Metasurfaces for Controlling Electromagnetic Fields

Transcription

1 Metasurfaces for Controlling Electromagnetic Fields Anthony Grbic Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor Graduate Students: Mohammadreza F. Imani, Gurkan Gok, Carl Pfeiffer AFOSR YIP (FA ) PECASE (FA ) Program Officer: Dr.Arje Nachman 1

2 AFOSR support Overall Goal: Extreme control of electromagnetic fields using subwavelength structured materials and surfaces. AFOSR Grant (FA ) My First Grant! Demonstration and Modeling of Material Responses Achieved Through Contradirectional Coupling, January 2007 to December AFOSR YIP (FA ) A New Approach to Manipulating Electromagnetic Fields: Near-Field Focusing Plates January 2008 to December DURIP (FA ) Millimeter-wave metamaterial characterization system, March PECASE (FA ) Tailoring the electromagnetic near field with patterned surfaces: near-field plates September 2009 August AFRL AMMTIAC contract with Alion Science and Technology (FA D003) Metamaterial Surfaces for Low-Loss Lens Applications March 2013-July Dr. N. Limberopoulos, Dr. B. Tomasic. 2

3 Metamaterials Metamaterials are engineered materials with tailored electromagnetic properties. They derive their properties from their subwavelength structure. Extreme control over electromagnetic fields can be achieved with metamaterials. Progress in metamaterials has enabled a myriad of devices: superlenses, invisibility cloaks, novel antennas and microwave/optical devices. However, the notable thickness of volumetric metamaterials can lead to bulky devices, fabrication challenges and even added losses. These concerns have driven the development of metamaterial surfaces: metasurfaces. 3

4 Metasurfaces Metasurfaces: two dimensional equivalents of metamaterials [1]. They are textured at a subwavelength scale, much like bulk metamaterials exhibit subwavelength granularity Metasurfaces can be described macroscopically in terms electric and magnetic polarizabilities, just as bulk metamaterials are described using effective material parameters. They can also be described in terms of electric and magnetic surface impedances / admittances. [1] C. L. Holloway, E. F. Kuester, J. A. Gordon, J. O Hara, J. Booth, and D. R. Smith, IEEE Antennas and Propagation Magazine, vol. 54, pp , Apr

5 Two broad categories of metasurfaces Metasurfaces that can tailor an incident wavefront upon reflection [2] or transmission [3,4]. Metasurfaces that guide or radiate waves. The metasurface acts as either a guiding structure or supports leaky waves that radiate directive radiation patterns [5,6]. [2] D. M. Pozar, Electronics Letters, vol.43, pp , Feb [3] N. Yu, P. Genevet, M. Kats, F. Aieta, J. Tetienne, F. Capasso, and Z. Gaburro,, Science, vol. 334, pp , Oct [4] X. Ni, N. Emani, A. Kilishev, A. Boltasseva, and V. Shalaev, Science, 335, pp. 427, Jan [5] B.H. Fong, J.S. Colburn, J.J. Ottusch, J.L. Visher, D.F. Sievenpiper, IEEE Trans. on Antennas and Propagation, vol. 58, pp , Oct [6] A. M. Patel, A. Grbic, IEEE Trans. on Antennas and Propagation, vol. 61, pp , Jan

6 Metasurfaces for manipulating the near field Objective: to achieve extreme electromagnetic field confinement: focus or detect fields with subwavelength resolution. A general class of aperture fields were proposed that can form a near-field focus. [7] R. Merlin, "Radiationless electromagnetic interference: evanescent-field lenses and perfect focusing," Science, 317, pp , August Near-field plates were introduced. Near-field plates are non-periodically patterned, grating-like surfaces that can focus electromagnetic waves to subwavelength dimensions. [8] A. Grbic and R. Merlin, Near-field focusing plates and their design, IEEE Transactions on Antennas and Propagation, 56, pp , October [9] A. Grbic, L. Jiang, and R. Merlin, Near-Field Plates: Subdiffraction focusing with patterned surfaces, Science, 320, , April

7 Aperture fields and subwavelength focal patterns Desired Focal Pattern x focal plane z I near-field focusing plate E x q L o ( y, z L) e q Lsin c( q y) Aperture Field (field at plate) o o line source L y L [10] A. Grbic, R. Merlin, E.M. Thomas, M.F. Imani, Nearfield plates: metamaterial surfaces / arrays for subwavelength focusing and probing, Proceedings of the IEEE, vol. 99, pp , Oct E total ( y) E x ( y, z 0) L[ Lcos( qo y) ysin( q 2 2 L y o y)] 7

8 Contour plot of the electromagnetic field x j x focal plane z current distribution L L y The near-field plate supports a highly oscillatory current distribution (aperture field) that focuses the electromagnetic near field to a subwavelength focus. 8

9 Design procedure for near-field plates 1) The current density needed to produce the focal pattern is computed. 2) The tangential field at the surface of the plate is found. 3) The surface impedance is calculated from the ratio of the current density to tangential field. 4) The surface is discretized into subwavelength elements. Each surface element is textured in order to realize the required impedance profile. 9

