Ultrafast Science
Yu‐Miin SheuDepartment of Electrophysics, National Chiao Tung University
許鈺敏交通大學電子物理系
Horse in Motion
Credit: Eadweard Muybridge (1830‐1904)
Outline
• Section I: Ultrafast optical science
• Section II: Light‐matter interaction Photo‐creating novel light‐matter interaction and metastable states in
multiferroics
Section I:
• Overview of ultrafast science and role of ultrafast lasers• Application of wide area: physics, chemistry, biology, etc.• Application and research in condensed matters:1. Electronic system and electron‐phonon coupling2. Phonons and coherent control basis3. Spins and spin flip
What is ultrafast science?
• Science, physics properties, chemical reactions on time scale of 10‐9‐10‐15 s (ns‐fs) or faster.
• Detectors are slow.• Science movie.• We need timing and snapshot.• Pulses, bandwidth, ultrafast lasers.
frp
~5‐100 fs
t
Timing
• Resolution • ∆t= ∆L/v• Most standard velocity: light c• Best characterized light source: lasers• CW lasers: single f , plane wave• Pulse lasers: superposition of many fs, broadband, large ∆ f when all phase locked: oscillator
• 1 ps = 10‐12 s (300 µm)• 1 fs = 10‐15 s (300 nm)
ULTRAFAST LASERS (PHOTO OF DEL MAR PHOTONICS)
& PUMP PROBE TECHNIQUES
Spontaneous and Stimulated Emission
T
Ground state level
Upper state levelEN
ERG
Y
L , LL
L , LL
L , LL
P , pp
Intermediate state level Photon emissionPhonon emission
L +, LL
Stimulated emission
Spontaneous emission
• Spontaneous emission: photon created from excited state with random phases and time intervals.
• Stimulated emission: photon created from excited state has the same frequency, phase and polarization.
Laser• Gain medium excited for spontaneous and stimulated
emission• Mirrors: high reflective, low loss • Cavity: forms standing wave at certain frequencies
resonator• Lasing modes: interference caused by cavity only allow
certain patterns and frequencies
Gain medium HROC
Pump energy
L
Pulse lasers
• A clean pulse can be obtained by summing many frequency components localized in space. They should be locked in phase.
• The shorter the pulse, the more the lasing modes. A transformed limited pulse has :
1 t
A pump‐probe idea of snapshot
Cartoon courtesy of Dario Polli’s webpage
• Two beams can be different light sources with additional setup. • Delay: mechanically or electronically controlled.• Detection: reflectivity, transmissivity, diffraction, PL, THz emission, etc. for visible to NIR
What can be studied
• Laser physics: faster, more power, more efficient for energy conversion …
• Biophysics: luminescence, absorption…
• Chemistry: molecular vibrations, absorption, luminescence…
• Condensed matters: electronic transfer, conductivity, luminescence, spin momentum transfer, phonons, energy conversion…
Second harmonic generation
High harmonic generationX‐UV, x rays…
THz generation
Surface probe, domain imagingCrystal symmetry studies
Element sensitive probes, Imaging, diffractions
Quasi‐particle excitation and detection, meta‐materials
Spectroscopy
Optical frequency mixing
Laser/Optical Physics
Biology
http://www.science.uwaterloo.ca/~qblu/qblu_website/Research.html
• Biophysics: fluorescence, absorption, biosensor applications…
• Ex. Radiology and thecause of DNA damage
Chemistry
Cartoon courtesy of C. B. Harris group Son et. al, ncomms5933
ULTRAFAST STUDIES OF CONDENSED MATTERS
From Atoms to Crystals
+ =
Basis formed from 2 atoms Space lattice Crystal
Periodicity is the core of most condensed matter physics, giving rise to interesting phenomena !
Condensed Matters
• Periodic structures band• Conductivity: metal, insulator, semiconductor, semimetal, superconductor, etc.
• Physical properties: 1. Electric: conductivity, mobility...2. Thermal: specific heat3. Permittivity : electric
susceptibility 4. Permeability: magnetic
susceptibility
Cartoon courtesy of Wiki
Important Systems of Condensed Matters
• Electrons, lattice, spins • Dedicates of band structure: lattice distortion: band or electronic change• Spin ordering
Which Path?
Cartoon courtesy of National Youth Commission and gammadata
• Measurements are made in steady state or when systems are in equilibrium.
• Frequency domain: history is hidden within the frequency width.
• Different path can leads to the same destination.• Time domain: we know how it goes.
Electrons (sub‐ps to few ps)
‐
+
e‐ph coupling
Broson et al., PRL 59, 17, 1962 (1987)
• Hot electrons relax through coupling to phonons.
• The emission of phonons creates lattice heating.
