Nonlinear optics in silicon: overview and future developments

Silicon PhotonicsSilicon-based micro and nanophotonic devices

http://silicon-photonics.ief.u-psud.fr/

Laurent Vivien

Pedro Damas, Xavier Le Roux, Eric Cassan, Delphine Marris-Morini,

Nicolas Izard, Alain Bosseboeuf, Thomas Maroutian, Philippe Lecoeur

Institut d’Electronique Fondamentale, CNRS UMR 8622,

Université Paris Sud, 91405 Orsay Cedex, France


Nonlinear optics in silicon:overview and future developments


Outline

� Motivation

� Nonlinear optics in silicon

� χ(3) in silicon� χ(2) in silicon

� Strained silicon photonics

� Conclusions


Global internet traffic

3

Source: L.Oxenlowe, Denmark Courtesy: D. Moss, CUDOS, Australia


Advanced video technology

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3D displays High Dynamic range

Future: Ultra high definition (4320p, 30bpp, 60Hz) needs 60 Gbps !


Power consumption

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Source: Shu Namiki, Japan Courtesy: D. Moss, CUDOS, Australia


Data centers

6

Source: L.Oxenlowe, Denmark Courtesy: D. Moss, CUDOS, Australia

Development of silicon photonics


Silicon photonic building blocks

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Tunable III-V laser on Si

Laser Modulator Detector

Emitter Receiver

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Po

we

r (

dB

m)

Wave length (n m)

40Gbt/s Silicon modulator

40Gbit/s Germanium photodetector


Photonic – Electronic circuits

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Silicon

PhotonicsCMOS Analog

& Digital Circuits+

=

Laser Beam splitter

detector

modulator

Electronic Photonic

Integrated circuit

Sourc

e: L

uxte

ra

Main challenges:

� Faster� Driving voltage of modulator� Power consumption

And also� Integration cost � Packaging

All-optical signal processing and low power consumption

EO modulators based on nonlinear properties in silicon


Why silicon waveguide?

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� High refractive index contrast between SiO2 (n~1.5) and Si (n~3.5)

� Strong light confinement of the guided mode (<0.1µm²)

� Si transparency in the near IR

� Lossless light propagation

� Dispersion can be engineered

400 nm

220 nm

SiO2

Si


Nonlinear optics in silicon

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Nonlinear Polarization:

√ Pockels effect:

�Linear electro-optic effect

√ Wavelength conversion

� Second Harmonic Generation (SHG)

√ Kerr effect:

�Nonlinear electro-optic effect


�Four wave mixing (FWM)

>>


Main drawback

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� Two photon absorption process leading to free carrier absorption� Excess loss at high power reducing nonlinear efficiency

Silicon FOM= 0.3 in telecom band

FOM >> 1 to be efficient in nonlinear regime

Courtesy: D. Moss, CUDOS, Australia


FWM amplification in NIR

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First observation of broadband gain in Si

Foster et al., Nature 441, 960 (2006).

To overcome this limitation:

� Sweep the photogenerated carriers out

� Reverse bias pin diode

�Mode confinement engineering

� Slot waveguides

�Mid-IR photonics

� Wavelength larger than 2.2µm

�Silicon nitride (Si3N4) photonics

�Wider gap energy (TPA negligible)


Towards Mid-IR wavelength range

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Silicon nitride photonics

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Outline

� Motivation




� Conclusions


Nonlinear optics in silicon

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Nonlinear Polarization:

√ Pockels effect:

�Linear electro-optic effect


� Second Harmonic Generation (SHG)

Break the symmetry of silicon crystal

Strained silicon

photonics

Without straining layer

With straining layer


χ(2) demonstration: State of the art

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� No EO effect

� Ridge waveguides did not work

� Comp. strain:

√

� Tensile strain:

√

� Comp. strain:

√

� Huge enhancement of EO effect

� Tensile strain:√

� SiO2 layer:

√ 0.3 GPa

√ 1.2 – 1.5 um

√ Compressive or tensile strain

*All χ2 units are in pm/V

� SiN layer:

√ 1.0 GPa

√ 0.75 um

√ Compressive strain

Jacobsen et al. Nature 441, 199-202 (11 May 2006)


χ(2) demonstration: State of the art

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� No EO effect

� Ridge waveguides did not work

� Comp. strain:

√

� Tensile strain:

√

� Comp. strain:

√

� Huge enhancement of EO effect

� Tensile strain:√

� SiO2 layer:

