Post on 28-Jun-2020
Optical Control in SemiconductorsOptical Control in Semiconductorsfor Spintronicsfor Spintronics
andandQuantum Information ProcessingQuantum Information Processing
Jun Kono
Department of Electrical & Computer Engineering,Rice University
October 8, 2003
Supported by NSF DMR-0325474 (ITR), NSF DMR-0134058 (CAREER), NSF INT-0221704, and DARPAMDA972-00-1-0034 (SpinS)
ECE Affiliates Meeting
October 8, 2003 J. Kono
Our Spin/Q.I.P. TeamOur Spin/Q.I.P. Team
Jigang Wang Ajit Srivastava Xiangfeng Wang Rahul Srivastava Giti Khodaparast
Dave ReitzeChris Stanton Young-Dahl Jho
Lu Sham Hiro Munekata
Univ. of Florida
UCSD Tokyo Inst.of Technology
October 8, 2003 J. Kono
Need for Need for QuantumQuantum Technologies Technologies
ß Miniaturization – approaching a physical limit
ß Quantum effects – statistical, fuzzy, strange – unavoidable
ß Novel quantum technologies being sought ‡ better performanceand new functionality and multi-functionality
ß Spin-based electronics, quantum electronics based on quantumcoherence, interference, and entanglement, … etc.
“Quantum physics holds the keyto the further advance ofcomputing in the postsilicon era.”
- J. Birnbaum and R. S. Williams
“Coherent spin packets may offergenuine quantum devicesthrough their wave-like properties.”
- D. D. Awschalom
October 8, 2003 J. Kono
OutlineOutline
n Semiconductor ‘Spintronics’n Towards Solid-State Realization of Quantum
Information ProcessingOur Approach: Ultrafast Optical ControlOur Recent Discoveries
Ultrafast Photoinduced Softening (UPS) in aFerromagnetic SemiconductorUltrafast Photoinduced Transparency (UPT) using theDynamic Franz-Keldysh Effect (DFKE)
Summary
October 8, 2003 J. Kono
Emerging Technologies forEmerging Technologies forSolid-State Information ProcessingSolid-State Information Processing
Spintronics
ß Use spinsspins in solid statedevices
ß Improve informationprocessing
ß Add new functionalities
Quantum InformationProcessing
ß Devise and implementquantum-coherent
strategies for computationcomputationand communicationcommunication
Use discrete, quantized degrees of freedom in a physicaldevice to perform information processing functions.
October 8, 2003 J. Kono
Magneto-ElectronicsMagneto-Electronics
Magnetic tunneling junction(MTJ) or “spin valve” ‡Nonvolatile MRAM:“Microchips that neverforget ”
S. Parkin (1990)Compatibility with Si andGaAs ‡ next phase:semiconductor spintronics
GMR ‡ read-out heads in hard drives
1st generation spintronic devices based onferromagnetic metals – already in commercial use
October 8, 2003 J. Kono
Recent Discoveries in SemiconductorsRecent Discoveries in Semiconductors
n A room temperature, optically induced, verylong lived quantum coherent spin state insemiconductors that responds at Terahertzwith no dissipation and can be transported bysmall electric fields (UCSB).
n Ferromagnetism in semiconducting GaMnAsat 120K (Japan, Europe, U.S.A.).
DARPA ‘Spins in Semiconductors’Program (2000 – present)
October 8, 2003 J. Kono
Spin-Enhanced and Spin-Spin-Enhanced and Spin-Enabled ElectronicsEnabled Electronics
• Quantum Spin Electronics– Tunneling/transport of quantum confined spin states
– Spin dependent resonant tunneling devices and spin filtering
– Spin FETs (“spin gating”)
– Spin LEDs, electroluminescent devices, and spin lasers
• Coherent Spin Electronics– Optically generated coherent spin states and coherent control of
propagating spin information - optical encoders and decoders
• Quantum Information Processing– Qubits using coherent spin states a|0> + b|1>, a2 + b2 = 1
– Spin based quantum computing, teleportation, code breaking andcryptography
October 8, 2003 J. Kono
Quantum Information ProcessingQuantum Information Processing
- Coherent superposition: 10 ba +=y
‡ Inherent parallelism ‡ can solve problems that arecomputationally too intensive for classical computers
- 2 examples of ‘quantum algorithms’:Shor’s factorization (1994)Grover’s search (1997)
- Quantum entanglement, EPR pair, quantum teleportation
- Physical Implementations:Trapped ions (1995)Cavity QED (1995)Bulk NMR (1997), …
Proof-of-principledemonstrations, butnot scalable
October 8, 2003 J. Kono
Toward Toward Solid-StateSolid-State Realization of QIP Realization of QIPThe Race is on!The Race is on!
