1. Ultrafast goes Ultra-Sensitive:
Cavity-Enhanced Transient Absorption Spectroscopy
Melanie A Roberts Reber*, Yuning Chen, Thomas K Allison
Allison Lab
Stony Brook University, Stony Brook, NY
*(soon to be at the University of Georgia)

2. Ultrafast Spectroscopy
Stroboscopic imaging
Detector is slow, but exposure time is short.
Femtosecond, and now attosecond, resolution.
Widely applied to solids, liquids, gasses.
But what is the “Probe-Related Signal”
Condensed Phase ( 1mM ~ 1017 molecules/cm3)
Absorbed probe light (transient absorption)
Nonlinear wave mixing (e.g. 2D spectroscopy)
Fluorescence (if lucky)
Molecular Beams (< 1013 molecules/cm3)
Photelectrons or ions.
Flourescence (if lucky).

3. Transient Absorption Spectroscopyτ
Pump light
Probe light
Sample
Detector
Standard experiment conditions
Solution phase
Molecular beams
Number density (molecules/cc)
1018
Less than 1014
Concentration (mmol/L) 1
Less than 1.5*10-4
Transient Absorption sensitivity is often low to study dilute samples!
Solution: Cavity-Enhanced Transient Absorption Spectroscopy using Ultrafast Frequency Combs

4. Transient Absorption Sensitivity}
Δ(Absorbance) = 10-6 }
Δ(Absorbance) > 10-6

5. Transient Absorption in Molecular Beams?
Pump light
Probe light
Detector
Sample τ
Signal-to-noise- calculation:
1013 #/cm3 density in molecular beams
Pump pulse excited 1% of sample
σ = cm2 (1 Megabarn)
Minimum detectable absorbance of 10-6
Absorbance = n l σ
n = number of molecules
l = pathlength through sample
σ = absorption cross section
How long a sample pathlength would we need for a signal to noise of 1:
10-6 = 1% x 1013 #/cm3 x l x cm2
pathlength, l= 10 cm for S/N = 1

6. Cavity Enhancement Frequency Domain Cavity modes CW Laser ω Laser
Free spectra range
Photon lifetime

7. Coupling into a Cavity Frequency Domain Time Domain Cavity modes
Femtosecond Laser Pulse
Time Domain

8. Coupling into a Cavity Frequency Domain frep Time Domain T =1/ frep
Cavity modes
Frequency comb …
frep
Time Domain
T =1/ frep
Finesses of > 1000 with cavity enhancements of several hundred are possible with bandwidth Δλ~ 30 nm
Adler, F., Thorpe, M., Cossel K.C., & Ye, J. Annu. Rev. Anal. Chem., 2010, 3, 8

9. Transient Absorption Cavity Enhancement
Pump Cavity Contribution
Probe Cavity Contribution
# of molecules excited
Beers Law absorption
For cavity Finesses of 200 and 400,
we get an enhancement of ~ 8100 over traditional Transient Absorption

10. Cavity-Enhanced Transient Absorption Spectrometer: CETAS
Yb:fiber laser - homebuilt 1060 nm, 10 W, 90 MHz repetition rate, ~100 fs pulse duration
arXiv:
Pump cavity and Probe cavity locked to laser
Employ lock-in detection and common-mode noise subtraction

11. Difficulties? … Cavity resonance
Comb Tooth
Cavity resonance
FM Noise → AM Noise conversion from the cavity.
Signal is enhanced… but also there is more noise!
Normally, you’d like to to do each experiment with a fresh sample, but molecular beam flow cannot replensish the sample within 11 ns. …
1/frep = 11.5 ns
6 ns
Time
SOLUTION FOR BOTH: Also couple a reference pulse train to the cavity and subtract:
SIGNAL = PROBE - REFERENCE

12. Demonstration: Molecular Iodine
Gas phase Iodine: Excitation to B state results in coherent ‘ringing’
(See for example: R.M. Bowman, M. Dantus, and A.H. Zewail, Chem. Phys. Lett. 161, 297 (1989))
ΔAbsorbance
Pump-Probe Delay …
550 fs
J. I. Steinfeld, R. N. Zare, L. Jones, and M. Lesk, and W. Klemperer J. Chem. Phys. . 42, 25 (1965)

