1. Four-Wave-Mixing Gain and All-optical Signal Processing in Silicon Nanowires
B. Kuyken 1,2, X. Liu3, S. Clemmen4, S. Selvaraja 1,2, W. Bogaerts1,2, D. Van Thourhout1,2, R. M. Osgood, Jr. 3, P. Emplit4 , S. Massar4, Y. A. Vlasov5, W. M. Green5, G. Roelkens1,2, R. Baets1,2
1Photonics Research Group, Ghent University, Ghent, Belgium 2Center for Nano- and Biophotonics, Ghent, Belgium 3Electrical Engineering Group, Columbia University, NY 4Laboratoire d’Information Quantique (LIQ), Université Libre de Bruxelles ,Belgium 5IBM Thomas J. Watson Research Center, Yorktown Heights, NY

2. Silicon as a nonlinear material
Chip-scale nonlinear optical applications using four-wave-mixing (FWM):
Ultra-fast digital systems: Tbps all-optical signal processing
Wavelength-provisioned networks: Wavelength conversion
On-chip light sources: OPOs, supercontinuum generation
Quantum computation: Entangled photon-pair generation
High confinement in Si nanophotonic waveguides enhances effective nonlinearity by :
~ 105 compared to single-mode fiber (SMF)
~ 104 compared to highly nonlinear fiber (HNLF)
~10 m
HNLF fiber
~1 mm
silicon waveguide

3. Effective nonlinearity in waveguide
Nonlinear index n2 (Optical Kerr Effect) n = n0 +n2I
Nonlinear absorption in material = 0 + βI
Refractive index change causes phase change in a waveguide
Nonlinear material needs a good figure of merit (FOM)

4. Downside of silicon: Two photon absorption
Fundamental two-photon absorption (TPA):
Limits effective pump power
Nonlinear efficiency is low in telecom band
Intrinsic
Two telecom
band photons
Silicon
bandgap
Eg = 1.12 eV

5. Comparison with other materials (@ 1550 nm)
(10-20 m2/W)
FOM (n2/λβ)
Typical Nonlinear Parameter of waveguide (1/Wm)
Crystalline Silicon
650
0.6
450
Amorphous Silicon
1300
2.1
770
Silicon Organic Hybrids: DDMEBT
1700
Slotted (1)
2.2
100
Strip (TM) (2)
1.2
108
Chalcogenide
As2S3 (3)
290
>10 10
As2Se3 (4)
1200 2
160
Ge11.5As24Se64.5(5)
860 60
150
Silicon Nitride (6) 24 -
1.5
Silica
2.7
<<1
Comparison with other materials 1550 nm)
1)C. Koos, et al. “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides”, Nature Photonics 3 (2009)
2)T. Vallaitis et al, “Optical properties of highly nonlinear silicon-organic hybrid (SOH) waveguide geometries”,Opt. Express 17(2009)
3) M. Lamont et al, "Supercontinuum generation in dispersion engineered highly nonlinear As2S3 chalcogenide planar waveguide, Opt. Express 16(2008)
4) V. Ta'eed et al, "Ultrafast all-optical chalcogenide glass photonic circuits," Opt. Express 15 (2007)
5) X. Gai, “Chalcogenide nanowire waveguides with a nonlinear parameter 150,000 /Wm”, CLEO (2010)
6) D. T. H. Tan, “Group velocity dispersion and self phase modulation in silicon nitride waveguides”, Appl. Phys. Lett. 96,(2010)

6. How can we overcome two photon absorption?
Use other CMOS compatible materials with less TPA Use longer wavelengths (2.2um)
a-Si:H

7. How can we overcome two photon absorption?
Use other CMOS compatible materials with less TPA Use longer wavelengths (2.2um)
a-Si:H

8. Hydrogenated amorphous silicon as a nonlinear material
Linear/nonlinear properties of a-Si:H waveguides
Nonlinear signal processing: amplification and sampling
Material degradation
a-Si:H

9. CMOS compatible a-Si:H platform @ imec
Confinement: TE mode
n~3.6
220 nm
a-Si:H
500 nm
Q>10000
Loss: dB/cm
Device layer deposited by PECVD
S. Selvaraja et al., ”Low-Loss Amorphous Silicon-On-Insulator Technology for Photonic Integrated Circuitry”,Optics Communications (2009)

10. Nonlinear properties of a-Si:H waveguides
What happens to short pulses travelling through these waveguides?

11. Results for a 1.1 cm waveguide
=770/Wm
=-28/Wm
FOM>2
(c-Si<~0.5)
B. Kuyken, S. Clemmen, S. Selvaraja, W. Bogaerts, S. Massar, R. Baets, G. Roelkens, Self phase modulation in highly nonlinear hydrogenated amorphous silicon,Photonics Society Annual Meeting, United States, (2010)

