clk08-sivan
TRANSCRIPT
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Yonatan Sivan, Tel Aviv University, Israelnow at Purdue University, USA
• G. Fibich, Tel Aviv University, Israel
• M. Weinstein, Columbia University, USA
• B. Ilan, UC Merced, USA
Stability and instability of solitons in inhomogeneous
media
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Motivation
• Light propagation in a nonlinear medium– z is the direction of propagation– transverse coordinate
• Look for solitons – waves that maintain their shape along the propagation in z
z
Nonlinear medium
x
1( , , ), 1,2,3dx x x d= =…
laser
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Solitons
• Appear in– Nonlinear optics– Cold atoms – Bose-Einstein Condensation (BEC) – Solid state– Water waves– Plasma physics
• Applications – communications, quantum computing, …
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Soliton stability• Ideally – get same soliton at the other end
• In practice, soliton must be stable (robust) under perturbations
z
Nonlinear medium
xlaser
E field
E field
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Stability analysis
• In this talk, a different approach– Qualitative approach: characterize instability dynamics– Quantitative approach: quantify strength of
stability/instability
• KEY question – is the soliton stable?– hundreds of papers…
• Typical answer – yes (stable)/no (unstable)– What is the instability dynamics?
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Outline of the talk
• Solitons in Nonlinear Schrödinger (NLS) Eq.
• Stability theory
• Qualitative approach
• Quantitative approach
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Paraxial propagation – NLS model
• A – amplitude of electric field • Initial condition:
– z is a “time”-like coordinate • Competition of diffraction ( ) and
focusing nonlinearity (F)
2 2( , ) (| | ) 0zdiffraction focusing
nonlinearity
iA z x A F A A+ ∇ + =
0( 0, ) ( )A z x A x= =
zNonlinear medium
xlaser
1
2 2 2dx x∇ = ∂ + + ∂
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Typical (self-) focusing nonlinearities
• Cubic (Kerr) nonlinearity
• Cubic-quintic nonlinearity
• Saturable nonlinearity
2 2( )F A A=
2 2 4( )F A A Aγ= −
22
2( )1
AF AAγ
=+
2 2( , ) (| | ) 0ziA z x A F A A+∇ + =2
0 ( )n n F A= +
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Physical configurations – slab/planar waveguide
•• no dynamics in y direction• 1+1 dimensions (x,z)
2( , ) (| | ) 0z xx
diffraction nonlinearity
iA z x A F A A+ + =
y
, 1x x d= =
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Physical configurations – bulk medium
•• 2+1 dimensions (x+y,z)
z
z=0
Nonlinear medium
2 2( , , ) (| | ) 0zdiffraction nonlinearity
iA z x y A F A A+ ∇ + =
xlaser
( , ), 2x x y d= =
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Physical configurations – pulses in bulk medium
•• β2 < 0, anomalous group velocity dispersion (GVD)• 3+1 dimensions (x+y+t,z)• Spatio-temporal soliton = “Light bullets”
2 22( , , , ) (| | ) 0z tt
diffraction dispersion nonlinearity
iA z x y t A A F A Aβ+ ∇ − + =
( , , ), 3tx x y d= =
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• Solitons are of the form
• Do not change their shape during propagation (in z)• Exhibit perfect balance between diffraction and
nonlinearity
( ) ( )2 2( , ) ; , ( ) 0;i zA x x xe u u F u u uz ν ν νν= ∇ + − =
Eq. for solitons2 2( , ) (| | ) 0ziA z x A F A A+∇ + =
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• Explicit solution• (propagation const.) proportional to
– amplitude– inverse width
Example – 1D solitons in a Kerr medium
( ); 2 sech( )u x xν ν ν=
2
( )
( , ) 0z xxKerr cubicnonlinearity
iA z x A AA+ + =
( ) ( )2 3( , ) ; , 0;i zA e u ux x x u uz ν ν νν= ∇ + − =
ν
~1/√ν~√ ~ν
x
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Outline of the talk
• Solitons in Nonlinear Schrödinger Eq.