10 Initial near-field plate implementation Design procedure was used to implement a near-field plate at microwave frequencies. [9] A. Grbic, L. Jiang, R. Merlin, Near-field plates: subdiffraction focusing with patterned surfaces, Science, 320, pp , Apr. 25, Frequency: 1 GHz FWHM= l o /18 10

11 Printed, concentric near-field plate Printed near-field plates (NFPs) consist of concentric annular slots, loaded with reactive elements, over a grounded dielectric substrate. The slots are non-periodically loaded to achieve a desired subwavelength focal pattern. [11] M.F. Imani and A. Grbic, Planar near-field plates, IEEE Trans. on Antennas and Propagation, 61, pp , Nov

12 Measured beams in the reactive near field (1 GHz) Measured FWHM Bessel beam NFP Airy pattern NFP Coaxial probe 12

13 Applications Microwave frequencies: At microwave frequencies, metasurfaces that manipulate the near field will find a number of applications: Probing devices for non-contact sensing. Targeting devices for medical devices. Wireless power transfer receivers and transmitters. THz and optical frequencies: Nanostructured implementations at these frequencies hold promise for: Microscopy Near-field optical data storage Lithography 13

14 Reflectionless metasurfaces Objective: to arbitrarily control electromagnetic waves using thin surfaces. Approach: employ the Surface Equivalence Principle (a rigorous form of Huygens Principle) to design metamaterial surfaces [7]. Characteristics: Textured at a subwavelength scale. Spatially non-periodic. Can exhibit electric, magnetic and even chiral responses. Can be reflectionless. Can fully manipulate co- and cross-polarized radiation. Can tailor the phase and polarization of an Impinging wavefront. Magnetic Current Electric Current [12] C. Pfeiffer and A. Grbic, Metamaterial Huygens' surfaces: tailoring wave fronts with reflectionless sheets, Physical Review Letters, vol. 110, , May

15 Experimental metamaterial Huygens surface Incident electric field is polarized in the y-direction. Sample dimensions: 7.53l x 7.53l Top side (electric response) Bottom side (magnetic response) 15

16 Beam deflection/refraction by a metasurface (10 GHz) Measured near field Simulated near field Refracted angle follows the generalized Snell s law [13]: r i sin sin sin 45 o [13] N. Yu, P. Genevet, M. Kats, F. Aieta, J. Tetienne, F. Capasso, and Z. Gaburro, Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction, Science, vol. 334, pp , Oct

17 Cascaded metasurfaces Cascading electric sheet impedances can also generate both electric and magnetic responses [14,15]. Three sheets can provide complete phase coverage with high transmission. Transmittance Phase [14] F. Monticone, N. M. Estakhri, and A. Alu, Full control of nanoscale optical transmission with a composite metascreen, Physical Review Letters, vol. 110, pp , May [15] C. Pfeiffer and A. Grbic, Cascaded metasurfaces for complete phase and polarization control, Applied Physics Letters, vol. 102, , June

18 Beam refraction and polarization conversion Anisotropic, inhomogeneous metasurface 5 different unit cells were employed to realize high transmission and linear phase shift. Perfect transmission and the desired phase dictate the relationship between the outer and middle sheet admittances. Unit cell dimensions 18

19 Refracting circular polarizer Wave is efficiently refracted. The electric field polarized along y + z is shown for x<0 and the left-handed circular component is shown for x>0. Circular Linear [16] C. Pfeiffer and A. Grbic, Millimeter-wave transmitarrays for wavefront and polarization control, IEEE. Trans. on Microwave Theory and Techniques, vol. 61, pp , December

20 Collimating lens with polarization control Radiation Pattern at 9.9 GHz collimating lens planar launcher 4.0 mm (foam spacer) 20

21 Applications The presented metasurfaces will find a wide range of applications. They will enable / advance, low profile and conformal antennas at microwave and millimeter-wave frequencies. flat or conformal quasi-optics and optics: lenses, beam refractors, and exotic polarization controlling devices. the efficient generation of various beams in the radiative near field (Bessel beams, self-accelerating Airy beams, vortex beams, and cylindrical vector beams) for micro/nano-manipulation and high resolution probing at optical frequencies. perfect absorbers stealth technology 21

22 Manipulating guided waves Recall the two broad classes of metasurfaces a) Metasurfaces that can tailor an incident wavefront upon reflection or transmission. The focus of the presentation so far. b) Metasurfaces that guide waves or support radiating (leaky) waves that produce directive radiation patterns. To shed some light on the design of metasurfaces that can control surface waves, let s examine controlling waves in two dimensions. 22

23 Controlling phase progression and power flow Objective: to develop a method for arbitrarily controlling the phase progression and power flow of electromagnetic fields within a 2D region of space. Desired spatial distributions of the wave vector k (phase) and direction of Poynting vector S (power) are set within the medium. Material parameters are solved for in terms of k and the direction of (S). TM z polarization Electric field is in z direction: E z. Magnetic field is in the x and y directions: H x, H y. μ = μ xx μ xy μ yx μ yy ε = ε z 23

