A 100 kHz Pulse Shaping 2D-IR Spectrometer Based on Dual Yb:KGW Amplifiers

Two-dimensional infrared spectroscopy using a fast-scanning interferometer and chirped pulse upconversion at 100 kHz

We report on a 100-kHz two-dimensional infrared (2DIR) spectrometer in the pump-probe geometry, which we apply to the measurement of the 2DIR spectrum of carboxyhemoglobin. The probe pulses are spectrally resolved by chirped-pulse upconversion (CPU) using a fast 2048-pixel line scan CMOS camera. The two-pulse pump sequence is generated using a conventional interferometer with a fast-scanning mechanical delay line allowing to achieve a scanning frequency of 2 Hz. The resulting modulation frequency of 3.1 kHz is large enough to shift the relevant signal away from the low-frequency noise of the laser source. The combined use of an interferometer on the pump side and of CPU on the probe side opens the way to an improved spectral resolution in both pump and probe dimensions, as compared to currently available 100-kHz 2DIR spectrometers based on pulse shapers and mercury-cadmium telluride detector arrays.

Two-Dimensional Electronic Spectroscopy of Rhodamine 700 Using an 8 fs Ultrabroadband Laser Source and Full-Wavelength Reference Detection

Two-dimensional electronic spectroscopy (2DES) is one of the premier tools for investigating photoinduced condensed phase dynamics, combining high temporal and spectral resolution to probe ultrafast phenomena. We have coupled an ultrabroadband laser source generated with a hollow-core fiber, compressing pulses to have a pulse duration of 8 fs, with a boxcars 2DES interferometer constructed from only conventional optics. The resulting ultrabroad bandwidth and high temporal resolution allow for superior spectral coverage of the typically broad molecular line shapes in the near-IR/visible region in room temperature solutions, and the exploration of the excited state dynamics at the earliest time epoch in complex systems. The new spectrometer is characterized by examining the dynamics of the dye molecule Rhodamine 700 in methanol solution. These data exhibit rich vibrational wavepacket dynamics, with 2DES data unraveling key molecular vibronic couplings between multiple vibrational modes. For the first time in a degenerate broadband 2DES experiment, we demonstrate the implementation of full-wavelength reference detection to correct wavelength-dependent laser intensity fluctuations. The net result is a 4-5× increased signal-to-noise (S/N) ratio compared to data acquired without reference detection, yielding a typical S/N ratio = 28. The increased S/N ratio facilitates more rapid data acquisition and examination of samples at lower optical densities, and thus concentrations, than typically used in 2DES experiments. These advances will help to alleviate the typical high demands on precious samples in 2DES measurements.

Machine-learning based classification of 2D-IR liquid biopsies enables stratification of melanoma relapse risk

Non-linear laser spectroscopy methods such as two-dimensional infrared (2D-IR) produce large, information-rich datasets, while developments in laser technology have brought substantial increases in data collection rates. This combination of data depth and quantity creates the opportunity to unite advanced data science approaches, such as Machine Learning (ML), with 2D-IR to reveal insights that surpass those from established data interpretation methods. To demonstrate this, we show that ML and 2D-IR spectroscopy can classify blood serum samples collected from patients with melanoma according to diagnostically-relevant groupings. Using just 20 μL samples, 2D-IR measures 'protein amide I fingerprints', which reflect the protein profile of blood serum. A hyphenated Partial Least Squares-Support Vector Machine (PLS-SVM) model was able to classify 2D-protein fingerprints taken from 40 patients with melanoma according to the presence, absence or later development of metastatic disease. Area under the receiver operating characteristic curve (AUROC) values of 0.75 and 0.86 were obtained when identifying samples from patients who were radiologically cancer free and with metastatic disease respectively. The model was also able to classify (AUROC = 0.80) samples from a third group of patients who were radiologically cancer-free at the point of testing but would go on to develop metastatic disease within five years. This ability to identify post-treatment patients at higher risk of relapse from a spectroscopic measurement of biofluid protein content shows the potential for hybrid 2D-IR-ML analyses and raises the prospect of a new route to an optical blood-based test capable of risk stratification for melanoma patients.

Two-Dimensional Infrared Spectroscopy Resolves the Vibrational Landscape in Donor-Bridge-Acceptor Complexes with Site-Specific Isotopic Labeling

Donor-bridge-acceptor complexes (D-B-A) are important model systems for understanding of light-induced processes. Here, we apply two-color two-dimensional infrared (2D-IR) spectroscopy to D-B-A complexes with a trans-Pt(II) acetylide bridge (D-C≡C-Pt-C≡C-A) to uncover the mechanism of vibrational energy redistribution (IVR). Site-selective 13C isotopic labeling of the bridge is used to decouple the acetylide modes positioned on either side of the Pt-center. Decoupling of the D-acetylide- from the A-acetylide- enables site-specific investigation of vibrational energy transfer (VET) rates, dynamic anharmonicities, and spectral diffusion. Surprisingly, the asymmetrically labeled D-B-A still undergoes intramolecular IVR between acetylide groups even though they are decoupled and positioned across a heavy atom usually perceived as a "vibrational bottleneck". Further, the rate of population transfer from the bridge to the acceptor was both site-specific and distance dependent. We show that vibrational excitation of the acetylide modes is transferred to ligand-centered modes on a subpicosecond time scale, followed by VET to solvent modes on the time scale of a few picoseconds. We also show that isotopic substitution does not affect the rate of spectral diffusion, indicating that changes in the vibrational dynamics are not a result of differences in local environment around the acetylides. Oscillations imprinted on the decay of the vibrationally excited acceptor-localized carbonyl modes show they enter a coherent superposition of states after excitation that dephases over 1-2 ps, and thus cannot be treated as independent in the 2D-IR spectra. These findings elucidate the vibrational landscape governing IR-mediated electron transfer and illustrate the power of isotopic labeling combined with multidimensional IR spectroscopy to disentangle vibrational energy propagation pathways in complex systems.

