Research On High Performance Photodetector Based On Subwavelength Grating

The rapid development of communication technology is one of the important driving forces for the profound change of information society.An optical communication technology with high-speed and large-capacity is always the goal of people’s pursuit.As one of the core optoelectronic devices in optical communication system,the research of photodetector has always been a hot direction in the field of optical communication.In order to meet the increasing demand of high-speed and large-capacity,the challenge of photodetector is increasingly severe.With the development of micro-nano structures,integrated optoelectronics have made great progress in materials,structures,processes and properties.It has been found that the integration with micro-nano structures can not only effectively improve the high-speed and high-response performance of the photodetector,but also realize the polarization,filtering,tuning and other characteristics.Planar dielectric subwavelength grating with high-contrast refractive index,as a typical micro-nano structure,has excellent beam-controlling ability and is of great significance for the development of integrated optical detectors.In this paper,we focus on the high performance photodetector based on SWGs.We studied the new optical phenomena and inside physical laws of the SWG,and clarified its light-beam control mechanisms.We also investigated the application of SWGs in photonic integration and optoelectronic integration.And in order to achieve the high speed,high quantum efficiency,narrow linewidth,and large tuning-range,we also researched on the integration of SWGs with photodetectors.The main innovations and research achievements are as follows:1.A TE-polarization subwavelength focusing grating lens is proposed.This grating adopts grating bars of two kinds of thicknesses,which can obtain richer phase selection,and is easier to achieve.A TE polarization focusing lens was designed with a focal length of 7.2μm,a transmittance of 72%and a half-maximum full width of 0.8 μm under 1550 nm incident light irradiation.However,it is difficult to design TE polarization focusing grating with one thickness under the same design requirements.In addition,the focusing mechanism of one-dimensional subwavelength focusing grating is investigated theoretically,and the focus shift phenomenon of subwavelength focusing grating is explained.Based on Fresnel edgediffraction approximation,a formula describing the intensity distribution of convergent light field is proposed,which can be used to characterize the focal shift.In order to guide the design of focusing grating,a fitting formula is proposed to predict the actual focal length.2.A flat transmitted serrated-phase(SP)high-contrast-index subwavelength grating beam splitter(HBS)is proposed.This beam splitter is formed by alternately arranging two kinds of grating bars with a phase difference of π.Compared to the typical linear-phase(LP)HBS,which consists of two symmetrical deflecting gratings,the SP-HBS is extensible in size,and can achieve excellent splitting ability regardless of normal incidence or small-angle oblique incidence with large deflection angles,higher diffraction efficiency,lower energy loss,and higher tolerance of fabrication accuracy.Furthermore,the incident light can be split in half at any part of the SP-HBS,and the output beams of light maintain the original shape.We designed an SP-HBS with a 44.8° deflection angle and a 90.28%transmissivity.3.A improved Fabry-Perot cavity based on symmetrical subwavelength grating deflected-reflector(SSWG-DR)is proposed.This resonant cavity can also be regarded as an equivalent multi-region F-P cavity by multiple mutual mirroring.It can not only achieve optical resonance with high quality factor,but also control the width of the oscillating light field and concentrate the oscillating light field in the middle of the resonant cavity.The resonance condition and the relationship between the width of the oscillating light field and the cavity length and the deflection-angle of the SWG-DR are derived.A SSWG-DR with a 1°deflection-angle is designed to form the SWG-DBR cavity.And by varying the cavity length(around 4,8,12,and 16 wavelengths),the oscillating light fields in different region is simulated and verified.4.A two-cavity-decoupled three-mirror two-cavity photodetector is proposed.This photodetector is based on the RCE photodetector structure.An reflector combined by the top mirror of filtering cavity and the bottom mirror of the RCE photodetector,take place of the middle mirror of the 3m2c structure,which forms the 3m2c photodetector.A 3m2c photodetector is designed and simulated,whose operation wavelength is 1550 nm,active layer thickness is 217 nm,quantum efficiency is 96.3%,and spectral linewidth is 0.12 nm.5.A one mirror inclined three-mirror cavity photodetector based on subwavelength grating(SWG-OMITMC-PD)is proposed.This photodetector structure is formed by replacing the SWG-DBR resonator with the absorption cavity structure in the 3m2c structure.SWG-OMITMC photodetector is based on another two-cavity decoupled mechanism,which can effectively relieve the mutual constraint between quantum efficiency,response speed and spectral linewidth,and achieve high speed,high quantum efficiency,narrow linewidth and larger wavelength-tuning range.A SWG-OMITMC photodetector is designed,its operating wavelength is 1550 nm,the thickness of the active layer is 400 nm,the quantum efficiency reaches 93.2%,the 3dB bandwidth is 33.5 GHz,and the spectral linewidth is 0.12 nm.Under the premise that the quantum efficiency is more than 80%,the wavelength-tuning range can reach 28nm.when at-3V bias,1550 nm working wavelength,32 μm mesa diameter and 400nm absorption layer,the inside PIN photodetector can achieve 33.5GHz 3dB bandwidth and 19%quantum efficiency.As for the SWG-OMITMC,its quantum efficiency is increased to 93.2%,the spectral linewidth is only 0.12nm,and the wavelength tuning range can reach 28 nm under the premise of ensuring the quantum efficiency is greater than 80%.6.A mushroom-mesa photodetector based on two-dimensional subwavelength grating is studied.The mushroom-mesa PIN structure reduces the junction capacitance of the device by reducing the area of the absorption layer,thereby improving the response speed.Then,by integrating a two-dimensional reflection-focusing grating,the unabsorbed incident light is reflected and concentrated on the absorbing layer,which can enhance light absorption and improve the responsivity of the device.The mushroom-mesa photodetector is designed and fabricated,and the fabrication process of mushroom-mesa structure is investigated.Then,the mushroom-mesa photodetector is integrated with a two-dimensional SWG.The mesa diameter of this device is 30 μm,the absorption thickness is 0.5μm,and the operation wavelength is 1550nm.The measurement results show that the 3-dB bandwidth of this device reaches 24.1 GHz under 3 V reverse bias voltage,which is 39.9%higher than that of the same size p-in photodetector.And the responsivity is 0.221 A/W and increased to 0.303A/W under the action of SWG.