Simulation results and fabrication details are presented for a two-dimensional photonic crystal membrane defect cavity for use as a laser in gas sensing and communications applications in the 3–4 µm wavelength range. The design is based on a modified L3 cavity in an aluminium–indium–gallium–antimonide membrane structure which contains two strained quantum wells. Peak emission is predicted at λ = 3.372 µm with a Q factor of 26 233 for an optimized membrane thickness of 1000 nm. A focused ion beam procedure is described for creating the membrane directly in a Fabry–Perot laser structure. A 400 nm thick membrane is fabricated and the photonic crystal pattern is etched into the membrane.