| The unidirectional optical transmission device is an important component of the integrated optical path,which has the function of preventing the backward scattering light in the optical path from interfering with the light source or photonic devices and maintaining the stability of the optical path.Photonic crystal unidirectional transmission device has the advantages of small size,adjustable working band,low transmission loss and easily integrated,which has been a topical research in domestic and international.At present,unidirectional optical transmission devices in visible band still need to solve the problems of low contrast ratio,low forward transmittance,narrow bandwidth and integration difficulties.Therefore,the design of high-performance unidirectional optical transmission devices in the visible band using different transmission mechanisms is of great research significance for the development of the optical communication field.This paper selects two-dimensional hexagonal boron nitride(hBN)materials with lower absorption in visible band,and proposes two hBN photonic crystal unidirectional transmission structures,which can realize unidirectional transmission of transverse electric(TE)polarized light and transverse magnetic(TM)polarized light with high contrast ratio in visible band,respectively.In addition,three valley photonic crystal unidirectional transmission structures are constructed,which can realize the high unidirectional transmission of circularly polarized light,unidirectional slow light transmission and bandpass notch filtering functions,respectively.The main research contents are as follows:(1)A two-dimensional hBN photonic crystal heterostructure is constructed based on the generalized total reflection principle.The band properties of the photonic crystal structure are studied by the plane wave expansion method,and the angle between the interface and the incident light is adjusted to realize the unidirectional transmission of TE polarized light in visible band.The effects of lattice constant,interface distance and structural thickness on the transmission spectrum of the heterostructure are investigated by the time-domain finite-difference method.The optimized photonic crystal heterostructure achieves unidirectional transmission with backward transmittance below 0.04 in visible band(610 nm-684 nm),forward transmittance of 0.66 and transmission contrast ratio of 0.98 at the peak wavelength of 652 nm.(2)In order to reduce the structural complexity and improve the forward transmission of the structure,a two-dimensional hBN collimated photonic crystal structure is proposed,which can realize the unidirectional transmission of TM polarized light in visible band.By analyzing the band structure and the equal frequency contours properties of the hBN photonic crystal,it is found that the structure has a horizontal photonic band gap and a collimation effect of the incident light in the 45° direction.The incident port of the photonic crystal is designed to be triangular shape to change the transmission direction of the incident light,thus realizing the self-collimation effect of the light wave in the photonic crystal structure.The overall size of the collimated photonic crystal structure is 5 μm × 11 μm,and the structure achieves unidirectional transmission with backward transmittance below 0.05 in visible band(590nm-632 nm),forward transmittance of 0.77 at the peak wavelengths of 590 nm and 632 nm,and transmission contrast ratio of 0.95.(3)A two-dimensional hBN valley photonic crystal unidirectional transmission structure is designed based on the spin-valley locking effect,which can realize the effective modulation of visible light.The honeycomb lattice photonic crystal with triangular air hole is selected to reduce the radius of air holes in three directions of the lattice cell to zero,break the spatial inversion symmetry of the structure,and broaden the photonic forbidden band of the valley photonic crystal.Straight-type,Z-type and Ω-type topological waveguides are designed by combining two valley photonic crystals with different topological properties.The straight-type waveguide achieves high unidirectional transmission of right-handed circularly polarized light in visible band(641 nm-714 nm)with a forward transmittance of 0.96 at the peak wavelength of 680 nm and a transmission contrast ratio of 0.99.The Z-type and Ω-type waveguides have narrower unidirectional transmission bandwidths due to the increase of 60°corners,with a forward transmittance of 0.75 at the peak wavelength of 680 nm.(4)The back-scattering of slow-light waveguides is a key problem that affects the insertion loss of the device,especially when the back-scattering becomes more obvious near the slow-light region.Based on the back-scattering suppression and anti-scattering transmission properties of the edge state of valley photonic crystal,a slow-light transmission waveguide structure of hBN photonic crystal is proposed.By changing the radius and refractive index of dielectric columns at the edge state of the valley photonic crystal,the adjustment of slow light wavelength in visible band(645 nm-671 nm)is realized.The Ω-type slow-light waveguide valley photonic crystal structure is designed with a size of5 μm × 2.7 μm.The structure has an optical delay transmission distance of 5.75 μm,a group refractive index of 629 at the slow-light wavelength of 645 nm,and an optical delay time of12 ps.The structure achieves unidirectional transmission in visible band(636 nm-646 nm)with a forward transmittance higher than 0.68 and a forward transmittance of 0.75 at the peak wavelength of 650 nm.(5)Due to the problems of interference of back-scattering light and the difficulty of coupling with the light source in micro-ring-resonators,a two-dimensional hBN valley photonic crystal ring-resonator and a bandpass notch filter structure are proposed.The number and position of resonance peaks in the topological bandwidth are tuned by changing the length and shape of the ring-resonator.The number and position of resonance peaks in the topological bandwidth are tuned by changing the length and shape of the ring-resonator.A single-ring bandpass notch filter structure consisting of a topological ring resonator and a topological straight waveguide is designed,and the effect of the distance between the ring-resonator and the straight waveguide on the optical wave coupling effect is investigated.Further,a dual-ring bandpass notch filter structure is formed by combining different lengths of ring-resonators and straight waveguides to achieve a zero transmittance of the notch wavelengths.The single-ring bandpass notch filter structure achieves unidirectional transmission in visible band(604 nm-645 nm)with a forward transmittance of 0.9,while filtering out the wavelengths of 611 nm and 626 nm with a quality factor of 679.3 at the notch wavelength of 611 nm.The design of this structure expands the application of unidirectional transmission devices in integrated optical circuits in visible band. |
