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Silicon nanophotonics: a new twist to silicon photonics
Lorenzo Pavesi
University of Trento
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TRENTO
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Last wednesday
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Silicon nanophotonics: a new twist to silicon photonics
Lorenzo Pavesi
University of Trento
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outline
• Silicon photonics
• Silicon nanophotonics
– Chaos in self induced oscillations in sequence of microrings
– Whispering gallery mode resonators for biosensing applications.
• Conclusions
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Moore’s law
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29 January 1969
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vs. Silicon Photonics
Silicon Photonics
Silicon
CMOS
LD,PD, microrings, ….
Silicon photonics is the standardization of photonics
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Silicon photonics
Photonic devices produced within standard silicon factory and with
standard silicon processing
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Silicon photonics timeline
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Projected integration density
Silicon photonic NOC
InP PLC
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outline
• Silicon photonics
• Silicon nanophotonics
– Chaos in self induced oscillations in sequence of microrings
– Whispering gallery mode resonators for biosensing applications.
• Conclusions
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Silicon nanophotonics: a platform
where photon or electron confinement enables new functionalities in silicon photonics
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• Confine carriers on nanoscale dimensions – Length scale =
electron DeBroglie wavelength
• Confine photons on nanoscale dimensions – Length scale =
light wavelength
Nanophotonics
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NanoSilicon photonics
• Confine carriers on nanoscale dimensions
• Confine photons on nanoscale dimensions
10 μm
Linnros talk
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Silicon Nanophotonics
• Confine carriers on nanoscale dimensions
• Confine photons on nanoscale dimensions
10 μm
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Introduction
n2 Whispering gallery mode (WGM) resonator
• Light is confined near the surface by total internal reflection
n1
• Circular geometry cavity
Quality factor (Q)
• Quantifies the stored energy and the light-matter interaction in the cavity
• Inverse value of the resonator loss (1/Q)
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Single cavity: Linear regime
• Add-drop filter
Input Through
Drop
Resonance condition:
Enhancements factor (EF):
k
𝑚 𝜆𝑚 = 𝑂𝑃 𝑛𝑒𝑓𝑓
𝑄 =𝜆𝑚
𝐹𝑊𝐻𝑀
𝜆𝑚 𝜆𝑚+1
𝑂𝑃 = 2π𝑅
𝐸𝐹 ∝ 𝑄
𝑃𝑖𝑛𝑠
𝑃𝑖𝑛𝑠 = 𝐸𝐹2𝑃𝑖𝑛 𝐸𝐹2 ≈ 1000
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SOI optical microresonators
Looped photonic wire Lot of applications Compact and easily integrable
Add-Drop filter All pass filter
Racetrack
SiO2 top cladding
Buried Oxyde (BOX)
Silicon Core SiO2 top cladding
Buried Oxyde (BOX)
Silicon substrate
Silicon on insulator photonics
205nm
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Input Through
Cavity
k
mrr Op
mrr neffOp
Op 2Rneff
Ring optical path
λ
Single optical resonator: Add-Drop filter
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outline
• Silicon photonics
• Silicon nanophotonics
– Chaos in self induced oscillations in sequence of microrings
– Whispering gallery mode resonators for biosensing applications.