2D-IR spectroscopy of azide-labeled carbohydrates in H2O

Carbohydrates constitute one of the key classes of biomacromolecules, yet vibrational spectroscopic studies involving carbohydrates remain scarce as spectra are highly congested and lack significant marker vibrations. Recently, we introduced and characterized a thiocyanate-labeled glucose [Gasse et al., J. Chem. Phys. 158, 145101 (2023)] demonstrating 2D-IR spectroscopy of carbohydrates using vibrational probes. Here, we build on that work and test azide groups as alternative for studies of carbohydrates to expand the available set of local probes. Many common carbohydrates with different azide labeling positions, such as galactose, glucose, or lactose, are readily available due to their application in click chemistry and hence do not require additional complex synthesis strategies. In this work, we have characterized azide-labeled glucose,, galactose, acetylglucosamine and lactose in water using IR and 2D-IR spectroscopy to test their potential for future applications in studies of carbohydrate-protein interactions. Our findings indicate that their absorption profiles and vibrational dynamics are primarily determined by the labeling position on the ring. However, we also observe additional variations between samples with the same labeling position. Furthermore, we demonstrate that their usage remains feasible at biologically relevant concentrations, highlighting their potential to probe more complex biological processes, i.e., enzymatic catalysis.

Chirped pulse upconversion for femtosecond mid-infrared spectroscopy at 100 kHz

We demonstrate that chirped pulse up-conversion (CPU), a method routinely used with systems based on 1-kHz Titanium:Sapphire lasers, can be extended to a repetition rate of 100 kHz with an Ytterbium diode-pumped femtosecond amplifier. Individual mid-infrared spectra can thus be measured directly in the near infrared using a fast CMOS linescan camera. After an appropriate Fourier processing, a spectral resolution of 1.1 cm-1 is reported, currently limited by our spectrometer. Additionally, we demonstrate the application of CPU to a pump-probe measurement of the vibrational relaxation in carboxy-hemoglobin, and we show that the combination of fast scanning and fast acquisition enables a straightforward removal of pump scattering interference.

Time-Resolved Dynamics of Metal Halide Perovskite under High Pressure: Recent Progress and Challenges

Metal halide perovskites have garnered significant attention in the scientific community for their promising applications in optoelectronic devices. The application of pressure engineering, a viable technique, has played a crucial role in substantially improving the optoelectronic characteristics of perovskites. Despite notable progress in understanding ground-state structural changes under high pressure, a comprehensive exploration of excited-state dynamics influencing luminescence remains incomplete. This Perspective delves into recent advances in time-resolved dynamics studies of photoexcited metal halide perovskites under high pressure. With a focus on the intricate interplay between structural alterations and electronic properties, we investigate electron-phonon interactions, carrier transport mechanisms, and the influential roles of self-trapped excitons (STEs) and coherent phonons in luminescence. However, significant challenges persist, notably the need for more advanced measurement techniques and a deeper understanding of the phenomena induced by high pressure in perovskites.

Optical Screening and Classification of Drug Binding to Proteins in Human Blood Serum

Protein-drug interactions in the human bloodstream are important factors in applications ranging from drug design, where protein binding influences efficacy and dose delivery, to biomedical diagnostics, where rapid, quantitative measurements could guide optimized treatment regimes. Current measurement approaches use multistep assays, which probe the protein-bound drug fraction indirectly and do not provide fundamental structural or dynamic information about the in vivo protein-drug interaction. We demonstrate that ultrafast 2D-IR spectroscopy can overcome these issues by providing a direct, label-free optical measurement of protein-drug binding in blood serum samples. Four commonly prescribed drugs, known to bind to human serum albumin (HSA), were added to pooled human serum at physiologically relevant concentrations. In each case, spectral changes to the amide I band of the serum sample were observed, consistent with binding to HSA, but were distinct for each of the four drugs. A machine-learning-based classification of the serum samples achieved a total cross-validation prediction accuracy of 92% when differentiating serum-only samples from those with a drug present. Identification on a per-drug basis achieved correct drug identification in 75% of cases. These unique spectroscopic signatures of the drug-protein interaction thus enable the detection and differentiation of drug containing samples and give structural insight into the binding process as well as quantitative information on protein-drug binding. Using currently available instrumentation, the 2D-IR data acquisition required just 1 min and 10 μL of serum per sample, and so these results pave the way to fast, specific, and quantitative measurements of protein-drug binding in vivo with potentially invaluable applications for the development of novel therapies and personalized medicine.

A versatile high-average-power ultrafast infrared driver tailored for high-harmonic generation and vibrational spectroscopy

We report on an ultrafast infrared optical parametric chirped-pulse amplifier (OPCPA), pumped by a 200-W thin-disk Yb-based regenerative amplifier at a repetition rate of 100 kHz. The OPCPA is tunable in the spectral range 1.4-3.9 [Formula: see text]m, generating up to 23 W of < 100-fs signal and 13 W of < 200-fs idler pulses for infrared spectroscopy, with additional spectral filtering capabilities for Raman spectroscopy. The OPCPA can also yield 19 W of 49-fs 1.75-[Formula: see text]m signal or 5 W of 62-fs 2.8-[Formula: see text]m idler pulses with active carrier-to-envelope-phase (CEP) stabilisation for high-harmonic generation (HHG). We illustrate the versatility of the laser design, catering to various experimental requirements for probing ultrafast science.

