1. fundamentals of ultrafast optics and...
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1. Fundamentals of ultrafast optics and lasers
2. Laser-based static spectroscopy
3. Time-resolved spectroscopy
Femtsecond pulse generation: active and passive mode-locking, ultrafast amplifiers
Laser Raman/Raleigh, multi-photon excitation spectroscopy; SWCNs
Ultrafast incoherent & coherent transient, magneto-optical, infrared & time-domain THz
SWCNs, (Ga,Mn)As, HTc superconductors
Today Today
Jigang Wang, Feb, 2009
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
A A femtosecondfemtosecond laser oscillator laser oscillator
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Higher IntensitiesHigher Intensities
Rep rate (pps)
Puls
e en
ergy
(J)
10910610310010-3
10-9
10-6
100
10-3
Oscillator
Cavity-dumped oscillator
RegA
Regenerative
Regen + multipass
Regen + multi-multi-pass
1 W average power
Francois Salin, CELIA, France
Ultrafast am
plifiers
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Ultrafast AmplifierUltrafast Amplifier
Pulse compressor
t
t
Solid state amplifiers
t
Dispersive delay linet
Short pulse
oscillator
Regenerative amplifier scheme
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Regenerative Regenerative amplifieramplifier
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Ultrafast AmplifiersUltrafast Amplifiers
PC1
PC2WP
Rod TFP
TFPSeed input
M1
M2
Before injection
Intra-cavity components:M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film PolarizerPC2 : Pockels Cell
Only the pulse to be amplified enters the cavity
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Ultrafast AmplifiersUltrafast Amplifiers
Intra-cavity components:M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film PolarizerPC2 : Pockels Cell
PC1
PC2WP
Rod TFP
TFPSeed input
M1
M2
Regen operation: pulse injection
V1=Vλ/2
V2=Vλ/4
Pulse is injected using the external Pockels cell PC1.Pulse is trapped using the internal Pockels cell PC2.
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Intra-cavity components:M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film PolarizerPC2 : Pockels Cell
PC1
PC2WP
Rod TFP
TFPSeed input
M1
M2
Regen operation: pulse ejection
V2=Vλ/4
output
V2=0
:internal Pockels cell PC2 is turned off
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
The worldThe world’’s largest lasers largest laserAlmost 10 years journey, due next month!
192 shaped pulses; 1.8 MJ total energy
National Ignition Facility(LLNL)
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Lasers as spectroscopy light sources Lasers as spectroscopy light sources
1. Static spectroscopy using CW lasers
2. Static spectroscopy using ultrashort pulsed lasersLaser Raman/Raleigh scattering, multi-photon excitation Spectroscopy; SWCNs
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Laser scattering experiment Laser scattering experiment -- basics basics
Excitation laser
Scattered light
Basic Instrumentation:
– Illuminate a sample with laser light (e.g. 532nm, 780nm)
– Scattered (no absorbed) light in two forms – collection and spectrally resolved detection
• Elastic (Rayleigh) → λscattered = λincident
• Inelastic (Raman) → λscattered ≠ λincident
Photon energy ωp
0
30
50
60
Inte
nsity
I s
Spectrally-resolved detection
RayleighRaman Raman
ωe
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Laser Rayleigh and Raman scatteringLaser Rayleigh and Raman scattering
tIRωααα sin10 +=
tEE eωααµ sin0==
])cos()[cos(2
1sin 010 ttEtE IReIRee ωωωωαωαµ +−−+=
Induces polarization P = N0µ oscillates at three frequencies!
