Optical Detection and Control of Excitons, Spin, and Magnetism in Nanostructures

Abstract

Coupled magnetic, electronic, and optical degrees of freedom, especially when afforded by nanostructures, have the potential to revolutionize scalable manufacturing, computing and memory storage, communication, and sensing. Two-dimensional (2D) magnets are particularly promising, but in many cases the optical spectra of such materials are complex and poorly understood. Chapter 2 reports on the optical properties of the A-type layered antiferromagnet, CrPS4. Very weak pure-electronic origins of the lattice Cr3+ are identified, across which the absorption and photoluminescence (PL) spectra mirror one another. Rich fine structure is assigned to exciton-magnon coupling, exchange splitting, and a second “defect” Cr3+. A radiative decay time of several microseconds is elucidated for this PL, consistent with the spin-forbidden 2E → 4A2 transition of lattice Cr3+. Energy migration dynamics are studied using Yb3+ dopants as deliberate traps, from which sub-picosecond inter-site hopping is estimated. This rapid hopping implies the formation and dispersion of Frenkel-type excitons based on multi-ion coupled 2E excitations. The observation of resolved magnon-coupled electronic transitions in CrPS4 may present new opportunities to explore and harness coupling between optical excitations and correlated spins in layered magnetic materials. Chapter 3 investigates the underlying magneto-optical coupling between Yb3+ and CrPS4. The collective spin properties of CrPS4 are encoded in the sharp f-f luminescence of isolated Yb3+ dopants via strong magnetic superexchange coupling between the two. The spontaneous magnetic ordering in CrPS4 induces large exchange splittings in the narrow Yb3+ f-f photoluminescence features below TN. Spin reorientation in CrPS4 via a "spin-flop" metamagnetic transition modulates the Yb3+ f-f luminescence energies and exchange splittings. This pronounced link between spin and optical properties enables the demonstration of optically driven spin-flop transitions in CrPS4. The high PL quantum yields and strong screening of the f electrons that benefit this work on CrPS4 are also advantageous for quantum applications. Chapter 4 explores interacting lanthanide dimers in Yb3+–Yb3+ and Yb3+–Er3+ molecular clusters toward 2-qubit systems. Simultaneous pair emission from Yb3+–Yb3+ dimers and its nonlinear power dependence and dynamics indicate coherent coupling. The Yb3+–Er3+ cluster exhibits inter-ion energy transfer evidenced by site-selective excitation experiments. From time-resolved PL experiments, the transfer efficiency is 56% and the incoherent electric dipole-dipole coupling strength is approximately 52 MHz. These results suggest the coherent dipole-dipole interaction in this molecular cluster may be strong enough for 2-qubit operations. Finally, Chapter 5 summarizes various strategies to synthesize ZnO nanostructures with high optical quality. Unlike the aforementioned vdW magnet and lanthanide dimer systems, ZnO already boasts extensive literature toward quantum applications and numerous synthetic routes. The preparation of optically bright nanostructures is crucial for device efficacy and integration. Various strategies toward preparing homogeneous ZnO nanostructures with high optical quality are summarized. In sum, this dissertation explores light-spin interactions in nanomaterials, with relevance to spintronic, magneto-photonic, and quantum applications.