Near-field Optical Properties And Manipulation Of Phonon Polaritons In α-MoO₃

Nanophotonics is an interdisciplinary subject that studies the interaction of light and matter at the nanoscale and is of great significance in many fields such as optoelectronics,information,material physics,and biosensing.Polaritons,as an important part of nanophotonics,are quasi-particle generated by the hybridization of free-space photons and polarized charges such as electrons,phonons,and excitons.They have played an important role in breaking through the diffraction limit in traditional optics,realizing subwavelength modulation of photons,and enhancing the interaction between light and matter.The emergence of two-dimensional materials in recent years has greatly enriched the material category of polariton and brought new vitality to the development of polariton.For polariton,lifetime and tunability are the keys to its wider application.Long-lived polariton could promote the performance of basic photonic devices,and rich tunability could enhance the manipulation of photonic devices.Therefore,this thesis starts with the natural hyperbolic phonon polariton material,and studies the lifetime and control of the phonon polariton.The thesis mainly consists of three parts:1.Gradient suspended substrate tunes the propagation properties of hyperbolic phonon polaritons.Near-field optical imaging shows that,compared toα-Mo O₃supported by a Si O₂ substrate,the phonon polariton in the suspendedα-Mo O₃ has a larger wavelength in RB₂ and has a larger wavelength in RB₃.We have systematically summarized the variation law of the phonon polaritons wavelength in response to the dielectric constant of the substrate by theoretical calculation,which is of great significance for regulating the propagation properties of phonon polaritons.2.The application of Mo-isotope-enrichedα-Mo O₃ greatly reduces the optical loss of hyperbolic phonon polariton.Compared with the naturally abundantα-Mo O₃crystals,the propagation length and lifetime of phonon polaritons in the Mo-isotope-enrichedα-Mo O₃ crystals are significantly improved to 22μm and 47 ps.Furthermore,we found that the atomic mass of Mo element,as a new degree of freedom,can effectively adjust the spectrum range of the Reststahlen band inα-Mo O₃.Our findings provide a new opportunity to reduce the propagation loss of hyperbolic phonon polaritons inα-Mo O₃ crystals at room temperature,which has potential realistic meaning for the fabrication of ultra-low loss polaritonic devices in the future.3.Reversibly tunable phonon polariton is realized by combining the phase-change material VO₂ andα-Mo O₃.When using VO₂ as the substrate material,the change of temperature could control the phase transition of VO₂,thereby realizing the reversible regulation of phonon polaritons inα-Mo O₃ crystals.We exploit the remarkable dielectric difference between the insulating and metallic states in VO₂ to realize the refraction effect of phonon polaritons and their switch control at the nanoscale,which provides a new idea for designing nanoscale compact and reconfigurable wavefront modulation optics.By combining other polaritonic materials and more phase-change materials,a wider range of subwavelength applications over a wider frequency range can be achieved.In this thesis,we systematically studied the near-field optical properties and control methods of hyperbolic phonon polaritons inα-Mo O₃ in the mid-infrared band based on a near-field optical system with ultra-high spatial resolution,which provides a useful reference for the basic research and application of polaritonic devices.