• Conclusions
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Equal cavities
…
1
1 4
1 8
mrr neff 2 R
L
Resonator
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SCISSOR
• Two spectral features:
– Resonance: depends on the optical path of the cavity
– Periodicity add the bragg band
– Bragg reflection: depends on the distance between the cavities
… 1 8
L
m bb neff 2 L
mrr neff 2 R
Side-Coupled Integrated Spaced Sequence of Resonators
R
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SCISSOR: Bragg band
• Coherent reflections from each resonator
m bb neff 2 L
… 1 8 R
L L L L
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SCISSOR: spectrum
… 1 8
L R r b
r
b
λ
m bb neff 2 L
mrr neff 2 R
Bragg
Resonator
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Through port transmission spectra
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The topic
• Study of the behaviour of microresonators under input power variations
• Linear to non-linear transition study
Input power
Output power
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Sub-bandgap absorption mechanisms
Linear Absorption
Two Photon Absorption (TPA)
Free Carrier Absorption (FCA)
• 3th telecom window (centered at 1550 nm) • Silicon bandagap 1.1 eV • Photons energy 0.8 eV
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Non linear index variations
• TOE: Thermo-optic effect
• FCD: Free carrier dispersion
• Kerr: Charge polarization
FCD 38.5%
Kerr 0.2%
Phenomena relation in our system
TOE 61.3%
𝑑𝑛
𝑑𝑇> 0
𝑑𝑛
𝑑𝑁< 0
𝑑𝑛
𝑑𝑇>
𝑑𝑛
𝑑𝑁
Red shift
Blue shift
Thermal is predominant
TOE
FCD
τTOE≈ ns
τFCA≈ ps
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Non linear regime: self lock
• High power density (TW/cm2) : non linear regime
𝑛𝑡𝑜𝑡 = 𝑛0 𝜆𝑚 ∝ 𝑛𝑡𝑜𝑡
𝜆𝑝
Resonance Self-Lock 𝑡 = 𝜏𝑇𝑂𝐸
𝜆𝑚 𝜆𝑝
𝑡 = 0
+Δ𝑛𝑇𝑂𝐸 𝑃
TOE
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Simplified view 𝑑𝑛
𝑑𝑇> 0
𝑑𝑛
𝑑𝑁< 0
𝑑𝑛
𝑑𝑇>
𝑑𝑛
𝑑𝑁 Red shift
Blu shift
Thermal is predominant TOE
FCD
λ
TOE FCD
τTOE≈ ns
τFCA≈ ps
𝑛𝑡𝑜𝑡 = 𝑛0 + Δ𝑛𝑇𝑂𝐸 𝑃 −𝛥𝑛𝐹𝐶𝐷 𝑃
Function of power
𝜆𝑝
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Dynamics of single cavity λ0 Pump
• Free carrier • Thermal • Total detuning
Tot. detuning
λp λ0
Posi
tive
N
egat
ive
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Drop temporal behaviour
CW Input Through
Drop
300 400 500 600 700 800 900
0.5
1.0
1.5
Dro
p Inte
nsity (
Arb
. U
.)
Time (ns)
𝑇
• The period T depends on the
– input CW power
– CW laser wavelength
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• R = 7 μm • Gap = 180 nm
100 200 300 400
0.12
0.14
0.16
0.18
0.20
Dro
p I
nte
nsity (
Arb
. U
.)
Time (nm)
Temporal measurements
300 400 500 600 700 800 900
0.5
1.0
1.5
Dro
p Inte
nsity (
Arb
. U
.)
Time (ns)
Simulation
Experiment
Thomas J. Johnson, Matthew
Borselli and Oskar Painter, Optics Express, Vol. 14, No.2
(2006)
104 ns
CW Input Through
Drop
R
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Summary: Non linear single cavity
• Periodic interplay between FCD and TOE
• CW to oscillating behaviour
• Tunable oscillating period
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• Single vs sequence
300 400 500 600 700 800 900
0.5
1.0
1.5
Dro
p Inte
nsity (
Arb
. U
.)
