Phone numbersMid-IR Physics and Devices
Probing the electronic and optical properties of quantum cascade lasers under operating conditions
Researchers: Dmitry Revin and Michael Soulby.
A new intersubband transmission spectroscopy technique, which provides details of the electronic carrier distribution inside operating QCLs over the entire range of mid-infrared transition energies, is described. In these experiments, the broadband thermal "Globar" emission from the FTIR spectrometer was used as an incident light. This emission was precisely focused onto one facet of QCL under investigation and only the light transmitted through the waveguide was collected from the other side of the laser ridge and detected with cooled MCT detector. This novel technique allows spectroscopic study of light transmission through the waveguide of QCLs in a very broad spectral range (~1.5-12µm), limited only by the detector response and by the interband absorption materials used in QCL core region. Waveguide transmittance spectra have been studied for both TE and TM polarisation, for a range of InGaAs/InAlAs/InP and Antimonide-based QCLs with different active region designs emitting from 4.5 to 10.3µm. The transmission measurements clearly show the depopulation of the lower laser levels as bias is increased, the onset and growth of optical amplification at the energy corresponding to the laser transitions as current is increased towards threshold, and the thermal filling of the second laser level and decrease of material gain at high temperatures. This technique also allows direct determination of key parameters such as the exact temperature of the laser core region under operating conditions, as well as the modal gain and waveguide loss coefficients.
