Time-resolved spectroscopy
Chih-Wei Luo (羅志偉)
Department of Electrophysics, National Chiao Tung University, Taiwan
Ultrafast Dynamics Lab
Outline
2. Spectroscopic methods for studying ultrafast dynamics
3. Some examples in time-resolved spectroscopy
1. Introduction of pulses
Prof. Chih-Wei Luo, NCTU EP
Prof. Chih-Wei Luo, NCTU EP
What is the ultrashort pulse?
~10-6 s
~10-9 s
~10-12 s ~10-15 s
Introduction of pulses
Introduction of pulses The shortest laser pulse
1987 6 fs Opt. Lett. 12, 483 (1987)
1997 4.5 fs Opt. Lett. 22, 102 (1997) 4.5 fs Opt. Lett. 22, 522 (1997)
2002 3.9 fs Opt. Lett. 27, 306 (2002)
Baltuška, Fuji, Kobayashi 3.8 fs Phys. Rev. Lett. 88, 203901 (2002)
2004 250 as Nature 427, 817 (2004) 2006 130 as Science 314, 443 (2006) 2008 80 as Science 320, 1614 (2008) 2013 67 as Opt. Lett. 37, 3891 (2013)
Visible extreme ultraviolet
Shorter pulse with shorter wavelength!! Prof. Chih-Wei Luo, NCTU EP
Introduction of pulses Timescales
1 minute 10 fs light
pulse Age of universe
Time (seconds)
Computer clock cycle
Camera flash
Age of pyramids
One month
Human existence
10-15 10-12 10-9 10-6 10-3 100 103 106 109 1012 1015 1018
1 femtosecond 1 picosecond
a pulse : 1 minute ~ 1 minute : age of universe
Prof. Chih-Wei Luo, NCTU EP
Introduction of pulses
femtosecond laser
Ultrafast camera!!
Prof. Chih-Wei Luo, NCTU EP
Introduction of pulses The possibility for nuclear fusion! Short pulse = intense peak power 100 mJ, 100 fs = 1 TW 1018 W/cm2 @ φ = 10 μm (1010 V/cm)
LegendLegend
AmplifierAmplifier
MiraMira
SeedSeed
VerdiVerdiPumpPump
EvolutionEvolutionPumpPump
Short pulse, low energy
Long pulse, high energy
Short pulse, high energy
LegendLegend
AmplifierAmplifier
MiraMira
SeedSeed
VerdiVerdiPumpPump
VerdiVerdiPumpPump
EvolutionEvolutionPumpPump
EvolutionEvolutionEvolutionEvolutionPumpPump
Short pulse, low energy
Long pulse, high energy
Short pulse, high energy
Introduction of pulses USA National Ignition Facility
Output power ~ 300 TW
Prof. Chih-Wei Luo, NCTU EP
Prof. Chih-Wei Luo, NCTU EP
Introduction of pulses
Free electron laser - Japan
Introduction of pulses
Prof. Chih-Wei Luo, NCTU EP
Outline
2. Spectroscopic methods for studying ultrafast dynamics
3. Some examples in time-resolved spectroscopy
1. Introduction of pulses
Prof. Chih-Wei Luo, NCTU EP
Section Outline
2.1 Pump-probe methods
2.2 Time-resolved Emission spectroscopy: Electronic methods
General principle Time-resolved absorption in the UV-visible range Time-resolved absorption in the IR range
Broad-bandwidth photodetectors The streak camera Single-photon counting
2.3 Time-resolved Emission spectroscopy: Optical methods
The Kerr shutter Up-conversion method
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
General Principles
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
General Principles
a(t) ∝ n(t)
time 13 ns
pump pulses
t (delay)
time probe pulses
Space time Who is the first one to use this idea?
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
Typical pump-probe system
prism
Ti:sapphire laser 20fs @ 75MHz
Ar+ laser, all lines
prism Delay stage
Lock-in amp.
Computer
RF F M
PD
AOM
AOM λ/2
P
λ/2 P
Chamber
Monitor CCD
D
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
Time-resolved absorption in the UV-visible spectrum Beer-Lambert law
( ) ( ) ( )lNνItν, I t0 10Δ ∆−×= νε
Where εν is the absorption coefficient of the sample at frequency ν, N(Δt) is the population absorbing at time t at frequency ν, l is the length of sample excited.
