Nonlinear Optical Spectroscopy Research Articles

Nonlinear optical spectroscopy and two-photon excited fluorescence spectroscopy reveal the excited states of fluorophores embedded in a beetle's elytra.

Upon illumination by ultraviolet light, many animal species emit light through fluorescence processes arising from fluorophores embedded within their biological tissues. Fluorescence studies in living organisms are however relatively scarce and so far limited to the linear regime. Multiphoton excitation fluorescence analyses as well as nonlinear optical techniques offer unique possibilities to investigate the effects of the local environment on the excited states of fluorophores. Herein, these techniques are applied for the first time to study of the naturally controlled fluorescence in insects. The case of the male Hoplia coerulea beetle is investigated because the scales covering the beetle's elytra are known to possess an internal photonic structure with embedded fluorophores, which controls both the beetle's coloration and the fluorescence emission. An intense two-photon excitation fluorescence signal is observed, the intensity of which changes upon contact with water. A third-harmonic generation signal is also detected, the intensity of which depends on the light polarization state. The analysis of these nonlinear optical and fluorescent responses unveils the multi-excited states character of the fluorophore molecules embedded in the beetle's elytra. The role of form anisotropy in the photonic structure, which causes additional tailoring of the beetle's optical responses, is demonstrated by circularly polarized light and nonlinear optical measurements.

Four-Dimensional Coherent Spectroscopy of Complex Molecular Systems in Solution

An understanding of microscopic interactions in solution is of fundamental importance in chemistry. However, the structure and dynamics of complex systems in the condensed phase, especially far from thermal equilibrium, are masked by broad and often featureless absorption and emission spectra. Nonlinear optical spectroscopy has proven to be a powerful and general approach to disentangling congested spectra by spreading information across multiple dimensions, revealing features oftentimes hidden in lower-order projections. As the dimensionality of the measurement is increased, the better the microscopic interactions are revealed, as spectral bands disperse in the large hyper-spectral volume. This capability, however, comes at a steep price, as the signal decreases exponentially at higher orders of optical response, and added experimental complexity increases noise. We discuss a 4D coherent spectroscopy known as gradient-assisted multidimensional electronic–Raman spectroscopy (GAMERS) that reveals coupling ...

Neat Water-Vapor Interface: Proton Continuum and the Nonresonant Background.

Whether the surface of neat water is "acidic" or "basic" remains an active and controversial field of research. Most of the experimental evidence supporting the preferential adsorption of H3O+ ions stems from nonlinear optical spectroscopy methods typically carried out at extreme pH conditions (pH < 1). Here, we use vibrational sum frequency spectroscopy (VSFS) to target the "proton continuum", an unexplored frequency range characteristic of hydrated protons and hydroxide ions. The VSFS spectra of neat water show a broad and nonzero signal intensity between 1700 and 3000 cm-1 in the three different polarization combinations examined. By comparing the SF response of water with that from dilute HCl and NaOH aqueous solutions, we conclude the intensity does not originate from either adsorbed H3O+ or OH- ions. Contributions from the nonresonant background are then critically considered by comparing the experimental results with many-body molecular dynamics (MB-MD) simulated spectra.

Inclusion of nuclear quantum effects for simulations of nonlinear spectroscopy.

The computation and interpretation of nonlinear vibrational spectroscopy is of vital importance for understanding a wide range of dynamical processes in molecular systems. Here, we introduce an approach to evaluate multi-time response functions in terms of multi-time double symmetrized Kubo transformed thermal correlation functions. Furthermore, we introduce a multi-time extension of ring polymer molecular dynamics to evaluate these Kubo transforms. Benchmark calculations show that the approximations are useful for short times even for nonlinear operators, providing a consistent improvement over classical simulations of multi-time correlation functions. The introduced methodology thus provides a practical way of including nuclear quantum effects in multi-time response functions of non-linear optical spectroscopy.

Gel Phase Membrane Retards Amyloid β-Peptide (1-42) Fibrillation by Restricting Slaved Diffusion of Peptides on Lipid Bilayers.

Plasma membranes in the human brain can interact with amyloid β-peptide (1-42; Aβ42) and induce Aβ42 fibrillation, which is considered to be a crucial process underlying the neurotoxicity of Aβ42 and the pathogenesis of Alzheimer's disease (AD). However, the mechanism of membrane-mediated Aβ42 fibrillation at the molecular level remains elusive. Here we study the role of adsorbed Aβ42 peptides on membrane-mediated fibrillation using supported lipid bilayers of varying phase structures (gel and fluid). Using total internal reflection fluorescence microscopy and interfacial specific second-order nonlinear optical spectroscopy, we show that the dynamics of 2D-mobile Aβ42 molecules, facilitated by the highly mobile lipids underneath the peptides, are critical to Aβ42 fibrillation on liquid phase membranes. This growth mechanism is retarded on gel phase membranes where the dynamics of 2D-mobile peptides are restricted by the "frozen" lipids with less mobility.

