Introduction to

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Chih-Wei Luo (羅志偉)

Department of Electrophysics, National Chiao Tung University, Taiwan

Introduction to

Ultrafast Science and Technology

October 06, 2020 at NTU

Ultrafast Dynamics Lab

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Outline

2. Nanoparticle fabrication

1. Introduction to femtosecond laser pulses 3. Nanostructure fabrication

4. Ultrafast dynamics in topological insulators

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The Nobel Prize in Physics 2018

Optical Tweezers & Chirped Pulse Amplification (CPA)

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反其道而行的創新 --啾頻脈衝放大

羅志偉、葉恬恬

物理雙月刊 2月號/2019 41卷第1期

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What is the ultrashort pulse?

~10

^-6

s

~10

^-9

s

~10

^-12

s

~10

^-15

s

Introduction to fs laser pulses

Vernier caliper Ruler

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The shorter pulse duration, the more papers!

1950 1960 1970 1980 1990 2000 2010 10^-18

10^-15 10^-12 10^-9

intra-cavity pulse compression XUV excitation pulse Colliding pulse

mode-locking Passive mode-locking Active mode-locking

Pulse duration (sec.)

Year

First laser (Ruby)

1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 0

500 1000 1500

2000 Femtosecond in Web of Science

No. of Publications

Year

Introduction to fs laser pulses

Prof. Ahmed Zewail The 1999 Nobel Prize in Chemistry

Prof. Theodor W. Hänsch

& Prof. John L. Hall The 2005 Nobel Prize in Physics

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Ultrafast camera!!

Introduction to fs laser pulses

femtosecond laser

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What is the ultrashort pulse?

~10

^-6

s

~10

^-9

s

~10

^-12

s

~10

^-15

s

Introduction to fs laser pulses

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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)

Legend Legend

Amplifier Amplifier

Mira Mira

SeedSeed

Verdi Verdi PumpPump

Evolution EvolutionPumpPump

Short pulse, low energy

Long pulse, high energy

Short pulse, high energy Legend

Legend

Amplifier Amplifier

Mira Mira

SeedSeed

Verdi Verdi PumpPump

Evolution EvolutionPumpPump

Evolution Evolution Evolution EvolutionPumpPump

Short pulse, low energy

Long pulse, high energy

Short pulse, high energy

Introduction to fs laser pulses

 USA National Ignition Facility

@192 laser beams

Institute of Laser Engineering Osaka University

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Introduction to fs laser pulses

USA National Ignition Facility

 Output power ~ 300 TW

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Introduction to fs laser pulses

Free electron laser - Japan

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Researches in Ultrafast Dynamics Lab

 Selected publications

 Femtosecond laser annealing

 Superconductors

 2D materials – Graphene, MoS₂

1) Adv. Optical Mater. 1, 804-808 (2013) 2) Nano Lett. 13, 5797 (2013)

3) Nanoscale 6, 8575 (2014) 4) Nano Energy 15, 625 (2015)

5) Advanced Materials 28, 876 (2016)

6) Advanced Functional Materials 26,729 (2016) 7) Optica 3, 82 (2016)

8) npj Quantum Materials, 2, 1 (2017) 9) Optics Express 25, 33134 (2017) 10) Nano Lett. 18, 7742 (2018)

11) Phys. Rev. Materials 3, 034802 (2019) 12) Optics Express 28, 685 (2020)

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4. Ultrafast dynamics in topological insulators

Outline

2. Nanoparticle fabrication

1. Introduction to femtosecond laser pulses

3. Nanostructure fabrication

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“Can we utilize the femtosecond pulses to obtain ZnSe nanoparticles?“

Pure!

Simple!

Fast!

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Legend Micra10

Amplifier Oscillator

Cylindrical lens

Mirror

Translation stage

80 fs, 0.8 W, 80 MHz 30 fs, 2 W, 5 KHz

Iris

Experimental setup

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Before laser process

After laser process Dispersion in ethanol TEM image measurement

Experimental procedure

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Composition of ZnSe nanoparticles

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Normalized count

Energy (keV)

Zn Se

The EDX spectrum

 The main elements in nanoparticles are zinc and selenium.

 The molar ratio of Zn

and Se ~ 1 : 1.

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Structural phase transition

XRD results

Cubic ^structure

ZnSe single crystal

Femtosecond laser process

Hexagonal ^structure

ZnSe nanoparticles

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Cubic (400)

Intensity (a.u.)

2θ (degree)

ZnSe single crystal

Cubic (200)

20 25 30 35 40 45 50 55 60

Cubic (311)

Cubic (220) Hexagonal (112)

Hexagonal (103)

Hexagonal (110)

Hexagonal (102)

Hexagonal (101)Hexagonal (002)Hexagonal (100) Cubic (111)

Intensity (a.u.)