The measured optical density (OD)
( ) ( )( ) ( )lN
tν, IνItν, tΔ
logΔOD 0 ∆== νε
( )( ) ( )
τεν
τ
tlNt
NtNΔ0lnΔODln
et-
0
−=∴⇒
=∆
Prof. Chih-Wei Luo, NCTU EP
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
Time-resolved absorption in the UV-visible spectrum Detection systems with lock-in technique (SNR~106)
detector & lock-in amplifier @ 87KHz
ΔI(t) / I0(t) = ΔR(t) / R(t) ∝ n(t)
n(t)
time
time
13 ns
t (delay)
pump pulses
probe pulses
0.01 ms time
I0 (t)
probe pulses (from sample)
ΔI (t)
AO modulator @ 87KHz at pump pulses
Typical noise spectrum
closedpumpi
r
closedpumpi
r
openpumpi
r
closedpump
closedpump
openpump
II
II
II
R
RR
RR
−
=
−=
∆ ( ) ( )( ) 0I
II
II
closedpumpr
closedpumpropen
pumpr ∆=
−=
( ) ( )closepumpiopen
pumpi II = ( ) 0IIclosedpumpr ≡Where and
2-1 Pump-probe methods
Time-resolved absorption in the UV-visible spectrum Experimental tricks to avoid artifacts Polarization of the pump and probe
(1)For liquid (isotropic)
(2)For solid materials
Pump//probe relaxation of transition moment Pump⊥probe relaxation of transition moment + reorientation of transition moment
pump probe
Pump//probe larger coherent peak during the pulse duration.
Pump⊥probe smaller coherent peak during the pulse duration.
This coherent effect is more serious for shorter pulse, smaller angle between pump and probe beam.
pump
pump
probe
probe
Prof. Chih-Wei Luo, NCTU EP
2-1 Pump-probe methods
Time-resolved absorption in the UV-visible spectrum
a(t) ∝ n(t)
time 13 ns
pump pulses
Zero delay time probe pulses
Probe and GVD the zero delay for each wavelength is different. lose time resolution
Experimental tricks to avoid artifacts
Prof. Chih-Wei Luo, NCTU EP
2-2 Time-resolved emission spectroscopy: electronic methods
Broad-bandwidth photodetectors
The streak camera
Time resolution ~ a few ps, limited by the bandwidth of the electronic system.
The sensitivity of these system is limited.
Time resolution ~ sub-ps. The main limitation is the dynamics range of single-shot measurement. Obtain the emission spectrum simultaneously with the associated
dynamics at each wavelength.
Prof. Chih-Wei Luo, NCTU EP
2-2 Time-resolved emission spectroscopy: electronic methods
Single-photon counting For high-repetition-rate laser source. Time resolution is limited by the pulse duration
and by the response function of the electronic devices.
An accurate measurement of the response function allows sophisticated deconvolution procedures to reach time resolutions of the order of ten ps.
Prof. Chih-Wei Luo, NCTU EP
2-3 Time-resolved emission spectroscopy: optical methods
The Kerr shutter Kerr cell: the ability of isotropic materials (CS2 or glass) to become anisotropic under
the action of an applied electric field (optical Kerr effect). Time resolution: depends on the opening pulse duration and on the relaxation time of
the anisotropy.
Experimental tricks (1) Respective polarizations of probe and pump
pulses (45°). (2) Leakage through polarizers P1 and P2. (3) Spectral dispersion of the transmission function. (4) Parasitic light from the opening beam. (5) Spectral limitation. (6) Time resolution. (7) The excited volume should be as small as
possible to avoid spatial dispersion. (8) The sample and Kerr cell should be as thin as
possible and one should reduce the aperture of optical collection.
