Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering

The motion of chemical bonds within molecules can be observed in real time, in the form of vibrational wavepackets prepared and interrogated through ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are commonly performed on large ensembles of molecules, and as such, are limited to the extent that ensemble coherence can be maintained. Here, we describe vibrational wavepacket motion on single molecules, recorded through time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The required sensitivity to detect the motion of a single molecule, under ambient conditions, is achieved by equipping the molecule with a dipolar nano-antenna (a gold dumbbell). In contrast with measurements in ensembles, the vibrational coherence on a single molecule does not dephase. It develops phase fluctuations with characteristic statistics. We present the time evolution of discretely sampled statistical states, and highlight the unique information content in the characteristic, early-time probability distribution function of the signal.Comment: 17 pages, 5 figure

Time-Resolved Measurement of Vibrational Coherences in the Single Molecule Limit

Time-resolved, surface-enhanced, coherent anti-Stokes Raman spectroscopy (tr-SECARS) isideally suited for preparing and interrogating vibrational coherences on single molecules.We have succeeded in the rst demonstration of this concept through measurements carriedout on molecules attached to gold nanosphere pairs which act as plasmonic nano-dumbellantennae. The tr-SECARS traces provide unique signatures of coherent evolution in discreteensembles. The signals are characterized by phase and amplitude noise, which can be cast interms of amplitude probability distribution functions (PDF), which allow rigorous distinctionbetween single, few, and many molecule coherences. We give a brief background on tr-CARS,the experimental system for carrying out tr-SECARS and the analysis of the results in termsof PDFs. The analysis makes it clear that we have, for the rst time, observed the coherentvibrational motion of a single molecule

Tomographic State Reconstruction and Time Resolved Surface Enhanced Coherent Anti-Stokes Raman Scattering in the Single Molecule Limit

Time-resolved, surface-enhanced, coherent anti-Stokes Raman spectroscopy (tr-SECARS) isideally suited for preparing and probing vibrational coherences in molecules. By enhancingthe local response of a single molecule with a dipolar nano-antenna, vibrational dynamicshave been measured at the single molecule limit. In contrast with tr-CARS measurementsin ensembles, the vibrational coherence of a single molecule is not subject to pure dephasing.It exhibits characteristic phase and amplitude noise, which allows the statistical distinctionbetween single, few, and many molecule sources to be determined. To build on the cur-rent work, by using three unique pulses to spectrally lter the response of the molecule,the characteristic noise can be isolated and measured background-free. If the probing of asuperposition state is carried out over a real resonance, then it is possible to tomographicallyreconstruct the complete description of quantum dynamics in phase space representation viathe Wigner Distribution Function(WDF). The WDF can be reconstructed from either thewavepacket via Wigner Transform, or an experimentally measured density, via an InverseRadon Transform. The calculations presented here highlight the necessary conditions inorder to reconstruct the WDF with delity from a proposed experiment and compare thedensity derived WDF with that of the wavepacket. The principle is rstly demonstrated us-ing a Kerr gated detection of emission from an evolving state on a bound harmonic potentialenergy surface. The model is then explained in the case of a proposed spectrally resolvedtransient grating experiment (SRTG). The WDFs generated from the limiting conditionsshow that the reproduction delity of the experimentally derived WDF are dependent onthe probe, utilized to measure the evolving superposition, and the curvature, or the vibra-tional frequency of the potential energy surfaces, and the dephasing time of the vibrationalsuperposition states. Given two potentials, I show that it is possible to optimize probepulse parameters to improve the delity of the state reconstruction. Due to the variationalprinciple, the negative volume of the WDF, or the Wigner hole, can only be reduced viameasurements - the pulse parameters can be optimized iteratively even when the molecularpotentials are not known

Using an emittance exchanger as a bunch compressor

A general architecture of an emittance exchanger (EEX) is considered, where the horizontal and longitudinal phase spaces are exchanged. A family of designs is described which can lead to extremely short final longitudinal lengths, even subfemtosecond. Using higher-order particle simulations, a preferred configuration is found, which has better compression capability and less emittance growth than the standard EEX design at high beam energy. An alternative design is also found which eliminates any final energy-phase coupling. Features of compression using an EEX are significantly different than with a chicane because the final longitudinal phase space is decoupled from the initial longitudinal phase space. Advantages of using an EEX for compression include less susceptibility to the coherent synchrotron radiation (CSR) microbunch instability, less susceptibility to bunch length broadening from CSR effects, and elimination of the initial energy-phase correlation that is needed for compression using a chicane as well as any residual energy-phase correlation after compression. A key disadvantage of using an EEX is that the final horizontal emittance tends to strongly depend on the initial bunch length and beam energy

Seeing the vibrational breathing of a single molecule through time-resolved coherent anti-Stokes Raman scattering

The motion of chemical bonds within molecules can be observed in real time, in the form of vibrational wavepackets prepared and interrogated through ultrafast nonlinear spectroscopy. Such nonlinear optical measurements are commonly performed on large ensembles of molecules, and as such, are limited to the extent that ensemble coherence can be maintained. Here, we describe vibrational wavepacket motion on single molecules, recorded through time-resolved, surface-enhanced, coherent anti-Stokes Raman scattering. The required sensitivity to detect the motion of a single molecule, under ambient conditions, is achieved by equipping the molecule with a dipolar nano-antenna (a gold dumbbell). In contrast with measurements in ensembles, the vibrational coherence on a single molecule does not dephase. It develops phase fluctuations with characteristic statistics. We present the time evolution of discretely sampled statistical states, and highlight the unique information content in the characteristic, early-time probability distribution function of the signal

Arbitrary emittance partitioning between any two dimensions for electron beams

The flat-beam transform (FBT) for round symmetric beams can be extended using the concept of eigenemittances. By tailoring the initial beam conditions at the cathode, including adding arbitrary correlations between any two dimensions, this extension can be used to provide greater freedom in controlling the beam’s final emittances. In principle, this technique can be used to generate extraordinarily transversely bright electron beams. Examples are provided where an equivalent FBT is established between the horizontal and the longitudinal beam dimensions

Ultrafast pump-probe force microscopy with nanoscale resolution

We perform time-resolved pump-probe microscopy measurements by recording the local force between a sharp tip and the photo-excited sample as a readout mechanism for the material's nonlinear polarization. We show that the photo-induced force is sensitive to the same excited state dynamics as measured in an optical pump-probe experiment. Ultrafast pump-probe force microscopy constitutes a non-optical detection technique with nanoscale resolution that pushes pump-probe sensitivities close to the realm of single molecule studies