Development and applications of nonlinear optical spectroscopy: 16th ECONOS/36th ECW meeting in Jena (Germany)

Development and applications of nonlinear optical spectroscopy: 16th ECONOS/36th ECW meeting in Jena (Germany)

Insights into Ice Formation via Immersion Freezing from Nonlinear Optical Spectroscopy

Ice formation is an exceedingly common phenomenon with quotidian, geological, biological, and atmospheric ramifications. Nevertheless, the nucleation and growth of ice is a poorly understood process. In environmentally relevant situations, nucleation and growth of ice often occurs on the surface of a heterogeneous phase that acts to catalyze the liquid-to-solid phase transition. Nonlinear optical spectroscopy is one of the few experimental techniques capable of accessing such buried interfaces. In this short perspective, insights into the dynamic growth of ice at solid surfaces are reviewed. Application of nonlinear optical spectroscopy to monitor ice formation is an emerging research topic. Ice formation may benefit from the presence of a solid surface. However, simple models of rigid interactions between surface and liquid must be modified to include dynamic interactions during formation and growth.

Shape Control of Electrochemically Deposited Metal Films and Nanostructures through Additive Effects

The use of electrolyte additives to affect nanocrystallite shape and film morphology in electroless and electrodeposited metal films is presented. Copper electrodeposited onto gold results in preferential square pyramidal copper nanostructure morphologies in the presence of the organic additive malachite green (MG). We have employed in-situ and ex situ methods to investigate the additive effects, including nonlinear optical spectroscopy, electrochemical scanning tunneling microscopy, linear sweep and cyclic voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). These shape-controlled additive effects are supported by additive adsorption energy calculations, which indicate preferential interactions, and differential growth kinetics on different facets of the film's growing nanostructures during electrodeposition. Facet-specific interactions have also been exploited in the epitaxial electroless deposition of noble metals onto single crystal Ag(100) substrates. Here, we examine the role of specific anions in determining the resulting film and nanostructure morphologies via SEM and high resolution transmission electron microscopy (HRTEM). These effects have been exploited to yield large area patterned, and shape-controlled nanoarrays of single crystal metal nanostructures for plasmonic and metamaterial applications. These approaches offer new and cost effective routes to achieve shape-controlled surface nanostructure.

Steady-State Linear and Non-linear Optical Spectroscopy of Organic Chromophores and Bio-macromolecules.

Bio-macromolecules as DNA, lipid membranes and (poly)peptides are essential compounds at the core of biological systems. The development of techniques and methodologies for their characterization is therefore necessary and of utmost interest, even though difficulties can be experienced due to their intrinsic complex nature. Among these methods, spectroscopies, relying on optical properties are especially important to determine their macromolecular structures and behaviors, as well as the possible interactions and reactivity with external dyes—often drugs or pollutants—that can (photo)sensitize the bio-macromolecule leading to eventual chemical modifications, thus damages. In this review, we will focus on the theoretical simulation of electronic spectroscopies of bio-macromolecules, considering their secondary structure and including their interaction with different kind of (photo)sensitizers. Namely, absorption, emission and electronic circular dichroism (CD) spectra are calculated and compared with the available experimental data. Non-linear properties will be also taken into account by two-photon absorption, a highly promising technique (i) to enhance absorption in the red and infra-red windows and (ii) to enhance spatial resolution. Methodologically, the implications of using implicit and explicit solvent, coupled to quantum and thermal samplings of the phase space, will be addressed. Especially, hybrid quantum mechanics/molecular mechanics (QM/MM) methods are explored for a comparison with solely QM methods, in order to address the necessity to consider an accurate description of environmental effects on spectroscopic properties of biological systems.

Polarization Control with Plasmonic Antenna Tips: A Universal Approach to Optical Nanocrystallography and Vector-Field Imaging.

Controlling the propagation and polarization vectors in linear and nonlinear optical spectroscopy enables us to probe the anisotropy of optical responses providing structural symmetry selective contrast in optical imaging. Here, we present a novel tilted antenna-tip approach to control the optical vector-field by breaking the axial symmetry of the nanoprobe in tip-enhanced near-field microscopy. This gives rise to a localized plasmonic antenna effect with significantly enhanced optical field vectors with control of both in-plane and out-of-plane components. We use the resulting vector-field specificity in the symmetry selective nonlinear optical response of second-harmonic generation (SHG) for a generalized approach to optical nanocrystallography and imaging. In tip-enhanced SHG imaging of monolayer MoS2 films and single-crystalline ferroelectric YMnO3, we reveal nanocrystallographic details of domain boundaries and domain topology with enhanced sensitivity and nanoscale spatial resolution. The approach is applicable to any anisotropic linear and nonlinear optical response and enables the optical nanocrystallographic imaging of molecular or quantum materials.

