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J.S. Colton, ODMR studies of n-GaAs
Optically-detected magnetic resonance studies of n-GaAs
Talk for APS March Meeting, Mar 20, 2009
John S. Colton, Brigham Young UniversityUndergraduate students: Benjamin Heaton
Michael JohnsonDaniel JensonMitch JonesSteve Brown
Samples provided by Allan Bracker, Naval Research LaboratoryBerry Jonker & Aubrey Hanbriki, also NRL
Funding:National Science Foundation
J.S. Colton, ODMR studies of n-GaAs
Outline
• Samples• Electron spins in n-GaAs: some basics• ODMR
– Resonant microwave cavity– Kerr rotation detection
• Optical power dependence• Pulsed microwaves and light–success and failure• Spin LED sample (planned)• Conclusions
J.S. Colton, ODMR studies of n-GaAs
AlGaAs barrier
1 m n-type GaAs
AlGaAs barrier
GaAs substrate
Lightly-doped n-GaAs
Electrons confined to donors—spin similarities with QDs
Free exciton Donor-bound exciton(higher energy)
3E14 cm-3 “bulk” sample 3E10 cm-2 quantum well, 14 nm
AlGaAs barrier
14 nm GaAs (mod.doped)
AlGaAs barrier
GaAs substrate
(other narrower wells on top)
exciton (higher energy)
negative trion
J.S. Colton, ODMR studies of n-GaAs
Dzhioev et al., Phys Rev B (2002)
T2* spin lifetimes of ~ 1-200 ns
(inhomogeneous dephasing)
Hanle effect measurements T1 measurements: 3 data points for bulk
• 3E15 cm-3 sample (Colton et al., 2004)
– T1 up to 1.4 s (1.5K, 5T)
• 1E15 cm-3 sample (Colton et al., 2007)
– T1 up to 19 s (1.5K, 3-7T)
• 5E13 cm-3 sample (Fu et al., 2006)
– T1 up to 3 ms (1.5K, 2-4T)
(Agrees with time-resolved Faraday/Kerr,magnetic resonance, etc.)
T2: thought to be T1
• Microseconds in QDs– Gated GaAs (Harvard, Delft)
– Self-assembled InGaAs (Dortmund)
Beff
Why is T2* shorter? Random nuclear spins
SIAH
J.S. Colton, ODMR studies of n-GaAs
8.5 – 12 GHz Microwave Resonant Cavity
Different “dielectric resonators”:• 5 possible resonant frequencies• Q-factors (without sample): 2000-5000
for more information see Colton & Wienkes, Review of
Scientific Instruments, 2009
Goals:• Use microwaves to measure and
manipulate spin• Develop techniques for seeing
coherent oscillations (Rabi) and, e.g., measuring T2
J.S. Colton, ODMR studies of n-GaAs
Kerr rotation optical detection
Sample in cryostat
Difference signal
Computer
data
control
Microwave source/amplifier
Resonantcavity
Microwave resonance affectsspin polarization
horiz
Linearly polarizedprobelaser
Polarizingbeam splitter
Balanced detector
vert
PIN diode switch
reference
Lockin amplifier
Pulse sequencegenerator
control
proportional to polarization angle proportional to spin polarization
J.S. Colton, ODMR studies of n-GaAs
Various optical powers
What’s going on?• Electron spins polarize the nuclei (when taken out of equilibrium)• Nuclear spins produce Beff
“Probe” is affecting system
Typical ODMR peak• QW sample• width: T2
* 10-15 ns
J.S. Colton, ODMR studies of n-GaAs
Microwave pulses needed to (hopefully) control spins
Problems: (1) Lockin response limited to 100 kHz (10 ms) (2) Need to use as little laser power as possible
Solutions: • use boxcar integrators gated signal detection • use pulsed light in addition to pulsed microwaves
Signal = BC1 – BC2
Later: BC2 not always needed
J.S. Colton, ODMR studies of n-GaAs
Pulsed Microwaves, cw light
Boxcar can be as good as lockin possibly better
Allows for very short pulse lengths, little loss of S/N
Still see ODMR peaks down to 10 ns gates!
J.S. Colton, ODMR studies of n-GaAs
Pulsed Light (pulsed microwaves, too)
• Why the difficulty?– Our current idea: the vast difference in signal between light
on/light off is “leaking through” the boxcar somehow– Limited success came only when light pulses long enough that
we could filter out that component of the signal and leave behind the faster response to microwaves
• Requiring that long of light pulses ruins the major benefit• Giving up nice idea…for now
J.S. Colton, ODMR studies of n-GaAs
Spin LED
• Iron spin contact– Spin polarized electrons into 10 nm QW
• Substantial optical/spin polarization when B 2 T• Eliminates probe beam altogether!• Also: doesn’t rely on low T to initialize spin• Experiments planned for immediate future
Image from Jonker, Proc IEEE 2003
J.S. Colton, ODMR studies of n-GaAs
Conclusions/What’s next?
• Much successful ODMR in GaAs, 8.5-12 GHz, using: – Resonant cavity– Kerr rotation detection
• Looking for coherent oscillations (next talk)• Dealing with strong nuclear effects (next talk)
– May revisit gating laser on/off• Other samples
– Spin LED (via PL polarization)– Narrower quantum wells– Self-assembled quantum dots (InAs)
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