Semiconductors vs. Superconductors
Spin Qubits vs. Charge Qubits
Electrical Manipulation vs. Optical Manipulation
• Intensive search for realistic approaches to building aquantum computer
• Solid-state systems offer a much greater degree ofcontrol over design and fabrication, necessary forconstructing large-scale devices
Cooper Pairs vs. Flux QubitsElectron Spin vs. Nuclear Spins
October 8, 2003 J. Kono
Decoherence Decoherence ProblemProblem
T : decoherence timet : operation timeR = T/t : figure of merit
How can we increase T and/or decrease t?
• Coherent states are very easily damaged by uncontrolledinteractions with the environment – decoherence
• Unavoidable decoherence will cause the quantuminformation to decay ‡ main obstacle
• Decoherence causes a collapse of the superposition stateinto a single eigenstate ‡ loss of parallelism
October 8, 2003 J. Kono
To develop novel ultrafast optical methodsin semiconductors that may find application inspintronics or quantum information sciencethrough coherent light-matter interactionsinvolving ferromagnetism, band structures,lattice vibrations, and excitons
Our GoalOur Goal
e-
October 8, 2003 J. Kono
Ultrafast Ultrafast Photoinduced SofteningPhotoinduced Softening(UPS) in InMnAs(UPS) in InMnAs
J. Wang, G. A. Khodaparast & J. Kono
Rice University
T. Slupinski, A. Oiwa & H. Munekata
Tokyo Institute of Technology
October 8, 2003 J. Kono
• Ultrafast demagnetization (~ hundred fs)and slow recovery
• Possible application to ultrafast magneto-optical recording
E. Beaurepaire et al., PRL 76, 4250 (1996); PRB 58, 12134 (1998).
Ni CoPt3
Ultrafast Optics in Ferromagnetic Ultrafast Optics in Ferromagnetic MetalsMetals
October 8, 2003 J. Kono
Low-temperature MBEgrown III1-xMnxV:
• InMnAs: Tc < 60 K
• GaMnAs: Tc < 170 K
Mn ions (Mn2+) = acceptors& local magnetic moments(3d5, S = 5/2)
Mn-Mn exchange:hole mediated
Carrier density tuning
External control of ferromagnetism
III-V Ferromagnetic SemiconductorsIII-V Ferromagnetic Semiconductors
October 8, 2003 J. KonoAdvantages ofAdvantages of
Ferromagnetic SemiconductorsFerromagnetic Semiconductorsover over Ferromagnetic MetalsFerromagnetic Metals
• Ultrafast pump ‡ primarily increasescarrier density (rather than carriertemperature)
• Created carriers interact with Mn ions ‡enhance Mn-Mn exchange interaction
• Circular-polarized pump ‡ spin polarizedcarriers
• Low-T MBE growth ‡ ultrashort lifetimes
October 8, 2003 J. Kono
Ultrafast Carrier Ultrafast Carrier DDynamicsynamics in InMnAs in InMnAs
(1) carriertrapping
(~ 2 ps)
(2) Carrierrecombinationof trappedcarriers
-3
-2
-1
0
1
2
?R
/R (
%)
200150100500
Time Delay (ps)
Pump = 1260 nmProbe = 775 nm
(1)
(2)
October 8, 2003 J. Kono
-6x10-5
-4
-2
0
2
4
6
MC
D S
igna
l (V
)
-0.4 -0.2 0.0 0.2 0.4
Magnetic field (T)
55 K 45 K 35 K 20 K 10 K
In0.91Mn0.09As(25nm)/GaSb(820nm) on GaAs(100)Tc = 55 K
Magneto-optical Kerr Effect (MOKE)Magneto-optical Kerr Effect (MOKE)
October 8, 2003 J. Kono
l Selective pumpingof InMnAs by fs MIRpulses
l Photogenerated,transient spin-polarized carriers
l Probe time-dependentferromagnetism
Two-Color Pump & ProbeTwo-Color Pump & Probe
InMnAs GaSb
MIRMIRpumppump
~150 fs~150 fs
NIR NIR probeprobe
holes
V
C
m
October 8, 2003 J. Kono
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
MO
KE
Sig
nal (
deg.
