Surface-enhancement Of Near-field Scattering Field By Nanostructures Supporting Phonon-polaritonic Modes

Surface phonon polaritons(SPh P)are collectively coupled resonances between phonons and photons,propagating on the surface of polar crystals.SPh P modes have the properties of strong localfield enhancement of electromagnetic waves,with the ultralow propagation loss,which can significantly prompt the light-matter interaction.These features make SPh P widely applied in the fields of biosensors,thermal emitters,quantum devices,metamaterials,etc.,in the mid-infrared regime.In the research of various SPh P devices,scanning near-field optical microscopy(SNOM)has been playing an important role in mode characterization.SNOM can directly provide the image of localized or propagative SPh P modes at 10 nm resolution,and become the major experimental tool to study the physical properties of SPh P.In recent years,as the widespread applications of SNOM in the area of biochemical material analysis,nanostructures or nano-devices supporting SPh P modes have been an important auxiliary tool to enhance the SNOM near-field scattering-field intensity.Especially,the Fourier transform infrared nanospectroscopy(nano-FTIR),as a derivative of SNOM,is always troubled by its low sensitivity and poor signal-to-noise ratio.However,with the help of nanostructured surfaces supporting SPh P modes,the weak signal peaks can be enhanced with better recognition of nanomaterials.Therefore,in this work,we try to explore the proper nanostructures for near-field scattering enhancement,with the purpose of manipulating and promoting the light-matter interaction between SPh P and SNOM tips.We first design a nanostructural resonant cavity on the hexagonal boron nitride(h-BN)nanoflakes,to enhance the scattering intensity of SNOM probes.The nano-cavity is built with a slit at a 1μm between two gold nanowires,where a piece of free-standing h-BN nanoflake is transferred onto the slit.The proposed cavity can be treated as a hollow slot waveguide,which can sustain slot SPh P modes that exhibit ultrahigh field confinement and enhancement.The SNOM images show that the SPh P mode can be excited,which produces the Fabry-Perot resonance in the cavity.After rigorous theoretical and experimental analysis and comparisons,we find that the internal stress within the freestanding h-BN nanoflake has a strong influence on the intrinsic SPh P modes.However,the slot SPh P modes do not lead to the increase of near-field scattering-field intensity,indicating that stronglyconfined modes may inefficiently stimulate the scattering process of SNOM tips.Moreover,we also design photonic crystals for SPh P consisting of arrayed nano-pillars fabricated on the surface of crystalline Si C.The photonic crystals can manipulate the propagative SPh P on the Si C surface,while the individual nano-pillars also support dipole modes that resonate transversely and longitudinally.All the modes can be used for the enhancement of nano-FTIR spectral signals.The experimental results show that nano-FTIR third-order spectral intensity has a 4-fold enhancement,if measured near the centre of the interpillar area,compared with bare Si C surface.However,on the contrary,the nano-FTIR signals are attenuated if measured on the top base of nanopillars.Consequently,we can conclude that the physical mechanisms for enhancing near-field scattering fields are intrinsically different from that of far-field enhancement like surface Raman enhancement.The SNOM probes have a strong influence on the local modes,making modal distortion and scattering-field attenuation.But if the SNOM probe is far away from local modes,the electric dipole scattering process becomes the dominant mechanism,presenting some complicate correlations between nano-FTIR intensity and photonic bandgap.These results could inspire the design of more nanostructures for the enhancement of near-field scattering signals for future works.