Coherent Raman spectroscopy of Cd1xMnxTe quantum wells

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Coherent Raman spectroscopy of Cd1xMnxTe quantum wells

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Universit t W rzburg, Am Hubland, 97074 W rzburg, Germany. M. Wiater, G. ... effects (which reduce the Mn2 magnetization and shift the excitonic band gap ... – PowerPoint PPT presentation

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Title: Coherent Raman spectroscopy of Cd1xMnxTe quantum wells

1
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
2
Plan

Spin Flip Raman Scattering (SFRS)
Coherent Raman (CRESR)
Mn2 in Cd1-xMnxTe quantum wells

3
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

4
Spin flip Raman of Cd1-xMnxTe QWs

Spin flip Raman scattering occurs between the
S5/2 Mn2 3d5 levels with Dms1
Resonance is again via an excitonic intermediate
state
Many multiples of the fundamental Dms1 signal
are seen.

...
Stühler et al, PRB 49 (1994) 7345 PRL 74 (1995)
2567 Koenig et al, PRB 61 (2000),16870
5
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 doesnt have the sensitivity of optical
techniques

Electron Spin Resonance
Coherent Raman ESR

Coherent Raman-detected ESR (CRESR) has both.

6
How does CRESR work?

Laser beam reflects from (or passes through)
sample and induces coherence between ground state
1gt and the intermediate excitonic state 3gt
At spin resonance field, microwaves induce
coherence between the spin states 1gt and 2gt
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.

7
Simulation of Mn2 in CdTe
Zeeman
Hyperfine, I5/2
Crystal field
Experimental CRESR results from bulk CdTe shown
in red
Biaxial strain or quantum confinement adds
(large) term of this form
8
First CRESR result on Mn2 in QWs

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
9
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.

10
Selectivity and sensitivity of CRESR

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 201

45Å
78Å

301Å
11
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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