• Stability theory
• Qualitative approach
• Quantitative approach
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Stability theory
• Vakhitov-Kolokolov (1973): necessary condition for stability of
is the slope condition
2( ) 0, ( ) ( ; )soliton power
P P u x dxν ν νν
∂> =
∂ ∫
( , )i zA e u xν ν=
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Example: homogeneous Kerr medium
•
1, 0
2, 0
3, 0
Pd
Pd
Pd
ν
ν
ν
∂= >
∂∂
= =∂∂
= <∂
⇒
⎫⇒⎬
⎭ins
stabili
tabi
ty
lity
22 2( ) ( ; ) ( )
d
P u x dx C dν ν ν−
= =∫
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Stability in a d=1 Kerr medium
• Incident beam is a perturbed d=1 soliton
• Solution stays close to the soliton• 1D Solitons are stable!
( )
( 0, ) (1 0.05) 2 sech( )u x
A z x xν ν= = +
| |A
0 200
2
z
| ( , 0) |A z x =
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Instability in a d=2 Kerr medium
•
• collapse at a finite distance!• 2D Solitons are unstable
( 0) (1 0.02) ( , )A z u x y= = +
• Incident beam is a perturbed d=2 soliton
0 1.20
0.2
0 1.20
50
z z
| ( ) |A z 1| ( ) |A z −
amplitude width
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• Vakhitov-Kolokolov (1973): Slope condition is necessary for stability
• Is it also sufficient?
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• Weinstein (1985-6), Grillakis, Shatah, Strauss (1987-9):
1. Slope (VK) condition
2. Spectral condition: the operator
must have only one negative eigenvalue• Two conditions are necessary and sufficient for
stability
Rigorous stability theory (u>0)
2 2( )L G uν+ = −∇ + −
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• Only one negative eigenvalue λmin
• Spectral condition is satisfied
continuous spectrummin 0λ < 0λ
0 ν
Spectrum of L+
2 2( )L G uν+ = −∇ + −
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Summary - stability in homogeneous medium
• Spectral condition is always satisfied • Stability determined by slope (VK) condition
2( ) 0, ( ) ( ; )soliton power
P P u x dxν ν νν
∂> =
∂ ∫
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Inhomogeneous media
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Light propagation in inhomogeneous medium
• Since late 1980’s: interest in inhomogeneous media– E. Yablonovitch, S. John (1986) – photonic crystals– Christodoulides & Joseph (1988) – discrete solitons– ...
• Goals:– Stabilize beams in high dimensions (“Light bullets”)– Applications – communications (switching, routing, …)
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• Varying linear refractive index– Waveguide arrays / photonic lattices
Inhomogeneous media
02
2( ) | |n xn n A= +02
2( ) | |n zn n A= +
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• Varying nonlinear refractive index– Novel materials
Inhomogeneous media
20 2 ( ) | |n n n x A= +2
0 2 ( ) | |n n n z A= +
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• This study – modulation in only• Refractive index = – Potential
NLS in inhomogeneous media
x
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• This study – modulation in only• Refractive index = – Potential
– arbitrary potentials (Vnl, Vl)• periodic/disordered potentials, periodic potentials with defects,
single/multi-waveguide potentials etc.
– any nonlinearity F– any dimension d
( )
NLS in inhomogeneous media
( )2 2( ), 1 (| ( )| ) 0lz nlV xiA z x A A V x AF A+∇ + − − =
x
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Inhomogeneities in BEC
• Same equation (Gross-Pitaevskii) in BEC
• Dynamics in time (not z)•• Inhomogeneities created by
– Magnetic traps– Feshbach resonance– Optical lattices
( ) ( )2 2( ), 1 (| ( )| ) 0lt nlV xiA t x A A V x AF A+∇ + − − =
( , , ), ( , ),x x y z x x y x x= = =
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How do inhomogeneitiesaffect stability?