Induced dipole Polarizability Incoming field
ωIR
ωe ωs
ωIR
ωe ωs
ωIR
ωs
E1
E2
VirtualState
Rayleigh Stokes Raman Anti-Stokes Raman
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Signals from the scattering experimentsSignals from the scattering experiments
• Spectrum – ωIR can be molecular vibrationsand low energy collective excitations such as
phonons, magnons, plasmons, spin flip transitions…
• Scattered intensity – ~ 0.1 part per million photons
• Cross section – ~ 10-30 cm2
R
θ462
2
24
2
2
0 )2
()2
1()
2(
2
cos1 −∝+−+= λ
λπθ d
n
n
RII
22
2
4
65
)2
1(
3
2
+−=
n
nds λ
πσ ωσ h/0 sph IN =
Reman/Rayleigh scattering – a net change in polarizability
Absorption, FTIR – a net change in dipole moment,
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
The origin of polarizabilityThe origin of polarizability
Tendency of charge distribution or wave function of a dipole to be distorted by local E field, i.e.,
Lex ENE 00 /χεα = χ: electrical susceptibility0/ ≠dtdα
oC
o oC
o oC
o0/ ≠dtdµ
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Selection rules: Raman vs. IRSelection rules: Raman vs. IR
ωe ωs
ωIR
ωe ωs
E1
Rayleigh Stokes Raman
V
)(),()( 0 ises EP ωωωχεω =
11),( EVVEse MM →→∝ωωχ
E2
E1
M is dipole transition element, e.g., where η is along E filed ><=→ irfM fi .η
21),( EVVEse MM →→∝ωωχ
ωe
IR absorptionE2
E1
One-photon
21)( EEe M →∝ωχ
Multi-photon, e.g., twophoton
1221
),(
EEEVVE
ee
MMM →→→∝ωωχ
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
SingleSingle--wall carbon nanotubeswall carbon nanotubes
Metallic Semiconducting
Ch = na + mb
n – m = 3M + ν
1) M = ν = 0
2) M ≠ 0, ν = 0
3) M ≠ 0, ν = ±1
Metal
Narrow Gap Semicond.
Large Gap Semicond.
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Example: Laser Raman in SWCNsExample: Laser Raman in SWCNs
“Dark-field Spectroscopy”
1. Presence of nanotubes 2. Orientation of isolated tubes or aligned samples3. Diameters of carbon nanotubes:4. Mechanical strain
Raman Intensity vs. shift
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
Rayleigh scatterings in individual SWCNsRayleigh scatterings in individual SWCNs
In-situ CVD growthacross etched slit
Rayleigh Spectra
Energy (eV)
Heinz, Brus, Colombia Univ
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
MultiMulti--photon Excitation Spectroscopy in SWCNsphoton Excitation Spectroscopy in SWCNs
α
hν
1s
2p
Eg
Heinz, Brus, Colombia Univ
Motivations and basic schemes
Transient transmission and reflection spectroscopy
Ultrafast magneto-optical spectroscopy
Ultrafast mid-infrared/THz spectroscopy
Coherent transient spectroscopy
Examples
1. Time-resolved (ultrafast) laser spectroscopy
TodayToday’’s Lectures Lecture
Jigang Wang, Feb, 2009
Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/
TimeTime--resolved laser spectroscopy: whyresolved laser spectroscopy: why
Ultra-fast
Ultra-broadband
Ultra-intensive
Manipulation
Fundamental time scales for key microscopic interactions
Energy scales of important collective excitations
Searching for new regimes of condensed matter physics
A new paradigm for condensed matter physics
Fundamental time scales in condensed matterFundamental time scales in condensed matter
Jigang Wang, Feb, 2009
10-9 s = 1 ns
10-12 s = 1 ps
10-15 s = 1 fs
Time
carrier recombination(100ps-1ns)
carrier cooling (1-100ps)e-acoustic phonon (1-100 ps)
e-opitcal phonon scattering (<1 ps)e-hole scattering (<1 ps)
h-optical phonon scattering (100 fs)e-e scattering (10 fs)
e correaltion time (<1 fs)
Electronic Magnetic (Atomic) Structural
Spin precession, dampingin in FM(100 ps-10ns)
Spin-phonon (1-100ps)Spin precession in AFM (1-100ps)
Spin-orbit (10 fs)Spin-spin exchange(1 fs)
vibration period (100 fs)
ultrafast chemical/biological reactions
Ultrafast melting(1-100 ps)
Rotations of Molecules (1ns)
Fundamental energy scales in condensed matterFundamental energy scales in condensed matter
Jigang Wang, Feb, 2009
3 eV
Energy
1 eV
100 meV X
0 meV
Mott gap, charge-transfer gap (1-3 eV)
Interband transition in most semiconductors (400 meV – 2 eV)Multi-phonons and multi-magnons (50-500 meV)
Intra-exciton trainsiton in semiconducting SWCNs(150 meV - 300 meV)
Polarons (20-300 meV)
Pseudogap excitation (30-300 meV) Optical phonons (40-70 meV)Magnons (10- 40 meV)
Superconduting gap (1-40 meV)
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: schemes Ultrafast laser spectroscopy: schemes
The most commonly used geometry is “pump and probe”.
It usually involves exciting the medium with one (or more) ultrashort laser pulse(s) and probing it a variable delay later with another.
