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