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“Applied” approach
• Slope (VK) condition• Numerics• Ignore spectral condition
Rigorous approach
• Slope (VK) condition• Spectral condition
) 2( ( ) (| | ) ( )nl lVL L V x G A V x+ + += +
Typical result – soliton stable (yes)/unstable (no)
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Qualitative approach
• Characterize the instability dynamics• Key observation: instability dynamics depends
on which condition is violated– Look at each condition separately
• Results for ground state solitons only (u>0)
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Outline of the talk
• Solitons in Nonlinear Schrödinger Eq.
• Stability theory
• Qualitative approach– Slope condition– Spectral condition
• Quantitative approach
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Instability due to violation of slope condition
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Violation of slope condition
• d=2 homogeneous Kerr medium –– Slope = 0, i.e., instability
–• Collapse
–• Total diffraction
( )P Constν =
0 1.20
50
z
| |A
0 100
1
z
| |A( 0) (1 0.02) ( , )A z u x y= = +
−5 100ν
( 0) (1 0.02) ( , )A z u x y= = −
power
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• d=2 Kerr medium with potential– Stable and unstable branches
–– 1% perturbation width decrease by factor of 15
Violation of slope condition – cont.
| ( ) |A z
−5 100ν
unstable stable
homogeneous medium
( 0) (1 0.01) ( , )
power
A z u x y= = +
0 52
30
z0 5
0
1
z
amplitude
1| ( ) |A z −
width
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Violation of slope condition – cont.
Conclusion:violation of slope condition focusing instability
0 52
30
z
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Instability due to violation of spectral condition
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Spectral condition
• The operator
must have only one negative eigenvalue
• No potential : spectral condition satisfied
( ) 2 2((1 ) () () )Vnl lV x u xL VG uν+ = −∇ + − − +
( )( , ) , 0i zA z x e u x uν= >
continuous spectrummin 0λ < 0λ
0 ν( )( )VL L+ +=
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• With potential:– remains negative– continuous spectrum remains positive– only can become negative
• Spectral condition determined by • Spectral condition not automatically satisfied
( )0Vλ
0
continuous spectrum
( )m in 0Vλ < ( )
0Vλ
?
( )minVλ
Spectral condition – cont.
( )0Vλ
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lattice
Generic families of solitons
soliton
Soliton at lattice min. Soliton at lattice max.
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Sign of λ0(V)
( )0Vλ
• Numerical/asymptotic/analytic observation
λ0(V) > 0 for solitons at a lattice
min. (potential well)spectral condition satisfied
ν
0
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Sign of λ0(V)
( )0Vλ
• Numerical/asymptotic/analytic observation
λ0(V) < 0 for solitons at a lattice
max. (potential barrier)spectral condition violated
λ0(V) > 0 for solitons at a lattice
min. (potential well)spectral condition satisfied
ν
0 0
( )0Vλ
ν
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• Stability only at potential min. – solitons are more “comfortable” at a potential min. (well) than at a potential max. (barrier)– stay near potential min.– tend to move from potential max. to potential min.
• Different type of instability– Lateral location rather width
x
Physical intuition
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• Soliton centered at a lattice min. – stays at lattice min.– lateral stability
0 25
<x>
z
max.
min.
0 25
<x>
z
min.
max.
• Soliton centered at a lattice max. – moves from lattice max.
to lattice min.– drift instability
x
Numerical demonstration
Center of
mass
( 0 , ) ( )sh ift
A z x u x δ= = −Input beam:
x
2x x A= ∫“Center of mass”:
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Conclusion:violation of spectral condition drift instability
0 25
<x>
z
min.
max.
