Optimizations for ultra small and wide incident angle metamaterial perfect absorbers at low frequency

Thesis for the Degree of Doctor of Philosophy Optimizations for ultra-small and wideincident-angle metamaterial perfect absorbers at low frequency by Bui Xuan Khuyen February 2018 Department of Physics Graduate School HANYANG UNIVERSITY Thesis for the Degree of Doctor of Philosophy Optimizations for ultra-small and wideincident-angle metamaterial perfect absorbers at low frequency by Bui Xuan Khuyen Supervised by Professor YoungPak Lee SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY AT HANYANG UNIVERSITY SEOUL, SOUTH KOREA February 2018 © Bui Xuan Khuyen, 2018 Acknowledgments I would like to take this opportunity to thank those people: without their helps and supports, this thesis would have not been possible My greatest thanks must express to my supervisor, Professor YoungPak Lee, for his invaluable guidance, and endless encouragement over past four years I am very proud to have the opportunity to study under his supports in this field of metamaterials My gratitude also goes to the q-Psi Laboratory, Department of Physics, Hanyang University for providing me a financial support in the professional research environment I would like to thank Associate Professor Vu Dinh Lam who has encouraged in my current researches, and Dr Nguyen Thanh Tung who has always shared with me many useful suggestions and research experiences in metamaterials It would like to express my deep thankfulness to Professor Joo Yull Rhee and Professor Ki Won Kim for lots of interesting discussions in my studies It is also a pleasure to thank the former and current members of the q-Psi Laboratory as well as my friends at Hanyang University for the friendship and all the supports in past four years In addition, I would like to thank to my close friends, both new and old, far away and nearby for their continuous encouragements, which are the great motivations of my life Thank you all Finally, I owe my deepest gratitude to the encouragement, the support, and the love from my parents, my wife and my grandfather I Contents List of Figures V Abstract IX Introduction 1.1 Historical overview 1.2 Framework of the thesis Theoretical background 2.1 Material classification 2.2 Metamaterials and effective-medium theory 2.3 Electromagnetic-parameter retrieval from metamaterials 2.4 Electric and magnetic responses in metamaterials 11 2.5 Impedance matching 16 2.6 Metamaterial perfect absorbers 18 Simulation and experiment of microwave metamaterials 22 3.1 Numerical simulation 22 3.2 Fabrication technique 23 3.3 Measurement configuration in the radio band 24 Reducing the size of single/dual-band MPA by increasing the effective inductance of the pattern in the UHF band 26 4.1 Introduction 26 4.2 Metamaterial model and fabrication 26 4.3 Mechanism of perfect absorption at 400 MHz 28 4.4 Examination on MPA for different incident radiations 32 II 4.5 Realization of dual-band perfect absorption via self-asymmetric structure 34 4.6 Summary 36 Scaling down the size of MPA by increasing the effective capacitance via the effective coupling cross-section of top and bottom layers in the VHF band 37 5.1 Introduction 37 5.2 Metamaterial scheme and experiment 37 5.3 Physical mechanism of the perfect absorption at 250 MHz 38 5.4 Estimation for the perfect absorption at a wide oblique incident angle 40 5.5 Summary 42 Miniaturization for single/dual-band MPA by integrating the parasitic capacitors and the through interconnects in the VHF band 43 6.1 Introduction 43 6.2 Proposed design and measurement 43 6.3 Mechanism of the energy consumption of incoming electromagnetic wave at 102 MHz 46 6.4 Investigation on the perfect absorption for wide incident angle 50 6.5 Realization of the dual-band MPA by utilizing super-cell structure 50 6.6 Summary 52 Study on the miniaturization for dual/triple-band MPA by utilizing only parasitic capacitors in the UHF band 53 7.1 Introduction 53 7.2 Metamaterial model and experimental setup 53 III 7.3 Mechanism analysis of the perfect dual-absorption peak at 305 and 360.5 MHz 54 7.4 Valuation of the dual-band perfect absorption for wide incident angle 58 7.5 Extended study on triple-band perfect absorption 59 7.6 Summary 61 Conclusions and perspective 62 List of publications 66 Bibliography 68 IV List of Figures Fig 2.1 Material classifications based on sign of ε and μ Fig 2.2 Diagram for S-parameters measurements on (a) a homogeneous 1D slab (b) an inhomogeneous asymmetric 1D slab and (c) a symmetric inhomogeneous 1D slab A single unit cell of structure has a thickness of d 10 Fig 2.3 (a) Lattice of thin metallic wires and (b) its effective permittivity (with radius r = 5.0 m and periodicity a = 40.0 mm) 12 Fig 2.4 Unit-cell structure of several electric elements: (a) single SRR (b) CW and (c) equivalent-circuit model 13 Fig 2.5 Schematic of the magnetic-resonant elements as (a) unit cell of CWP structure and (b) equivalent LC-circuit model Simplified LC-circuit models for (c) magnetic and (d) electric resonant frequencies, respectively 16 Fig 2.6 (a) Schematic of unit cell for the first MPA (b) Distributions of Ohmic and dielectric losses at the resonant frequency 19 Fig 3.1 Fabrication process of MPA samples 24 Fig 3.2 Schematic of the reflection-measurement 25 Fig 4.1 (a) Unit cell of the proposed MPA Photos of the fabricated (b) single-peak