Universal scheme for optically- detected T 1 measurements (…and application to an n = 3E14 cm -3 GaAs sample)

1. Universal scheme for optically-detected T1 measurements(…and application to an n = 3E14 cm-3 GaAs sample) John S. Colton Brigham Young University Students: (grad)Tyler Park (undergrads) Ken Clark David Meyer Daniel Craft Dallas Smith Jane Cutler Scott Thalman Funding acknowledgement: NASA EPSCoR program Talk for APS March Meeting Mar 22, 2011

2. Bloch Sphere Spin up • Spin: • Can visualize direction via “Block Sphere” • Spin up / spin down energy splitting: DE = gmBB • T1: longitudinal lifetime; transition time from spin up to spin down Magnetic field Image from Wikipedia

3. Motivation 1: Materials Characterization • Which materials are best?

4. Motivation 2: Help understand physics Hayes group, Wash Univ St Louis – Optically-pumped NMR • Mui et al., “Effects of optical absorption on 71Ga optically polarized NMR in semi-insulating GaAs: Measurements and simulations”, Phys Rev B 2007. • Hayes et al., “Optically pumped nuclear magnetic resonance of semiconductors”, J Chem Phys 2008. • Mui et al., “Manifestation of Landau level effects in optically-pumped NMR of semi-insulating GaAs”, Phys Chem Chem Phys, 2009. • Ramaswamy et al., “Optically pumped NMR: Revealing spin-dependent Landau level transitions in GaAs”, Phys Rev B, 2010.

5. Motivation 3: Three particular samples 2004: 3E15 cm-3 2006: 5E13 cm-3 2007: 1E15 cm-3 Colton et al., PRB 2004 Fu et al., PRB 2006 (Stanford) Colton et al., PRB 2007 3E14 cm-3 ?

6. How to measure T1? Simplest version: Like “Time Resolved Faraday/Kerr Rotation” Problem!! Crooker et al., Phys Rev B 1997 Kikkawa & Awschalom, Phys Rev B 1998 (but use longitudinal field, of course)

7. (detector) (excitation) (detector) Pulsed light 2004 & 2007 experiments • Single beam • Electronic pulse generator to modulate • Pump/probe controlled by length of pulse • Spins detected via PL polarization Problem! – for lowest doped sample, probe beam needed to be too weak

8. Experimental Setup 2-channel pulse gen. Circ. polar. pump laser: 781 nm diode (fast mod. input) PEM field magnet/ cryostat Lin. polar. probe laser: 821 nm cw Ti-sapphire AOM sample reference Polarizing beam splitter signal Balanced photodiode detector Lockin amplifier

9. LCP pump RCP pump RCP pump Timing Diagram 12 ms PEM: ~ 1 ms (~10 periods) Pump: Spin polarization (expected): Probe: scan relative delay

10. What we saw: 0T Spin polarization stops as soon as pump stops. No decay! T1 << 20 ns As expected… T2* = 5 ns from resonance experiments end of pump start of pump

11. What we saw: 1.5 T end of pump Signal exists well after pump stops. Spins preserved! Exponential decay! start of pump

12. 100 scans later (summary of data) real? expected 20+ ms gets very short real?

13. What we saw: 20 ns probe pulse, low field probe “leaving” pump start of pump 180 phase change probe “entering” pump end of pump

14. Conclusions • Successful demonstration of new technique • Should work with any material for which Kerr rotation can be detected • …which is a lot! • Unexpected results for 3E14 cm-3 sample. • Why is lifetime so low? • Odd phase behavior seen with very short probe • Plans: • Revisit high field values… are features real? • Other temperatures • Other samples • Implement EOM (Pockel’s Cell) for longer T1’s