| Quantum Cascade Laser(QCL)is a unipolar semiconductor device,utilizing electron transitions between sub-bands coupled by multiple quantum wells,the wavelength of which covers mid-,far-infrared and THz region.There are many characteristic absorption peaks of molecules within long-wave infrared spectrum range,which is called ‘fingerprint' region of molecules.Besides,the atmosphere windows of 8-14 ?m is also within this spectrum range.The development of long-wave infrared QCLs is very important for the gas sensing,free space optical communication and directed infrared countermeasure.To date,watt-level outputs at wavelengths in the middle-wave infrared(MWIR,?= 3-5 ?m)range have been obtained.However,because of the limitation of the intrinsic technological characteristics of long-wave devices(such as increased free-electron optical losses at longer wavelengths,the lower intersubband gain,the decreased optical confinement),the progress of LWIR QCLs had been slower than that of the MWIR QCLs.This work focuses on improving output power and optimizing beam quality of QCLs by active design and device structure design.The main results are summarized as follows:1.Based on the heat-conduction equation(Fourier's law),the thermal properties of QCLs were studied using the finite element method.The effects of different device parameters on thermal properties were investigated,and the optimal device configuration with best heat-dissipating capacity is obtained which provide fundamental basis for improving the device heat-dissipation.2.Based on design demands,the secondary epitaxial technique used in quantum cascade materials was systematically studied,and MOCVD growth including intrinsic and semi-insulating In P epitaxy has been investigated.Based on the ??8.3 um epitaxial wafer,a buried heterostructure FP quantum cascade laser was fabricated.Compared to the traditional structured,the optimized laser has a continuous output optical power of 520 m W.3.A high performance long wave infrared quantum cascade laser based on the bound-to-continuum active region is designed.The laser presented here exhibits a full-width at halfmaximum of electroluminescence spectra as large as 44 me V,which is well suited for applications of a long wave infrared laser that requires wide tuning range.For a 5 mm-long and 8 ?m –wide buried heterostructure laser,continuous wave output power of 401 m W at 283 K and 108 m W at 323 K have been achieved with the threshold current densities of 2.03 and 2.85 k A/cm2,and the lasing wavelength is ?8 ?m.4.An In P-based room-temperature high-average-power quantum cascade lasers emitting at 14 ?m is designed.Using a novel active region design,a diagonal bound-to-bound lasing transition is guaranteed by efficient electron injection into the upperlaser level and fast nonresonant electron extraction through a miniband from the lower laser level.For a 4 mm long and 40 ?m wide double channel ridge waveguide laser with 55 stages of the active region,the threshold current density is only 3.13 k A/cm2 at room temperature.At 293 K,the maximum singlefacet peak power and average power are up to830 m W and 75 m W,respectively.The laser exhibits a characteristic temperature T0 of 395 K over a temperature range from 293 to 353 K.5.A Littrow type EC-QCL configuration was implemented.In order to suppress the Fabry-Pérot(FP)modes inside the QCL cavity,a two-layer AR coating was designed for our EC-QCL.The two layers coating consisting of Al2O3 and Zn Se can lower the front facet reflectivity,making the tuning range of our EC-QCL increase by 62%,from 36 cm-1(13.57-14.26 cm-1)to 58 cm-1(13.26?14.35 cm-1)in pulsed operation. |
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