Terahertz Quantum Cascade Laser Resonator:From Phase Locked Array To Dipole Antenna Array

Since the first terahertz quantum cascade laser(THz-QCL)was invented in 2002,it has become the focus of THz radiation source due to its high energy conversion efficiency,compactness,portability and easy integration.After nearly 20 years of development,THz-QCL has been greatly improved in terms of output power,maximum operating temperature,far-field quality,single-mode property and frequency tuning range.These advances not only benefit from the design of quantum cascade active region and the improvement of epitaxial growth technology,but also benefit from the exploration of THz-QCL waveguide structures and resonator structures.In order to improve the output power,far-field quality and single-mode property of THz-QCL,this thesis focuses on two technical routes: THz-QCL phase locked array and metal-metal waveguide dipole antenna array.Aiming to increase the maximum output power of single-mode THz QCL through the phase locked THz QCL array technology route,the research is as follows:1)Investigated experimentally and theoretically the mode competition and output power characteristics in first-order distributed feedback terahertz quantum cascade lasers(1st-DFB-THz-QCLs)based on a buried grating.The simulations obtain the relationships between the etching depth of buried grating and the waveguide loss,optical confinement factor,radiation loss and radiation efficiency of two band-edge modes.Experimentally,the relationship between the radiation wavelength of the laser and the period of the buried grating is linear and the devices work stably in the transverse and longitudinal fundamental mode in the whole dynamic range.2)Based on the forementioned work,a phase-locked laser pair containing two 1stDFB-THz-QCLs has been realized via evanescent-wave coupling.Investigated theoretically the thereshold condition and the mode competition of in-phase and out-of-phase supermodes in the phase-locked laser pair.Experimentally,a reliable singlemode phase-locked laser pair was achieved,with the peak output power of 8.06 mW,and an amplification factor of about 1.72.The spectra and far-field measurement confirmed the coherent phase-locked operation of the laser pair,and realized the emission direction with the highest brightness scaning from-10° to 10°by controlling the bias conditions of the laser elements.Aiming to get a THz-QCL resonator with high heat dissipation performance,high quality far-field,high radiation efficiency,and wide electrically frequency tuning through the metal-metal waveguide dipole antenna array technology route,the research is as follows:3)Combined with the theory of dipole antenna,the theoretical basis of single linear dipole antenna and dipole antenna array are discussed.The radiation angle and radiation loss of antenna structure are discussed.The electric field distribution and power flow characteristics of metal-metal waveguide dipole modes are studied,and then the characteristics of in-phase mode and out-of-phase mode in metal-metal waveguide dipole antenna array structure are investigated,reveal the reason why the two modes can not be used in THz-QCL resonator directly.4)Based on in-phase mode,a novel THz-QC-VCSEL resonator structure is designed,which combines divinylsiloxane-bis-benzocyclobutene(DVS-BCB)dielectric layer and metal mirror.The resonator uses the characteristics of the bidirectional radiation dipole antenna and combines the natural inversed phase relationship of the bidirectional radiation.The anti phase electric field is reflected back to the top of the dipole antenna through the metal mirror,which significantly reduces the radiation efficiency of the resonator and realizes the lasing of THz radiation.The simulation results indicated that the resonator can flexibly control the laser frequency and radiation loss,and has a single lobe far-field radiation spot.The simulation results show that the full width half maximum of the far-field spot can be as small as 6.5° ? 8°.A new type of boundary condition is proposed,and named as radiation boundary condition(RBC).The simulation results indicated that the RBC can reduce the power consumption of the device and improve the maximum operating temperature of the device.The BCB bonding and curing process of Ga As substrate is explored,BCB dielectric layer and mirror with precise thickness are constructed,and the process flow of THz-QC-VCSEL is designed and explored.5)Based on out-of-phase mode,a THz-QCL resonator with a non-uniform dipole antenna is designed.The relationship between the structure parameters and the lasing frequency and radiation loss is obtained by numerical simulation,which guided the experiment.A single-mode,vertical surface lasing THz-QCL is experimentally obtained.The device has a high quality far-field spot,and the full width half maximum can be as small as 4.5??16?.The structure tuning and power characteristics of THzQCL are investigated experimentally.The device with maximum output power of 33.2mW,power slope efficiency of 77.2 mW/A and electro-optical conversion efficiency of0.14 % % is obtained.Finally,the prospect and assumption of THz-QCL with high output power,high quality far-field spot and wide tuning range in continuous flow mode is proposed to further improve the performance of THz-QCL with vertical surface emission of nonuniform dipole antenna.