Artificial photonic band gap materials
1.PT symmetry in photonic systems
Parity-time (PT) symmetry is crucial in quantum mechanics. In contrast to conventional quantum mechanics, it has been proposed that non-Hermitian Hamiltonians where can still have an entirely real spectrum with respect to the PT symmetry. Due to the equivalence between the Schrödinger equation in quantum mechanics and the wave equation in optics, PT symmetry has been studied in the realm of optics with non-Hermitian optical potentials (gain/loss). By keeping or breaking of PT symmetry, photonic systems show plentiful phenomenon.
One-way photonic mode conversion by breaking parity–time (PT) symmetry
　　
Optical communications and computing require on-chip nonreciprocal light propagation to isolate and stabilize different chip-scale optical components. By breaking PT symmetry, we have designed and fabricated a metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide such that nonreciprocal light propagation is obtained on a silicon photonic chip. Nonreciprocal light transport and one-way photonic mode conversion are demonstrated at the wavelength of 1.55 micrometers in both simulations and experiments. Our system is compatible with conventional complementary metal-oxide-semiconductor processing, providing a way to chip-scale optical isolators for optical communications and computing.
L. Feng et al, Science 333, 729 (2011)
http://science.sciencemag.org/content/333/6043/729
2.Topological property of photonic crystals
In electronics, symmetry related topological properties on behalf of the global property of energy band always indicate the emergence of novel phases, which play an important role in various physical systems. For example, in systems with broken time-reversal symmetry (TRS), non-zero Chern number leads to quantum Hall effect. However, in systems with unbroken TRS, non-trivial topological invariant, such as spin Chern number and  invariant, helps to realize quantum spin Hall effect (QSHE) and topological insulator (TI. Since both solid-states physics and photonics obey the common band theory, a number of creative works have been proposed in the realm of bosonic systems, including photonics, to explore the non-trivial photonic topological phases.
Photonic topological insulator with broken time-reversal symmetry
　　
　　
A topological insulator is a material with an insulating interior but time-reversal symmetry-protected conducting edge states. Since its prediction and discovery almost a decade ago, such a symmetry protected topological phase has been explored beyond electronic systems in the realm of photonics. Electrons are spin-1/2 particles, whereas photons are spin-1 particles. The distinct spin difference
between these two kinds of particles means that their corresponding symmetry is fundamentally different. It is well understood that an electronic topological insulator is protected by the electron’s spin-1/2 (fermionic) time-reversal symmetry Tf2 = −1. However, the same protection does not exist under normal circumstances for a photonic topological insulator, due to photon’s spin-1 (bosonic) time-reversal symmetry Tb2 = 1. In this work, we report a design of photonic topological insulator using the Tellegen magnetoelectric coupling as the photonic pseudospin orbit interaction for left and right circularly polarized helical spin states. The Tellegen magnetoelectric coupling breaks bosonic time-reversal symmetry but instead gives rise to a conserved artificial fermionic-like-pseudo time-reversal symmetry, Tp (Tp2 = −1), due to the electromagnetic duality. Surprisingly, we find that, in this system, the helical edge states are, in fact, protected by this fermionic-like pseudo time-reversal symmetry Tp rather than by the bosonic time-reversal symmetry Tb. This remarkable finding is expected
to pave a new path to understanding the symmetry protection mechanism for topological phases of other fundamental particles and to searching for novel implementations for topological insulators.
Parity-time electromagnetic diode
　　
　　Schematic of square lattice of YIG semi-cylinders (black) on copper plate (yellow).
　　
One-way propagation for phase velocity with x-cut NRPC. Electric field distribution with (b) +10o from left-down and (c) -10o from right-up.
Phys. Rev. B 83, 075117 (2011)
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.83.075117
One-way Tunable cross-waveguide
　　
One-way cross-waveguides of time average energy flow with different directions of external magnetic field. Red (blue) line represents the one-way direction for 2nd gap (3rd gap respectively), and +, - means the counter directions of external magnetic field along z-axis.