E
K
Ultrafast excitation –Highly non-equilibrium state
Time-delayed probe –build-up of transient state
and recovery of the ground state
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: how Ultrafast laser spectroscopy: how
∆t
10 fs – 100 fs
k2
DetectorSpectrometer
k1TimeTime
SignalSignal
∆∆tt = = --100 fs100 fs ∆∆tt = 0 fs= 0 fs ∆∆tt = 100 fs= 100 fs
λλ λλλλ
Sub-10 fs, sub-1 nm, B field up to 10T, Low temperature
down to 1.2K
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: types Ultrafast laser spectroscopy: types
∆t
T
2k2-k1
k1
R
EEUltrafast incoherent Spectroscopy: Transient reflection/transmission
Ultrafast mid-infrared
Ultrafast THz Spectroscopy
Ultrafast magneto-optical
Coherent transient spectroscopy
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: signalsUltrafast laser spectroscopy: signals
TransmissionTransmission
ReflectionReflection
EmissionEmission
Signals -> M, p, σ, χ(2) , χ(3) ...
Absorption
magnetization, conductivity, electrical polarization, 2nd and 3rd order nonlinearity……
Let the unexcited medium have an absorption coefficient, α0.Immediately after excitation, the absorption decreases by ∆α0.
∆α(τ) = ∆α0 exp(–τ /τex) for τ > 0
where τ is the delay after excitation, and τex is the excited-state lifetime.
So the transmitted probe-beam intensity—and hence pulse energy and average power—will depend on the delay, τ, and the lifetime, τex:
Itransmitted(τ) = Iincident exp–[α0 – ∆α0exp(–τ /τex)]L where L = sample length
= Iincident exp–α0L exp∆α0exp(–τ /τex)L
≈ [ Iincident exp–α0L] 1+∆α0exp(–τ /τex)L assuming ∆α0 L << 1
≈ Itransmitted(0−) 1+∆α0exp(–τ /τex)L
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: modelingUltrafast laser spectroscopy: modelingExample: transient transmission
Jigang Wang, Feb, 2009
Ultrafast laser spectroscopy: modelingUltrafast laser spectroscopy: modelingExample: transient transmission
∆T(τ) /T0 = [Itransmitted(τ) – Itransmitted(0−)] /Itransmitted(0−)
The relative change in transmitted intensity vs. delay, τ, is:
Cha
nge
in p
robe
-be
am in
tens
ity
Delay, τ0
Itransmitted(τ) ≈ Itransmitted(0−) 1+∆α0exp(–τ /τex)L
∆T(τ) /T0 ≈ ∆α0 exp(–τ /τex)L
⇒
InGaAs
Jigang Wang, Feb, 2009
Example: transient transmission in LT InGaAs
-0.8
-0.4
0.0
0.4
0.8
∆R/R
(%
)
20016012080400
Time Delay (ps)
-0.8
-0.4
0.0
0.4
∆R/R
(%
)
100Time Delay (ps)
InGaMnAs/InGaAsT= 20K
Ultrafast carrier dynamicsUltrafast carrier dynamics
Pump: 2 µmProbe: 775 nm
1. Initial drop in reflectivity
2. Very rapid rise (~2 ps) + sign change
3. Periodic oscillations(~ 23 ps)
4. Very slow decay(100’s of ps)
Jigang Wang, Feb, 2009
Carrier trapping: two regimesCarrier trapping: two regimes
V.B
C.B
As+Ga
Antisite
Ga Vacancies
As0Ga
Antisite
(1) carrier trapping by mid- bandgapdefects (~2 ps)
(2) reexcitation and recombination of trapped carriers
-6
-4
-2
0
2
103 *∆
R/R
3210-1Time delay [ps]
(1)
(2)
Jigang Wang, Feb, 2009
-1.0
-0.5
0.0
0.5
1.0
∆R/R
(%
)
1208040Time Delay (ps)
650nm
775nm
850nm
Propagating coherent acoustic phonons Propagating coherent acoustic phonons
EF
~100 fs
Phonon package
Cs
InAs GaSb
Jigang Wang, Feb, 2009
Ultrafast MagnetoUltrafast Magneto--optical Spectroscopyoptical Spectroscopy
Magnetic IonsMagnetic Ions
CarriersCarriers
ExcitationExcitation
Detection Detection kθ
kη
M
