J.S. Colton, Universal scheme for opt.-detected T1 measurements
Universal scheme for opticallydetected T1 measurements
(…and application to an n = 3E14 cm-3 GaAs sample)
John S. Colton
Brigham Young University
Students:
(grad) Tyler Park
(undergrads) Ken Clark
Daniel Craft
Jane Cutler
Funding acknowledgement:
NASA EPSCoR program
David Meyer
Dallas Smith
Scott Thalman
Talk for APS March Meeting
Mar 22, 2011
J.S. Colton, Universal scheme for opt.-detected T1 measurements
• Spin:
– Can visualize direction via
“Block Sphere”
– Spin up / spin down energy
splitting: DE = gmBB
– T1: longitudinal lifetime;
transition time from spin up to
spin down
Bloch Sphere
Spin up
Magnetic field
Image from Wikipedia
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Motivation 1: Materials Characterization
• Which materials are best?
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Motivation 2: Help understand physics
Hayes group, Wash Univ St Louis – Optically-pumped NMR
• Mui et al., “Effects of optical absorption on 71Ga optically
polarized NMR in semi-insulating GaAs: Measurements and
simulations”, Phys Rev B 2007.
• Hayes et al., “Optically pumped nuclear magnetic resonance
of semiconductors”, J Chem Phys 2008.
• Mui et al., “Manifestation of Landau level effects in opticallypumped NMR of semi-insulating GaAs”, Phys Chem Chem
Phys, 2009.
• Ramaswamy et al., “Optically pumped NMR: Revealing spindependent Landau level transitions in GaAs”, Phys Rev B,
2010.
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Motivation 3: Three particular samples
2004:
3E15 cm-3
2006:
5E13 cm-3
Colton et al., PRB 2004
2007:
1E15 cm-3
Colton et al., PRB 2007
Fu et al., PRB 2006
(Stanford)
3E14 cm-3 ?
J.S. Colton, Universal scheme for opt.-detected T1 measurements
How to measure T1?
Simplest version: Like “Time Resolved Faraday/Kerr Rotation”
Problem!!
Crooker et al., Phys Rev B 1997
Kikkawa & Awschalom, Phys Rev B 1998
(but use longitudinal field, of course)
J.S. Colton, Universal scheme for opt.-detected T1 measurements
(detector)
(excitation)
(detector)
2004 & 2007 experiments
• Single beam
• Electronic pulse generator to
modulate
Pulsed light
• Pump/probe controlled by
length of pulse
• Spins detected via PL
Problem! – for lowest doped sample,
polarization
probe beam needed to be too weak
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Experimental Setup
2-channel
pulse gen.
Circ. polar. pump
laser: 781 nm diode
(fast mod. input)
PEM
field
magnet/
cryostat
reference
Lin. polar. probe laser:
821 nm cw Ti-sapphire
Lockin amplifier
AOM
Balanced
photodiode
detector
sample
Polarizing
beam splitter
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Timing Diagram
12 ms
PEM:
RCP pump
~ 1 ms
Pump:
Spin polarization (expected):
Probe:
scan relative delay
LCP pump
(~10 periods)
RCP pump
J.S. Colton, Universal scheme for opt.-detected T1 measurements
What we saw: 0T
8.0x10
GaAs: n = 3E14 cm
-6
0 T, 1.5 K
Lockin signal (V)
6.0x10
-3
overall period = 600 ns
probe: 20 ns
pump: 120 ns
-6
D
###
###
###
###
###
###
###
###
###
file 18
overall period =
lockin tau=1s; 3
probe: 20 ns, 1.
pump: 120 ns, P
delay 1
Spin polarization
stops
file 18: noas
decay soon as
pump stops.
No decay!
T1 << 20 ns
16 Mar 2011 - file 18
4.0x10
-6
2.0x10
-6
Data: f17_f16and17ave
Model: ExpDec1
Equation: y = A1*exp(-x/t1) + y0
Weighting:
y
No weighting
As expected…
T2* = 5 ns
from resonance
experiments
Chi^2/DoF
= 2.9693E-15
R^2
= 0.99679
0.0
y0
A1
t1
0
100
200
300
400
500
Probe delay (ns)
start of pump
end of pump
600
-7.8974E-7
0.00022
64.16573
±3.0515E-8
±0.00003
±2.17496
J.S. Colton, Universal scheme for opt.-detected T1 measurements
What we saw: 1.5 T
end of pump
Lockin signal (V)
GaAs: n = 3E14 cm
1.4x10
-5
1.2x10
-5
1.0x10
-5
8.0x10
-6
6.0x10
-6
4.0x10
-6
2.0x10
-6
1.5T, 1.5 K
overall period = 3500 ns
probe: 150 ns
pump: 200 ns
Signal exists well
after pump stops.
Equation: y = A1*exp(-x/t1) + y0
Weighting:
y
No weighting
decay time: 414 ns
Exponential
decay!
Chi^2/DoF
= 3.2502E-15
R^2
= 0.99963
y0
A1
t1
-6
-6
-4.0x10
-500
B
ExpDec1 fit of f42_B
###
###
###
###
###
###
###
###
###
Data: f42_B
###
Model: ExpDec1
Spins preserved!
0.0
-2.0x10
-3
0
500
1000 1500 2000 2500 3000 3500
Delay (ns)
start of pump
-1.789E-6
0.00005
414.40633
±1.5063E-8
±6.0594E-7
±3.0797
J.S. Colton, Universal scheme for opt.-detected T1 measurements
100 scans later (summary of data)
GaAs: n = 3E14 cm
700
1.5 K
600
-3
real?
Lifetimes (ns)
500
gets
very
short
expected
20+ ms
400
300
200
100
real?
0
0
1
2
3
4
5
Magnetic Field (T)
6
7
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Lockin Signal (V)
What we saw: 20 ns probe pulse, low field
start of pump
4.0x10
-6
3.0x10
-6
2.0x10
-6
GaAs: n = 3E14 cm
0.6 T, 1.5 K
-3
16 Mar 2011 - file 21,22
overall period = 600 ns
pump: 120 ns
probe: 20 ns
-6
1.0x10
0.0
-1.0x10
-6
-2.0x10
-6
-3.0x10
-6
-4.0x10
-6
-5.0x10
-6
-6.0x10
-6
-7.0x10
-6
f21and22ave
ExpDec1 fit of f22_f2
file 21,22
overall period = 600 ns = 1667 kH
lockin tau=1s; 3 sec per point
probe: 20 ns, 1.4mW
pump: 120 ns, PEM
delay 185 ns = 785 mod
decay time: 41 ns
probe
“leaving”
pump
0
100
180 phase change
probe “entering” pump
200
300
400
500
Probe delay (ns)
end of pump
600
Data: f22_f21and22ave
Model: ExpDec1
Equation: y = A1*exp(-x/t1) + y0
Weighting:
y
No weighting
Chi^2/DoF
= 3.9898E-15
R^2
= 0.98803
y0
A1
t1
-4.6075E-7
0.00252
35.78558
±1.4637
±0.0007
±1.5166
J.S. Colton, Universal scheme for opt.-detected T1 measurements
Conclusions
• Successful demonstration of new technique
– Should work with any material for which Kerr rotation
can be detected
– …which is a lot!
• Unexpected results for 3E14 cm-3 sample.
– Why is lifetime so low?
– Odd phase behavior seen with very short probe
• Plans:
– Revisit high field values… are features real?
– Other temperatures
– Other samples
– Implement EOM (Pockel’s Cell) for longer T1’s