The gas concentration can be detected through a parity-time (PT) -symmetry structure. The structure is constructed by placing a resonance cavity between two PT-symmetry Bragg reflectors. The cavity and some specific layers in the structure are filled with the air doped with another gas. The transmittance spectra of the structure are calculated by the transfer matrix method. The peak value of the defect mode is dependent on the concentration of the gas. The sensor detects the gas concentration by the transmittance of the defect mode, instead of the peak wavelength of the defect mode. The defect mode has a large peak value and high Q value because of the amplification effect of the PT-symmetry structure, which apparently enhances the sensor sensitivity.
In order to achieve tunable parity-time (PT) symmetry system, we design a PT-symmetry structure including magneto-optical material. We use the transfer matrix method to study the optical properties of the designed structure under the modulation of magnetic field. The structure system takes on twofold unidirectional properties, i.e., the reflectance and the transmittance are dependent on both the incidence direction and the incidence angle. The unique band-edge modes with the system have both enhanced reflectance and transmittance in one incidence direction or angle but lead to an absorption effect in the opposite incidence direction or angle. The reversal of magnetic field direction will all lead to the exchanging of the left band-edge modes and the right band-edge modes.
In order to obtain the waveguide of multiple functionalities, we design a coupled system of two unidirectional air waveguides and find it is a system of multiple modes through band calculations. Through numerical simulations, we also find that the mode excitation is dependent on the position of the source. With the same frequency the line source can excite either the even mode or the odd modes in one single waveguide or two waveguide just by changing the positions of the source. Such a system provides us the way to control the excitation of mode and obtain the waveguide modes with special applications.
Negative refractive-photonic crystal (NR-PC) lenses that can exceed the diffraction limit of focus resolution for imaging and target detection in the near field have gotten more and more special attention in recent years. Three flat lens groups with Ag defects based on NR-PC are designed, and the focusing imaging in the NR-PC three flat lens groups is concluded with the extension of Snell’s law, and the influence on the resolution for a target detection dynamic scanning scheme is simulated by using the finite difference time domain method. An optimal-doped structure with Ag defects is achieved by different simulation combinations. The refocusing resolution 0.18834λ is achieved in the optimal structure and there is approximately a 0.06806λ improvement in the refocusing resolution compared to those undoped with Ag (0.2564λ); it also possesses distinct smaller side-lobes than a single flat lens doped with Ag. This means the optimal detecting ability for the three NR-PC flat lens groups with Ag defects is more improved than that for a single undoped and doped with Ag. This is significant for the perfect imaging being achieved for a particle structure.
Nonreciprocal transmission is achieved through an optical prism coupling system composed of magneto-optical (MO) media. The transmission properties of the system are studied through the developed transfer matrix method for magnetic materials. The unusual result is that a tunable nonreciprocal resonance tunneling occurs if two MO cavities have reversal magnetization. The results are verified through an electromagnetic field simulation based on the finite element solver.
The characteristics of localized fields of doped photonic crystal fibers (PCFs) are studied by numerical simulation method in the paper. An interesting phenomenon is produced with the enhancement of stimulated radiation, which is the transmittivity being greater than one. And the numerical results show clearly the relation between the characteristics of localized fields, the abnormal group velocity in photonic band gaps and the negative imaginary component of the complex effective index of refraction of doped medium. Based on the relations the amplification of stimulated emission can be realized by introducing active impurities into the defect media of PCFs. Furthermore, the narrow transmission bands can be obtained by introducing line defects into the doped PCFs, which are used as channels in wavelength-division multiplexed (WDM) communication systems. And the doped PCFs can also be used to make optical amplifiers employed in dense WDM communication systems.