XIIth International Workshop on Polarized Sources, Targets & Polarimetry Highly Effective Polarized Electron Sources Based on Strained Semiconductor Superlattice with Distributed Bragg Reflector Leonid G. Gerchikov Laboratory of Spin-Polarized Electron Spectroscopy Department of Experimental Physics State Polytechnic University St.-Petersburg, Russia Highly Effective Polarized Electron Sources Based on Strained Semiconductor Superlattice with Distributed Bragg Reflector DBR
30
Embed
XIIth International Workshop on Polarized Sources, Targets Polarimetry Highly Effective Polarized Electron Sources Based on Strained Semiconductor Superlattice.
Introduction –High-Energy spin physics requirements –Photocathodes based on strained semiconductor superlattices –Optical resonator with DBR Design of photocathode Strain-compensated superlattice photocathode with DBR Superlattice with strained QW and DBR Summary & OutlookOUTLINE
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
XIIth International Workshop on Polarized Sources, Targets & Polarimetry
Highly Effective Polarized Electron Sources Based on Strained Semiconductor
Superlattice with Distributed Bragg Reflector
Leonid G. GerchikovLaboratory of Spin-Polarized Electron Spectroscopy
Department of Experimental PhysicsState Polytechnic University
St.-Petersburg, Russia
Highly Effective Polarized Electron Sources Based on Strained Semiconductor
Superlattice with Distributed Bragg Reflector DBR
CollaboratorsCollaboratorsDepartment of Experimental Physics, St.Petersburg State Polytechnic University, Russia, Yurii A. Mamaev, Yurii P.Yashin, Vitaly V. Kuz’michev, Dmitry A. Vasiliev, Leonid G. Gerchikov
Stanford Linear Accelerator Center, Stanford, CA, USA, James E. Clendenin , Takashi Maruyama
A.F. Ioffe Physicotechnical Institute RAS, Russia, Viktor M. Ustinov, Aleksey E. Zhukov, Vladimir S. Mikhrin, Alexey P. Vasiliev
Department of Electronic and Electrical Engineering, University of Sheffield, UK, John S. Roberts
Institute of Nuclear Physics, Mainz University, Mainz, Germany, Kurt Aulenbacher
• Introduction– High-Energy spin physics requirements– Photocathodes based on strained
semiconductor superlattices – Optical resonator with DBR
• Design of photocathode
• Strain-compensated superlattice photocathode with DBR
•Large electronic current requirement•Light energy limitations:•Surface charge saturation•Heating
High QE
High polarization of electron emission from High polarization of electron emission from strained semiconductor SL at the expense of QEstrained semiconductor SL at the expense of QE
Spectra of electron emission: Polarization P and Quantum Efficiency QE
•Polarization is maximal at photoabsorption threshold where QE is small.•Strain relaxation does not allow to produce thick photocathode with high QE.•Rise of the vacuum level increases P and decreases QE
Goal: considerable increase of QE at the main polarization maximum.Method: Resonance enhancement of photoabsorption in SL integrated into Fabry-Perot optical cavity. Photoabsorption in the working layer:L 1, - photoabsorbtion coefficient,L - thickness of SL
Resonant enhancement by factor 2/(1-(RDBRRGaAs) 1/2)2
Resonant enhancement of polarized electron Resonant enhancement of polarized electron emission from strained semiconductor layeremission from strained semiconductor layer
Resonant enhancement of polarized electron Resonant enhancement of polarized electron emission from strained semiconductor layeremission from strained semiconductor layer
J. C. Groebli, D. Oberli, F. Meier, A. Dommann, Yu. Mamaev, A. Subashiev and Yu. Yashin, Phys. Rev. Lett. 74, 2106 (1995).