fs- & vectorially resolved Magnetization dynamics at H < 8.0 T and T > 1.5K
Jigang Wang, Feb, 2009
Transient Demagnetization in Transient Demagnetization in InMnAsInMnAs
0.4
0.2
0.0
− ∆θ
K /θ
K
1 10 100 1000Time Delay (ps)
(1) (2) (4)(3)
-∆M
/MSpin-spin Spin-phonon Heat diffusion
Jigang Wang, Feb, 2009
Ultrafast Rotation in Ultrafast Rotation in GaMnAsGaMnAs
-80
-60
-40
-20
0
20
∆θ k
(µr
ad)
8006004002000Time Delay (ps)
30
20
10
0
∆θ k
(µr
ad)
3.02.01.00.0-1.0Time Delay (ps)
Y+(0)
|B|=0T, T = 5K
Jigang Wang, Feb, 2009
-10
1
-8-6
-4-2
02
-1
0
1
∆Mz (10 -5
rad)
∆My (10
-5 )
∆Mx (10
-5 )
Z (001)
X (110)
Y (1-10)
(100)MHA
3.1 eV ~ 120 fs
Ultrafast Rotation in Ultrafast Rotation in GaMnAsGaMnAs
-10
-8
-6
-4
-2
0
∆M
z (1
0-5 ra
d)
-1.0 0.0 1.0
∆My (10-5 )
-1.5
-1.0
-0.5
0.0
0.5
1.0 ∆
My
(10-5
)
-1.5 -1.0 -0.5 0.0 0.5 1.0
∆Mx (10-5 )
-10
-8
-6
-4
-2
0
∆M
z (1
0-5 ra
d)
-1.5 -1.0 -0.5 0.0 0.5 1.0
∆Mx (10-5 )
9.3 ps
Amplitude Amplitude andand Phase informationPhase informationReal & Imaginary Part of Real & Imaginary Part of σσ((ωω), ), εε((ωω))
( ) ωωω σ ω
≡ ≈+ + 0
( ) 2( ) 1 ( )
OUT
IN S
Et
E n d Z
Complex transmission coefficientComplex transmission coefficient
( )tE t( )iE t
thin filmthin film
FieldField--resolved Detectionresolved Detection
1 THz = 300 µm = 33 cm-1 = 4.1 meV
ZnTe
ETHz(t) ∝dt2
d2P(t)
near-IR pulse
nonlinear crystal(ZnTe)
Jigang Wang, Feb, 2009
Optical Pump and THz probeOptical Pump and THz probe
Jigang Wang, Feb, 2009
THz probes of THz probes of excitonexciton formation and ionization formation and ionization ∆σ
1(Ω
-1cm
-1) 20
10
∆t (ps)
0
100
200
300
Photon Energy (meV)
0
4 8 4 8
4 8
TL = 6 K 30 K 60 K
T = 6 K T = 6 K ⇒⇒ recombinativerecombinativepopulation decaypopulation decay
High THigh TLL: : ee--hh pairs become conductingpairs become conducting
ExcitonicExcitonic component remainscomponent remains
⇒⇒ excitonexciton ionization (via phonons)ionization (via phonons)
Quantum beatQuantum beat detected by 3detected by 3--Pulse fourPulse four--wavewave--mixingmixing
k2
k3
k1
k1+k2- k3
Δt12
Coherent transient Spectroscopy Coherent transient Spectroscopy
LL0
LL1ћωcE
ner
gy
Optical excitation mostly on LL1
The LL0 signal is small in comparison to LL1, but with strong oscillations
LL1 signal – no clear oscillations.
Quantum Beats of 2D MagnetoQuantum Beats of 2D Magneto--excitonsexcitons
Coherent Quantum Beats Coherent Quantum Beats
+k2 +k1-k3
∆t12
ks
ω1 phase accumulated
+k2+k1
-k3
∆t12
ks
ω0 phase accumulated
•• k2 create dipole k2 create dipole coherence , k1 & k3 coherence , k1 & k3 probesprobes
•• The oscillations of The oscillations of ΩΩ00--ΩΩ1 will decay as 1 will decay as ΓΓ0+0+ΓΓ1 1
Jigang Wang, Feb, 2009
Other ultrafast spectroscopic techniquesOther ultrafast spectroscopic techniques
Photon Echo, three pulse photon-echo peak shift
Heterodyne detected four-wave mixing
Transient grating spectroscopy,
Transient Coherent Raman Spectroscopy
Ultrafast electron scattering,
ultrafast X-ray scattering/absorption
Transient Surface SHG Spectroscopy
Transient photo-emission Spectroscopy
Time-resolved fluoresce spectroscopy
Heterodyned ultrafast polarization spectroscopy
……
Almost any physical effect that can be induced and thereby
probed by ultrashort light pulses!
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