Opt. Express 22, 274–282 (2014)
Michael Krall and co-workers from Vienna University of Technology in Austria have demonstrated a micropillar-array terahertz laser with a pillar size that is much smaller than the emission wavelength. A bulk heterostructure with a 10-μm-thick GaAs/Al0.15Ga0.85As quantum cascade structure designed for an optical gain of around 4 THz is etched to become a pillar array by reactive ion etching. The 5-μm-diameter micropillars have a filling factor of 0.75 and are arranged in a hexagonal lattice with array sizes up to 1,000 × 120 μm. Lasing emission is observed up to a maximum temperature of 55 K. Because the lattice periodicity is significantly smaller than the emission wavelength, the micropillar array can be considered a homogeneous effective medium rather than a photonic crystal. Consequently, the array resonator is not defined by the distributed feedback of a photonic crystal; instead it operates as a Fabry–Pérot resonator defined by the boundaries of the array. The observed threshold current density of the laser is approximately 0.65 kA cm−2. As each pillar can be pumped individually, the researchers claim that the pumping geometry can be changed dynamically.
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