Terahertz Metamaterial based Thin Film Sensor with ground plane Eppa Akhilesh Reddy(1), Sabyasachi Banerjee(1,2), Chillamcherla Sai Amith(1), Nakka Lok Abhishikth(1), Dibakar Roy Chowdhury(1)* Mahindra Ecole Centrale, Jeedimetla, Hyderabad, India, 500043 BITS Pilani – Hyderabad, Jawahar Nagar, Shameerpet Mandal, Hyderabad, Telangana 500078 *dibakar.roychowdhury@mechyd.ac.in Abstract: In this work, we have studied a metamaterial based thin film absorber which consists of a cross-shaped resonator and a ground plane. It displays sharp resonance peaks in the frequency spectrum which resulted in high figure of merit (FoM) when employed in thin film sensing. Introduction Simulation Procedure[1] At each thickness, refractive index of the polyimide (analyte) is varied from 1.2 to 2.0 with a step of 0.2 and simulations are carried out using CST microwave studios. Frequency shift of resonant peak is calculated for each refractive index of polyimide with reference refractive index of polyimide as 1.2. The slope of the Frequency Shift Vs Refractive Index gives sensitivity at a particular thickness. The average FWHM is calculated at each thickness with different refractive indices. FoM is then obtained by taking the ratio of sensitivity to that of FWHM. Metamaterials are man-made materials that could be engineered to respond to the Electromagnetic radiations in novel ways which can essentially be different from naturally occurring materials. Normally naturally occurring materials do not trigger magnetic component whereas metamaterials can effect both the magnetic and electric components of EM waves depending on the design. Several metamaterial based sensors have been demonstrated till date [1,2]. However those sensors often suffer from a lack of high FoM due to low Q factor caused by losses or due to the lack of strong electromagnetic interactions. Recently, it is shown that Perfect metamaterial absorbers(PMA’s) can also be employed in sensing the analytes. Fig 3(b) Fig 2. (a) illustrates Reflection amplitude Vs Frequency plot. Maximum resonant peak can be observed at 0.86 Thz for thickness 5.5um of polyimide. Fig 3(c) Fig3. (a) Sensitivity for different thicknesses at different resonant peaks.(b) FWHM for different thicknesses of polyimide.(c) Figure of Merit of the sensor Design Conclusions In the proposed design, the unit cell consists of three layers in which the analyte layer is embedded in between the cross shaped resonators and ground plane(metal sheet). We have used GaAs as substrate and polyamide as analyte for our studies. The advantage of using a tri-layered structure over the conventional single layered metamaterials is that the former has a better chance of inciting a Fabry Perot-like behaviour[2]. Multiple reflection of incident EM waves in the cavity leads to the absorption, which further leads to significant field trapping and enhancement. Thickness of polyimide layer is also changed as demanded in the course of the study. We have designed and simulated a novel multi layer structure based on Terahertz metamaterials towards thin film sensing which shows relatively higher FoM when compared to the other metamaterials based thin film sensors. Currently experimental efforts are going on to validate our numerical findings. We are attributing stronger electric and magnetic field confinements to higher FoM values. These kinds of perfect metamaterial absorber- based sensors can be effectively and efficiently used for sensing purposes in the finger print region of the electromagnetic spectrum with several organic, explosive, and bio-molecules that have unique spectral signature at the terahertz frequencies. Fig 2.(b) shows the Frequency shift Vs R.I of polyimide Sensitivity and FoM We get different resonance peaks at distinct frequencies as we change the refractive index of the material. The dependence of resonance peaks with the R.I of polyimide for different thicknesses of polyimide are shown in Figure 2,(b) Figure of merit is a quantity which determines the performance of a device, system or method in comparison to its alternatives. Higher the FoM value better the system it is. The greater FoM can be attributed to the fact that the resonant electric and magnetic fields present in the vicinity of metamaterial gets redistributed as we change the refractive index of embedded analyte and leads to a strong modulation of absorber resonance frequency and amplitude. Acknowledgements The authors gratefully acknowledge the financial support from SERB, Department of Science and Technology, India (EMR/2015/001339). Authors also gratefully acknowledge support from Mahindra Ecole Centrale. References Maidul Islam , S. Jagan Mohan Rao , Gagan Kumar , Bishnu P.Pal & Dibakar Roy Chowdhury,”Role of resonance Modes on Terahertz Metamaterials based Thin Film Sensors”, Scientific Reports 7: 7355 DOI: 10.1038/s41598-017-077209. Longqing cong, Siyu Tan, Riad Yahiaoui, FengpingYan, Weili Zhang and Ranjan Singh; Experimental demonstration of ultrasensitive sensing with Terahertz metamaterials absorbers: A comparison with the metamaterials; Applied Physics Letters 106,031107(2015). Fig 1 Fig 1 Schematic of the proposed unit cell consisting of cross shaped resonator (CSR), Spacer and the ground plane. Fig 3(a)