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Register a new userElectron and Light Induced Stimulated Raman Spectroscopy for Nanoscale Molecular Mapping
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We propose and theoretically analyze a new vibrational spectroscopy, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of electron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. With ELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband but spatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterize this technique, we develop a numerical model and conduct full-field electromagnetic simulations to investigate two distinct nanoparticle geometries, nanorods and nanospheres, coated with a Raman-active material. Our results show the significant ($10^6$-$10^7$) stimulated Raman enhancement that is achieved with dual electron and optical excitation of these nanoparticle geometries. Importantly, the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determined by the nanoparticle geometry and the plasmon mode volume. Our results highlight the promise of ELISR for simultaneous high-resolution electron microscopy with sub-diffraction-limited Raman spectroscopy, complementing advances in superresolution microscopy, correlated light and electron microscopy, and vibrational electron energy loss spectroscopy.
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Excited-state vibrations are crucial for determining photophysical and photochemical properties of molecular compounds. Stimulated Raman scattering can coherently stimulate and probe molecular vibrations with optical pulses, but it is generally restricted to ground state properties. Working in resonance conditions, indeed, enables cross-section enhancement and selective excitation to a targeted electronic level, but is hampered by an increased signal complexity due to the presence of overlapping spectral contributions. Here, we show how detailed information on ground and excited state vibrations can be disentangled, by exploiting the relative time delay between Raman and probe pulses to control the excited state population, combined with a diagrammatic formalism to dissect the pathways concurring to the signal generation. The proposed method is then exploited to elucidate the vibrational properties of ground and excited electronic states in the paradigmatic case of Cresyl Violet. We anticipate that the presented approach holds the potential for selective mapping the reaction coordinates pertaining to transient electronic stages implied in photo-active compounds.
Stimulated Raman spectroscopy has become a powerful tool to study the spatiodynamics of molecular bonds with high sensitivity, resolution and speed. However, sensitivity and speed of state-of-the-art stimulated Raman spectroscopy are currently limited by the shot-noise of the light beam probing the Raman process. Here, we demonstrate an enhancement of the sensitivity of continuous-wave stimulated Raman spectroscopy by reducing the quantum noise of the probing light below the shot-noise limit by means of amplitude squeezed states of light. Probing polymer samples with Raman shifts around 2950 $cm^{-1}$ with squeezed states, we demonstrate a quantum-enhancement of the stimulated Raman signal-to-noise ratio (SNR) of 3.60 dB relative to the shot-noise limited SNR. Our proof-of-concept demonstration of quantum-enhanced Raman spectroscopy paves the way for a new generation of Raman microscopes, where weak Raman transitions can be imaged without the use of markers or an increase in the total optical power.
A simple physical mechanism of stimulated light scattering on nanoscale objects in water suspension similar to Langmuir waves mechanism in plasma is proposed. The proposed mechanism is based on a dipole interaction between the light wave and the non-compensated electrical charge that inevitably exists on a nanoscale object (a virus or a nanoparticle) in water environment. The experimental data for tobacco mosaic virus and polystyrene nanospheres are presented to support the suggested physical mechanism. It has been demonstrated that stimulated amplification spectral line frequencies observed experimentally are well explained by the suggested mechanism. In particular, the absence of lower frequency lines and the generation lines shift when changing the pH are due to ion friction appearing in the ionic solution environment. The selection rules observed experimentally also confirm the dipole interaction type. It has been shown that microwave radiation on nanoscale object acoustic vibrations frequency should appear under such scattering conditions. We demonstrate that such conditions also allow for local selective heating of nanoscale objects by dozens to hundreds degrees K. This effect is controlled by the optical irradiation parameters and can be used for affecting selectively certain types of viruses.
We demonstrate spatially-resolved measurements of spontaneous and stimulated electron-photon interactions in nanoscale optical near fields using electron energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and photon-induced near-field electron microscopy (PINEM). Specifically, we study resonant surface plasmon modes that are tightly confined to the tip apexes of an Au nanostar, enabling a direct correlation of EELS, CL, and PINEM on the same physical structure at the nanometer length scale. Complemented by numerical electromagnetic boundary-element method calculations, we discuss the spontaneous and stimulated electron-photon interaction strength and spatial dependence of our EELS, CL and PINEM distributions. We demonstrate that in the limit of an isolated tip mode, spatial variations in the electron-near field coupling are fully determined by the modal electric field profile, irrespective of the spontaneous (in EELS and CL) or stimulated nature (in PINEM) of the process. Yet we show that coupling to the tip modes crucially depends on the incident electron energy with a maximum at a few keV, depending on the proximity of the interaction to the tip apex. Our results provide elementary insights into spontaneous and stimulated electron-light-matter interactions at the nanoscale that have key implications for research on (quantum) coherent optical phenomena in electron microscopy.
Spectroscopic analysis of large biomolecules is critical in a number of applications, including medical diagnostics and label-free biosensing. Recently, it has been shown that Raman spectroscopy of proteins can be used to diagnose some diseases, including a few types of cancer. These experiments have however been performed using traditional Raman spectroscopy and the development of the Surface enhanced Raman spectroscopy (SERS) assays suitable for large biomolecules could lead to a substantial decrease in the amount of specimen necessary for these experiments. We present a new method to achieve high local field enhancement in surface enhanced Raman spectroscopy through the simultaneous adjustment of the lattice plasmons and localized surface plasmon polaritons, in a periodic bilayer nanoantenna array resulting in a high enhancement factor over the sensing area, with relatively high uniformity. The proposed plasmonic nanostructure is comprised of two interacting nanoantenna layers, providing a sharp band-edge lattice plasmon mode and a wide-band localized surface plasmon for the separate enhancement of the pump and emitted Raman signals. We demonstrate the application of the proposed nanostructure for the spectral analysis of large biomolecules by binding a protein (streptavidin) selectively on the hot-spots between the two stacked layers, using a low concentration solution (100 nM) and we successfully acquire its SERS spectrum.
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