The Study On The Fano Resonance,Surface Enhanced Raman Scattering And Nonlinear Optical Properties Of Metallic Nanoparticles Fabricated By Nanosphere Lithography

Various metallic nanostructures have been designed and fabricated out due to the increasing advances in nano-fabrication techniques.Those noble metal nanostructures exhibit unique functional properties,which hold great potential for applications in the research fields of biosensors,surface enhanced Raman scattering(SERS),information processing and optical antennas.Meanwhile,it has merged into a new rapidly growing discipline,called surface plasmonics,which covers the research area of physics,chemistry,material science,information science,biology,and their inter-disciplines.In this thesis,we will give detailed studies on Fano resonance(FR)and SERS of novel metallic nanostructures fabricated by nanosphere lithography(NSL),and the physical mechanism will be explored as well.In addition,the enhancement of nonlinear optical absorption of metallic nanoparticle array with ultrasmall nanogap caused by plasmonic near-field interaction will also be investigated.The thesis is mainly composed of three sections that are arranged as following:1.We first proposed a very simple ?-shaped nanostructure without the particular requirement of nano-gaps to generate a strong FR under the excitation of a linearly polarized light with its electric field direction parallel to the arms of the ?-shaped nanostructure.We show that the FR stems from the coupling between a broad-band magnetic dipolar(bright)mode and a narrow-band electric quadrupolar(dark)mode,which are supported by the L-shaped and straight nanorods,respectively.Experimentally,the angle-resolved nanosphere lithography(ARNSL)is employed to realize the design of ?-shaped nanostructure by producing partially overlapped double metallic nanotriangles with large area(?cm2).The effect of the geometry parameters(e.g.,the overlap degree of the two nanotriangles and the size of ?-shaped nanostructure)on the FR is studied in detail.Furthermore,we provided the first experimental demonstration of the generation of double Fano resonances of the partially overlapped asymmetric double metallic nanotriangles(POADT)in the near-infrared region by tuning the two different nanotriangle sizes,which is also fabricated by the angle-resolved nanosphere lithography(ARNSL).Both experimental results and theoretical analysis demonstrate that the double Fano resonances occur and stem from the interference between a spectrally overlapping broad super-radiant dipole modes and narrow quadrupolar modes of the same nanostructure of POADT when the degree of asymmetry of the POADT(defined as the ratio of the nanotriangle sizes of the large triangle and the small triangle)is around 1.67.The degree of asymmetry and the size of POADT were further varied to tune the line shape and spectral position of Fano resonance.In addition,we demonstrated that both the fabricated Au-PODT array and Au-POADT array can serve as efficient LSPR sensors for refractive index sensing in the near-infrared region.The Au-PODT array possesses a figure of merit(FoM)of 3.5,a value 3-fold higher than that of a single-nanotriangle sensor.The Au-POADT shows an unprecedented sensitivity with a LSPR figure of merit(FoM)of 35 and 13 for the double Fano resonances modes.It is noted that the best LSPR based on the nanostructure of plasmonic nanoclusters has been reported to experimentally yield a FoM of 5.7,which was achieved in the nanostructure of plasmonic nanoclusters reported by Halas's research group.The inherent spectral tunability and the ease of being patterned into large-area substrates with high reproducibility by the fast and inexpensive fabrication technique will pave a way to design realizable on-chipplasmonic sensing device for potential applications.2.We designed and prepared a broadband and polarization independent perfect absorber and demonstrated its application in enhancing the Raman signals.First,the perfect absorber consisting of the large area Au nanotriangles array on top of an optically opaque gold film separated by a SiO2 dielectric layer was fabricated by nanosphere lithography process.For such a typical metal/dielectric/metal(MIM)nanostructure,its broad absorption band can be tuned from visible to near infrared by controlling the size of the microsphere.According to our numerical calculations,we find that the broad absorption band results from the generation of multi-plasmonic modes in the MIM nanostructure accompanying with the enhancement of local electromagnetic field.We further characterized the surface-enhanced Raman scattering(SERS)performance of the MIM nanostructure and obtained the value of the SERS enhancement factor(EF)as large as 4.9×106 with an improvement of 22 times increase compared to the same Au nanotriangles array deposited directly on a quartz substrate.By controlling the thickness of the SiO2 dielectric layer of our nanotriangle based MIM nanosturctures,the SERS EF could increase as the increasement of the product of the optical absorptions at both the excitation wavelength and Raman scattering wavelength.Owing to the simple,productive,and inexpensive fabrication technique,our MIM nanostructure could be a potential candidate for vaious applications such as SERS and solar cell.3.We investigated the near-field coupling effect on the nonlinear absorption(NLA)of gold bowtie nanoantennas.First,the cost-effective nanosphere lithography combined with oxygen plasma etching process was utilized to prepare two-dimensional(2D)arrays of bowtie nanoantennas consisting of gold nanotriangle dimers with an ultrasmall nanogap(<10 nm)between adjacent gold nanotriangles.The measured and calculated linear optical absorption spectra reveal the existence of the near-field coupling in bowtie nanoantennas nanostructure.We observed that upon decreasing the gap width,nonlinear absorption is enhanced not only due to the minimum of the spectral detuning between the center wavelength of the laser source and the LSP resonance but also the enhancement of near-field coupling,under the excitation of femtosecond pulses,using an open aperture Z-scan technique.It is further observed that while keeping the same spectral detuning between the center wavelength of the laser source and the LSPRs resonance for different samples,the smaller gap width generates a stronger NLA.When the gap width approaches to the sub 10 nm,the antenna hot spot in the near resonant regime contributes noticeably to the local electrical field enhancement at the nanotriangle corners,leading to an additional enhancement of the nonlinear optical absorption.The role of near-field resonant plasmonic coupling in bowtie nanoantenna is analyzed and confirmed by simulations.4.We investigated the fabrication,the optical linear and nonlinear properties of the stacked nanoprism dimer(SPD)nanostructure.First,Au SPD nanostructures with different thickness of interstitial gap layer(IGL)of SiO2 film in between the two Au nanoprisms were fabricated by the combination of ARNSL and standard planar deposition.The thickness of IGL was obtained as small as 10 nm.The linear optical transmission spectra reveal the existence of the strong near-field coupling accompanying with the strong local electrical field enhancement in the IGL of the Au SPD nanostructure when the polarization of the incident light is along its overlapped base.When the polarization of the incident light is perpendicular to the overlapped base,the Fano resonance of Au SPD nanostructure is generated by the near-field coupling effect,which is controlled by the SiO2 thickness of IGL.Then we observed the prominent enhancement of optical nonlinear absorption(NLA)of Au SPD nanostructure with the small nanogap is due to the enhancement of near-field coupling,under the excitation of femtosecond pulses,using an open aperture Z-scan technique.It is found that the effective NLA coefficient P of Au SPD nanostructure with the gap distance of 10 nm is increased by approximately ten times compared with that of the single Au nanoprism arrays.Moreover,the technique of ARNSL combined with atomic layer deposition was utilized to achieve the IGL thickness of the Au SPD nanostructure as small as 1 nm.The ultrastrong enhanced near-field coupling can cause the increase in magnitude of both the redshift of the dipole resonance wavelength and the local electrical field enhancement in the interstitial gap layer as an exponential growth trend,when the IGL thickness of the Au SPD nanostructure was decreased from 10 nm to 1 nm,which is still behaved as the classical physics picture.We will plan to further achieve the IGL thickness down to the subnanometer scale and to investigate the extreme nonlinear interactions as well as the optical quantum tunnelling.