Cavity-Enhanced Ultrafast Transient Absorption Spectroscopy

Kevin Keleher, Yuning Chen, Melanie Roberts Reber, Thomas K. AllisonDepartments of Chemistry and Physics, Stony Brook University

IntroductionOur group is to develop the new technique of cavity-enhanced ultrafast

transient absorption spectroscopy to observe ultrafast dynamics of

molecules. This technique uses high-finesse optical cavities and Ytterbium

fiber laser frequency combs to allow for high sensitivity in the absorption

spectroscopy. In this experiment we will analyze the dynamics of visible

chromophores in the gas phase, and later on observe the vibrational

dynamics of hydrogen-bonded clusters.

Ytterbium Fiber Laser Design

Yb Oscillator OutputThe laser can be mode locked in several dispersion regimes by manipulating

the distance between the gratings. Mode locking in the anomalous regime,

where shorter wavelengths have a faster group velocity, allows for a wider

spectrum.

Spectrometer Probe Cavity Supersonic

Expansion

System Setup

Yb Oscillator

Stable YAG

To fo stabilization

electronics

Fiber

Stretcher

AOM

OPO

Pump Cavity

Vacuum Chamber

Tunable PDH

CavityLock

Delay Stage

Doubling

Crystal

Doubling

Crystal

Beam

Splitter

Grating

Compressor

Nonlinear polarization evolution is used to mode lock the laser passively

(no external signal required).

Actual

Laser

AutocorrelatorAn intensity autocorrelator is used to determine the pulse duration of the

laser system’s output. A time delay τ is given to one of the split beams. A second harmonic signal, S(τ), is produced in a BBO crystal. Mathematically, S(τ) is the autocorrelation of the incident pulse.

𝑆 = 𝐼 𝑡 𝐼 𝑡 − 𝜏 𝑑𝑡

WDM

Pump Laser

Yb-doped Fiber

λ/4

λ/4

λ/2

GratingsPolarizingBeam-Splitter

To Amplifier

HI-1060 Fiber

Piezo

Output Power: 60 mW (average)Pump Power: 150 mWRepetition Rate of 60 MHz

Frequency combs in CE spectroscopy

inT

Time difference between pulses can

be adjusted to allow for constructive

interference in the cavity.

Frequency comb teeth line up with

the resonate modes of the cavity.

Dispersion changes the optical cavity modes, shifting them out of

alignment with the comb’s teeth, because of this the dispersion limits the

attainable optical bandwidth in the cavity.

Vacuum ChamberThe chamber has a 4 foot by 2 foot design, and will have a background

pressure of 100 mtorr. Due to the large external forces acting on the

chamber, the aluminum chamber is supported by perpendicular struts on

both top and bottom of the chamber. The throughput of the system will be as

much as 23 TorrL/s and can be calculated by using the following equation.

𝑄 = 𝑆𝑒𝑓𝑓 × 𝑃𝐶Where Q = throughput, 𝑃𝐶 = chamber pressure, 𝑆𝑒𝑓𝑓 = pumping speed

Vacuum pumps currently used are an Edwards EH-1200 roots blower and a

Edwards E2M80 roughing pump. Testing the newly set up pumps resulted in

pressures about 10 mTorr.

Funding

Lab Photos

Increased Sensitivity in ExperimentHaving high finesse optical cavities (F ≈ 1000), both pump comb and probe

comb are improved by a factor of ~ 𝐹

𝜋, and improving the signal received by

the spectrometer by (𝐹

𝜋)2. This makes this method approximately 105 times

more sensitive than traditional ultrafast transient absorption spectroscopy.

Actual Chamber

Normal Dispersion Anomalous Dispersion

Comb

Amplifier

AmplifierThe Ytterbium doped fiber of the amplifier increases the signal’s power

through stimulated emission of the doping agent. More than 15 W output can

be obtained for 30 W of pump power. We use chirped pulse amplification,

with ~ 100 ps pulses, to avoid nonlinear effects in the amplifier fiber. A pulse

compressor, consisting of two diffraction gratings and a retroreflector, reduce

the pulse duration follow the amplifier.

Symbolic Representation