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Mathematical intuition
• Eigenfunction associated with λ0(V) is odd
+ =
eigenfunction
x
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Mathematical intuition
• Eigenfunction associated with λ0(V) is odd
+ =
eigenfunctionsoliton
xx x
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Mathematical intuition
• Eigenfunction associated with λ0(V) is odd
• Its growth causes lateral shift of beam center
+ =
eigenfunctionsoliton
xx x
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Mathematical intuition
• Eigenfunction associated with λ0(V) is odd
• Its growth causes lateral shift of beam center
+ =
eigenfunctionsoliton
xx x
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Qualitative approach – summary
• Characterization of instabilities– Violation of slope condition focusing instability– Violation of spectral condition drift instability
• Distinction between instabilities is useful for more complex lattices
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• Created experimentally by optical induction
V
Example
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• Created experimentally by optical induction• Consider solitons centered at a shallow local
maximum
V
Example
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Example
x
y
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• Narrow solitoncentered at a lattice max.
x
y
Example
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• Narrow solitoncentered at a lattice max.
x
y
Example
x
y
• Wide solitons:solitoncentered at a lattice max.
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x
y
Example
Drift instability
0 1.3z
<x> <y>
global max.
global max.
shallow max.
x
y
• Wide solitons:solitoncentered at a lattice max.
• Narrow solitoncentered at a lattice max.
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x
y
Example
Drift instability Stability
0 1.3z
<x> <y>
global max.
global max.
shallow max.
0 20z
<x> <y>
global max.
global max.
shallow max.
x
y
• Narrow solitoncentered at a lattice max.
• Wide solitons:solitoncentered at a lattice max.
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x
y
Example
Drift instability Stability
0 1.3z
<x> <y>
global max.
global max.
shallow max.
0 20z
<x> <y>
global max.
global max.
shallow max.
x
y
• Narrow solitoncentered at a lattice max.
• Wide solitons:solitoncentered at a lattice max.
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• Narrow solitons: centered at a lattice max.
• Wide solitons:effectivelycentered at a lattice max. min.x
y
Example
Drift instability Stability
0 1.3z
<x> <y>
global max.
global max.
shallow max.
0 20z
<x> <y>
global max.
global max.
shallow max.
x
y
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• Narrow solitons: centered at a lattice max.
• Wide solitons:effectivelycentered at a lattice max. min.x
y
Example
x
y
At which width is the transition between drift instability and stability?
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x
y
Example
0.06 1.5Width
λ0(V)
0( )0Vλ
• Wide solitons:effectivelycentered at a lattice min.
• Narrow solitons: centered at a lattice max.
x
y
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x
y
Example
0.06 1.5Width
λ0(V)
0( )0Vλ
• Wide solitons:effectivelycentered at a lattice min.
• Narrow solitons: centered at a lattice max.
x
y
transition
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Conclusion
• Dynamics “deciphered” using the qualitative approach– “effective centering” determines violation/satisfaction
of spectral condition – In turn, determines dynamics of “center of mass”– Dynamics is determined by slope condition
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Outline of the talk
• NLS and solitons - review
• Stability theory
• Qualitative approach– Slope condition – width instability– Spectral condition – drift instability
• Quantitative approach
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Motivation: 2d nonlinear lattices
• Can the nonlinear potential stabilize the solitons?– spectral condition satisfied only at potential min. – slope condition satisfied only for
• narrow solitons• specially designed potential
( ) ( )2 2, , 1 | |( , ) 0nz lViA z A xx yy A A+∇ + − =
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Narrow solitons in 2d nonlinear lattices• u(x,y) is a stable narrow soliton• Test stability of u(x,y) numerically:
– add extremely small perturbation
collapse instability!
( 0, , ) 1.0001 ( , )A z x y u x y= =
0 20
ampl
itude
z
![Page 68: CLK08-Sivan](https://reader034.vdocument.in/reader034/viewer/2022052522/5478e10fb4af9fef158b4696/html5/thumbnails/68.jpg)
Narrow solitons in 2d nonlinear lattices• u(x,y) is a stable narrow soliton• Test stability of u(x,y) numerically:
– add extremely small perturbation
collapse instability!
