Investigation of metamaterial absorber in the thz region

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATED UNIVERSITY OF SCIENCE AND TECHNOLOGY ----------------------------- Dang Hong Luu INVESTIGATION OF METAMATERIAL ABSORBER IN THE THz REGION MAJOR: ELECTRONIC MATERIALS NUMBER: 9440123 THESIS ABSTRACT Hanoi - 2018 This thesis was studied in: Graduated University of Science and Technology, Vietnam Academy of Science and Technology. Supervisor: 1. Assoc. Prof. Vu Dinh Lam 2. Dr. Le Dac Tuyen Peer review 1: Peer review 2: Peer review 3: This thesis will be defended at the Graduated University of Science and Technology - Vietnam Academy of Science and Technology, h/ /./2019 This thesis will be saved in: - Library of Graduated University of Science and Technology - Vietnam National Library 1 INTRODUCTION 1. Necessaries of thesis Metamaterial (MM) is artificial structure having extraordinary electromagnetic features not found in nature. Structure of MM is designed with meta-atoms, which interacts with both the electric and magnetic fields of the electromagnetic wave. Therefore, MM can create many interesting properties. Nowday, some properties of MM were demonstrated in both theorical and experimental experiments by many research groups independently. However, novel properties of MM are discovered and significantly affect to science and physics. Many significant studies are focus mostly in negative refractive index metamaterial (NRM), metamaterial absorber (MA), or a combination of those in particular applications. MA is able to absorb unity electromagnetic wave with geometric scale much smaller than operating wavelength. In Vietnam, research on metamaterials is mostly GHz region according to limitations of fabrication and measurement. In Terahertz (THz) region, an interaction between electromagnetic wave with metamaterial in micrometer and nanometer scale is much complicated because of quantum effects. Beside, THz technology is appying in many fields: military, information technology, media, biology, medicine, security, environment, etc. Therefore, MM operating in THz region is gaining much attention of researchers in worldwide, with many significant applications in Laser in THz region, scanning system, national defence. Otherwise, it is a fundamental field for investigating metamaterial in visible region. With these reasons, 2 studying metamaterial in THz region is extremely significant. 2. Purposes of the thesis - Propose physical mechanism to investigate metamaterial absorber operating in THz region. - Design, simulation, and characterizations of MA in THz region. Optimize parameter structure to increase absorption and broaden operating wavelength region, modify operating wavelength by external factors. - Fabricate MA operating in THz region. Study electromagnetic properties and applications. 3. Main research - Investigate MAs operating in THz region. - Simplify MA structures, which will be available for fabrication in THz region with broadband and tunable by external factors. - Study on applications of MAs. Accomplishment: This thesis proposed MA structure operating in THz region: 1) Optimize MA structure to improve absorption and broaden operating wavelengths; 2) Propose MA model controlling absorbing properties by optical pumping and temperature in THz region; 3) Demonstrate ability of MA to enhance oscillating signal of molecules and applied to sensitively detect Bovine Serum Albumin. This thesis consists Chapter 1: Introduction Chapter 2: Research method Chapter 3: Optimization for MA structures 3 Chapter 4: Controlling the operating wavelength of MAs and applying it for sensor Chapter 5: MA based on near-field coupling and Babinet effects. CHAPTER 1: INTRODUCTION 1.1 Introduction to metamaterial Metamaterial (MM) is artificial material constructed by meta-atom in particular arrangement, similar to unit-cell in crystal lattice of conventional materials. The scale of meta-atom is much smaller than operating wavelength. In several recent years, researchs on metamaterial developes considerably, related to other scientific fields as fundamential physics, optics, material science, electronic engineering, etc. 1.2. Classification of MM Fig. 1.3. Classification of MMs based on permittivity and permeability 1.3. Effective medium theory According to effective medium theory (EMT), one could consider MM as homogenous medium with effective permittivity 4 and permeability characterizing to whole medium. This hypothesis bases on the fact that the scale of constitutive component is much smaller than operating wavelengths, then the interaction between incident waves to medium could be accounted as average of all constitutive parts. 