Phys 590B Condensed Matter Physics: Experimental Methods Optical Spectroscopy of Advanced Materials Basic optics, nonlinear and ultrafast optics Jigang Wang Department of Physics, Iowa State University and Ames Lab- USDoE
Outline (Feb 9 th -20 1. Feb 9 th, 11 th and 13 th : overview, basic optics and spectroscopy 2. Feb 16 th,18 th and 20 th : Advanced optics, ultrafast and nonlinear spectroscopy - femtosecond lasers: case study; spectroscopy techniques: incoherent & coherent transient, magneto-optical, infrared & time-domain THz General References: Demtroder, Laser Spectroscopy: Basic Concepts and Instrumentation Diels and Rudolph (DR), Ultrashort Laser Pulse Phenomena 20 th ) Shah, Ultrashort Spectroscopy of Semiconductors and Semiconductor Nanostructures Chemla, D.S., Ultrafast Transient Nonlinear Optical Processes in Semiconductors For example, see Copper, S.L, Optical Spectroscopy Studies of Metal-Insulator Transitions in Perovskite-related Oxides Jigang Wang, Feb, 2009
Today Overview and Introduction Jigang Wang, Feb, 2009
Light is, in short, the most refined form of matter. Louis de Broglie Berkeley, California
The first spectroscopy experiment
A brief history of optics 17th-century 18 th -century 19th-century 20th-century Kepler, Newton Huygens. Total internal reflection, Telescope, geometrical optics, the wave theory, prism dispersion, the particle theory of light Fresnel, Young Maxwell Michelson Interference, diffraction, expressions for reflected and transmitted waves, unified electricity and magnetism Einstein Light is (1) a phenomenon of empty space (2) both a wave and a particle
The equations of optics Maxwell s equations r r r r E = / E = ρ ε r r r r B= 0 B= µε r B t r E t ε is the permittivity, µ is the permeability of the medium
Solving Maxwell s s equations r r E µε t 2 2 E = 0 2 r r r r Ert (, ) cos( ωt± k ) Light is an Electromagnetic Wave
Wave Properties Velocity E x E t 2 2 µε = 0 2 2 Phase velocity ω k = v = 1 µε Group velocity v / g [ dk dω] 1 ω dn vg = v phase / 1+ n dω
Wave Properties Spectrum 1 THz = 300 µm = 33 cm -1 = 4.1 mev
Refraction At an oblique angle, light can be completely transmitted or completely reflected. "Total internal reflection" is the basis of optical fibers, a billion dollar industry.
Conventional Spectroscopy Methods E T R - Absorption, Reflection, Emission, Interference, Scattering - Spectrally resolved before or after sample
Absorption Spectroscopy Penetration depth into water vs. wavelength Dispersion elements 1 km Penetration depth into water 1 m 1 mm 1 µm Radio Microwave IR UV X-ray 1 km 1 m 1 mm 1 µm 1 nm Wavelength Visible spectrum
Scattering Spectroscopy I Raman 4 λ
Fourier Transform Infrared (FTIR) Spectrometer White light Interference
Nature can do similar tricks by itself
Nature knows interference
The amazing light Laser Light amplification of stimulated emission of radiation (Laser) A laser will lase if the beam increases in irradiance during a round trip: that is, if I 3 > I 0.
Continuous vs. ultrashort pulses of laser Continuous beam: Ultrashort pulse:
How fast is Ultra-fast? milli 10-3 micro 10-6 nano 10-9 pico 10-12 femto 10-15 atto 10-18
The evolution of pulse Lasers 10 6 Time resolution (seconds) 10 9 10 12 10 15 Electronics Optics 1960 1970 1980 1990 2000 Year Courtesy of Trebino
Long vs. short pulses of laser Long pulse Short pulse
Generating short pulses = mode-locking Locking the phases of the laser frequencies yields an ultrashort pulse.
A generic ultrashort-pulse laser A generic ultrafast laser has a broadband gain medium,a pulseshortening device, and two or more mirrors: Pulse-shortening devices include: Saturable absorbers Phase modulators Dispersion compensators Optical-Kerr media
Ultrashort laser pulse broadening Different fquencies travel at different group velocities in materials, causing pulses to expand to highly "chirped" (frequency-swept) pulses. Input ultrashort pulse Any medium Chirped output not-so-ultrashort pulse
Ultrafast Optics is Nonlinear Optics
Ultrashort & Ultrabroadband Spectral Density 1 0 1 10 100 1000 R E T T, α, R PL, FWM Photon Energy (mev) Signal t = -100 fs t = 0 fs λ = 0 fs t = 100 fs λ Time HHG... λ Signals -> M, p, σ, χ (2), χ (3)... Simultaneous high temporal and spectral resolution
Strategic advantages Ultrafast Ultrabroadband e - Manipulation
Ultrafast Magneto-optical optical Spectroscopy Excitation J Wang, LBNL Carriers M Detection θ k Magnetic Ions η k Tunable pump-probe from MIR, NIR to visible Highly sensitive to time reversal symmetry breaking
Ultrafast THz Spectroscopy Electric field in time-domain FFT spectrum Field-resolved Detection EO Signal S/N~10000:1 0.5-3 THz -2 0 2 4 0 1 2 3 Time delay (ps) Frequency (THz) Amplitude and Phase information Real & Imaginary Part of σ(ω), ε(ω) Power Spectrum Complex transmission coefficient t ( ω) Ei EOUT ( ω) ω 2 E ( ) 1 + n + d Z σ( ω) 0 IN () t thin film S Et () t
Coherent Transient Spectroscopy Ultrafast FWM k1 k3 k2 k1+k2- k3 k k k-q H V (q) int k + q ( a b) Residual coulomb interactions the dynamics between quasiparticles
Ultrafast demagnetization (1) (2) (3) (4) θ K /θ K 0.4 0.2 0.0 1 10 100 1000 Time Delay (ps)
Ultrafast spectroscopy of HTc superconductor Cuprate SC: Generic Phase Diagram R. A Kaindl et al., Science, 287, 470 (2000)
Many-body Effects in SWCN α O 1D Band picture e h E g van Hove singularity Energy α - T/T (%) 1.0 0.5 0.0 (A) Transmittance 1.0 0.5 0.0 Photon Energy (mev) 120 160 200 240 280 320 t = -500 fs 200 fs (B) α 1D Exciton picture 1000 fs Energy O 2000 fs O Energy 120 160 200 240 280 320 Photon Energy (mev)
Ultrafast Laser Lab Tour www.cmpgroup.ameslab.gov/ultrafast/ Short pulse oscillat or t Dispersive delay line Solid state amplifiers t t Pulse compressor Spectral Density 1 0 1 10 100 1000 HHG... t Photon Energy (mev)