)
6040200Delay Time (ps)
_K
_K
B (T)
T = 16 KTc = 55 KH = 0 OePump: 2 mmProbe: 775 nm
0.4
0.2
0.0
-0.2
-0.4-0.2 -0.1 0.0 0.1 0.2
0.4
0.2
0.0
-0.2
-0.4
-0.2 -0.1 0.0 0.1 0.2
J. Wang et al., J. Supercond. 16, 373 (2003); Physica E, in press; cond-mat/0305017
Ultrafast Photoinduced MOKEUltrafast Photoinduced MOKE
October 8, 2003 J. Kono
t ~ -4 ps t ~ 0 ps t ~ 11 ps
n Loop shrinks horizontally and then comes back!
n First demonstration of ultrafast optical manipulationof coercivity
-4
-2
0
2
4
MOK
E Si
gnal
(10-6
V)
-0.2 -0.1 0.0 0.1 0.2Magnetic Field (T)
-78
-76
-74
-72
-70
-68
-66
MOK
E Si
gnal
(10-6
V)
-0.2 -0.1 0.0 0.1 0.2Magnetic Field (T)
-4
-2
0
2
4
MOK
E Si
gnal
(10-6
V)
-0.2 -0.1 0.0 0.1 0.2Magnetic Field (T)
J. Wang et al., J. Supercond. 16, 373 (2003); Physica E, in press; cond-mat/0305017
Ultrafast Photoinduced SofteningUltrafast Photoinduced Softening
October 8, 2003 J. Kono
NonthermalNonthermal MagnetoopticalMagnetooptical Recording Recording
Hc0
B
M
Hcmod < Hex < Hc
0
B
M~100 fs
Hcmod
t = 0
t > 0
Spin flipping ‡ ultrafast information recording
Dynamic Franz-Keldysh EffectDynamic Franz-Keldysh Effect(DFKE) in GaAs:(DFKE) in GaAs:
Optical absorption in AC-driven solidsOptical absorption in AC-driven solids
A. Srivastava, R. Srivastava, J. Wang, and J. Kono
Department of Electrical & Computer Engineering,Rice University, U.S.A.
1. Predictions
2. Observations
3. Interpretation
4. Significance
MIRpump
c
v
NIRprobe
Ultrafast bandgap engineering
October 8, 2003 J. Kono
Wiggling Wiggling EElectronlectron in a in a LLaser aser FFieldield
Classical energy Classical energy UUpp
e-
potential iveponderomot : 42
11K.E.
2
20
2
0
2p
TU
m
Eedtmv
T≡=˜
¯
ˆÁË
Ê= Ú w
( )
( ) tmeE
tv
tEtE
ww
w
sin
cos
0
0
-=fi
=
– wiggly motion
Quantum energyQuantum energy wh
transitionpU<<wh
pU>>wh
: DC-FKE
: Multiphoton
October 8, 2003 J. Kono
Time-Periodic Potentialvs.