1.4. Negative refractive index metamaterial Negative refractive index metamaterial (NRI-MM) is a combination of both electrical and magnetic components inside MM, leads to simultaneously negative permittivity and permeability (μ < 0, ε < 0). In order to create negative refractive index, the mentioned structure need to consist of two parts: 1) magnetic part to create negative permeability (μ < 0); 2) electrical part for negative permittivity (ε < 0) in THz region under plasma frequency. 1.5. Metamaterial absorber In case of achieving impedance matching at resonant frequency, MM express some interesting properties, such as perfect energy absorption of incident EM waves in operating wavelengths. This MM was defined as metamaterial perfect absorber (MPA). At resonant frequency, energy is stored and dissipate into thermal or inside dielectric medium of MPA, then the transmission and the reflection are destructed. In order to investigate mechanism of MA in THz region, we analyzed split-ring resonator (SRR) structure. Specifically, SRR structure could be considered as resonator structures operating based on LC oscillating model and electrical dipole. According to this, dielectric loss and metallic loss are two main dissipating mechanisms of MAs (metal-dielectric- 5 metal) operating in THz region. 1.6. Electromagnetically induced transparency effect Electromagnetically induced transparency (EIT) is a quantum effect that make an absorptive medium become transparent in a narrow frequency (with negligible absorption). Fundamentally, MM is made of electromagnetic resonant structures. Therefore, MM is possible to create EIT effect without any restricted quantum condition. Till now, there are two basic methods to create EIT MM. The first method is known as “bright-bright” interaction, in which both resonances are stimulated by external electric field. Another way to create EIT effect in MM is based on “bright-dark” interaction, in which only one resonance is excited by incident wave and another one is excited by near-field coupling of the first resonance. Because of a difference in resonant stimulation, the first resonance is so- called bright mode and the second is dark mode. 1.7. Applications of MM 1.7.1. Super lens Based on NRI-MM, super lens can rehabilitate not only transmitted part but also evanescent part of incident wave. This is a fundamental difference between super lens and conventional lens. Therefore, resolution of super lens is increase considerably. 1.7.2. Invisibility cloak By manipulating refractive index of a metamaterial around covered objects, direction of EM wave in this shell could be bended intentionally. Then, metamaterial shell can direct incident wave around them without being affected by object itself, and make the object invisible. This behaviour is notable and could be applied to both civil engineering and military. 6 Fig. 1.1. (a) MM which has refractive index changing around covered object (b) Fundamental mechanism of invisibility cloak 1.7.3. MM using in sensors MM using in sensor operates based on surface plasmon resonance in THz region, in which molecule could be detected by changes in obtained spectrum because of molecule’s absorption. CHAPTER 2. RESEARCH METHOD 2.1. Design structure and material Disk structure and SRR structure are consider as appropriate solution to create NIR-MM, MPA and MM based on EIT effect in high frequency region. This is also basic structure which was chosen for researching, investigating and optimizing in this thesis. 2.2. Simulation method In the thesis, we used commercial simulating software named CST Microwave Studio (Computer Simulation Technology) to simulate electromagnetic properties of MM because of its effectivity and accuracy, which are agree well with published experimental results. 2.3. Equivalent LC circuit method One of effective ways to study operating mechanism of 7 MM based on geometric parameters structure is oscillating LC resonant circuit. Under a stimulation of external EM field, effective inductance (L) is determined by metallic layer’s shape, while effective capacitor (C) is determined by an arrangement of constitutive components of MM (dielectric-metal). Then, resonant frequency could be predicted through geometric parameter of each structure. 2.4. Data analysis We using calculation method proposed by X. D. Chen et al. to defined effective parameters (refractive index or impedance) of MM operating in GHz and THz frequency region. 2.5. Experimental method In the limitation of this thesis, we fabricated MM operating in THz region intended to apply in biosensor. The proposed structure consists of three layers, Ag-Si-Ag, which was fabricated in sapphire substrate of 11 cm2 by using photolithography method. Thickness of layers from the bottom to the top are 0.5 μm, 3.0 μm, and 0.2 μm, respectively. This structure was optimized in simulation by CST software. CHAPTER 3. OPTIMIZATION OF MA STRUCTURES 3.1. Optimize absorption by using cavity structure (MAC) The proposed MAC structure was obtained by removing the central disk of array 3x3 disk resonators. By create a resonant cavity in MM structure, we were successful in optimizing absorption to nearly 100%, much higher than previous researches. 