Nonlinear effects in ultrasmall SilicononInsulator ring resonators

1
Nonlinear effects in ultrasmall
Silicon-on-Insulator ring resonators

• G. Priem, P. Dumon, W. Bogaerts, D. Van
  Thourhout, G. Morthier and R. Baets

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Outline

• Introduction ultrafast NL effects
• Fabrication and experiments in SOI rings
• Analysis of competing phenomena
• Conclusions

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Ultrafast nonlinear effects

• Much effort into ultrafast nonlinear effects
• ? all-optical signal processing
• Kerr effect
• Mostly theoretically (study of resonant
  enhancement, bistability, ...)
• Materials e.g. AlGaAs with
• Two-photon absorption
• Also experimentally
• 80 Gb/s modulation (92) in 10 mm SOI wire
  (NICT-UGent-IMEC)
• Materials e.g. Silicon with

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Issues

• Ultrafast nonlinear effects are typically small
• ? optical confinement (HC systems) !!
• Two-photon absorption leads to secondary effects
• Free carriers
• additional absorption
• free-carrier dispersion (also called plasma
  effect)
• Temperature change
• dispersion

strong, but slow
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Visual picture
FCA ?a
FCD ?r
conduction band
thermalization

• thermal dn/dT

2PA ?
valence band
valence band
surface recombination ?carr
Kerr effect n2
thermal relaxation ?th
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Visual picture
FCA ?a
FCD ?r
conduction band
thermalization
thermal dn/dT
2PA ?
valence band
surface recombination ?carr
valence band
Kerr effect n2
thermal relaxation ?th

• Which effects dominate the Silicon system?
• Magnitude of time constants?

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Fabrication SOI structures

• Good NL interaction requires strong confinement
• Transversal wires, PhC waveguides
• Longitudinal resonators (ring, PhC, DBR, ...)
• Our best results
• low-loss SOI ring resonators (R?3?m)
• Processing (IMEC-Leuven)
• photo resist deposition
• 248nm deep UV litho
• resist development
• reactive ion etching (RIE)

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Low-power measurements

• Example ring resonator (4?m radius)
• FSR of 21.25 nm
• BW of 6.03 GHz BW of 3.77 GHz
• Q 32000 Q 51000
• splitted resonance

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High-power CW measurements

• bistability at 0.27 mW (upper arm shown only)
• NL loss small compared to NL index effects
• clearly thermal effects dominate (?n gt 0)

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Absorption analysis

• Nonlinear loss (2PA and FCA) gives information
  about carrier concentration and lifetime.
• Good agreement with other groups
• Timeconstants carrier dependent 1-10 ns

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Dispersion analysis

• thermal effects indeed dominate (lifetime 65 ns)
• free carrier dispersion also important
• linear (thermal) term required for good fitting,
  corresponding to linear loss of 0.22 dB/mm

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Comparison experiment - theory

• Good overall agreement is obtained.
• Detailed analysis shows that not all solutions
  are stable!

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Thermal-carrier pulsations

• Two opposite refractive index effects with
  different time constants may give rise to
  periodic pulsations.
• Due to noise quasi-periodic (visualization
  period on oscilloscope not possible)
• Measurement of standard deviation at 0.76 mW

sim.
exp.
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Oscilloscope example
?1
exp.
exp.
?2
?1
?2
sim.
sim.
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Pump-probe experiments

• FCD based switching (carrier lifetime of 1-10
  ns)
• Pump signal 10ns pulse, 10001000 pattern,
  peak 0.66mW, 5dB extinction ratio,
  on-resonance (1557 nm)
• Probe signal CW, off- and on-resonance (1536
  nm)

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Inverted and non-inverted ? conversion
on-resonance

• off-resonance

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Instabilities at higher pump power
on-resonance

• off-resonance

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Conclusions (1)

• Inside SOI resonators, two-photon absorption
  gives rise to secondary effects which are
  dominant.
• Thermal effects (100ns)
• Free-carrier effects (1-10ns)
• We demonstrated
• Thermal bistability with only 270 ?W
• FCD based inverted and non-inverted switching
• Carrier-thermal pulsations limit optical
  switching, but can be used for pulse generation.

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Conclusions (2)

• All results were confirmed by simulation.
• To obtain higher bitrates and avoid unstability,
  carrier extraction by means of a reverse biased
  p-i-n structure
• Shorter carrier lifetime (below 100 ps)
• No thermal effects ? no unstability
• Carrier bistability possible