Studies On Ultrasensitive Characterizations Of Plasmonic Modes And Their Applications

Surface plasmons(SPs)have attracted enormous interest due to its focus on outstanding light-trapping and large electromagnetic-field enhancement properties.It has proven to be instrumental in numerous significant applications,such as physics,chemistry,biology,communication,and energy.Although rapid developments and encouraging progresses have been achieved in numerous plasmon-related researches,many unsolved scientific issues,e.g.,plasmon sensors,plasmon-induced hot electrons,and plasmon-enhanced sum-frequency generation among others,should be answered.Based on above issues,we select gold nanohole arrays as a platform,and systematically study resonance linewidths,hot-electron dynamics and nonlinear optical applications in this thesis.This thesis is divided into five parts.We briefly introduce elementary knowledge and research progress into SPs,provides extended applications in sensors,hot electrons,and nonlinear optics,raises several scientific questions,and then describes our ideas in the first chapter of this thesis.Then,we display corresponding experimental setups and theoretical methods in the subsequent chapter.In the third chapter,we explore the effects of excitations(e.g.,incident angles,azimuthal angles,and polarizations)on plasmon wavelengths and linewidths,and perform relevant biosensors experiments.By utilizing home-built femtosecond transient reflection spectroscopy,we quantitatively investigate the contribution of conduction-band electron distribution to the SPP intensity of the array with ultrahigh sensitivity in the fourth chapter.The thesis closes with investigation on characterizing SPR modes based on all-metal nanostructures without any probed molecule by using plasmon-enhanced sum frequency generation(PESFG)spectroscopy.The main results and significance are provided below.1.The integrated manipulations of resonance wavelengths and linewidths in 2D periodic nanohole arrays have been explored.Through designing and fabricating 2D gold nanohole arrays,we systematically investigate the effects of incident angles,azimuthal angles,and polarizations of excitations on resonance linewidths,and propose the optimal method for tailoring linewidths.We observe that the linewidth becomes narrower when the incident angle increases,the resonance wavelength and linewidth vary monotonously when the azimuthal angle increases,and that two modes interfere at the 45° azimuthal angle with the occurrence of abruptly changing of linewidths.Also,we find that the linewidths are barely affected by polarizations.Though conducting biosensor experiments,we confirm that the integrated method of tailoring linewidths can be applied to biosensors.2.The response of surface plasmon polariton(SPP)modes in Au nanohole arrays has been effectively tuned by properly adjusting the sample orientation without changing the geometrical parameters,and a very narrow linewidth down to 8 nm is achieved via the strong interference of two(0,-1)and(-1,0)SPP modes in the ?-M direction under transverse magnetic polarization.These results have been validated excellently by finite-element-method numerical simulations.More importantly,we have quantitatively investigated the contribution of conduction-band electron distribution to the SPP intensity of the array within a 20 ps timescale with ultrahigh sensitivity by utilizing home-built femtosecond transient absorption spectroscopy,and observed the minimum SPP intensity at?700 fs following excitation with a 0.2 ?J pulse.This study may help enhance the understanding toward the intrinsic micromechanism of SPR,thus offering opportunities for potential applications in strong coupling and new-style optical wave manipulations.3.We have experimentally demonstrated ultrahigh-sensitive characterization of the SPR modes based on all-metal nanostructures without any probed molecule by plasmon-enhanced sum frequency generation(PESFG).Through designing two-dimensional Au nanohole arrays as a platform,we have performed PESFG measurements with the lab-built reflection configuration.As a result,PESFG signals evolve closely with the reflectance spectra at both excitation and emission wavelengths,and satisfactory qualitative agreements have been obtained.Finally,we also demonstrated that the resonant wavelengths of the SPR modes and their spatial distributions exert an influence on PESFG,and that these results are validated excellently by the phase-matching equation.Our study may benefit the understanding toward the mechanism that governs PESFG,and offer opportunities for exploring new-style SPR modes by PESFG.