| Terahertz quantum cascade laser(THz-QCL)presents high performance in terms of high output power,narrow linewidth,tunable frequency and high integration,which shows important roles in the applications including nondestructive testing,security inspection,microscopy,biomedicine and communication.Therefore,THz-QCL is an ideal terahertz source.With the development of the terahertz applications,the requirement of the high performance,integrated and multifunctional THz-QCL becomes increasingly urgent.The thesis focuses on the terahertz master-oscillator power amplifier quantum cascade laser(THz-MOPA-QCL).The generation and radiation of the THz wave in the master-oscillator power amplifier(MOPA)structure are segregated,thus the MOPA structure is a perfect platform to control the frequency,amplitude,polarization and orbital angular momentum of the THz wave.The thesis built the model to describe the gain saturation of the THz-MOPA-QCL and analyse the power amplification,and finally improved the output power and the beam quality.On this basis,the terahertz quantum cascade amplifier(THz-QCA)that can amplify the free space THz wave is proposed,in which the grating couplers are adopted instead of the cleaved facets.The THz-MOPA-QCL with controllable polarization has been realized,in which the 2D antenna array acted as the coupler.Finally,we realized the THz-MOPA-QCL that emits the vector vortex beam directly,in which The ring resonator integrated with periodicity scatters can modulate the orbital angular momentum of the emission.The representative innovations are as follows:1)The thesis built the quantitative model of the THz-MOPA-QCL based on the carrier rate equation of the QCL,which described the gain saturation of the THz-QCL and predicted the influence of the material and structure parameters on the output power of the QCL.The agreement between the calculated and experimental results certified the validity of the quantitative model.Guided by the model,we analyzed systemically the power amplification in the THz-MOPA-QCL and proposed the design rules of the MOPA structure.Experimentally,the demonstrated THz-MOPA-QCL realizes single mode emission with a side-mode suppression ratio of 23dB,the peak output power achieves 153 m W,the wall-plug efficiency is 0.22% and the divergence angle is～6°’16°,all measured in the pulse mode at 77 K.2)The thesis proposed the terahertz quantum cascade amplifier(THz-QCA),which is capable of amplifying the free space THz wave.The THz-QCA consists of the receiver,the amplifier and the emitter.The receiver and emitter adopt the diffraction grating instead of the cleaved facets,which exhibit high coupling efficiency and low reflectivity,and thus suppress effectively the self-lasing and increase signal intensity.The preliminary experimental results show that,although the THz-QCA exhibits selflasing,the threshold current density of the THz-QCA is significantly higher than that of the FP cavity laser.3)The thesis realized the THz-MOPA-QCL with accurately controllable lasing frequency and polarization state.The MOPA device consists of a first-order distributed feedback(DFB)laser,a preamplifier and a 2D antenna array.The first-order DFB laser acted as the seed source adopts the buried grating structure.The each unit of the 2D antenna array contains one or two magnetic dipoles,and the polarization of the emission is determined by the orientation of the antennas and the phase difference between them.The optimized 2D antenna array exhibits high radiation efficiency and low reflectivity,which avoids to inducing field oscillation in the array or influencing the mode oscillation in the DFB section.Each demonstrated device exhibits single-mode emission with a side mode suppression ratio of 26 d B and a single-lobed beam with a low divergence of ～23°’30°.Realized in different devices,the degree of linear or circular polarization reaches as high as 97.5% or 99.3%.Both the operation frequency and the polarization state of the radiation are lithographically tunable.4)The thesis realized the THz-MOPA-QCL with controllable orbital angular momentum(OAM),which consists of a first-order DFB laser,a buswaveguide and a ring resonator.The first-order DFB laser acted as the seed source,the THz wave is coupled into the ring resonator through the buswaveguide.The ring resonator integrated with periodicity arranged dual-slits acted as scatters,in which the fully destructive interference between the reflected wave caused by the dual-slits reduces effectively the reflectivity of the ring resonator,ensures the unidirectional propagation of the THz wave and thus extracts the THz vector vortex beam with high OAM mode purity into the free space.The demonstrated devices exhibit single mode emission,operate on the DFB mode,and the far-field pattern is the hollow ring.The topological charges of the left circular polarization component and the right circular polarization component are respectively +3 and +1,and the OAM mode purities of these two components are 61.5%and 85.5%,respectively.At the last,the thesis discussed the approaches to further improve the performance of the THz-MOPA-QCL,and realize more accurate control of the amplitude,polarization and phase of the THz wave. |