Prof. Chih-Wei Luo, NCTU EP
2-3 Time-resolved emission spectroscopy: optical methods
Up-conversion methods Is well suited to low-energy laser pulses with a high repetition rate. The spectral range of this detection technique is wider in the infrared region than that
of photocathodes. Time resolution: <100 fs
Prof. Chih-Wei Luo, NCTU EP
2-3 Time-resolved emission spectroscopy: optical methods
Up-conversion methods Frequency mixing in the nonlinear crystal
LSLS and kkk +=+= ΣΣ ωωω
( ) ( ) ( )2
2
2
22
eff
cossin1ΣΣΣ
+=
λλλ
θθθ
oe nnn
( ) ( )( ) ( )
−
−= −−
−−−
ΣΣ
ΣΣ
22
2eff
21cos
λλ
λλ
θoe
o
nnnn
Phase-matching conditions
Calculate the phase-matching angle θ between the propagation direction and the optical axis (ne<no)
( ) ( ) ( )L
Lo
S
Soeff ,λλ
λλ
λλθ nnn
+=Σ
Σ
LS
111λλλ
+=Σ
KS
KL KΣ
θ
Optical axis
Σλen Σλ
on
SλonLλ
on
Prof. Chih-Wei Luo, NCTU EP
Outline
2. Spectroscopic methods for studying ultrafast dynamics
3. Some examples in time-resolved spectroscopy
1. Introduction of pulses
Prof. Chih-Wei Luo, NCTU EP
Section Outline
3.1 Optical pump-probe 3.2 Optical pump Mid-IR probe 3.3 Optical pump X-ray probe 3.4 Time-resolved angle resolved photoemission spectroscopy (tr-ARPES)
物理雙月刊,2010六月號。 Prof. Chih-Wei Luo, NCTU EP
Prof. Chih-Wei Luo, NCTU EP
3-1 Optical pump-probe
Electron-phonon coupling in metals
P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987). S. D. Brorson, et al. , Phys. Rev. Lett. 64, 2172 (1990).
)(3
)(3)(
2
2
-TTωλπkdt
dTC
-TTωλπk
-dt
dTTC
eB
eB
eee
=
=
Ce is the electronic specific heat, Cℓ is the bosonic specific heat, λ is the electron-boson coupling constant is the second moment of the boson spectrum 2ω
Standard scattering rate formulas - Two temperature model
3 2ωλ
TπkB
=τAt high temperature
TbTaR e ∆+∆=∆
Copper (Cu)
The explanation for the sign change in ΔR/R from the energy band point of view.
G. L. Eesley, Phys. Rev. Lett. 51, 2140 (1983).
smearing effect
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
High-Tc superconductor YBa2Cu3O7
Identify how many relaxation processes occur by the slope in semi-logarithmic scale.
C. W. Luo, Dissertation, National Chiao Tung University, Taiwan (2003).
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
0 20 40 60 80
T=60K
T=67K
T=71K
T=80K
T=100K
T=140K
T=180K
T=220K
T=290K
∆R
/R (a
rb. u
nits
)
Delay Time (ps)
-10 0 10 20 30 40 50 60 70 80
1
3
T = 170 K
∆R/R
(arb
.uni
ts)
Delay time (ps)
λ = 800 nm
2
Oscillation
relaxation
d-d excitation by photon
H. C. Shih, T. H. Lin, C. W. Luo, et al., PRB 80, 024427 (2009)
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
Charge transfer from e2g to a1g by pump pulses
223 rzd−
levels3Mn3 d+
)(),( 22 xyyxd−
E
Pump energy :1.52 eV
Pum
p energy
Pum
p energy
T=290K
T=140K
Room temperature Low temperature
Woo Seok Choi, et al PRB 78 ,054440 (2008) Observed the blueshift of energy gap !
0 50 100 150 200 250 300
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Norm
aliz
ed a
mpl
itude
of ∆
R/R
Temperature (K)
815nm
T0=140 K
0 20 40 60 80
∆R/
R
Delay time (ps)
T=290 K
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
levels3Mn3 d+ Pump energy :1.55 eV
Room temperature Low temperature
0 50 100 150 200 250 300
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Norm
aliz
ed a
mpl
itude
of ∆
R/R
Temperature (K)
815nm 800nm
T0=117 K
223 rzd−
)(),( 22 xyyxd−
E
Pum
p energy
Pum
p energy
T=290K
T=117K
T=140K
Charge transfer from e2g to a1g by pump pulses
Observed the blueshift of energy gap !
