universal scheme for optically- detected t 1 measurements (…and application to an n = 3e14 cm...

J.S. Colton, Universal scheme for opt.-detected T1 measurements

Universal scheme for optically-detected T1 measurements

(…and application to an n = 3E14 cm-3 GaAs sample)

Talk for APS March MeetingMar 22, 2011

John S. ColtonBrigham Young University

Funding acknowledgement:NASA EPSCoR program

Students: (grad) Tyler Park (undergrads) Ken Clark David Meyer

Daniel Craft Dallas SmithJane Cutler Scott Thalman


Bloch Sphere• Spin:– Can visualize direction via

“Block Sphere”– Spin up / spin down energy

splitting: E = gB

– T1: longitudinal lifetime; transition time from spin up to spin down

Spin up

Image from Wikipedia

Magnetic field


Motivation 1: Materials Characterization

• Which materials are best?


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.


Motivation 3: Three particular samples

2006: 5E13 cm-3

2007: 1E15 cm-3

2004: 3E15 cm-3

3E14 cm-3 ?

Colton et al., PRB 2004

Colton et al., PRB 2007

Fu et al., PRB 2006(Stanford)


How to measure T1?

Simplest version: Like “Time Resolved Faraday/Kerr Rotation”

Crooker et al., Phys Rev B 1997Kikkawa & Awschalom, Phys Rev B 1998

Problem!!

(but use longitudinal field, of course)


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


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

refe

renc

e

Lockin amplifier

signal

2-channelpulse gen.

field


Timing Diagram

(~10 periods)

Pump:

Probe:

scan relative delay

PEM: RCP pump RCP pumpLCP pump

Spin polarization (expected):

12 s

~ 1 s


What we saw: 0T

start of pump end of pump

Spin polarization stops as soon as pump stops. No decay!T1 << 20 ns

As expected…T2

* = 5 nsfrom resonance experiments


What we saw: 1.5 T

start of pump

end of pump

Signal exists well after pump stops.

Spins preserved!

Exponential decay!


100 scans later (summary of data)

expected 20+ s

real?

gets very short

real?


What we saw: 20 ns probe pulse, low field

start of pump

end of pumpprobe “entering” pump

180 phase change

probe “leaving” pump


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

universal scheme for optically- detected t 1 measurements (…and application to an n = 3e14 cm...

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