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.

/pdf/observation-of-asymmetric-transport-in-structures-with-4ijqeqnibr.pdf

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

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

Unidirectional transmission in one dimensional photonic crystal composed of PT symmetric and magneto-optical materials

A multicavity based quasi-parity-time (PT) symmetric structure is theoretically investigated for realizing highly sensitive gas detection. In the PT-symmetric broken phase, the singular pole effect can be achieved with much enhanced transmission, based on which low concentration of gas in the air can be efficiently sensed. In addition, the sensing signal is directly recorded by the change of the transmittance (T) of the sensor, real-time monitoring can be realized in our proposed structure. The sensitivities of -ΔT/ΔnD can reach 79437.2602 RIU−1 and 24753.7366 RIU−1 for benzene and methane, respectively. Such high sensitivity just occurs in the low concentration range. Our structures show great promise in constructing sensitive gas sensors to detect various harmful gases, such as volatile organic compounds (VOC) etc. 

Highly sensitive gas optical sensing in a multicavity based quasi-parity-time-symmetric structure

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.

Rong-kun Ma

Papers

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