Title: Coherent Raman spectroscopy for the detection of electron spin resonance
1Coherent Raman spectroscopy for thedetection of
electron spin resonance
- Daniel Wolverson
- Department of Physics
- University of Bath
2Spintronics motivation
- Magnetic semiconductors from well-known to highly
controversial... - CdMnTe (Mg)
- ZnMnSe (Be,S)
- GaMnAs (Al)
- GaMnN
- Other transition metals besides Mn
- Cr, Co, Fe...
dilute magnetic semiconductor (DMS)
- M Oestreich et al, Appl. Phys. Letts 74 (1999)
1251 (Marburg / Hull) - R Fiederling et al, Nature 402 (1999) 787
(Wuerzburg) - D Ferrand et al., Sol. St. Commun. 119 (2001) 237
(Grenoble / Wuerzburg / Warsaw)
3Optical measurement of g factors
Electron Spin Resonance
(resolution)
(sensitivity and resolution)
(sensitivity)
4SFRS experimental system
- resolution limited either by spectrometer or
laser - corresponds to 0.01 in the g-factor (which is of
order 1 to 2 typically) - Fabry-Perot detection and single-frequency dye
laser might be a conventional next step.
5Example electron SFRS of cubic CdSe
6Spin flip Raman introduction
- Inelastic light scattering via spin flip
- Sidebands are produced shifted up and down in
energy with respect to the laser line a Zeeman
splitting is measured. - Spin relaxation lifetimes, band non-parabolicity
effects, constraints on momentum transfer of free
carriers, and even collective 2DEG excitations in
QWs can all modify SFRS energy or linewidth - SFRS is strongly resonant at free or bound
exciton transition energies.
7Spin flip Raman spectra
- Electron SFRS seen in modulation-doped 15nm
layers of ZnMnSe - Transitions between Mn2 3d levels (PMR) also
seen (g2.0) - Electron SFRS Raman shift follows Brillouin
function of field - Doping levels from 109 to 3 x 1011 cm-2 were
studied. - Bulk-like layers with 1015 lt n lt 1019 cm-3 were
also studied.
8Electron SFRS in Cd1-xMnxTe
- Nominal x 0.005 (0.5).
- SFRS measures CB splitting only (in contrast to
PL, PLE, reflectivity). No VB or diamagnetic
effects. - CB splitting here follows a Brillouin function.
- Fitting yields effective x, electron temperature
(and, in general, bound magnetic polaron energy).
- Mn2 3d5 has pure spin S5/2, g-factor 2.00
Dms1 signal also well-known in SFRS.
9What is CRESR?
- At spin resonance, the microwave field B1 induces
the precession of the magnetization about the
static magnetic field B0 - The component of the magnetization along the
laser beam direction oscillates in sign - The circular dichroism (circularly polarized
absorption) oscillates - The transmitted beam is modulated at the
precession frequency (in the microwave region).
Magnetic field B0
Microwave magnetic field B1
Laser, RCP / LCP
10Experimental setup for CRESR
- Laser beam passes through or reflects from sample
and onto a fast photodiode - At resonance, microwaves induce coherence between
the two spin states - The Raman scattered beam propagates co-linearly
with laser beam - These mix on the photodiode to produce a
microwave signal optical heterodyne detection.
Magnetic Field
Specimen
Raman
Laser
Fast photo-detector
Microwaves
dc
Microwave mixer
Microwave source
11Why optical heterodyne detection?
- High (near single photon) sensitivity for
coherent optical signals - Blind to broadband incoherent backgrounds (i.e.
luminescence) - Allows both amplitude and phase measurements of
the optical signal - Highly efficient detectors with bandwidths of
several hundred GHz have now been developed.
12Our first semiconductor CRESR
- ZnSe single resonance seen near 0.88 Tesla
- g-factor is 1.1162
- Precision ? 0.0001
- Accuracy ? 0.001
- Comparison of energy scales between CRESR and
SFRS (inset) shows CRESR has very much higher
resolution.