24 Material parameters in terms of phase and power flow Using plane-wave relations in anisotropic media, along with an impedance matching process, the required spatially-varying material parameters can be found. μ = Permeability tensor ε z k x + κk y κε z k x + κk y 2 + k 2 y ε z 2 k xk y ε z κε z k x + κk y κ 2 ε z k x + κk y 2 k xk y ε z 2 + k 2 x ε z Permittivity constant ε z κ = tan θ s θ s is the angle between the Poynting vector and the x axis. Unknowns: unit cell permittivities ε z. They can be solved for by minimizing inter-cell reflections. Once permittivities are found, the permeability entries can be computed. [17] G. Gok and A. Grbic, Tailoring the Phase and Power Flow of Electromagnetic Fields, Physical Review Letters, vol. 111, , Dec

25 Tailoring the electromagnetic field Time snapshot of the vertical electric field E z Unit cell dimension λ o 7.2. Transformation region: 1.4 λ o x 8.4 λ o. The field emanating from a cylindrical source is reshaped over the transformation region. The incident field at the input boundary (boundary 1) is transformed to a desired field distribution at the output boundary (boundary 2). 25

26 Example 1: trapezoidal amplitude & linear phase Desired output power density Desired output phase : S direction : k k x = 0k o Input power density Input phase 26

27 Example 1: trapezoidal amplitude & linear phase Material parameters A time snapshot of vertical electric field E z Power density Phase 27

28 Example 2: triangular amplitude & uniform phase Material parameters A time snapshot of vertical electric field E z Power density Phase 28

29 Implementation: tensor transmission-line metamaterials Parallel plate waveguide (PPWG) μ xx yx xy yy Tensor transmission-line (TL) unit cell Circuit model Microstrip implementation z Permeability tensor Impedance tensor yy jd yx xx xy Z Z2 2Z3 2Z Z 2Z 2Z Permittivity scalar jd z Admittance scalar Y [18] G. Gok and A. Grbic, Tensor transmission-line metamaterials, IEEE Trans. on Antennas and Propagation, vol. 58, no. 5, pp , May [19] G. Gok and A. Grbic, A printed beam-shifting slab designed using tensor transmission-line metamaterials, IEEE Trans. on Antennas and Propagation, vol. 61, no. 2, pp , Feb

30 Antenna beamformer Displaced by 1 unit cell Displaced by 2 unit cells Displaced by 3 unit cells Trapezoidal power density distribution with uniform phase output The beam scans as the cylindrical source is laterally displaced. 30

31 Fabricated antenna beamformer ( 8 GHz) 31

32 Applications The proposed design approach allows independent spatial control of phase progression and power flow. It could find use in the design of electromagnetic devices such as: antennas and beam forming networks, scattering control: manipulation of radar signatures signal routing on optical circuits mode conversion devices for microwave/optical interconnects and transitions, extreme aperture distributions for super-directive radiation, and the excitation of Airy and Bessel beams. 32

33 Honors Ernest and Bettine Kuh Distinguished Faculty Scholar Endowed Associate Professorship, Department of EECS, University of Michigan, May May Early Tenure, Department of EECS, University of Michigan, September Booker Fellowship, US National Committee, International Union of Radio Science. Presented to a U.S. scientist every three years. Citation: For outstanding contributions to radio science. July Outstanding Young Engineer Award, IEEE Microwave Theory and Techniques Society. Citation: For outstanding early career contributions to the microwave profession. June Henry Russel Award, University of Michigan. One of the highest honors the university bestows upon junior faculty, March

34 Summary Metasurfaces for Near Field Manipulation: Near-Field Plates Introduced near-field plates: finely-structured non-periodic surfaces (metamaterial surfaces) that can focus electromagnetic energy to extreme subwavelength resolutions. Reflectionless Metamaterial Surfaces for Wavefront and Polarization Control Developed metasurfaces that provide extreme control of electromagnetic wave fronts across optically thin layers. These reflectionless surfaces allow phase and polarization control, providing new beam shaping, steering, and focusing capabilities. Method to Control the Phase and Power Flow of Electromagnetic Waves Formulated a method for arbitrarily controlling the phase progression and power flow of electromagnetic fields in 2D using metamaterials. Circuit-Based Metamaterials: Tensor Transmission-Line Metamaterials Devised transmission-line/circuit-based metamaterials that exhibit tensorial effective material parameters. Used these planar metamaterials to control surface waves. 34

35 Acknowledgements AFOSR Dr. Arje Nachman, Program Officer, Electromagnetics For continued support over the years, and fruitful technical discussions. Dr. Harold Weinstock, Program Officer, Quantum Electronic Solids For invitations to his review meetings. AFRL Dr. Nicholaos I. Limberopoulos, Electronics Engineer, Air Force Research Laboratory (AFRL/RYDP), Sensors Directorate, Electro-Optic and Infrared Sensing Technology Branch Dr. Boris Tomasic, Principal Electronics Engineer, Air Force Research Laboratory (AFRL/RYH), Sensors Directorate, Antenna Technology Branch Dr. Augustine M. Urbas, Research Physicist Air Force Research Laboratory, (AFRL/RX) Materials and Manufacturing Directorate, 35

36 Metamaterials and Metasurfaces 36