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Silicon nanophotonics: a new twist to silicon photonics

Lorenzo Pavesi

University of Trento


TRENTO


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Silicon nanophotonics: a new twist to silicon photonics

Lorenzo Pavesi

University of Trento


outline

• Silicon photonics

• Silicon nanophotonics

– Chaos in self induced oscillations in sequence of microrings

– Whispering gallery mode resonators for biosensing applications.

• Conclusions


Moore’s law


29 January 1969




vs. Silicon Photonics

Silicon Photonics

Silicon

CMOS

LD,PD, microrings, ….

Silicon photonics is the standardization of photonics


Silicon photonics

Photonic devices produced within standard silicon factory and with

standard silicon processing


Silicon photonics timeline


Projected integration density

Silicon photonic NOC

InP PLC


outline





• Conclusions


Silicon nanophotonics: a platform

where photon or electron confinement enables new functionalities in silicon photonics


• Confine carriers on nanoscale dimensions – Length scale =

electron DeBroglie wavelength

• Confine photons on nanoscale dimensions – Length scale =

light wavelength

Nanophotonics


NanoSilicon photonics

• Confine carriers on nanoscale dimensions

• Confine photons on nanoscale dimensions

10 μm

Linnros talk


Silicon Nanophotonics

• Confine carriers on nanoscale dimensions

• Confine photons on nanoscale dimensions

10 μm


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)


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


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


Input Through

Cavity

k

mrr Op

mrr neffOp

Op 2Rneff

Ring optical path

λ

Single optical resonator: Add-Drop filter


outline





• Conclusions


Equal cavities

…

1

1 4

1 8

mrr neff 2 R

L

Resonator


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


SCISSOR: Bragg band

• Coherent reflections from each resonator

m bb neff 2 L

… 1 8 R

L L L L


SCISSOR: spectrum

… 1 8

L R r b

r

b

λ

m bb neff 2 L

mrr neff 2 R

Bragg

Resonator


Through port transmission spectra


The topic

• Study of the behaviour of microresonators under input power variations

• Linear to non-linear transition study

Input power

Output power


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


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


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


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


Dynamics of single cavity λ0 Pump

• Free carrier • Thermal • Total detuning

Tot. detuning

λp λ0

Posi

tive

N

egat

ive


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


• 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


Summary: Non linear single cavity

• Periodic interplay between FCD and TOE

• CW to oscillating behaviour

• Tunable oscillating period


• 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


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


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


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


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)


outline





• Conclusions




An all-integrated device

Principal coupling schemes

In-plane coupling

resonator wg

Vertical (bus) coupling

resonator wg


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


An all-integrated device with vertical coupling

A generic process flow

cladding

substrate

strip waveguide

upper cladding


An all-integrated device with vertical coupling

A generic process flow

Top view


The device (waveguide & resonator)


The free-standing device

Sacrificial layer


Free standing structures


Mode selection


Depending on the gap (NIR or VIS)


Wedge vs. Sharp edges


Application: biosensing

Primary recognition molecule

Body fluid with a target molecule

Specific binding







Silicon photonics biosensor

Symphony: aflatoxin in diary products


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!


Acknowledgments

SIMPHONY


acknowledgments


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Silicon nanophotonics: a new twist to silicon photonics/Menu/general/column... · Silicon nanophotonics: a new twist to silicon photonics Lorenzo Pavesi University of Trento . NanoScience