Session B03: Spectroscopy in Space and Time I; Frequency Comb Spectroscopy

Frequency comb spectroscopy with coherent and incoherent light

Optical frequency combs are a unique spectroscopic tool, providing an unparalleled combination of frequency accuracy, high-resolution and broad spectral coverage. The field of frequency comb spectroscopy has grown rapidly to encompass multiple spectroscopic scenarios, ranging from trace gas detection for atmospheric sensing to fundamental spectroscopy in chemistry, physics and biology. In this talk, I will highlight our recent development of frequency comb spectroscopy techniques that apply to both the active detection of coherent laser light, as well as passive detection of incoherent thermal light. In the case of coherent spectroscopy, we have developed extremely broad bandwidth mid-infrared frequency combs that span from 3 microns to nearly 25 microns. Read out of spectral information imprinted on the infrared frequency comb is enabled by electro-optic sampling, where a second comb at a wavelength of 1.5 micron, and a slightly different repetition rate, stroboscopically samples and upconverts the mid-infrared comb to the near-infrared. This approach has the benefit of shot-noise limited detection with robust 1.5 micron detectors that have significantly greater efficiency, and spectral and electrical bandwidth when compared to midinfrared detectors. In a second case, we employ laser-based heterodyne radiometry to measure incoherent light sources in the near-infrared and introduce techniques for absolute frequency calibration with a laser frequency comb. Here we measure solar and atmospheric spectral features with signal-to-noise ratio that matches the fundamental quantum-limited prediction given by the thermal photon distribution and our system's efficiency, bandwidth, and averaging time. Additionally, we explore the direct heterodyne of incoherent thermal light with the frequency comb itself, thereby bringing the power of telecommunications photonics and the precision of frequency comb metrology to laser heterodyne radiometry, with implications for solar and astronomical spectroscopy, remote sensing, and precise Doppler velocimetry.   

Charge transfer states govern the excited state dynamics of melanin-like materials

The broadband absorption of melanin is vital for living organisms, functioning as the coloration and photoprotection survival mechanisms. Beside this, a possible carcinogenic action of melanin upon UV irradiation has been extensively debated. Here we  investigate the origin of the optical properties of melanin-like materials in order to explain this dual behaviour.

We demonstrate that the optical absorption of melanin-like materials, and in particular of polydopamine (PDA), is due, to a large extent, to delocalize charge-transfer (CT) states involving electron donor and electron acceptor molecular unit.

The ultrafast photophysical dynamics of PDA were investigated in detail by multidimensional spectroscopic techniques. The results pinpoint the importance of a joint model to reconstruct the excited state response, considering CT states and heterogeneity within chromophores. Interestingly, under visible excitation a 40-fs formation time of CT-states is observed, while UV excitation results in at least twofold faster formation. While an opposite trend is observed for the recombination lifetimes as a function of the excitation wavelength. Our data highlight the photoprotective role of the visible transitions and give a hint to the debated possible photodamaging pathways under UV excitation.

    

Weak Coupling as a Tool for Enhancing Nonlinear Infrared Signal of Chemical Species

The existing literature contains numerous examples of vibrational strong coupling (VSC) in which the coupling of a vibrational mode and a cavity mode results in new transitions termed vibration-cavity polaritons. These vibration-cavity polaritons possess both molecular and photonic character. VSC has previously proven useful in applications such as modifying reaction kinetics and altering product ratios. Herein we explore a different regime of vibrational-cavity coupling for systems where the optical and vibrational mode are only weakly coupled. Assuming a cavity mode is resonant with the vibrational mode of interest, the act of inserting a molecule into a cavity can enhance the nonlinear signal by over an order of magnitude. This effect manifests as increased amplitude in two-dimensional infrared (2D-IR) and pump-probe spectra. The signal enhancement occurs despite the lack of energetically-split polaritonic modes commonly associated with VSC. Herein we use the carbonyl stretch of W(CO)₆ to compare the dynamics for weakly coupled molecules with bare molecules via their respective frequency-frequency correlation function (FFCF) parameters. FFCF values such as the homogeneous linewidth contribution are reported in a range of concentrations with various solvents to determine their dependence on the phenomena responsible for the signal enhancement. We also use our measurements to estimate a limit of detection for dilute weakly-coupled solutions such as W(CO)₆. A more thorough understanding of the weak coupling regime could give rise to new methods of chemical sensing for especially dilute systems and extend 2D-IR to previously inaccessible species with particularly weak transition dipole moments.   

Mid-Infrared Frequency Comb Spectroscopy for Chemical Reaction Kinetics

The 2005 Nobel Prize in Physics was awarded in part to John Hall and Theodor Hänsch for their development of optical frequency combs. As a simultaneously spectrally broadband and high-resolution light source, frequency comb lasers have been successfully implemented in a variety of scientific fields ranging from metrology to high resolution spectroscopy. In this talk, I will highlight the use of infrared frequency comb laser spectroscopy in chemical reaction kinetics and high-resolution spectroscopy, particularly as it pertains to studying gas-phase chemistry. I will give you an overview of frequency comb lasers, the design and implementation of a spatially dispersive infrared frequency comb spectrometer, and results from our newly built system.

