The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.
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