Space-Periodic Potential
a0
Relevant length scale:
Bloch Electron Bloch Electron in a in a LLaser aser FFieldield
a
whªpU
20
wm
eEa =
ww ªB 0aa ª
fl Oscillation amplitude
)/( 00 hEeaB =w
October 8, 2003 J. Kono
How to realize How to realize aa ~ ~ aa00
022//aaEIgwl=µµ
0.1 1 10 100
1E7
1E8
1E9
1E10
1E11
1E12
1E13
1E14
1E15
m* = 0.07m0
Req
uire
d In
tens
ity (
W/c
m2
)
Wavelength (mm)
1=g
Ti:S
apph
ireN
d:Y
AG
OPA
FEL
22334pUeEImlww=µhh
easier at longerwavelengths
Intense MIRfs pulses
October 8, 2003 J. Kono
-1.0 -0.5 0.0 0.5 1.00.00.20.40.60.81.01.21.41.6
with pump without pump
Tra
nsi
tio
n r
ate
(a.u
)
hW - Eg (in units of hw)
Blue shiftBlue shift of band edge ‡transparency
Exponentialtail
Dynamic Franz-Keldysh Dynamic Franz-Keldysh EEffectffectY. Yacoby, PR 169, 610 (1968).
0aa ª
October 8, 2003 J. Kono
Photoinduced Photoinduced TTransparencyransparency~40%~40% photoinduced photoinduced transparency transparency
Band edge of GaAsAbsorption below band edge
GaAs film
Room temp.
Pump energy = 138 meV
October 8, 2003 J. Kono
DFKE Simulation for a GaAs FilmDFKE Simulation for a GaAs Film1.0
0.8
0.6
0.4
0.2
0.0
Tra
nsm
ittan
ce
no pump pumped
-0.6
-0.4
-0.2
0.0
0.2
T ?
T0
1.551.501.451.401.351.30
Energy (eV)
• Undoped GaAsfilm (2.7 mm thick)
• Multiple-reflectionincluded
• lpump = 9 mm (138meV)
• Ipump = 1010 W/cm2
ll InducedInducedtransparencytransparency
October 8, 2003 J. Kono
DFKE: DFKE: ConclusionsConclusions
n Intense MIR laser fields can coherentlymodify electronic states in solids throughnon-resonant pumping
n No sample damage, no real carriers
n Ultrafast transmission quenching belowband edge observed
n First observation of induced transparencyabove the band edge
n Main features of observations qualitativelyagree with theory
October 8, 2003 J. Kono
Accomplished:
• Photogenerated transient carriers ‡ modify magneticproperties in InMnAs ‡ first demonstration of ultrafastsoftening: Hc (coercivity) decreases (‘hard’ ‡ ‘soft’)
• Intense and coherent midinfrared radiation modified bandstructure through DFKE ‡ first observation of ultrafastphotoinduced transparency
SummarySummaryUltrafast Optical Control in SemiconductorsUltrafast Optical Control in Semiconductors
In Progress:
• Demonstration of ultrafast photoinduced magnetizationreversal
• Transient modifications of Tc and photoinduced transientpara- to ferromagnetic transition
October 8, 2003 J. Kono
Realization of Spin-Based DevicesRealization of Spin-Based Devices
Technical issuesTechnical issues
• How strongly can one create carriers of agiven spin?
• How long can one sustain the spinpolarization?
• How can one modulate or control the spin?
• How sensitively can one detect the spin?
October 8, 2003 J. Kono
Decoherence Decoherence ProblemProblem
T : coherence timet : operation timeR = T/t : figure of merit
Factoring a 4-bit number using Shor’s algorithmRequires ~2 x 104 gate operations on 20 qubits ‡R has to be > 4 x 105
• Coherent states are very easily damaged by uncontrolledinteractions with the environment – decoherence
• Unavoidable decoherence will cause the quantuminformation to decay, thus inducing errors in thecomputation
• Decoherence occurs rapidly in complex big systems, whichis why we never observe macroscopic superpositions
October 8, 2003 J. Kono
Ultrafast Optics in Ferromagnetic Ultrafast Optics in Ferromagnetic MetalsMetals
Ni and Co: E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996). M. Aeschlimann et al., Phys. Rev. Lett. 79, 5158 (1997). A. Scholl et al., Phys. Rev. Lett. 79, 5146 (1997). J. Hohlfeld et al., Phys. Rev. Lett. 78, 4861 (1997). J. Güdde et al., Phys. Rev. B 59, R6608 (1999). B. Koopmans et al., Phys. Rev. Lett. 85, 844 (2000).
