Modal Symmetry Controlled Second-Harmonic Generation by Propagating Plasmons

Coherent control of enhanced second-harmonic generation in a plasmonic nanocircuit using a transition metal dichalcogenide monolayer

Nonlinear nanophotonic circuits, renowned for their compact form and integration capabilities, hold potential for advancing high-capacity optical signal processing. However, limited practicality arises from low nonlinear conversion efficiency. Transition metal dichalcogenides (TMDs) could present a promising avenue to address this challenge, given their superior optical nonlinear characteristics and compatibility with diverse device platforms. Nevertheless, this potential remains largely unexplored, with current endeavors predominantly focusing on the demonstration of TMDs' coherent nonlinear signals via free-space excitation and collection. In this work, we perform direct integration of TMDs onto a plasmonic nanocircuitry. By controlling the polarization angle of the input laser, we show selective routing of second-harmonic generation (SHG) signals from a MoSe2 monolayer within the plasmonic circuit. Routing extinction ratios of 14.86 dB are achieved, demonstrating good coherence preservation in this hybrid nanocircuit. Additionally, our characterization indicates that the integration of TMDs leads to a 13.8-fold SHG enhancement, compared with the pristine nonlinear plasmonic nanocircuitry. These distinct features-efficient SHG generation, coupling, and controllable routing-suggest that our hybrid TMD-plasmonic nanocircuitry could find immediate applications including on-chip optical frequency conversion, selective routing, switching, logic operations, as well as quantum operations.

Near-Field Photodetection in Direction Tunable Surface Plasmon Polaritons Waveguides Embedded with Graphene

2D materials have manifested themselves as key components toward compact integrated circuits. Because of their capability to circumvent the diffraction limit, light manipulation using surface plasmon polaritons (SPPs) is highly-valued. In this study, plasmonic photodetection using graphene as a 2D material is investigated. Non-scattering near-field detection of SPPs is implemented via monolayer graphene stacked under an SPP waveguide with a symmetric antenna. Energy conversion between radiation power and electrical signals is utilized for the photovoltaic and photoconductive processes of the gold-graphene interface and biased electrodes, measuring a maximum photoresponsivity of 29.2 mA W-1 . The generated photocurrent is altered under the polarization state of the input light, producing a 400% contrast between the maximum and minimum signals. This result is universally applicable to all on-chip optoelectronic circuits.

Nonlinear chaotic dynamics in nonlocal plasmonic core-shell nanoparticle dimer

Plasmonic nanoparticles can be employed as a promising integrated platform for lumped optical nanoelements with unprecedentedly high integration capacity and efficient nanoscale ultrafast nonlinear functionality. Further minimizing the size of plasmonic nanoelements will lead to a rich variety of nonlocal optical effects due to the nonlocal nature of electrons in plasmonic materials. In this work, we theoretically investigate the nonlinear chaotic dynamics of the plasmonic core-shell nanoparticle dimer consisting of a nonlocal plasmonic core and a Kerr-type nonlinear shell at nanometer scale. This kind of optical nanoantennae could provide novel switching functionality: tristable, astable multivibrators, and chaos generator. We give a qualitative analysis on the influence of nonlocality and aspect ratio of core-shell nanoparticles on the chaos regime as well as on the nonlinear dynamical processing. It is demonstrated that considering nonlocality is very important in the design of such nonlinear functional photonic nanoelements with ultra-small size. Compared to solid nanoparticles, core-shell nanoparticles provide an additional freedom to adjust their plasmonic property hence tuning the chaotic dynamic regime in the geometric parameter space. This kind of nanoscale nonlinear system could be the candidate for a nonlinear nanophotonic device with a tunable nonlinear dynamical response.

Enhancing on/off ratio of a dielectric-loaded plasmonic logic gate with an amplitude modulator

Plasmonic waveguides allow focusing, guiding, and manipulating light at the nanoscale and promise the miniaturization of functional optical nanocircuits. Dielectric-loaded plasmonic (DLP) waveguides and logic gates have drawn attention because of their relatively low loss, easy fabrication, and good compatibility with gain and active tunable materials. However, the rather low on/off ratio of DLP logic gates remains the main challenge. Here, we introduce an amplitude modulator and theoretically demonstrate an enhanced on/off ratio of a DLP logic gate for XNOR operation. Multimode interference (MMI) in DLP waveguide is precisely calculated for the design of the logic gate. Multiplexing and power splitting at arbitrary multimode numbers have been theoretically analyzed with respect to the size of the amplitude modulator. An enhanced on/off ratio of 11.26 dB has been achieved. The proposed amplitude modulator can also be used to optimize the performance of other logic gates or MMI-based plasmonic functional devices.