2θ (degree)

ZnSe particles at F =290.52mJ/cm2

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The size of ZnSe particles are< 100 nm for laser fluence = 127 mJ/cm²

20nm

20nm 50nm

TEM image of ZnSe nanoparticles

H. I. Wang, et al., Journal of Nanomaterials 2012, 278364 (2012)

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The size of ZnSe nanoparticles

20nm 50nm

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Se nanoparticle prepared by fs Laser-induced plasma shock wave deposition

Wen-Yen Tzeng, et al., Optics Express 28, 685 (2020)

trigonal Se: t-Se monoclinic Se: m-Se amorphous-Se: a-Se Trigonal Monoclinic / Amorphous

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Outline

2. Nanoparticle fabrication

1. Introduction to femtosecond laser pulses 3. Nanostructure fabrication

4. Ultrafast dynamics in topological insulators

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Nanostructure on ITO films

 Pulse number-dependent nanostructure

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Nanostructure on ITO films

 Transport properties

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Nanostructure on ITO films

 X-ray photoelectron spectroscopy (XPS)

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Application I

 Effects on organic photovoltaics using fs-laser-treated ITO

Mei-Hsin Chen, et al., ACS Applied Materials & Interfaces 8, 24989 (2016)

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Application II

 Anisotropic optical properties

Chih Wang, et al., Applied Physics Letters 101, 101911 (2012)

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Application III

 The colors of ITO films before and after laser processing.

Ya-Hsin Tseng, et al., Optics Express 25, 33134-33142 (2017)

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Application IV

 The image that is displayed on the LCD can be selectively screened by varying the view angle.

Ya-Hsin Tseng, et al., Optics Express 25, 33134-33142 (2017)

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 The hexagonal ZnSe & Se nanoparticles can be fabricated by properly controlling the fluences of the irradiating fs laser.

 The nanostructure with anisotropic transmission characteristics on ITO films induced by fs laser can be used for the alignment layer , polarizer and conducting layer in LCD cell.

 The nanostructure on the surface of ITO films significantly attenuates blue light, which are suited to eye protection and the screening of images behind ITO films for information security .

Summary I

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Outline

2. Nanoparticle fabrication

1. Introduction to femtosecond laser pulses 3. Nanostructure fabrication

4. Ultrafast dynamics in topological insulators

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Platform for ultrafast dynamic study in Taiwan

Superconductors

Heterostructures e.g., water splitting

Intermetallics

 Strongly correlated electron systems

Spin-glass systems

 2D materials

Perovskite

2D transition metal dichalcogenide

 Topological insulators

Thin films

Crystals

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Topological insulators (TIs)

 3D TIs: Bi₂Se₃, Bi₂Te₃, … etc.

H. Zhang, et al., Nat. Phys. 5, 438 (2009) M. Z. Hasan, et. al., Nat .Phys. 5, 398 (2009)

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THz emission from topological insulators

Chih-Wei Luo, et al., Advanced Optical Materials 1, 804-808 (2013) sample

B.S.

Delay stage

ZnTe

balanced diodes λ/4

lens

WP

Teflon Chopper

5.2 MHz Ti:Sapphire 50 fs @ 800 nm 1.7 W

EO sampling

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C. M. Tu et al., Physical Review B 96, 195407 (2017)

J. McIver et al, Nature Nanotech. 7, 96 (2012).

Mechanism of THz emission from TIs

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Ultrashort-pulse light sources in UDL

Y. Normura, et al., Optics Letters 40, 423-426 (2015)

7.5 fs

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800 nm pump & ultrabroadband mid-IR probe

H. Shirai, et al., Phys. Rev. Appl. 3, 051002 (2015)

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800 nm pump & ultrabroadband mid-IR probe

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800 nm pump & ultrabroadband mid-IR probe

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 Sb₂Te₂Se single crystals (p-type)

Ultrafast dynamics in topological insulators

 Bi₂Te₂Se single crystals (n-type) Wavenumber (cm^-1)

Delay time(ps)

Wavenumber (cm^-1)

Delay time(ps)

∆R/R Pump beam fluence：101 (μJ/cm²)

ARPES images: measured by Dr. Cheng-Maw Cheng (NSRRC)

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 Surface carrier transition

The sign changes of ∆ R / R

 Free carrier absorption

Reflectivity

E: energy level, T: temperature

Drude model

Carrier concentration:

Reflectivity

Wavenumber (cm^-1)

Delay time(ps)

Wavenumber (cm^-1)

Delay time(ps)

Sb₂Te₂Se

Bi₂Te₂Se

Falkovsky model

L. A. Falkovsky, and A. A. Varlamov, A. Eur. Phys. J. B. 56, 281(2007).

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Relaxation processes in Sb

₂

Te

₂

Se

Wavenumber (cm^-1)

Delay time(ps)

1000 1500 2000 2500 3000 3500 -1.6

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

5 ps 4 ps 3 ps 2 ps 1 ps

Shifted ∆R/R

Wavenumber (cm^-1)

0 ps

T(K)|E| (eV)Γ(cm-1 )ωp(cm-1 )

0 2 4 6 8

2000 2500 500 1000 0.00 0.05 0.10 0.15 500 1000

1500 0 2 4 6 8

y ()

Delay time (ps)

DF

Fitted by Falkovsky model

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T(K)|E| (eV)Γ(cm-1 )ωp(cm-1 )

0 2 4 6 8

2000 2500 500 1000 0.00 0.05 0.10 0.15 500 1000

1500 0 2 4 6 8

y ()

Delay time (ps)

DF

Relaxation processes in Sb

₂

Te

₂

Se

Surface carrier transition

Free carriers

T. T. Yeh, et al., Scientific Reports 10, 9803 (2020)

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Summary II

 Ultrabroadband mid-IR generation & detection

Time-resolved + FTIR

 Reveal the full ultrafast dynamics in topological insulators.

Wavenumber (cm^-1)

Delay time(ps)

Apply to study the vibration dynamics of molecules in femtosecond timescale.

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Acknowledgements

TCECM

Group members

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Thank you

for your attention ^!!

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