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Emission intensity from terahertz quantum cascade lasers under near-infrared optical pulse injection

We have investigated intensity change of terahertz emission from terahertz quantum cascade lasers under optical injection of near-infrared femtosecond laser pulses. THz intensities were suppressed by optical injections and their extinction ratios were dependent on the time delay of optical pulses. Recovery times from the suppressed states to the pre-excited states, for interband and intersubband excitations, were 40ns and 400ns, respectively.