We report the observation of ultrafast intensity oscillations in the time resolved optical emission from a semiconductor microcavity resonantly pumped with intense femtosecond pulses. The observed oscillations are pronounced, persist for several picoseconds, and have frequencies varying from 1.4 to 2.7 THz, depending upon the incidence angle of the pump beam. The angular dependence and spectra show that the oscillations result from interference between the pump and surface-normal modes of the microcavity.

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On the assumption that the portion of the power absorbed in the sample can be determined from the difference between the incident and the reflected spectra (about 7%), we estimate a carrier density of ≈4.8×1012cm−2 per QW for an incident power of 20 mW with a focus diameter of 15 μm. This value is clearly high enough to provide gain inside the cavity. However, no clear evidence of laser action was observed under the conditions where the emission intensity shows strong oscillations.
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The cryostat and the experimental system limited the angle of incidence to <27°, and led to the highest observed frequency of ≈2.7 THz. A more intrinsic limitation would occur as a result of the decreasing reflectivity of the Bragg mirrors at large incident angles.
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These results imply that the frequency of the oscillations should be independent of the center frequency of the incident laser pulse.
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Our measurements of the emitted intensity on direction of polarization of the pump beam shows that the polarization direction of oscillating emission is the same as that of the pump beam. These results suggest that the oscillation frequency and direction of polarization are determined by the polarization of the pump pulse, which eliminates a picture of simple relaxation oscillation of lasing action in the microcavity.
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