and (c) dual-band samples Insets in (b) and (c) are the unit cells, corresponding to the single-peak and the dual-band MPA structures, respectively (d) Experimental setup for the measurement 27 Fig 4.2 (a) Simulated and measured absorption spectra of the single-peak MPA Red and blue dashed arrows display the FWHM values corresponding to the simulated and the experimental absorption spectrum, respectively Induced surface currents on (b) the front and (c) the back layers at 400 MHz 3-dimensional distributions of (d) V the magnetic energy, (e) the electric energy and (f) the power loss at the resonant frequency 28 Fig 4.3 (a) Equivalent circuit of the discussed MPA (b) Simulated and calculated absorption frequencies according to the radius of the quarter of disk (r) and the width of slender wire (w) Red-square and blue-triangle symbols represent the simulated results, while the calculated results are denoted in black dots and green diamonds 30 Fig 4.4 (a) Simulated and (b) measured absorption spectra of the single-peak MPA according to the incident angle of EM wave for TE polarization (c) The same simulated absorption spectra for TM polarization 33 Fig 4.5 (a) Dependence of the absorption on the gap (g2) of proposed dual-band MPA (b) Simulated absorption spectra according to the polarization angle (ϕ) of EM wave The inset is the schematic view of varying polarization angle ϕ Induced surface currents on the front and the back metallic layers at two absorption peaks for ϕ = (c) -45o and (d) 45o Red and black arrows are designated for the current flows on the surface of front metallic layer at low and high absorption frequencies, respectively 35 Fig 5.1 (a) Structural specifications of the unit cell (b) Zoomed image of × unit cells for the fabricated sample (c) Arrangement for the experimental setup 38 Fig 5.2 (a) Simulated and experimental absorption spectra of the suggested MPA Distributions of (b) the induced surface currents on the top and the bottom layers, 3-dimensional (c) magnetic energy, (d) electric energy and (e) power loss at 250 MHz 39 VI Fig 5.3 (a) Simulated and (b) measured absorption spectra of the proposed MPA for different oblique incident angles of TE polarization (c) Similar simulated absorption spectra for TM polarization 42 Fig 6.1 3-dimensional periodic structure of the unit cell of single MPA with the polarization of EM wave 44 Fig 6.2 (a) Schematic of the proposed dual-band MPA structure with the polarization of EM field (b) Illustrated arrangement for the experimental configuration (bottom) and magnification of the fabricated sample (top) 45 Fig 6.3 (a) Simulated effective impedance and absorption spectrum of the proposed MPA 3-dimensional distributions for (b) the induced surface currents, (c) the magnetic energy, and (d) the power loss at the resonant frequency 46 Fig 6.4 (a) Equivalent circuit of the discussed MPA (b) Simulated and calculated absorption frequencies according to the value of lumped capacitor Red-square and blue-circle symbols represent the calculated and the simulated results, respectively Green-triangle symbols mark the simulated absorption 48 Fig 6.5 Simulated absorption spectra of the ultrathin MPA according to the incident angle of EM wave for (a) TE and (b) TM polarizations 50 Fig 6.6 (a) Simulated and (b) measured absorption spectra according to the incident angle of EM wave, of the proposed dual-band MPA 52 Fig 7.1 Schematic of the simulation and the measurement for ultrathin DMPA (a) 3dimensional periodic structure of the unit cell, (b) experimental configuration of the proposed DMPA with the polarization of EM wave 54 Fig 7.2 Physical mechanism of the dual-band perfect absorption (a) Simulated effective impedance and absorption spectrum of the DMPA Distributions of (b) the induced VII 7.6 Summary I demonstrated a new type of ultrathin DMPA by experiment and simulation in the UHF region By integrating external capacitors to tune the magnetic resonance at very low frequencies, the thickness of dual-band absorber was efficiently miniaturized to be 0.0026λ (at 304.9 MHz) and 0.0031λ (at 358.5 MHz) The practical operation (absorption over 91% in TE and TM polarizations) of this DMPA was presented for a wide range of incident angles (up to 55o) I also developed a tri-element-integrated model that recognized an ultrathin triple-band MPA (thickness of 0.0017λ at 181.2 MHz) working in a lowerfrequency region 61 Chapter Conclusions and perspective In this thesis, I studied four different models of ultra-small and wide-incident-angle MPAs at very low frequencies First of all, I have designed, fabricated, and demonstrated an MPA working at 400 MHz The absorption is nearly unchanged over a relatively wide range (up to 30o) of incident angles for both TE and TM polarization The roles of structural components in the MPA design to obtain low-frequency absorption are also explained in detail by both calculation and simulation Furthermore, by flexibly controlling the geometry of MPA structure, I obtained a dual-band perfect absorption in the same region Specifically, the extension of the absorption bandwidth (for absorption of 90%) is well maintained at incident angles up to 30o for both TE and TM polarization At 250 MHz, I have demonstrated a special design for an ultrathin MPA (thickness of 1/240 with respect to the operational wavelength) Both simulation and experiment confirm that the