Anisotropic dynamics of an interfacial enzyme active site observed using tethered substrate analogs and ultrafast 2D IR spectroscopy

Enzymes accelerate the rates of biomolecular reactions by many orders of magnitude compared to bulk solution, and it is widely understood that this catalytic effect arises from a combination of polar pre-organization and electrostatic transition state stabilization. A number of recent reports have also implicated ultrafast (femtosecond-picosecond) timescale motions in enzymatic activity. However, complications arising from spatially-distributed disorder, the occurrence of multiple substrate binding modes, and the influence of hydration dynamics on solvent-exposed active sites still confound many experimental studies. Here we use ultrafast two-dimensional infrared (2D IR) spectroscopy and covalently-tethered substrate analogs to examine dynamical properties of the promiscuous Pyrococcus horikoshii ene-reductase (PhENR) active site in two binding configurations mimicking proposed "inactive" and "reactive" Michaelis complexes. Spectral diffusion measurements of aryl-nitrile substrate analogs reveal an end-to-end tradeoff between fast (sub-ps) and slow (>5 ps) motions. Fermi resonant aryl-azide analogs that sense interactions of coupled oscillators are described. Lineshape and quantum beat analyses of these probes reveal characteristics that correlate with aryl-nitrile frequency fluctuation correlation functions parameters, demonstrating that this anisotropy is an intrinsic property of the water-exposed active site, where countervailing gradients of fast dynamics and disorder in the reactant ground state are maintained near the hydration interface. Our results suggest several plausible factors leading to state-selective rate enhancement and promiscuity in PhENR. This study also highlights a strategy to detect perturbations to vibrational modes outside the transparent window of the mid-IR spectrum, which may be extended to other macromolecular systems.

Adding a second AgGaS2 stage to Ti:sapphire/BBO/AgGaS2 setups increases mid-infrared power twofold

We present a method for increasing the power of mid-infrared laser pulses generated by a conventional beta-barium borate (BBO) optical parametric amplifier (OPA) and AgGaS2 difference frequency generation (DFG) pumped by a Ti:sapphire amplifier. The method involves an additional stage of parametric amplification with a second AgGaS2 crystal pumped by selected outputs of the conventional DFG stage. This method does not require additional pump power from the Ti:sapphire laser source and improves the overall photon conversion efficiency for generating mid-infrared light. It merely requires an additional AgGaS2 crystal and dichroic mirrors. Following difference frequency generation, the method reuses near-infrared light (∼1.9 µm), typically discarded, to pump the additional AgGaS2 stage and amplifies the mid-infrared light twofold. We demonstrate and characterize the power, spectrum, duration, and noise of the mid-IR pulses before and after the second AgGaS2 stage. We observe small changes in center frequencies, bandwidth, and pulse duration for ∼150-fs pulses between 4 and 5 µm.

Breaking Barriers in Ultrafast Spectroscopy and Imaging Using 100 kHz Amplified Yb-Laser Systems

ConspectusUltrafast spectroscopy and imaging have become tools utilized by a broad range of scientists involved in materials, energy, biological, and chemical sciences. Commercialization of ultrafast spectrometers including transient absorption spectrometers, vibrational sum frequency generation spectrometers, and even multidimensional spectrometers have put these advanced spectroscopy measurements into the hands of practitioners originally outside the field of ultrafast spectroscopy. There is now a technology shift occurring in ultrafast spectroscopy, made possible by new Yb-based lasers, that is opening exciting new experiments in the chemical and physical sciences. Amplified Yb-based lasers are not only more compact and efficient than their predecessors but also, most importantly, operate at many times the repetition rate with improved noise characteristics in comparison to the previous generation of Ti:sapphire amplifier technologies. Taken together, these attributes are enabling new experiments, generating improvements to long-standing techniques, and affording the transformation of spectroscopies to microscopies. This Account aims to show that the shift to 100 kHz lasers is a transformative step in nonlinear spectroscopy and imaging, much like the dramatic expansion that occurred with the commercialization of Ti:sapphire laser systems in the 1990s. The impact of this technology will be felt across a great swath of scientific communities. We first describe the technology landscape of amplified Yb-based laser systems used in conjunction with 100 kHz spectrometers operating with shot-to-shot pulse shaping and detection. We also identify the range of different parametric conversion and supercontinuum techniques which now provide a path to making pulses of light optimal for ultrafast spectroscopy. Second, we describe specific instances from our laboratories of how the amplified Yb-based light sources and spectrometers are transformative. For multiple probe time-resolved infrared and transient 2D IR spectroscopy, the gain in temporal span and signal-to-noise enables dynamical spectroscopy measurements from femtoseconds to seconds. These gains widen the applicability of time-resolved infrared techniques across a range of topics in photochemistry, photocatalysis, and photobiology as well as lower the technical barriers to implementation in a laboratory. For 2D visible spectroscopy and microscopy with white light, as well as 2D IR imaging, the high repetition rates of these new Yb-based light sources allow one to spatially map 2D spectra while maintaining high signal-to-noise in the data. To illustrate the gains, we provide examples of imaging applications in the study of photovoltaic materials and spectroelectrochemistry.

Increasing Pump-Probe Signal toward Asymptotic Limits

Optimization of pump-probe signal requires a complete understanding of how signal scales with experimental factors. In simple systems, signal scales quadratically with molar absorptivity, and linearly with fluence, concentration, and path length. In practice, scaling factors weaken beyond certain thresholds (e.g., OD > 0.1) due to asymptotic limits related to optical density, fluence and path length. While computational models can accurately account for subdued scaling, quantitative explanations often appear quite technical in the literature. This Perspective aims to present a simpler understanding of the subject with concise formulas for estimating absolute magnitudes of signal under both ordinary and asymptotic scaling conditions. This formulation may be more appealing for spectroscopists seeking rough estimates of signal or relative comparisons. We identify scaling dependencies of signal with respect to experimental parameters and discuss applications for improving signal under broad conditions. We also review other signal enhancement methods, such as local-oscillator attenuation and plasmonic enhancement, and discuss respective benefits and challenges regarding asymptotic limits that signal cannot exceed.

