Impact of local-field effects on the plasmonic enhancement of vibrational signals by infrared nanoantennas.

We have developed an analytical model that provides a mechanistic description of the plasmonic enhancement of vibrational signals by infrared nanoantennas. Our treatment is based on a coupled-point-dipole model which considers the interaction between a point-like nanoantenna and a single vibrational dipole moment. This idealized model is refined in two consecutive steps. The first step generalizes the model to make the treatment of non-point-like nanoantennas possible. The second step deals with local-field effects originating from the mutual interaction of the molecular vibrations. We have compared the results of our model with finite-difference time-domain simulations, and we find that our model predicts both the lineshapes and the amplitudes of the vibrational signals in a quantitative manner. Our analysis shows that the local-field effects play a surprisingly dominant role in the plasmonic enhancement, and we discuss possibilities of engineering this local field in order to further boost the plasmonic amplification.

Ultrasensitive Ultrafast Vibrational Spectroscopy Employing the Near Field of Gold Nanoantennas.

We introduce a novel method to perform nonlinear vibrational spectroscopy on nanoscale volumes. Our technique uses the intense near field of infrared nanoantennas to amplify the nonlinear vibrational signals of molecules located in the vicinity of the antenna surface. We demonstrate the capabilities of the method by performing infrared pump-probe spectroscopy and two-dimensional infrared spectroscopy on 5 nm layers of polymethylmetacrylate. In these experiments we observe enhancement factors of the nonlinear signals of more than 4 orders of magnitude. We discuss the mechanism underlying the amplification process as well as strategies for further increasing the sensitivity of the technique.

Single-photon spectroscopy of a single molecule.

Efficient interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single-photon streams with specific wavelengths, bandwidths, and power as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons.

Quantum interference of tunably indistinguishable photons from remote organic molecules.

We demonstrate two-photon interference using two remote single molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we tune and explore the effect of photon distinguishability. We discuss future improvements on the brightness of single-photon beams, their integration by large numbers on chips, and the extension of our experimental scheme to coupling and entanglement of distant molecules.

Destabilization of the hydrogen-bond structure of water by the osmolyte trimethylamine N-oxide.

We use femtosecond mid-infrared pump-probe spectroscopy to investigate the effects of the osmolyte trimethylamine N-oxide (TMAO) on the structural dynamics of water. As a comparison, we also investigate the effects of other amphiphilic molecules: tetramethylurea (TMU), urea, proline, and N-methylacetamide (NMA). Our measurements show that TMAO has the unique property of increasing the orientational mobility of part of the water molecules in the solution, indicating that TMAO reorganizes the hydrogen-bond network of water in a special way. We also investigate the influence of the simultaneous presence in a solution of TMAO and urea, and of TMAO and NMA. It turns out that the effects of TMAO and urea are additive, whereas those of TMAO and NMA are nonadditive.

Strong slowing down of water reorientation in mixtures of water and tetramethylurea.

We use mid-infrared pump-probe spectroscopy to study the ultrafast dynamics of HDO molecules in mixtures of tetramethylurea (TMU) and water. The composition of the studied solutions ranges from pure water to an equimolar mixture of water and TMU. We find that the vibrational relaxation of the OD-stretching vibration of HDO proceeds via an intermediate level in which the molecule is more strongly hydrogen bonded than in the ground state. As the TMU concentration is increased, the lifetime of the excited state and of the intermediate increase from 1.8 to 5.2 ps and from 0.7 to 2.2 ps, respectively. The orientational relaxation data indicate that the solutions contain two types of water molecules: bulk-like molecules that have the same reorientation time constant as in the pure liquid (taurot = 2.5 ps) and molecules that are strongly immobilized (taurot > 10 ps). The immobilized water molecules turn out to be involved in the solvation of the methyl groups of the tetramethylurea molecule. The fraction of immobilized water molecules grows with increasing TMU concentration, reaching a limiting value of 60% at very high concentrations.

Molecular reorientation of liquid water studied with femtosecond midinfrared spectroscopy.

The molecular reorientation of liquid water is key to the hydration and stabilization of molecules and ions in aqueous solution. A powerful technique to study this reorientation is to measure the time-dependent anisotropy of the excitation of the O-H/O-D stretch vibration of HDO dissolved in D2O/H2O using femtosecond midinfrared laser pulses. In this paper, we present and discuss experiments in which this technique is used to study the correlation between the molecular reorientation of the water molecules and the strength of the hydrogen-bond interactions. On short time scales (<200 fs), it was found that the anisotropy shows a partial decay due to librational motions of the water molecules that keep the hydrogen bond intact. On longer time scale (>200 fs), the anisotropy shows a complete decay with an average time constant of 2.5 ps. From the frequency dependence of the anisotropy dynamics, it follows that a subensemble of the water molecules shows a fast reorientation that is accompanied by a large change of the vibrational frequency. This finding agrees with the molecular jumping mechanism for the reorientation of liquid water that has recently been proposed by Laage and Hynes.

