Probing Vibrational Strong Coupling of Molecules with Wavelength‐Modulated Raman Spectroscopy

Abstract

that are shifted from that of the incident laser light. In particular, peaks appear in the spectrum of the scattered light that correspond to vibrational resonances of the molecules/materials of which the sample is composed. These spectral peaks form the characteristic fingerprints of the molecules/materials under investigation; the study of such spectra is known as Raman spectroscopy and has become a well-established and powerful analytic technique. [5] Unfortunately, Raman signals are very weak with cross-sections of order 10 −30 cm 2 for non-resonant Raman and 10 −24 cm 2 for resonant Raman. [6] In many situations, these very weak signals are swamped by light produced by other processes, notably fluorescence, which can be many orders of magnitude stronger than the Raman signal. Background fluorescence is particularly problematic when the samples to be investigated are dyes/pigments, [3] for example, resonant Raman, [6] and also when they involve metals. [7] In both cases, fluorescence can be so significant as to mask the desired Raman signals.Various approaches have been adopted to try and deal with the background problem, [8,9] and they fall into three broad categories. First, post-processing can be used to try and subtract the background. [10,11] Although appealing and commonly used, this approach has its drawbacks, especially in that it does not overcome the problem of noise in the background signal. Second, one can move to the time-domain. Raman scattering is an instantaneous process, whereas fluorescence occurs over a protracted time-scale; however, this approach comes at a very significant increase in complexity. [12] Third, a differential approach can be adopted. Here, the idea is to modify the way the data are acquired so as to directly filter out the background and its associated noise. A number of such approaches have been successfully deployed. One technique is based on acquiring spectra for slightly different grating settings in the spectrometer. [7] Another approach is to subtract two Raman spectra obtained with different polarization configurations. [13] Then, there are techniques that make use of the fact that if the pump wavelength is shifted the Raman peaks will also move in wavelength, but the fluorescence will not. Spectra acquired at two or more closely spaced pump wavelengths can thus be processed to provide a differential signal that largely eliminates the background; [14] this technique is known as shifted excitation Raman difference spectroscopy (SERDS). We make use of one variant of this approach here, that of wavelength-modulated Raman spectroscopy (WMRS). WMRS [15] is a significant improvement on Raman spectroscopy is a powerful technique that enables fingerprinting of materials, molecules, and chemical environments by probing vibrational resonances. In many applications, the desired Raman signals are masked by fluorescence, either from the molecular system being studied, or from adjacent metallic nanostructures. Here, it is shown that wavelength-modulated Raman spectroscopy provides...

Chasing Vibro-Polariton Fingerprints in Infrared and Raman Spectra Using Surface Lattice Resonances on Extended Metasurfaces

“…Some regions of high intensity are present on top of the gold disk, especially in the measurements performed at a height of 500 nm (Figure h), and are most likely caused by gold fluorescence (see the Supporting Information, Figure S8). The absence of polaritons in the Raman spectra and micro-Raman maps, even in regions with high SLR field enhancement, confirms previous results in Fabry–Perot cavities. , A possible explanation for this absence could be found in the less-than-ideal spatial overlap of the electromagnetic modes at optical and infrared wavelengths. ,, This is a consequence of the inhomogeneity of the field profile in our periodic array, as shown in Figure c–e. However, it has been shown that even in Fabry–Perot cavities strongly coupled to molecular vibrations, with rather homogeneous field distributions and good mode overlap, vibro-polariton signatures in Raman spectra remain elusive.…”

“…The absence of polaritons in the Raman spectra and micro-Raman maps, even in regions with high SLR field enhancement, confirms previous results in Fabry–Perot cavities. 38 , 49 A possible explanation for this absence could be found in the less-than-ideal spatial overlap of the electromagnetic modes at optical and infrared wavelengths. 42 , 43 , 50 This is a consequence of the inhomogeneity of the field profile in our periodic array, as shown in Figure 2 c–e.…”

Complementary Surface‐Enhanced Raman Scattering (SERS) and IR Absorption Spectroscopy (SEIRAS) with Nanorods‐on‐a‐Mirror

“…[39] One puzzling observation is the clear and straightforward detection of spectral splitting and coupled polaritonic states with IR absorption spectroscopy while in Raman spectra, only elusive to nonexistent detection of these coupled states has been observed. [40][41][42][43] One proposed reason for the absence of polaritonic features in the Raman spectra is that large numbers of uncoupled molecules co-exist with the coupled molecules and dominate the Raman spectra. NRoMs could offer a unique platform to probe specifically vibrationally coupled molecules through SERS, provided that VSC conditions can be reached in an optimized configuration.…”

Coherent anharmonicity transfer from matter to light in the THz regime

“…In analogy with classical phase modulation processes in bulk nonlinear optical materials [50], which depend on the anharmonic response of the medium to strong fields [51], cavity-assisted phase shifts are possible due to photon blockade effects that arise due to intrinsic spectral anharmonicities of strongly coupled light-matter systems [52,53]. In the infrared regime, despite the growing interest in cavity QED phenomena with molecular vibrations [25,26,40,[54][55][56][57][58][59] and semiconductors [16,[60][61][62], viable physical mechanisms for implementing conditional phase dynamics with infrared fields have yet to be developed.…”