Study On Terahertz Sources Based On Optical Rectification And Novel Terahertz Guided Wave Structures

Terahertz range refers to electromagnetic waves with frequencies betweenmicrowave and infrared. This range is the only spectrum in electromagnetic spectrum，which is not fully developed. The development of terahertz spectrum is one of the hotspot subject in current scientific and technological research. Terahertz technology,known as one of the10emerging technologies that will change the world, which hasfound applications with very important academic value in many different fields, such ascommunication, radar, security monitoring, imaging, bio-medical, and spectral analysis.Both terahertz radiation generation technique and terahertz guidied wave technique arethe most important terahertz technology; terahertz sources and terahertz guidied wavedevices are key components of terahertz systems. Therefore, in order to speed up thepace of the emerging terahertz technology and promote the development and utilizationof the terahertz spectrum, it is urgent needed for systematic and in-depth study on highefficiency and high reliability terahertz radiation sources as well as low loss and lowdispersion terahertz guidied wave structures. The area of focus in this dissertation willbe high power broadband terahertz sources and center frequency tunable narrowbandterahertz sources based on optical rectification methods, dielectric slab waveguides,parallel plate dielectric waveguides （PPDW） and graphene surface plasmon polaritons（SPP） waveguides. The main contents of this dissertation are concluded as follows:1. High power broadband （quasi-single-cycle） terahertz sources based on opticalrectification. First of all, the principles and the phase matching conditions of broadbandterahertz wave generation via optical rectification have been studied. Since the collinearphase matching condition in LiNbO₃during optical rectification process is not fulfilled,a tilted optical pulse front technique is used to achieve noncollinear phase matching.Then, a broadband terahertz wave generation setup based on discrete tilted pulse fronttechnique via Echelon is built. By using0.15mJ pump pulse, the generated terahertzpulse energy is0.012μJ and the optical-to-terahertz energy conversion efficiency isabout8×10-5. In order to enhance the conversion efficiency, another broadband terahertzwave generation setup based on continuous tilted pulse front technique via grating is built. With7mJ pump pulse, a quasi-single-cycle terahertz pulse is generated with pulsepeak field up to150kV/cm （at the focus point of off-axis parabolic mirror pair）, whichcorresponding spectrum with peak frequency of0.5THz, bandwidth of1.6THz. Thegenerated terahertz pulse energy is as high as7μJ and the energy conversion efficiencyis up to1×10^-3. Comparing to optical rectification terahertz generation based on normalpulsed front technique method, this continuous tilted pulse font technique has more than1000times lager in energy conversion efficiency.2. Center frequency tunable narrowband （quasi-multi-cycle） terahertz sourcesbased on optical rectification. The mechanism of narrowband terahertz wave generationby using Chirp-Delay method has been studied. Due to narrowband terahertz wavesgeneration setup usually suffers from instability in center frequency and difficulty inalignment, according to different optical path length will produce a particular relativedelay, an delay generator, called Etalon, with high phase stability is designed toguarantee frequency stability of the narrowband terahertz sources; by introducing amirror pair on translation stage, the amount of optical path length change of the gratingpair will be cancelled by the amount of path length change of the mirror pair, then anadaptive optical path length compensation parallel grating compressor is built, which isof great convenience for optical rectification narrowband terahertz wave generationsetup adjustment and experimental data collection. In this case, a LiNbO₃slabwaveguide is selected as an optical rectification crystal, and two Chirp-Delay schemes’setups with compressor in pump arm and compressor in probe arm are built. Theexperimental results show that both setups are able to generate terahertz wave efficiently,and the tunable center frequencies are range from0.25THz to1.2THz with bandwidthof100GHz.3. Terahertz guided wave structures based on dielectric slab waveguides. Thetransmission characteristics of three-layer dielectric slab waveguides have been studiedby using mode matching methods. The general expressions for guided modes’ fielddistributions, dispersion relation, power confinement factor and attenuation coefficientsare obtained. By using polarization gating imaging system, terahertz waves transmissioncharacteristics in LiNbO₃and LiTaO₃slab waveguides are tested. The experimentalresults are consistent with the theoretical results. Meanwhile, terahertz generation,transmission, detection and control on single electro-optic crystal chip have been investigated. Furthermore, a tapered slit array LiTaO₃slab waveguide structure with anair gap on the tip is proposed to reduce the fragility of the tapered structure sample andincrease the field enhancement. The experimental results show that this structure is notonly able to improve the mechanical stability, but also able to significantly enhance theelectric fields （about5.5times） and improve the power confinement of the terahertzwave in the air gap in the tip.4. Terahertz guided wave structures based on parallel plate dielectric waveguides（PPDWs）. In order to eliminate the poor cross-sectional electromagnetic fielddistribution of parallel plate waveguide， by taking advantage of the good fielddistribution of slab waveguides, a PPDW is proposed as an promising terahertz guidedwave structure. The investigation results show that this PPDW can be used to guideterahertz waves efficiently with broardband, low attenuation，low dispersion and highfield concentration, which can be fabricated and utilized easily. Afterwards, terahertzwave transmission characteristics of generalized parallel plate dielectric waveguide arestudied, and the general expressions for guided modes’ field distributions, dispersionrelation, power confinement factor, attenuation coefficients and characteristicimpedance are also obtained. Moreover, a terahertz coax-PPDW transition （during0.45⁰.75THz, S11<-20dB，S21>-0.7dB，insertion loss as low as0.18dB）, a3dBdirectional coupler based on bridged PPDW （with isolation better than27dB, couplerloss of0.9dB, center frequency of1THz, bandwidth of12.5%, compared toconventional PPDW coupler, this bridged PPDW coupler’s bandwidth is about4.2timesbroader, the coupling length is about61%shorter） and a broadband PPDW filter aredesigned, which may have potential applications for the terahertz integrated circuits andsystems.5. Terahertz guided wave structures based on graphene surface plasmon polaritons（SPP） waveguides. As we know, accurate electromagnetic characteristic parameters ofgraphene are the foundation of studying electromagnetic transmission characteristics ofgraphene waveguides. By using Kubo formula, the surface conductivity, equivalentcomplex permittivity and electrical tenability of graphene at terahertz frequencies areobtained. Meanwhile, a numerical simulation is used to verify the transmissioncharacteristics of graphene SPP with a good agreement. Then, several terahertz passivedevices such as splitters, reflectors, tapered structures based on single-atomic-layer graphene are proposed and simulated, which will lay solid theoretical foundation forgraphene based terahertz devices and systems.