absorption is polarization-insensitive at the normal incidence and nearly unchanged over a relatively wide range (up to 45o) of incident angles I also studied a new type of ultrathin MPA by experiment, simulation and theory in the VHF region By exploiting the magnetic resonance in the special structure, the unit cell of absorber is efficiently miniaturized to a periodicity of 0.012λ and thickness of 0.0012λ at 102 MHz The operation of designed MPA (absorption over 90%) is entirely stable for a wide range of incident angles (up to 55o) for TE and TM polarization The simulation also suggests that the absorption is polarization-insensitive at the normal incidence Particularly, by tuning the lumped capacitance of initial MPA structure, I effectively realized the dualband perfect absorption in the same frequency region Both experiment and simulation 62 confirm that two absorption peaks (absorption about 90%) are well maintained for incident angles up to 50o In the UHF region, an improvement for ultrathin DMPAs was investigated by experiment and simulation By integrating only external capacitors to tune the magnetic resonance at very low frequency, the thickness of dual-band absorber was efficiently miniaturized to be 0.0026λ (at 304.9 MHz) and 0.0031λ (at 358.5 MHz) The practical operation (absorption over 91% for TE and TM polarization) of this DMPA was tested over a wide range of incident angles (up to 55o) In addition, the simulations and the experiments show that the dual-band absorption is polarization-independent at the normal incidence I also developed a tri-element-integrated model to create an ultrathin triple-band MPA (thickness of 0.0017λ) working at 181.2 MHz Thus, I believe that the ultra-subwavelength structures are useful and can be applied to future meta-devices in the radio band Since the study of multi-band absorption in MMs is a rather new topic, it is more difficult to make an MPA sample with multi-bands in the MHz range Most of the unit-cell size of the recent MPAs are in the order of several cm, in the GHz region, to a few m in the MHz region, which are too large for the fabrication or even the measurement Therefore, I suggested four specific cases, with discussion on the physical mechanism and the possible applications of single/triple-band MPA for telecommunication All the structures are based on planar magnetic-resonant elements A further study could consider the problems of multi-band perfect absorption by adding the interaction with electric-resonance elements However, it might be complicated task, since many elements should be added into a MPA structure 63 Fig 8.1 Examples of flexible/broad-band MPA, which can be adapted in the military and the civilian applications If the whole battleship (or drone) is covered by a low-frequency flexible/broad-band MPA, their existence cannot be detected by the low-frequency radars The perspective of this thesis, especially, of the work presented in Chapter 7, might be the applied scope of the integrated model to reduce the size of absorber while keeping the same operational frequency, and to scale down the size of absorber to make a higher 64 operational frequency in the LTE (long-term-evolution) band, should be studied more These further studies might allow me to create a flexible/broad-band MPA in the radio band and to overcome the dependence of absorption on the size of practical lumped components, and to improve the current soldering techniques in fabrication These researches are expected to be useful in the military and the civilian tasks As shown in Fig 8.1, the lowfrequency flexible/broad-band MPAs can provide a stealth function for the next-generation military and civilian vehicles In addition, these MPAs might be also promising candidates for future health-care applications, since they can absorb the harmful EM radiation from LTE smart phones, Wi-Fi and smart home devices 65 List of publications [1] B X Khuyen, B S Tung, N V Dung, Y J Yoo, Y J Kim, K W Kim, V D Lam, J G Yang, and Y P Lee , “Size-efficient metamaterial absorber at low frequencies: Design, fabrication, and characterization,” Journal of Applied Physics 117, 243105 (2015) [2] B S Tung, B X Khuyen, N V Dung, V D Lam, Y H Kim, H Cheong, and Y P Lee, “Multi-band near-perfect absorption via the resonance excitation of dark metamolecules,” Optics Communications 356, 362 (2015) [3] N V Dung, B S Tung, B X Khuyen, Y J Yoo ,Y J Kim, J Y Rhee, V D Lam, and Y P Lee, “Simple metamaterial structure enabling triple-band perfect absorber,” Journal of Physics D: Applied Physics 48, 375103 (2015) [4] N V Dung, B S Tung, B X Khuyen, Y J Yoo, Y P Lee, J Y Rhee, and V D Lam,“Metamaterial perfect absorber using the magnetic resonance of dielectric inclusions,” Journal of the Korean Physical Society 68, 1008 (2016) [5] B X Khuyen, B S Tung, Y J Yoo, Y J Kim, V D Lam, J G Yang, and Y P Lee,“Ultrathin metamaterial-based perfect absorbers for VHF and THz bands,” Current Applied Physics 16, 1009 (2016) [6] B X Khuyen, B S Tung, Y J Yoo, Y J Kim, K W Kim, L.-Y Chen, V D Lam, and Y P Lee, “Miniaturization for ultrathin metamaterial perfect absorber in the VHF band,” Scientific Reports 7, 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Estimation for the perfect absorption at a wide oblique incident angle 40 5.5 Summary 42 Miniaturization for single/dual-band MPA by integrating the parasitic capacitors and the