Optimization of Photocathode structureOptimization of Photocathode structure
Buffer
GaAsSubstrate
DBR
SL
BBR
760 780 800 820 840 860 880 9000.0
0.2
0.4
0.6
0.8
1.0
Ref
lect
ivity
Wavelength, nm
• SL structure: layers composition and thickness are chosen to assure Eg= for P()=Pmax Ehh-lh > 60meV for high polarization Ee1 > 40meV for effective electron transport
• DBR structure: 20x(AlAs(/4)/ (AlxGa1-xAs(/4)) Layer thickness l = /4n() for Bragg reflection x 0.8 for large reflection band width = 2n/n
• Fabry-Perot resonance cavity: BBR + SL + buffer layer Effective thickness = k /2 for QE() = QEmax Effective thickness of BBR+SL /4
Simulation of resonant photoabsorptionSimulation of resonant photoabsorption
• SL’s energy band structure, photoabsorption coefficient, polarization of photoelectrons.
Method: kp – method within 8-band Kane model. A.V. Subashiev, L.G. Gerchikov, and A.I. Ipatov. J. Appl. Phys., 96, 1511
(2004).
• Distribution of electromagnetic field in resonance cavity, reflectivity, QE.
Method: transfer matrixes. M.Born and E.Wolf. Princeples of Optics, Pergamon Press, New York,
1991
• Even small in-plane anisotropy leads to resonant polarization losses. High quality structure of Fabry-Perot cavity is required.
• The optical thickness of Fabry-Perot cavity can not be adjusted after fabrication.
“Resonance Enhancement of Spin-Polarized Electron Emission from Strain Compensated AlInGaAs GaAsP Superlattices” J.S. Roberts, Yu.P. Yashin, Yu. A. Mamaev, L.G.Gerchikov,T. Maruyama, D.-A. Luh, J.E. Clendenin, Proceedings of the 14th international conference “Nanostructures: Physics and Technology”, St.Petersburg, 26-30 June 2006.
ReflectivityReflectivity
Experiment, QT 1890 DBR Theory , QT 1890 DBR
780 800 820 840 860 880 9000.0
0.2
0.4
0.6
0.8
1.0
Ref
lect
ivity
Wavelength, nm
550 600 650 700 750 800 850 900 950
10-5
10-4
10-3
10-2
10-1
100
101
0
20
40
60
80
QE
, %
W avelength, nm
QE-4, SL QT 1890 non DBR QE-2, SL QT 1890 DBR
P-4, SL QT 1890 non DBR P-2, SL QT 1890 DBR
Pol
ariz
atio
n, %SPTU data
Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())
Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())
550 600 650 700 750 800 850 900
10-4
10-3
10-2
10-1
100
101
0
20
40
60
80
W avelength, nm
QE
, %
QE, SL 7-396 DBR QE, SL 7-395 no DBR
P, SL 7-396 DBR P, SL 7-395 no DBR
Pol
ariz
atio
n, %
550 600 650 700 750 800 850 900
10-4
10-3
10-2
10-1
100
101
0
20
40
60
80
W avelength, nm
QE
, %
QE, Experiment SL 7-396 DBR QE, Theory SL 7-396 DBR
P, Experiment SL 7-396 DBR P, Theory SL 7-396 DBR
Pol
ariz
atio
n, %
Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())
Resonant enhancement of QE Resonant enhancement of QE
750 800 850 9000
10
20
30
W avelength, nm
QE
Enc
hanc
emen
t
Experiment SL 7-396 DBR Theory SL 7-396 DBR
Summary & OutlookSummary & Outlook
• We have developed a novel type photocathode based on strain compensated superlattices integrated into a Fabry-Perot optical cavity of high structural quality.
• We demonstrate a tenfold enhancement of quantum efficiency without polarization losses due to the multiple resonance reflection from DBR layer.
• The obtained results demonstrate the advantages of the developed photocathode as a perspective candidate for spin polarized electron sources.
AcknowledgmentsAcknowledgments
This work was supported by • Russian Ministry of Education and
Science under grant N.P. 2.1.1.2215 in the frames of a program “Development of the High School scientific potential”
• Swiss National Science Foundation under grant SNSF IB7420-111116