Sub-wavelength antireflection gratings on quantum cascade lasers

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Quantum Cascade (QC) lasers are used in many ways such as external cavity mode in the mid-infrared regime. They require at least one of the facets to have very low reflectivity in order to suppress the formation of coupled cavities and also enhance the output power. Although conventional thin film antireflection (AR) coatings have been used to reduce facet reflectivity, they tend to degrade under thermal cycling of the QC lasers. In order to alleviate this problem, we demonstrate the use of sub-wavelength gratings acting as antireflective structures. These gratings have two advantages, (i) they are etched into the QC laser facet and thus avoid adhesion problems (ii) they can, in theory, reduce the facet reflectivity to 0%. Because the sub-wavelength grating period is much smaller than the incident wavelength, it acts as a homogeneous medium. This allows us to combine the thin film theory with the effective medium approach to compute the grating parameters, such as the fill factor and the depth, that result in minimum reflectivity. These gratings were fabricated on &lgr; = 4.9 &mgr;m and &lgr; = 9.8 &mgr;m QC lasers using focused ion beam milling. The lasers were characterized before and after milling the gratings by measuring the light-current-voltage characteristics. A facet reflectivity of about 1-3% was determined from the theory fitted to data. Although this reflectivity is comparable to AR coatings on QCL facets, further optimization is possible.