3.2. Broadening the absorption bandwidth of MA 3.3.1. Near-field coupling effect 8 To achieve perfect absorption in broadband MA, we proposed new model of MPA, which consists of supercell of array 3x3 disk resonators. By removing 4 disks in position of 1, 3, 7, 9 in supercell, obtained bandwidth raised to 1.0 THz with absorptivity over 90%. The highest absorption is 98% at 14.6 THz. MPA model may provide potential applications in the near future. 3.3.2. Defect wall MAs using disk structure was optimized by combining with square structure as in Fig 3.17(a). Unit cell of this structure consists of FR-4 dielectric layer with ε = 4.3 and thickness td = 1.5 mm sandwiched between 2 copper layers with thickness ts = 0.03 mm, electric conductivity σ = 5.82 × 107 S/m. Top layer is a disk with diameter D = 3.5 mm surrounded by a square with outer length is 9.0 mm and inner length is 6.5 mm. The bottom layer is covered fully by copper. Fig. 3.17. (a) Unit cell, (b) Unit cell with different defect walls. We investigated absorption of structure with 100 units cell and 2 defect walls in different polarized angles. The obtained result reveals ultra-band absorption with absorption over 95% from 20 THz to 25 THz (Fig. 3.21). 9 Fig. 3. 21. Simulation result of MA structure with 2 defect walls 3.3. Conclusion According to disk structure, we investigated cavity resonance, interaction of 5 disks and defect wall in THz region. Absorption and broadening mechanisms were studied by current distribution, electric and magnetic field. Simulation results are in agreement with LC equivalent circuit model and numerical study. Then, we proposed optimization method by geometric parameters. Optimizing MA combined with defect wall broadened absorption band by 5 THz with absorptivity over 90%. CHAPTER 4: TUNEABLE METAMATERIAL ABSORBERS AND SENSOR APPLICATION 4.1. Control metamaterial absorber by optical pumping In this study, VO2 was used as an intermediary to control MPA by optical stimulation. When optical stimulating amplitude changes, VO2 switches over from metal to dielectric and vice 10 versa. As the VO2 conductivity changes, the electromagnetic response of the metamaterial structure is also affected. 4.1.1. Split ring resonator To study effectively about ability to control MPA by optical injection method, split-ring resonator (SRR) was chosen to optimize operation at the corresponding frequency range (around 0.5 THz). 4.1.2. Disk structure with vacancy Figure 4.4(a) depicts MPA including 3 layers: (1) the top layer consists of gold disks with a radius R1 = 4.0 µm and thickness tm = 0.072 µm; (2) the middle layer is made of polyimide with thickness of td = 0.6 µm and electric permittivity ε = 3.5; (3) the bottom layer is a gold thin film with thickness of tm covering whore area. To investigate the absorption and frequency control ability of this structure, the disk with radius of R2 was cut from the disk with radius R1 and filled with VO2 (Fig. 4.4(a)). Fig 4.4. (a) Disk structure with vacancy; (b) Equivalent circuit diagram Figure 4.5(a) shows frequency shift of absorption spectra when R2 increases. It can be explained based on LC equivalent circuit model in Fig 4.4(b). When R2 is larger, an effective area 11 of gold disk with radius R1 decreases, then an inductance Lm and Cm of the structure also decreases. As a result, magnetic resonant frequency of this structure raises. Fig 4.5 reveals that in case of R2 = 0 μm, the absorption peak is at 10,8 GHz, and when R2 = 0.3 μm, 1.2 μm, 2.4 μm, 3.6 μm, 4.8 μm, the absorption frequency is at 10.9 THz, 11.0 THz, 12.2 THz, 13.8 THz and 15.8 THz, respectively. Fig. 4.5. Dependence of absorption spectra of the MA on radius of vacant disk 4.1.3. Control absorption frequency and absorption In this part, according to a change in electric conductivity of vacancy VO2 (vacant disk R2) in frequency range from 10 THz to 25 THz, MA structure can be easily manipulated in absorption frequency and absorption in THz region. 4.2. Control metamaterial absorber by thermal stimulation 4.2.1. Thermal properties of InSb To investigate control ability on operating effectivity of MPA by thermal factor in THz region, InSb was selected. When temperature raises, charge density also increases, therefore, InSb behaves as metal and affect noticeably to an interaction of metamaterial to surrounding electromagnetic field. 