Design and optimization of photonic shields for atmospheric re-entry. N. Komarevskiy, V. Shklover, L. Braginsky, J. Lawson and Ch.

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1 Design and optimization of photonic shields for atmospheric re-entry N. Komarevskiy, V. Shklover, L. Braginsky, J. Lawson and Ch. Hafner

2 Application of interest re-entry Optimized structures: 1D, inverse opals, woodpiles, porous-reflectors Leaky modes Conclusions 2

3 Application of interest - Reentry Irradiation of vehicle Reflecting photonic system (RPS) Goal: Reduce radiative heating => high reflection 3

4 Radiative spectra of shock gas A. M. Brandis et al Major fraction of radiation is incident normally Goal: reflector for B= nm = THz 4

5 Optimization goal Total reflection of the unpolarized radiation for normal incidence Polarization averaged reflection Total reflection for each polarization For homogeneous radiation 5

6 Dielectic permittivity of the chosen materials Silicon Carbide or Glassy Carbon Transparent in THz Strongly absorptive in THz 6

7 1D SiC/air and GC/air Glassy Carbon reflection is limited to 38% 7

8 Glassy carbon as a single frequency reflector High reflection can be achieved 8

9 Glassy Carbon inverse opal Zakhidov et al

10 10 Guided-mode resonance structure (GMR) Planar waveguide Corrugated waveguide Waveguide modes Modes folded into 1 st Brillouin zone (empty lattice) Resonance reflection

11 Guided-mode resonance structure, silicon carbide 11

12 Guided-mode resonance structure, silicon carbide 12

13 Leaky modes in transparent waveguides Planar waveguide z d/2 0 -d/2 eigenfrequencies ω real Field decays exponentially outside Poynting vector S z ( z >d/2)=0 13

14 Leaky modes in transparent waveguides z d/2 -d/2 0 Corrugated waveguide Two types of solution 1) ω real, confined modes 2) ω complex, leaky modes (radiative modes) In notation exp(-iωt), eig ω= ω 1 i ω 2, so E(t)~exp(-i ω 1 t- ω 2 t) decays Plane waves outside of the waveguide: kz n m ; n, m 0, 1, 2 c d x d y k a ib, a 0 or a 0; b 0 z k ( k i k ) z 2 Poynting vector S z >0, z>d/2 (radiation condition), therefore, k z = k 1 -i k 2 E( z) ~ exp( i k z k z), z d / 2 exponentially increasing wave

15 Leaky modes in transparent waveguides ε=6.62 Eigenfrequencies ω= ω 1 i ω 2, ω 2 is full width of the resonance 15

16 Leaky modes in transparent waveguides 344 THz 357 THz 344 THz 357 THz ε=

17 Silicon Carbide woodpiles 17

18 Origin of the peaks above 300 THz (Leaky modes) 18

19 Silicon Carbide inverse woodpiles 8-layered inverse woodpile α d x 10 parameters for optimization: d x =d y, r 1 -r 8, α Difficult to optimize a structure with many leaky modes 19

20 Silicon Carbide porous-reflector 20

21 Sensitivity to geometrical imperfections Statistical approach: calculations of are made Each geometrical parameter is varied randomly near optimal: imperfection strength random number average sensitivity maximum sensitivity 21

22 Sensitivity to geometrical imperfections 1) GMR structures: 12 14%, 29 31% max 6%, 16.5% 3%, max 8% 2) Woodpiles: 4-layered:, 8-layered max 3) Porous-reflector: 4 6%, 15 20% max 4) 1D structures SiC/air: 0.5%, 4% ; 1D structures GC/air: 0.8%, max 3% max 22

23 Coating of individual fibers in the heat shield Bare carbon fiber Coated fibers enhance reflectivity Pica - Phenolic Impregnated Carbon Ablator 23

24 Coating of individual fibers in the heat shield Bare carbon fiber r=3 μm Coating with 4 SiC layers decrease of absorption in a carbon fiber 82%-18% 24

25 Conclusions SiC gives opportunities to design ideal reflector in THz Reflectivity of GC is limited to 38% GC can be designed to reflect one strong spectral line 1D structures are optimal for normal reflection and the most robust to imperfection Leaky modes are important in the optimization and can have positive and negative role 25

26 Danke! 26