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
levels3Mn3 d+ Pump energy :1.68 eV
Room temperature Low temperature
T=63K
Pum
p energy
223 rzd−
EP
ump energy
T=290K
)(),( 22 xyyxd−
0 50 100 150 200 250 300
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Norm
aliz
ed a
mpl
itude
of ∆
R/R
Temperature (K)
815nm 800nm 785nm 770nm 755nm 740nmT0
40 60 80 100 120 140 160 1800.00
0.04
0.08
0.12
0.16
0.20
Temperature (K)
Slop
e
40 60 80 100 120 140 160 1801.481.501.521.541.561.581.601.621.641.661.681.701.72
Temperature (K)
AFM
Ener
gy ga
p Edd
(eV)
0 20 40 60 80 100
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
1.2x10-5
0 50 100 150 200 250 300
1/χ
(Oe /
emu )
Temperature (K)
Curie-Weiss Law
TSRχ
(em
u/O
e)
Temperature (K)
ZFC 100 OeH//c-axis
THo
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
levels3Mn3 d+ Pump energy :1.68 eV
Room temperature Low temperature
EP
ump energy
0 50 100 150 200 250 300
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Norm
aliz
ed a
mpl
itude
of ∆
R/R
Temperature (K)
815nm 800nm 785nm 770nm 755nm 740nmT0
40 60 80 100 120 140 160 1800.00
0.04
0.08
0.12
0.16
0.20
Temperature (K)
Slop
e
40 60 80 100 120 140 160 1801.481.501.521.541.561.581.601.621.641.661.681.701.72
Temperature (K)
AFM
Ener
gy ga
p Edd
(eV)
T=63K
Pum
p energy
223 rzd−
T=290K
)(),( 22 xyyxd−
T=63K
Pum
p energy
)(),( 22 xyyxd−
Extra-blueshift comes from AFM ordering!!
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
Demagnetization dynamics
60 90 120 150 180 210 240
1
2
3
4
5
6
7
τ m
Temperature (K)
800nm 785nm 770nm 755nm 740nm
0 20 40 60 80
0 100 200 300 400 500 600
75K
180K
290K
∆R/R
(arb
. uni
ts)
Delay time (ps)
Te Tl Tsτm
T=75K
T=180K
Delay time (ps)
T=290K
τm
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Oscillation component
Strain Pulse Model )sin2/( 22 θυλτ −≅ nsoundprobeosc
Multiferroics HoMnO3
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Multiferroics HoMnO3
Oscillation component
H. C. Shih, T. H. Lin, C. W. Luo, et al., New J. Phys. 13, 053003 (2011)
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
0 10 20 30 40 50
0
2
4
6
8
10Sample: Bi2Se3
∆R/R
[x10
-4] (
arb.
uni
ts)
Delay time (ps)
Topological insulator Bi2Se3
0 2 4 6 8 10 12-0.20.00.20.40.60.81.01.21.41.6 Bi2Se3 (Bridgeman)
Fit curve
Delay time (ps)
∆ R/R
[x10
-3] (
arb.
uni
ts)
0 2 4 6 8 10 12-10
-8
-6
-4
-2
0
2
4
6
8
Delay time (ps)
∆R/R
[x10
-5] (
arb.
uni
ts)
20 40 60 80 100 120 140 160 180 200
0
1
2
3
Wavenumber (cm-1)
Inte
nsity
[x1
0-14 ] (
arb.
