Cavity-enhanced ultrafast spectroscopy: ultrafast meets ultrasensitive

Abstract

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...

Surface‐Enhanced Dual‐Comb Coherent Raman Spectroscopy with Nanoporous Gold Films

“…On the other hand, though ultrasensitive absorption DCS for gas‐phase samples has been demonstrated with high‐finesse cavities, hollow‐core fibers, multi‐pass cells, or by using mid‐ and far‐infrared combs directly interrogating fundamental molecular ro‐vibrational transitions, surface‐enhanced DCS, with orders of magnitude sensitivity improvements, has not been previously introduced. Thus, our work highlights new opportunities for the field of DCS.…”

“…Our scheme provides an enabling solution to such predicament and may lead to new spectroscopic applications, beyond static chemical sensing, such as studies of nonrepetitive chemical reactions or fast characterization of trace amounts of hazardous chemicals in water samples. On the other hand, though ultrasensitive absorption DCS for gas-phase samples has been demonstrated with highfinesse cavities, [29][30][31][32] hollow-core fibers, [7] multi-pass cells, [5,33] or by using mid-and far-infrared combs directly interrogating fundamental molecular ro-vibrational transitions, [2,6,[34][35][36][37][38][39] surface-enhanced DCS, with orders of magnitude sensitivity improvements, has not been previously introduced. Thus, our work highlights new opportunities for the field of DCS.…”

High-power ultrafast Yb:fiber laser frequency combs using commercially available components and basic fiber tools

“…In our laboratory, the primary application of these frequency combs involves coupling them to passive optical cavities 30,96 for either performing spectroscopy 10 or generating high-order harmonics. 97 Here we give examples using these lasers in both applications.…”

“…10 we reported measurements taken in a heliumseeded supersonic expansion using a 700 µm diameter round nozzle, where there is no clustering between the He and the I 2 molecules. If the expansion is instead seeded with Argon under the right conditions, the effects of clustering can be observed as the backing pressure is increased.…”

“…In our lab, for example, we have recently demonstrated a large improvement in the sensitivity of ultrafast optical spectroscopy using frequency comb methods. 10 Most frequency comb lasers operate in the near infrared, based on Ti:sapphire, Er:fiber, or Yb-based gain media, but most spectroscopic applications of frequency combs lie in other regions of the electromagnetic spectrum. It is then desirable to shift the comb to other spectral regions using nonlinear optical techniques [11][12][13][14] but doing this with high efficiency requires high peak powers.…”