　　
Transmission at lower port. (a) Various frequency excited by left source without using the central rod, where coupled-mode theory (red circles) agree well with simulation (blue line). The inset pictures represent time average energy flow.
Phys. Rev. B 83, 075117 (2011)
http://scitation.aip.org/content/aip/journal/apl/96/11/10.1063/1.3358386
One-way left-handed and right-handed propagation
　　
Phase analysis near edge Dirac point in 2nd band-gap. The lower and upper cladding is a square lattice of YIG PC and alumina PC. Ez is plotted with t=0, 1/2T, 1/4T, 3/4T at frequencies below (0.521), at (0.522) and above (0.524) edge Dirac Point respectively, where T is the optical period.
Phys. Rev. B 83, 075117 (2011)
http://www.sciencedirect.com/science/article/pii/S0038109810005077
One-way edge modes in different band-gaps
　　
One-way edge modes of gyromagnetic PC. Band-structure (TM) of a square lattice of YIG cylinders in air with (a) zero dc magnetic field and (b) 1600 Gauss +z dc magnetic field. The red numbers represent the nonzero Chern numbers. (c) Zoom-in projected band-structure, where blue and green area are for YIG and cladding alumina PCs respectively. The red and green curves in second and third band-gaps are the edge modes of upper (left) and lower (right) interface respectively. Field distributions at frequencies (d) 0.552 and (e) 0.628.
Phys. Rev. B 83, 075117 (2011)
http://scitation.aip.org/content/aip/journal/jap/107/12/10.1063/1.3374470
One-way cloak
　　
Spatial coordinate transformation for the square-shaped cloak based on the nonreciprocal gyromagnetic PC (a) original space and (b) transformed space
　　
One-way cloak at the frequency of 0.51 with a pair of incident light: (a) +30o and (b) -30o.
Appl. Phys. Lett. 99, 151112-151112-3 (2011)
http://scitation.aip.org/content/aip/journal/apl/99/15/10.1063/1.3648112
3 silicon photonic devices
As electronic integrated circuits have been in accordance with Moore's Law for 30 years, the quantum critical effect and power consumption issues have make it gradually approach its physical limits. Exploring new information carriers has become an important topic in the development of information technology, where photonic technology has become an important candidate. The past 20 years have witnessed great progress of artificial microstructures like photonic crystals, plasmonics and metamaterials which provide people new materials, ideas and opportunities to manipulate light in demand. On the other hand, in order to make photonic technology compatible with traditional silicon microelectronics technology, silicon photonics has been proposed from the application point of view, which will give a solid foundation for further optoelectronic integration. 
3.1 Broadband and wide angle infrared wire-grid polarizer
An infrared polarizer consisting of metal-insulator-metal (MIM)gratings is designed with transmittance exceeding 85% and polarization extinction ratio (ER) higher than 70 dB in the wavelength range from 1.5 to 8 μm. Moreover, the polarizer retains excellent performance even when the incident angle increases to as large as 80°. The MIM gratings support magnetic polariton (MP) in the near-infrared regime and operate non-resonantly in the mid-infrared regime, both of which result in the advantages of high extinction ratio, broadband, and wide angle.
　　
3.2 Highly efficient and perfectly vertical chip-to-fiber dual-layer grating coupler
A novel high-efficiency silicon-chip-to-fiber grating coupler is investigated here. By introducing a dual layer grating structure with an inter-layer lateral shift to mimic 45° tilted mirror behavior, perfectly vertical coupling is successfully demonstrated. Our numerical results show that a peak silicon-chip-to-fiber coupling efficiency about 70% is possible near 1550 nm. Meanwhile, for the entire telecom C-band, i.e. wavelengths from 1530 nm to 1565 nm, the coupling efficiency is > 50% and the back reflection is less than < 1%.