Unidirectional Transmission Of Light Waves In Photonic Crystal Waveguide Heterostructure

Compared with electrons,photons as information carriers have the advantages of low energy consumption,large bandwidth,fast transmission speed and high confidentiality.Thus,quantum photonic technology has become the one of the trends of future development,and one of its important developing directions is the miniaturization and on-chip integration.Optical asymmetric transmission devices（ATDs）,which are like the diodes in integrated electric circuitry,play an important role in quantum information processing and scalable quantum nanophotonic networks.As a result,optical ATDs have emerged as a research focus.A number of designs based on nanophotonic structures have been proposed to achieve asymmetric transmission,including metamaterial,photonic crystals（Ph Cs）,slab waveguides,surface plasmon and resonant effects.Compared with other methods,Ph Cs have the advantages of tunable optical properties,especially the photonic band structures,and low propagation loss.In addition,the Ph Cs is fully compatible with current CMOS nanofabrication technique and suitable for the on-chip integration,which paves the way towards integrated photonic chips for quantum computing and information processing.One of the most unique characteristics of Ph Cs is the photonic bandgap（PBG）.With proper design,it is possible to generate complete photonic bandgap（CPBG）with high refractive index materials（n>2.1）,such as silicon and germanium,which suppresses the light propagation from all the directions.Thus,it has been demonstrated to create low loss optical waveguides and high quality factor（Q-factor）nanocavities based on CPBGs of Ph Cs,most of which are demonstrated in 2D Ph C structures.However,it is challenging to apply CPBG in the design of ATD,as the forward incident light may be suppressed by the CPBG,resulting low forward transmittance,which is undesired.Thus,to the best of our knowledge,there is no demonstration of using CPBGs in the design of ATDs.In this paper,three kinds of waveguide unidirectional transmission devices based on two-dimensional Ph C heterostructures are proposed.Under the condition of satisfying total reflection,the plane wave expansion method is mainly used to calculate the energy band structure,and the two-dimensional time-domain finite difference method is used to calculate the transmission efficiency of the structure.A heterogeneous structure with wide working bandwidth and high transmission efficiency is obtained.The main content and calculation results are divided into the following three parts:1.Here,we theoretically present an on-chip nanophotonic asymmetric transmission device（ATD）based on the photonic crystal（Ph C）waveguide structure with complete photonic bandgaps（CPBGs）.The ATD comprises two-dimensional（2D）silica and germanium Ph Cs with CPBGs,within which line defects are introduced to create highly efficient waveguides to achieve high forward transmittance.In the meantime,the total internal reflection（TIR）principle is applied to block the backward incidence achieving asymmetric transmission.We optimize the design of the Ph Cs and the waveguide structure by scanning different structure parameters.The optimized ATD shows a high forward transmittance of 0.581 and contrast ratio of 0.989 at the wavelength of1582 nm for TE mode.The results deepen the understanding and open up the new possibility in designing novel ATDs.The on-chip ATD will find broad applications in optical communications and quantum computing.2.A waveguide-type photonic crystal heterostructure capable of realizing broadband unidirectional high transmission of light waves is composed of photonic crystal 1（PC1）and photonic crystal 2（PC2）.Among them,PC1 is a periodic arrangement of germanium cylinders in the silica background,the filling rate is r1/a1=0.32,r1=0.256um;PC2 is a periodic arrangement of silica cylinders in the germanium background,the filling rate is r2/a2=0.4,r2=0.334um.The angle between the heterojunction interface and the incident direction of the light wave is 60°.At the optical communication center wavelength of 1550 nm,the forward transmittance in the TE polarization state is0.90,and the transmission contrast is 0.98.In the range of 1450 nm～1650 nm,the forward transmittance in TE polarization state is above 0.78 and the transmission contrast is above 0.86.3.A heterogeneous structure of a coupled-cavity photonic crystal that can realize wide-band circular polarization is composed of photonic crystal 1（PC1）and photonic crystal 2（PC2）.Among them,PC1 is a Ge cylinder formed in a triangular lattice periodic arrangement in the background of Si O₂.PC1 removed7 rows of lattices and introduced a coupling cavity structure.The lattice constant a1=800 nm,r1b=0.256μm,r1s=0.05μm;The Si O₂ cylinder forms a triangular lattice periodic arrangement in the background of Ge.In PC2,13 rows of lattices are removed and the optical waveguide structure is introduced.The lattice constant a2=835 nm,r2=0.334μm.The angle between the heterojunction interface and the incident direction of the light wave is 60°.At a wavelength of 1550 nm at the center of optical communication,the forward transmittance under circular polarization is 0.88,and the transmission contrast is0.97.In the range of 1000 nm to 2000 nm,the forward transmittance is above0.76 and the transmission contrast is above 0.87 under the circular polarization state.