( 0, , ) 1.0001 ( , )A z x y u x y= =
0 20
ampl
itude
z
What is going on?
![Page 69: CLK08-Sivan](https://reader034.vdocument.in/reader034/viewer/2022052522/5478e10fb4af9fef158b4696/html5/thumbnails/69.jpg)
Strength of stability• Slope = 0 instability• Slope > 0 stability
– What happens for a very small positive slope?• Strength of stability determined by magnitude of
slope– small slope leads to weak stabilization– stronger stability for larger slope
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Strength of stability: nonlinear lattices
0 80
ampl
itude
z
• Fixed input beam
• Change lattice – slope = 0.01 (stable)
( 0) 1.0001 ( , )A z u x y= =
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0 80
ampl
itude
z
Strength of stability: nonlinear lattices
• Fixed input beam
• Change lattice – slope = 0.01 (stable)
– slope = 0.006 (less stable)
( 0) 1.0001 ( , )A z u x y= =
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0 80
ampl
itude
z
Strength of stability: nonlinear lattices
• Fixed input beam
• Change lattice – slope = 0.01 (stable)
– slope = 0.006 (less stable)
– slope = 0.002 (unstable)
( 0) 1.0001 ( , )A z u x y= =
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0 80
ampl
itude
z
• Fixed input beam
• Change lattice – slope = 0.01 (stable)
– slope = 0.006 (less stable)
– slope = 0.002 (unstable)
– slope = 0.0002 (even more unstable)
( 0) 1.0001 ( , )
Strength of stability: nonlinear lattices
A z u x y= =
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0 80
ampl
itude
z
Strength of stability: nonlinear lattices
• Fixed input beam
• Change lattice – slope = 0.01 (stable)
– slope = 0.006 (less stable)
– slope = 0.002 (unstable)
– slope = 0.0002 (even more unstable)
• Strength of stability is determined by magnitude of slope
( 0) 1.0001 ( , )A z u x y= =
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Narrow solitons in 2d nonlinear lattices• Original soliton was weakly stable
– Slope = 0.01( 0) 1.0001 ( , )A z u x y= =
0 80
ampl
itude
z
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Narrow solitons in 2d nonlinear lattices• Original soliton was weakly stable
– Slope = 0.01
– Stability for a smaller perturbation • Soliton is “theoretically” stable, but “practically”
unstable
( 0) 1.0001 ( , )A z u x y= =0 80
ampl
itude
z( 0) 1.00004 ( , )A z u x y= =
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• “Old” approach – check only sign of slope• “New” quantitative approach – check also the
magnitude of the slope.
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Slope condition: “old” approach
instability stability
Discontinuous transition between stability and instability
0 Slope
width
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instability stability
Continuous transition between stability and instability
0 Slope
Slope condition: quantitative approach
width
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Spectral condition: “old” approach
instability stability
Discontinuous transition between stability and instability
0 λ0(V)
drift
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instability stability
Continuous transition between stability and instability
λ0(V)
0
Spectral condition: quantitative approach
drift
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instability stability
Continuous transition between stability and instability
λ0(V)
0Can we get an analytical prediction for the drift rate as a function of ?
Spectral condition: quantitative approach
drift
λ0(V)
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• soliton width/potential period <<1
• Can use perturbation analysis!
• Expand potential as
• Solve for
( )21( ) (0) ''(0)2
V x V V xε= + +
Narrow solitons
soliton
ε =
2x x A= ∫
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Narrow solitons – cont.
• Center of mass obeys an oscillator equation!
2
22 22
2
Ehrenfest law
2 2 ''(0) ,x
d x d A V dV A xdz
εΩ
= − ∇ −∫ ∫
2 21( ) (0) ''(0)2
V x V V xε= + +
• Compute effective force
22 2 2
2 , 2 ''(0)d x x dVdz
ε= Ω Ω = −
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Narrow solitons - cont.