Temperature-Induced Effects on the Structure of Gramicidin S

We report on the structure of Gramicidin S (GS) in a model membrane mimetic environment represented by the amphipathic solvent 1-octanol using one-dimensional (1D) and two-dimensional (2D) IR spectroscopy. To explore potential structural changes of GS, we also performed a series of spectroscopic measurements at differing temperatures. By analyzing the amide I band and using 2D-IR spectral changes, results could be associated to the disruption of aggregates/oligomers, as well as structural and conformational changes happening in the concentrated solution of GS. The ability of 2D-IR to enable differentiation in melting transitions of oligomerized GS structures is attributed to the sensitivity of the technique to vibrational coupling. Two melting transition temperatures were identified; at T_m1 in the range 41-47 °C where the GS aggregates/oligomers disassemble and at T_m2 = 57 ± 2 °C where there is significant change involving GS β-sheet-type hydrogen bonds, whereby it is proposed that there is loss of interpeptide hydrogen bonds and we are left with mainly intrapeptide β-sheet and β-turn hydrogen bonds of the smaller oligomers. Further analysis with quantum mechanical/molecular mechanics (QM/MM) simulations and second derivative results highlighted the participation of active GS side chains. Ultimately, this work contributes toward understanding the GS structure and the formulation of GS analogues with improved bioactivity.

Ultrafast 2D-IR spectroscopy of intensely optically scattering pelleted solid catalysts

Solid, powdered samples are often prepared for infrared (IR) spectroscopy analysis in the form of compressed pellets. The intense scattering of incident light by such samples inhibits applications of more advanced IR spectroscopic techniques, such as two-dimensional (2D)-IR spectroscopy. We describe here an experimental approach that enables the measurement of high-quality 2D-IR spectra from scattering pellets of zeolites, titania, and fumed silica in the OD-stretching region of the spectrum under flowing gas and variable temperature up to ∼500 ^◦C. In addition to known scatter suppression techniques, such as phase cycling and polarization control, we demonstrate how a bright probe laser beam comparable in strength with the pump beam provides effective scatter suppression. The possible nonlinear signals arising from this approach are discussed and shown to be limited in consequence. In the intense focus of 2D-IR laser beams, a free-standing solid pellet may become elevated in temperature compared with its surroundings. The effects of steady state and transient laser heating effects on practical applications are discussed.

2D-IR spectroscopy of proteins in H2O-A Perspective

The form of the amide I infrared absorption band provides a sensitive probe of the secondary structure and dynamics of proteins in the solution phase. However, the frequency coincidence of the amide I band with the bending vibrational mode of H₂O has necessitated the widespread use of deuterated solvents. Recently, it has been demonstrated that ultrafast 2D-IR spectroscopy allows the detection of the protein amide I band in H₂O-based fluids, meaning that IR methods can now be applied to study proteins in physiologically relevant solvents. In this perspective, we describe the basis of the 2D-IR method for observing the protein amide I band in H₂O and show how this development has the potential to impact areas ranging from our fundamental appreciation of protein structural dynamics to new applications for 2D-IR spectroscopy in the analytical and biomedical sciences. In addition, we discuss how the spectral response of water, rather than being a hindrance, now provides a basis for new approaches to data pre-processing, standardization of 2D-IR data collection, and signal quantification. Ultimately, we visualize a direction of travel toward the creation of 2D-IR spectral libraries that can be linked to advanced computational methods for use in high-throughput protein screening and disease diagnosis.

Spectrophotometric Concentration Analysis Without Molar Absorption Coefficients by Two-Dimensional-Infrared and Fourier Transform Infrared Spectroscopy

A spectrophotometric method for determining relative concentrations of infrared (IR)-active analytes with unknown concentration and unknown molar absorption coefficient is explored. This type of method may be useful for the characterization of complex/heterogeneous liquids or solids, the study of transient species, and for other scenarios where it might be difficult to gain concentration information by other means. Concentration ratios of two species are obtained from their IR absorption and two-dimensional (2D)-IR diagonal bleach signals using simple ratiometric calculations. A simple calculation framework for deriving concentration ratios from spectral data is developed, extended to IR-pump-probe signals, and applied to the calculation of transition dipole ratios. Corrections to account for the attenuation of the 2D-IR signal caused by population relaxation, spectral overlap, wavelength-dependent pump absorption, inhomogeneous broadening, and laser intensity variations are described. A simple formula for calculating the attenuation of the 2D-IR signal due to sample absorption is deduced and by comparison with 2D-IR signals at varying total sample absorbance found to be quantitatively accurate. 2D-IR and Fourier transform infrared spectroscopy of two carbonyl containing species acetone and N-methyl-acetamide dissolved in D2O are used to experimentally confirm the validity of the ratiometric calculations. Finally, to address ambiguities over units and scaling of 2D-IR signals, a physical unit of 2D-IR spectral amplitude in mOD/c m - 1 is proposed.

