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Coherent Raman spectroscopy of Cd 1-x Mn x Te quantum wells

Coherent Raman spectroscopy of Cd 1-x Mn x Te 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,

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Coherent Raman spectroscopy of Cd 1-x Mn x Te quantum wells

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  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 S=5/2 Mn2+ 3d5 levels with Dms=±1; • Resonance is again via an excitonic intermediate state; • Many multiples of the fundamental Dms=±1 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 doesn’t 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 |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.

  7. B B Simulation of Mn2+ in CdTe Zeeman Hyperfine, I=5/2 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 20:1 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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