrate and the other glass as a substrate.For both geometries, different antenna structures are simulated and a figure of merit which depends on antenna peak gain, half-power beam width, and fractional bandwidth is defined and compared for all structures.

Antenna Designs
We consider glass superstrate (pyrex) with indium tin oxide (ITO) and gold imprinted on lower side as shown in Figure 1.
Drude model of gold is used in the simulations.CST Microwave studio, a 3D electromagnetic field solver based on finite integration technique, is used for simulations.Drude model of gold has been extensively studied before, and the relative dielectric constant of gold is approximated as where   and  are the plasma frequency and collusion frequency, respectively.The values of   and  are 1.216 × 10 16 rad/s and 0.2 × 10 14 rad/s.We considered crossed dipole (CD) antenna, bowtie antenna, spiral antenna, and modified crossed dipole (MCD) antenna for infrared applications.The structures are shown in Figure 2.
To assess the performance of each antenna structure a figure of merit (FOM) is defined as International Journal of Antennas and Propagation where HPBW is the half-power beam width and FBW is the fractional bandwidth, defined as where   is the center frequency for the frequency band.The impedance frequency bandwidth is defined as the band of frequencies where the magnitude of the input reflection coefficient drops below −8 dB.This FOM is unitless and is used for all optimized antenna structures for comparison.

Results and Discussion
Crossed dipole with 50 nm ITO and 50 nm gold thickness on a 20 × 20 × 5 m glass substrate is considered first.On half arm length is taken as 950 nm and width of the arm is taken as 80 nm.Normally, resonant antenna length at free space is expected to be around 5 m for 30 THz radiation, but due to glass superstrate, the resonant dimension is reduced  Realized gain, that is, mismatch loss subtracted from directivity, is shown in Figures 5(a) and 5(b).The peak gain for vertically polarized pattern is 4.9 dBi.The FOM comes about 0.0365.
Next, we studied bowtie antenna element on 20 × 20 × 5 m glass superstrate.The ITO and gold thicknesses are 5 nm and 150 nm, respectively.Bowtie has 74 ∘ flare angle, and the length of one arm is 900 nm, again smaller than its free-space counterpart by about 1/5th.The magnitude of input reflection coefficient with 50 Ω reference is given in Figure 6.The input impedance is displayed in Figures 7(a  The radiation patterns of bowtie antenna are shown in Figures 8(a) and 8(b).Peak gain is found to be 5.3 dBi and HPBW is 21.9 ∘ .Calculated FOM for bowtie antenna is 0.129.
Next, we considered balanced Archimedean spiral antenna.The glass superstrate is taken as 8 × 8 × 4 m.The ITO and gold antenna thicknesses are 5 nm and 50 nm, respectively.The width and arm length of the spiral arm are 200 nm and 500 nm, respectively.Feed gap of 50 nm is taken in the simulations.The input reflection coefficient magnitude with reference to 50 Ω is shown in Figure 9.
The input impedance of spiral antenna is shown in Figures 10(a The radiation patterns of balanced Archimedean spiral antenna are shown in Figures 11(a) and 11(b).The HPBW is about 69 ∘ and peak gain is 6.3 dBi.Calculated FOM is 0.41.Balanced spiral antenna has larger bandwidth because its active surface has rotational symmetry at different operation frequencies.While the longest length is resonant at lower end of the frequency band, its active area shrinks as frequency is increased.Because it preserves its structural symmetry as  frequency changes, the antenna phase center at the input terminals experiences less variation, which, in turn, makes it more wideband.It is also less sensitive to polarization as rotational structure captures all antenna-plane polarizations compared to bowtie and dipole-like structures.
Last structure we studied is modified crossed dipole (MCD) antenna for dual polarization applications.Contrary to previous structures, the glass is taken as a substrate and its dimensions are taken as 10 × 10 × 2 m.The ITO and gold thicknesses are 5 nm and 100 nm, respectively.The dimension of each arm is 1.68 m.The magnitude of input reflections coefficient is presented in Figure 12.
The impedance of MCD antenna is shown in Figure 13.Radiation patterns of MCD antenna are shown in Figures 14(a) and 14(b).The HPBW is 56 ∘ and the peak gain at 10 m is 1.6 dBi.The FOM is calculated as 0.051.

Conclusions
Glass superstrate as opposed to glass substrate can offer substantial improvement for antenna characteristics, which are measured in terms of antenna match bandwidth, peak realized gain, and half-power beamwidth.We combined  all these metrics into a single one and called it FOM.Comparison of FOMs for all structures is shown in Table 1.It is obvious that balanced Archimedean spiral antenna performs best among all other structures.In addition, this balanced spiral antenna is almost polarization insensitive for
) and 7(b).International Journal of Antennas and Propagation

Figure 12 :
Figure 12: Magnitude of input reflection coefficient for modified crossed-dipole.

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Figure 13 :
Figure 13: Input impedance of MCD antenna as a function of frequency: (a) resistance and (b) reactance.

Table 1 :
Performance comparison of antennas.