The transient response of short-cavity semiconductor lasers excited by 250 fs optical pulses is investigated. The emission is time-and spectrally-resolved by an up-conversion technique. The experimental evidence is compared with simulations employing a simple resonator model, which allow for a comparison of the energetic position, width, and intensity of the laser emission as a function of time. The time scale of the lasing process is governed by heating due to stimulated emission. Caused by the changing carrier distribution functions, inducing changes of the refractive index, the resonance frequency of the cavity experiences a rapid red shift in time, which cannot be followed adiabatically by stimulated emission. This transient mismatch qualitatively explains the observed temporal evolution of the linewidth.
ExperimentThe short-cavity semiconductor lasers (SCSL) in our experiment consist of a 1 pm thick layer of bulk In,Ga,-,As lattice-matched to an InP substrate. After the latter has been removed in an area of z 1 mm', dielectric mirrors with a reflectivity of ~9 9 % at 1.55 pm wavelength, each one consisting ofeight pairs of A/4 layers of SiO, and Ta,O,, are evaporated on either sides. Only one longitudinal mode around a photon energy of 0.83 eV is active, the adjacent longitudinal modes are already outside the spectral regime of high reflectivity of the mirrors [l].The optical excitation at 1.56 eV photon energy takes place within the transparency region of the dielectric mirrors. We use transform-limited, Gaussian pulses with a duration of 250 fs which are focused to a spot diameter of about 30 pm onto the sample held at room temperature. The SCSL emission is inspected on an infrared camera system and exhibits a Gaussian profile for high and low excitation levels, indicating not only longitudinal, but also transverse single mode operation. In order to study the temporal dynamics we up-convert the SCSL emission with a fraction of the pump light in a 0.5 mm thick crystal of LiIO,. The sum-frequency is detected with a spectrometer connected to a CCD camera. The spectra are recorded as a function of time delay between excitation and reference pulse in steps of 0.5 ps.Data for excitation slightly above (Fig. la, pump energy 2.5 nJ) and well above the threshold energy of ~2 nJ (Fig. lb, pump energy 15 nJ) are depicted in Fig. 1. For low excitation the SCSL emission exhibits a rather large time delay of more than 15 ps with respect to the pump pulse arriving at t = 0. Both, rise and decay extend over several ps showing a small red shift of the laser line with time. This scenario gradually changes towards higher excitation. At very high excitation levels the SCSL emission exhibits a sharp (400 fs ') Otto-Hahn-Str. 4, D-44221 Dortmund, Federal Republic of Germany.