12. Comparison with other materials (@ 1550 nm)
(10-20 m2/W)
FOM (n2/λβ)
Typical Nonlinear Parameter of waveguide (1/Wm)
Crystalline Silicon
650
0.6
450
Amorphous Silicon
1300
2.1
770
Silicon Organic Hybrids: DDMEBT
1700
Slotted (1)
2.2
100
Strip (TM) (2)
1.2
108
Chalcogenide
As2S3 (3)
290
>10 10
As2Se3 (4)
1200 2
160
Ge11.5As24Se64.5(5)
860 60
150
Silicon Nitride (6) 24 -
1.5
Silica
2.7
<<1
Comparison with other materials 1550 nm)
1)C. Koos, et al. “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides”, Nature Photonics 3 (2009)
2)T. Vallaitis et al, “Optical properties of highly nonlinear silicon-organic hybrid (SOH) waveguide geometries”,Opt. Express 17(2009)
3) M. Lamont et al, "Supercontinuum generation in dispersion engineered highly nonlinear As2S3 chalcogenide planar waveguide, Opt. Express 16(2008)
4) V. Ta'eed et al, "Ultrafast all-optical chalcogenide glass photonic circuits," Opt. Express 15 (2007)
5) X. Gai, “Chalcogenide nanowire waveguides with a nonlinear parameter 150,000 /Wm”, CLEO (2010)
6) D. T. H. Tan, “Group velocity dispersion and self phase modulation in silicon nitride waveguides”, Appl. Phys. Lett. 96,(2010)

13. Hydrogenated amorphous silicon as a nonlinear material
Linear/nonlinear properties of a-Si:H waveguides
Nonlinear signal processing: amplification and sampling
Material degradation
a-Si:H

14. Phasematching in high index contrast waveguides
Small correction for SPM/XPM
Figure: MA Foster et al., “Broad-band optical parametric gain on a silicon photonic chip”, Nature, 2006

15. Dispersion engineeringβ ω
Δω1
Δω1
500 nm
Air
a-Si
220 nm
SiO2

16. Broadband amplification in 1.1 cm a-Si:H waveguide
Pump probe Experiment:
Pump
n~3.6
220 nm
a-Si:H
Probe
500 nm
Looking at synchronized and not synchronized pulses reveals on/off gain

17. Broadband amplification in 1.1 cm a-Si:H waveguide
Gain/conversion as a function of wavelength at peak power of 5.2W
Max On/off gain dB
Max On chip gain dB
c-Si: on/off gain 4.2 dB
on chip gain 1.8 dB
B. Kuyken, et al., “On-chip parametric amplification with 26.5dB gain at telecommunication wavelengths using CMOS-compatible hydrogenated amorphous silicon waveguides”,Optics Letters (2011)

18. 320 Gbit/s sampling of data in 4 mm a-Si:H waveguide
Succesfull sampling with conversion efficiency of +12 dB!
(c-Si -7.5dB)
H. Ji et al., “Optical Waveform Sampling of a 320 Gbit/s Serial Data Signal using a Hydrogenated Amorphous Silicon Waveguide” ECOC (2011)

19. Hydrogenated amorphous silicon as a nonlinear material
Linear/nonlinear properties of a-Si:H waveguides
Nonlinear signal processing: amplification and sampling
Material degradation
a-Si:H

20. Modulation instability in a-Si:H waveguides
Impossible to observe in c-Si
FWM amplification of background noise

21. Degradation of material: Modulation instability vs Time
Coupled Peak power 5.2 W, 4 ps, 10 MHz
Material is photosensitive!!!
How is this possible?

22. Staebler-Wronski effect in amorphous silicon solar cells
1976: first cells developed
1977: Staebler-Wronski effect: dark conductivity decreases after illumination
Staebler, Wronski, Appl. Phys. Lett. 31 (1977)

23. Degradation is caused by recombination of electron-hole pairs in amorphous silicon solar cells
Recombination energy is enough to break weak Si-Si bonds
Leads to the creation of dangling bonds
Efficiency a-Si solar cells decreases over time
Heating can bring efficiency to its initial level

24. Staebler-Wronski in a-Si:H waveguides?
Electron-Hole pairs created by residual TPA
Heating chip for 30 min at 200 °C “resets” the material
without penalty
a-Si:H
Original
Annealed: 200˚C/30min

25. How can we overcome two photon absorption?
Use other CMOS compatible materials with less TPA Use longer wavelengths (2.2um)
a-Si:H

26. Reducing the TPA by using longer wavelengths
Amplification of mid infrared and telecom signals
Silicon as broadband gain medium for long wavelength sources

27. Long wavelength to overcome TPA
Simple idea: Use a pump at long wavelengths where TPA is absent
Puts a lot of stress on the dispersion engineering of waveguides
Dispersion engineering ?
Small correction for SPM/XPM

