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  • 1. Fundamentals of ultrafast optics and lasers

    2. Laser-based static spectroscopy

    3. Time-resolved spectroscopy

    Femtsecond pulse generation: active and passive mode-locking, ultrafast amplifiers

    Laser Raman/Raleigh, multi-photon excitation spectroscopy; SWCNs

    Ultrafast incoherent & coherent transient, magneto- optical, infrared & time-domain THz

    SWCNs, (Ga,Mn)As, HTc superconductors

    Today Today

    Jigang Wang, Feb, 2009

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    A A femtosecondfemtosecond laser oscillator laser oscillator

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Higher IntensitiesHigher Intensities

    Rep rate (pps)

    Pu ls

    e en

    er gy

    (J )

    10910610310010-3 10-9

    10-6

    100

    10-3

    Oscillator

    Cavity-dumped oscillator

    RegA

    Regenerative

    Regen + multipass

    Regen + multi- multi-pass

    1 W average power

    Francois Salin, CELIA, France

    U ltrafast am

    p lifiers

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Ultrafast AmplifierUltrafast Amplifier

    Pulse compressor

    t

    t

    Solid state amplifiers

    t

    Dispersive delay line t

    Short pulse

    oscillator

    Regenerative amplifier scheme

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Regenerative Regenerative amplifieramplifier

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Ultrafast AmplifiersUltrafast Amplifiers

    PC1

    PC2WP

    Rod TFP

    TFPSeed input

    M1

    M2

    Before injection

    Intra-cavity components: M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film Polarizer PC2 : Pockels Cell

    Only the pulse to be amplified enters the cavity

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Ultrafast AmplifiersUltrafast Amplifiers

    Intra-cavity components: M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film Polarizer PC2 : Pockels Cell

    PC1

    PC2WP

    Rod TFP

    TFPSeed input

    M1

    M2

    Regen operation: pulse injection

    V1=Vλ/2

    V2=Vλ/4

    Pulse is injected using the external Pockels cell PC1. Pulse is trapped using the internal Pockels cell PC2.

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Intra-cavity components: M1, M2 : End mirrors Rod : Ti:Sapphire rod WP : ¼ Waveplate TFP : Thin Film Polarizer PC2 : Pockels Cell

    PC1

    PC2WP

    Rod TFP

    TFPSeed input

    M1

    M2

    Regen operation: pulse ejection

    V2=Vλ/4

    output

    V2=0

    :internal Pockels cell PC2 is turned off

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    The worldThe world’’s largest lasers largest laser Almost 10 years journey, due next month!

    192 shaped pulses; 1.8 MJ total energy

    National Ignition Facility (LLNL)

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Lasers as spectroscopy light sources Lasers as spectroscopy light sources

    1. Static spectroscopy using CW lasers

    2. Static spectroscopy using ultrashort pulsed lasers Laser Raman/Raleigh scattering, multi-photon excitation Spectroscopy; SWCNs

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Laser scattering experiment Laser scattering experiment -- basics basics

    Excitation laser

    Scattered light

    Basic Instrumentation:

    – Illuminate a sample with laser light (e.g. 532nm, 780nm)

    – Scattered (no absorbed) light in two forms – collection and spectrally resolved detection

    • Elastic (Rayleigh) → λscattered = λincident • Inelastic (Raman) → λscattered ≠ λincident

    Photon energy ωp 0

    30

    50

    60

    In te

    ns ity

    I s

    Spectrally-resolved detection

    Rayleigh Raman Raman

    ωe

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Laser Rayleigh and Raman scatteringLaser Rayleigh and Raman scattering

    tIRωααα sin10 +=

    tEE eωααµ sin0==

    ])cos()[cos( 2

    1 sin 010 ttEtE IReIRee ωωωωαωαµ +−−+=

    Induces polarization P = N0µ oscillates at three frequencies!