The effect of caesium alloying on the ultrafast structural dynamics of hybrid organic-inorganic halide perovskites

Hybrid inorganic-organic perovskites have attracted considerable attention over recent years as promising processable electronic materials. In particular, the rich structural dynamics of these 'soft' materials has become a subject of investigation and debate due to their direct influence on the perovskites' optoelectronic properties. Significant effort has focused on understanding the role and behaviour of the organic cations within the perovskite, as their rotational dynamics may be linked to material stability, heterogeneity and performance in (opto)electronic devices. To this end, we use two-dimensional IR spectroscopy (2DIR) to understand the effect of partial caesium alloying on the rotational dynamics of the methylammonium cation in the archetypal hybrid perovskite CH₃NH₃PbI₃. We find that caesium incorporation primarily inhibits the slower 'reorientational jump' modes of the organic cation, whilst a smaller effect on the fast 'wobbling time' may be due to distortions and rigidisation of the inorganic cuboctahedral cage. 2DIR centre-line-slope analysis further reveals that while static disorder increases with caesium substitution, the dynamic disorder (reflected in the phase memory of the N-H stretching mode of methylammonium) is largely independent of caesium addition. Our results contribute to the development of a unified model of cation dynamics within organohalide perovskites.

Inline amplification of mid-infrared intrapulse difference frequency generation

We demonstrate an ultrafast mid-infrared source architecture that implements both intrapulse difference frequency generation (iDFG) and further optical parametric amplification (OPA), in an all-inline configuration. The source is driven by a nonlinearly compressed high-energy Yb-doped-fiber amplifier delivering 7.4 fs pulses at a central wavelength of 1030 nm, at a repetition rate of 250 kHz. It delivers 1 µJ, 73 fs pulses at a central wavelength of 8 µm, tunable over more than one octave. By enrolling all the pump photons in the iDFG process and recycling the long wavelength pump photons amplified in the iDFG in the subsequent OPA, we obtain an unprecedented overall optical efficiency of 2%. These performances, combining high energy and repetition rate in a very simple all-inline setup, make this technique ideally suited for a growing number of applications, such as high harmonic generation in solids or two-dimensional infrared spectroscopy experiments.

Generative Adversarial Neural Networks for Denoising Coherent Multidimensional Spectra

Ultrafast spectroscopy often involves measuring weak signals and long data acquisition times. Spectra are typically collected as a "pump-probe" spectrum by measuring differences in intensity across laser shots. Shot-to-shot intensity fluctuations are most often the primary source of noise in ultrafast spectroscopy. Here, we present a novel approach for denoising ultrafast two-dimensional infrared (2D IR) spectra using conditional generative adversarial neural networks (cGANNs). The cGANN approach is able to eliminate shot-to-shot noise and reconstruct the line shapes present in the noisy input spectrum. We present a general approach for training the cGANN using matched pairs of noisy and clean synthetic 2D IR spectra based on the Kubo-line shape model for a three-level system. Experimental shot-to-shot laser noise is added to synthetic spectra to recreate the noise profile present in measured experimental spectra. The cGANNs can recover line shapes from synthetic 2D IR spectra with signal-to-noise ratios as low as 2:1, while largely preserving the key features such as center frequencies, line widths, and diagonal elongation. In addition, we benchmark the performance of the cGANN using experimental 2D IR spectra of an ester carbonyl vibrational probe and demonstrate that, by applying the cGANN denoising approach, we can extract the frequency-frequency time correlation function (FFCF) from reconstructed spectra using a nodal-line slope analysis. Finally, we provide a set of practical guidelines for extending the denoising method to other coherent multidimensional spectroscopies.

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm

We present a setup for generating broadband (up to 1050 cm^-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Detection of paracetamol binding to albumin in blood serum using 2D-IR spectroscopy

Two-Dimensional Infrared (2D-IR) spectroscopy is used to detect binding of paracetamol with proteins in blood serum. Quantitative peak patterns are observed indicating structural changes of the albumins' secondary structure when paracetamol bound.

Least-Squares Fitting of Multidimensional Spectra to Kubo Line-Shape Models

We report a comprehensive study of the efficacy of least-squares fitting of multidimensional spectra to generalized Kubo line-shape models and introduce a novel least-squares fitting metric, termed the scale invariant gradient norm (SIGN), that enables a highly reliable and versatile algorithm. The precision of dephasing parameters is between 8× and 50× better for nonlinear model fitting compared to that for the centerline-slope (CLS) method, which effectively increases data acquisition efficiency by 1–2 orders of magnitude. Whereas the CLS method requires sequential fitting of both the nonlinear and linear spectra, our model fitting algorithm only requires nonlinear spectra but accurately predicts the linear spectrum. We show an experimental example in which the CLS time constants differ by 60% for independent measurements of the same system, while the Kubo time constants differ by only 10% for model fitting. This suggests that model fitting is a far more robust method of measuring spectral diffusion than the CLS method, which is more susceptible to structured residual signals that are not removable by pure solvent subtraction. Statistical analysis of the CLS method reveals a fundamental oversight in accounting for the propagation of uncertainty by Kubo time constants in the process of fitting to the linear absorption spectrum. A standalone desktop app and source code for the least-squares fitting algorithm are freely available, with example line-shape models and data. We have written the MATLAB source code in a generic framework where users may supply custom line-shape models. Using this application, a standard desktop fits a 12-parameter generalized Kubo model to a 10⁶ data-point spectrum in a few minutes.

