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.
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: Assoc Prof Vu Dinh Lam 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 INTRODUCTION 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, studying metamaterial in THz region is extremely significant 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 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 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 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- 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 socalled 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 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 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 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 11 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 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 To achieve perfect absorption in broadband MA, we proposed new model of MPA, which consists of supercell of array 3x3 disk resonators By removing disks in position of 1, 3, 7, 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 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 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) 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 = 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 individual layers: (1) a top layer consists of periodicity (the lattice constant a = 50 μm), l = 40 μm 0, g = μm and w = μm; (2) a middle layer is made of dielectric with thickness ts = 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 = μm To manipulate MA by thermal factor, a gap between 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°-tiledview 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 THz, which resonates with the absorption signal of the targeted BSA molecules Different thicknesses (0.2 and 0.5μm) and different widths (10 and μm) were chosen for the top and bottom Ag disk resonators, respectively The thickness of the Si insulator and the periodicity were 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 wettability on specific substrates The BSA spectrum was plotted with the MM spectrum to emphasize the spectral matching of the MM resonance and the target molecules’ signal Although the initial purpose to obtain perfect matching of the M1 mode to the vibration signal of BSA) was not realized, the discrepancy between the simulation and experiments (displayed in Fig 4.14) might be small enough to neglect The BSA signal is located between the two modes of the MM and at a higher energy than the M1 mode of the MM 17 Fig 4.15 (a) Normalized transmittance spectrum of a submicron-thick BSA protein layer (black circles), which was measured before the experiments; the signal strength shows a transmittance of approximately 25% The spectrum was plotted 18 with the transmittance of the MM sample (red circles) to show the matching of the protein signal and MM resonance Red line shows the Fano fit for the signal of the submicron-thick BSA (b) Normalized transmittance spectra of an ultrathin layer of BSA molecules absorbed on the MM sample and a reference sapphire substrate (c) Spectra of the organic molecules DTTCI and Rh6G measured in conditions similar to those used for the BSA sample 4.4 Conclusion In this chapter, we investigate ability to control MPA by optical stimulation on VO2 and thermal stimulation on InSb Obtained results reveal that we could control resonant frequency and absorption effectively We also demonstrated that MMs in THz region with appropriate structure could detect molecules as well as in enhanced surface spectra in optical frequency Enhanced spectra by MMs is useful now, since it provides a simple method for sensing without destroy large bio-molecule, opens new way to apply THz sensor in industry applications CHAPTER 5: METAMATERIAL ABSORBER BASED ON NEAR-FIELD COUPLING AND BABINET EFFECTS 5.1 Multi-band MA based on near-field coupling effect Figure 5.1 illustrates the unit-cell design of the proposed MPA The absorber consists of three layers, which are a front patterned metallic layer, a back continuous metallic film and a middle sandwiched dielectric layer Both metallic layers have a thickness tm = 0.5 μm, while the thickness of dielectric layer is td = 1.4 m The patterned structure is the combination of a short vertical cut-wire (VCW) and two long horizontal cut-wires 19 (HCWs) The length l1 and width w1 of VCW are 5.5 and 2.5 m, while these parameters of HCW are l2 = 13.5 and w2 = 1.5 m, respectively The distance d between VCW and HCW is 0.5 m The periodicity p of the structure is 15 m The dielectric is silicon with a dielectric constant of 11.9, while the metal is modeled as silver with a conductivity of 6.3 × 107 S/m Fig 5.1 Illustration of periodic structure and unit cell of the structure In the studied frequency range from 6.0 to 8.5 THz, the HCWA does not show any response while the VCWA exhibits a single absorption peak of 80 % at 7.49 THz 20 Fig 5.2 Absorption spectra of VCW absorber, HCW absorber and the proposed MPA 5.2 Multi-band absorption induced by defects In this section, we propose another way to improve the bandwidth of the MPA By removing resonators on the center horizontal and vertical lines, it is expected that the absorption spectrum of MPA can be changed because of the defects The absorption of the DMPA is also enhanced with the absorption of 98%, 100% and 97% at 6.88, 7.53 and 7.84 THz, respectively 5.3 Babinet effect for MM absorber MPA structure in Fig 5.8 consists of: top layer is made of silver with thickness tm = 0.1 µm, unit cell area is 340ì340 àm2 with rectangular holes (2 vertical holes and horizontal hole with length l = 220 µm, width w = 20 µm, distance between vertical bar and horizontal bar is d = 25 µm) Dielectric layer is pyrex with thickness td = 50 µm and refractive index n = 2.195 The bottom layer is made of silver with thickness tm = 0.1 µm Fig 5.8 Schematic for a unit cell of MPA in (a) threedimensional view and (b) 2-D view s indicates the position of horizontal hole from the center 21 Fig 5.10 Dependence of absorption spectra in values of s Figure 