Spectroscopic studies of optical and electronic processes in quantum cascade lasers

• Main Author: Hassan, Randa Saad Ali
• Published: University of Sheffield 2012
• Subjects: 621.366
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.575367

In this thesis, two sets of quantum cascade lasers were studied. The first set is three Ga0.3In0.7As/A1As(Sb)/lnP strain compensated QCLs emitting at 3.3 μrn at room temperature. The three QCL samples have the same active region design but differ in their waveguide doping. The losses within the samples were investigated by the Fabry- Perot fringes contrast technique which provides probing the QCLs with both transverse electric (TE) and transverse magnetic (TM) polarizations for measuring the waveguide losses and the resonance intersubband absorption respectively. We found that reducing the waveguide doping slightly, results in a reduction in the waveguide losses by more than 50%. Additionally, when the injector doping is reduced, the resonance absorption within the QCL structures is reduced too. The second set of the studied quantum cascade lasers is three Gao41n06As/ Alo58Ino42As strain-compensated on InP substrates emitting in the vicinity of 5 urn. The three lasers have different active region designs. Those active regions are double phonon design, single phonon design and zero phonon design. The main difference among those designs is their extraction rate. The double phonon design has the most efficient extraction while the zero phonon design has least and the single phonon is in the middle. The three lasers show performance with the same order as their active region extraction efficiencies which means that the laser with the active region design having the most efficient extraction shows the highest performance. A comparative investigation of those samples was performed by using the broad-band mid infrared transmission technique. This technique enables the mid-infrared spectroscopic study of the electronic distribution in quantum cascade lasers whilst under operating conditions. Using the TE polarization, the waveguide of the samples was investigated in a wide range of wavelength (1.2-15 μrn). On the other hand, the TM polarization was used for investigating the electron distribution in a range of temperature and bias condition by probing the intersubband transitions in the QCL devices. The gain build- up, the resonance intersubband absorption evolution and the transparency current were also investigated in a range of temperature and bias for the three samples. The dispersive gain was observed for all the samples at high temperatures however for the zero phonon design it commenced at a temperature as low as 80 K. A new experimental method for measuring the transparency current from the transmission spectra is introduced. The transparency current measured contributes about 70% to the threshold current for our samples. The analysis of the obtained transmission spectra also revealed that gain onset is achieved at a certain voltage value for all the temperatures which we call "voltage threshold". The electron temperature for the zero phonon design was also predicted from the analysis of the obtained transmission spectra. It was found that the electron temperature measures about a 100 K higher than the lattice temperature.