CoPt3: G. Ju et al., Phys. Rev. B 57, R700 (1998). E. Beaurepaire et al., Phys. Rev. B 58, 12134 (1998). L. Guidoni et al., Phys. Rev. Lett. 89, 017401 (2002).
Ultrafast Ultrafast ppump ump ‡‡ Electronic heating Electronic heating‡‡ Microscopic understanding elusive Microscopic understanding elusive
October 8, 2003 J. Kono
S. Koshihara et al., PRL 78, 4617 (1997); A. Oiwa et al., APL 78, 518 (2001).
CW Optical Control of FerromagnetismCW Optical Control of Ferromagnetism
n Light-induced ferromagnetism
n Light-induced coercivity decrease
n Persistent photoeffect
October 8, 2003 J. Kono
InMnAs GaSb
~0.8 eV
~0.4 eV
• Electron-holeseparation
• Hole densityincreasespersistently
• Slow
• Not reversible
Persistent Persistent PhotoeffectPhotoeffect
m
October 8, 2003 J. Kono
= tunable source of intense MIR pulses
MIR3 mm to 18 mmup to ~ 3 mJ
140 fs, 1 mJ
775 nm Ti:SRegenerative
Amplifier
up to 100 mJ
signal + idler signal1.1 mm to 1.6 mm
idler1.6 mm to 3 mm
RegenRegen + OPA + DFG + OPA + DFG
Optical ParametricAmplifier
DifferenceFrequencyGenerator
PRB 65, 121307(R) (2002)PRL 86, 3292 (2001) PRL 85, 3293 (2000)
October 8, 2003 J. Kono
Time Delay
MO
KE
Sig
nal q
t1t2t3
MO
KE
Sig
nal
Time Delay
DR
/R
H
Experimental Experimental SSetupetup
October 8, 2003 J. Kono
VE
CE
FE
InMnAsx < 9.5%
GaSb
~0.8 eV
~0.4 eV
holes
9-25 nm l Type-II broken gapheterostructures withAlGaSb
l First ferromagnetic p-type films, Tc = 7 K(1991), Tc = 35 K in p-InMnAs/GaSb (1993)
l Light-inducedferromagnetism (1997)
l Electrical tuning offerromagnetism (2000)
InMnAs-Based HeterostructuresInMnAs-Based Heterostructures
October 8, 2003 J. Kono
Carrier Carrier DDynamicsynamics in InMnAs in InMnAs
(1) carrier trapping
(~ 2 ps)
(2) recombinationof trappedcarriers
Pump 2Pump 2 mmm, 0.5m, 0.5 mW mWProbe 775 nmProbe 775 nm
V.B
C.B
(1)(2)
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
DR
/R (
%)
20151050
Time Delay (ps)
(1)
(2)
October 8, 2003 J. Kono
-8x10-5
-6
-4
-2
0
2
4
MO
KE
Sig
na
l (V
)
840-4
Time delay (ps)
-1
0
1
x10-5
-0.2 -0.1 0.0 0.1 0.2
Magnetic Field (T)
8
7
x10-5
-8
-7
x10-5
s+ pumping
s- pumping
20K
s+ pumping
s- pumping
t = 0
t = 0
t = -4 ps
Pump-Polarization DependencePump-Polarization Dependence
October 8, 2003 J. Kono
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Pho
to-in
duce
d M
OK
E S
igna
l (V
)
6420-2Time Delay (ps)
s+ pumping
s- pumping
122 K
MOKE Dynamics above MOKE Dynamics above TTcc
‡ Not related to ferromagnetism‡ This is due to carrier spin
October 8, 2003 J. Kono
What determines coercivity? -- Anisotropy & Exchange
CWCW
-8
-7
MO
KE
Sig
nal
0.20.10.0-0.1-0.2
Magnetic Field (T)
-1
0
1
(10 -
5 V
)
UltrafastUltrafast
• Anisotropy (K) does NOTchange with carrier density
• Exchange increases
Decreased domain wall energy‡ Smaller coercivity
22sincosiiijijEKJSqf=-Â
Origin of Ultrafast SofteningOrigin of Ultrafast Softening
