Boyu Ji

Resonance hybridization and near field properties of strongly coupled plasmonic ring dimer-rod nanosystem

Combined effects of polarization, split gap, and rod width on the resonance hybridization and near field properties of strongly coupled gold dimer-rod nanosystem are comparatively investigated in the light of the constituent nanostructures. By aligning polarization of the incident light parallel to the long axis of the nanorod, introducing small split gaps to the dimer walls, and varying width of the nanorod, we have simultaneously achieved resonance mode coupling, huge near field enhancement, and prolonged plasmon lifetime. As a result of strong coupling between the nanostructures and due to an intense confinement of near fields at the split and dimer-rod gaps, the extinction spectrum of the coupled nanosystem shows an increase in intensity and blueshift in wavelength. Consequently, the near field lifespan of the split-nanosystem is prolonged in contrast to the constituent nanostructures and unsplit-nanosystem. On the other hand, for polarization of the light perpendicular to the long axis of the nanorod...

Subwavelength imaging and control of ultrafast optical near-field under resonant- and off-resonant excitation of bowtie nanostructures

We demonstrate subwavelength imaging and control of localized near-field distribution under resonant and off-resonant excitation of identical gold bowtie nanostructures through photoemission electron microscopy. Control of the near-field distribution was realized by polarization rotation of single femtosecond laser pulse and variation of the phase delay of two orthogonally polarized femtosecond laser pulses. We show that the localized optical near-field distribution can be well controlled either among the corners of the nano-prisms in the bowtie for resonant excitation or the edges for off-resonant excitation. A better visualization of the PEEM image is achieved for resonant excitation than in the case of off-resonant excitation. 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, a shift of the near-field excitation position with inverted or unchanged phase, alternatively an un-shift of the excitation position but only with inverted phase of the near-field, can be realized by rotating the polarization angle of a single pulse and coherent control of two orthogonally polarized fs laser pulses.

Controlling optical field enhancement of a nanoring dimer for plasmon-based applications

Control of resonance dynamics and gap plasmons of coupled nanostructures beyond commonly used parameters such as the dimer gap has a paramount importance in practical applications where a fixed feed-gap is needed. In this report, we show control of resonance peak shift and gap plasmon intensity of a closely spaced nanoring dimer through polarization and illumination angle of incident light, geometry of the constituent nanorings and refractive index of the substrate underneath. For fixed outer radii and constant dimer gap, the resonance peak of the nanodimer shows universal redshift as the inner radii of nanorings increase and polarization of the incident light approaches the dimer axis. Furthermore, we show that increasing inner dimer radii and introducing a small split gap to the nanodimer results in highly enhanced gap plasmon intensity. Finally, at optimized dimer geometry and illumination of the incident light, 682 nm RIU−1 refractive index sensitivity of the nanodimer was obtained and its implication for surface-based sensing is discussed in detail.

Features of Local Electric Field Excitation in Asymmetric Nanocross Illuminated by Ultrafast Laser Pulse

Features of the asymmetric nanocross including extinction spectrum, local electric field intensity, and temporal response of the local electric field under ultrashort laser illumination are investigated in this paper. It is found that, due to the simultaneous excitation of local electric fields in the arms that are perpendicular and parallel to the laser polarization direction of the asymmetric nanocross, extinction spectrum exhibits multiple resonant peaks and the position of the peaks can be tuned by changing the lengths of the arms. Simulation results disclose that there is a strong connection between optical response of the parallel and perpendicular arms. Moreover, temporal response of electric field in arms of the asymmetric nanocross shows that oscillations in the parallel arms start earlier than that of the perpendicular arms, and they are in phase when one of the parallel arms resonantly excited, which further reflects the relationship between the parallel and perpendicular arms. Therefore, we demonstrate that the perpendicular arm excitation is attributed to that of the nonresonant parallel arm in the asymmetric structure which cannot keep the overall electric neutrality of the nanostructure, and thus, perpendicular arms are activated to maintain this balance.

