1. Optical study of Spintronics in III-V semiconductors
Xiaodong Cui
University of Hong Kong

2. Collaborators Spin Dynamics • Magneto-photocurrent
Dr. Yang Chunlei Mr. Dai Junfeng
Theorist:
Dr. Lu Hai-Zhou
Prof. Shen Shun-Qing
Prof. Zhang Fu-Chun

3. Conventional electronic devices ignore the spin property and rely strictly on the transport of the electrical charge of electrons
Adding the spin degree of freedom provides new effects, new capabilities and new functionalities

4. No Need for Moving Spin: Potential for Low Power Dissipation!
Present Applications: Giant Magneto Resistivity in Ferromagnetic materials.
Charge
Spin
No Need for Moving Spin: Potential for Low Power Dissipation!

5. Outline
Time resolved Kerr-rotation spectroscopy in the Spin dynamics study
Spin Photocurrent in two dimensional electron gases of InGaAs

6. Kerr Rotation spectroscopy
Classical picture:
Change in the polarization state when a linearly
polarized light reflected from a strong magnet.
Magnetization ↔Bound currents boundary conditions E M

7. Microscopic origin – selection rule
mj=-1/2
mj=+1/2 2 1 3
-1/2
+1/2
mj=-3/2
mj=+3/2
mj=-1/2
mj=+1/2
Pump beam: Creating Spin Polarization via Optical injection.
Probe beam: A linearly polarized light is a superposition of a left and right circularly
Polarized lights.

8. PBS: polarized beam splitter LC: lock-in amplifier L: lens
DET M1
YAG
Ti:Sapphire M2 M3 M4 M5 M6 M7 M8 M9
M10
M11
Sample I1 I2 I3 I4
PEM
Chopper
Pump
Probe
PBS2
PBS1
BS1
BS2
/2 Plate L3 L4 L5
I: Iris
DET: Twin detector
M: Mirror
PBS: polarized beam splitter
LC: lock-in amplifier
L: lens

9. g-factor Kerr-rotation spectroscopy
Existing techniques to study g factor:
Electric transport
Low temperature, high requirements for sample quality
Electron spin resonance
unpaired electron
Magneto-photoluminescence
complex origins, signal reflects information of exciton
Kerr-rotation spectroscopy
Magnitude, NO sign information

10. g factor study by Kerr rotation spectroscopyz y x
Torque driving precession
Spin projection along Z

11. (a) GaAs thin film
g= (T=5K)
(b) GaAs 2DEG
g= (T=5K)
(c) GaAsN/GaAs quantum well (N~1.5%)
g=+0.97

12. Phase shift is determined by the experimental configuration 
GaAsN/GaAs quantum well
Phase shift is determined by the experimental configuration 
For g>0
Phase term gBBt/ħ+ for B>0
gBBt/ħ- for B<0

13. Another Approach – magnetic field scan at fixed time delay
Magnetic field shift is determined by the experimental configuration 
Advantage against time scan:
time shift in time scan ~ ps
magnetic shift in field scan ~ Gauss

14. Spin flip mechanisms
Electron-impurity scattering -- Elliott and Yafet (EY) mechanism
Electron-hole exchange scattering -- Bir–Aranov–Pikus (BAP)
Spin-orbital coupling -- D’yakonov–Perel’ (DP) mechanism

15. Sample Descriptions
Monolayer (ML) and Sub-Monolayer InAs embedded in GaAs
A) (001) oriented: 1/3 ML, 1/2 ML, 1ML
GaAs
GaAs
sub monolayer (1/3, 1/2…) InAs
1 monolayer InAs
B) (311) oriented: 1ML
GaAs
1 monolayer InAs

16. Electronic states of Monolayer and sub-monolayer InAs

17. Lateral size reduction strongly suppressed spin relaxation process
Low pumping (pump/probe =1 , about 5x1016 /cm3 in GaAs)
T=77K, Laser: 1.52 eV, B=0T and 0.82 T
Lateral size reduction strongly suppressed spin relaxation process

20. Electric current and spin current
The electric current
The spin current

21. Generation of Spin current Spin injection
Spin polarized charge current
Non-local spin injection
Optical injection
Intra-band Linearly polarized light:
Ganichev et al., Nature Physics 2, 609 (2006).
Inter-band Linearly polarized light (one photon, two photon):
H. Zhao et al., PHYSICAL REVIEW B 72, ; Phys. Rev. Lett. 96, (2006).
Bhat et al., Phys. Rev. Lett. 85, 5432 (2000).
Spin pumping (ferromagnetic resonance)
Spin Hall effect

22. Generation and Detection of Spin current -- Spin Hall effect
Converting to magnetization
Converting to charge current
Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179 (2006).
Awschalom, Science 306, 1910–1913 (2004)
Kimura, Phys. Rev. Lett, 98, (2007)
Wunderlich; Phys. Rev. Lett. 94, (2005)
Wunderlich, Nature Physics, 5,675 (2009)

23. Zero-bias spin separation
Ganichev et al., Nature Physics 2, 609 (2006).
Intra-band excitation with linearly polarized THz radiation
Spin dependent excitation and relaxation process

24. C2V symmetry H=(xky- ykx) (001)
Incident light: 0.8eV Linearly polarized light (Band edge excitation)
Rashba coefficient =4.3E10-12 eVm

26. J(Bx, By, )= C0By + CxBxsin2 + CyBycos2

27. (c)

28. Estimate the spin current
Measurement of Photocurrent with Hall Effect J~ 1.5X10-2A/m at 1mW
Estimate the spin current from SdH oscillation
Estimate the ratio of field induced charge current
Vs. zero field spin current

29. The magnetic field induced charge current vs. pure spin current
Magnetic field induced charge current density ~
Pure Spin photocurrent density(ħ) ~
The ratio ~
In our case, Fermi energy ~ 10-1~10 -2eV (n=9E11cm-1),
Zeeman energy hu=1.2E-4 eV/Telsa (g= -0.4)
The Ratio ~ ~10-3 /Tesla

30. Conclusion
Magnetic field induced photocurrent via direct inter-band transition by a linearly polarized light
Our experiments support that the spin photocurrent could be generated by linearly polarized light absorption in material with spin-orbit coupling.
The conversion of spin current to magnetic field induced photocurrent is around 10-2~10-3 per Tesla.