We present a new technique using a frequency comb laser and optical cavities for performing ultrafast transient absorption spectroscopy with improved sensitivity. Resonantly enhancing the probe pulses, we demonstrate a sensitivity of ∆OD = 1 × 10 −9 / √ Hz for averaging times as long as 30 s per delay point (∆OD min = 2 × 10 −10 ). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at high repetition-rate, so that signals can be recorded from samples with optical densities as low as OD ≈ 10 −8 , or column densities < 10 10 molecules/cm 2 . This high sensitivity enables new directions for ultrafast spectroscopy.The advent of the mode-locked Ti:Sapphire laser in the early 1990's [1] made ultrafast pumpprobe measurements routine and widely accessible to a broad range of scientists. This was largely due to the Ti:Sapphire laser's robustness compared to the dye-lasers it replaced.However, another aspect of the Ti:Sapphire laser that made many experiments possible was its capability of low-noise performance. This allowed for measurements of very small changes in the optical properties of a sample induced by a pump pulse, which is necessary when the sample is either dilute [2,3,4] or must be excited weakly to probe the desired these authors contributed equally to this work 1 arXiv:1511.02973v1 [physics.optics] 10 Nov 2015physics [5,6]. Even with relatively noisy chirped pulse amplified systems, one can measure changes in absorbance or reflection to a few parts in 10 6 , and ultrafast transient absorption spectroscopy is the simplest and most widely applied form of ultrafast spectroscopy [7].Despite this enormous progress, there remain many samples for which ultrafast optical spectroscopy is still prohibitively difficult. Most directly related to the current work are the "designer" gas-phase molecules and molecular clusters that can be produced in a supersonic expansion. With optical spectroscopy seemingly hopeless, ultrafast experimenters perform measurements on these systems by ionizing the molecules with UV pulses or strong fields [8,9], detecting the resulting ions and electrons. This is indeed extremely sensitive due to the capabilities of single particle detection and background free signals. However, ionization projects the molecular state of interest onto a very different manifold of final states than optical measurements, and this can make the comparison of experimental data from gas phase and condensed phase highly non-trivial [10,11]. Furthermore, while dynamics of electronically excited states can be probed by ionization, there exists no ionization-based methods for probing purely vibrational dynamics analogous to the powerful tools of ultrafast infrared spectroscopy [12].It was realized in the early days of lasers that an optical resonator is useful for absorption enhancement due to the fact that light passes through a sample many times [13]. Today, cavities are widely used in many different contexts for the enhancement of optical signals [14].Recently, several groups have...
We present a detailed description of the design, construction, and performance of high-power ultrafast Yb:fiber laser frequency combs in operation in our laboratory. We discuss two such laser systems: an 87 MHz, 9 W, 85 fs laser operating at 1060 nm and an 87 MHz, 80 W, 155 fs laser operating at 1035 nm. Both are constructed using low-cost, commercially available components, and can be assembled using only basic tools for cleaving and splicing single-mode fibers. We describe practical methods for achieving and characterizing low-noise single-pulse operation and long-term stability from Yb:fiber oscillators based on nonlinear polarization evolution. Stabilization of the combs using a variety of transducers, including a new method for tuning the carrier-envelope offset frequency, is discussed. High average power is achieved through chirped-pulse amplification in simple fiber amplifiers based on double-clad photonic crystal fibers. We describe the use of these combs in several applications, including ultrasensitive femtosecond time-resolved spectroscopy and cavity-enhanced high-order harmonic generation. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C5H9) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to Trot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N − 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40–90 #/cm−1) in the critical regime (ρ ≈ 100 #/cm−1) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C2 symmetry double minimum 1D energy surface over a C2v transition state. The inversion barrier [Vbarrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C2 symmetry equilibrium geometry and tunneling splittings for the ground state too small (<1 MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a “deceleration” of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding “acceleration” of IVR processes in linear hydrocarbons.
Multidimensional spectroscopy has been shown to be a powerful tool to study dynamics of complex systems in the condensed phase. However, 2D spectroscopy in the gas phase, specifically of dilute species in molecular beams, has yet to be realized. There are many complex systems, such as small clusters or transient intermediates, for which the added information from 2D spectroscopy would aid in the understanding of structures and dynamics. We use the unique properties of frequency comb lasers to improve multidimensional spectroscopy with the goal of ultrafast, 2D-spectroscopy of dilute species in molecular beams. First, we are creating a novel 2D spectrometer utilizing a homebuilt Yb-fiber frequency comb laser and an electro-optic modulator-based frequency comb. Inspired by dual-comb spectroscopy, this converts the signal from optical to radio frequencies via heterodyne detection and eliminates the need for a traditional spectrometer. A second benefit of using frequency comb lasers is that the ultrafast pulses can be coupled into enhancement cavities, greatly increasing the sensitivity of the technique. By improving the sensitivity, ultrafast 2D spectroscopy of dilute species in molecular beams will be possible for the first time. Current progress towards cavity-enhanced 2D spectroscopy will be discussed.
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