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(Top) Heterodyne detection scheme. AOM: acousto-optic modulators. (Bottom) Sketch of sample structure.
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We have compared the gain dynamics of the ground state excitonic transition between undoped and p-doped electrically-pumped InGaAs quantum-dot optical amplifiers, for temperatures from 300 K to 20 K. A pump-probe differential transmission technique in heterodyne detection with sub-picosecond time resolution was used. The comparison shows that in th...
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Context 1
... gain (and refractive index 10 ) dynamics are measured using a pump-probe differential transmission technique in heterodyne detection, described in detail in Refs. [11,12]. Briefly, 100 fs Fourier-limited laser pulses at 76 MHz repetition rate in resonance with the GS transition (see spectrum of the optical pulse in Fig. 2) are divided into pump, probe and reference beams (see sketch in Fig. 1). Pump and probe pulses are coupled into the transverse electric waveguide mode with a relative delay time τ p (positive for pump leading) and the transmitted probe is detected using a heterodyne technique. For this technique, probe and pump beams are shifted by a radio-frequency amount using acoustic optical modulators. The probe transmitted through the device interferes with the unshifted reference pulse and is detected at the corresponding radio-frequency by two balanced photodiodes and a lock-in amplifier. 13,14 Due to the interferometric detection, we are sensitive to both amplitude and phase changes of the transmitted probe field induced by the pump ...
Context 2
... investigated samples are p-type-intrinsic-n-type (p-i-n) ridge waveguide diode structures of 4 μm width and 0.5 mm length containing 10 InGaAs dot-in-well layers, with a dot GS transition wavelength around 1.3 μm at room temperature, separated by 33 nm GaAs spacers and embedded between 1.5 μm thick AlGaAs cladding layers (see sketch in Fig. 1). p-doping near the QDs is achieved by incorporating a 10 nm thick region of carbon-doped GaAs in the spacer layer, ending 9 nm below each dot-in-well layer. We estimate a doping level of ∼ 8 (p-doped sample) acceptors per dot, while a second sample had undoped GaAs spacers. All samples were processed with tilted facets (∼ 7 • ) to avoid back-reflections into the waveguide mode and lasing. 4 From amplified spontaneous emission spectra, as shown in Fig. 2, we have inferred on both samples an inhomogeneous broadening of the GS transition of ∼ 36 meV due to fluctuations in dot size and alloy composition and an emission from the first optically-active excited state (ES) transition at about 60 meV above the GS transition. In Fig. 2 two spectra are compared for the undoped and p-doped devices at an injection current corresponding to the same GS modal gain Γg = 14 cm −1 . The comparison clearly shows a suppression of the ES emission in the p-doped sample, similar to what reported in Ref. ...
