Study of Optical Gain and Carrier Dynamics in Quantum-Dot Laser Structures

Quantum-dot lasers have attracted much interest in recent years due to their
superior properties, such as low threshold current density, high optical gain and high
characteristic temperature. Several approaches have been reported in the literature to
measure the gain spectra of semiconductor lasers. Henry’s method deduces the gain
indirectly from the spontaneous emission spectra under some assumptions. Using
Hakki-Paoli method, the gain is calculated from the contrast of modal spectrum
oscillations due to the Fabry-Perot cavity below the threshold. A major drawback of this
method is the requirement of a high-resolution spectrograph to obtain the true contrasts.
In this thesis, net modal gain and absorption spectra were measured by a variable
stripe length (VSL) method for an electrically pumped multisection device. This
measurement is not limited by the lasing threshold and can be applied even at high
excitation densities. Meanwhile, a modified segmented contact (MSC) method is
implemented by manipulating the data from single, double, and triple biased sections,
respectively. The new approach subtracts background signals from the unguided
spontaneous emission, resulting in clean, accurate gain spectra. Besides, the inaccurate
estimation of injection current due to current leakage through the finite gap resistance
between two adjacent sections is minimized by connecting the unused sections to the
ground. The exact solutions of MSC method were also calculated and applied to
estimate the magnitude of unguided spontaneous emission of each section.
On the other hand, high frequency response and large modulation bandwidth are
the other superiorities of quantum dot lasers and amplifiers. Ultrahigh bit rates in the
amplifier require ultrafast gain recovery, which is mainly limited by carrier capture and
relaxation process in quantum dots1. Time resolved pump-probe experiments were carried out under room temperature in order to understand the carrier dynamics, such as
carrier life time, capture time, and relaxation time of our devices. Using optical
excitation into GaAs barrier through the window on the top of the device by two
degenerate pump and probe laser pulses, we can obtain the carrier life time under
spontaneous emission or stimulated emission from semiconductor optical amplifiers or
laser devices, respectively. The gain saturation and state filling phenomena were also
been observed in our experiments.