A mechanism for asymmetric transport which is based on parity-time-symmetric nonlinearities is presented. We show that in contrast to the case of conservative nonlinearities, an increase of the complementary conductance strength leads to a simultaneous increase of asymmetry and transmittance intensity. We experimentally demonstrate the phenomenon using a pair of coupled Van der Pol oscillators as a reference system, each with complementary anharmonic gain and loss conductances, connected to transmission lines. An equivalent optical setup is also proposed.

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Observation of Asymmetric Transport in Structures with Active Nonlinearities

We propose a method of fabricating subwavelength structures based on multi-exposure surface plasmon interference lithography. This new nanolithography technique fabricates various subwavelength structures breaking the diffraction limit, which differs from the common method of writing various micro structures by using a two-laser-beam interference with sample rotation. Analysis of the optical field distributions obtained using the proposed nanolithography technique, which was performed using the theory of surface plasmon interference lithography in combination with coordinate matrix transformation, is also provided. Various special subwavelength structures are demonstrated by presenting simulated optical field distributions corresponding to different sample rotations, including two-dimensional lattice structures and periodic quasi-hexagonal structures. These structures are potentially useful in micro- and nano-optics applications.

Theoretical investigation of subwavelength structure fabrication based on multi-exposure surface plasmon interference lithography

We present an induced reflection response analogue to electromagnetically induced transparency (EIT) in a novel Tamm plasmon system, consisting of a thin metal film and a Bragg grating with a defect layer. The results show that an induced narrow peak can be generated in the original broad reflection dip, which is attributed to the coupling and interference between the Tamm plasmon and defect modes in the grating structure. It is found that the EIT-like induced reflection is strongly dependent on the thickness of defect layer, grating period number between the metal and defect layers, thickness of Bragg grating layer, refractive index of defect layer, and thickness of metal film. Additionally, the induced reflection can be dynamically tuned by adjusting the angle of incident light. The numerical simulations agree extremely well with theoretical calculations. The coupling strength between the Tamm plasmon and defect modes is determined by the above parameters. These results will provide a new avenue for light field control and devices in multilayer photonic systems.

Induced reflection in Tamm plasmon systems.

The pole is the unique phenomena in the parity-time (PT) symmetry structure. To explore the application of the pole effect, a PT-symmetry resonator coupling system is designed as a gas sensor. The large transmittance and reflectance near the pole are found extraordinary sensitive to the refractive index of the polluted air in the system. Through measuring the reflectance, the refractive index of the polluted air and the concentration of gas in the air can be detected. The sensor has an extraordinary sensitivity in detecting the gases with low concentration. The maximum sensitivities $\Delta R/R$ for the methane, acetylene, and ethane are 0.112, 0.190, and 0.282, respectively. The limitation of detectable concentration change is 1/1500 for methane with the photodetector that can sense the relative change rate of the reflectance of 10−3.

Sensing Gases by the Pole Effect of Parity-Time Symmetric Coupled Resonators

Silicon nanowire optical rectangular waveguide (SNORW)-based biosensor is proposed for the first time to detect deoxyribonucleic acid (DNA) hybridization. It works on the principle of detecting the change in refractive index during the hybridization process when double strand DNA splits into single strands of DNA or two single strands renaturate into a double strand DNA. Considering the fabrication on 220- and 330-nm silicon technology, first a typical configuration of SNORW is optimized to provide physical compactness and high-power confinement. Next, parameters characterizing link layer and DNA are added, and change in power confinement is ascertained when double strand DNA (dsDNA) splits into two single strand DNA (ssDNA) or two ssDNA renaturate to form a dsDNA. A mean to enhance the difference in power confinement in the presence of 100% ssDNA and 100% dsDNA is also proposed. As the splitting/renaturation of DNAs is a slow process, power confinement when different percentages of dsDNA and ssDNA coexist is also evaluated.

Silicon Nanowire Optical Rectangular Waveguide Biosensor for DNA Hybridization

In order to achieve enhanced transmittance of parity-time (PT)-symmetric system, we design a layered PT-symmetric structure including resonators. We use the scatter matrix method to study the optical properties of the designed structure under the modulation of resonators. The structure system takes on a singular pole effect, i.e., the huge reflectance and transmittance occur at a special wavelength and period number. The field distribution reveals that the singular pole results from the coupling resonance of single cavity and the whole structure. Because of the coupling resonance, the total gain in layer A is much larger than the total loss in layer C. The reflectance and transmittance at the singular pole take on a high sensitivity on the refraction index of the resonators.

Singular pole and enhanced sensitivity of PT-symmetric layered structure with resonators

In order to study the coupling and evolution law of the waveguide mode and two plasmonic surface modes, we construct a line defect waveguide based on hexagonal honeycomb plasmonic photonic crystal. Through adjusting the radius of the edge dielectric rods, the competition and evolution behaviors occur between dielectric waveguide mode and plasmonic waveguide mode. There are three status: only plasmonic waveguide modes occur for r A a ; only dielectric waveguide modes occur for r A > 0.25 a ; two kinds of modes coexist for 0.09 a r A a . The plasmonic waveguide mode has advantages in achieving slow light.

Competition and evolution of dielectric waveguide mode and plasmonic waveguide mode

Tunable All-Optical Diode Based on a Coupled System of Two Nonsymmetrical Cavities and a Metal Film

All-optical diode actions through a coupled system of Tamm plasmon-polariton and nonlinear cavity mode

The transmission properties of a resonator based on one air layer sandwiched by two-layered anisotropic metamaterials with different negative-permittivity tensors are studied through analytical analysis and numerical calculations. The resonator has orthogonally polarized dual wavelength output. The difference of the two wavelengths can be adjusted through changing the permittivity tensors, and the ratio of the intensities of the two orthogonally polarized waves can also be adjusted through changing the polarization direction of incident wave. The study results may find applications in the design of laser resonator.

Yun-Tuan Fang

Papers

Singular pole and enhanced sensitivity of PT-symmetric layered structure with resonators

Competition and evolution of dielectric waveguide mode and plasmonic waveguide mode

Tunable All-Optical Diode Based on a Coupled System of Two Nonsymmetrical Cavities and a Metal Film

All-optical diode actions through a coupled system of Tamm plasmon-polariton and nonlinear cavity mode

Resonator with orthogonally polarized dual wavelength output based on layered anisotropic metamaterials