cavity solitons : the semiconductor experimentalist's point of view
DESCRIPTION
Cavity Solitons : the semiconductor experimentalist's point of view. Robert Kuszelewicz. Laboratoire de Photonique et de Nanostructures LPN-CNRS/UPR20, Marcoussis, France. What is at stake ?. Cavity solitons have a double concern : Fundamental : - PowerPoint PPT PresentationTRANSCRIPT
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Cavity Solitons : the semiconductor experimentalist's
point of view
Robert KuszelewiczLaboratoire de Photonique et de Nanostructures
LPN-CNRS/UPR20, Marcoussis, France
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
What is at stake ?
Cavity solitons have a double concern :● Fundamental :
Phenomena, concepts and theoretical approaches of non linear pattern formation
● Applied :Original Functions, all-optical signal processing.
III-V Semiconductor materials are at the crossroad of two streams of interest.
• Strong intensity-dependent nonlinear optical properties near the band gap edge
• Integrability and ability to realise a large variety of optical devices with a complete crystal compatibility (compacity)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Purpose of the lecture
● Show how theoretical concepts pertaining to TNO can be implemented into physical systems
● Draw a short history of the most striking advances with various materials :
● Na-vapor, LCLV, III-V semiconductors
● Concentrate on semiconductor systems :
● State of the art
● Advantages, limitations, drawbacks
● Capabilities and expectations
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Outlines
Materials : • Na-vapour, LCLV and III-V semiconductors
Mechanisms for Transverse Nonlinear Optics:• Nonlinearity• Competition mechanisms• Transverse mechanisms
Phenomena : • From bistability to spatial pattern formation
... Transverse Nonlinear Optics
Description of various systems :• Passively injected systems • Amplifying injected systems • Laser systems : injected or saturable absorber
Limitations :• Uniformity : thickness, current, defects• Thermal effects : Production, dependance and dissipation
What to do with ?• Functions, processing
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Material systems and related optical models
Liquid Crystal Light Valves
Kerr-like dispersive medium
Sodium Vapor :
Two-level system : saturable dielectric function
III-V semiconductors
Electronic bands : Dynamical responseMany-body interactionsDielectric function
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
LCLV : the Pure Kerr-model
0 2n I n n I
Residori et al., J. Opt. B: Quantum Semiclass. Opt. 6 (2004) S169–S176
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
The two level system : optical response
'0
I
'
0I
● Positive or negative nonlinear dispersion
● Inversion of properties above transparency
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
The bulk semiconductor model
Parabolic model
+ Exciton
Many-body effects
+ Band filling
+ Coulomb interactionsElectron screening, Band gap renormalisationExchange interaction
Direct gap semiconductors
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Recombination mechanisms
Radiative
Stimulated
Spontaneous
Auger
phonon-assisted
Non radiative
Recombinations on defects
Electron-phonon
nrnr
nn
t
3
Auger
nCn
t
2
rad
nBn
t
2,
stim
nn E
t
Direct Indirect
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Dynamical regimes
Dynamical regimes : ultra short pulses.
Coherent transients : photons écho, superradiance, self-induced transparency.. Dynamical Stark Effect : : Dynamic nonlinearities. Polarization quasi-steady state vs Electric field. NL response ruled by the evolution of excited populations : intraband relaxation
: quasi-stationnary situation : adiabatic elimination of the carriers
Characteristic timescales Dephasing time
Time after which phase relation is lost between the polarisation and the exciting field
Populations lifetimeRelaxation time of excited populations (probabilities)
In general For III-V materials ,
Excitation duration pT
2 1pT T T
2 1 pT T T
1T
2T
2 1T T 2 200T fs 1 1 10T ns
2pT T
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Carrier dynamics
Excitation of optically coupled states
Intraband dynamics of carriers
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
The direct gap semiconductor dielectric function
The Carrier density-dependant susceptibility with and without many-body effects (Koch model)
Real and imaginary part are connected by the Kramers-Kronig transform :
-factor derives as so that Re nN NIm gN Nc
i
220
cn d
'0
I
'
0I
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
QD susceptibility
Energy
WL
TEd
γrad
QD γNR
QD
Energy
WL
TEd
γrad
QD γNR
QD
InAlAs/GaAlAs QD
Dynamic model of QD
Density ~ 1011 cm-3
Inhomogeneous broadening
Inhomogeneously broadened optical response
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Observations
The properties of the susceptibility in III-V semiconductor are multifactorial :
● High background index n~3.5,
● Strong NL in the vivinity of the band gap,
● In the passive case, the NL dispersion is defocusing
● Above transparency, focusing NL appear
● III-V semiconductor are highly temperature-sensitive
via the shift of the band gap
● Quantum dots seem quite a promising alternative for focusing NL
0gE
T
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Mechanisms of TNO
Transverse Nonlinear Optical phenomena require :
• Kerr-like or saturable intensity-dependent susceptibility
• Positive feedback : Fabry-Perot resonance or feedback mirrorenforces light-matter interaction duration yielding a “catastrophy”. Mechanism with threshold :
NL + feedback PW bistability
• Transverse effects : diffraction, diffusionCreate the conditions of non locality.
