Rudra Sankar Dhar
Nanoscopic Probing of Operating Nano-Photonic Devices
The external performance of quantum optoelectronic devices is governed by the three-dimensional profiles of electric potentials determined by the distribution of charge carriers (electrons and holes) within the active regions of the devices. Dynamic charge carriers play a vital role in active photonic quantum/nano devices, such as electrically-pumped semiconductor lasers. As an example, in quantum cascade lasers (QCLs) the electric field domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially non-uniform and temporally unstable. Unfortunately, there exists no prior means of probing the inner potential profile directly and as a result the mechanisms responsible for sub-par device performance of QCLs remain the subject of speculation. Another example is interband cascade lasers (ICLs), in which the distribution of gain-providing charge carrier governs the operation and performance of the devices, but has not been experimentally measured prior to this study.
This work presents a systematic experimental study of gain-providing charge carrier distribution in a lasing interband cascade laser and electric potential distribution in THz QCLs. The unique charge carrier distribution profile in the quantum-well active region is quantitatively measured at nanometer scales by using the non-invasive scanning voltage microscopy (SVM) technique. Experimental results clearly confirm the accumulation and spatial segregation of holes and electrons in the core of the ICL device. The SVM technique has been applied to lasing THz QCLs to verify the hypothesis of electric field domains in semiconductor quantum structures. The experimental results reveal that the multi-quantum-well active region is divided into multiple sections having distinctly different electric fields due to the formation of electric field domain (EFD) which shifts in a hopping-style with increase in device bias. The SVM findings in THz QCLs indicate the importance of quantum active region design for intrinsically more uniform and stable electric field profiles.
The two showcase study examples demonstrate that the cryogenic-temperature SVM is an enabling technique, being able to measure and resolve nanometer scale features non-destructively on operating devices. This experimental approach connects the inner dynamics with the external measures of the operating quantum devices by directly mapping the electric field distribution as well as the charge carrier distribution in nano regime. The SVM technique is expected to facilitate a deeper understanding of fundamental processes that are governing the operation and performance of a wide range of nanoelectronic and nanophotonic devices.