RESUMEN

Potential energy landscape (PEL) concepts have been useful in conceptualizing the effects of intermolecular interactions on dynamic and thermodynamic properties of liquids and glasses. "Basins", or regions of reduced potential energy associated with locally preferred molecular packing are important PEL features. The molecular configurations at the bottom of these basins are referred to as inherent structures (ISs). Experimental methods for directly characterizing PEL features such as these are rare, largely relegating PEL concepts to theory and simulation studies, and impeding their exploration in real systems. Recently, we showed that quasielastic neutron scattering (QENS) data from propylene carbonate (PC) exhibit signatures of picosecond timescale motion that are consistent with intrabasin motion and interbasin transitions [Cicerone et al., J. Chem. Phys., 2017, 146, 054502]. Here we present optically-heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy studies on PC. The data exhibit signatures of motion within and transitions between basins that agree quantitatively with and extend the QENS results. We show that the librational component of the OKE response corresponds to intrabasin dynamics, and the enigmatic intermediate OKE response corresponds to interbasin transition events. The OKE data extend the measurement range of these parameters and reveal their utility in characterizing PEL features of real systems.

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Preliminary studies have shown that silk fibroin can protect biomacromolecules from thermal degradation, but a deeper understanding of underlying mechanisms needed to fully leverage the stabilizing potential of this matrix has not been realized. In this study, we investigate stabilization of plasma C-reactive protein (CRP), a diagnostic indicator of infection or inflammation, to gain insight into stabilizing mechanisms of silk. We observed that the addition of antiplasticizing excipients that suppress ß-relaxation amplitudes in silk matrices resulted in enhanced stability of plasma CRP. These observations are consistent with those made in sugar-glass-based protein-stabilizing matrices and suggest fundamental insight into mechanisms as well as practical strategies to employ with silk protein matrices for enhanced stabilization utility.

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We investigate the transfer of orbital angular momentum among multiple beams involved in a coherent Raman interaction. We use a liquid crystal light modulator to shape pump and Stokes beams into optical vortices with various integer values of topological charge, and cross them in a Raman-active crystal to produce multiple Stokes and anti-Stokes sidebands. We measure the resultant vortex charges using a tilted-lens technique. We verify that in every case the generated beams' topological charges obey a simple relationship, resulting from angular momentum conservation for created and annihilated photons, or equivalently, from phase-matching considerations for multiple interacting beams.

RESUMEN

Coherent Raman sidebands have the potential to serve as a source of single cycle pulses. We generate these sidebands by crossing two-color femtosecond laser pulses in a Raman-active crystal. We design a reflection scheme using spherical mirrors to combine coherent Raman sidebands. The sidebands and the driving pulses are refocused back to the Raman crystal and the relative spectral phases are retrieved from an interferogram based on the nonlinear Raman interaction. Furthermore, using a deformable mirror to adjust the spectral phases, we demonstrate that our setup is capable of synthesizing ultrafast waveforms using coherent Raman sidebands.

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We generate broadband light by focusing two femtosecond pulses into a Raman-active crystal. By reflecting Raman sideband beams together with the two driving beams back to the same crystal (with a slight spatial offset), we generate sidebands covering a broader spectral range, compared to a single pass. In this novel double-path configuration, multiple Raman sideband beams interact with each other since the phase-matching condition is automatically fulfilled. This scheme enables an enhanced cascaded coherent anti-Stokes scattering process and also doubles the interaction length, thus it allows one to use relatively weak energy pump pulses and thereby avoid optical damage.

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An octave-spanning coherent supercontinuum is generated by non-collinear Raman-assisted four-wave mixing in single-crystal diamond using 7.7 fs laser pulses that have been chirped to about 420 fs in duration. The use of ultrabroad bandwidth pulses as input results in substantial overlap of the generated spectrum of the anti-Stokes sidebands, creating a phase-locked supercontinuum when all the sidebands are combined to overlap in time and space. The overall bandwidth of the generated supercontinuum is sufficient to support its compression to isolated few-to-single cycle attosecond transients. The significant spectral overlap of adjacent anti-Stokes sidebands allows the utilization of straight-forward spectral interferometry to test the relative phase coherence of the anti-Stokes outputs and is demonstrated here for two adjacent pairs of sidebands. The method can subsequently be employed to set the relative phase of the sidebands for pulse compression and for the synthesis of arbitrary field transients.

