Study of dynamics of mid-infrared quantum cascade lasers pumped by near-infrared light

Abstract

Exploiting the intrinsic ultra-fast carrier relaxation life time in quantum cascade laser (QCL) nanostructures, all-optical modulation has recently gained large attraction due to its ability to circumvent the problem of parasitic effects associated with electrical modulation, and thus, resulting in a high modulation bandwidth. All-optical modulation of QCL offers an unique way to control intersubband transition through interband transition induced by injecting near-infrared (NIR) light into the cavity. In this thesis, we have modeled the changed carrier dynamics of a QCL due to photo-excited carriers created by NIR absorption. Electron-hole pairs are created when the NIR light excites an electron from valence to conduction band, thus, carrier densities of the conduction subbands increase. The spatial distribution of the carriers gives rise to additional band-bending, which changes the carrier transport in the QCL heterostructure significantly. These extra carriers also decrease the effective refractive index of the cavity, which shifts the photon modes to higher values. Using our transport model for GaAs/AlGaAs and In- AlAs/InGaAs QCLs, we find that gain modulation depth of QCLs is proportional to injected NIR power. The emission wavelength of QCL also shows a blue shift, which varies proportionally with optical pumping power, thus enabling fast wavelength modulation by external light injection. Gain increases when the NIR light wavelength is close to QCL bandgap as the photoexcited carriers increases the population at the upper laser level. However, for NIR pumping energy larger than band-gap, gain decreases due to generation of hot-carriers. These high-k electrons eventually thermalize to injector ground but can not populate the upper lasing level because of the misalignment due to the bandbending created by excited carrier. NIR pulse duration significantly changes extra carrier generation and we achieve similar results for both pico-second and femto-second range pulses.