Optimization of Semiconductor-Based SRR Metamaterials as Sensors

The development of hybrid sensor media is needed to achieve more efficient, sensitive, and accurate performance. Efforts to modify the structure of conventional metamaterials are carried out by integrating semiconductor materials which aim to improve the characteristics of optical properties, electrical properties, and sensitivity as sensors. This study aims to analyze and investigate changes in the optical properties of semiconductor-based metamaterials. The research was conducted through simulation and numerical methods to design and characterize the SRR metamaterial geometry, with a modified Nicolson-Ross-Weir approach, especially the optical parameters of refractive index. The single-cell square pattern SRR metamaterial geometry with a ring radius of 2.2 – 2.8 mm on quartz glass substrate designed at a smaller wavelength based on a maximum frequency source of 9 GHz. The square SRR metamaterial is integrated with several semiconductor materials such as silicon (Si), gallium arsenide (GaAs), and aluminum nitride (AlN). Changes in radius size cause a redshift with respect to radius enlargement. The increasing ring radius of SRR causes a higher resonance depth of the refractive index. Combining hybrid semiconductors with metamaterial results in more negative metamaterial properties as the refractive index becomes larger and negative. The addition of semiconductor material to the metamaterial substrate causes a negative refractive index to shift to a lower frequency.


Introduction
Metamaterials are artificial materials with unique characteristics (negative refractive index) and high resonance sensitivity.The use of metamaterials as an advanced technological breakthrough has a very high and broad potential because the material and its structure can be updated [1].Pure metamaterial structures such as split ring resonator (SRR) have been reported by Saktioto et al., where the application of hexagonal SRR metamaterials with FR-4 substrate (flame retardant with woven glassreinforced epoxy gresin, ε = 4.3) can improve the sensor performance of low profile microstrip antennas with a large frequency width of 8 GHz [2].However, pure metamaterials in their current application are still under the requirements of modern technology with high quality.This problem needs to be investigated to improve metamaterials into superior materials.On the other hand, the high engineering properties of metamaterials can create hybrid metamaterials from a combination of other materials such as semiconductors [3].
Semiconductor materials have a very good opportunity to improve sensor performance efficiency compared to other models of conventional materials.Therefore, this study aims to analyze and investigate changes in the optical properties of semiconductor-based metamaterials.The research method was carried out by simulation and numerical by designing and characterizing the SRR metamaterial geometry with a Nicolson-Ross-Weir (NRW) electromagnetic field function approach, especially the optical parameters of permittivity, permeability, and refractive index.The single-cell square pattern SRR metamaterial geometry as shown in Figure 1 is designed with a ring radius of 2.2 -2.8 mm at a smaller wavelength based on a maximum frequency source of 9 GHz.Quartz glass is used as a substrate that has high heat resistance and high transmittance, making it suitable for coupling with semiconductors to improve optical, electronic, and sensor performance properties.

Design and characterization
This step is carried out in a simulation manner by designing a metamaterial (5.5 × 5.5 mm 2 ) with a square split ring resonator (SRR) structure as shown in Figure 1 (a).This simulation uses CST Studio Suite software with finite-difference time-domain (FDTD) method for a single-cell metamaterial.The conventional metamaterial structure consists of a dielectric substrate made of quartz (ε = 3.8) and SRR metal inclusions of copper as shown in Figure 1(b).The metamaterial structures each have a thickness of 1 mm (quartz) and 0.05 mm (copper).SRR metamaterials have geometric characteristics that depend on the smallest wavelength of the given frequency [4].In this research, the maximum source frequency is 9 GHz.The geometry of the square SRR structure on the inner and outer metal ring radii is varied as detailed in Table 1.
Semiconductors have a characteristic band gap that lies between the valence and conduction bands.This material will be active if given energy or electric current.One of the semiconductor materials in the form of thin layers of silicon (Si), gallium arsenide (GaAs), and aluminum nitride (AlN) will be integrated into the SRR radius variation metamaterial model with the best characteristics.In addition, the thickness of the semiconductor film was set at 500 nm and placed between the square SRR and the quartz substrate as shown in Figure 1(c).Identification of the physical optical properties of the metamaterial is carried out by simulation using a waveguide designed based on the wavelength of the source [5].The waveguide design consists of a waveport, a perfect electric conductor boundary, and a perfect magnetic conductor each of which is located on the x, y, and z axes (see in Figure 2).The metamaterial characteristics are determined by processing S parameter data (spectrum) in the form of S12 (reflection) and S21 (transmission).Parameter S data is displayed in the form of absolute value and phase for each type of spectrum.The two data are then transformed into complex numbers with the following equation [2]: (1) where, |11| and |21| is a spectrum in absolute numbers, while 11 and 21 is a spectrum in phase numbers.
The characteristics of metamaterials have a relative constant (permittivity) from the ratio between the electrostatic density of the material by the electric field to the vacuum state [7].The characteristics of metamaterials also have other relative constants (permeability) from the comparison between the magnetic response of the material by the magnetic field [8].The permittivity and permeability can be calculated using Equations ( 3) and (4) of the following NRW model [2]: where,   is the relative permittivity,   is the relative permeability,  is the speed of light,  is the source frequency, and   is the waveport distance.The metamaterial medium constant has an influence on the refraction of EM wave propagation [9].The refractive index can be calculated using Equation (5) of the following NRW model [2]:

