Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a black art. Although there are now numerous examples of control using laser-induced nucleation, a physical understanding is absent and preventing progress. We will show that concentration fluctuations in the neighborhood of a liquid-liquid critical point can be harnessed by an optical-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in liquid mixtures using a low-power laser diode confirm the effect. Phase separation and nucleation through an optical-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter.
We describe the use of a range of modern spectroscopic techniques—from terahertz time-domain spectroscopy (THz- TDS) to high dynamic-range femtosecond optical Kerr-effect (OKE) spectroscopy—to study the interaction of proteins, peptides, and other biomolecules with the aqueous solvent. Chemical reactivity in proteins requires fast picosecond fluctuations to reach the transition state, to dissipate energy, and (possibly) to reduce the width and height of energy barriers along the reaction coordinate. Such motions are linked with the structure and dynamics of the aqueous solvent making hydration critical to function. These dynamics take place over a huge range of timescales: from the nanosecond timescale of diffusion of water molecules in the first solvation shell of proteins, picosecond motions of amino-acid side chains, and sub-picosecond librational and phonon-like motions of water. It is shown that a large range of frequencies from MHz to THz is accessible directly using OKE resulting in the reduced anisotropic Raman spectrum and by using a combination of techniques including THz-TDS resulting in the dielectric spectrum. Using these techniques, we can now observe very significant differences in the spectra of proteins in aqueous solvent in the 3-30 THz range and more subtle differences at lower frequencies (10 GHz-3 THz).
To exploit the great potential of room-temperature ionic liquids (RTILs) as solvents that offer both low environmental
impact and product selectivity, an understanding of the liquid structure, the microscopic dynamics, and the way in which
the pertinent macroscopic properties, such as viscosity, thermal conductivity, ionic diffusion, and solvation dynamics
depend on these properties, is essential. We have measured the intermolecular dynamics of the 1,3-dialkylimidazoliumbased
RTILs [emim][BF4], [emim][DCA], and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical
Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and
far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively
featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and
intermolecular vibrations. In the terahertz region, the signal-to-noise ratio of the OKE spectra is particularly high and the
data show that there is a greater number of librational and intermolecular vibrational modes than previously detected. Of
greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in strong accordance with
recent simulations that observe mesoscopic structure arising from aggregates or clusters; structure that explains the
anomalous and inconveniently-high viscosities of these liquids.
The second-order processes of optical rectification and photoconduction are well known and widely used to produce
ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification relying on the excitation of surface plasmons (SPs) in metallic nanostructures. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the SPs, results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher order in the incident field.
We demonstrate a new application of multiphoton excitation of fluorescence; the measurement of silica particle growth during sol-gel polymerization. Recently we have reported the use of fluorescence anisotropy as an alternative approach to nm particle sizing using conventional techniques such as small angle light, neutron or x-ray scattering. Advantages of our approach include near angstrom resolution, minimal interference from the gel network as well as the additional benefits of providing microviscosity and hydrodynamic information. Fluorescence anisotropy applied to particle sizing in colloids in general is still in its infancy, but in this paper we show using probes fluorescein and rhodamine 6G, under conditions unsuitable for one-photon excitation, can be successful with multiphoton excitation.
Propagation of free-space femtosecond THz pulses (T rays) through and past metal structures with dimension on the order of a wavelength has been studied. In waveguides with diameters close to one wavelength, it is found that the phase velocity can become superluminal and even infinite or negative. T rays that propagate past a 100-micrometers metal wire are delayed when the polarization is perpendicular and advanced when it is parallel. In this case, it is also observed that the centroid velocity can become superluminal. Many of the result do not conform to simple waveguide theory, because of multiple reflections of the evanescent waves inside the waveguide. This 'Fabry-Perot' effect for evanescent waves is the cause of the negative phase velocities below the waveguide cutoff.
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