| Born the gap between microwave electronics and infrared photonics,terahertz(THz)band is the one last electromagnetic spectrum being visited and has shown many superior features.The generation,propagation,modulation,together with detection forms the key nodes of terahertz science,which plays arising roles in many important fields such as high-speed broadband communication,homeland security,medical diagnostics,biological sensing and material research.However,due to the low energy of terahertz photons,most natural materials respond poorly to it,which results in the lack of highly efficient and active terahertz devices.Meanwhile,the generation and detection system of THz radiation is also faced with a lot of imperfections;to explore in the frontiers of THz science,it's essential to build a home-made THz generation-detection system with high performance and also great extensibility.In this thesis,firstly,the tunable THz active device is investigated.A novel method of active modulating the THz radiation is proposed and then demonstrated by analyzing the performance of a prototype following the proposed method.Secondly,the design and construction of two sets of the THz generation and detection system is introduced,together with their applications in the material science.Here,the main work and conclusions of this thesis are listed below:1.We proposed a novel method of active modulating THz radiations based on the utilization of graphene under simultaneously applied magnetic field and gate electric field.This graphene-based dual field method relies on the fact that graphene possesses an unequally spaced Landau levels system in the presence of magnetic field,therefore by tuning the optical transitions between different Landau levels,the frequency-agile absorbance of THz photons can be realized.Hence,by choosing a proper combination of magnetic field and gate electric field,the devices following our method can actively modulate THz radiations in a great freedom.2.We designed an active THz modulator prototype to demonstrate our proposed graphene based dual field method.The simulation results confirm the excellent frequency agility of the prototype,which can modulate THz radiations on four individual channels with relative modulation depth exceeding 35 dB,clearly demonstrating the great frequency tunability of our proposed method.Moreover,the energy required in switching between the four channels is down to 10 meV,which demonstrates our proposed method the extreme modulation efficiency.3.By introducing the surface plasmon polaritons(SPPs)into an active graphene-based dual field THz transmission modulator,we successfully enhanced the modulation depth of the device.The simulation results clearly shows an enhanced amplitude modulation depth of 95.1%.The underlying mechanism is the near field enhancement between the graphene layer and the metallic complementary split ring resonators(CSRRs)layer which is inserted to excite the SPPs.Moreover,this method can also be used to enhance the polarization modulation of the device.A switch from linearly polarization state to circularly polarization state is then demonstrated.4.We designed and constructed two sets of THz time domain spectroscopy system.The first one is based on the photoconductive antenna technique and featured with broadband spectrum,high SNR and great stability.The system is working between 0.1 THz and 4.8 THz,with SNR of 30000:1 in the time domain,dynamical range of 90 dB in a single scan,and frequency resolution of 5GHz.Compared with the other reported TDS systems,our TDS system is among the best.The second one is based on the optical rectification effect and is featured with high extensibility and high THz pulse energy.In future studies,by replacing different nonlinear crystals,this TDS system can realize an ultra-broadband spectrum coverage and sub-mJ level THz pulse.Moreover,two working modes are realized in both the TDS systems:fast scan mode establishes one scan in only 2 seconds and precise scan mode measures the THz signal with SNR up to 30000:1 and frequency resolution of 5GHz.5.We applied our TDS systems to measure the THz dielectric response of several typical Perovskite and layer-structured Perovskite oxides.Through the measurement,analysis and perception of those materials in the range of THz,we laid the ground work for both the high-throughput near-field material characterization in THz regime and the research of quantum functional oxide materials.Specially,we found that the La0.7Sr0.3MnO3 thin film is of both good conductivity and well THz transmittance,which makes it to be a new transparent electrode material in the THz range.6.By utilizing home-made temperature controlling module we upgraded one THz-TDS into a temperature-controlled THz-TDS system.The new system is then used to characterize the phase transition behavior of VO2 thin film in the angle of THz spectrum.Inversely,we also demonstrated active THz amplitude modulation on the VO2 thin film with a modulation depth of 84.6%,via temperature controlling.7.By utilizing the dynamic aperture technique and a 2D scanning stage,we endowed one THz-TDS with the ability of imaging beyond the diffraction limit.The new system successfully captured a 2D scanning image of a PCA sample with spatial resolution of 40 microns,which is about one eighth of the wavelength.The sub-wavelength imaging ability of our system is thus demonstrated.Moreover,we designed and fabricated a THz/visible dichroic beam splitter which does not absorb THz waves when being pumped by lasers.The beam splitter is based on the ultra-thin silver film grown on a Mylar flexible substrate,and can simultaneously reflect visible light and transmit THz radiation without any pump absorption.This beam splitter plays an important role in the dynamical aperture based THz near field imaging system. |
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