Arbitrarily control of electromagnetic field in femto-nano spatiotemporal scale can be realized by illuminating metallic nanoparticle with well-defined femtosecond laser pulse, which is at heart of current ambitious research endeavors in nanophotonics. However, the quantitative relation of the behavior of localized field with the incident laser pulse hasn’t yet been revealed. Here, active switching of localized near field is achieved by single chirped laser pulse in asymmetric Au nanocross system within the pulse duration using Finite Differential Time Domain algorithm. Temporal interval of energy switching between the two poles of nanocross is determined by the chirp of the laser pulse According to the temporal evolution, we found that field enhancement is asymmetric for positively and negatively chirped pulses as the consequence of the imbalance response of the plasmonic field around resonant frequency. It is demonstrated that under the excitation pulse of specific spectrum, field enhancement can also be effectively modulated by chirped laser pulse, providing a new degree of freedom for manipulating the dynamics of localized surface plasmons in nanoparticle.
Accurately capturing the spatiotemporal information of surface plasmon polaritons (SPPs) is the basis for expanding SPPs applications. We record time-resolved nonlinear photoemission electron microscopy (TR-PEEM) images of weakly excited femtosecond SPPs launched from a rectangular trench milled into a flat silver film. Experimental results show that the overall photoelectron yield is greatly enhanced (typically 6-fold enhancement with the comparison of that without 400nm pulse) in this configuration. The spatiotemporal evolution of SPP can be easily investigated with its carrier wavelength, group velocityand phase velocity. The improvement of photoemission yield is interpreted by the quantum pathway for two-color with changing the nonlinear for photoemission. It is found that the contrast between bright and dark fringes of SPPs is significantly improved compared to the single-color scheme due to the improvement of quantum pathway channels for photoemission. These findings complete the underlying physics of two-color PEEM optimized SPP spatiotemporal imaging.
Realizing multiple beam shaping functions in a single plasmonic device is the key to realizing photonic integration. Surface Plasmon Polariton (SPP) Bessel-like Beams and Bottle Beams have potential applications in nanophotonics, especially in near-field optical trapping, micro-manipulation, and on-chip interconnect circuits. Thus, it is very interesting to find new approaches for simultaneous generation of surface plasmon polariton Bessel-like beams and bottle beams in a single photonic device. The wavelength-manipulated Bessel SPP beam and Bottle beam emitters devices are composed of four and five compact coupling elements with a specific spatial distribution, respectively. Besides, as the wavelength of the incident light is changed, the generated Bessel-like SPP beam and SPP Bottle beam can be directionally excited on one side of the transmitter and the launching direction can be dynamically selected. The design scheme of the proposed device provides a new means for constructing plasmonic devices with multiple beam shaping functionalities.
The remote excitation based on the propagating surface plasmons (PSP) is becoming increasingly extensive. Here, we study the dynamics of the localized surface plasmons (LSP) remotely excited by PSP on the gold film using the finitedifference time-domain numerical simulations method. The results show that the spectra of LSP excited by PSP change along with its excitation location, showing a unique phenomenon that the spectra exhibit a redshift tendency compared to the LSP excited by the traditional laser source. By comparing with the dynamics of LSP excited by a laser source, the unique near-field characteristics of that excited by PSP can be obtained. Furthermore, we find that the dephasing time of LSP excited by the PSP is longer than by the traditional laser source. We believe the results of this study can be used to improve the efficiency of remote catalytic reactions and provide new ways to prolong the dephasing time.
Accurately grasping and controlling the plasmon dynamics and dephasing time is a prerequisite for the application of plasmons. Here, we report on the investigation of dynamics and dephasing time of different plasmonic hot spots in a single bowtie structure under varied light polarization using time-resolved photoemission electron microscopy (PEEM). In contrast to those previous global-parameter descriptions, we here report the experimental observation of apparently spatially diverse plasmon dynamic characteristics and spatially different dephasing time within a plasmonic bowtie. We experimentally obtain different plasmon dynamics in the tips of the bowtie nanostructure with different light polarization and actively control dephasing time by changing the light polarization which transforms the plasmon mode. Experimental results got the minimum dephasing time of 8.5fs and the maximum dephasing time of 17fs, which has a large adjustment range. In addition, we found that structural defects can prolong the dephasing time, and we analyzed its role in the influence of plasmon dynamics and dephasing time.
The plasmon effect is of great significance for photoemission in metallic nanostructure. We introduced the photoemission electron microscope (PEEM) in detail, and used it to study the effects of polarization on the far-field and near-field of the plasmon. We further investigate the photoelectron energy spectrum obtained by PEEM and demonstrated the spatial distribution of photoelectrons with different energies. These experimental results help us to further understand the mechanism of photoemission and laid the foundation for the future development of plasmon device and technology.
The precise understanding of the spatiotemporal characteristics of ultrafast surface plasmons is a prerequisite for applications of plasmonics. Here, we report on the investigation of near-field imaging and dynamics of propagating and localized surface plasmons (PSPs and LSPs) using photoemission electron microscopy (PEEM) of the trench on the silver film and gold bowtie nanostructure. The actual propagation direction of PSPs is directly obtained by reading PEEM images via the non-collinear exciting method by the trench. The results have demonstrated that the trench structure is potential as a 2D plasmonic dispersion element. Moreover, we experimentally obtain different LSPs dephasing times in the tips of the bowtie nanostructure by interferometric time-resolved PEEM. Experimental result reveals the dynamics of the LSP field initially oscillate at the laser field frequency and finally develop into its eigenfrequency after experiencing a few periods of frequency fluctuation.
Subwavelength imaging and control of localized near-field distribution under off-resonant excitation within identical gold bowtie structure, and of dark mode distribution within nanoring were demonstrated. The near-field control was established by coherent control of two orthogonally polarized fs laser pulses in bowtie and by varying polarization direction and wavelength of single femtosecond laser beam in the nanoring structure. We found that the hot spot under off-resonant wavelength illumination mainly distributed along the edges of the nanoprism in the bowtie and quadruple mode formation in the nanoring. The obtained results show that the PEEM images correspond generally to the simulated patterns of the plasmonic modes for the both structures and difference exists between experimental and simulated images. The responsible reasons for difference are discussed in terms of band structure near Fermi level and of surface imperfects of the structure. Our finding for the near field control of the nanostructure provides a fundamental understanding of the non-radiative optical near field and will pave the ways for the applications such as sensing, SERS, biomedicine and plasmonic devices.
KEYWORDS: Near field, Ultrafast phenomena, Plasmons, Electron microscopy, Near field optics, Gold, Ultrafast imaging, Polarization, Femtosecond phenomena, Plasmonics
We report the direct imaging of plasmon on the tips of nano-prisms in a bowtie structure excited by 7 fs laser pulses and probing of ultrafast plasmon dynamics by combining the pump-probe technology with three-photon photoemission electron microscopy. A series of images of the evolution of local surface plasmon modes on different tips of the bowtie are obtained by the time-resolved three-photon photoemission electron microscopy, and the result discloses that plasmon excitation is dominated by the interference of the pump and probe pulses within the first 13 fs of the delay time, and thereafter the individual plasmon starts to oscillate on its own characteristic resonant frequencies. On the other hand, control of the near-field distribution was realized by variation of the phase delay of two orthogonally polarized 200fs laser pulses. The experimental results of the optical near-field distribution control are well reproduced by finite-difference time-domain simulations and understood by linear combination of electric charge distribution of the bowtie by s- and p- polarized light illumination. In addition, an independent shift of the excitation position or the phase of the near-field can be realized by coherent control of two orthogonally polarized fs laser pulses.
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