Results and discussions
The difference in the square pattern SRR ring radii causes a shift in the resonant frequency at a wider refractive index around 0.3 -0.7 GHz as shown in Figure 3.A square pattern that has an SRR ring radius of 2.8 mm tends to highly resonate at a lower frequency.The results show that the frequencydependent characteristics of the metamaterial (its refractive index) vary with the dimensions of the metal inclusions.Whereas resonance at high frequencies has a smaller wavelength based on the small area of metal inclusions [10].The large dimension of the metamaterial is due to its structure being proportional to the specified SRR ring radius.Therefore, a shift in the resonant frequency for larger SRR ring radii occurs at lower frequencies or according to the wavelength of the structure.In addition, the larger the radius of the SRR ring causes the resonance depth of the permittivity, permeability, and index of refraction to be higher.This, of course, cannot be avoided due to changes in the dimensions of the metamaterial structure which relatively change based on the SRR ring radius.The resonator built from the SRR pattern aims to generate a resonance at a certain frequency.The optimal depth of resonance frequency can be obtained by constructing the dimensions of the metamaterial structure according to the size of the minimum wavelength of a given frequency [11].So if the microdimensional metamaterial is operated at a longer wavelength range or at a lower frequency, then the metamaterial structure does not experience optimal resonance.A square SRR metamaterial integrated with several semiconductor materials has been successfully simulated.The addition of semiconductor materials to the metamaterial structure does not change the optical properties based on the shift in the resonant frequency.However, upon closer examination, there is an increase in depth and a significant shift in the negative refractive index resonant frequency as shown in Figure 4.The addition of semiconductor material to the metamaterial substrate causes a negative refractive index to shift to a lower frequency.Si material with a bandgap energy of 1.1 eV has a larger negative resonance of -144.3 followed by GaAs (1.42 eV) of -138.7 at a resonant frequency of 3.44 GHz.Whereas AlN material with an energy band gap of 6.42 eV has a smaller negative refractive index -126.2at a resonant frequency of 3.46 GHz followed by Clear sample (without semiconductor) of -125.9 at 3.48 GHz.The use of a semiconductor with a wide band gap will increase the number of electrons flowing from the conduction band to the valence band because a wide band gap will make the photocatalyst reaction space more easily absorbed, or in other words the absorption spectrum will be broad.This proves that the addition of semiconductor materials offers the advantages of their electronic, optical, and magnetic properties which allow the metamaterial properties to respond constructively [12].Therefore, semiconductor-based metamaterial models can strengthen sensor performance for applications in several fields of science based on the material and frequency used.

Conclusions
The SRR metamaterial structure has been successfully designed in a square pattern and has acquired the characteristics of a unit cell.Changes in radius size cause a redshift with respect to radius enlargement, where the resonant frequency shift from 5.16 GHz for R1 = 2.2 mm, 4.55 GHz for R2 = 2.4 mm, 3.93 GHZ for R3 = 2.6 mm, and 3.47 GHz for R4 = 2.8 mm.The addition of semiconductor material to the metamaterial substrate causes a negative refractive index to shift to a lower frequency.Si material with a bandgap energy of 1.1 eV has a larger negative resonance followed by GaAs at a resonant frequency of 3.44 GHz.Then, AlN and clear (only metamaterial) has resonant frequency of 3.46 GHz and 3.48 GHz, respectively.

Figure 1 .
Figure 1.(a) Front view of square SRR metamaterial, (b) structure of original model SRR metamaterial, and (c) metamaterial structure of semiconductor addition model.

Figure 3 .
Figure 3. Resonant frequency of negative refractive index of metamaterials at different ring radius.

Figure 4 .
Figure 4. Resonant frequency of negative refractive index of semiconductor based metamaterials.

Table 1 .
Square pattern SRR ring radius for each metamaterial sample.

Table 2 .
Extreme negative refractive index resonant frequency in semiconductor-based metamaterials.