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1. Nonlinear infrared polaritonic interaction between cavities mediated by molecular vibrations at ultrafast time scale

   https://doi.org/10.1126/sciadv.abf6397  

   Xiang, Bo ; Wang, Jiaxi ; Yang, Zimo ; Xiong, Wei ( May 2021 , Science Advances)

   null (Ed.)

   Realizing nonlinear interactions between spatially separated particles can advance molecular science and technology, including remote catalysis of chemical reactions, ultrafast processing of information in infrared (IR) photonic circuitry, and advanced platforms for quantum simulations with increased complexity. Here, we achieved nonlinear interactions at ultrafast time scale between polaritons contained in spatially adjacent cavities in the mid-IR regime, altering polaritons in one cavity by pumping polaritons in an adjacent one. This was done by strong coupling molecular vibrational modes with photon modes, a process that combines characteristics of both photon delocalization and molecular nonlinearity. The dual photon/molecule character of polaritons enables delocalized nonlinearity—a property that neither molecular nor cavity mode would have alone. 

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2. Polarized Raman spectra of hematite and assignment of external modes

   https://doi.org/10.1002/jrs.5824  

   Marshall, Craig P. ; Dufresne, William J. B. ; Rufledt, Carson J. ( January 2020 , Journal of Raman Spectroscopy)

   Notwithstanding the significant practical importance of hematite, α‐Fe₂O₃, the complete assignment and understanding of the Raman spectrum acquired on this crystalline solid are uncertain. Above all, only one of the two externalE_gphonons arising from theΓpoint has been resolved and hence assigned. It is well known that theE_gmode at 294 cm⁻¹has been attributed to one of the two external phonons arising from theΓpoint. To this end, we have undertaken studies to examine the polarized Raman scattering on a pure single crystal to gain a better understanding and assignment of phonons arising from theΓpoint in the Raman spectrum of hematite. Here, we resolve and assign the previously unidentified externalE_gphonon at 245 cm⁻¹and, additionally, confirm that the band at 294 cm⁻¹is an externalE_gphonon, in the first‐order Raman spectrum of hematite. Further, our polarized Raman spectra display interesting polarization behavior of the 2LO mode at 1320 cm⁻¹, where this band is only Raman active ine_i||e_spolarization configurations.

    

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3. Raman study of layered breathing kagome lattice semiconductor Nb 3 Cl 8

   https://doi.org/10.1088/2053-1583/acfa10  

   Jeff, Dylan A ; Gonzalez, Favian ; Harrison, Kamal ; Zhao, Yuzhou ; Fernando, Tharindu ; Regmi, Sabin ; Liu, Zhaoyu ; Gutierrez, Humberto R ; Neupane, Madhab ; Yang, Jihui ; et al ( September 2023 , 2D Materials)

   Niobium chloride (Nb₃Cl₈) is a layered two-dimensional semiconducting material with many exotic properties including a breathing kagome lattice, a topological flat band in its band structure, and a crystal structure that undergoes a structural and magnetic phase transition at temperatures below 90 K. Despite being a remarkable material with fascinating new physics, the understanding of its phonon properties is at its infancy. In this study, we investigate the phonon dynamics of Nb₃Cl₈in bulk and few layer flakes using polarized Raman spectroscopy and density-functional theory (DFT) analysis to determine the material’s vibrational modes, as well as their symmetrical representations and atomic displacements. We experimentally resolved 12 phonon modes, five of which areA_1gmodes while the remaining seven areE_gmodes, which is in strong agreement with our DFT calculation. Layer-dependent results suggest that the Raman peak positions are mostly insensitive to changes in layer thickness, while peak intensity and full width at half maximum are affected. Raman measurements as a function of excitation wavelength (473–785 nm) show a significant increase of the peak intensities when using a 473 nm excitation source, suggesting a near resonant condition. Temperature-dependent Raman experiments carried out above and below the transition temperature did not show any change in the symmetries of the phonon modes, suggesting that the structural phase transition is likely from the high temperatureP$\overline{\text{3}m}$1 phase to the low-temperatureR$\overline{3m}$phase. Magneto-Raman measurements carried out at 140 and 2 K between −2 and 2 T show that the Raman modes are not magnetically coupled. Overall, our study presented here significantly advances the fundamental understanding of layered Nb₃Cl₈material which can be further exploited for future applications.

    

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4. Vibrational anatomy of C90, C96, and C100 fullertubes: probing Frankenstein's skeletal structures of fullerene head endcaps and nanotube belt midsection

   https://doi.org/10.1039/d2nr01870e  

   Schiemenz, Sandra ; Koenig, Ryan M. ; Stevenson, Steven ; Avdoshenko, Stanislav M. ; Popov, Alexey A. ( July 2022 , Nanoscale)

   Fullertubes are tubular fullerenes with nanotube-like middle section and fullerene-like endcaps. To understand how this intermediate form between spherical fullerenes and nanotubes is reflected in the vibrational modes, we performed comprehensive studies of IR and Raman spectra of fullertubes C90-D5h, C96-D3d, and C100-D5d. An excellent agreement between experimental and DFT-computed spectra enabled a detailed vibrational assignment and allowed an analysis of the localization degree of the vibrational modes in different parts of fullertubes. Projection analysis was performed to establish an exact numerical correspondence between vibrations of the belt midsection and fullerene headcaps to the modes of nanotubes and fullerene C60-Ih. As a result, we could not only identify fullerene-like and CNT-like vibrations of fullertubes, but also trace their origin in specific vibrational modes of CNT and C60-Ih. IR spectra were found to be dominated by vibrations of fullerene-like caps resembling IR-active modes of C60-Ih, whereas in Raman spectra both caps and belt vibrations are found to be equally active. Unlike the resonance Raman spectra of CNTs, in which only two single-phonon bands are detected, the Raman spectra of fullertubes exhibit several CNT-like vibrations and thus provide additional information on nanotube phonons. 

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5. The Role of Optical Phonon Confinement in the Infrared Dielectric Response of III–V Superlattices

   https://doi.org/10.1002/adma.202305106  

   Matson, Joseph R. ; Alam, Md Nazmul ; Varnavides, Georgios ; Sohr, Patrick ; Knight, Sean ; Darakchieva, Vanya ; Stokey, Megan ; Schubert, Mathias ; Said, Ayman ; Beechem, Thomas ; et al ( December 2023 , Advanced Materials)

   Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon‐polaritons that allow for low‐loss, subdiffractional control of light. The properties of phonon‐polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most “bulk” materials. Superlattices composed of alternating atomically thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so‐called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm⁻¹. These results provide an alternative pathway toward designer IR optical materials.

    

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