Tip-Enhanced Multipolar Raman Scattering

Wang, Chih-Feng; Cheng, Zhihua; O'callahan, Brian; Crampton, Kevin T.; Jones, Matthew R.; El‐Khoury, Patrick Z.

doi:10.1021/acs.jpclett.0c00559

Cited by 26 publications

(51 citation statements)

References 49 publications

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“…The quadrupole moment would play an important role if the radius of curvature of the tip could be sharpened to the few-nanometer regime, or operated in the plasmonic gap mode. [48][49][50][51] In principle, out-of-plane χ (2) components (χ (2) xyz , χ (2) xzy , χ (2) zxy ) are allowed based on the point group symmetry (D 3 ) of 3R-homobilayer TMD. However, if the out-of-plane contribution was significant, we would expect the intensity ratio to be significantly higher than four, since the additional out-of-plane χ (2) components are absent in a monolayer.…”

Section: Resultsmentioning

confidence: 99%

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

Yao

Zhang

Yanev

et al. 2022

Advanced Optical Materials

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Second harmonic generation (SHG) is a nonlinear optical response arising exclusively from broken inversion symmetry in the electric‐dipole limit. Recently, SHG has attracted widespread interest as a versatile and noninvasive tool for characterization of crystal symmetry and emerging ferroic or topological orders in quantum materials. However, conventional far‐field optics is unable to probe local symmetry at the deep subwavelength scale. Here, near‐field SHG imaging of 2D semiconductors and heterostructures with the spatial resolution down to 20 nm is demonstrated using a scattering‐type nano‐optical apparatus. It is shown that near‐field SHG efficiency is greatly enhanced by excitons in atomically thin transition metal dichalcogenides. Furthermore, by correlating nonlinear and linear scattering‐type nano‐imaging, nanoscale variations of interlayer stacking order in bilayer WSe2 are resolved, and the stacking‐tuned excitonic light–matter interactions are revealed. This work demonstrates nonlinear optical interrogation of crystal symmetry and structure–property relationships at the nanometer length scales relevant to emerging properties in quantum materials.

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Section: Resultsmentioning

confidence: 99%

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

Yao

Zhang

Yanev

et al. 2022

Advanced Optical Materials

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“…Field gradients are marked by Ramanforbidden transitions. 2,35 Several pioneering SERS reports provided evidence for multipolar Raman scattering; early analyses suggested local electric field gradients that vary over molecular length scales result in multipolar Raman scattering signals that are comparable to conventional (dipolar) Raman scattering. 37 The latter is certainly possible in TERS, as shown in Figures 4C and 6, wherein local optical fields that vary over the single nanometer length scale can be directly visualized.…”

Section: Gradients In Tersmentioning

confidence: 97%

“…A comparison between spectra simulated following eq 4, akin to those shown in Figure 7, and TERS spectra allowed us to track 3D molecular orientations with fewnanometer spatial resolution, as described in early 28,29 and more recent reports from our group. 4 In addition to tensorial Raman scattering, optical field gradients at the apex of a plasmonic probe can lead to signatures in TERS 2,35,36 that are not observable in ensemble-averaged Raman scattering. Field gradients are marked by Ramanforbidden transitions.…”

Section: Gradients In Tersmentioning

confidence: 99%

Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation

El‐Khoury

2021

Acc. Chem. Res.