Multiphoton Microscopy for Caries Detection with ICDAS Classification

Dentin carious lesion is a dynamic process that involves demineralization and collagen denaturation. Collagen type I is the major protein in dentin and it has been investigated based on its optical properties. Multiphoton microscopy (MPM) is a nonlinear imaging technique that reveals the caries process using the collagen two-photon excitation fluorescence (2PEF) and its second-harmonic generation (SHG). Combining the histological and the International Caries Detection and Assessment System (ICDAS) classifications with nonlinear optical spectroscopy (NLOS), 2PEF and SHG intensities of enamel and dentin were highly altered during the caries process. It has been proven that the ratio SHG/2PEF is a relevant indicator of the organic matrix denaturation [Terrer et al.: J Dent Res 2016; 96: 574–579]. In the present study, a series of measurable signals is made to detect early stages of carious lesion according to the ICDAS classification and to explore the relationship between these measures and the ICDAS scale. Comparison of the efficiency of nonlinear optical signals for caries detection with the ICDAS classification is essential to evaluate their potential for clinical application. In our study, the use of the NLOS measured by MPM allowed us to monitor a quantitative parameter (SHG/2PEF ratio) according to the dentin carious lesion state (ICDAS and histological examination). Three coherent new groups were defined (ICDAS 0/1; ICDAS 2/3; ICDAS 4/5/6), where the carious process can be clearly described with a statistically significant decrease of the SHG/2PEF ratio.

Quantum coherence measurement with femtosecond time-resolve two-dimensional electronic spectroscopy: principles, applications and outlook

Two-dimensional electronic spectroscopy is a kind of nonlinear optical spectroscopy with both high time resolution and high frequency resolution. It can be used to observe the complex dynamics of a condensed molecular system. Meanwhile it is a very powerful tool to study the coherence between the electronic states or electronic and vibration states. In 2007, Flemming's group reported the long-lived quantum coherence observed in the energy transfer process in the light-harvesting antenna protein complex Fenna-Matthews-Olson at 77 K by means of two-dimensional electronic spectroscopy. Though it has been proved not to arise from the pure electronic coherence later, this discovery has greatly stimulated the exploration of the coherent energy transfer pathways possibly existing in the natural and artificial photosynthetic systems, and this is still a very active area nowadays. Here in this paper we briefly review the principle and set-up of the two-dimensional electronic spectroscopy, and also some of its applications in investigating coherent energy transfer in the photosynthetic and artificial systems, aiming to bring this novel spectroscopic tool into a wider application.

DNA and the chiral water superstructure

A few months after the publication of our paper Chiral micron-sized H2O aggregates in water: Circular dichroism of supramolecular H2O architectures created by perturbing pure water, WATER, 8, 1–29 (2017) submitted on June 6, 2016 and published on March 2, 2017, the paper DNA's Chiral Spine of Hydration by M. L. McDermott, H. Vanselous, S. A. Corcelli, and P. B. Petersen, was published in ACS Central Science, on May 24, 2017. The paper reports the first observation of a chiral water superstructure surrounding DNA at ambient conditions. Second-order nonlinear optical spectroscopy revealed the structure. These experimental findings validate the significance for DNA of our results obtained by circular dichroism (CD) measurement.

Sub-μeVdecoherence-induced population pulsation resonances in an InGaN system

We report on high frequency resolution coherent nonlinear optical spectroscopy on an ensemble of InGaN disks in GaN nanowires at 300 K. Sub-$\mu$eV resonances in the inhomogeneously broadened third order ($\chi^{(3)}$) absorption spectrum show asymmetric line shapes, where the degree of asymmetry depends on the wavelength of the excitation beams. Theory based on the Optical Bloch Equations (OBE) indicates that the lineshape asymmetry is a result of fast decoherence in the system and the narrow resonances originate from coherent population pulsations that are induced by decoherence in the system. Using the OBE, we estimate that the decoherence time of the optically induced dipole (formed between the unexcited ground state the excited electron-hole pair) at room temperature is 125 fs, corresponding to a linewidth of ~10 meV. The decay time of the excitation is ~5-10 ns, depending on the excitation energy. The lineshapes are well fit with the OBE indicating that the resonances are characterized by discrete levels with no evidence of many body physics.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E′ and E″ phonon modes.