uni
ts)
FFT
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Topological insulator
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Topological insulator
Time delay (ps)
Bi Bi2Se2 Bi2Se3
A1g1
A1g1
V. Chis et al., Phys. Rev. B 86, 174304 (2012). M. Hase, et al., Appl. Phys. Lett. 69 2474 (1996)
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
Topological insulator
Time delay (ps)
Bi Bi2Se2 Bi2Se3
A1g1
A1g1
A1g1
QL chain 11.752 Å
11.797 Å
2.987 Å
ΔL ~ 0.38% Δω=-4.11 cm-1
ΔL ~ -2.26% Δω=13.03 cm-1
Δω=f(ΔL)
Bi-Bi bond 3.056 Å
H. Lind, et al., Phys. Rev. B 72, 184101 (2005)
3-1 Optical pump-probe
Prof. Chih-Wei Luo, NCTU EP
3-2 Optical pump Mid-IR probe
Bi2Se3 #3
Topological insulator Bi2Se3
Prof. Chih-Wei Luo, NCTU EP
3-2 Optical pump Mid-IR probe
Topological insulator Bi2Se3
80 100 120 140 1600.0
0.5
1.0
Spec
tral
den
sity
(mor
m.)
Photon energy (meV)
47° 50° 52° 55° 58° 61° 64°
(a)
16 14 12 10 8Wavelength (μm)
Pump Beam Probe Beam
Central Wavelength
800 nm 8 ~ 14 μm
Spot Size (FWHM) 485 μm 392 μm
Pulse Width ~ 100 fs ~ 500 fs
Prof. Chih-Wei Luo, NCTU EP
3-2 Optical pump Mid-IR probe
Topological insulator Bi2Se3
Estimate the shift of absorption peak by
We can obtain the energy loss rate near Dirac point is ~ 1 meV/ps.
Energy loss rate = 15 (meV) / 14.76 (ps)
∫∫
∆
∆=
photon
photonphoton
dERR
dEERRmomentFirst
)/(
)/(
Prof. Chih-Wei Luo, NCTU EP
Prof. Chih-Wei Luo, NCTU EP
3-2 Optical pump Mid-IR probe
Topological insulator Bi2Se3
Energy- and time-resolved pump probe spectroscopy
3-3 Optical pump X-ray probe
Coherent Femtosecond Motion in Laser-Excited Bismuth
Bismuth unit cell (Peierls distorted)
Optical Pump X-ray probe setup
S. L. Johnson, et al., Phys. Rev. Lett. 100, 155501 (2008) Prof. Chih-Wei Luo, NCTU EP
3-3 Optical pump X-ray probe
Coherent Femtosecond Motion in Laser-Excited Bismuth
Dependence of the dynamics of the diffracted intensity on absorbed laser fluence with α=0.45°: 0.56 mJ/cm2 (blue squares), 1.10 mJ/cm2 (red circles), and 2.24 mJ/cm2 (purple triangles).
X-ray: 7.1 keV
Time resolution: ~195 fs
S. L. Johnson, et al., Phys. Rev. Lett. 100, 155501 (2008) Prof. Chih-Wei Luo, NCTU EP
3-3 Optical pump X-ray probe
Ultrafast inter-ionic charge transfer of transition-metal complexes
[Fe(bpy)3]2+(PF6-)2
B. Freyer, et al., J. Chem. Phys. 138, 144504 (2013) Prof. Chih-Wei Luo, NCTU EP
3-3 Optical pump X-ray probe
Ultrafast inter-ionic charge transfer of transition-metal complexes
B. Freyer, et al., J. Chem. Phys. 138, 144504 (2013)
charge redistribution
Prof. Chih-Wei Luo, NCTU EP
3-4 Time-resolved angle resolved photoemission spectroscopy (tr-ARPES)
The setup of tr-ARPES for ultrafast optical excitation of a persistent surface-state (SS) population in the topological insulator Bi2Se3.
J. A. Sobota, et al., Phys. Rev. Lett. 108, 117403 (2012) BCB: Bulk conduction band BVB: Bulk valance band
Prof. Chih-Wei Luo, NCTU EP
3-4 Time-resolved angle resolved photoemission spectroscopy (tr-ARPES)
Prof. Chih-Wei Luo, NCTU EP
Transient photoemission intensities within the integration windows indicated in the subsequent panel
J. A. Sobota, et al., Phys. Rev. Lett. 108, 117403 (2012)
Schematic of the transitions and scattering processes, including the (c) direct optical transition, (d) scattering into SS and BCB, (e) intraband scattering of the BCB and SS, and (f) the BCB-to-SS scattering responsible for the persistent SS population.
SS: Surface state BCB: Bulk conduction band