Quantum Interference Control of Photoexcited Carriers for K-Space Microscopy

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Non-technical Description:Microscopy is a ubiquitous and familiar tool in science and technology. It provides information by determining the spatial location of objects within a sample being studied. For optical microscopy, light must be spatially localized, i.e., focused, to enable the determination of spatial locations. This project will explore an alternate form of optical microscopy in solids where the momentum of an object, here electrons, will be determined, rather than their positions. To resolve the momentum of the electrons, the light must be focused in momentum, which is achieved by interference between absorption of a single photon and absorption of multiple photons. The information gained by this method will give insight into the momentum structure, also known as the band structure, of solids, which determine their fundamental properties such as whether they are insulators, conductors or semiconductors. It will also characterize changes in momentum, which are relevant for operating properties of devices made from the solids. The broader impacts of this project include unique training for graduate and undergraduate students in sophisticated optical methods and providing new tools for the semiconductor industry to characterize and perfect materials and devices used for electronics and optoelectronic products.Technical Description:This proposal describes an experimental project to address the lack of momentum-space sensitivity in optical methods by using quantum interference between absorption pathways involving different numbers of photons to achieve carrier distributions that are both asymmetric and localized in k-space. The interference between two-photon and three-photon absorption processes provides localization in the transverse momentum, opening the door to selectively probing regions of k-space, or performing “microscopy" in momentum-space. This ability would ultimately allow optical methods to be sensitive to band structure and carrier distributions within the bands. Beyond providing information about the band structure, the ability to selectively excite regions of k-space will provide unique information about how non-equilibrium electronic excitations thermalize and return to equilibrium.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.