1. 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 University Undergraduate 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”)

2. J.S. Colton, ODMR of self-assembled InAs QDs Two things we easily learn from ODMR: g-factor via peak position –hf = g  B B resonant T 2 * spin lifetime (“inhomogeneous dephasing time”) via peak width –  (T 2 * ) -1 = g  B B halfwidth |g| = 0.349, T 2 *  9.4 ns 14 nm GaAs quantum well Energy B +½+½ –½  E = g  B B Spin Resonance: Zeeman effect

3. J.S. Colton, ODMR of self-assembled InAs QDs Compare to time-resolved Faraday/Kerr rotation: g-factor through frequency of oscillation –hf = g  B B T 2 * through decay of oscillation –e -t/T2* Kikkawa & Awschalom, 1997 Two things we easily learn from ODMR: g-factor via peak position T 2 * spin lifetime via peak width Bloch Sphere

4. J.S. Colton, ODMR of self-assembled InAs QDs ODMR with Kerr rotation detection Sample in cryostat/ superconducting magnet Difference signal Computer data control Microwave source/amplifier Resonant cavity Microwave resonance affects spin polarization horiz Linearly polarized cw probe laser Polarizing beam splitter Balanced detector vert PIN diode switch reference Lockin amplifier Pulse sequence generator control  proportional to polarization angle  proportional to spin polarization

5. 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 B eff Various optical powers Heaton, et al., Solid State Comm. 150 (2010), “Nuclear effects in Kerr rotation-detected magnetic resonance of electrons in GaAs” B eff Well-known effect. Shortens T 2 *, for example. Shifting and Broadening

6. 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:

7. 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: 1.0  10 10 cm -2 –Approx. 1 electron/dot 20  Figure from Kennedy, et al. Proc. SPIE, Vol. 6903 (2008)

8. 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

9. J.S. Colton, ODMR of self-assembled InAs QDs Peak position: |g // | = 0.485 Peak width: T 2 * = 2.9 ns Frequency: |g  | = 0.43 Decay at 2T: T 2 * = 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:

10. J.S. Colton, ODMR of self-assembled InAs QDs What’s going on here? Single peak T 2 * 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) T 2 * = 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 B eff (nuclear)?

11. 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)

12. 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?