Youjiang Zhu

Dynamically tunable plasmon induced transparency in a graphene-based nanoribbon waveguide coupled with graphene rectangular resonators structure on sapphire substrate

In this paper, we propose dynamically tunable plasmon induced transparency (PIT) in a graphene-based nanoribbon waveguide coupled with graphene rectangular resonators structure on sapphire substrate by shifting the Fermi energy level of the graphene. Two different methods are employed to obtain the PIT effect: one is based on the direct destructive interference between a radiative state and a dark state, the other is based on the indirect coupling through a graphene nanoribbon waveguide. Our numerical results reveal that high tunability in the PIT transparency window can be obtained by altering the Fermi energy levels of the graphene rectangular resonators. Moreover, double PITs are also numerically predicted in this ultracompact structure, comprising series of graphene rectangular resonators. Compared with previously proposed graphene-based PIT effects, our proposed scheme is much easier to design and fabricate. This work not only paves a new way towards the realization of graphene-based integrated nanophotonic devices, but also has important applications in multi-channel-selective filters, sensors, and slow light.

High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators

A novel nanoscale structure for high sensitivity sensing which consists of a graphene nanoribbon waveguide coupled with detuned graphene square-nanoring resonators (GSNR) based on edge mode is investigated in detail. By altering the Fermi energy level of the graphene, the plasmon-induced transparency (PIT) window from the destructive interference between a radiative square-nanoring resonator and a dark square-nanoring resonator can be easily tailored. The coupled mode theory (CMT) is used to show that the theoretical results agree well with the finite difference time domain (FDTD) simulations. This nanosensor yields a ultrahigh sensitivity of ∼2600 nm/refractive index unit (RIU) and a figure of merit (FOM) of ∼54 in the mid-infrared (MIR) spectrum. The revealed results indicate that the Fermi energy level of the graphene and the coupling distance play important roles in optimizing the sensing properties. Our proposed structure exerts a peculiar fascination on the realization of ultra-compact graphene plasmonic nanosensor in the future.

Low-power and ultrafast all-optical tunable plasmon induced transparency in metal-dielectric-metal waveguide side-coupled Fabry-Perot resonators system

In this paper, low-power and ultrafast all-optical tunable plasmon induced transparency in metal-dielectric-metal (MDM) waveguide side-coupled Fabry-Perot (FP) resonators system with nonlinear optical Kerr medium is investigated both analytically and numerically. High tunability in transparency window magnitude and phase responses is obtained when nonlinear optical Kerr material is embedded in the MDM waveguide. In order to reduce the pump intensity, traditional nonlinear optical Kerr material is replaced by graphene. A shift of 64 nm in the central wavelength of the transparency window is achieved when the FP resonators are covered with monolayer graphene with pump intensity increasing from 9.2 to 10 MW/cm2. An ultrafast response time of the order of 1 ps is reached because of ultrafast carrier relaxation dynamics of graphene. This work not only paves the way towards the realization of on-chip integrated nanophotonic devices but also opens the possibility of the construction of ultrahigh-speed informatio...

Tunable Triple Plasmon-Induced Transparencies in Dual T-Shaped Cavities Side-Coupled Waveguide

In this letter, all-optical dynamically tunable triple plasmon-induced transparency responses are proposed in an ultracompact plasmonic structure, which comprises dual T-shaped resonators side-coupled to a metal-insulator-metal waveguide in the near infrared range. High tunability in magnitudes of optical channel 1 and optical channel 3 can be achieved, when the waveguide is filled with a nonlinear optical Kerr material, while that of optical channel 2 remains the same approximately. It is also possible to realize low-power tunability in central wavelengths of all-optical channels, when the resonators are covered by a single graphene layer under excitation with a pump light at various intensities. This letter opens up the possibility for the realization of a dynamically tunable multichannel filter or slow light devices in highly integrated optical circuits.

