Sub-diffraction Photonics:Two-dimensional Polaritons And Nanofocusing

Technologies capable of evading diffraction limit and squeeze electromagnetic energy into subwavelength scales have potential to yield far-reaching influence on the future of photonics and other related disciplines.At the heart of this remarkable advancement lie-surface polaritons-electric dipole(plasmons in metal,optical phonons in dielectrics,and excitons in semiconductors)that can be excited by efficient illumination with light.The promises of polaritons led to the design of novel nanoscale devices capable of efficient subwavelength delivery,confinement,harnessing and manipulation of electromagnetic energy,and hold future applications in nanoscale integration of optoelectronics devices,super resolution imaging,quantum computing and single molecule trapping.The possibilities of efficient delivery and manipulation of energy in nanoscale region largely depends on the nanofocusing device.This makes nanofocusing device a crucial in the field of nanophotonics,and ignites numerous techniques to design,optimize,and fabricate novel focusing devices.This thesis,predicted and characterized new phenomena(transverse electric phonon polaritons mode)capable of subwavelength confinement of energy in ultrathin material,and proposed a new approach that bridges the gap between theoretical design and practical realization of nanofocusing device.A brief summary of this thesis is highlighted in the following subsections:1.Despite the tremendous significance of the polaritons in subwavelength confinement,most of the reported literature and devices focus on transverse magnetic modes.The recent rediscovery of graphene predicted the existence of transverse electric polaritons in two-dimensional materials.In this work,we predicted for the first time,transverse electric phonon polaritons in ultrathin hexagonal boron nitrides slab with subwavelength confinement better than the transverse electric plasmons in graphene.This work brings hope of possible practical realizations of transverse electric polaritons in ultrathin materials.2.Photonics devices experience performance gap between theoretical design and practical realization that is threatening its development.To solve this challenge,we proposed a new approach(fragmented high-order transformation optics)and designed a hyperlens capable of focusing mid-infrared electromagnetic energy into few nanometers.The hyperlens can be fabricated by stacking few metal-dielectric nanolayers.The conceptual idea is achieved by fragmenting the lenslet into N(N>1)discrete regions,and carry out high-order transformation optics in each fragment.