Metal-Dielectric Photonic Multilayers for Beam Control

Transcription

1 Metal-Dielectric Photonic Multilayers for Beam Control Andrey Chabanov 1,2 1 Department of Physics and Astronomy University of Texas at San Antonio 2 AFRL/RY, WPAFB Funded by AFOSR (Program Director Dr. Ali Sayir)

2 UTSA: Rodion Kononchuk, Kyle Smith AFRL/RY: Igor Anisimov, Nicholaos Limberopoulos, Ilya Vitebskiy, Vladimir Vasilyev AFRL/RD: Brad Hoff, Martin Hilario, Anthony Baros Wesleyan U: Tsampikos Kottos, Elena Makri 2

3 Selective control of light-matter interactions in complex system H 2 E 2 Nano-layer can be: - dielectric/metallic - magnetic - nonlinear/phase change Defect layer The localized mode has E and H nodes and antinode in the layered structure. - Design complex multilayers with suitable spatial distributions of electric and magnetic fields - Introduce functional material at specific location(s) in the host multilayer to enhance desired or/and suppress undesired responses 3

4 Potential (maximum) transmittance: P. H. Berning and A. F. Turner, J. Opt. Soc. Am. 47, (1957) 4

5 Induced Transmission through Cobalt thin film without metal with metal Q factor = 800 MW ceramics: 1 mm; ε 1 = 37; loss tangent, 5 x 10-4 Glass: 3 mm; ε 2 = 3.8; loss tangent, 7 x nm Co 90 Fe 10 on glass wafer R. Kononchuk et al. (2016) 5

6 Faraday rotation in FM metal-dielectric multilayer Faraday rotation enhancement scheme: placing a ferromagnetic metal layer F at the location of the E - node (also, H - antinode) of the localized mode eliminates ohmic losses and enhances magnetic Faraday rotation by a factor of Q. K. Smith et al. JoP D (2013) 6

7 Magnetophotonic response of FM metal-dielectric multilayer, K. Smith et al. JoP D (2013) 7

8 Transmission and Faraday rotation of FM metal-dielectric multilayer K. Smith et al. JoP D (2013) 8

9 Transmission (db) Frequency (GHz) Faraday effect in Cobalt-dielectric multilayer LHP RHP T tot Transmission (db) Frequency (GHz) Transmission (db) Frequency (GHz) Transmission (db) Transmission (db) Frequency (GHz) Transmission (db) Frequency (GHz) Faraday effect (deg.) Frequency (GHz) HMFL (Tallahassee, FL) 9

10 Faraday rotator designs using Cobalt-dielectric multilayer [HL] 3 HFH [LH] 3 [HL] 3 HFH [LH] 7 FH [LH] T 0.4 Transmission LH component RH component T Transmission LH component RH component (units of Faraday rotation LH phase RH phase o FR (units of Faraday rotation LH phase RH phase o FR Coupled-cavity structure: - increased isolation band - increased transmission - decreases losses in cobalt layer f (MHz) F = 180 nm Co f (MHz) F = 70 nm Co 0 K. Smith et al. JoP D (2013) 10

11 Co-Pt systems Leon Abelmann (Twente, Netherland) Pt(17.5 nm) / [Co(0.4 nm) / Pt(0.6nm)] 25 / Pt(1.8 nm) m [mam 2 ] moment = ± [mam 2 ] background x= ± [mam 2 /T] background y= ± [mam 2 /T] T2 perpendicular m m^ B [T] Beth Stadler (UM), Co-Pd systems 5 T, RT 4 bore magnet (5 ppm) expected on-line in Fall

12 Asymmetric vs Symmetric Metal-Dielectric Multilayers Symmetric cavity Asymmetric cavity A A Unperturbed state A C accidental spatial degeneracy ε A ε A ε A ε C A A Perturbed state A C ε' A = ε A + Δε ε' A = ε A + Δε ε' A = ε A + Δε ε C E-node remains at the same position E-node is shifted from the position of metallic layer, resulting in suppression of a resonant transmission mode R. Kononchuk et al, Photonic Metamaterials, Greece (2016) 12

13 Oblique transmission through asymmetric metal-dielectric multilayer Transmission, T peak T Incident angle (deg.) Frequency (GHz) Strong transmission directionality R. Kononchuk et al, Photonic Metamaterials, Greece (2016) 13

14 Concept of wide-aperture omnidirectional isolator Standard free-space microwave/optical isolator Forward Incident wave Polarizer 1 45º Faraday rotator Polarizer 2 Transmitted wave No transmitted wave Polarizer 1 45º Faraday rotator Polarizer 2 Backward Incident wave Basic problems with existing free-space isolators 1. Weakness of magneto-optical light-matter interactions leads to large size 2. Absorption causes loss of power and poor isolation 3. All existing isolators only perform for plane-parallel waves and at normal incidence K. Smith et al, Photonic Metamaterials, Copenhagen (2014) 14