S. J. Bingham, J. J. Davies and D. Wolverson,
Phys. Rev. B 65 155301 (2002)
13CRESR dependence on excitation energy
- Excitation profiles of SFRS and CRESR are
similar - PL spectra of the donor-bound exciton show the
effects of changing the strain state - a strained to substrate
- b free-standing ZnSe
- c ZnSe attached to silica.
SFRS
CRESR
a
b
c
14CRESR applied to heterostructures
- ZnSe on GaAs (reflection geometry substrate now
not removed) - Complex lineshape may indicate more than one
shallow donor species
- ZnSe quantum well in ZnBeMgSe barriers
- Detectable signal from a single quantum well
15CRESR of bulk Cd1-xMnxTe (i)
- First application to a magnetic semiconductor
- Archetypical CdMnTe chosen
- Magnetic field swept through microwave (spin)
resonance condition for a laser energy in the
exciton region - Internal Dms1 transition of Mn2 ions is seen,
as noted in our SFRS spectra earlier, at B
0.48T for 13.7GHz - Lineshape results from hyperfine interaction with
Mn I5/2 nucleus (bars on figure) - Dispersion- and absorption-like components shown.
16CB and VB splittings, Cd0.995Mn0.005Te
- Vertical meV
- Horiz. Tesla
- Dots CB
- Lines VB
- T0 0.3K
- Effective x is 0.0048
17Excitation energy dependence
- Laser energy swept through optical resonance
condition for a set of magnetic fields near the
g2.00 spin resonance field (0.49T) - Compare structure seen to the predicted exciton
energies (which are also measured via PLE) - lh, hh degenerate in bulk, therefore ? 8
transitions (S1/2 ? J3/2) - Exciton electron-hole exchange and diamagnetic
shift taken into account.
18Simulation of CRESR
- On wider magnetic field scale, see a second broad
signal - Is it CB electron spin flip?
- Can simulate data with
- set of usual ESR lineshapes at Mn2 g2.00 line
(including hyperfine interaction with Mn nucleus)
and - one line at the position of the broad signal.
- So far, consistent with electron spin flip Raman.
19CRESR of bulk Cd1-xMnxTe (ii)
- Magnetic field swept through microwave (spin)
resonance condition for a set of laser energies
in exciton region - Internal Dms1 transition of Mn2 ions seen at B
0.5T (centre of figure), independent of optical
excitation energy - Broad signal has g-factor dependent on
excitation energy (!) - CB electron SFRS or optical detuning effect?
20Excitation-energy dependent broad peak
- Positions of broad signal marked by ?
- Do not observe a signal at field corresponding to
spin resonance condition for CB electrons - The microwave frequency (13.7 GHz) is too low for
this (35 GHz available) - Symmetrical shift to higher field with detuning
from resonance follows the outer exciton energy
levels (red).
21Summary
- First application of CRESR technique to a dilute
magnetic semiconductor demonstrates feasibility,
high sensitivity, high resolution and shows
expected signals for a simple bulk sample - New features also seen Mn hyperfine structure,
complex resonance behaviour - Extension to multiple Mn2 signals with Dmsgt1
planned - First results on GaMnAs also obtained (not
presented) and are completely different
Funding EPSRC, DFG, INTAS
22Team and collaborators
- Lowenna Glover
- Dr. Stephen Bingham
- Prof. J. John Davies
- Shanshan Zeng
- Dr. Gazi Aliev
- Dr David Richards, KCL.
- Prof. Jean Geurts and Prof. Laurens Molenkamp,
Würzburg. - Dr. Richard Harley, Southampton.
- U. Bremen
- CEA-Grenoble
- Heriot-Watt U.
- A.F. Ioffe Institute
- U. Lecce
- Philipps-U. Marburg
- U. Nottingham
- Polish Acad. Sci., Warsaw
- U. Würzburg