High radio-frequency field strength nutation NMR of quadrupolar nuclei.

Owing to the introduction of microcoils, high RF field strength nutation NMR is a viable candidate for the study of quadrupolar nuclei with strong quadrupolar couplings, not accessible using contemporary NMR techniques. We show powder 23Na nutation spectra on sodium nitrite for RF field strengths of up to 1170kHz, that conform to theoretical predictions. For lanthanum fluoride powder, 139La nutation spectra taken at elevated RF field amplitudes show clear discrepancies when compared to the theory. These errors are shown to be mainly caused by pulse transients at the end of the pulse, which proved to be detrimental to the shape of the nutation spectra. Using a nutation pulse which ends in a sudden frequency jump, we show that these errors can be reduced, and nutation spectra that conform to theory can be readily acquired. This enables nutation NMR for the study of quadrupolar nuclei with a strong quadrupolar coupling, bridging the gap between NMR, which can only analyse nuclei with a weak to medium quadrupolar coupling, and NQR, were extensive searching for the right quadrupolar frequency is the limiting factor.

Observation of immobilized water molecules around hydrophobic groups.

We have used femtosecond midinfrared spectroscopy to study the orientational mobility of water molecules in the hydration shells of hydrophobic groups. Our results show that hydrophobic groups are surrounded by a number of water molecules that display much slower orientational dynamics than the bulk liquid and that are therefore effectively immobilized. It turns out that each methyl group is surrounded by four immobilized water OH groups.

Orientational dynamics of isotopically diluted H2O and D2O.

We use femtosecond midinfrared pump-probe spectroscopy to compare the ultrafast dynamics of HDO dissolved in D2O and H2O. For both systems the vibrational energy relaxation proceeds through an intermediate state. The relaxation leads to heating of the sample, which is observed in the transient spectra. In order to obtain the correct anisotropy decay, the ingrowing heating signal is subtracted from the raw data. For the OD vibration this procedure works well. For the OH vibration, however, we find an additional effect that leads to a severe distortion of the anisotropy. We show that this effect can be explained by a slightly faster reorientation of excited molecules during their relaxation as compared to unexcited molecules. We construct a model that includes this effect and is able to reproduce the experimental data. Using this model we show how the distorted anisotropy can be corrected.

Effect of urea on the structural dynamics of water.

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the effect of urea on the structure and dynamics of water. Surprisingly, we find that, even at high concentrations of urea (8 M), the orientational dynamics of most water molecules are the same as in pure liquid water, showing that urea has a negligible effect on the hydrogen-bond dynamics of these molecules. However, a small fraction of the water molecules (approximately one water molecule per urea molecule) turns out to be strongly immobilized by urea, displaying orientational dynamics that are more than six times slower than in bulk water. A likely explanation is that these water molecules are tightly associated with urea, forming specific urea-water complexes. We discuss these results in light of the protein denaturing ability of aqueous urea.

On the orientational relaxation of HDO in liquid water.

We use femtosecond mid-infrared pump-probe spectroscopy to study the orientational relaxation of HDO molecules dissolved in H2O. In order to obtain a reliable anisotropy decay we model the effects of heating and correct for these effects. We have measured the reorientation time constant of the OD vibration from 2430 to 2600 cm(-1), and observe a value of 2.5 ps that shows no variation over this frequency interval. Our results are discussed in the context of previous experiments that have been performed on the complementary system of HDO dissolved in D2O.

Orientational dynamics of hydrogen-bonded phenol.

We use femtosecond mid-infrared pump-probe spectroscopy to study the effects of hydrogen bonding on the orientational dynamics of the OD-stretch vibration of phenol-d. We study two samples: phenol-d in chloroform and phenol-d in chloroform to which we added excess acetone. For phenol-d in chloroform, we observe rotational diffusion of the OD group around the CO bond, with a correlation time of 3.7 ps. For phenol-d hydrogen bonded to acetone, the reorientation time is strongly dependent on the probe frequency, varying from 3 ps on the blue side of the spectrum to more than 30 ps on the red side. (c) 2004 American Institute of Physics.