We passivate the surface of the InGaAs quantum well gain medium and GaAs membrane in a photonic crystal nanocavity laser through a (NH$_4$)S treatment. The passivated laser shows a five-fold reduction in surface recombination loss, resulting in a four-fold reduction in the laser threshold. A three-level laser model explains the results well and shows that for this material system, surface recombination losses are as important as cavity $Q$ in determining the lasing threshold. Surface passivation therefore appears vital in operating such lasers under practical conditions.
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