Mechanism with threshold : MI
The conjunction of these three mechanisms generate the spatio-temporal dynamics.
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Phenomena
From bistability to spatial pattern formation ... Transverse Nonlinear Optics
• PW bistability : thermal / electronic NL
• Switching waves
BISTABILITY + LARGE FRESNEL NUMBER
• Pattern formation
• Localised states
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Devices
Description of various classes :
• Passive injected systems : Na vapours and III-V semiconductors
• Injected amplifying systems : optically- and electrically- pumped VCSEL
• Laser systems : injected or saturable absorber systems
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Passive injected systems
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Na-vapor feedback mirror systems (Muenster univ.)
Drift in a gradient
Modulated landscape
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
LCLV in a feedback loop
Non local interactions
Patterns and localised states
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Semiconductor systems
Experimental system (LPN, PTB)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Switching waves
Iinc
0 I0 I
downI
up
Iref
Outward front
Inward front
Maxwell point
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Patterns and dressed solitons
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Injected semiconductor amplifiers
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Injected semiconductor amplifiers
Arguments on the interest for over transparency operation
● Positive : dispersive confinement
● Amplification : cascadability
Two similar approaches :
● Electrical pumping
INLN
● Optical pumping
LPN
Discussion on the respective interest of each
( )
N
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Electrical vs Optical pumpingPump
I
Advantages Disadvantages
Electrical
Optical
Pumping
- High current densities- Integration
- Joule heating- Technological steps- Spatial inhomogeneities at the borders
- Few technological steps- No Joule heating- Choosing the pump spatial profile
- External laser source- Coupling to the cavity- Thermal management due to the substrate (850 nm)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Theoretical model for field and carriers
[M. Brambilla, L. Lugiato, F. Prati, L. Spinelli, W. Firth, PRL 79, 2042 (1997)]
C : bistability parameter: pumpHenry factor
2
22 2
1 2 1
1
i
Ei E E i i N E i E
tN
N N N E D Nt
C
= 1 transparency = 1+1/2C laser threshold
Good compromise : C ≈ 0.5high finesse cavity
C large small excursion range in terms of
small
C small large excursion range in terms of
large
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Electrically injected VCSELs
Bottom emitting laser (ULM)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Patterns and localised states (INLN)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
The initial demonstration : independance of 2CS
Writing Erasure sequence of 2 CS (Barland et al., Nature 419, 699(2002)
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Interplay with non uniformity
80 m
The thickness gradient scans the state space through the detuning parameter
Seven CS
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Cavity design for optical pumping
[S. Barbay, Y. Ménesguen, I. Sagnes, R. Kuszelewicz, APL 86, 151119 (2005)]
Active layer
Absorbing spacers
Aperiodic back mirror Front mirror
Pump window
Cavity resonance
[Y. Ménesguen, R. Kuszelewicz, to appear IEEE-JQE 41,N°7 (2005)]
Optical pump fewer technological steps, less heating
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Experimental setup
Large-area semiconductor amplifier in AlGaAs/GaAs
Laboratoirede Photonique et de Nanostructures
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Results / pattern formation
890.98 nm 889.95 nm 889.27 nm
888.23 nm
888.62 nm
Decreasing
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Spontaneous formation of CS
Pump
120 m Increasing Pumping
Pump + injection 888.38nm
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Independance and multiplicity of CSs
Y. Ménesguen, S. Barbay, X. Hachair, L. Leroy, I. Sagnes and R. Kuszelewicz, submitted PRA (2006)
Independence of 2 CSW/E in the vicinity of 3 other CS
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Hysteresis
HB power
Loca
l re
flect
ed
in
ten
sity
Field Carriers
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Fast incoherent writing/erasure : 60ps pulses
CS off
CS on~2nsCS on
CS off
~5 ns
60 ps writing 60 ps writing pulsepulse
Repeated writing and reset
writing erasure
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Semiconductor Laser devices
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Laser systems
Why going above threshold ?
