The Excitation And Tuning Of Surface Phonon Polaritons In 4H-SiC Enabled By Grating Structures

Surface plasmons(SPP) are crucial components for nano-optics and have been extensively used to enhance sensing, light emission, photodetection, surface enhanced spectroscopy, and others. However, the resonant frequencies of SPPs excited on the surface of precious metals(e.g., gold, silver, copper, and so on) are mainly limited to visible and near-infrared frequency range. At lower frequencies(i.e., mid- and far-infrared region), metal-based plasmonic systems generally behave high optical, which has limited further progress. Recently, the study found that polar materials(e.g., SiC) have a spectral region of high reflectivity in mid- and far-infrared frequency range(e.g., the reflectance of SiC approaching 100% within the “Reststrahlen” band, i.e., the spectral range of ~793cm-1-969cm-1), where the permittivity is negative. It’s within this spectral range that SiC and other polar dielectric materials can support SPhP resonant modes. SPhP resonances arise from the coupling between the electromagnetic fields and optical phonons(lattice vibrations) of the polar crystals, and thus inherit some features of phonons, such as high Q-factors, low optical loss inherent, and so on. Therefore, SPhP resonance, a different type of surface waves, may be a promising alternative to plasmonic modes for realizing widespread applications at mid-infrared frequencies.Generally speaking, the optical excitations of SPhP waves can be achieved at the interface of air/SiC by micropatterning the surface of polar materials.In this paper, four types of novel SiC/Au periodic microstructures to excite SPhP wavesat mid-infrared frequencies are fabricated by using improved processes in order to avoid damage to the lattice of SiC. We systematically investigate the spectral tuning of SPhP resonance excited optically. The main research work is as follows:1) The preparation process of polaritonic devices is optimized. Four types of novel periodic microstructures(i.e., one-dimensional gratings, a periodic square microcavity array, a periodic hole array with large spacing between adjacent holes and a periodic hole array with short spacing between adjacent holes) to excite SPhP waves are fabricated on SiC substrates by standard UV-lithography, metallization, and subsequent lift-off processes. The SPhP resonances excited all exhibit high Q factors.2) The passive control of SPhP resonance properties in phonon polaritonic material. We find that the SPhP modes are shifted as the fill factor varies and the Q factor becomes larger as the thickness of gold film increases, which facilitates the the design for the light-phonon devices with high efficiency.3) The active control of SPhP resonance properties in phonon polaritonic material. In view of the fact that plasmonic structures coupled with graphene is dynamically tunable, we combine graphene and phonon polaritonic materials in a hybrid structures, and exploit the tunable electrical properties, sensitive optical response of graphene sheet to achieve blue shift tunability of SPhP resonances via chemical doping graphene.4) Finally, on the basis ofintense near-field induced by SPhPs on SiC/air interface, we further explore the potential of SPhP modes as a dielectric optical sensor in mid-infrared index-sensing applications. Experiments demonstrate that the optical response of the SPhP modes is more sensitive comparing with SPP, and thus SPhP can be used to detect adsorption of atomic layer films. In addition, we notice that SiC-based 2D gratings(including a periodic hole microcavity array and periodic square microcavity array) are more appropriate for ultrasensitive index sensing comparing with the SiC-based one-dimensional grating structure.