Anisotropic negative refractive index material (NRM) S. T Anisotropic negative refractive index material (NRM) S. T. Chui Bartol Research Institute University of Delaware

Outline: Left handed material, existing material Magnetic composites: a kind of anisotropic NRM Inverse total internal reflection. Anisotropic NRM with positive definite permittivity. Negative refraction and omidirectional total transmission. Photonic Hall effect. Enhanced localization effect at low frequencies.

Research in collaboration with L. B. Hu and Z. F. Lin Research in collaboration with L. B. Hu and Z. F. Lin. Chui was partly supported by the ARMY research lab through the center of composite studies at the University of Delaware, by DARPA and by the NSF.

Left-Handed Materials Poynting vector S = E£ H Convention Materials(RHM): Left-Handed Materials(LHM): S¢ k > 0 S ¢ k <0 Wave propagates(phase velocity) in the same direction of energy flow(k) Wave propagates(phase velocity) in the opposite direction of energy flow(k) UD MAGNETS CMR2

Original Idea: Negative dielectric constant. Negative magnetic susceptibility. Because the velocity of light is inversly proportional to the square root of the product of these two susceptibilities, light propagation is not damped . This argument focuses on the real parts of the susceptibilities.

Some references: V. G. Veselago, Sov. Phys. Usp. 10, 509 (1968) J. B. Pendry, A. J. Holden, W. J. Stewart and I. Young, Phys. Rev. Lett. 76, 4773 (1996). D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemet-Nasser, S. Schultz, Phys. Rev. Lett. 67, 3578 (2000).

Unusual Physical Properties Reversed Doppler effect – microwave radiation or light shift to lower frequencies as a source approaches and to higher frequencies as it recedes. Reversed Cerenkov effect – light emitted in the backward direction (forward direction in a right-handed materials) when a charged particle passes through a medium. Reversed Snell’s law – light that enters a LHM from a normal material will undergo reflection, but opposite to that usually observed. Unusual lens: UD MAGNETS CMR2

Negative index of refraction UD MAGNETS CMR2  

Current Material UD MAGNETS Since these materials are made by microstructure, they are very difficult to be used UD MAGNETS CMR2

Magnetic nanocomposite as left-handed material Figure . Proposed structures with (a) metallic nanowires; (b) metal/insulator multilayer nanowires; (c) metallic nanoparticles; (d) compacted metallic nanoparticles; and (e) metallic needles embedded in a dielectric matrix.

Negative magnetic susceptibility comes from a resonance Magnetic susceptibility: In current material, resonance is from a resonantor. Our material: it is from the intrinsic Ferromagnetic Resonance due to spin waves: ώ LHM ώ0 Dielectric constant of metal is negative: damping 1/τ in metallic phase UD MAGNETS CMR2

Relationship between the magnetic field b and the macroscopic field h

The resonance form of the susceptibilities

Magnetic nanocomposites are examples of anisotropic LHM’s Magnetic nanocomposites are examples of anisotropic LHM’s. Its possible advantages are: Easier to manufacture. Lower loss. Magnetization direction can be locally tuned. Anisotropy offers more degrees of freedom.

Effective medium approximation result: For left handed circularly polarized radiation propagating along the direction of the magnetization. For metal concentration below the conducting percolation threshold but above the magnetic percolation threshold. The direction of energy flow is opposite the wavevector above the ferromagnetic resonance. The damping turns out to be small!

Some references: S. T. Chui and L. B. Hu, Phys. Rev. B65, 144407 (2002) S. T. Chui, L. B. Hu and Z. F. Lin, Phys. Lett. A319, 85 (2003).

An idea that we have used: Imaginary parts of the susceptibilities were included in our calculation. For a given frequency w there are two possible wave vectors §k with k=k’+ik’’. The direction of energy flow is controlled by the imaginary part of the wave vector. E=E0 exp (ik’¢ x –k’’x) For k’’>0 the wave moves in the direction of increasing x; for k’’<0 the wave moves in the direction of decreasing x

Imaginary wave vector reamins small and does not change sign (energy flow direction is unchanged)

Real wave vector becomes negative

Crucial physics At the resonance, the relative sign between the real and the imaginary part of the wave vector changes.

Terminology: Positive definite (indefinite) dielectric constants Positive definite: all ei are positive.

