J.S. Colton, ODMR of self-assembled InAs QDs

Optically-Detected Electron Spin Resonance of

Self-Assembled InAs Quantum Dots

Talk for APS March Meeting, Mar 15, 2010

John S. Colton, Brigham Young UniversityUndergraduate students: Mitch Jones, Steve Brown,

Dallas Smith, Scott Thalman, and Ken Clark

Samples provided by Allan Bracker, Naval Research Laboratory

(ODESR = ODMR, “optically-detected magnetic resonance”)

J.S. Colton, ODMR of self-assembled InAs QDs

Two things we easily learn from ODMR:• g-factor via peak position

– hf = gBBresonant

• T2* spin lifetime (“inhomogeneous

dephasing time”) via peak width– (T2

*)-1 = gBBhalfwidth

• |g| = 0.349, T2* 9.4 ns

14 nm GaAs quantum well

Energy

B

+½

–½

E = g B B

Spin Resonance: Zeeman effect

J.S. Colton, ODMR of self-assembled InAs QDs

Compare to time-resolved Faraday/Kerr rotation:

• g-factor through frequency of oscillation– hf = gBB

• T2* through decay of oscillation

– e-t/T2*

Kikkawa & Awschalom, 1997

Two things we easily learn from ODMR:• g-factor via peak position

• T2* spin lifetime via peak width

Bloch Sphere

J.S. Colton, ODMR of self-assembled InAs QDs

ODMR with Kerr rotation detection

Sample in cryostat/superconductingmagnet

Difference signal

Computer

data

control

Microwave source/amplifier

Resonantcavity

Microwave resonance affectsspin polarization

horiz

Linearly polarizedcw probelaser

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 of self-assembled InAs QDs

Quantum Well ODMR

What’s going on?• Electron spins polarize the nuclei

(when taken out of equilibrium)• Nuclear spins produce Beff

Various optical powers

Heaton, et al., Solid State Comm. 150 (2010), “Nuclear effects in Kerr rotation-detected magnetic resonance of electrons in GaAs”

Beff

Well-known effect. Shortens T2

*, for example.

SIAH

Shifting and Broadening

J.S. Colton, ODMR of self-assembled InAs QDs

Quantum well, cont.: Wavelength Dependence

Idea: select out particular QDs via wavelength

Kerr.Rot. (non-resonant) Kerr.Rot. ODMR

Exciton Trion

Tune laser to here:

J.S. Colton, ODMR of self-assembled InAs QDs

Sample: Self-assembled InAs QDs (A. Bracker, NRL)

• “Cap-flush” method: – In layer (SAQDs)– GaAs partial layer– In flush– GaAs continued growth

• Doping: 1.01010 cm-2

– Approx. 1 electron/dot

20

Figure from Kennedy, et al.Proc. SPIE, Vol. 6903 (2008)

J.S. Colton, ODMR of self-assembled InAs QDs

Some observations:• Peaks always start about 1.6-1.7 T• Peak widths always about the same

– No finer structures • Peak positions never reproducible• Higher microwave/laser powers

– more peaks, higher field peaks

Laser here

J.S. Colton, ODMR of self-assembled InAs QDs

• Peak position: |g//| = 0.485 • Peak width: T2

* = 2.9 ns

• Frequency: |g| = 0.43

• Decay at 2T: T2* = 0.4 ns

• Faster decay at high fields due to g-factor inhomogenity– In a more heavily doped sample,

g 0.1 (possibly less than that, here)– Pulsed laser (more spectral bandwidth)

From Kennedy, et al. (unpublished)

Compare to time-resolved Kerr.Rot. of this sample:

Weakest microwave power:

J.S. Colton, ODMR of self-assembled InAs QDs

What’s going on here?

• Single peak T2* values of ~3 ns (vs. 0.4 ns) argue that

we are seeing signal from a much more homogeneous subset of QDs, perhaps individual dots– Close to theoretical value expected for isolated QDs

(from nuclear spin fluctuations changing during measurement)

• T2* = 4.6 ns for nearly isolated donors in GaAs

- Colton et al., Solid State Comm (2004)

• Peaks from 1.6 - 2.8 T would correspond to |g| = 0.30 to 0.53.– A bit higher than g = 0.1 from time-resolved Kerr rot.– Are these the true resonant fields? Or shifted due to

Beff (nuclear)?

J.S. Colton, ODMR of self-assembled InAs QDs

The $64,000 questions

• Are we seeing spin resonance of individual QDs at these different fields?– Don’t know– Would be remarkable– If not, each peak is at least from a very

homogeneous group, with essentially the same g-factor

• Could nuclear polarization perhaps explain why peaks are occurring at different fields?– Don’t know– Would require all electrons in group to be shifting

together (peaks are apparently merging together, not broadening)

J.S. Colton, ODMR of self-assembled InAs QDs

Conclusion

• Saw ODMR from doped electrons in an InAs SAQD sample.

• Wavelength selected out a small subset of QDs– Individual QDs?

• May/may not have had nuclear spin effects causing ODMR peaks to shift

• At the very least, it’s interesting!

Where do we go from here?

• More precise optical/microwave power dependence• Simultaneous rf to resonate nuclei• Sample with density gradient?