Yinxiao Xiang

Tunable Band-Stop Filters for Graphene Plasmons Based on Periodically Modulated Graphene.

Tunable band-stop filters based on graphene with periodically modulated chemical potentials are proposed. Periodic graphene can be considered as a plasmonic crystal. Its energy band diagram is analyzed, which clearly shows a blue shift of the forbidden band with increasing chemical potential. Structural design and optimization are performed by an effective-index-based transfer matrix method, which is confirmed by numerical simulations. The center frequency of the filter can be tuned in a range from 37 to 53 THz based on the electrical tunability of graphene, while the modulation depth (−26 dB) and the bandwidth (3.1 THz) of the filter remain unchanged. Specifically, the bandwidth and modulation depth of the filters can be flexibly preset by adjusting the chemical potential ratio and the period number. The length of the filter (~750 nm) is only 1/9 of the operating wavelength in vacuum, which makes the filter a good choice for compact on-chip applications.

Flexible modulation of plasmon-induced transparency in a strongly coupled graphene grating-sheet system.

General actively tunable near-field plasmon-induced transparency (PIT) systems based on couplings between localized plasmon resonances of graphene nanostructures not only suffer from interantenna separations of smaller than 20 nm, but also lack switchable effect about the transparency window. Here, the performance of an active PIT system based on graphene grating-sheet with near-field coupling distance of more than 100 nm is investigated in mid-infrared. The transparency window in spectrum is analyzed objectively and proved to be more likely stemmed from Aulter-Townes splitting. The proposed system exhibits flexible tunability in slow-light and electro-optical switches, promising for practical active photonic devices.

Optical bistability based on Bragg grating resonators in metal-insulator-metal plasmonic waveguides

An optically bistable device based on a Bragg grating resonator with a nonlinear medium in metal-insulator-metal waveguides is proposed. Its properties are numerically investigated by a finite-difference time-domain method and further qualitatively analyzed by adopting Airy equation. Cavities with different Q factors are compared with respect to bi-stability. Cavities with a small Q factor lead to a high transmission and a narrow hysteresis loop. The response time of such cavities is found to be in the sub-picosecond region. Our nano-scale switching structure is comparatively easy to fabricate and integrate in plasmonic circuits and promises to be useful for future all-optical computing and communication technology.

Mid-infrared optical near-field switching in heterogeneous graphene ribbon pairs

The spatial switching of mid-infrared light near-fields is proposed in coupled graphene heterogeneous ribbon pairs. By using the coupled plasmon modes in graphene ribbon pairs, the electric near-field enhancement can be spatially controlled in graphene ribbons as the tuning of the external bias voltage difference. More specifically, due to symmetry breaking, the anti-symmetrically coupled dipolar plasmons exist except for the symmetrically coupled modes in homogeneous graphene pairs. Moreover, the gap distance is one of the key parameters for switching of electric near-fields, strong coupling means the efficient control of near-fields.

Design methodology for all-optical bistable switches based on a plasmonic resonator sandwiched between dielectric waveguides

We present a bistable device consisting of a Bragg grating resonator with a Kerr medium sandwiched between two dielectric slab waveguides. The resonator is situated in a nanometer-scaled metal?insulator?metal plasmonic waveguide. Due to the dimensional confinement from the dielectric waveguide to the nanoscaled plasmonic waveguide, electric fields are enhanced greatly, which will further reduce the threshold value. Moreover, a semi-analytic method, based on the impedance theory and the transfer matrix method, is developed to study the transmission and reflection spectra as well as the bistability loop of such a switch. Our method is fast and accurate, as confirmed by the finite-difference time-domain simulation.

Ultra-strong enhancement of electromagnetic fields in an L-shaped plasmonic nanocavity.

Enhancements up to four orders of magnitude for electric intensity and three orders of magnitude for magnetic intensity are realized in a novel 2D L-shaped nanocavity. This structure makes full use of the dimension confinement, cavity resonance and tip enhancement to increase the electromagnetic intensity. An impedance matching model is developed to design this cavity by regarding the cavity as a load impedance where electromagnetic fields are maximally enhanced when maximum electromagnetic energy is delivered to the load impedance. Our L-shaped nanocavity promises a variety of useful functionalities in sensing, nonlinear spectroscopy and signal processing.