28. Dispersion engineeringβ ω
Δω1
Δω1

29. Phasematching close to pump in 2 cm waveguide
Interesting for mid infrared applications
-Sensing/spectroscopy
-Free space communication..
-nonlinear processing in mid IR??
How to connect this with telecom?
-Use higher order dispersion terms
Pump: 2ps, Rep 78 MHz, Probe: low power CW
X. Liu, et al., Mid-infrared broadband modulation instability and 50dB Raman assisted parametric gain in silicon photonic wires,CLEO (2011)

30. Extensive dispersion engineeringβ ω
Δω1
Δω1
Δω2
Δω2

31. Phase-Matching Between Mid-IR and Telecom Band
Waveguide: 2 cm long Low loss: 2.5 dB/cm Pumped at 1946 nm: 2 ps pulse train, 76 MHz rep rate, Pp = 37 W b2 < 0 and b4 > 0
Waveguide engineered for TE polarized mode, anomalous beta2 between the two zero dispersion wavelengths of 1810 nm and 2410 nm, and has positive fourth order dispersion due to the positive curvature of the dispersion graph.
B. Kuyken, et al., “Frequency conversion of mid-infrared optical signals into the telecom band using nonlinear silicon nanophotonic wires”,OFC (2011)

32. Up-Conversion of Mid-IR Signals to Telecom Band
Up-conversion across > 800 nm
Parametric gain probed at 2440 nm
18.8 dB on-chip parametric signal gain
Transparency bandwidth ~ 150 nm
Mid-IR conversion to telecom L/U band
Simultaneous 19.5 dB gain
Transparency bandwidth ~ 45 nm
Important to note here that the parametric gain boosts even further the “effective detectivity” of this kind of device when used as a mid-IR detector.
CW tunable Cr2+:ZnSe laser
Ps < 0.15 mW

33. Down-Conversion of Telecom Signals to Mid-IR Band
Down-conversion across > 800 nm
Parametric gain probed at 1620 nm
8.4 dB on-chip parametric signal gain
Transparency bandwidth ~ 20 nm
First demonstration of telecom-band gain
with mid-IR pump
Telecom conversion to mid-IR band
Simultaneous 8.0 dB gain
Transparency bandwidth ~ 40 nm
Efficient generation of high-speed optical data with telecom components
Compared with the case of converting mid-IR signal to telecom, this result here shows a reduction about 11 dB in the magnitude of the gain. It is caused by the fact that the input telecom signal, unlike the mid-IR signal, suffers from non-degenerate two-photon absorption (cross-TPA) where silicon could absorb one signal photon and one pump photon at a wavelength of 1946 nm to reach its band-gap.
The cross-TPA coefficient is expected to be larger for the down-conversion pump-signal configuration than it is for the up-conversion configuration.
CW tunable diode laser
Ps < 0.15 mW
S. Zlatanovic et al., Nature Photonics 4, 561 (2010).
A. C. Turner-Foster et al., Opt. Express 18, 1904 (2010). 33

34. Reducing the TPA by using longer wavelengths
Amplification of mid infrared and telecom signals
Silicon as broadband gain medium for long wavelength sources

35. Telecom to Mid IR Supercontinuum Power in 2cm silicon waveguide
>1000 nm
Pump powers
Pump pulses: 12.7 W, 2 ps, 78 MHz Rep
B. Kuyken et al., “Generation of a telecom-to-mid-infrared spanning supercontinuum using silicon-on-insulator wire waveguides” CLEO (2011)

36. Building an Silicon based tunable light source: Silicon based optical parametric oscillator
Image: RP photonics

37. Building an Silicon based OPO
Optical Parametric Oscillator
Use robust fiber
Use silicon chip

38. Design of the silicon based OPO
Fiber length ~5m
Due to lack of WDM (DE)MUX beyond 2000 nm

39. Using dispersion in fiber to tune the OPO
Animate animate and stretches
12.8 ns

40. The OPO is a widely tunable source
Output spectra for different delays
Pulse Energy (pJ) at output of the chip
>70 nm
Make a back up graph on chip power
Mention that is on chip: inset figure gain spectrum of p 8 such and label this. In stead show spectra of pulse at different wavelengths
Offset gebruiken
In function of?
The fonts van de grafieken zijn niet gelijk. Vertikale as als titel
B. Kuyken, et al., “Widely Tunable Silicon Mid-Infrared Optical Parametric Oscillator”, in Group IV Photonics (2011)

41. Conclusion
Hydrogenated amorphous silicon waveguides have excellent linear/nonlinear properties Understanding degradation effect is ongoing work Using long wavelength pumps to process telecom signals is achieved by extreme dispersion engineering Nonlinear optics in c-si waveguides can be used to make new sources at long wavelengths.