    Induced dipole Polarizability Incoming field

    ωIR

    ωe ωs

    ωIR

    ωe ωs

    ωIR

    ωs

    E1

    E2

    Virtual State

    Rayleigh Stokes Raman Anti-Stokes Raman

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Signals from the scattering experimentsSignals from the scattering experiments

    • Spectrum – ωIR can be molecular vibrations and low energy collective excitations such as

    phonons, magnons, plasmons, spin flip transitions…

    • Scattered intensity – ~ 0.1 part per million photons

    • Cross section – ~ 10-30 cm2

    R

    θ 462

    2

    2 4

    2

    2

    0 )2 ()

    2

    1 ()

    2 (

    2

    cos1 −∝ + −+= λ

    λ πθ d

    n

    n

    R II

    2 2

    2

    4

    65

    ) 2

    1 (

    3

    2

    + −=

    n

    nd s λ

    πσ ωσ h/0 sph IN =

  • Reman/Rayleigh scattering – a net change in polarizability

    Absorption, FTIR – a net change in dipole moment,

    Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    The origin of polarizabilityThe origin of polarizability

    Tendency of charge distribution or wave function of a dipole to be distorted by local E field, i.e.,

    Lex ENE 00 /χεα = χ: electrical susceptibility 0/ ≠dtdα

    o C

    o o C

    o o C

    o 0/ ≠dtdµ

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Selection rules: Raman vs. IRSelection rules: Raman vs. IR

    ωe ωs

    ωIR

    ωe ωs

    E1

    Rayleigh Stokes Raman

    V

    )(),()( 0 ises EP ωωωχεω =

    11 ),( EVVEse MM →→∝ωωχ

    E2 E1

    M is dipole transition element, e.g., where η is along E filed >

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    SingleSingle--wall carbon nanotubeswall carbon nanotubes

    Metallic Semiconducting

    Ch = na + mb

    n – m = 3M + ν

    1) M = ν = 0

    2) M ≠ 0, ν = 0

    3) M ≠ 0, ν = ±1

    Metal

    Narrow Gap Semicond.

    Large Gap Semicond.

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Example: Laser Raman in SWCNsExample: Laser Raman in SWCNs

    “Dark-field Spectroscopy”

    1. Presence of nanotubes 2. Orientation of isolated tubes or aligned samples 3. Diameters of carbon nanotubes: 4. Mechanical strain

    Raman Intensity vs. shift

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    Rayleigh scatterings in individual SWCNsRayleigh scatterings in individual SWCNs

    In-situ CVD growth across etched slit

    Rayleigh Spectra

    Energy (eV)

    Heinz, Brus, Colombia Univ

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    MultiMulti--photon Excitation Spectroscopy in SWCNsphoton Excitation Spectroscopy in SWCNs

    α

    1s

    2p

    Eg

    Heinz, Brus, Colombia Univ

  • Motivations and basic schemes

    Transient transmission and reflection spectroscopy

    Ultrafast magneto-optical spectroscopy

    Ultrafast mid-infrared/THz spectroscopy

    Coherent transient spectroscopy

    Examples

    1. Time-resolved (ultrafast) laser spectroscopy

    TodayToday’’s Lectures Lecture

    Jigang Wang, Feb, 2009

  • Jigang Wang, http://www.cmpgroup.ameslab.gov/ultrafast/

    TimeTime--resolved laser spectroscopy: whyresolved laser spectroscopy: why

    Ultra-fast

    Ultra-broadband

    Ultra-intensive

    Manipulation

    Fundamental time scales for key microscopic interactions

    Energy scales of important collective excitations

    Searching for new regimes of condensed matter physics

    A new paradigm for condensed matter physics

  • Fundamental time scales in condensed matterFundamental time scales in condensed matter

    Jigang Wang, Feb, 2009

    10-9 s = 1 ns

    10-12 s = 1 ps

    10-15 s = 1 fs

    Time

    carrier recombination (100ps-1ns)

    carrier cooling (1-100ps) e-acoustic phonon (1-100 ps)

    e-opitcal phonon scattering (

  • Fundamental energy scales in condensed matterFundamental energy scales in condensed matter

    Jigang Wang, Feb, 2009

    3 eV

    Energy

    1 eV

    100 meV X

    0 meV

    Mott gap, charge-transfer gap (1-3 eV)

    Interband transition in most semiconductors (400 meV – 2 eV) Multi-phonons and multi-magnons (50-500 meV)

    Intra-exciton trainsiton in semiconducting SWCNs (150 meV - 300 meV)

    Polarons (20-300 meV)

    Pseudogap excitation (30-300 meV) Optical phonons (40-70 meV) Magnons (10- 40 meV)

    Superconduting gap (1-40 meV)

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