Chih Chih-Wei L Wei Luo ( ) Department of Electrophysics, - PowerPoint PPT Presentation

Introduction to Ultrafast Science and Technology Chih Chih-Wei L Wei Luo ( 羅志偉 ) Department of Electrophysics, National Chiao Tung University, Taiwan Ul Ultraf rafas ast Dynami namics Lab October 06, 2020 at NTU

Outline 1. Introduction to femtosecond laser pulses 2. Nanoparticle fabrication 3. Nanostructure fabrication 4. Ultrafast dynamics in topological insulators

The Nobel Prize in Physics 2018  Optical Tweezers & Chirped Pulse Amplification (CPA)

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

Introduction to fs laser pulses What is the ultrashort pulse? ~10 -6 s ~10 -9 s Ruler ~10 -12 s ~10 -15 s Vernier caliper

Introduction to fs laser pulses The shorter pulse duration, the more papers! 2000 Femtosecond in Web of Science -9 10 Pulse duration (sec.) No. of Publications 1500 Active mode-locking -12 10 Passive mode-locking 1000 intra-cavity pulse compression Colliding pulse -15 10 mode-locking 500 XUV excitation pulse First laser (Ruby) -18 10 0 1950 1960 1970 1980 1990 2000 2010 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 Year Year Prof. Theodor W. Hänsch Prof. Ahmed Zewail & Prof. John L. Hall The 1999 Nobel Prize The 2005 Nobel Prize in Chemistry in Physics

Introduction to fs laser pulses Ultrafast camera!! femtosecond laser

Introduction to fs laser pulses What is the ultrashort pulse? ~10 -6 s ~10 -9 s ~10 -12 s ~10 -15 s

Introduction to fs laser pulses The possibility for nuclear fusion! Institute of Laser Engineering  Short pulse = intense peak power Osaka University  100 mJ, 100 fs = 1 TW  10 18 W/cm 2 @ φ = 10 μm (10 10 V/cm) Long pulse, Long pulse, Pump Pump Pump Pump Pump Pump high energy high energy Verdi Verdi Verdi Verdi Verdi Verdi Pump Pump Pump Pump Pump Pump Evolution Evolution Evolution Evolution Evolution Evolution Evolution Evolution Seed Seed Seed Seed Mira Mira Mira Mira Short pulse, Short pulse, low energy low energy Amplifier Amplifier Amplifier Amplifier Legend Legend Legend Legend Short pulse, Short pulse,  USA National Ignition Facility high energy high energy @192 laser beams

Introduction to fs laser pulses USA National Ignition Facility  Output power ~ 300 TW

Introduction to fs laser pulses Free electron laser - Japan

Researches in Ultrafast Dynamics Lab  Superconductors  Femtosecond laser annealing  Selected publications  2D materials – Graphene, MoS 2 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)

Outline 1. Introduction to femtosecond laser pulses 2. Nanoparticle fabrication 3. Nanostructure fabrication 4. Ultrafast dynamics in topological insulators

“Can we utilize the femtosecond pulses to obtain ZnSe nanoparticles?“ Pure! Simple! Fast!

Experimental setup 80 fs, 0.8 W, 80 MHz 30 fs, 2 W, 5 KHz Oscillator Amplifier Mirror Micra10 Legend Iris Iris Cylindrical lens Translation stage

Experimental procedure TEM image measurement Before laser process After laser process Dispersion in ethanol

Composition of ZnSe nanoparticles The EDX spectrum The main elements in  nanoparticles are zinc Zn and selenium. Normalized count Se The molar ratio of Zn  and Se ~ 1 : 1. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Energy (keV)

Structural phase transition XRD results ZnSe particles at F =290.52mJ/cm2 Cubic (111) ZnSe single crystal Cubic (400) Hexagonal (100) Hexagonal (002) Cubic (220) Intensity (a.u.) Intensity (a.u.) Cubic (311) Hexagonal (101) Hexagonal (110) Hexagonal (103) Hexagonal (112) Hexagonal (102) Cubic (200) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 20 25 30 35 40 45 50 55 60 2 θ (degree) 2 θ ( degree ) Cubic structure Hexagonal structure ZnSe single crystal ZnSe nanoparticles Femtosecond laser process

TEM image of ZnSe nanoparticles 50nm 20nm 20nm The size of ZnSe particles are < 100 nm for laser fluence = 127 mJ/cm 2 H. I. Wang, et al., Journal of Nanomaterials 2012, 278364 (2012)

The size of ZnSe nanoparticles 50nm 20nm

Se nanoparticle prepared by fs Laser-induced plasma shock wave deposition Trigonal Monoclinic / Amorphous trigonal Se: t-Se monoclinic Se: m-Se amorphous-Se: a-Se Wen-Yen Tzeng , et al., Optics Express 28 , 685 (2020)

