Research On Terahertz Modulators With Surface/Interface Enhancement

Terahertz radiation with various unique properties has attracted widespread attention owing to its prospective applications in many fields such as material identification,security inspection,non-destructive detection,biomedicine,radar and communications,etc.Although the technology of terahertz wave generation and detection is increasingly matured and even commercialized,however due to the lack of natural materials that can effectively respond to this special radiation,the development of terahertz modulators is relatively lagging behind,consequently limiting its further actual applications.Nowadays,there have been a diversity of novel materials,structures and techniques developed to improve terahertz modulators’performance,but how to simultaneously realize high-efficiency,low-loss,high-speed and broadband terahertz modulation with easily fabricated and cost-effective devices is still on the way.This dissertation focuses on the interaction of terahertz wave with surface/interface-enhanced semiconductors and then performs corresponding researches on theory,materials,devices,and applications based on carrier concentration modulation to regulate the amplitude of transmited terahertz waves.1)Starting with the general equation of carrier dynamics in semiconductors,the modulation mechanism of all-optical semiconductor-based terahertz modulators was explored.Combining with Drude model and Fresnel transmission matrix method,the corresponding theoretical model was built and simulated to investigate device parameters’influence on modulation performance including the semiconductor thickness,carrier lifetime,pumping wavelength and laser reflectivity.It proves the decisive role that a long effective carrier lifetime plays on the enhancement of modulation performance in theory.A definitive idea is suggested that prolonging carrier lifetime and suppressing pumping reflection can sygnergistically enhance the efficiency of all-optical Si-based terahertz modulators.This simulation result also provides a theoretical foundation for further investigating the enhancement of modulation performance in these semiconductor-based modulators hybriding with heterogeneous coatings.2)A novel mechanism“surface passivation”was proposed to interpretate the enhanced modulation observed in semiconductor modulators based on heterostructures,and claimed that it is intrinsically orginated from the reduced recombination and lengthened lifetime of carriers as a consequence of Si’s surface passivation by the heterogeneous coatings.As a proof of concept,two Si-based terahertz modulators with surface passivators of Si N_x and Si N_x/Si O₂overlayers were designed,respectively.By exquisitely design the thickness of these coatings,longer carrier lifetimes and extremely low pumping reflection were simultaneously observed in these proposed modulators,both of which contributed to a remarkable improvement in modulation depth and decline in required laser power.Especially for the Si N_x/Si O₂-coated one,under a low photodoping of～0.7 W/cm~2,as high as 97%modulation depth was obtained in a wide frequency range from 0.2 THz to 1.0 THz.This result successfully certified that surface passivation has a critical impact on determining the modulation performance of these devices,which offers a universal explanation for the mechanism of enhanced modulation in such semiconductor-based hybrid structures.3)To solve the inherent drawbacks such as high insertion loss and low pumping laser utilization in Si-based modulators due to the abrupt change of refractive index at the air-Si interface,an improved modulator architecture based on Si surface-texturing and surface-passivation was designed.Traditional photolithography and wet chemical etching technique were first adopted to produce homogeneous microtextures.Benefitting from the anisotropic property of alkaline etching,these microstructures are regular truncated pyramids of subwavelength size,named as TPA,on one side of Si wafer.This TPA functions as an inhomogeneous antireflection design,which was adopted as the incident side of terahertz radiation to enhance the zero-order diffraction and the intial transmission of terahertz radiation.Then,Si N_x/Si O₂ overlayer producted by PECVD was coated onto another surface of Si to act as the pumping side,which benefits for harvesting pumping laser and enhancing modulation efficiency.Consequently this micro/nano hybrid structure shows a decreased loss of 0.51-d B in a broadband of 0.3 THz～1.0 THz and an increased modulation depth of>90%.Especially for a special sub-band of 0.84 THz～0.91THz,this value could be further enhanced to 95%,whilst the loss was declined to～1 d B with the initial transmittance high to 79%.This modulation architecture with low loss but high efficiency is of great significance for the current situation status that the output power of terahertz sources is still rather limited.Moreover,this low-reflection characteristic also benefits to eliminate the interference and noise of reflected terahertz radiation to terahertz systems,hence improving the stability and reliability.4)Considering the fact that Si and Ge have a relatively long carrier lifetime which will limit the response speed of modulators,Ga As whose carrier lifetime as low as ns or even ps was proposed to realize high-speed spatial terahertz modulation.Also to eliminate the dependence of Ga As-based devices on intense laser,surface passivation was participated in.Passivated by sulfur,this Ga As-based modulator realized a comparative modulation depth（～95%）to its Si/Ge-based counterparts but rather faster response speed（69-MHz）,which is also the highest one that have been reported in all-optical terahertz spatial modulators.To a certain degree,this study solves the bottleneck of the mutual restriction between modulation depth and rate in semiconductor-based all-optical terahertz modulators,which demonstrates a new strategy to alleviate the compromise between high modulation depth and speed.5)As an extension,this dissertation also explored how to realize a flexible terahertz modulation.A flexible GFET-based THz modulator was developed with the graphene as the modulating layer,ion-gel as the gate dielectric layer and PET as the substrate.By coating graphene with high-κdielectric DMSO and forming DMSO-GFET,the intrinsic modulation depth of graphene was increased by～2 folds.By stacking two DMSO-GFET structures onto both sides of PET,as high as 70%flexible terahertz modulation was obtained.By exquisitely integrating GFET and DMSO-GFET to each side of PET,a 4-level broadband terahertz amplitude modulation was observed.These studies demonstrate that this special GFET modulating structure can be used as a basic element or block to build more complex and multifunctional THz devices.