1. 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 University Undergraduate students: Benjamin Heaton Michael Johnson Daniel Jenson Mitch Jones Steve Brown Samples provided by Allan Bracker, Naval Research Laboratory Berry Jonker & Aubrey Hanbriki, also NRL Funding: National Science Foundation

2. 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

3. 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

4. J.S. Colton, ODMR studies of n-GaAs Dzhioev et al., Phys Rev B (2002) T 2 * spin lifetimes of ~ 1-200 ns (inhomogeneous dephasing) Hanle effect measurements T 1 measurements: 3 data points for bulk 3E15 cm -3 sample (Colton et al., 2004) –T 1 up to 1.4  s (1.5K, 5T) 1E15 cm -3 sample (Colton et al., 2007) –T 1 up to 19  s (1.5K, 3-7T) 5E13 cm -3 sample (Fu et al., 2006) –T 1  up to 3 ms (1.5K, 2-4T) (Agrees with time-resolved Faraday/Kerr, magnetic resonance, etc.) T 2 : thought to be  T 1 Microseconds in QDs –Gated GaAs (Harvard, Delft) –Self-assembled InGaAs (Dortmund) B eff Why is T 2 * shorter? Random nuclear spins

5. 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 T 2

6. J.S. Colton, ODMR studies of n-GaAs Kerr rotation optical detection Sample in cryostat Difference signal Computer data control Microwave source/amplifier Resonant cavity Microwave resonance affects spin polarization horiz Linearly polarized 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

7. 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 B eff “Probe” is affecting system Typical ODMR peak QW sample width: T 2 *  10-15 ns

8. 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

9. 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!

10. 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

11. 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

12. 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)