Research
Frequency Comb Development
Our Technologies:
Ultrafast Spectroscopy with Frequency Combs
Cavity-Enhanced Transient Absorption Spectroscopy
We use ultrafast fiber-laser frequency combs and couple them to external enhancement cavities to both increase the power and effective absorption pathlength, thus giving signal enhancements of several orders of magnitude over traditional transient absorption spectroscopy. This technique allows us to study samples in molecular beams on the femtosecond timescale with transient absorption spectroscopy. Before the development of this technique, ultrafast transient absorption was limited to concentrated condensed phase samples and study of gas phase samples generally required an indirect probe, such as measuring the energy of an ejected electron, or was limited to molecules and states that fluoresce. Now, not only can we now do the same experiment on condense phase molecules as gas phase molecules via transient absorption spectroscopy, we are able to directly probe the molecular states undergoing the dynamics we wish to study. .
Cavity-Enhanced 2D Spectroscopy
Building off of our success with transient absorption spectroscopy, we are building a cavity-enhanced two-dimensional spectrometer. It will soon be coupled to a molecular beam chamber for 2D spectroscopy of dilute species in molecular beams.
Electro-Optical Modulation-Based Frequency Comb
We start with a stable continuous-wave laser and modulate (or shake) it with electro-optic modulators.
Schematic of Cavity-Enhanced Transient Absorption Spectrometer
Cartoon depiction of Cavity-Enhanced 2D Spectroscopy and corresponding pulse sequence
Our Homebuilt Ytterbium-doped Frequency Comb Fiber Lasers
45 fs pulse duration
10 W output power after amplification
50-90 MHz Repetition Rate
1030 nm -1060 nm center wavelength
— X. Li, M.A.R. Reber, C. Corder, P. Zhao, T.K. Allison. Rev. Sci. Inst. 2016; 87:093114
— N.D. Cooper, U.M. Ta, M.A.R. Reber. Appl. Opt. 2023
Current Research Projects
Excited State Dynamics of Small Hydrocarbon Radicals
Hydrocarbon combustion mechanisms are dominated by reactions involving radicals, yet these highly-reactive and short-lived species have been challenging experimentalists for decades. While, linear spectroscopy has been a powerful method for the detection and characterization of these key reactive intermediates, many radicals are seen to have broad and unstructured electronic absorptions, as a result of ultrafast dynamics, and only minimal information can be obtained from these spectra.
Cavity-enhanced transient absorption spectroscopy has sufficient sensitivity for observing femtosecond electronic excited state dynamics of radicals with absorption, for the first time. Allyl radical is our first combustion radical to study because of the integral importance in hydrocarbon combustion and use as model theoretical systems.
Coherent Control using 2D Spectroscopy
This project will develop spectroscopy-driven approach to designing coherent control experiments, specifically improve the sensitivity and resolution of two-dimensional spectroscopy through the use of optical enhancement cavities and frequency comb lasers.
Dual Comb Spectroscopy
We built a frequency comb based upon electro-optic modulation of a continuous-wave (cw) Nd:YAG laser, called an EO-comb or EOM comb. We are working to combine this with our home-built Yb:fiber lasers to do dual comb spectroscopy. The advantage of using an EOM comb is the comb parameters are specified through the electronics instead of through the optics, which should simplify the locking of the two lasers. Stay tuned for first results from our new dual comb, and dual laser, spectrometer!
Publications
Todd Eliason, Payton A. Parker, and Melanie A. R. Reber, "Electro-optic frequency comb generation via cascaded modulators driven at lower frequency harmonics," Opt. Express 32, 36394-36404 (2024)
Walker M. Jones and Melanie A. R. Reber, "Ultrafast structured light through nonlinear frequency generation in an optical enhancement cavity," Opt. Lett. 49, 4999-5002 (2024)
Nicholas D. Cooper, Uyen M. Ta, and Melanie A. R. Reber, "Spectral shaping of an ultrafast ytterbium fiber laser via a passive intracavity optical filter: a simple and reliable route to sub-45 fs pulses," Appl. Opt. 62, 2195-2199 (2023)