polarization of exciton polariton condensates in lateral traps c. trallero-giner, a. v. kavokin and...
TRANSCRIPT
Polarization of exciton Polarization of exciton polariton condensates in polariton condensates in
lateral trapslateral traps
C. Trallero-Giner, A. V. Kavokin and T. C. H. Liew
Havana University and CINVESTAV-DF
University of Southampton
Ecole Polytechnique Fédérale de Lausanne
OUTLINEOUTLINE
• Introduction
• Scalar BEC in a two dimensional trap
• Spinor condensates of exciton-polaritons
• Conclusions
Introduction
Es posible obtener alta densidad de un gas de “átomos” ligeros. La coherencia cuántica debe ser a las altas tempertaturasLos polaritones cuya masa es 0.0001 me
POLARITON CONDENSATION IN POLARITON CONDENSATION IN TRAP MICROCAVITIESTRAP MICROCAVITIES
-Photons from a laser create electron-hole pairs or excitons.
-The excitons and photons interaction form a new quantum state= polaritonpolariton.
Peter Littlewood SCIENCE VOL 316
2 dimensional GaAs-based microcavity structure.Spatial strep trap ( R. Balili, et al. Science 316, 1007 (2007))
REVIEWS OF MODERN PHYSICS, VOL. 82, APRIL–JUNE 2010
two dimensional Gross-Pitaievskii equation
The description of the linearly polarized exciton polariton condensate formed in a lateral trap semiconductor microcavity:
α1 and α2 – self-interaction parameter ω – trap frequency m – exciton-polariton mass
Scalar BEC
-Explicit analytical representations for the whole range of the self-interactionparameter α1+α2.
The main goal
-To show the range of validity.
Thomas-Fermi approach
Experimentally it is not always the case
Analytical approaches
Variational methodFor non-linear differential equation the variationalmethod is not well establish.
-5 -4 -3 -2 -1 0 1 2 3 4 5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Numeric solution
ThomasFermi
VariationalMethod
x / l0
Norm
ali
zed
ord
er p
aram
ete
r (l 0)1
/2 x/l 0)
a)
Gross-Pitaievskii integral equation
-Green function
Green function formalism
-spectral representation
-Integral representation
-harmonic oscillator wavefunctions
Perturbative method
It is useful to get simple expressions for μIt is useful to get simple expressions for μ00
and Φand Φ00 through a perturbation approach. through a perturbation approach.
∫|Φ0(r)|2dr=N
Ψ0=Φ0/√N
-small term
∫| Ψ0|2dr=1
Using the integral representation for the 2D GPE
The general solution for the order parameter Ψ0
has an explicit representation as
{φn1;n2 (r)} -2D harmonic oscillator wave functions
-must fulfill the non-linear equation system
T is a fourth-range tensor
The eigenvector C is sought in the form of a series of the nonlinear interaction parameter Λ
-small term
Energy Λ/2
-3 -2 -1 0 1 2 3 4 5
-0,5
0,0
0,5
1,0
1,5Numerical solution Analytical solution
ner
gy/
Universal result
The normalized order parameter Ψ0
Hn(z) the Hermite polynomial
Ei(z)-the exponential integral; γ-the Euler constant
Ψ(r)= Φ(r)/√N
r→r/l
0.4 0.8 1.2 1.6 2.0 2.4 2.8
0.1
0.2
0.3
0.4
0.5
r
Norm
alized o
der para
mete
r
In typical microcavities the values of the interaction constants can change with the exciton-photon detuning, δ
Eb-the exciton binding energy, ab -the exciton Bohr radius X -the excitonic Hopfield coefficientV the exciton-photon coupling energy
GaAs
GaAs
The polaritons have two allowed spin projections
If the absence of external magnetic field the ‘‘parallel spins’’ and ‘‘anti-parallel spin’’ states of noninteracting polaritons are degenerate.
The effect of a magnetic fieldThe effect of a magnetic field
To find the order parameter in a magnetic field we start with the spinor GPE:
We are in presence of two independent circular polarized states Φ±
Spinor condensates of exciton-polaritons
-Ω is the magnetic field splitting
-two coupled spinor GPEs for the two circularly polarized components Φ±
-α1 the interaction of excitons with parallel spin-α2 the interaction of excitons with anti-parallel spin
The normalization ∫|Φ±|dr = N±Ψ± (r)= Φ± (r)/√N ±
Λ1=α1N+ /(2l2ћω)
Λ12=α2N- /(2l2ћω)
η=N+/N-
EnergiesEnergies
μ +=(E+-Ω))/ ћω =1+0.159*(Λ1+Λ12)+ 0.0036*F+(Λ1,Λ12)
μ -=(E-+Ω))/ ћω =1+0.159*(Λ1/ η +Λ12 η)+ 0.0036*F-(Λ1/ η , Λ12 η)
F+=(3Λ1+2Λ12)(Λ1/η+ηΛ12)+Λ12(Λ1+Λ12)
F-=(3Λ1/η+2Λ12η)(Λ1+Λ12)+(Λ1/η+ηΛ12)Λ12η
0.5 1.0 1.5 2.0 2.5
1.2
1.3
1.4
1.5
μ +=(E+-Ω))/ ћω
μ -=(E-+Ω))/ ћω
Λ1=α1N+ /(2l2ћω)
Λ12=α2N- /(2l2ћω)
0.4 0.8 1.2 1.6
1.1
1.2
1.3
1.4
+= ( E+-
-= ( E-+
μ +=1+0.159*(Λ1+Λ12)+0.0036*F+(Λ1,Λ12)
μ -=1+0.159*(Λ1/ η +Λ12 η)+0.0036*F-(Λ1, Λ12)
Order parameter for the two circularly Order parameter for the two circularly polarized polarized ΨΨ±± components. components.
Λ1=1Λ12=0.4
Ψ± = Φ±/√N±
η=N+/N- =1
=0.6 =0.40.5 1.0 1.5 2.0 2.5
0.1
0.2
0.3
0.4
0.5_(r):N+=0.6N--
r
Norm
alized o
der para
mete
r
_(r):N+=0.4N-
r)
The circular polarization degree
If the condensate is elliptically polarized we find a nonuniform distribution of the Polarization in space.
The circular polarizationdegree at r = 0
Polariton number The polarization changes from circular to ellipticaland approaches a linear polarization asymptoticallyat high polariton number.
Conclusions-We have provided analytical solution for the exciton-polariton condensate formed in a lateral trap semiconductor microcavity.
-An absolute estimation of the accuracy of the method
−3 < Λ < 3
ΛΛ versus versus the detuning parameter the detuning parameter δδTypical Values GaAs
N~105-106
-We extended the method to find the ground state of the condensate in a magnetic field
3/
N+/N
-<1
3
--Validity of the methodValidity of the method
THANKSTHANKS
40 80 120 160 200
-40
-30
-20
-10
0
10
20
30
40
Theory without lattice with lattice
Experiment magnetic trap +optical lattice magnetic trap
Cente
r m
ass p
osition [
m]
Time [ms] PRL. 86, 4447 (2001)