    

Transient absorption spectroscopy in molecular beams

Molecular beams are the ideal environment for observing and understanding molecular dynamics on ultrafast timescales. The lack of solvent interactions and lowered vibrational temperatures make experimental signals easier to interpret and theoretical modeling more tractable enabling more direct comparisons. Unfortunately, optical ultrafast measurements of gas-phase samples are typically not possible due to the low sample densities. To address this gap, we have developed a new spectrometer using femtosecond enhancement cavities and frequency comb lasers with a demonstrated detection limit of ΔOD = 1 x 10^-9/√Hz, making transient absorption measurements in molecular beams possible. This technique also allows for direct comparison to conventional solution phase transient absorption studies as well as opens up new sample avenues possible only in molecular beams such as clusters or radicals. In this talk, we will discuss ultrafast transient absorption measurements in molecular beams and direct comparison to solution-phase data. We explore the effect of argon clustering on excited-state proton transfer and internal conversion in salicylideneaniline and the competition between intersystem crossing and internal conversion in 2-thiouracil.   

Influence of the molecular structure on the electronic coherence triggered by ionization

Ultrafast dynamics of the electron density of a molecule can be initiated by the creation of an electronic states superposition. The strong correlation between the nuclear and electronic motion induces a fast decoherence within just a few femtoseconds, making the experimental observation of electronic oscillations challenging. In order to find suitable molecules we presented in a previous publication an efficient algorithm to compute the time evolution of the electronic coherence created by the ionization of a molecule using an efficient semiclassical method for the nuclear dynamics in combination with a high-level ab initio evaluation of the electronic structure. We propose here to apply this algorithm to a new molecular dataset to investigate the influence of the molecular structure on the duration of the electronic coherence. The explored chemical space has been constructed by defining various carbon-based skeletons and substituents to systematically build all possible combinations. In addition we studied molecules found in the literature for which the electronic dynamics has been only evaluated in the absence of nuclear motion.   

Invited Speaker: Jun JiangTitle: Two-color cavity ringdown spectroscopy

In this talk, we introduce the two-color cavity ringdown (2C-CRD) technique. The method utilizes cavity-enhanced pump power and probe absorption pathlength, and offers generally applicable, high-sensitivity, high-selectivity optical detection. With the pump radiation switched off during every other probe ringdown, the net 2C signal from the difference between the pump-on and pump-off decay rates is immune to drifts of the CRD rates and spectral overlaps from non-target one-photon transitions.

The measurement capabilities of our 2C-CRD method are demonstrated with the first-ever room-temperature optical detection of radiocarbon dioxide (¹⁴CO₂) below its natural abundance concentration. The baseline compensation of the 2C technique allows us to simultaneously achieve high detection sensitivity (k_min~5×10^-13 cm^-1) and orders-of-magnitude reduction in spectral congestion compared to one-color detection. Two quantum cascade lasers are used to excite a pair of ν₃=1←0 and ν₃=2←1 rovibrational transitions of ¹⁴CO₂. With 2 minutes of averaging, the technique differentiates, with an accuracy of 8 parts-per-quadrillion (10¹⁵) (equivalent to 0.7% of the ¹⁴CO₂ natural abundance), six combusted CO₂ samples with ¹⁴CO₂ concentrations ranging from zero to approximately 1.5X of the contemporary abundance. Room temperature optical detection of ¹⁴CO₂ at our demonstrated accuracy is not possible with any other existing techniques.

In addition to its use in ultra-trace analysis, our cavity-enhanced 2C technique is well-suited for rovibrational-state-resolved measurements in chemical dynamics and high-resolution spectroscopic studies. The sensitivity and selectivity of our 2C technique could potentially enable spectroscopic investigations of chemical species at internal energies and molecular conformations that are inaccessible with other methods. One potential extension of our cavity-enhanced pump-probe technique is the use of broadband radiation as the probe, such as a mid-IR frequency comb, to achieve rapid, multiplexed detection with high sensitivity and selectivity.   

Author not Attending

Quantum states of the light, e.g., single photons, entanglement and squeezing, opens up a new avenue for spectroscopy by utilizing the parameters of quantum optical fields as novel control knobs and through the variation of photon statistics. With the advance of cavity quantum electrodynamics and light source technology, imaging and controlling the electron and vibrational motions of molecules can be achieved, towards unprecedented resolution and precision, not accessible by the classical light pulses [1]. In this talk, I will present our recent works on multidimensional spectroscopy using quantum states of light. Several schemes will be discussed: multidimensional coherent probe and Raman spectra with quantum optical fields [2,3,4]. Microscopic models for molecular relaxation using density matrix will be incorporated for a unified understanding of the signals.   

Capturing tunnelling and wavepacket splitting effects on electronic spectra with Hagedorn wavepackets

Vibrationally resolved electronic spectroscopy has improved enormously our understanding of physical and chemical processes taking place in molecules. Semiclassical methods such as the thawed Gaussian approximation (TGA) have been developed to efficiently model quantum dynamics involved in spectroscopy in order to better understand experimental results and guide further investigations.

In TGA, a single Gaussian wavepacket (GWP) is propagated using classical-like equations under the local harmonic approximation (LHA), which keeps the wavepacket in the Gaussian form. To explore wavepackets that are not well represented by a single GWP, we can use a class of semiclassical wavepackets introduced by Hagedorn (Hagedorn wavepackets, or HWP) that are constructed from a complete orthonormal basis in the form of polynomials times a particular GWP. The time evolution of the GWP parameters associated with such wavepackets retains the simple propagation form as in TGA, while a variational method can be applied to the coefficients of basis functions to account for effects beyond LHA. As a result, the Hagedorn wavepackets can be applied to much higher-dimensional systems than grid-based quantum approaches.

On several examples of double-well and anharmonic potentials, we demonstrate the convergence of the HWP approach to exact quantum results with a sufficient number of basis functions and its ability to treat spectroscopy problems involving quantum tunnelling and wavepacket splitting that commonly arise in chemical dynamics but are beyond the reach of a single thawed GWP.