Nonlinear Optics in Silicon Core Fibers A. C. Peacock 1, P. Mehta 1, T. D. Day 2, J. R. Sparks 2, J. V. Badding 2, and N. Healy 1 POEM:2012 Nov Optoelectronics Research Centre, University of Southampton, UK 2 Department of Chemistry and Materials Research Institute, Pennsylvania State University, Pennsylvania, USA

2 Outline Silicon fibers and their fabrication Nonlinear propagation equations Nonlinear properties of silicon fibers –Absorption (TPA) –Spectral broadening (SPM) –Optical modulation via TPA –Comparison of different core sizes Tapered silicon core fibers –Nonlinear pulse shaping

Nonlinear Silicon Photonics - on chip…. Raman amplifiers/lasers (Claps et al., Opt. Express, v.11, 2003) Wavelength conversion: FWM/XPM/THG All-optical control: TPA, FCD Supercontinuum: SPM And much more… 3 WOK Publication Data Breakthroughs in Nonlinear Si Photonics 2011 Y. Okawachi et al., IEEE Photon. J. 4, 601 (2011)

Why Fiberize Silicon? 4 Fibers are the backbone of telecommunications industry Silicon waveguides largely used as a nonlinear element Incorporation inside the fiber geometry negates some of the coupling issues Allow for the construction of cheap/robust devices Exploit wide variety of fiber templates for novel designs New fiber materials extend applications to medicine, imaging, sensing, and security

Penn State & ORC 1 Clemson Univ. 2 Virginia Tech. 3 5 A Brief History of Silicon Fibers Mar 06 Oct 07 Nov 08 Jan 10 Oct 09 Jul 10 Jun 11 Jan 12 dB/cm 1.P. Sazio et al., Science 311,1583 (2006) 2.J. Ballato et al., Opt. Express 16, (2008) 3.B. Scott et al., IEEE Photon. Techn. Lett (2009)

6 A Brief History of Silicon Fibers Mar 06 Oct 07 Nov 08 Jan 10 Oct 09 Jul 10 Jun 11 Jan 12 dB/cm 1.L. Lagonigro et al., Appl. Phys. Lett. 96, (2010) 2.P. Mehta et al., Opt. Express 18, (2010) 3.P. Mehta et al., CLEO 2012, CTh1C.2 Penn State & ORC 1-3 Clemson Univ. Virginia Tech.

7 Chemical Deposition High Pressure Chemical Fluid Deposition Pressure –35MPa Precursor –SiH 4 +Hydrogen Temperature –low (<400 o C) for a-Si –high (>500 o C) for p-Si

Silicon Optical Fibers 8 1.N. Healy et al., Opt. Express 17, (2009) 2.N. Healy et al., Opt. Express 19, (2011) 3.J. R. Sparks et al., JLT 29, 2005 (2011) 4.N. Healy et al., Opt. Express 18, 7596 (2010) 12 34

Hydrogenated Amorphous Silicon High nonlinear refractive index n~3.6 Bandgap ~1.7eV Transparent from ~800nm-6  m Hydrogen can passivate dangling bonds for optical low loss Lowest loss to date at –0.8 dB/cm (1.55  m) –0.7 dB/cm (2.8  m) 9 D=5.7  m

10 Nonlinear Propagation in Silicon

11 Nonlinear Propagation in Si Fibers

12 Nonlinear Loss 1.P. Mehta et al., Opt. Express 16, (2010). Hyperbolic secant input

Self-Phase Modulation 13 1.P. Mehta et al., Opt. Express16, (2010) 2.A. C. Peacock et al., Opt. Lett. 37, 3351 (2012)

High power pump induces an absorption dip on weak probe One photon from the pump and one photon from the probe –Total energy must be greater than the bandgap E g Makes use of the imaginary component of the third order nonlinearity Im[  (3) ] – ultrafast! 1 –All-optical modulation –Wavelength conversion 14 Modulation via TPA 1.D. J. Moss et al., Electron. Lett. 41, 2005

15 Modulation via TPA Simplified pump-probe equations High power pump I 1 at 1 Weak probe A 2 at 2

16 Modulation via TPA 16 Degenerate pump-probe technique 1.P. Mehta et al., Opt. Express, vol. 19, (2011).

17 Modulation via TPA Non-degenerate pump-probe technique Highly Nonlinear Fibre (HNLF) Bandwidth Variable Tuneable Filter (BVF)

18 Modulation via TPA Non-degenerate pump-probe technique

19 Cross-Absorption Modulation Extinction:  ~ 3 dB Pulse width ~ 1 ps  ~87ns Pump Probe

20 Cross-Phase Modulation 1.R. Dekker et al., Opt. Express 14, 8336, E. Tien et al., Appl. Phys. Lett. 95, , H. Hsieh et al., Opt. Express 18, 9613, 2010 See next presentation: IF5B.4 Real part of the third order nonlinearity Re[  (3) ] High power pump induces a phase shift on a weak probe due to intensity dependent refractive index change –Optical switching 1 –Gating 2 –Regeneration 3

21 Towards smaller core fibers - Nonlinear Absorption 1.7µm core diameter A eff = 1.24  m 2 L = 6mm

22 Towards smaller core fibers -Self-Phase Modulation 1.7µm core diameter A eff = 1.24  m 2 L = 6mm

Combine large nonlinearity with reduced  for low power high-speed devices Core Size Comparison Core sizes: 1.7  m (green), 5.7  m (red) 23 Nonlinear parameter:

Tapered Silicon Core Fibers 24 BIT communications fusion splicer Arc current in the range: 8-15 mA Duration: 5.5 s – heat silicon above melting point 1410 o C Pull distance selected for desired ratio

Tailor Waveguide Parameters 25 Decreasing dispersion is formally equivalent to dispersion and gain –Pulse shaping Applications of tapered fibers –Short pulse generation –Phase matched four-wave mixing –Supercontinuum generation

26 Dispersion and Nonlinearity Tailoring =1.55  m NormalAnomalous High core/cladding index contrast allows for tailoring of the waveguide dispersion Normal dispersion regime ₋decreasing dispersion ₋increasing nonlinearity ₋parabolic pulses? Anomalous dispersion regime ₋decreasing dispersion ₋decreasing nonlinearity ₋soliton solutions?

Normal Dispersion - Parabolic Pulse Shaping Silicon Taper ₋L = 2mm ₋D in = 2.5  m ₋D out = 1  m Input Pulse ₋Gaussian ₋T in = 200fs ₋P 0 = 200W 27 1.A. Peacock, N. Healy, “Parabolic pulse generation in tapered silicon fibers,” Opt. Lett., vol. 35, 1780 (2010). Self-similar solutions for high power pulse propagation –strict linear chirp

28 Anomalous Dispersion - Soliton Propagation Silicon Taper ₋L = 10mm ₋D in = 640nm ₋D out = 850nm Input Pulse ₋N = 1 ₋T in = 170fs ₋P 0 = 1W 1.A. Peacock “Soliton propagation in tapered silicon core fibers,” Opt. Lett., vol. 35, 3697, Compensate for loss induced broadening of fundamental soliton

Conclusions Demonstrated the smallest core silicon fibers –losses comparable with on-chip technologies First nonlinear characterization of silicon fibers –demonstrate device functionality Moving towards nanoscale waveguides –low power operation and faster device speeds Exploit fiber geometry for novel nonlinear functionality –tapered fibers 29

Thank You Acknowledgments EPSRC (EP/G051755/1 and EP/J004863/1) Royal Academy of Engineering NSF (DMR and DMR )