Experimental setup

21aTH-4
スペクトルおよび
時間分解光誘起ファラデー回転による
磁気ポーラロンスピン配向過程
Spin polarization dynamics on magnetic polaron
by means of spectrum- and time-resolved Faraday rotation
橋本
佑介、三野弘文、山室智文、神原
、松末俊夫A、嶽山正二郎B,C
大蔵
千葉大院自然科学、千葉大工A、千葉大理B、
東大・物性研C
Motivation
Localized energy
Free exciton magnetic polaron
(FEMP) in CdMnTe
Localized energy of
Magnetic Polaron
Alloy
Potential
5 10
Mn Concentration [%]
x = 5 ~ 10% → FEMP energy : Large
Alloy potential fluctuation : small
Method :
High quality sample with low Mn concentration
Spectrum- and time-resolved
Photo-induced Faraday rotation
(SRPIFR, TRPIFR)
Aim :
Photo-induced Magnetism
Sample
Bulk-Cd1-xMnxTe
x = 5%
GaAs
Cd1-xMgxTe
Cd1-xMnxTe
Quartz disk
The opaque GaAs substrate was removed
CdMgTe layer is transparency in CdMnTe
resonance of this sample
Absorption and Photoluminescence
Absorption
4.2K
Photoluminescence
O. D.
PL
PL
Light source：
He-Ne 633nm
FEMP
BMP
BMP'
1.650
1.660
FX
1.670
1.680
Photon energy [eV]
Distinct PL line of the FEMP appear !!
PL position [eV]
Binding energy [meV]
Absorption
1.6750
FX
1.6740
FEMP
1.6722
1.8
BMP
1.6657
8.3
BMP’
1.6558
18.2
Adiabatic Potential (meV)
Theoretical study of
the self-trapped FEMP
Mn concentration = 10 %
1/aB
Inversion of the extension of
the center of mass motion
Masakatsu Umehara
Experimental setup of PIFR
Time Delay
Pump
Probe
λ/2
B.S.
76MHz
Ti:Sapphire
Laser
λ/2
λ/4
EX absorption
Sample
1.4 ~ 300K
0 ~ 6.9T
Laser spectrum
1.670 1.675 1.680
Energy
Lock-in
Amplifier
Polarization
Beam Splitter
Optical Bridge
Faraday Rotation [a. u.]
Temporal profiles of PIFR

5K

0
5
10
15
Time [ps]
F  
l
2c
n: refractive index
frequency
(n  n )
l: sample thickness
c: light speed
Laser spectrum
Fourier transfer spectrum filter
Grating
Mirror
lens
slit
Mirror
Probe beam
EX
Band edge exciton
resonance absorption
FWHM
Pump：6.2meV
(2.8nm)
Probe：1.6meV
(0.7nm)
1.670 1.675 1.680
Energy
Transmission [a. u.]
PIFR excitation energy dependence
Pump
FEMP
FX
Probe
Energy
0
FX：
FEMP：
10
20
30
Delay Time [ps]
40
Exciton spin relaxation (8ps)
Exciton spin relaxation
＋
Long decay process > 13ns
Mn spin relaxation ≈100ns
T. Strutz et.al, Phys. Rev. Lett 68, 3912
Ferromagnetic spin alignment of
the Mn ions via the FEMP !!!
PIFR spectrum at 13ns
PIFR() - PIFR() [a. u.]
FEMP FX
Excitation energy
Δt = 13ns
FEMP resonance
Long decay signal
FX resonance
0
No signal
1.670 1.672 1.674 1.676
Photon energy [eV]
Exciton feels the effective magnetic field
of the ferromagnetically aligned Mn spins
FEMP
Free
exciton
Mn ion
summary
The excitation energy dependence of the
spectrum- and time-resolved photo-induced
Faraday rotation have been observed.
Under the FEMP resonance excitation, the
PIFR signal shows the quite long decay signal
that last more than 13ns. We attribute this
long decay process to the Mn spin orientation
via the FEMP formation.
In the case of the FEMP resonance excitation,
the PIFR spectrum at 13ns indicates the
exciton energy splitting via the effective
magnetic field that should be induced by the
ferromagnetically aligned Mn spins.
Experimental setup
Balancing unit OUT
Pump beam
(circularly polarized)
Polarization beam splitter
Sample
Probe beam
(linear polarized)
Intensity
O. D.
Band edge exciton
absorption spectrum
Excitation Laser spectrum
736 738 740 742 744
Wavelength [nm]
Absorption and Photoluminescence
Experimental setup
FEMP
Absorption 3.0
2.8
4.2K
2.6
2.4
2.2
BMP
2.0
1.8
1.6
1.660
1.670
1.680
Energy [eV]
Absortption [O. D.]
PL [Arb. Units]
Absorption
・Spectrophotometer
Photoluminescence
・Ar-ion laser Excitation
・1/3 meter grating spectrometer and
a charge-coupled device
Magnetic Polaron
Free Exciton Magnetic Polaron (FMP)
Mn spin
h
e
Exciton spin
Localization only by sp-d exchange interaction
Bound Magnetic Polaron (BMP)
E
e
h
Electron or hole
localized to
impurities, then
formation of
magnetic
polaron
PIFR in Magnetic field
To remove the back ground,
the PIFR measured with  and  pumping was subtracted
0.25T, 5K
Rotation [arb. units]
The PIFR
signal that
remain in
negative
time region
is appear
FX resonance
1.678 eV
FMP resonance
1.673 eV
0
10
20
30
Time [ps]
Decay time of the signal in FMP resonance is longer
than the repetition time of light source (13ns)
The exciton life time is no more than 300 pico seconds
The photoinduced ferromagnetic alignment
of Mn spin by FMP formation

Download Report