Characterizing surface phonon polaritons in quartz by near-field microscopy

Surface phonon polaritons (SPhPs) are hybrid light-matter states originating from strong coupling between photons and optical phonons which are quasiparticles of lattice vibration. SPhP can be excited in the strong reflection bands in the infrared (IR) region (Reststrahlen region) of polar crystal, where the real part of the dielectric constant is negative. Control of SPhP is expected to be one of the promising ways to improve the thermal conductivity of nanoscale devices because the propagation length of SPhP is as long as $100 \mu m. SiC [1]$ and quartz [2, 3] are polar crystals that are often used for SPhP excitation. Due to crystal lattice vibrations inherent in a quartz [2], the reflectivity of a quartz is significantly decreased at around $1160 cm^{-1}$, although these frequencies are within Reststrahlen region. In the previous work, SPhP excitation was not observed at around $1160 cm^{-1}$ in the attenuated total reflection (ATR) spectrum [3]. For further understanding of the fundamental characteristics of SPhP in a quartz, the direct visualization of SPhP in real space is crucial. In this work, we investigated the frequency dependence of SPhP propagation on a quartz surface by using the scattering-type scanning near-field optical microscopy ($s -$SNOM), which enables nanoscale spectroscopic imaging [1].