5.10 illustrates changes of absorption spectra when moving horizontal hole according to E direction When s = 0, MPA structure is symmetric and create single band of absorption spectrum When s increases, a symmetry of MPA structure is broken Initial resonant peak is split into separated peaks, The larger value of s, the bigger distance between new peaks With s = 80 µm, we can obtain resonant absorption peaks at 0.32 THz and 0.34 THz with absorption of 97% for both peaks 5.4 Conclusion By making use of EIT effect, near field interaction and defects in metamaterial structure, we are successful in building up MPA model with multi-band absorption of over 90% Absorption mechanism can be explained based on rearrangement of electromagnetic field at resonant frequency, beside the interaction between inherent resonance and guide-mode resonance (GMR) Our study also reveals that, defect not only increases number of resonant peaks, but also enhances absorption 22 of each resonance Moreover, MPA structures designed based on Babinet theory also create resonant peak with high absorption By changing geometric parameter, we can manipulate the position of resonant peaks These results are initial significant steps in process of optimizing, controlling and fabricating MMs operating in THz region CONCLUSION Project “Study on metamaterial absorbers operating in the THz region” was studied at Graduated University of Science and Technology – Vietnam Academy of Science and Technology The achieved results of thesis were published in 06 international journals (04 ISI journals, 02 Scopus journal) and 04 papers are published in proceeding of scientific conferences Main conclusions according to these results: Optimized metamaterial structures in order to enhance both absorption and bandwidth in the THz region: MA structure based on cavity, 5-disk resonator with a bandwidth of 1.0 THz and absorption over 90%, defect MA model with a bandwidth of THz and absorption over 95% Explained the physical mechanism for perfect absorption and for wide-band absorption of these structures Investigated the ability to tune the resonant frequency and the absorption by using the optical and thermal excitations (based on VO2 and InSb materials) The absorption peak of MPA can be controlled from 10.8 to 15.5 THz, when the conductivity of VO2 is changed from 1.8.107 S/m to 5.10-19 S/m Since the temperature 23 of InSb is changed from 260 to 380 K, the absorption peak can be tuned from 0.5 to 0,65 THz Designed and studied multi-band MPA with absorption over 90% based on the near-field interaction Explained in detail the physical mechanism of near-field coupling, which can be applied to manipulate resonant frequency via geometrical parameters Successfully realized a metamaterial model, which can be used for sensing molecule (bovine serum albumin) as same as optical surface enhancement in the optical region This result confirmed that this THz metamaterial can be useful for applications in organic-molecular sensing OUTLOOK Continue to study metamaterial absorbers in the THz and the visible region, which are available to be fabricated Study and fabricate efficient metamaterial-based sensors, which not destroy the bio-molecules and can be applied into the modern industry LIST OF PUBLICATIONS IN THESIS Manh Cuong Tran, Dinh Hai Le, Van Hai Pham, Hoang Tung Do, Dac Tuyen Le, Hong Luu Dang, and Dinh Lam Vu, Controlled Defect Based Ultra Broadband Full-sized Metamaterial Absorber, Scientific Reports 8, 9523 (2018) Tung S Bui, Thang D Dao, Luu H Dang, Lam D Vu, Akihiko Ohi, Toshihide Nabatame, YoungPak Lee, Tadaaki Nagao, and Chung V Hoang, Metamaterial-enhanced 24 vibrational absorption spectroscopy for the detection of protein molecules, Scientific Reports 6, 32123 (2016) Dang Hong Luu, Bui Son Tung, Bui Xuan Khuyen, Le Dac Tuyen and Vu Dinh Lam, Multi-band absorption induced by near-field coupling and defects in metamaterial, Optik International Journal for Light and Electron Optics 156, 811816 (2018) H L Dang, V C Nguyen, D H Le, H T Nguyen, M C Tran, D T Le, and D L Vu, Broadband metamaterial perfect absorber obtained by coupling effect, Journal of Nonlinear Optical Physics and Materials, 26(3), 1750036 (2017) H L Dang, H T Nguyen, V D Nguyen, S T Bui, D T Le, Q M Ngo, and D L Vu, Cavity induced perfect absorption in metamaterials, Advances in Natural Sciences: Nanoscience and Nanotechnology 7(1), 015015 (2016) Dang Hong Luu, Nguyen Van Dung, Pham Hai, Trinh Thi Giang, Vu Dinh Lam, Switchable and tunable metamaterial absorber in THz frequencies, Journal of Science: Advanced Materials and Devices 1, 65-68 (2016) Dang Hong Luu, Trinh Thi Giang, Nguyen Van Cuong, Le Dinh Hai, Le Dac Tuyen, and Vu Dinh Lam, Optically manipulated metamaterial absorber in THz frequencies, Proceeding of the 8th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN-2016), 209-213 (2016) Dang Hong Luu, Pham The Linh, Nguyen Van Cuong, Tran Manh Cuong, Le Dac Tuyen, Vu Dinh Lam, Visible 25 metamaterial absorber with hexagonal structutre, Tuyển tập hội nghị Vật lý chất rắn Khoa học vật liệu toàn quốc lần thứ (SPMS 2017), 68-71 (2017) Trần Mạnh Cường, Lê Đình Hải, Đặng Hồng Lưu, Lê Đắc Tuyên, Vũ Đình Lãm, Ultra broadband and polarizationinsensitive metamaterial THz absorber full-sized structure using meta via wall, Tuyển tập hội nghị Vật lý chất rắn Khoa học vật liệu toàn quốc lần thứ (SPMS 2017), 408-411 (2017) 10 Đặng Hồng Lưu, Bùi Sơn Tùng, Trịnh Thị Giang, Phạm Thế Linh, Nguyễn Văn Cường, Trần Mạnh Cường, Bùi Xuân Khuyến, Lê Đắc Tuyên, Vũ Đình Lãm, Vật liệu biến hóa hấp thụ sóng điện từ dựa hiệu ứng suốt cảm ứng điện từ, Tuyển tập hội nghị Vật lý chất rắn Khoa học vật liệu toàn quốc lần thứ (SPMS 2017), 38-41 (2017) ... defence Otherwise, it is a fundamental field for investigating metamaterial in visible region With these reasons, studying metamaterial in THz region is extremely significant Purposes of the thesis. .. process of optimizing, controlling and fabricating MMs operating in THz region CONCLUSION Project “Study on metamaterial absorbers operating in the THz region was studied at Graduated University of. .. 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