Full spectrum 2D IR spectroscopy reveals below-gap absorption and phonon dynamics in the mid-IR bandgap semiconductor InAs

While the mid-infrared spectral region spans more than 3000 cm^-1, ultrafast mid-IR spectroscopies are normally limited to the spectral bandwidth that can be generated in optical parametric amplifiers-typically a few hundred cm^-1. As such, the spectral coverage in conventional two dimensional infrared (2D IR) spectroscopy captures only about 1% of the full potential 2D mid-IR spectrum. Here, we present 2D IR spectra using a continuum source as both the excitation and probe pulses, thus capturing close to the full 2D IR spectrum. While the continuum pulses span the entire mid-IR range, they are currently too weak to efficiently excite molecular vibrational modes but strong enough to induce electronic responses and excite phonons in semiconductors. We demonstrate the full spectrum 2D IR spectroscopy of the mid-IR bandgap semiconductor indium arsenide with a bandgap at 2855 cm^-1. The measured response extends far below the bandgap and is due to field-induced band-shifting, causing probe absorption below the bandgap. While the band-shifting induces an instantaneous response that exists only during pulse overlap, the 2D IR spectra reveal additional off-diagonal features that decay on longer timescales. These longer-lived off-diagonal features result from coherent phonons excited via a Raman-like process at specific excitation frequencies. This study illustrates that the full spectrum 2D IR spectroscopy of electronic states in the mid-IR is possible with current continuum pulse technology and is effective in characterizing semiconductor properties.

Structure-Dependent Electron Transfer Rates for Dihydrophenazine, Phenoxazine, and Phenothiazine Photoredox Catalysts Employed in Atom Transfer Radical Polymerization

Organic photocatalysts (PCs) are gaining popularity in applications of photoredox catalysis, but few studies have explored their modus operandi. We report a detailed mechanistic investigation of the electron transfer activation step of organocatalyzed atom transfer radical polymerization (O-ATRP) involving electronically excited organic PCs and a radical initiator, methyl 2-bromopropionate (MBP). This study compares nine N-aryl modified PCs possessing dihydrophenazine, phenoxazine, or phenothiazine core chromophores. Transient electronic and vibrational absorption spectroscopies over subpicosecond to nanosecond and microsecond time intervals, respectively, track spectroscopic signatures of both the reactants and products of photoinduced electron transfer in N,N-dimethylformamide, dichloromethane, and toluene solutions. The rate coefficients for electron transfer exhibit a range of values up to ∼10¹⁰ M^-1 s^-1 influenced systematically by the PC structures. These rate coefficients are an order of magnitude smaller for catalysts with charge transfer character in their first excited singlet (S₁) or triplet (T₁) states than for photocatalysts with locally excited character. The latter species show nearly diffusion-limited rate coefficients for the electron transfer to MBP. The derived kinetic parameters are used to model the contributions to electron transfer from the S₁ state of each PC for different concentrations of MBP. Comparisons of singlet and triplet reactivity for one of the phenoxazine PCs reveal that the rate coefficient k_ET(T₁) = (2.7 ± 0.3) × 10⁷ M^-1 s^-1 for electron transfer from the T₁ state is 2 orders of magnitude lower than that from the S₁ state, k_ET(S₁) = (2.6 ± 0.4) × 10⁹ M^-1 s^-1. The trends in bimolecular electron transfer rate coefficients are accounted for using a modified Marcus theory for dissociative electron transfer.

An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN^-) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to H₂O. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN^- suggests two sub-ensembles. Vibrational relaxation rates extracted from the 2D-IR spectra provide evidence of the dynamical differences between the two sub-ensembles. We support the interpretation of two sub-ensembles with response function simulations of two overlapping bands with different vibrational relaxation rates and, otherwise, similar dynamics. The measured rates for spectral diffusion depend on polarization, indicating reorientation-induced spectral diffusion (RISD). A model of restricted molecular rotation (wobbling in a cone) fully describes the observed spectral diffusion in EAN. In H₂O, both RISD and structural spectral diffusion contribute with similar timescales. This complete characterization of the dynamics at room temperature provides the basis for the temperature-dependent measurements in Paper II of this series.

Detection of Glycine as a Model Protein in Blood Serum Using 2D-IR Spectroscopy

Glycine (Gly) is used as a model system to evaluate the ability of ultrafast two-dimensional infrared (2D-IR) spectroscopy to detect and quantify the low-molecular-weight proteinaceous components of blood serum. Combining data acquisition schemes to suppress absorption bands of H₂O that overlap with the protein amide I band with analysis of peak patterns appearing in the off-diagonal region of the 2D-IR spectrum allows separation of the Gly spectral signature from that of the dominant protein fraction of serum in a transmission-mode 2D-IR measurement without any sample manipulation, e.g., filtration or drying. 2D-IR spectra of blood serum samples supplemented with varying concentrations of Gly were obtained, and a range of data analysis methods compared, leading to a detection limit of ∼3 mg/mL for Gly. The reported methodology provides a platform for a critical assessment of the sensitivity of 2D-IR for measuring the concentrations of amino acids, peptides, and low-molecular-weight proteins present in serum samples. We conclude that, in the case of several clinically relevant diagnostic molecules and their combinations, the potential exists for 2D-IR to complement IR absorption methods as the benefits of the second frequency dimension offered by 2D-IR spectroscopy outweigh the added technical complexity of the measurement.

Shot-to-shot 2D IR spectroscopy at 100 kHz using a Yb laser and custom-designed electronics

The majority of 2D IR spectrometers operate at 1-10 kHz using Ti:Sapphire laser technology. We report a 2D IR spectrometer designed around Yb:KGW laser technology that operates shot-to-shot at 100 kHz. It includes a home-built OPA, a mid-IR pulse shaper, and custom-designed electronics with optional on-chip processing. We report a direct comparison between Yb:KGW and Ti:Sapphire based 2D IR spectrometers. Even though the mid-IR pulse energy is much lower for the Yb:KGW driven system, there is an 8x improvement in signal-to-noise over the 1 kHz Ti:Sapphire driven spectrometer to which it is compared. Experimental data is shown for sub-millimolar concentrations of amides. Advantages and disadvantages of the design are discussed, including thermal background that arises at high repetition rates. This fundamental spectrometer design takes advantage of newly available Yb laser technology in a new way, providing a straightforward means of enhancing sensitivity.