A dynamic and ultrafast group delay tuning mechanism in two microcavities side-coupled with a waveguide system

We theoretically propose a dynamic and ultrafast group delay tuning mechanism in two microcavities side-coupled to a waveguide system through external optical pump beams. The optical Kerr effect modulation method is applied to improve tuning rate with response time of subpicoseconds or even femtoseconds. The group delay of an all-optical analog to electromagnetically induced transparency effect can be controlled by tuning either the frequency of photonic crystal microcavities or the propagation phase of line waveguide. Group delay is controlled between 5.88 and 70.98 ps by dynamically tuning resonant frequencies of the microcavities. Alternatively, the group delay is controlled between 1.86 and 12.08 ps by dynamically tuning the propagation phase of line waveguide. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and coupled-mode formalism. Results show a new direction toward microstructure integration optical pulse trapping and all-optical dynamical storage of light devices in optical communication and quantum information processing.

Plasmon-induced transparency effect in metal-insulator-metal waveguide coupled with multiple dark and bright nanocavities

Abstract. We used bright and dark nanocavities coupled to a metal-insulator-metal waveguide to realize a plasmonic analogue of electromagnetically induced transparency (EIT) in integrated plasmonics. The bright nanocavity is directly coupled to the waveguide, while the dark nanocavity is achieved with the help of evanescent coupling. The numerical simulation shows a typical EIT-like line in the transmission spectrum. Using the model of EIT effect in a three-level atomic system, this phenomenon is well explained. Adding the number of dark nanocavities, we get multiple transparent peaks in the transmission spectrum of plasmon-induced transparency (PIT) effect, and we can realize control of the PIT effect by changing the geometric parameters of the plasmonic structure.

Plasmon-induced transparency effect in a single circular split-ring core ring resonator side-coupled to a metal-isolator-metal waveguide

We theoretically and numerically investigated the plasmon-induced transparency (PIT) effect in a single circular split-ring core ring resonator (CSRCRR) side coupled to a metal–isolator–metal waveguide, in which we can realize a single PIT effect window. The transmission line theory and the coupled mode theory are used. The results (transmission peak varies from 10% to 75%) show that the PIT window results from the destructive interference between the resonance modes in the CSRCRR. Then, the limit of wavelength detuning of the two modes in CSRCRR is studied. This work provides a new structure to realize the PIT effect and shows a new way to judge whether the phenomenon is the real PIT effect or not.

Slow light with large group index–bandwidth product in ellipse-hole photonic crystal waveguides

In this study, we propose a new type of slow light photonic crystal waveguide structure to achieve wideband slow light with low dispersion. The waveguide is based on a triangular lattice ellipse-hole photonic crystal imposed simply by a selective altering of the locations of the holes adjacent to the line defect. Under a constant group index criterion of ±10% variation, when group indices are nearly constants of 54, 69, and 80, their corresponding bandwidths of the flat band reach 12.7, 10.0, and 8.6 nm around 1550 nm, respectively. A nearly constant large group index-bandwidth product of 0.44 is achieved for all cases. Low dispersion slow light propagation is confirmed by studying the relative temporal pulse-width spreading with the two-dimensional finite-difference time-domain method.

Enhancement of magneto‐optic recorded domain image by mathematical morphology

To observe the magneto‐optic (MO) recording domains precisely it is necessary to erase noise and overcome poor contrast. By applying image averaging and subtracting nonmagnetic contrast in digital image processing system, the sharp domain image can be obtained. This is the first time the domain image has been combined with morphological signal processing. By using mathematical morphology in the processing algorithms, the nonmagnetical contrast (background image) can be generated from a real image directly. Subtracting it from the real image, the target domains can be segmented and enhanced.

Low-power, ultrafast, and dynamic all-optical tunable plasmonic analog to electromagnetically induced transparency in two resonators side-coupled with a waveguide system

We theoretically and numerically investigate a low-power, ultrafast, and dynamic all-optical tunable plasmonic analog to electromagnetically induced transparency (EIT) in two nanodisk resonators side-coupled to a metal-insulator-metal plasmonic waveguide system. The optical Kerr effect is enhanced by the slow light effect of the plasmonic EIT-like effect and the plasmonic waveguide based on graphene-Ag composite material structures with giant effective Kerr nonlinear coefficient. The optical Kerr effect modulation method is applied to improve tuning rate with response time of subpicoseconds or even femtoseconds. With dynamically tuning the propagation phase of the plasmonic waveguide, π-phase shift of the transmission spectrum in the plasmonic EIT-like system is achieved under excitation of a pump light with an intensity as low as 5.85 MW/cm2. The group delay is controlled between 0.09 and 0.4 ps. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and coupled-mo...