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Conspectus Historically, molecular spectroscopists have focused their attention to the right-hand side of the Schrödinger equation. Our major goal had and still has to do with determining a (bio)molecular system’s Hamiltonian operator. From a theoretical spectroscopist’s perspective, this entails varying the parameters of a model Hamiltonian until the predicted observables agree with their experimental analogues. In this context, less emphasis has been put on the left-hand side of the equation, where the interplay between a system and its immediate local environment is described. The latter is particularly meaningful and informative in modern applications of optical microscopy and spectroscopy that take advantage of surface plasmons to enhance molecular scattering cross-sections and to increase the attainable spatial resolution that is classically limited by diffraction. Indeed, the manipulation of light near the apex of a metallic nanotip has enabled single molecule detection, identification, and imaging. The distinct advantages of the so-called tip-enhanced optical nanospectroscopy/nanoimaging approaches are self-evident: ultrahigh spatial resolution (nanometer or better) and ultimate sensitivity (down to yoctomolar) are both attainable, all while retaining the ability to chemically fingerprint one molecule at a time (e.g., through Raman scattering). An equally interesting aspect of the same approach stems from using the properties of a single molecule to characterize the local environment in which it resides. This concept of single molecule spectroscopy on the left-hand side of the Schrödinger equation is certainly not novel and has been discussed in pioneering single molecule studies that ultimately led to a Nobel prize in chemistry. That said, local environment mapping through ultrasensitive optical spectroscopy acquires a unique flavor when executed using tip-enhanced Raman scattering (TERS). This is the subject of this Account. In a series of recent reports, our group utilized TERS to characterize different properties of nanolocalized and enhanced optical fields. The platforms that were used to this end consist of chemically functionalized plasmonic nanostructures and nanoparticles imaged using visible-light-irradiated gold- or silver-coated probes of an atomic force microscope. Through a detailed analysis of the recorded spectral nanoimages, we found that molecular Raman spectra may be used to track the magnitudes, resonances, spatiotemporal gradients, and even vector components of optical fields with nanometer spatial resolution under ambient conditions. On the other side of the equation, understanding how spatially varying optical fields modulate molecular nano-Raman spectra is of utmost importance to emerging areas of nanophotonics. For instance, tracking plasmon-enhanced chemical transformations via TERS necessitates a deeper fundamental understanding of the optical signatures of molecular reorientation and multipolar Raman scattering, both of which may be driven by local optical field gradients th...

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“…[ 2–4,6,8 ] Furthermore, the tip‐enhanced multipolar Raman scattering spectrum has also been reported, due to surface plasmonic gradient effect at plasmonic tip–sample nanojunctions, which induced the electric dipole–electric quadrupole‐driven transitions. [ 5,6,11–16 ]…”

Section: Introductionmentioning

confidence: 99%

“…[2][3][4]6,8] Furthermore, the tip-enhanced multipolar Raman scattering spectrum has also been reported, due to surface plasmonic gradient effect at plasmonic tip-sample nanojunctions, which induced the electric dipole-electric quadrupole-driven transitions. [5,6,[11][12][13][14][15][16] Plexcitons are polaritonic modes that result from the strong coupling interaction between plasmons and excitons. [17][18][19][20][21][22][23][24][25][26][27][28] Absorption and emission properties of hybrid system can be designed and controlled by excitonplasmon interaction.…”

Section: Introductionmentioning

confidence: 99%

Plexciton and electron–phonon interaction in tip‐enhanced resonance Raman scattering

Song

Cheng

et al. 2021

J Raman Spectroscopy

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We investigate physical mechanism on plexciton and electron–phonon interaction in tip‐enhanced resonance Raman scattering (TERRS). Plexcitons are polaritonic modes that result from the strong coupling between excitons and plasmons. Exciton and plasmon coupling interaction can result in two plexciton peaks; and the plexciton peak at low energy (bright mode) is greatly enhanced, and the plexciton peak at high energy (dark mode) is suppressed in TERRS. The bright mode of plexciton can coherently strongly enhance resonance Raman scattering in TERRS. The total enhancement in TERRS can reach up to the order of 1011, including plexciton enhancement of 108 and molecular resonance Raman enhancement of 103. Interaction between voltage/current and molecular vibrations (electron–phonon interaction) can also further strongly enhance the intensity of Raman scattering in TERRS. The electron–phonon interaction at different incident light is theoretically revealed for plexciton‐enhanced resonant Raman scattering in TERRS. Our theoretical results can promote the deeper understanding the contributions of plexciton and electron–phonon interaction to TERRS.

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Tip-Enhanced Multipolar Raman Scattering

Cited by 26 publications

References 49 publications

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation

Plexciton and electron–phonon interaction in tip‐enhanced resonance Raman scattering

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