Photonic Crystal Distributed Feedback Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are promising candidates as bright sources of spectrally pure, diffraction limited radiation in the mid- to far-infrared range. Standard Fabry-Perot QCLs can achieve very high peak powers with increasing cavity width. However, this increase in power is eventually limited by the appearance of multiple lateral lasing modes inside the cavity. As such, broad area lasers suffer from a wide emitting spectrum and a broad far field profile. One dimensional photonic crystal (1D PhC) structures can be integrated with lasers to improve their spatial or spectral properties. 1D Distributed Feedback (DFB) PhCs can be used to maintain lasing single mode at a Bragg wavelength determined by the grating period. However, 1D DFBs still suffer from multiple lateral modes and a broad far field as the cavity width is increased. Another approach, the 1D angled-grating DFB (&agr;-DFB), uses gratings positioned at an angle with respect to the cavity facet. The coupling of lateral modes with the longitudinal modes results in nearly diffraction limited output and suppression of higher order lateral modes. However, achieving single longitudinal mode output becomes a big challenge in 1D &agr;-DFBs. True single mode (both longitudinal and lateral) operation can be obtained for a broad area laser by combining both PhC types within a 2D photonic crystal distributed feedback (PCDFB) design. In the limit of weak index modulation, a 2D PhC can be considered as a 2D Bragg grating. The effectiveness of PCDFB design increases with a smaller linewidth enhancement factor (LEF). Owing to their intersubband nature, quantum cascade lasers generate TM polarized light with a very small LEF. Hence, quantum cascade lasers are very suitable candidates for realization of high performance PCDFB lasers.;In the first part of this thesis, the mathematical foundations of photonic crystal lattice design are presented. The novel features that two dimensional photonic crystals can offer are explained. Then mode selection processes and design merits for lasers with longitudinal and transverse Bragg gratings are presented for distributed feedback lasers accommodating weakly modulated 1D longitudinal and 2D photonic crystal structures.;In the second part, the realization of broad area QCLs with 2D photonic crystals are demonstrated. By using a 2D rectangular photonic crystal structure, the dependence of transverse mode selection processes with cavity dimensions is experimentally shown. By optimizing the photonic crystal structure and device dimensions, a total peak power of 34 W was obtained from a 400 &mgr;m wide, 2D PCDFB QCL with spectrally pure, nearly diffraction limited and normal to the facet emission at 4.36 &mgr;m under pulsed operation at room temperature. These results present the highest power obtained from a broad area laser with the desired nearly diffraction limited far field and spectrally pure spectrum.;The third part of thesis focuses on investigation of high power single mode DFB QCL lasers with 1D longitudinal gratings and narrow ridge widths under continuous wave operation at room temperature. DFB QCLs with first and second order buried gratings are demonstrated by using patterning with holographic lithography and MOCVD based regrowth. For an edge emitting laser design, implementation of the second order grating is used to realize complex coupled long cavity DFB lasers which are easier to fabricate and allows single mode stability at smaller |kappa|L values. Single mode 5 mm long DFB lasers emitting up to 700 mW at lambda = 4.8 &mgr;m under continuous wave operation at room temperature were demonstrated.;In the last part, a widely tunable continuous wave DFB QCL Array implementation was presented by using monolithically integrated DFB QCL bars with high reflection coated, dry etched back facet. Continuous wave single mode emission covering 200 nm range was demonstrated on such a single DFB QCL Array chip fabricated from QCL material whose gain curve was centered at 4.6 &mgr;m.