• Use perturbation analysis to compute eigenvalue
• Combine with Ω2 = - 2dV''(0)ε2 and get:
( )2 2 ( ) 2
04,(0) (0)
V dC CV V
λν
Ω = − =+
( )2
( )0
''(0)(0) (0)
V VV V
ελν
≈+
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Narrow solitons – cont.• Conclusion:
22 2 2 ( )
02 , Vd x x Cdz
λ= Ω Ω = −
( ) 20
( ) 20
stability0 0 oscil
ins
latory solutions
0 0 expone tabntial solution is il ty
V
V
λ
λ
⎧ > ⇒ Ω < ⇒ ⇒⎪⎨⎪ < ⇒ Ω > ⇒ ⇒⎩
(Lattice min.)
(Lattice max.)
Analytical quantitative relation between spectral condition ( ) and dynamics (Ω)λ0
(V)
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• But so far only for narrow beams • Can we compute the drift rate also for wider
solitons?
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Calculation of drift rate in a general setting
• Valid for– any soliton width – any potential (periodic/non-periodic, single/multi waveguide, …)– any nonlinearity (Kerr, cubic-quintic, saturable, …)– any dimension
( )
( ) ( )2 ( ) ( ) ( ) ( )
01( ) ( ) ( )
,0,
,
V VV V V V
V V V
f fC L f f
f L fλ+−
−
= > =
22 2 2 ( )
02 , Vd x x Cdz
λ= Ω Ω = −
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• Soliton slightly shifted from lattice max.
0 200
1
z
<x>
Examples of lateral dynamics (1)(0, ) ( )
shift
A x u x δ= −
max
min
max numerics
z/Ldiff400
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• Soliton slightly shifted from lattice max.• Solution of oscillator equation
– Ω is the drift (=instability) rate
– agreement up to the lattice min.– up to many diffraction lengths
Examples of lateral dynamics (1)
cosh( )x zδ= Ω
0 20
0
1
z
<x>
max
min
maxtheorynumerics
z/Ldiff400
(0, ) ( )shift
A x u x δ= −
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Drift rates
• Excellent agreement between analysis and numerics
d=1, weak lattice
d=2, strong lattice
10 100 3000
3
ν
Ω
0 50 1000
17
ν
Ω
theorynumerics
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• So far, studied instability dynamics• Oscillator equation good also for stability
dynamics
Examples of lateral dynamics (2)
max
max
min
z
– soliton moving at an angle from a lattice min.
sin( )tgx zθ= Ω
Ω• Solution of oscillator equation is
– Ω determines the maximal deviation (=strength of stability)
θ
![Page 93: CLK08-Sivan](https://reader034.vdocument.in/reader034/viewer/2022052522/5478e10fb4af9fef158b4696/html5/thumbnails/93.jpg)
Examples of lateral dynamics (2)
• d=2, periodic lattice
Kerr (cubic) medium Cubic-Quintic medium
0 200
max
max
min
z / Ldiff
0 10
<x>
z / Ldiff
max
max
min
theory
numerics
sin( )tgx zθ= ΩΩ
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Implications of quantitative study
• Experimental example (Morandotti et al. 2000): – experiment in a slab waveguide array – soliton centered at a lattice max. does not drift to a
lattice min. over 18 diffraction lengths• Explanation: absence of observable drift due to
small drift rate• Theoretical instability but practical stability
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Summary• Qualitative approach
– Slope condition focusing instability– Spectral condition drift instability
• Quantitative approach– continuous transition between stability and instability– analytical formula for lateral dynamics– still open: find analytical formula for width dynamics
• “Theoretical” vs. “practical” stability/instability• Results valid for any physical configuration of
lattice, dimension, nonlinearity