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)