Photonic Crystal Devices for Communication Photonics

Transkript

1 Photonic Crystal Devices for Communication Photonics R. Kappeler, P. Strasser, P. Kaspar, H. Jäckel Communication Photonics Group, Electronics Laboratory, ETH Zurich

2 Outline Introduction / motivation optical integration Photonic crystals for communication photonics slow light losses Computing realistic losses 2

3 The Vision in Communication Photonics Shift from electronics to optics - compete with speed (huge potential in optics) - compete with complexity - and hence with device size (difficult) Monolithic integration technology for passive and active photonic devices Baba T., Nature Photon., vol. 1, pp (2007) 3

4 Miniaturization Strategies Photonic Wires Photonic Crystals Xia F. et al., Nature Photon., vol. 1, pp (2007) Plasmonics Kaspar P. et al, to be published in Proc. SPIE Sergey I. et al., Nature Lett., vol. 440, pp (2006) 4

5 Photonic Wires vs. Photonic Crystals Waveguides 220nm nm 1μm 400nm 11 periods Photonic Wire PhC Plasmonics Prop. Loss ~ 1dB/cm ~ 4dB/cm ~ 900dB/cm Confinement ~ λ/(2*n) ~ λ/(2*n) << λ Separation ~ 2 μm 11 periods ~ λ Fabrication Stand. CMOS challenging? Daoxin Dai. et al., Appl. Opt., 46, 1126 (2007) 5

6 Photonic Wires vs. Photonic Crystals Bends Xudong C. et al., Opt.Ex., vol.14, pp Smaijc J. et al.,opt.ex., vol.11, pp Photonic Wire PhC Plasmonics Bend. Radius ~ 2 μm ~ 4 μm ~1 μm? Device width ~ 2 μm ~ 11 periods < λ Fabrication Stand. CMOS challenging? Daoxin Dai. et al., Appl. Opt., 46, 1126 (2007) 6

7 So, what is the motivation to use photonic crystals for communication photonics? 7

8 So, what is the motivation to use photonic crystals for communication photonics? Dispersion of PhC guided modes! 8

9 Photonic Crystals: Tailoring Dispersion Coupler Reduced Coupling Length Reduced coupling length due to dispersion of the guided modes 20μm Strasser P. et al., Opt. Express, 15, (2007) 9

10 Photonic Wires vs. Photonic Crystals Slow Light V gr = c/n eff Krauss T.F., J. Phys. D, 40 (2007) Low group velocity field enhancement Device length shrinks by the slow down factor s Device length Photonic wire L Photonic crystal ~ L/s 10

11 Photonic Wires vs. Photonic Crystals Slow Light All devices based on strong light matter interaction benefit from slow light all devices based on non-linear processes Often the field enhancement can be Example: SOA achieved by resonant P in SOA P out cavities Examples: Switches, SOA, s 11

12 Conclusions The huge effort to implement PhCs is only justified if the slow light mode can be used. Open issues preventing the use of the slow light modes in PhC: - coupling into slow light mode -large propagation losses The losses don t scale just proportional with the group velocity Kuramochi E. et al., Phys. Rev. B72, (R) (2005) 12

13 Electrically Pumped PhC Waveguides Huge Losses of Metallic Contacts Loss in a W3 waveguide: ~90dB/cm Loss in a contacted W3 waveguide: > 1500dB/cm Kaspar P. et al., Proc.SPIE, submitted (2008) 13

14 Facts Losses in a singlemode slab PhC waveguide are high >500dB/cm, losses in membranes are lower than in slabs (>4dB/cm) Imperfections of the technology have a strong influence on the propagation losses Many loss channels have been reported and investigated how do they relate quantitatively? which ones are negligible? where can technology improve the situation? 14

15 Reported Loss Mechanisms Intrinsic losses Out-of plane scattering (Mode coupling) Losses due to slab asymmetry Material absorption (e.g. metallic contacts) Backscattering Design losses Radiation leakage Interface losses Technology Hole depth Hole shape Hole roughness Hole position disorder Rauscher K., ETH Diss Nr

16 Technology Issues Experimental separation difficult Individual loss causes (especially technology-related losses) cannot be controlled independently by the technology. Hole shape Surface roughness conical shape, outliers at the bottom Strasser P. et al., Nanotechnology (2007) 16

17 Available Simulation Tools to Assess Losses Plane-wave expansion tool (MPB) Lossless band computations (of periodic structures) Cannot calculate losses FDTD (MEEP) Wave propagation problems The required simulation domain together with the fine grid (required for metals) make loss calculations computational intensive. FEM (COMSOL) Wavepropagation (complex ε ) Eigenmode only for real k The required simulation domain together with the fine structures of PhC make loss calculations computational intensive. 17

18 What Can Simulation Do? Intrinsic losses Out-of plane scattering (Mode coupling) Losses due to slab asymmetry ~ Material absorption Backscattering Design losses ~ Radiation Leakage Interface losses Technology Hole depth ~ Hole shape x Hole roughness x Hole position disorder x ~ Relative strength unknown Do we work on/improve the correct loss channel? 18

19 Conclusion: Key Question (for the use of PhCs) Photonic crystals offer highly interesting features for the miniaturization of optical devices but optical losses must be mastered first. How can we reduce optical losses? Clever design? Improvement of the technology? We need first to understand the loss mechanisms. Loss channels Strength of these loss channels How can simulations contribute to answer those questions? 19

20 Thank you for your attention! 20