Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy

Abstract

Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in “real time”. Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy-5-(2-ethylhexyloxy)−1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the real-time information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.

Time-Domain Observation of Surface-Enhanced Coherent Raman Scattering with 105–106 Enhancement

“…We carried out ISRS measurements for trans -1,2-bis­(4-pyridyl)­ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs) and successfully observed surface-enhanced Raman signals with drastic intensity enhancement. Enhancement factors as high as 10 5 –10 6 were obtained, which is in stark contrast to the small enhancement reported before . The present study on surface-enhanced impulsive stimulated Raman spectroscopy (SE-ISRS) can be extended to femtosecond time-resolved SERS measurements in the time domain by introducing the actinic pump pulse and hence paves a way for studying ultrafast structural dynamics at the single molecule level.…”

“…Actually, Kobayashi and co-workers recently reported a pump–probe study of the adsorbate on the SERS substrate using sub-10 fs pulses, aiming at obtaining surface-enhanced coherent Raman signals in the time domain. However, they only observed small enhancement factor of <5 …”

“…To achieve this goal, it is desirable to combine SERS with coherent nonlinear Raman spectroscopy because high time resolution can be achieved by utilizing femtosecond pulses for triggering the Raman process. In fact, several attempts to combine SERS with coherent nonlinear Raman spectroscopy have already been reported, − with the aim to extend SERS to time-resolved measurements. In particular, Frontiera et al employed broadband stimulated Raman spectroscopy using femtosecond and picosecond pulses and successfully obtained surface-enhanced Raman spectra of adsorbed molecules with an enhancement factor as high as ∼10 4 –10 6 .…”

“…However, they only observed small enhancement factor of <5. 32 In this study, we report on the observation of surfaceenhanced Raman signals with impulsive stimulated Raman spectroscopy (ISRS) using sub-8 fs pulses. In this time-domain Raman technique, Raman-active vibrations of molecules are coherently excited by the ultrashort pump pulse, and they are monitored directly in the time domain as the oscillation of the time-resolved absorption signal.…”

Universal route to optimal few- to single-cycle pulse generation in hollow-core fiber compressors

“…In particular, such pulses have paved the way for attosecond physics and metrology [1][2][3][4][5][6], via the extreme ultraviolet (XUV) attosecond pulse trains and isolated attosecond pulses [7] that can be obtained by high-harmonic generation (HHG). The use of few-cycle optical pulses with durations close to or shorter than 10 fs in the near-infrared, visible and near-ultraviolet spectral regions has been extended in recent years to a wide range of spectroscopic techniques such as impulsive vibrational spectroscopy [8][9][10], time-resolved stimulated Raman spectroscopy [11][12][13][14], and ultrafast pump-probe absorption spectroscopy [15,16]. Few-cycle optical pulses have also become an interesting tool for transient absorption microscopy [17], near-field imaging techniques [18] and for generating ultrashort terahertz radiation [19].…”