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Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals. Hyperbolic phonon polaritons occurring in anisotropic materials exhibit strong light confinement and propagation directionality. Matson et al. report real-space imaging and control of recently discovered hyperbolic shear phonon-polaritons in beta-Ga2O3, arising from symmetry breaking in the dielectric response. [ABSTRACT FROM AUTHOR]

Polaritons can provide strong optical field enhancement allowing them to boost light–matter interaction. Here, we experimentally observe enhancement in mid-infrared second-harmonic generation (SHG) using grating-coupled surface phonon polaritons of the 6H-SiC surface. In our experiment, we measure the SHG along the polariton dispersion by changing the incidence angle of the excitation beam. We observe hybridization between the propagating surface phonon polaritons and localized plasmon resonances in the gold grating, evidenced by the modification of the polariton dispersion as we change the area ratio of grating and substrate. Design options for engineering the plasmon–phonon polariton hybridization are discussed. Overall, we find a rather low yield of polariton-enhanced SHG in this geometry compared to prism-coupling and nanostructures and discuss possible origins. [ABSTRACT FROM AUTHOR]

Super-resolution microscopy in the visible is an established powerful tool in various disciplines. In the long-wave infrared (LWIR) spectral range, however, no comparable schemes have been demonstrated to date. In this work, we experimentally demonstrate super-resolution microscopy in the LWIR range ( λ I R ≈ 10 – 12 μ m) using IR-visible sum-frequency generation. We operate our microscope in a wide-field scheme and image localized surface phonon polaritons in 4H-SiC nanostructures as a proof-of-concept. With this technique, we demonstrate an enhanced spatial resolution of ∼ λ I R / 9 , enabling to resolve the polariton resonances in individual sub-diffractional nanostructures with sub-wavelength spacing. Furthermore, we show that this resolution allows us to differentiate between spatial patterns associated with different polariton modes within individual nanostructures. [ABSTRACT FROM AUTHOR]

We investigate the influence of carrier cooling dynamics in TiO2 on the excited-state potential energy surface along the Alg optical phonon coordinate after above band-gap excitation using ultrashort ultraviolet pulses. The large amplitude coherent oscillation observed in a pump-probe transient reflectivity measurement shows a phase shift of -0.2π with respect to a purely instantaneous displacive excitation. The dynamic evolution of the potential energy surface minimum of the coherent phonon coordinate is explored using accurate density functional theory calculations, which confirm a shift of the potential energy surface minimum upon resonant laser excitation and reveal a significant positive contribution to the displacive force due to the cooling of the excited hot electron-hole plasma. We show that this noninstantaneous effect can quantitatively explain the experimentally observed phase using reasonable assumptions for the parameters characterizing the excited carriers. Our work demonstrates that the fast equilibration dynamics of laser-excited nonequilibrium carrier populations can have a pronounced effect on the initial structural response of crystalline solids. [ABSTRACT FROM AUTHOR]

In stimulated Raman scattering, two incident optical waves induce a force oscillating at the difference of the two light frequencies. This process has enabled important applications such as the excitation and coherent control of phonons and magnons by femtosecond laser pulses. Here, we experimentally and theoretically demonstrate the so far neglected up-conversion counterpart of this process: THz sum-frequency excitation of a Raman-active phonon mode, which is tantamount to two-photon absorption by an optical transition between two adjacent vibrational levels. Coherent control of an optical lattice vibration of diamond is achieved by an intense terahertz pulse whose spectrum is centered at half the phonon frequency of 40 THz. Remarkably, the carrier-envelope phase of the THz pulse is directly transferred into the phase of the lattice vibration. New prospects in general infrared spectroscopy, action spectroscopy, and lattice trajectory control in the electronic ground state emerge. [ABSTRACT FROM AUTHOR]

We study the effects of crystal anisotropy on optical phonon resonances in the second harmonic generation (SHG) from silicon carbide (SiC) in its reststrahl region. By comparing experiments and simulations for isotropic 3C-SiC and anisotropic 4H-SiC in two crystal cuts, we identify several pronounced effects in the nonlinear response, which arise solely from the crystal anisotropy. Specifically, we demonstrate that the axial and planar transverse optical phonon resonances selectively and exclusively appear in the corresponding tensor elements of the nonlinear susceptibility, enabling observation of an intense SHG peak originating from a weak phonon mode due to zone folding along the c axis of 4H-SiC. Similarly, we identify an anisotropy factor ζ≡ε⊥/ε∥ responsible for a steep enhancement of the transmitted fundamental fields at the axial longitudinal optical phonon frequency, resulting in strongly enhanced SHG. We develop a general recipe to extract all these features that is directly applicable to all wurtzite-structure polar dielectrics, where a very similar behavior is expected. Our model study illustrates the opportunities for utilizing the crystal anisotropy for selectively enhancing nonlinear-optical effects in polar dielectrics, which could potentially be extended to built-in anisotropy in artificially designed hybrid materials. [ABSTRACT FROM AUTHOR]

The article offers information on a study regarding dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation. It mentions that spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. It states that transfer of angularmomentumbetween spin sublattices is also a key process inmagnetic switching of ferrimagneticmetallic alloys by optical femtosecond laser pulses.