Outline 1. Introduction to femtosecond laser pulses 2. Nanoparticle fabrication 3. Nanostructure fabrication 4. Ultrafast dynamics in topological insulators

Nanostructure on ITO films  Pulse number-dependent nanostructure

Nanostructure on ITO films  Transport properties

Nanostructure on ITO films  X-ray photoelectron spectroscopy (XPS)

Application I  Effects on organic photovoltaics using fs-laser-treated ITO Mei-Hsin Chen, et al., ACS Applied Materials & Interfaces 8 , 24989 (2016)

Application II  Anisotropic o ptical properties Chih Wang, et al., Applied Physics Letters 101 , 101911 (2012)

Application III  The colors of ITO films before and after laser processing. Ya-Hsin Tseng, et al., Optics Express 25 , 33134-33142 (2017)

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)

Summary I  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.

Outline 1. Introduction to femtosecond laser pulses 2. Nanoparticle fabrication 3. Nanostructure fabrication 4. Ultrafast dynamics in topological insulators

Platform for ultrafast dynamic study in Taiwan  Strongly correlated  Topological insulators electron systems Crystals Superconductors Thin films Spin-glass systems  2D materials Intermetallics Perovskite Heterostructures 2D transition metal e.g., water splitting dichalcogenide

Topological insulators (TIs)  3D TIs: Bi 2 Se 3 , Bi 2 Te 3 , … etc. H. Zhang, et al., Nat. Phys. 5, 438 (2009) M. Z. Hasan, et. al., Nat .Phys. 5, 398 (2009)

THz emission from topological insulators 5.2 MHz Ti:Sapphire 50 fs @ 800 nm 1.7 W sample B.S. lens Chopper Teflon lens B.S. ZnTe λ/4 WP balanced diodes EO sampling Delay stage Chih-Wei Luo, et al ., Advanced Optical Materials 1, 804-808 (2013)

Mechanism of THz emission from TIs J. McIver et al, Nature Nanotech. 7 , 96 (2012). C. M. Tu et al ., Physical Review B 96, 195407 (2017)

Ultrashort-pulse light sources in UDL 7.5 fs Y. Normura , et al., Optics Letters 40, 423-426 (2015)

800 nm pump & ultrabroadband mid-IR probe H. Shirai, et al., Phys. Rev. Appl. 3, 051002 (2015)

800 nm pump & ultrabroadband mid-IR probe H. Shirai, et al., Phys. Rev. Appl. 3, 051002 (2015)

800 nm pump & ultrabroadband mid-IR probe H. Shirai, et al., Phys. Rev. Appl. 3, 051002 (2015)

Ultrafast dynamics in topological insulators  Sb 2 Te 2 Se single crystals (p-type) Pump beam fluence ： 101 ( μ J/cm 2 ) Delay time (ps) ∆ R/R Wavenumber (cm -1 )  Bi 2 Te 2 Se single crystals (n-type) Delay time (ps) Wavenumber (cm -1 ) ARPES images: measured by Dr. Cheng-Maw Cheng (NSRRC)

The sign changes of ∆ R / R  Surface carrier transition Sb 2 Te 2 Se Delay time (ps) Reflectivity E: energy level , T: temperature Falkovsky model Wavenumber (cm -1 ) L. A. Falkovsky, and A. A. Varlamov, A. Eur. Phys. J. B. 56 , 281(2007).  Free carrier absorption Bi 2 Te 2 Se Delay time (ps) Reflectivity Drude model Carrier concentration: Wavenumber (cm -1 )

Delay time (ps) Relaxation processes in Sb 2 Te 2 Se Wavenumber (cm -1 ) 0 2 4 6 8 1500 0.2 1000 0 ps T (K) 0.0 F Fitted by 500 Falkovsky -0.2 1 ps 0.15 model |E | (eV) -0.4 0.10 Shifted ∆ R / R 2 ps 0.05 -0.6 0.00 1000 -0.8 3 ps Γ (cm -1 ) D -1.0 500 4 ps ) -1.2 ω p (cm -1 ) 2500 y ( -1.4 5 ps -1.6 2000 1000 1500 2000 2500 3000 3500 0 2 4 6 8 Delay time (ps) -1 ) Wavenumber (cm

Relaxation processes in Sb 2 Te 2 Se 0 2 4 6 8 1500 1000 Surface carrier transition T (K) F 500 0.15 |E | (eV) 0.10 0.05 0.00 1000 Γ (cm -1 ) D Free carriers 500 ) ω p (cm -1 ) 2500 y ( 2000 0 2 4 6 8 Delay time (ps) T. T. Yeh, et al., Scientific Reports 10, 9803 (2020)