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We propose and experimentally validate a technique for four-wave mixing background suppression in coherent anti-Stokes Raman spectroscopy. It is based on the interaction of the signals generated from the Kerr third-order nonlinearity and the cascaded quadratic process in a nonlinear crystal. Theoretical analysis agrees well with the experimental results, which provide a quantitative assessment of different contributions and allow extraction of the nonlinearity parameters.

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We have generated multi-color optical vortices in a Raman-active crystal PbWO4 using two-color Fourier-transform limited femtosecond laser pulses. This setup overcomes some of the limitation of our previous research by allowing for the production of subcycle femtosecond optical vortices without the need for compensating for added chirp. In addition, the use of an OPA allows for greater flexibility in exciting different Raman modes. We verified the topological charges using two different methods. These diagnostic experiments verify not only theoretically predicted OAM algebra but demonstrated instabilities in high-order OVs. We have also studied factors which affect the high-order vortex sidebands such as the diameter and intensity of the input beams.

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We experimentally investigated the nonlinear optical interaction between the instantaneous four-wave mixing and the cascaded quadratic frequency conversion in commonly used nonlinear optical KTP and LiNbO3 with the aim of a possible background suppression of the non-resonant background in coherent anti-Stokes Raman scattering. The possibility of background-free heterodyne coherent anti-Stokes Raman scattering microspectroscopy is investigated at the interface formed by a liquid (isopropyl alcohol) and a nonlinear crystal (LiNbO3).

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We develop a technique for optimizing the phase of broad spectrally-separated frequency sidebands-a "holey" spectrum. We use a source of multiple-order coherent Raman sidebands, obtained by crossing femtosecond pump and Stokes beams in synthetic single-crystal diamond. We combine the sidebands into a single beam and show the phase coherence among the sidebands by investigating the interference between them in groups of three while varying one sideband phase by an acousto-optics pulse shaper. We then show how we optimize the broad "holey" spectrum by overcoming the limited temporal shaping window of the pulse shaper. We also explore how the resultant second harmonic/sum frequency generation of the full combined broadband spectrum varies as we vary different sideband phases. This step-by-step phase optimization of the "holey" spectrum can be applied to sidebands with similar structure to synthesize arbitrary optical waveforms.

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We study interference between a local oscillator and coherent anti-Stokes Raman scattering signal fields by controlling their relative phase and amplitude. This control allows direct observation of the real and imaginary components of the third-order nonlinear susceptibility (chi((3))) of the sample. In addition, we demonstrate that the heterodyne method can be used to amplify the signal.

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We demonstrate broadband light generation in diamond pumped by two-color femtosecond laser pulses. We find that phase matching plays a critical role in the output angle and frequency of the generated sidebands. When a third femtosecond probe pulse is applied to the crystal in the boxed Coherent anti-Stokes Raman Scattering geometry, a two-dimensional array of multi-color beams is generated through the Raman, four-wave mixing, and six-wave-mixing processes. We test the mutual coherence between the generated sidebands. Such coherence, maintained over the broad spectrum, opens possibilities for synthesis of subfemtosecond light waveforms.

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We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO(4)). As many as 20 anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from the infrared, through the visible region, to the ultraviolet, and it consists of discrete spatially separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields and open possibilities for synthesis of subfemtosecond light waveforms.

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We introduce a hybrid technique that combines the robustness of frequency-resolved coherent anti-Stokes Raman scattering (CARS) with the advantages of time-resolved CARS spectroscopy. Instantaneous coherent broadband excitation of several characteristic molecular vibrations and the subsequent probing of these vibrations by an optimally shaped time-delayed narrowband laser pulse help to suppress the nonresonant background and to retrieve the species-specific signal. We used this technique for coherent Raman spectroscopy of sodium dipicolinate powder, which is similar to calcium dipicolinate (a marker molecule for bacterial endospores, such as Bacillus subtilis and Bacillus anthracis), and we demonstrated a rapid and highly specific detection scheme that works even in the presence of multiple scattering.

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We use time-resolved coherent Raman spectroscopy to obtain molecule-specific signals from dipicolinic acid (DPA), which is a marker molecule for bacterial spores. We use femtosecond laser pulses in both visible and UV spectral regions and compare experimental results with theoretical predictions. By exciting vibrational coherence on more than one mode simultaneously, we observe a quantum beat signal that can be used to extract the parameters of molecular motion in DPA. The signal is enhanced when an UV probe pulse is used, because its frequency is near-resonant to the first excited electronic state of the molecule. The capability for unambiguous identification of DPA molecules will lead to a technique for real-time detection of spores.

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