Research On The Dynamics And Control Of Ultrafast Plasmon

Due to the unique optical properties of sub-wavelength localization,near-field enhancement associated with surface plasmons,which enables us to manipulate photons at nanoscale,ultrafast plasmon opens the way for realization of integrated all-optical circuit and development of much smaller,faster and more efficient nanophotonics.Ultrafast plasmon has potentials in the areas of optical calculation,optical storage,photo-catalytics,nano-photonics integration,optical sensing,biological labeling,biological imaging,solar cells and surface enhanced Raman spectroscopy.Nowdays,it is one of the most attractive research fields in science.Based on the unique physical features of ultrafast surface plasmon,the interferometric time-resolved photoemission electron microscopy(ITR-PEEM)technology is used to map ultrafast plasmonic near-field(hot-spots)realized by concentrating ultrafast laser pulses in metal nanostructures,with capability of probing temporal evolution of ultrafast plasmon dynamic and controlling ultrafast plasmon field distribution.The main contents of this thesis are as follows:Firstly,ITR-PEEM technology is used to image ultrafast plasmon dynamics within bowtie nanostructure and nanowire excited by p-polarized 7 fs laser pulses.When the delay time between two laser pulses is shorter than 13 fs,the interference of the pump and probe dominates the oscillation of the plasmon at the tip of the right nanoprism and lower corner of the left nanoprism,and the interference field forces them to resonate at the laser frequency.As the delay time increases,plasmon starts to oscillate towards its own characteristic resonance frequency.For hot-spots in the bowtie nanostructure illuminated by 7 fs laser pulses with varied polarization directions,the plasmon resonance frequencies show difference.Furthermore,if the polarization angle of incidence laser increases relative to the p-polarization,the excited plasmon resonance frequency becomes larger.We observe the temporal evolution traces of the plasmon at the different areas in a nanowire and have found that the plasmons show a difference in oscillation frequency starting from the second optical cycle as a result of the phase propagation of a plasmonic excitation through whole nanowire.From the results of the plasmon dynamics at the lower left corners of nanowires with different sizes,we get to know that the temporal evolution traces have a phase delay due to the phase propagation and resonance frequency of the plasmon.Secondly,direct imaging of ultrafast plasmon distributions within gold dolmen nanostructures excited by p-polarized 7 fs laser using PEEM system and probing of dynamics at different areas in the dolmen nanostructures using ITR-PEEM have been performed.It is seen that the hot spots mainly locate at left ends of the dimer slabs,meanwhile,there are also hot spots appeared in the right end of the upper dimer as well as upper end of the monomer.In a dolmen nanostructure with the dimer length of 220 nm and monomer length of 500 nm,plasmon evolution traces show that the relative phase of the photoemission signal from the two left ends of the dimer is always zero during the whole delay time period.Moreover,it is interesting to note that plasmon evolution traces on both ends exhibit small side wings in addition to the main peaks.The appearance of side wing structure is typical feature of plsmon mode beat pattern resulting from the coherent superposition of multiple localized surface plasmon modes.For the dolmen nanostructure with the dimer length of 220 nm and monomer length of 300 nm,it is found that the plasmon temporal evolution from upper end of the monomer is always in phase with that in right end of the upper dimer slab during whole time period,which is attributed to a strong local field-based plasmon coupling between two closely-situated modes.It is interesting to note that a higher photoelectron yield region can be shifted from the left to the right end in the upper dimer slab when the delay between two femtosecond laser pulses is changed from 11.35 to 12.68 fs,and then it shifts back to the left end again when the delay is set to 14.01 fs.The shift of higher photoelectron yield in PEEM images indicates that the manipulation of the plasmon distribution on a nanometer spatial and femtosecond temporal precision can be realized by varying the delay time between two laser pulses in the ITR-PEEM technology.In addition,from the plasmon temporal evolution in the dolmen nanostructures containing two parallel slabs of different lengths,we observe that the plasmon oscillation from two left ends go completely out of phase.Thirdly,we have investigated the controlling of ultrafast plasmon localization within silver cuboid microstructure excited by picosecond laser pluses using a photoemission electron microscopy.The experiment results show that both location and intensity of the plasmonic field depend sensitively on the polarization state of the incident light,and the plasmon can further be switched from the upper to the lower edge by turning the polarization of the picosecond laser pulses at a step of 90°.We have succeeded in achieving the control of ultrafast plasmon localization in silver micro-cuboid through rotating the polarization direction of the exciting light pulses.Lastly,experiments have been performed to control plasmon localization within bowtie nanostructure by changing polarization direction of single femtosecond laser beam and the relative time delay between two femtosecond laser beams.The hot-spot could be switched from the lower to the upper corner of the left nanoprism within the bowtie nanostructure when the polarization angle is changed from 30° to 120°.In addition,the photoemission signal changes significantly when the time delay step between two cross-polarized pulses is changed by 0.67 fs.Especially,the hot-spot at the outer corners of the left nanoprism moves from the lower to the upper corner when the time delay is changed from-0.67 fs to 0.67 fs.The FDTD simulations are performed to confirm the control process of the plasmon,and the simulated images are in excellent agreement with experimental results.The optical near-field can be manipulated from the upper corner to the lower edge when the time delay step between two non-orthogonal pulses is changed from 0.67 fs to 1.33 fs.Coherent control of the plasmon localization in the bowtie nanostructure at femtosecond and nanosized spatiotemporal resolution has been achieved by changing the delay time between the two pulses.The work conducted in this thesis lays a firm foundation for disclosing of the optical properties of the ultrafast plasmon,a deep understanding of coherent control techniques of plasmonic field,and also manipulating of the plasmon distribution on a nanometer spatial and femtosecond time scale.It opens the way for the applications of ultrafast plasmon in many emerging fields.