Photon echoes and two dimensional spectra of the amide I band of proteins measured by femtosecond IR - Raman spectroscopy

Infrared (IR) and Raman spectroscopy are fundamental techniques in chemistry, allowing the convenient determination of bond specific chemical composition and structure. Over the last decades, ultrafast multidimensional IR approaches using sequences of femtosecond IR pulses have begun to provide a new means of gaining additional information on molecular vibrational couplings, distributions of molecular structures and ultrafast molecular structural dynamics. In this contribution, new approaches to measuring multidimensional spectra involving IR and Raman processes are presented and applied to the study of the amide I band of proteins. Rephasing of the amide I band is observed using dispersed IR-Raman photon echoes and frequency domain 2D-IR-Raman spectra are measured by use of a mid-IR pulse shaper or over a broader spectral range using a tuneable picosecond laser. A simple pulse shaping approach to performing heterodyned time-domain Fourier Transform 2D-IR-Raman spectroscopy is introduced, revealing that the 2D-IR-Raman spectra distinguish homogeneous and inhomogeneous broadening in the same way as the well-established methods of 2D-IR spectroscopy. Across all datasets, the unique dependence of the amide I data on the IR and Raman strengths, vibrational anharmonicities and inhomogeneous broadening provides a fascinating spectroscopic view of the amide I band.

New ultrafast 2D-IR-Raman photon echo spectroscopy techniques are introduced and applied to the structural analysis of proteins.

Two-dimensional infrared spectroscopy: an emerging analytical tool?

Ultrafast two-dimensional infrared (2D-IR) spectroscopy has provided valuable insights into biomolecular structure and dynamics, but recent progress in laser technology and data analysis methods have demonstrated the potential for high throughput 2D-IR measurements and analytical applications. Using 2D-IR as an analytical tool requires a different approach to data collection and analysis compared to pure research applications however and, in this review, we highlight progress towards usage of 2D-IR spectroscopy in areas relevant to biomedical, pharmaceutical and analytical molecular science. We summarise the technical and methodological advances made to date and discuss the challenges that still face 2D-IR spectroscopy as it attempts to transition from the state-of-the-art laser laboratory to the standard suite of analytical tools.

Edge-pixel referencing suppresses correlated baseline noise in heterodyned spectroscopies

Referencing schemes are commonly used in heterodyned spectroscopies to mitigate correlated baseline noise arising from shot-to-shot fluctuations of the local oscillator. Although successful, these methods rely on careful pixel-to-pixel matching between the two spectrographs. A recent scheme introduced by Feng et al. [Opt. Express 27(15), 20323-20346 (2019)] employed a correlation matrix to allow free mapping between dissimilar spectrographs, leading to the first demonstration of floor noise limited detection on a multichannel array used in heterodyned spectroscopy. In addition to their primary results using a second reference spectrometer, Feng et al. briefly demonstrated the flexibility of their method by referencing to same-array pixels at the two spectral edges (i.e., edge-pixel referencing). We present a comprehensive study of this approach, which we term edge-pixel referencing, including optimization of the approach, assessment of the performance, and determination of the effects of background responses. We show that, within some limitations, the distortions due to background signals will not affect the 2D IR line shape or amplitude and can be mitigated by band narrowing of the pump beams. We also show that the performance of edge-pixel referencing is comparable to that of referencing to a second spectrometer in terms of noise suppression and that the line shapes and amplitudes of the spectral features are, within the measurement error, identical. Altogether, these results demonstrate that edge-pixel referencing is a powerful approach for noise suppression in heterodyned spectroscopies, which requires no new hardware and, so, can be implemented as a software solution for anyone performing heterodyned spectroscopy with multichannel array detectors already.

2D-Infrared Spectroscopy of Proteins in Water: Using the Solvent Thermal Response as an Internal Standard

Ultrafast two-dimensional infrared (2D-IR) spectra can now be obtained in a matter of seconds, opening up the possibility of high-throughput screening applications of relevance to the biomedical and pharmaceutical sectors. Determining quantitative information from 2D-IR spectra recorded on different samples and different instruments is however made difficult by variations in beam alignment, laser intensity, and sample conditions. Recently, we demonstrated that 2D-IR spectroscopy of the protein amide I band can be performed in aqueous (H₂O) rather than deuterated (D₂O) solvents, and we now report a method that uses the magnitude of the associated thermal response of H₂O as an internal normalization standard for 2D-IR spectra. Using the water response, which is temporally separated from the protein signal, to normalize the spectra allows significant reduction of the impact of measurement-to-measurement fluctuations on the data. We demonstrate that this normalization method enables creation of calibration curves for measurement of absolute protein concentrations and facilitates reproducible difference spectroscopy methodologies. These advances make significant progress toward the robust data handling strategies that will be essential for the realization of automated spectral analysis tools for large scale 2D-IR screening studies of protein-containing solutions and biofluids.