Nonlinear Optical Signal Generation Mediated by a Plasmonic Azimuthally Chirped Grating

Plasmonic gratings are simple and effective platforms for nonlinear signal generation since they provide a well-defined momentum for photon-plasmon coupling and local hot spots for frequency conversion. Here, a plasmonic azimuthally chirped grating (ACG), which provides spatially resolved broadband momentum for photon-plasmon coupling, was exploited to investigate the plasmonic enhancement effect in two nonlinear optical processes, namely two-photon photoluminescence (TPPL) and second harmonic generation (SHG). The spatial distributions of the nonlinear signals were determined experimentally by hyperspectral mapping with ultrashort pulsed excitation. The experimental spatial distributions of nonlinear signals agree very well with the analytical prediction based on photon-plasmon coupling with the momentum of the ACG, revealing the "antenna" function of the grating in plasmonic nonlinear signal generation. This work highlights the importance of the antenna effect of the gratings for nonlinear signal generation and provides insight into the enhancement mechanism of plasmonic gratings in addition to local hot spot engineering.

Nonscattering Photodetection in the Propagation of Unidirectional Surface Plasmon Polaritons Embedded with Graphene

Recently, nanoscale light manipulation using surface plasmon polaritons (SPPs) has received considerable research attention. The conventional method of detecting SPPs is through light scattering or using bulky Si or Ge photodetectors. However, these bulky systems limit the application of nanophotonic circuits. In this study, the light-matter interaction between graphene and SPP was investigated. For realizing an improved integration in nanocircuits, single-layer graphene was added to asymmetric SPP nanoantenna arrays for nonscattering detection in the near field. The developed device is capable of detecting the controlled propagation of SPPs with a photoresponsivity of 15 mA/W, which paves the way for the new-generation on-chip optical communication.

Efficient four-wave mixing based on multiple plasmonic resonance

Plasmonic nanostructures provide a new way to improve nonlinear optical effects. As a mode of surface plasmons (SP), localized SPs can highly localize and enhance electromagnetic fields within a subwavelength volume. In this work, we developed a one-dimensional V-groove Ag nanograting. Through simulation, we realized triple-resonance enhanced four-wave mixing (FWM), in which both the excitation and signal waves are in resonance with LSPRs modified by propagating SPs, and can perfectly overlap with each other in each single nanogroove. Compared with that from a flat Ag plate, the FWM enhancement factor can be over six orders of magnitude. Next, we filled the Ag V-groove with nonlinear polymer 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene and further improved the enhancement factor to eight orders of magnitude, together with a conversion efficiency of 1.02×10^-2. Finally, by changing the water filling ratio, the FWM signal is tuned over 180 nm, while keeping the enhancement factor over seven orders of magnitude.

Nanoscale Electrical Excitation of Distinct Modes in Plasmonic Waveguides

The electrical excitation of guided plasmonic modes at the nanoscale enables integration of optical nanocircuitry into nanoelectronics. In this context, exciting plasmons with a distinct modal field profile constitutes a key advantage over conventional single-mode integrated photonics. Here, we demonstrate the selective electrical excitation of the lowest-order symmetric and antisymmetric plasmonic modes in a two-wire transmission line. We achieve mode selectivity by precisely positioning nanoscale excitation sources, i.e., junctions for inelastic electron tunneling, within the respective modal field distribution. By using advanced fabrication that combines focused He-ion beam milling and dielectrophoresis, we control the location of tunnel junctions with sub-10 nm accuracy. At the far end of the two-wire transmission line, the guided plasmonic modes are converted into far-field radiation at separate spatial positions showing two distinct orthogonal polarizations. Hence, the resulting device represents the smallest electrically driven light source with directly switchable polarization states with possible applications in display technology.

A Polarization-Actuated Plasmonic Circulator

A circulator for surface plasmon polaritons (SPPs) based on a plasmonic two-wire transmission-line (TWTL) structure is experimentally realized. A TWTL offers two distinct plasmon modes that can be independently excited, solely determined by the polarization of the laser field. Through controlled superposition of the two modes, TWTLs are exploited to enable polarization-actuated plasmonic circulators. In the first demonstration, the coupling antennas to the plasmonic circulator are designed to circulate SPPs sensitive to linearly polarized excitation. In the second design, the circulator reacts to the spin angular momenta carried by circularly polarized laser excitations. In both cases, the SPP circulation directions are directly controlled by the laser polarization, and the number of ports is easily expandable. Experimentally, a wide optical operational bandwidth of ∼100 nm is achieved. The results show a major step toward the realization of multifunctioning photonic nanocircuitry.