Time (ns)
… ? SCISSOR
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Modeling the SCISSOR in time Building the SCISSOR model:
Ring i Ring i+1 Ring i+-1 Ring 1 Ring N
FCD
TOE
𝜏𝑖 𝜅 𝜅 𝑃𝑖𝑛𝑐
𝑑𝑎𝑖(𝑡)
𝑑𝑡=
𝑗 𝜔𝑝 − 𝜔0𝑖 +𝑗
𝜏𝑖(𝑡)𝑎𝑖(𝑡)
𝑗 𝜔𝑝 − 𝜔0𝑖 1 + Δ𝜔𝑇𝑂𝐸 𝑡 + Δ𝜔𝐹𝐶𝐷 𝑡 +
𝑗
𝜏𝑖(𝑡)𝑎𝑖(𝑡)
𝑗 𝜔𝑝 − 𝜔0𝑖 1 + Δ𝜔𝑇𝑂𝐸 𝑡 + Δ𝜔𝐹𝐶𝐷 𝑡 +
𝑗
𝜏𝑖 𝑡𝑎𝑖 𝑡 + 𝑗𝜅 𝑎𝑖−1 𝑡 + 𝑎𝑖+1 𝑡 + 𝛽(𝒂𝒊≠𝒊,𝒊+𝟏,𝒊−𝟏, 𝑃𝑖𝑛𝑐)
Start from the single damped linear oscillator, forced by a laser at frequency 𝜔𝑝
Introduce the resonance frequency shifts (FCD and TOE)
Introduce coupling with rings and input force
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Random number generation
• Chaotic optical signals are particularly suited for generating random bit sequences.
• Random bit sequences find applications in many fields • Code security • Monte Carlo simulations • Cryptography • Stochastic Evolutionary Algorithms
Reason of success:
• Sub-nanosecond time scale variations (high bitrates)
• Extremely fast decorrelation times (High unpredictability)
• Wide post processing possibilities with easily implementing algorithms and hardware
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NIST* standard test of randomness Test type 1Gbps Chaotic signal Noise signal Quasi-periodic signal
Frequency
Block Frequency
Cumulative Sums
Runs
Longest runs of ones
Rank
FFT
Non overlapping template
Overlapping template
Random excursion
Random excursion variant
Universal test
Approximate entropy test
Serial test
Linear complexity test
Passed Not passed *National Institute of Standards and Technology
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Performances State of art: 300 Gbps random bit generation*
*Kanter, Ido, et al. "An optical ultrafast random bit generator." Nature Photonics4.1 (2009): 58-61.
But...
• No CMOS compatibility
• No easy cascadability
• Large footprint (cm size)
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outline
• Silicon photonics
• Silicon nanophotonics
– Chaos in self induced oscillations in sequence of microrings
– Whispering gallery mode resonators for biosensing applications.
• Conclusions
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An all-integrated device
Principal coupling schemes
In-plane coupling
resonator wg
Vertical (bus) coupling
resonator wg
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An all-integrated device
Principal coupling schemes
In-plane coupling
resonator wg
1. Requires reduced (~100nm) coupling-gap
2. Gap defined through E-beam or deep-UV Lithography
3. A 1-mask process imposes equal waveguide and resonator thicknesses, because of a single deposition
4. A 1-mask process imposes the same material for both the waveguide and the resonator
Vertical (bus) coupling
resonator wg
1. Requires mach larger (> 800nm) coupling-gap
2. Coupling gap defined and controlled through deposition, use of conventional optical Lithography
3. A 2-mask process allows for independent waveguide and resonator thicknesses, multiple depositions
4. A 2-mask process allows for use of different materials for the waveguide and the resonator
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An all-integrated device with vertical coupling
A generic process flow
cladding
substrate
strip waveguide
upper cladding
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An all-integrated device with vertical coupling
A generic process flow
Top view
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The device (waveguide & resonator)
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The free-standing device
Sacrificial layer
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Free standing structures
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Mode selection
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Depending on the gap (NIR or VIS)
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Wedge vs. Sharp edges
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Application: biosensing
Primary recognition molecule
Body fluid with a target molecule
Specific binding
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Silicon photonics biosensor
Symphony: aflatoxin in diary products
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Conclusions
• nanoSilicon photonics
• A lot of new physics can be found in an already mature research field such as Silicon Photonics
• Silicon photonics is not only data com… is much more!
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Acknowledgments
SIMPHONY
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acknowledgments
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