The Nonreciprocal Properties In Photonic Crystals

Recently, the nonreciprocal properties in photonic crystals have drawn much attention in both fundamental research and applications, due to their inherent novel phenomena. In this dissertation, by introducing the time-reversal symmetry and spatial inversion symmetry (separately or both) into photonic crystals, the nonreciprocal properties would be realized, such as the one-way edge modes, one-way bulk modes and nonreciprocal transmission, which might trigger some exotic applications, such as one-way splitters, optical isolators and one-way cloaks1. Analogous to the quantum Hall effect of electrons, a two-dimensional magneto-optical photonic crystal shows two pairs of one-way edge Dirac cones, whereby only clockwise and anti-clockwise circle wave scattering (facing the direction of the external applied magnetic field) are allowed in the second and third band-gaps respectively. These two different chiral pathways originate from the different exchange of Chern numbers, which are attributed to the broken time-reversal symmetry. Moreover, below and above the edge Dirac point, the edge modes show the left-handed (counter propagation of group velocity and phase velocity) and right-handed characters respectively.2. The influence of boundary conditions on the one-way edge modes in two-dimensional magneto-optical photonic crystals is studied theoretically by supercell methods. The numerical results reveal that it could bring some new properties by tailoring the boundary, but would not change the intrinsic one-way character for edge modes in the band-gap generally. Evenmore, there are more than one edge modes and waveguide modes to appear, which could be coupled each other and split into new ones. Two independent channels for one-way edge modes can be realized. The frequency of the edge Dirac point can be tunable.3. We design a tunable one-way cross-waveguide splitter, resulting from the broken time-reversal symmetry, based on the edge modes of the gyromagnetic photonic crystal. Mode control can be realized by altering the radius or refractive index of a single central electro-optical rod. The on-off switch of the channels can be manipulated by external electric or magnetic field. An coupled-mode theory between defect modes and waveguide modes has been used to characterize the transmission efficiency of the channels.4. We propose a kind of electromagnetic diodes based on a two-dimensional nonreciprocal gyrotropic photonic crystal. This periodical microstructure is of separately broken symmetries in both parity and time-reversal but obeys the parity-time symmetry. This kind of diode could support the bulk one-way propagating modes either for group velocity or phase velocity with various types of negative and positive refraction. This symmetries broken system might be a platform to realize abnormal photoelectronic devices and analogy to electron counterpart with one-way features.5. Transformation optics is recently of great interest since it can help to create various applications that were considered illusions in history, such as an invisible cloak. Here we propose a novel physical concept of simultaneously breaking the parity and time-reversal symmetries in transformation optics, by which nonreciprocal one-way invisible cloak is designed and validated. The one-way invisible cloak is made of a coordinate-transformed nonreciprocal photonic crystal, showing a perfect cloaking for wave incident from one direction but acting as a perfect reflector for wave from the counter direction. The proposed nonreciprocal transformation optics shows a high promise of applications in military, as protecting the own information to be detected but efficiently grabbing the information from the "enemy" side.6. Analogous to the spin quantum Hall effect of electrons, a two-dimensional photonic crystal constructed by magneto-optical medium with both gyroelectric and gyromagnetic properties was proposed, which breaks time-reversal symmetry separately for a single polarization but obeys the time-reversal invariant for the circular polarized wave. The polarization transportation can be realized in a pair of time-reversal invariant gapless edge states, which can be treated as an optical counterpart of spin quantum Hall effect. The symmetric or anti-symmetric edge states can be excited at the boundary, which can be tunable by using different exciting sources. This time-reversal invariant model might be a platform to mimic the spin property of electrons to realize some abnormal photonic devices with "spin" quantum features.7. We theoretically and experimentally reveal that the enhanced resonant optical transmission can be realized through a one-dimensional photonic crystal adjacent to a thin metal film at a frequency in the original band-gap of the photonic crystal. The influence of the periodic number of photonic crystal and the thickness of the adjacent metal on the transmission frequency and intensity is studied in details. An optimum design is given to reach the maximum transmission efficiency meanwhile a mechanism underlining the resonant optical transmission phenomenon is proposed. An effective admittance-matching theory is used to understand this effect and quantitatively determine the resonant frequency, which matches very well with the simulated and measured results. The effects might be very useful to realize some optical filters and sensor devices since the structure is easy for mass production and is matured technique to be prepared in industry. 8. We report our design of nonreciprocal transmission/reflection based on a one-dimensional photonic crystal adjacent to the gyroelectric metal, which is stemming from the broken time-reversal symmetry. With broken time-reversal symmetry by applying an external magnetic field, the dielectric function of gyroelectric metal would be both nonreciprocal and negative in optical window, which could excite the nonreciprocal surface modes between the PC and metal in the band-gap of original PC. An effective admittance-matching theory based on nonreciprocal systems is proposed to understand this effect and quantitatively determine the work frequency. This1DPC-metal model might be very helpful to realize the optical diode with the advantage of the low loss, wide frequency window, small external magnetic field area, mature technique in industry, etc..9. We theoretically investigated a format of two-dimensional dielectric-chiral photonic crystal structure that is composed of a dispersive chiral medium embedded in a dielectric background. The photonic band-structure shows distinctive dispersion relationship for circularly polarized electromagnetic waves, leading to a number of intriguing wave properties, namely chirality dependent’node switching’, polarization sensitive transmission and handedness dependent mode localization. All of these effects are attributed to the strong interaction between the local resonant modes around the dispersive chiral rods and the Bloch modes of the bulk waves. The chirality dependent properties might find tremendous applications in polarization based optoelectronics devices and rapid separation of chiral compounds in pharmaceutical industry.