Int. Conf. II-VI 2007 Coherent Raman spectroscopy of Cd 1-x Mn x Te quantum wells Lowenna Smith, Daniel Wolverson, Stephen Bingham and J. John Davies Department.

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