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

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Bloch Sphere Spin: –Can visualize direction via “Block Sphere” –Spin up / spin down energy splitting:  E = g   B –T 1 : longitudinal lifetime; transition time from spin up to spin down Spin up Image from Wikipedia Magnetic field

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Motivation 1: Materials Characterization Which materials are best?

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Motivation 2: Help understand physics Hayes group, Wash Univ St Louis – Optically-pumped NMR Mui et al., “Effects of optical absorption on 71 Ga optically polarized NMR in semi-insulating GaAs: Measurements and simulations”, Phys Rev B Hayes et al., “Optically pumped nuclear magnetic resonance of semiconductors”, J Chem Phys Mui et al., “Manifestation of Landau level effects in optically- pumped NMR of semi-insulating GaAs”, Phys Chem Chem Phys, Ramaswamy et al., “Optically pumped NMR: Revealing spin- dependent Landau level transitions in GaAs”, Phys Rev B, 2010.

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Motivation 3: Three particular samples 2006: 5E13 cm : 1E15 cm : 3E15 cm -3 3E14 cm -3 ? Colton et al., PRB 2004 Colton et al., PRB 2007 Fu et al., PRB 2006 (Stanford)

J.S. Colton, Universal scheme for opt.-detected T 1 measurements How to measure T 1 ? Simplest version: Like “Time Resolved Faraday/Kerr Rotation” Crooker et al., Phys Rev B 1997 Kikkawa & Awschalom, Phys Rev B 1998 Problem!! (but use longitudinal field, of course)

J.S. Colton, Universal scheme for opt.-detected T 1 measurements 2004 & 2007 experiments Single beam Electronic pulse generator to modulate Pump/probe controlled by length of pulse Spins detected via PL polarization (detector) (excitation) (detector) Pulsed light Problem! – for lowest doped sample, probe beam needed to be too weak

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Experimental Setup magnet/ cryostat sample Circ. polar. pump laser: 781 nm diode (fast mod. input) Lin. polar. probe laser: 821 nm cw Ti-sapphire AOM Polarizing beam splitter PEM Balanced photodiode detector reference Lockin amplifier signal 2-channel pulse gen. field

J.S. Colton, Universal scheme for opt.-detected T 1 measurements Timing Diagram (~10 periods) Pump: Probe: scan relative delay PEM: RCP pump LCP pump Spin polarization (expected): 12  s ~ 1  s

J.S. Colton, Universal scheme for opt.-detected T 1 measurements What we saw: 0T start of pump end of pump Spin polarization stops as soon as pump stops. No decay! T 1 << 20 ns As expected… T 2 * = 5 ns from resonance experiments

J.S. Colton, Universal scheme for opt.-detected T 1 measurements What we saw: 1.5 T start of pump end of pump Signal exists well after pump stops. Spins preserved! Exponential decay!

J.S. Colton, Universal scheme for opt.-detected T 1 measurements 100 scans later (summary of data) expected 20+  s real? gets very short real?

J.S. Colton, Universal scheme for opt.-detected T 1 measurements What we saw: 20 ns probe pulse, low field start of pump end of pump probe “entering” pump 180  phase change probe “leaving” pump

J.S. Colton, Universal scheme for opt.-detected T 1 measurements 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 T 1 ’s