12 4.2.2. Control resonant frequency and absorption of ring resonator (a) (b) Fig. 4.11. (a) MPA with SRR structure filled with InSb (b) LC equivalent circuit diagram Figure 4.11(a) shows MA structure including 3 individual layers: (1) a top layer consists of periodicity (the lattice constant a = 50 μm), l = 40 μm 0, g = 5 μm and w = 6 μm; (2) a middle layer is made of dielectric with thickness ts = 8 μm; (3) a bottom layer is cover totally by a gold thin film. The thickness of gold in top and bottom layers is set to be tm = 1 μm. To manipulate MA by thermal factor, a gap between 2 slits of SRR is filled with InSb. Fig. 4.11(b) depicts LC equivalent circuit of this structure. 13 Fig. 4.12. Resonant frequency and absorption of MPA depended on temperature When temperature increase from 260 K to 380 K, resonant frequency is shifted from 0.5 THz to 0.65 THz. Since temperature increases, charge density (N) of InSb raises causing larger effective inductances L1 and L3, therefore, total value of L of MPA decreases. As a result, magnetic resonant frequency of this MPA changes as can be seen as Fig. 4.12. 4.3. Applications of metamaterial on sensors 4.3.1. Operating mechanism of sensors in THz region In this part, we provide ways to apply metamaterial structure with thickness in micrometer scale, operating as an amplifier for enhancing the absorption signal of the THz vibration of an ultrathin adsorbed layer of large organic molecules. 4.3.2. Metamaterial structure in sensing bovine serum albumin (BSA) 14 Fig. 4.13. (a) Schematic illustrations of the MM sample in this study. (b) Cross-sectional illustration of the sample design with detailed dimensions of the sample. (c) SEM image of a typical sample. Small steps at the corners of the samples were mistakenly created during fabrication. Our proposed Ag–Si–Ag tri-layered MM structure is shown in Fig. 4.13(a) and (b). Figure 4.13(c) shows a 30°-tiled- view scanning electron microscope (SEM) image of the fabricated MM device. Two Ag disk arrays, used as back and top resonators that sandwich a Si insulator, were placed on a sapphire substrate. The geometrical parameters of the MM structure were optimized using an electromagnetic simulation. Here the MM is aimed at a dual-band resonance at approximately 5 THz, which resonates with the absorption signal of the targeted BSA molecules. Different thicknesses (0.2 and 0.5μm) and different widths (10 and 6 μm) were chosen for the top and bottom Ag disk resonators, respectively. The thickness of the Si insulator and the periodicity were 3 and 20 μm, respectively. 4.3.3. Optical properties of MM Figures 4.14(a) and (b) present the measured and simulated transmittance spectra of the fabricated MM, respectively. The measured transmittance of the MM shows a dual-band resonance at 4.2THz (or 140 cm-1, called M1, low frequency) and 5.8 THz (or 194 cm-1, called M2, high frequency). In a dual-band resonance of a metal–insulator–metal trilayered MM disk, the low-frequency peak is typically attributed to the magnetic dipole resonance, and the high-frequency peak is attributed to the electric dipole resonance. Figure 4.14(c) shows the results of further simulations of the electromagnetic field 15 distribution, which were performed to obtain more insight into the relationship between these two modes. Fig 4.14. (a) Measured and (b) simulated transmittance spectra of the MM structure. There were two resonant peaks, M1 (at low frequency) and M2 (at high frequency), which were related to the photonic–magnetic dipole coupling and magnetic resonances, respectively. For details, see the text. (c) Simulated electric and magnetic field distributions at the MM structure with excitations in the low-frequency (M1) and high-frequency (M2) modes. Color scale bars in (c) show the enhanced electric and magnetic fields compared to the incident fields; arrows indicate the maximum field enhancements for low-frequency (M1) excitation. 4.3.4. Sensing characteristics of MMs Figure 4.15 presents the results of BSA protein sensing using our MM. As previously stated, before the experiment, submicron-thick bulky samples of organic molecules [BSA, 3,3′ -diethylthiatricarbocyanine iodide (DTTCI), and Rhodium 6G 16 (Rh6G)] were measured. The bulk molecular layers were prepared by dropping solutions of the corresponding molecules onto the substrates and drying them in a stream of N2 gas. Between 50 and 2000 cm-1, BSA is the only molecule to display a vibration signal, which is located at 4.8 THz, as shown in Fig. 4.15(a). The spectral position and features of the BSA signal presented here is close to those described in an earlier report by Yoneyama. However, the absorption spectra of BSA in the THz may vary depending on the preparation (treatment temperature) of the films as well as the molecule’s conformation at the interface and the wett