13. I2 in Helium Transient Absorption Spectroscopy
Absorbance Change (∆OD)
Pump-Probe Delay (ps)
Vibrational Quantum Beats
Rotational Dephasing

14. Low Noise Performance: Allan Deviation
Experimental Sensitivity = 2 x (∆OD)
Overlap and Average of 60 scans
Line with slope = 1/2
For 60 scans, the standard deviation of mean is *10-10, the standard deviation of that is e-11
Allan Deviation is used to describe the amount of noise and type of noise
We are dominated by white noise:
1. We can average and still decrease noise
2. Noise decreases with a slope of 1/2

15. I2 in Argon Clusters Nozzle backing pressure
200 micron slit nozzle, probed 6 mm from nozzle
As Ar pressure is increased,clusters form and excited-state vibrational wavepacket is quenched

16. Cavity Enhanced Transient Absorption Spectroscopy
Acknowledgements
Starting up a lab at the
University of Georgia
Building a new instrument based upon
Cavity Enhanced Transient Absorption Spectroscopy
to study ultrafast dynamics relevant to light harvesting in solar cells
Looking for a Post-Doc!
Thomas Allison
Yuning Chen
Xinlong Li
Peng Zhao
Chris Corder
Equipment Loans:
Michael White
Trevor Sears
Christopher Johnson
Thomas Weinacht and Group

17. Fourier Transform Analysis
Red: Fourier Transform of I2 Data
Blue: Vibrational Beat Frequencies Calculated from Spectroscopic Data
Strongly excite 3 upper state transitions, so see 2 oscillations. Excite 5 total

18. Pre-Time Zero Signal Less Iodine More Iodine
Cavity Transmitted Light Spectrum

19. Frequency Combs Time domain frep = Comb spacing fo = Comb offset from
fo +Df
Time domain
frep = Comb spacing
fo = Comb offset from
harmonics of frep
T =1/ frep
Frequency domain
νn= n frep – fo
frep
I(n) n
Thorpe, M. & Ye, J. Applied Physics B, Springer-Verlag, 2008, 91,

20. Pump Cavity Locking and Modulation
input
Feed forward
Line driver
Function Generator:
3.2 kHz Sine Wave
Error out
integrator
Loop filter
(PI2D)
Vescent D2-125
Modulated Side-of-Line Lock
Modulation
Error in
Cavity Transmitted Light Signal
Frequency

21. Comb-Cavity Locking and Stabilization
Utilize a 2-point lock to lock comb and cavity:
Cavity Reflected Light
grating
Frequency Comb n
Cavity Resonances

22. input Cavity Lock Scheme: Pound-Drever-Hall lock Transmitted light
Slow detector
Transmitted light
Camera
Reflected light
input
grating
Reflected light
Slow PZT
PDH Error Signal
Transmitted light
Loop filter
(PI2D)
Vescent D2-125
Oscillator
EOM
Grating PZT R L I
“fo lock”
Mixer R L I
Function Generator:
2 MHz sine wave
To EOM in
oscillator
“frep lock”

23. Cavity-Enhanced Transient Absorption Spectrometer: CETAS
Yb:fiber laser - homebuilt 1060 nm, 10 W, 90 MHz repetition rate, ~100 fs pulse duration
arXiv:
Pump cavity and Probe cavity locked to laser
Employ lock-in detection and common-mode noise subtraction

24. Coupling into a Cavity Frequency Domain Time Domain Cavity Finesse
Cavity modes
CW Laser
Time Domain
Free spectra range
Cavity Finesse
Photon lifetime

25. What Do You Measure?
Photoelectron Spectroscopy
Transient Absorption Spectroscopy
Clusters are particularly problematic for Photoelectron Spectroscopy because cluster ion nearly always dissociates

26. Ultrafast Spectroscopy
Applied questions:
The initial events in vision
Light-harvesting systems
Photoprotection of DNA
Fundamental quantum mechanics and quantum chemistry:
Proton Transfer,
Isomerization,
Solvation Dynamics, …
Conical Intersections
Vapour Pressure: 1.91E-07 mmHg at 25°C.
Geminate recombination
S. Hahn, JPCB 2000