Time-resolved spectroscopy - NSRRC · The spectral range of this detection technique is wider in...

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