Modulation of the optical properties of quantum dots by surface acoustic waves

Abstract: Semiconductor quantum dots (QDs) are considered to be a crucial part of future quantum technologies due to their enormous potential as efficient sources of single and indistinguishable photons. Single photons are necessary to transport quantum information over large distances through the existing global fibre optic network. A major requirement for these single photon sources is the possibility to precisely control and tune their emission wavelength. Due to the solid-state nature of quantum dots, this is achievable through the use of optomechanical coupling. The focus of this thesis is to modulate the optical properties of single quantum dots via the deformation potential induced by surface acoustic waves (SAWs). These mechanical waves, which propagate along the surface of a crystal, can easily be excited electrically by interdigital transducers (IDTs) on a piezoelectric substrate. Here, we study the generation of SAWs by self-made IDTs on lithium niobate (LiNbO3). The frequency responses of multiple IDTs of different designs are detailed using the delta-function model. These theoretical predictions are corroborated by our numerical results based on a finite element method, as well as our experimental measurements at room and cryogenic temperatures. We show that although a SAW frequency of 1 GHz can be achieved with a single-electrode IDT with an electrode width of 1 μm, similar excitation frequencies can be reached through harmonic generation using multiple-electrode IDTs, without requiring as thin electrodes. We demonstrate that internal mechanical reflections are significantly reduced for floating-electrode IDTs, and measure efficient low-noise SAW generation at 1.54 GHz using a Split-52 design. Transducers with a frequency chirped design, allowing for SAW excitation over broad frequency bands rather than only at discrete frequencies, are discussed, and a resonance bandwidth of 140 MHz is measured for one such IDT. The modulation of the optical properties of QDs interfaced with the resulting SAWs is studied theoretically using the k・p method. Based on an eight-band Hamiltonian approach, the band structures of several III-V semiconductors under different strain conditions are computed. The photoluminescence (PL) spectrum of a QD coupled to a SAW is derived, and exhibits a spectral broadening of a few meVs consistent with recent experimental findings from the literature. Finally, we interface single InAsP QDs embedded in InP tapered nanowires with our IDTs in the perspective of measuring the modulation of their PL emission.