15 Concept of wide-aperture omnidirectional isolator T (db) [HL] 3 HCH [LH] 3 Isolation would fail at large oblique incident angles Frequency (GHz) [HL] 3 HCH* [L*H*] 3 T (db) T (db) Frequency (GHz) [HL] 3 HFH* [L*H*] Frequency (GHz) Transmissive at normal and opaque at oblique incidence Would not provide omnidirectional isolation K. Smith et al, Photonic Metamaterials, Copenhagen (2014) 15

16 Optical Limiters Safety device to protect the human eye and sensitive optical devices from high power radiation. Transmit low-intensity light, while blocking radiation above a certain limiting intensity threshold. Utilize NL optical material that scatter, refract, or absorb high power radiation. Existing limiters have serious limitations Threshold energy for NL behavior is higher than laser-induced damage threshold. Limiting threshold (LT) provided by NL materials is not far away from their damage threshold (DT). Dynamic range of the existing limiters, defined as DT/LT 1, is usually small. When DT/LT=1, the limiter is sacrificial (i.e., destroyed at LT and needs replacing). 16

17 Transmission in asymmetric metal-dielectric multilayer Steady-state (CW) regime: Material A, ε A =10 Material B, ε B = 1 Material C, ε C = 22.5 Material D, ε D = 2.25 ε A = 0 ε A ε A = 2% M, Cobalt, 180 nm turns opaque in very broad frequency range E. Makri et al, Sci. Rep (2016) 17

18 Transfer-Matrix Calculations of Pulse Propagation ε A T = ε A + T d dt T t = 1 C A T W I(t), where W I t = E I (t) 2. A(t) initially reaches a maximum around t=0.1 and then abruptly decays to less than -30 db. Transmission also decays while reflection reaches unity. E. Makri et al, Sci. Rep (2016) 18

19 Pros: Reflect rather than absorb high-power radiation => no overheating/destruction Intensity enhancement at the NL layer can be adjusted to a required level => adjustable LT Host multilayer can be made of high DT materials => protection for NL layer => dramatically increased dynamic range, DT/LT 1. Cons: Asymmetric metal-dielectric reflective limiter Require optical materials with pure NL absorption; such materials do exist, but can only work in a specific frequency range Enhanced NL absorption in the crossover to high reflectance => some heating Based on resonance phenomenon, so has narrow band May require significant ε to achieve high reflectance. 19

20 Multilayered Structures with Phase Change Materials VO 2 has a reversible metal-insulator transition at T c = 67 0 C Low intensity High intensity T=29 C T=90 C VO 2 nanolayer does not significantly effect the defect mode in the insulating state Under exposure to high-intensity radiation, the VO 2 nanolayer is heated and undergoes the transition to the metallic state In the metallic state, VO 2 strongly interacts with radiation at the E-antinode plane, suppressing the resonant mode and resonant transmission and resulting in a broadband reflection 250 μm Sapphire, ε = i 780 μm Air, ε = nm VO 29 0 C, ε = i 200 nm VO 90 0 C, ε = i 20

21 Millimeter-wave interaction with VO 2 in (heated) multilayer Spectral measurements are at normal incidence in a temperature-controlled furnace 21

22 High-power transmission through multilayer with VO 2 w/0.75 aperture Klystron amplifier, 95 GHz, 120 W max Stand-alone VO 2 did not switch to reflective state -15 db 22

23 Outstanding question: Is there a negative, or balancing, feedback in the limiting process in the multilayered structure with VO 2? Is there a equilibrium transmission due to the balancing feedback? 1. Heat production in VO 2 depends on temperature of VO At the MIT in VO 2 (at a given temperature), there can exist a metastable state of two phases (dielectric and metallic), leading to equilibrium transmission. Requires: high-quality, uniform VO 2 films, simultaneous modelling of EM wave propagation and heat transport in the multilayered structure, and temperature distribution measurement in VO 2 in real time. 23

24 Summary Metallic nanolayers introduced in dielectric multilayers can be made transmissive or opaque depending on the position of nanolayer. Ferromagnetic metallic nanolayer, such as Cobalt, can produce 45 0 Faraday rotation commonly used in free-space MW isolators. In asymmetric metal-dielectric multilayer, a small change in the refractive index of one of the constitutive layers and/or in the oblique incident angle can induce a transition from resonant transmission to broadband reflectance. The phenomenon can be used in omnidirectional isolators, optical switches, modulators and limiters. Reflective optical limiter has key advantages over existing absorbing limiters: The LT can be adjusted and made significantly lower, and the DT can be significantly higher than for stand-alone NL absorbing layer. Multilayers involving phase-change materials, like VO 2, can act as a reflective limiter for MMWs. Temporal behavior of the reflective multilayered limiter requires better understanding of heat generation and transport in the VO 2 film. 24