Study On The Surface Plasmon Optical Chirality Of Noble Metal Micro-nanostructure

When the light excites on the interface between the metal and the dielectric,the free electrons on the metal surface collectively oscillate,and the electromagnetic wave is coupled with the free electrons on the metal surface to form a near-field electromagnetic wave propagating along the metal surface,that is,surface plasmon（Surface Plasmon,SP）.Metal micro-nanostructures based on the characteristics of LSP and SPP have attracted the interest of researchers and have been applied in the fields of biomolecule detection,integrated photonic chips,solar cells,optical storage,and so on.In this paper,the metal nanostructures are designed to realize asymmetric transmission（AT）,circular dichroism（CD）,and chial electromagnetic field enhancement,and explor new mechanism for the generation of strong optical chirality.In addition,the electron-beam lithography and oblique angle deposition techniques are used to prepare the chiral metal nanostructures with different heights and double-layer complementary metal nanostructures to explore the new strategies for preparing chiral metal nanostructure,which provide a reference for polarization state regulation and chiral molecule detection.The main research works of this paper are as follows:The first part is the study of AT effect of connected gammadion-like nanostructure and rectangular/semicircular nanohole arrays.The connected gammadion-shaped nanostructure and rectangular/semicircular nanohole arrays are designed to induce and enhance AT effects by trapped mode.The simulated results show that the mechanism of AT effect in connected gammadion-like nanostructure is caused by the difference of resonance intensity of trapped mode,and the resonance intensity of trapped mode can be tuned by asymmetry of connected gammadion-like nanostructure.Meanwhile,the enhancement mechanism of AT effect in rectangular/semicircular nanohole arrays is caused by the trapped mode around the rectangular nanohole,which splits the original AT peak of rectangular nanohole array and enhances the AT signal.These results provide a new way to realize strong AT signal,and provide idea to regulate the AT characteristics of metal micro-nano structure.The second part is the study of the AT effect of S-shaped nanostructures.The S-shaped nanostructure is designed to generate prominent AT signal by different resonance under different circularly polarized light illumination.The simulated results show that a new AT signal generated while the length of two arms of S-shaped nanostructure is different.Under left-circularly polarized and right-circularly polarized light illumination.,the electric dipole and magnetic dipole resonance modes are formed at the arms of S-shapde nanostructure,leading to the resonance peak and the resonance valley at the same wavelength,and the considerable AT effect is generated.In addition,the grapheme strips are adding on the one of the arm of S-shaped nanostructure to tuned the AT signal by regulating the Fermi energy of graphene strips.These results provide another new way to realize AT effect,and also provide a new idea to regulate the AT effect of metal nanostructure.The third part is the study of CD effect of nanostructures with different height.The L-shaped nanostructures with different height is fabricated using the the electron-beam lithography and oblique angle deposition techniques to realize the strong CD signal.The deposition angle and time of oblique angle deposition can be tuned to change the different height of nanostructure,and realizing the regulation of CD signal.The numerical results show that the polarization direction of the bottom surface is different duo to different height of two arms of L-shaped nanostructure under left-circularly polarized and right-circularly polarized light illumination,which increases the CD signal.In addition,the MoS₂ layer is added on the L-shaped nanostructures with different height to increase the difference of charge polarization direction,which considerably furthered increases the CD signal.These results not only provide a new way to realize strong CD signals,but also provide a new strategy to prepare highly differential chiral nanostructures.The fourth part is the study of CD effect of double-layer complementary nanostructures.The double-layer complementary nanostructures,which is consisting of bottom rectangular nanoplates,top metal films with complementary nanoholes,and space layers,is designed and fabricated by the electron-beam lithography and normal electron beam deposition techniques.The simulated results show that the physical mechanisms underlying the CD effect is the difference coupling between localized surface plasmon resonances excited in complementary nanostructures and surface plasmon polaritons excited on metal nanofilm under left-circularly polarized and right-circularly polarized light illumination.The preparation of bilayer chiral nanostructures can be realized in only once electron-beam lithography and normal electron beam deposition technology,which greatly simplifies the manufacturing process and reduces the manufacturing cost.These results not only provide a new way to realize strong CD signal,but also provide a new strategy to prepare highly differential chiral nanostructures.The fourth part is the study of the near-field chirality of U-shaped groove nanostructure.The U-shaped groove nanostructure is proposed to generate uniform chiral near-fields under circularly polarized light illumination.The simulated results show that the magnetic polaritons mode excited in the nanocavity of U-shaped groove nanostructure due to the direct conduction of electric charges of two parallar nanostructure,and induce uniform chiral near-fields in the nanocavity.In addition,the U-shaped groove nanostructure is explored as a chiral sensor.Chiral molecules are introduced into the cavity of the U-shaped groove nanostructure,and the CD signal of the coupled system can be enhanced considerably.These results provide a new way to realize chiral electromagnetic field and provide a reference for chiral molecular detection.