Ultrafast Spectroscopy Gabriela Schlau-Cohen Fleming Group

Why femtoseconds? timescale = distance/velocity ~~~~~~ E ≈ hν ≈ (6.626*10-34kg*m2/s)*(3*108m/s /6*10-7m) ≈ 3*10-19kg*m2/s2 E= ½mv2 v=√(2*E*/m) =√(2*E*/9*10-31kg) =√(2*3*10-19/(9*10-31 ) m2/s2) v=8*105 m/s timescale ≈ (10*10-10m)/(8*105m/s) ≈ 10-15 sec

Ultrafast examples: Photosynthesis: energy transfer in <200 fs (Fleming group) Vision: isomerization of retinal in 200 fs (Mathies group) Dynamics: ring opening reaction in ~100s fs (Leone group) Transition states: Fe(CO)5 ligand exchange in <1 ps (Harris group) High intensity: properties of liquid carbon (Falcone group)

How can we measure things this fast? 1960 1970 1980 1990 2000 10 –6 –9 –12 –15 Timescale (seconds) Year Electronics Optics

Laser Basics Population inversion Pump energy source Lasing transition Four-level system Population inversion Pump energy source Lasing transition Fast decay Pump Transition Laser Transition Fast decay Level empties fast!

What we need for ultrashort pulse generation: Method of creating pulsed output Compressed output Broadband laser pulse

Ultrafast Laser Overview pump Laser oscillator Amplifier medium

3 pieces of ultrafast laser system: Oscillator Regenerative Amplifier Tunable Parametric Amplifier

Oscillator generates short pulses with mode-locking Prisms Cavity with partially reflective mirror Ti:Sapphire laser crystal Pump laser

Titanium: Sapphire 4 state system Upper state lifetime of 3.2 μs for population inversion Broadband of states around lasing wavelength Kerr-Lens effect (non-linear index of refraction) oxygen aluminum Al2O3 lattice

Ti:Sapphire spectral properties Absorption and emission spectra of Ti:Sapphire (nm) Ti:Sapphire spectral properties Intensity (au) FLUORESCENCE (au)

Mode-locking

Mechanism of Mode-locking: Kerr Lens Effect

Compression Prism compression t t Gratings, chirped mirrors

Chirped Pulse Amplification Pulse compressor t Solid state amplifiers Dispersive delay line Short pulse oscillator Stretch Amplify Recompress

Regenerative Amplifier Pulsed pump laser Pockels cell Pulsed seed Ti: Sapph crystal s-polarized light Faraday rotator thin-film polarizer Pockels cell p-polarized light

Optical Parametric Amplification (OPA) OPA/NOPA Parametric amplification Non-linear process Energy, momentum conserved w1 w1 “seed" "signal" w2 w3 "idler" “pump" Optical Parametric Amplification (OPA)

Non-linear processes wsig Emitted-light frequency

Time Resolution for P(3) “Excitation pulses” Variably delayed “Probe pulse” “Signal pulse” Medium under study Signal pulse energy Delay

Two-Dimensional Electronic Spectroscopy can study: Electronic structure Energy transfer dynamics Coupling Coherence Correlation functions

2D Spectroscopy Dimer Model (Theory) Excitation at one wavelength influences emission at other wavelengths Diagonal peaks are linear absorption Cross peaks are coupling and energy transfer Excited State Absorption Homogeneous Linewidth ωt (“emission”) Cross Peak Inhomogeneous Linewidth ωτ (“absorption”)

Electronic Coupling E E e2 J J D e1 e2 e1 g1 g2 1 Dimer 2

Principles of 2D Spectroscopy SIGNAL Time ABSORPTION FREQUENCY EMISSION FREQUENCY Recovered from Experiment

2D Heterodyne Spectroscopy meter coh. time pop. time echo time 1 2 3 4 t T t spherical mirror 4=LO 1 2 3 sig sample OD3 4 1&2 3 3&4 2 1 diffractive optic (DO) delay 2 Opt. Lett. 29 (8) 884 (2004) 2 f delay 1

Experimental Set-up

Fourier Transform

Future directions of ultrafast Faster: further compression into the attosecond regime More Powerful: higher energy transitions with coherent light in the x-ray regime

2D spectrum with cross-peaks A measurement at the amplitude level Positively Correlated Spectral Motion Negatively Correlated Spectral Motion