√ 0.3 GPa

√ 1.2 – 1.5 um

√ Compressive or tensile strain

*All χ2 units are in pm/V

� SiN layer:

√ 1.0 GPa

√ 0.75 um

√ Compressive strain

Jacobsen et al. Nature 441, 199-202 (11 May 2006)


Mach-Zehnder modulator based on

Pockels effect

� Pockel’s effect:

√

√

� Waveguide width effects:

� Conditions to achieve strain:

√ Bottom of waveguide fixed and top is strained

√ SiN induces strain all around the waveguide

√ Thermal annealing

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Waveguide width (nm)

χ2 (pm/V)

450 71.5

400 122

Bartos Chmielak et al. (2011) Opt. Express


SHG in silicon

� Several SiN techniques:

√ Low pressure CVD

�150nm

�780ºC

�1.2 GPa, tensile

√ Parallel-Plate PECVD

�500 nm

�300ºC, 308KHz

�500 MPa, compressive

√ Two-frequency PECVD

�500 nm

�13.56 MHz 60s – 308 KHz 10s

�60 MPa, compressive

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M. Cazzanelli et al., Nature Materials, 2011


SHG in silicon

� Second Harmonic Generation

� Pumping pulsed Lasers:

√ 4 ns

√ 100 fs

� Efficiency:

� Nonlinearity depends on:

√ Intensity of strain

√ Inhomogeneity of strain

√ Distribution of strain vs optical mode

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What is the relationship between strain and

2nd order nonlinear coefficient?


Outline

� Motivation




� Conclusions


Linear covalent crystal

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� Centro-symmetric 1D covalent crystal: χ2 =0

� Non Centro-symmetric 1D covalent crystal: χ2�0

√ Ionic character of the bonds

√ Result: The higher ionic character, the higher χ2

� Homogeneous strain:

√ Centro-symmetry is not broken!

� Inhomogeneous strain:

√ Centro-symmetry is broken

√ Electrons do not “see” the same potential in both directions

� Creation of some ionic character of each bond


Method for comparing structures

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Strain

� Simulations using ANSYS software

√ Calculation of strain maps

� Initial conditions:

√ Initial stress of the straining layer:

√ Obtained after fabrication conditions

�Reported in the literature

�Fabrication studies

Optical Mode

� Calculated using Mode-solver software

� Mode profile

√ Approximated by a 2D gaussiansurface

� Gaussian parameters imported to ANSYS

√ Overlaps are performed

� Structures Comparison

√ Looped in ANSYS

√ Extraction of the effective value


Simulations

� Nonlinear effects:

√ Overlap between optical mode and χ2 distributions

√ Overlap between optical mode and strain gradient distributions

Optical mode YY Strain Map

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Strain gradients

� StrainYY map:

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Optical mode and overlap

� Optical mode

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Final effective value

� … now we integrate over all the path

Final effective number:

Sum of the contributions of every path:

The final will be used to

compare different structures

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Final effective value of the considered path


Symmetry considerations

� In a symmetric structure

√ Strain and profiles are symmetric in x direction

√ Strain gradient profile is antisymmetric in x direction

�Symmetric but opposite signs

√ The gradients in X direction do not contribute to χ2

� In X symmetric waveguides, only the Y direction

gradients contribute to χ2

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Thus


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T (

db

)

λ (nm)

First experimental tests: Mzi modulator

� Technique PECVD

√ Expected 1GPa stress

� Assymetric MZI

√ Different arm lengths

� Transmission Spectra

� Very clear transmission

spectra from the MZI

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T (

db

)λ (nm)

0V

25V


Conclusion:

Most important results so far…

� Χ2 depends on the

gradients of strain

√ Every component of

strain

√ Gradients in every

direction

� Homogeneous strain

√ Does not contribute to χ2

generation

� Waveguide symmetries

in one direction

√ No contribution of

strain gradients in that direction

√ Symmetric waveguides have less contributions

to χ2 generation

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� Look for structures with…

√ Higher Gradients of strain

√ Higher structural asymmetries

http://silicon-photonics.ief.u-psud.fr/ 32

Acknowledgements:

Funding and collaborations

National Research Agency

SILVER, MICROS, GOSPEL, MASSTOR, ULTIMATE, Ca-Re-Lase, POSISLOT

Plat4M

photonic libraries and technology for manufacturing

SASER

Safe and Secure European Routing

HELIOS

Photonics Electronics functional integration on CMOS

Silicon photonics group

Documents

Nonlinear optics in silicon: overview and future developments