int. conf. ii-vi 2007 coherent raman spectroscopy of cd 1-x mn x te quantum wells lowenna smith,...
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Int. Conf. II-VI 2007
Coherent Raman spectroscopy ofCd1-xMnxTe quantum wells
Lowenna Smith, Daniel Wolverson,
Stephen Bingham and J. John Davies
Department of Physics,
University of Bath, Bath, UK
M. Lentze and J. Geurts
Physikalisiches Institut, EP III,
Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
M. Wiater, G. Karczewski, and
T. Wojtowicz
Institute of Physics, Polish Academy of Sciences, Warsaw, Poland
Int. Conf. II-VI 2007
Plan
Spin Flip Raman Scattering (SFRS) Coherent Raman (CRESR) Mn2+ in Cd1-xMnxTe quantum wells
Int. Conf. II-VI 2007
Spin flip Raman scattering
• Sample excited by laser in resonance with excitonic intermediate state;
• When a magnetic field is applied, weak sidebands at the Zeeman splitting appear on either side of the laser line;
• k.p theory predicts g-factors of band carriers;
• carriers at point defects must be modelled by spin Hamiltonians relevant to their symmetry
f i e Bh h g B
d
df
e
mc E
s
i
i
i
2
2
2
2
0
2 22
/
fi
Int. Conf. II-VI 2007
Spin flip Raman of Cd1-xMnxTe QWs
• Spin flip Raman scattering occurs between the S=5/2 Mn2+ 3d5 levels with ms=±1;
• Resonance is again via an excitonic intermediate state;
• Many multiples of the fundamental ms=±1 signal are seen.
Stühler et al, PRB 49 (1994) 7345; PRL 74 (1995) 2567Koenig et al, PRB 61 (2000),16870
...
Int. Conf. II-VI 2007
What is coherent Raman ESR?
• ESR can provide higher resolution than SFRS...
• but ESR does not have the selectivity of SFRS (no excitonic resonance);
• ESR also doesn’t have the sensitivity of optical techniques;
• Coherent Raman-detected ESR (CRESR) has both.
e Bhv g B
Electron Spin Resonance
CoherentRamanESRi
f
v
v
Int. Conf. II-VI 2007
How does CRESR work?
Laser beam reflects from (or passes through) sample and induces coherence between ground state |1> and the intermediate excitonic state |3>
At spin resonance field, microwaves induce coherence between the spin states |1> and |2>;
The Raman scattered beam propagates co-linearly with the reflected laser beam;
These mix on the photodiode to produce a microwave signal; this is optical heterodyne detection.
High (near single photon) sensitivity for coherent optical signals;
Blind to luminescence background;
Allows both amplitude and phase measurements of the optical signal.
PEMcontroller
Laser
PEM
Ref.
Photodiode
Low noiseamplifierPower
amplifier
Microwavesource
Lock-inamplifier
Lock-inamplifier
Cavity
S
Computer
Quadraturemixer
1
2
3
Int. Conf. II-VI 2007
Simulation of Mn2+ in CdTe
Mn
4 4 4 4
2 10 3
3
5
1
B
x y z
z
g
A
a S S S S
D S S S
S .B
I .S
H
B
Zeeman
Crystal
field
Hyperfine,
I=5/2
Experimental CRESR results from bulk CdTe
shown in redBiaxial strain orquantum confinementadds (large) term of this form
B
Int. Conf. II-VI 2007
First CRESR result on Mn2+ in QWs
0.000000
0.000001
0.000002
0.000003
0.000004
0.000005
CR
ES
R s
ign
al /
arb
. u
nits
1.00 1.05 1.10 1.15 1.20 1.25 1.30-0.000003
-0.000002
-0.000001
0.000000
0.000001
0.000002
0.000003
Magnetic field / T
See absorption-like and dispersion-like components (by analogy with ESR)
At low microwave power, can minimise saturation effects (no further absorption possible)
Must also avoid microwave heating effects (which reduce the Mn2+ magnetization and shift the excitonic band gap so that the optical resonance condition is lost)
Heating is most efficient at the spin resonance condition (where microwave absorption is strong).
absorption
dispersion
Int. Conf. II-VI 2007
CRESR from 3 SQWs in one structure
No sign of any hyperfine structure or crystal field effects – in contrast to dilute bulk CdMnTe, we only see a single Lorentzian line;
Crystal field is now dominated by axial term representing quantum confinement and strain effects (with inhomogeneous broadening);
Hyperfine effects are not resolved because of the relatively high Mn concentration (x ~ 1%) which leads to interactions between Mn ions;
Hence, absence of fine structure is not surprising.
Int. Conf. II-VI 2007
Selectivity and sensitivity of CRESR
0 100 200 3001.60
1.62
1.64
1.66
1.68
1.70
a
Well width / Å
Ene
rgy
/ eV
x5
0
b
CRESR signal
0 .0 0 0 0 0 0
0 .0 0 0 0 0 1
0 .0 0 0 0 0 2
0 .0 0 0 0 0 3
0 .0 0 0 0 0 4
0 .0 0 0 0 0 5
1.0 1.1 1.2 1.3-0 .0 0 0 0 0 3
-0 .0 0 0 0 0 2
-0 .0 0 0 0 0 1
0 .0 0 0 0 0 0
0 .0 0 0 0 0 1
0 .0 0 0 0 0 2
0 .0 0 0 0 0 3
Magnetic field / T
- 0 . 0 0 0 0 1 0
- 0 . 0 0 0 0 0 8
- 0 . 0 0 0 0 0 6
- 0 . 0 0 0 0 0 4
- 0 . 0 0 0 0 0 2
0 . 0 0 0 0 0 0
0 . 0 0 0 0 0 2
0 . 0 0 0 0 0 4
0 . 0 0 0 0 0 6
0 . 0 0 0 0 0 8
0 . 0 0 0 0 1 0
0 . 0 0 0 0 1 2
- 0 . 0 0 0 0 0 3 0
- 0 . 0 0 0 0 0 2 5
- 0 . 0 0 0 0 0 2 0
- 0 . 0 0 0 0 0 1 5
- 0 . 0 0 0 0 0 1 0
- 0 . 0 0 0 0 0 0 5
0 . 0 0 0 0 0 0 0
0 . 0 0 0 0 0 0 5
0 . 0 0 0 0 0 1 0
0 . 0 0 0 0 0 1 5
0 . 0 0 0 0 0 2 0
0 . 0 0 0 0 0 2 5
0 . 0 0 0 0 0 3 0
0 . 0 0 0 0 0 3 5
{
Signals just discussed came from 3 different QWs of the same heterostructure;
We are able to select these because of their different resonance energies; can demonstrate this by tuning the laser through the excitonic transitions of the set of QWs (the magnetic field is kept at the spin resonance field, ~1.2 T);
Also establishes sensitivity is high: single QW with ~1% Mn gives SNR of 20:1
301Å
78Å
45Å
Int. Conf. II-VI 2007
Summary
• Reviewed SFRS of Mn2+.
• Introduced coherent Raman-detected ESR.
• Reviewed Mn2+ in bulk CdTe.
• Presented first results of application of CRESR to a magnetic semiconductor quantum well; fine structure not observed but sensitivity and selectivity demonstrated.
• thanks to EPSRC, Royal Society, INTAS, NATO
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