Tailorable reflection of surface plasmons in defect engineered graphene

The electrical, optical, mechanical and thermal properties of graphene can be significantly altered by defects, thus engineering the defects in graphene is promising for applications in functionalized materials and nanoscale devices. Here the propagations of surface plasmon waves near graphene defect boundaries created by ion beams are studied. Specifically, plasmon reflections are observed near the induced defect boundaries for the first time, which implies that ion-irradiation induced defects act as efficient scattering centers for the plasmonic waves, just like the native grain boundaries. Moreover, engineering the defects with varied ion doses results in tailorable plasmon reflection properties due to changed defect degrees. The controllable plasmon reflections near ion induced defect boundaries open up a new avenue for plasmon wave engineering.

Dynamic spontaneous emission control of an optical emitter coupled to plasmons in strained graphene

Spontaneous emission control of an optical emitter is critical for many applications, such as in the fields of sensing, integrated photonics and quantum optics. Integrating optical emitters with a mechanical system can provide an avenue for strain sensors as well. Here, the dynamic spontaneous emission modification of an emitter coupled to graphene by uniaxial strain is demonstrated. Our results show that the emission rate can be controlled by tuning the strain of graphene, which depends on the polarized orientation of the emitter. More specifically, the decay rate can be enhanced for several times if the emitter is polarized perpendicular to graphene under strain. Azimuthal angle dependent oscillation of decay rate exists for the emitter polarized parallel to the graphene. Moreover, the controllable decay of the emitter comes from the anisotropic plasmons excitation in strained graphene, which is verified by the corresponding isofrequency contours of plasmons. The strain engineering provides a new platform for dynamic spontaneous emission modulation of emitters coupled with graphene, which opens up intriguing possibilities for the design of strain sensors and quantum devices.

In‐Plane Electrical Connectivity and Near‐Field Concentration of Isolated Graphene Resonators Realized by Ion Beams

Graphene plasmons provide great opportunities in light-matter interactions benefiting from the extreme confinement and electrical tunability. Structured graphene cavities possess enhanced confinements in 3D and steerable plasmon resonances, potential in applications for sensing and emission control at the nanoscale. Besides graphene boundaries obtained by mask lithography, graphene defects engineered by ion beams have shown efficient plasmon reflections. In this paper, near-field responses of structured graphene achieved by ion beam direct-writing are investigated. Graphene nanoresonators are fabricated easily and precisely with a spatial resolution better than 30 nm. Breathing modes are observed in graphene disks. The amorphous carbons around weaken the response of edge modes in the resonators, but meanwhile render the isolated resonators in-plane electrical connections, where near-fields are proved gate-tunable. The realization of gate-tunable near-fields of graphene 2D resonators opens up tunable near-field couplings with matters. Moreover, graphene nonconcentric rings with engineered near-field confinement distributions are demonstrated, where the quadrupole plasmon modes are excited. Near-field mappings reveal concentrations at the scale of 3.8×10-4λ02 within certain zones which can be engineered. The realization of electrically tunable graphene nanoresonators by ion beam direct-writing is promising for active manipulation of emission and sensing at the nanoscale.

Nanofocusing of the free-space optical energy with plasmonic Tamm states

To achieve extreme electromagnetic enhancement, we propose a plasmonic Tamm states (PTSs) configuration based on the metal-insulator-metal Bragg reflector, which is realized by periodically modulating the width of the insulator. Both the thick (2D) and thin (3D) structures are discussed. Through optimization performed by the impedance-based transfer matrix method and the finite difference time domain method, we find that both the electric field and magnetic field intensities can be increased by three orders of magnitude. The field-enhancement inside the PTSs configuration is not limited to extremely sharp waveguide terminal, which can greatly reduce processing difficulties.