Imaging Ultrafast Plasmon Dynamics within a Complex Dolmen Nanostructure Using Photoemission Electron Microscopy

We report direct nanoscale imaging of ultrafast plasmon in a gold dolmen nanostructure excited with the 7 fs laser pulses by combining the interferometric time-resolved technology with the three-photon photoemission electron microscopy (PEEM). The interferometric time-resolved traces show that the plasmon mode beating pattern appears at the ends of the dimer slabs in the dolmen nanostructure as a result of coherent superposition of multiple localized surface plasmon modes induced by broad bandwidth of the ultrafast laser pulses. The PEEM measurement further discloses that in-phase of the oscillation field of two neighbor defects are surprisingly observed, which is attributed to the plasmon coupling between them. Furthermore, the control of the temporal delay between the pump and probe laser pluses could be utilized for manipulation of the near-field distribution. These findings deepen our understanding of ultrafast plasmon dynamics in a complex nanosystem.

Coherent Control of Gap Plasmons of a Complex Nanosystem by Shaping Driving Femtosecond Pulses

Geometry-based control of local field of coupled plasmonic nanostructures is efficient for optimization of the field intensity. However, it provides weak control over spatial and temporal dynamics of the field and thus unsuitable for experimental studies and practical applications where fixed geometries are needed. In this study, we report on pulsed excitation of strongly coupled plasmonic nanosystem comprised of nanorod and split-ring antenna. The near-field intensities are manipulated by controlling time delay, relative phase, and polarization of the ultrafast excitation pulses. We show that the spectral and spatial intensities of the local fields at the gap regions of the coupled nanosystem can be pronounced by using two identical pulses with least time delay and phase difference. The corresponding temporal intensities of electric near-fields for both parallel and orthogonal polarization of the illumination fields are also briefly discussed. These findings might have implications for controlled excitation of complexly coupled plasmonic nanosystems.

Control and mapping ultrafast plasmons with PEEM

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.

Subwavelength imaging and control of ultrafast optical near field in nanosized bowtie and ring

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.

Efficient Modulation of Multipolar Fano Resonances in Asymmetric Ring-Disk/Split-Ring-Disk Nanostructure

Generation of multiple Fano resonances are theoretically investigated in asymmetry ring-disk and asymmetry split-ring-disk. The effects of structural parameter on multiple Fano resonances are analyzed in detail, and it is found that the wavelength of multipole Fano resonances can be extensively and accurately controlled by changing the gap size and the relative offset between the ring/split-ring and disk in asymmetric ring-disk/split-ring-disk nanostructure. Simulation on scattering spectra of the asymmetric structure show that Fano dips generally exhibit redshift in resonant wavelength and simultaneously with a varied modulation depth as symmetry of the structure is further broken. The results of the near-field distribution and phase simulation disclose that multiple Fano resonances are caused by interference of dipolar bright mode of whole asymmetric structure with the combined high-order dark-dark modes, and the dip on the shorter resonant wavelength side corresponds to higher-order dark mode. Furthermore, it is found that the multiple Fano resonances of asymmetry ring-disk are polarization-independent. However, for the asymmetric split-ring-disk, resonances are sensitive to polarization angle and number of the dips can be switched on and off by tuning the polarization angle. The proposed asymmetric nanostructures could find wide applications in plasmon line shaping, multiband sensing, electromagnetic-induced transparency and many other fields.

Numerical study on an efficient coupler of metal-dielectric nano-grating

A subwavelength metal grating with single layer and double layer structure are proposed and analyzed. The two gratings are evaluated and compared in terms of the reflectance spectra and the coupling efficiency. Numerically simulation shows that the absorption peak wavelength shows some red-shift when duty cycle of the grating increases for the single layer grating. The height modulated reflectance spectra of the double layer grating is discussed too, the shift of coupling efficiency is deeply affected by the Fabry-Pérot cavity modes and LSPR modes. The formation mechanism of low reflection band is discussed by modulating stripe thickness of the grating. It has significant applications in optical communication, biomedical sensors, plasmonic circuitry, and so on.