Transcript Large Rabi splitting in ZnSe

A ZnSe multi quantum well microcavity structure
optical size
at  = 538 nm
200 nm
The front side of the structure
is covered with high reflection
(R > 0.95) distributed BraggMirrors of ZnSe and YF3. The
microcavity is completed with
backside Bragg-Mirrors after
substrate wet etching.
100 nm
Resonant medium: Up to four
(Zn,Cd)Se quantum wells.
Luminescence selection is
possible with a variation of the
Cd-content or the well width.
real layer
thickness
ZnS /4-layer
YF3 /4-layer
6x

ZnSe-Cavity
1/2 
(Zn,Cd)Se-QWs
0 nm
8x
/4 YF3 layer
/4 ZnS layer
A standing wave is manifested in the cavity: quantum
wells or quantum dots are placed near its antinodes which
guarantees an effective coupling efficiency between
excitonic and photonic mode.
 The structure is particularly suited for investigation of
the strong and weak coupling regime in semiconductor
microcavites.
Structural characterisation of ZnSe/(Zn,Cd)Se QWs
The most important structural parameters of the microcavities are
quantum well length L, cadmium content x and the cavity size d.
Further interesting parameters for the optimization of our structures
are the alloy scattering in the (Zn,Cd)Se wells as well as the strain
status of the microcavity.
Growth process parameters
Ex-situ X-ray diffraction data
rel. simulation error ~ 2 %
14 nm
ZnSe spacer
12.5 nm
ZnSe spacer
82 nm ZnSe
82 nm ZnSe
7 nm
Zn0.65Cd0.35Se
quantum wells
81 nm ZnSe
81 nm ZnSe
5.8 nm
Zn0.65Cd0.35Se
quantum wells
GaAs
GaAs
101
0
10
(004)-Reflex w/2-scan
GaAs
-1
10
ZnSe
-2
Intensity
10
-3
10
(Zn,Cd)Se
1x10-4
1x10-5
-6
10
10-7
61
62
63
64
65
2 (deg)
66
67
68
300 K reflection measurements of a typical microcavity
The figure below shows the room temperature reflectivity spectra of the
microcavity obtained at different sample positions.
MQW emission
300 K
Lc=205.5 nm
Q = 210
reflectivity (arb. units)
Lc=203.3 nm
2,20
Lc=201.9 nm
The dots depict the
experimental data, the
curves represent
Lorentzian fits. The
respective cavity length
values, which are also
indicated in the figure,
were calculated from the
shift of the photonic mode
(cavity mode), which is
caused by the layer
thickness gradient.
In dependence on the
cavity length, the photonic
mode shifts to higher
Lc=200.5 nm
energies. At Lc = 200.5 nm
the photonic mode
approaches the
luminescence energy and a
splitting of the reflectivity
spectrum into two peaks at
 = 41 meV
2.290 eV and 2.331 eV is
2,24 2,28 2,32 2,36 2,40 2,44 observed. The energy
difference between both
photon energy (eV)
peaks is  = 41 meV.
Temperature dependent photoluminescence
330 K
normalized Pl intensity (a.u.)
320 K
310 K
min = 44 meV
300 K
290 K
280 K
The picture to the left shows
photoluminescence spectra of a
complete microcavity structure at
temperatures between 270 - 330 K.
The variation of the temperature
leads to a shift of the quantum well
luminescence energy according to
the bandgap shift. Therefore the
excitonic mode approaches the
photonic mode at a constant cavity
length of Lc = 200.5 nm. It is evident
from Fig. 4 that the luminescence
peaks show a clear anticrossing
behaviour.
The curves below are a fit of the
experimental data using a model of
270 K
the polariton dispersion. In our
calculation we used Ex = 2.627 – 1.1
2.20 2.25 2.30 2.35 2.40 2.45 2.50 · 10-3 T (eV) for the temperature
dependence of our quantum well
photon energy (eV)
2.38
calculated dispersion
experimental data
2.36
2.34
photon energy (eV)
luminescence and Ep =
2.298 eV as the constant
cavity mode energy,
yielding Rabi = 45 meV.
This value is in good
agreement to the
experimental data of the
reflection measurements
and confirms in addition
the existence of the
strong coupling regime.
2.32
2.30
2.28
2.26
2.24
2.22
260
270
280
290
300
310
temperature (K)
320
330
340