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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 t 1 measurements and application to an n 3e14 cm 3 gaas sample

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

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

Motivation 1: Materials Characterization

  • Which materials are best?


Motivation 2 help understand physics

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

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 t 1

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)


Universal scheme for optically detected t 1 measurements and application to an n 3e14 cm 3 gaas sample

(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

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


Timing diagram

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

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

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

100 scans later (summary of data)

real?

expected

20+ ms

gets very short

real?


What we saw 20 ns probe pulse low field

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

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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