October 8, 2003 J. Kono
Ultrafast Photoinduced SofteningUltrafast Photoinduced Softening
October 8, 2003 J. Kono
n Curie temperature (Tc) of InMnAs below roomtemperature ‡ explore new materials (e.g.,GaMnN)
n Small band gap of InMnAs requires powerfulmid-infrared (MIR) laser pulses ‡ requiresdevelopment of compact MIR source or largeband gap materials (e.g., GaMnN)
n Kerr rotation angle of InMnAs small ‡ newmaterials or sophisticated opticalarrangements necessary
LimitationsLimitations
October 8, 2003 J. Kono
E-field-induced changes in optical absorption near the E-field-induced changes in optical absorption near the bandedgebandedge
rEeÇrr
⋅-='Additional term in the Hamiltonian
Exponential for positive arguments
Oscillations fornegativearguments
Exponential tail below bandgap
Oscillationsabovebandgap
Franz-Keldysh Franz-Keldysh EEffectffect
absorption
wavefunction
dc fielddc field
October 8, 2003 J. Kono
Unusual Power DependenceUnusual Power Dependence
Dec
reas
ing
inte
nsi
ty
Incr
easi
ng
eff
ect!
!
Below band gap absorption in bulk GaAs at 300 KBelow band gap absorption in bulk GaAs at 300 K
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Tra
nsm
issi
on
1.3901.3801.3701.360
Photon Energy (eV)
4.7 mm 6 mm 8 mm 9 mm
October 8, 2003 J. Kono
Extent of Induced AbsorptionExtent of Induced AbsorptionPolycrystalline ZnSe, 3.5 mm driving field, g ~ 1
0.5
0.4
0.3
0.2
0.1
0.0
tran
smis
sion
2.62.42.22.01.8photon energy (eV)
Induced absorption extends almost 1 eV below Eg!
October 8, 2003 J. Kono
DFKE is adramatic effect
Other Strong-Field-Induced EffectsOther Strong-Field-Induced Effectsin Semiconductorsin Semiconductors
DC Franz-Keldysh effect AC Stark effect
shift ~ only a few meV at 100 kV/cm !
B.G. Yacobi et al.A. Mysyrowicz et al.
October 8, 2003 J. Kono
Based on: Y. Yacoby, Phys. Rev. 169, 610 (1967).
DFKE Simulation for GaAsDFKE Simulation for GaAs
2.5
2.0
1.5
1.0
0.5
0.0
?
(106
cm-1
)
1.601.551.501.451.401.351.30
Energy (eV)
no pump pumped
GaAs? pump = 9 ?m (h? = 138 meV)
Ipump = 1010 W/cm2 (Up~ h?)
Blue shift of band edge ‡Inducedtransparencytransparency
Inducedabsorption
October 8, 2003 J. Kono
Pump Pump WWavelength avelength DDependenceependence
Band edge of GaAs
GaAs film
Room temp.
October 8, 2003 J. Kono
ExperimentsExperiments
• A. H. Chin, J. M. Bakker, and J. Kono, Phys. Rev. Lett. 85, 3293 (2000).• A. H. Chin, O. G. Calderón, and J. Kono, Phys. Rev. Lett. 86, 3292 (2001).• O. G. Calderón, A. H. Chin, and J. Kono, Phys. Rev. A 63, 053807 (2001).• M. A. Zudov, J. Kono, A. P. Mitchell, and A. H. Chin, Phys. Rev. B 64,
121204(R), (2001).• A. H. Chin, J. Kono, and G. S. Solomon, Phys. Rev. B 65, 121307(R),
(2002).• A. Srivastava and J. Kono, in: Quantum Electronics and Laser Science
Conference, OSA Technical Digest (Optical Society of America,Washington DC, 2003), QFD2.