Development and applications of nonlinear optical spectroscopy: 15th ECONOS/35th ECW meeting in Gothenburg (Sweden)

The joint conference of the European Conference on Nonlinear Optical Spectroscopy (ECONOS) and European CARS Workshop (ECW) covers all theoretical and experimental aspects of nonlinear optical spectroscopy and microscopy. Topics include Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy, and other forms of coherent Raman scattering, other approaches to the detection of nonlinear optical responses in time and frequency domain, and investigations of the physical and chemical processes defining the spectral signatures in different nonlinear media. Also, emerging analytical techniques and their applications in biomedical research, material science, and engineering are discussed. The 15th ECONOS conference and 35th CARS workshop were held in Gothenburg, Sweden, on 24–27 April 2016, chaired by David Sedarsky and Stina Hemdal (Chalmers University of Technology). Altogether, 50 participants shared their research in oral sessions and poster sessions and participated in lively discussions and social activities. After an invited lecture and a welcome reception in the historic Chalmerska Huset on Sunday, the regular scientific program began on Monday morning at the same place. Five invited talks were given during the course of the conference, followed by contributed oral presentations, and two poster sessions. This special section of the Journal of Raman Spectroscopy constitutes a selection of three papers presented at ECONOS/ECW 2016. Due to the broad scope of the conference, the papers cover a broad range of methods and their applications to a variety of systems. The special section starts with a contribution by Martinez et al. 1 who applied a pump-probe Raman-Raman technique to obtain high-resolution Raman spectra of vibrationally excited states in a series of acetylene isotopologues. They utilized the naturally occurring process of vibration-to-vibration collisional relaxation to populate excited states of interest. Some of these states were observed and studied for the very first time. Meißner et al. 2 carried out a feasibility study on the use of a Raman-active crystal in picosecond-CARS of nitrogen. The Raman crystal facilitated narrowband stimulated Raman scattering (SRS) to produce a coherent beam for three-color CARS thermometry. The concept was demonstrated by temperature measurements at ambient conditions and in a fuel-rich sooting ethylene/air flame. The third paper represents a theoretical study of hybrid fs/ps-CARS by Marrocco 3. Hybrid fs/ps-CARS is a time-domain and frequency-resolved method, where interaction of a Raman medium with simultaneous femtosecond (fs) pump and Stokes pulses is interrogated by means of a delayed picosecond (ps) probe. The resulting anti-Stokes signal is usually studied by means of numerical methods built on the conventional time-dependent model of Raman coherence. The paper presents analytical closed-form solutions to the time model and validates their reliability against published experimental results. The results show that the method allows a fast signal reconstruction and provides adequate spectral accuracy. This ECONOS/ECW special section highlights the continuous progress in the area of nonlinear optical spectroscopy and its applications across the science and engineering disciplines. The enthusiasm and creativity of the scientific community as well as the rapid development of novel and improved experimental equipment will ensure an exciting future of nonlinear spectroscopy research. We would like to thank all the delegates, presenters of talks and posters, helpers in the background, and sponsors, which made the 2016 ECONOS/ECW meeting in Sweden an enjoyable and memorable event. Special thanks to the Editor-in-Chief of the Journal of Raman Spectroscopy, Larry Nafie, for making this special section possible, and of course, to all the authors and reviewers of the papers for their invaluable contribution.

Giant Rashba splitting in 2D organic-inorganic halide perovskites measured by transient spectroscopies.

Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural "multiple quantum wells" that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to "Rashba splitting" close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E’ and E” phonon modes.

Eye-safe near-infrared trace explosives detection and imaging.

We report the development of a non-contact no-reagents system operating in the eye-safe 1560-1800 nm wavelength range for standoff trace detection of explosives and high-speed imaging. Experimental results are provided for a number of chemicals including explosives on a variety of surfaces at sub-microgram per cm2 concentration. Chemically specific images were collected at 0.06 ms per pixel. Results from this effort indicate that the combination of modern industrial fiber lasers and nonlinear optical spectroscopy can address next generation eye-safe trace detection of chemicals including explosives.

Nonlinear optical signals and spectroscopy with quantum light

Conventional nonlinear spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies or time delays. Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and through the variation of photon statistics by coupling to matter. We present an intuitive diagrammatic approach for calculating ultrafast spectroscopy signals induced by quantum light, focusing on applications involving entangled photons with nonclassical bandwidth properties - known as "time-energy entanglement". Nonlinear optical signals induced by quantized light fields are expressed using time ordered multipoint correlation functions of superoperators. These are different from Glauber's g- functions for photon counting which use normally ordered products of ordinary operators. Entangled photon pairs are not subjected to the classical Fourier limitations on the joint temporal and spectral resolution. After a brief survey of properties of entangled photon pairs relevant to their spectroscopic applications, different optical signals, and photon counting setups are discussed and illustrated for simple multi-level model systems.