E- polarized wave satisfy k . E=0 For anisotropic materials with indefinite susceptibilities, NRM and LHM conditions are different: E- polarized wave satisfy k . E=0 For E-polarized wave in materials with uniaxial anisotropy perpendicular to the plane normal, exy<0 implies LHM; mz<0 implies NRM provided additional constraints on the angles are satisfied. A similar set of conditions applies for H-polarized waves. Similar relationships exist when the axis is parallel to the interface.

References: L. B. Hu, S. T. Chui and Z. F. Lin, Phys. Rev. B66, 085108 (2002). V. Lindell et al., Microwave and Opt. Tech. Lett. 31, 129 (2001) D. R. Smith and D. Schurig, Phys. Rev. Lett. 90, 077405-1, (2003). L. Zhou, C. T. Chan and P. Sheng, Phys. Rev. B68,115424 (2003).

Inverse total internal reflection Under some conditions, light will go through only if the angle of incidence t is smaller (not larger!) than some critical value Sometimes reentrant behaviour can also be exhibited. t

Anisotropic materials with positive definite susceptibilities can also exhibit negative refraction Y. Zhang, B. Fluegel and A. Mascarenhas, Phys. Rev. Lett. 91, 157404 (2003) Twinned anisotropic YVO4 crystal on both sides.

Idea behind negative refraction in anisotropic material Geometry of refraction shown on top, with the direction of the anisotropy axis as illustrated. Constant frequency contour in wavevector space shown in lower graph. Solid and dashed lines are for opposite sides Group velocity is the normal to this curve. X component of the wave vector is conserved. As illustrated Si and St, the incident and transmitted energy flow exhibit negative refraction.

Illustrative results from a quantitative analysis Range of incident angle (between the solid and the dashed curves) for negative refraction. The anisotropy parameter u=(e1-e2)/e1; e=(e1,e1,e2). Lower curve is for only one side anisotropic Top curve is for both sides anisotropic Z. Liu, Z. F. Lin and S. T. Chui, Phys. Rev. B69, 115402 (2004).

Multilayer structure as negatively refracting material Incoming direction, surface normal and anisotropy axis in the same plane.

Omidirectional total transmission When the dielectric constants on the left and the right satisfies certain conditions, all incoming radiation will be transmitted, none will be reflected.

Photonic Hall Effect: Mie scattering by magnetic particles: r£r£(s -1¢BI )- ks2BI=0. As the magnetization is reversed, ’ changes sign.

Mie scattering of magnetic particles BI=n,m dmnMmn(1)(k,r)+ cmnNmn(1)(k,r), not a function of L because r¢ B=0 r£r£ N(J)mn - k2 N(J)mn =0, M(J)mn = r £ N(J)mn /k The usual bais function satisfies the equations: r¢ M(J)mn=0, r¢ N(J)mn=0, r£ L(J)mn=0.

Photonic Hall effect: F(,)= d(,)/d}|-d(,)/d}|=0. Polar plot of magneto-transverse cross section F(,) at =/2 for two values of . Solid line (dotted line) denotes positive (negative) values for F(,). The applied magnetic field is in z direction (normal to the plot) and incident wave vector in x direction. Phys. Rev. E69, 056614 (2004).

Localization of light can be enhanced by left-handed material To enhance localization, the parameter =kl should be reduced. Here l , the mean free path is inversely propotional to the impurity scattering cross section . For a spherical impurity of radius a,  / x4 when x=ka <<1. Hence / 1/x3 for small x. It is difficult to localize light in the long wavelength limit.

Enhanced localization For left-handed material, there are scattering resonance at low frequencies. P/ E/(+2). When =-2, P is very big E

Enhanced localization: More detailed calculation (a)  and  of the NIM (b) The efficiency for scattering Q_s vs frequency. (c) The inverse of the localization parameter 1/kl vs frequency. Phys. Rev. E69, 016619 (2004)

Possible LHM Base on Nanomagnetic composite Equation of motion: Wave equation: UD MAGNETS CMR2

Possible LHM Base on Nanomagnetic composite may be negative UD MAGNETS CMR2

Granular Materials and Fabrication Granular materials (films, and bulk materials) Insulating: Teflon Ferrites Magnetic: NiFe Metallic Thin Films: Vapor deposition (magnetron Sputtering) Bulk Materials: Ball milling, chemical synthesis, and microcompounder (arriving in Oct.-Nov.) FeNi: Low loss, resonant frequency can be tuned with composition, and large negative permeability. Teflon: Low loss and low dielectric constants (Bulk materials have been sent out for fabrication a month ago and will arrive soon. Granular films have been fabricated)