Cascadability
Diversification of the bistable mechanisms
Injection or feedback lasers : mode competition
Saturable absorber (SA): gain loss competition
With SA, no holding beam is necessary
Incoherent switching
Optically pumped monolithic active cavity with SA
Laboratoirede Photonique et de Nanostructures
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Injected lasers above Threshold (INLN)
Laboratoirede Photonique et de Nanostructures
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Bistability of CS above threshold
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Feedback Lasers (USTRAT)
Laboratoirede Photonique et de Nanostructures
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Feedback laser
80m Laser (ULM) spatially filtered feedback,appear spontaneously at
preferential locations
With 200 m, LS can be written without spatial filtering
Bistable localised states(Y. Tanguy, T. Ackemann, USTRAT)
Frequency tuning dependence
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Saturable Absorber Semiconductor lasers (LPN)
Need a cavity with special requirements
Very good cavity finesse around lasing wavelength (use QW)
pump window (OP) + optimized pumping
Saturable absorber section
no pump field but laser field
Gain section
pump field & laser field
Optically pumped monolithic active cavity with SAOP-VCSELSA
QW gain medium(/QD?)
QW or QD saturable absorber
Pump field
Laser field
Laboratoirede Photonique et de Nanostructures
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OP-VCSELSA cavity design
Simplex optimization procedure on layer thicknesses :front and back mirror R,Tfront and back mirror j
R, j
T
overall cavity A
Front mirrorBack mirror
SA sectionSA section Gain sectionGain section
Laser field~980 nm
Pump fields795-805 nm
1 InGaAs/AlGaAs QW2 InAs/GaAs QW
Experiment :Optically pumped monolithic active
cavity with SA
Theory : Laser with SA (coll. INFM/Como) QD model (coll. INFM/Bari)
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Laser L(P) curve
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External cavity Laser with SA
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Stability analysis vs Fresnel number
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Transverse mode mapping
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Nearly self-imaging cavity
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Limitations
Semiconductor structures are subject to very stringent requirementsWith respect to :
• Uniformity : ~ (0.1nm)l/(10m)t
• thickness, • current, • defects
• Thermal management : • production, • dependance on temperature• dissipation
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
Thickness Uniformity
~40 GHz/100 m
885 nm
Ti:Sa injection @ 883.93nm
12 mm
~400 GHz/150 m
-> 40GHz/200 m
MBE grown (Ulm)
MOVPE grown (LPN)
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Pump uniformity
Optical pumping
Use flat top incoherent beams
Electrical pumping :
bottom emitting structure (Ulm)
metallic grids
Indium Titanium Oxide (ITO) (cf. LAAS)
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Thermal management
Substrate transfer on SiC, or C
Sub. GaAs
Ti deposition SiC + AuIn2
Mechanical polishing
Chemical etching
Heat source Heat dissipation
Laboratoirede Photonique et de Nanostructures
Spring School on Solitons in Optical Cavities, Cargèse May 8-12 2006
General Conclusion
Semiconductors are indeed promising candidates for the exploitation towards applications of the rich panel of NL dynamical properties.
High NL response
Integrability and compacity for devices
Number of limitations presently arisen such as those pertaining to
Uniformity, purity, thermal management
must be circumvented but solutions exist from other domains of semiconductor physics.
Hope that this school will have convinced at least some of you to contribute to this very exciting development.