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


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Universal scheme for optically- detected T 1 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

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

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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


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


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

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 ?


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)


(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


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


LCP pump

RCP pump

RCP pump

Timing Diagram

12 ms

PEM:

~ 1 ms

(~10 periods)

Pump:

Spin polarization (expected):

Probe:

scan relative delay


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


What we saw: 1.5 T

end of pump

Signal exists well after pump stops.

Spins preserved!

Exponential decay!

start of pump


100 scans later (summary of data)

real?

expected

20+ ms

gets very short

real?


What we saw: 20 ns probe pulse, low field

probe

“leaving”

pump

start of pump

180 phase change

probe “entering” pump

end of 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


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