Mechanisms of IR amplification in radical cation polarons

Break down of the Born-Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs). Here, we investigate the mechanism of this amplification, and show that it provides insights into intramolecular charge migration. Spectroelectrochemical time-resolved infrared (TRIR) and two-dimensional infrared (2D-IR) spectroscopies were used to investigate the radical cations of two butadiyne-linked conjugated porphyrin oligomers, a linear dimer and a cyclic hexamer. The 2D-IR spectra reveal strong coupling between all the IRAVs and the electronic π-π* polaron band. Intramolecular vibrational energy redistribution (IVR) and vibrational relaxation occur within ∼0.1-7 ps. TRIR spectra show that the transient ground state bleach (GSB) and excited state absorption (ESA) signals have anisotropies of 0.31 ± 0.07 and 0.08 ± 0.04 for the linear dimer and cyclic hexamer cations, respectively. The small TRIR anisotropy for the cyclic hexamer radical cation indicates that the vibrationally excited polaron migrates round the nanoring on a time scale faster than the measurement, i.e. within 0.5 ps, at 298 K. Density functional theory (DFT) calculations qualitatively reproduce the emergence of the IRAVs. The first singlet (S1) excited states of the neutral porphyrin oligomers exhibit similar IRAVs to the radical cations, implying that the excitons have similar electronic structures to polarons. Our results show that IRAVs originate from the strong coupling of charge redistribution to nuclear motion, and from the similar energies of electronic and vibrational transitions.

Dual spectral phase and diffraction angle compensation of a broadband AOM 4-f pulse-shaper for ultrafast spectroscopy

AOM-based pulse shaping as a method has been shown to provide many advantages in the field of ultrafast spectroscopy, in particular for the creation of phase matched pulse pairs for two-dimensional IR and electronic spectroscopy. In this paper we demonstrate the capabilities of a quartz-based AOM pulse-shaper to provide fine control over the phase and spatial dispersion of ultrafast supercontinuum pulses. We show that by using the Bragg condition, we can define a mask function for our AOM such that the angle of diffraction is constant for all frequencies. By summing all the contributions to spectral phase due to normal and anomalous dispersion of our optical components, and taking into account the intrinsic frequency dependent phase as a result of the acoustic sine wave propagating through the AOM, we can determine an optimal mask function that meets the Bragg condition for all frequencies, and generates compressed (∼50 fs) supercontinuum pulses.

Measuring proteins in H2O with 2D-IR spectroscopy

The amide I infrared band of proteins is highly sensitive to secondary structure, but studies under physiological conditions are prevented by strong, overlapping water absorptions, motivating the widespread use of deuterated solutions. H/D exchange raises fundamental questions regarding the impact of increased mass on protein dynamics, while deuteration is impractical for biomedical or commercial applications of protein IR spectroscopy. We show that 2D-IR spectroscopy can avoid this problem because the 2D-IR amide I signature of proteins dominates that of water even at sub-millimolar protein concentrations. Using equine blood serum as a test system, we investigate the significant implications of being able to measure the spectroscopy and dynamics of proteins in water, demonstrating relevance in areas ranging from fundamental science to the clinic. Measurements of vibrational relaxation dynamics of serum proteins reveals that deuteration slows down the rate of amide I vibrational relaxation by >10%, indicating a dynamic impact of isotopic exchange in some proteins. The unique link between protein secondary structure and 2D-IR amide I lineshape allows differentiation of signals due to albumin and globulin protein fractions in serum leading to measurements of the biomedically-important albumin to globulin ratio (AGR) with an accuracy of ±4% across a clinically-relevant range. Furthermore, we demonstrate that 2D-IR spectroscopy enables differentiation of the structurally similar globulin proteins IgG, IgA and IgM, opening up a straightforward spectroscopic approach to measuring levels of serum proteins that are currently only accessible via biomedical laboratory testing.

Optimized reconstructions of compressively sampled two-dimensional infrared spectra

Compressive sampling has the potential to dramatically accelerate the pace of data collection in two-dimensional infrared (2D IR) spectroscopy. We have previously introduced the Generic Iteratively Reweighted Annihilating Filter (GIRAF) reconstruction algorithm to solve the reconstruction in 2D IR compressive sampling. Here, we report a thorough assessment of this method and comparison to our earlier efforts using the Total Variation (TV) algorithm. We show that the GIRAF algorithm has some distinct advantages over TV. Although it is no better or worse in terms of ameliorating the impacts of compressive sampling on the measured 2D IR line shape, we find that the nature of those effects is different for GIRAF than they were for TV. In addition to assessing the impacts on the line shape of a single oscillator, we also test the ability of the algorithm to reconstruct spectra that have transitions from more than one oscillator, such as the coupled carbonyl oscillators in rhodium dicarbonyl. Finally, and perhaps most importantly, we show that the GIRAF algorithm has a distinct denoising effect on the signal-to-noise ratio (SNR) of the 2D IR spectra that can increase the SNR by as much as 4× without any additional signal averaging and collecting fewer data points, which should further enhance the acceleration of data collection that can be achieved using compressive sampling and enable even more challenging experimental measurements.

Broadband vibrational sum-frequency generation spectrometer at 100 kHz in the 950-1750 cm-1 spectral range utilizing a LiGaS2 optical parametric amplifier

We present a 100 kHz broadband vibrational sum-frequency generation (VSFG) spectrometer operating in the 5.7-10.5 µm (950-1750 cm^-1) wavelength range. The mid-infrared beam of the system is obtained from a collinear, type-I LiGaS₂-crystal-based optical parametric amplifier seeded by a supercontinuum and pumped directly by 180 fs, ~32 µJ, 1.03 µm pulses from an Yb:KGd(WO₄)₂ laser system. Up to 0.5 µJ mid-infrared pulses with durations below 100 fs were obtained after dispersion compensation utilizing bulk materials. We demonstrate the utility of the spectrometer by recording high-resolution, low-noise vibrational spectra of Langmuir-Blodgett supported lipid monolayers on CaF₂. The presented VSFG spectrometer scheme offers superior signal-to-noise ratios and constitutes a high-efficiency, low-cost, easy-to-use alternative to traditional schemes relying on optical parametric amplification followed by difference frequency generation.