• Strain can be generated due to boundary conditions.
• All the absorbed energies becomes lattice heating that dissipates through bulk or heat sink of systems.
• Two effects (transport and e‐ph coupling) compete for strongly absorbing materials, such as metal.
film
substrate
pump
Electron‐Phonon Coupling of Au metal
film
substrate
pump
• Both transport and e‐ph coupling can be understood.• An example of methods to separate various effect through time‐space considerations.
film
substrate
pump
Phonons
a/2
a
C C
M2 M1
+
+
+
+
+
‐ ‐
‐ ‐
‐ ‐
‐‐
++
+
+
+‐π/a π/a
ω(k)
ω2
ω1
ω3
Phonon= quantized lattice motion = lattice eigen vibration mode
Acoustic
Optical
Spontaneous and Stimulated Raman Scattering
p ‐ s
Cartoon courtesy of UIC
Spontaneous:• Thermally induced• Vibration not in phase
Stimulated:• Electromagnetic force drives vibrations• Driving force ∆ t <1/ 2• Oscillates in phase
Topics :• Electron‐phonon coupling• Coherent control
Coherent Phonon of Bi
• Coherent phonon oscillation modulates refractive index. • Strong absorption materials require the reflectivity measurements.• Transparent materials can be measured through transmissivity. • Phonon lifetime, dephasing time versus temperature, fluence, etc..
Data of Ishioka group’s web, related materials: Phys. Rev. B 58, 5448 (1998), Appl. Phys. Lett. 78, 3965 (2001).
Selective Control of SrTiO3
• Selection rule followed for both pump and probe polarization.
• Eg is from off‐diagonal term , requiring pump along 45oto induce change in birefringence.
• Probe beam is along the 45oand then split into two orthogonal beam for A1g subtraction.
Kohmoto et al, original source: DOI: 10.5772/52140
Ag Eg
Coherent Phonon Control• One pulse excitation.
• Two pulse separated 1 periodin phase and constructive.
• Tow pulse separated 1.5 periodout of phase and destructive.
• Multiple‐pulse sequence.
• Pulse shaping techniques: modulating phase and intensity of frequency components of optical pulses
Cartoon from web of Fedor Mitschke groupLiebig et al, Optic express 18, 19, 20498 (2010)
Spin and Magnetization I
B+
• Single spin in a magnetic field:
• Spins of nonmagnetic materials in a magnetic field:
BB
DiamagnetismParamagnetism
• Magnetic materials without magnetic fields:
Ferromagnetism Antiferromagnetism
M M
Spin and Magnetization II
Magnetic material
• Spin ferromagneticordered < Tc (Curie temperature)
• Spin antiferromagneticordered < TN (Neel temperature)
)
C. Pfleiderer, Nature Physics 7, 673–674 (2011)Cartoon of Padget, Nature Photonics 5, 673–674 (2011)
Spiral Skyrmion
Spin Flip and Demagnetization
a a
a
energy costly
lower energy
∆E
Ground state 1st excited state?
1st excited state
Quantized spin wave: magnon
Ultrafast Demagnetization
• Optical Kerr (or Faraday) rotation ∝
• Can be induced from linear polarized light
• Demagnetization requires the change of total spin angular momentum
• Spin angular momentum relaxation require spin‐orbital coupling and spin‐impurity/phonon scattering Elliott‐Yafetmechanism
• 20 years later, still not coherently agreed and many experiments are going on
PRL
Spin Precession
Kirilyuk et al, Rev. Mod. Phys. 32, 2731 (2010)
• Instantaneous laser heating induce a change in magnetic anisotropy, creating change in effective field.
• The spin precession starts if the anisotropic change is faster than precession period.
NiFe/NiO bilayer cobalt particles in Al2O3Ga,MnAs slabs
Nonthermal Spin Coherent Control
Kimel et al, Nature 435, 655 (2005) ω
ω1
ω1
White et al, PRB 25, 1822 (1982)
?
Some Timescales10‐18as
10‐15fs
10‐12ps
10‐9ns
10‐6µs
10‐3ms
100s
Period of visible light ~ 1 fs
Bohr period of e‐ ~ 150 as
Electronic scattering
Charge transfer
Vibrational motion
Spin reorientationSpontaneous emission
phonons
Structure change
Thermal transfer
Metastable states
Trap states
Much More in Condensed Matters…
• Intriguing materials with metal‐insulator transition, phase transition, topological states, and so on.
• Photo‐induced phase transition.
• Photo‐induced exotic phenomena.
• Subtle spin relaxation process.
• Subtle interface phenomena.
• Delicate coupling that paves new avenue of material control.
order
disorder
Fundamental light‐matter interactions
Cartoon courtesy of Nphoton v7,n 4, 2013