1.5×1.5mm quantum cascade laser has a surface emission of 5W
Their paper “Large area photonic crystal quantum cascade laser with 5 W surface-emitting power” published in the Optics Express journal explains how they improved their buried heterostructure and buried grating technique through several design tweaks, notably by optimizing current injection using a grid-like overlay electrode.
showing the square lattice of active-region (InGaAs/AlInAs)
pillars (red) surrounded by semi-insulating InP with a lower
refractive index. The shape of the pillars is illustrated in the
inset at the left-top corner.
The large-area PhC-QCL consists of an array of square pillars (over 500 periods in each in-plane dimension), comprising a 2.6μm-thick active region based on a strain-balanced InGaAs/AlInAs structure. One peculiarity of the design is that the top-view shape of each PhC pillar is a square with a missing corner, which the authors explain, creates an asymmetry that enhances the surface power extraction. The empty space of the PhC layer is filled with InP:Fe building the PhC layer with an index contrast between 3.08 (InP) and 3.35 (InGaAs/AlInAs). After growing an InP:Si cladding layer on top, the laser cavity is defined by wet-etching the cladding layer into 1.5×1.5mm square mesas, with SiNx deposited on the boundaries of the mesa to create absorbing boundaries.
While the epi-side of the laser is fully covered by a 4μm-thick gold contact, the window contact is initially a square window with a width of 100μm and a thickness of 4μm, but here an additional grid-like gold contact only 268nm thick is deposited on top of the former to optimize the current injection. The 1.5×1.5mm device was demonstrated to operate at room temperature at a wavelength of 8.75μm and a maximum peak power up to 5W.
dry-etching, before HVPE regrowth.
The authors aim to further improve power output thanks to a more powerful electrical driver and by improving the regrowth planarization process, but also by experimenting with other PhC pillar shapes less sensitive to fabrication imperfections. The device didn’t have any anti-reflection coating on the substrate surface. The paper concludes that although the PhC-QCL process used is far from being mature, the laser already competes with other state of the art devices in the mid-infrared region.
ETH Zurich – ethz.ch/en.html
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