Modulation of Quantum Dot Energy Levels by Surface Acoustic Waves for use as a Triggered Photon Source

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Hubert, Colin
Photoluminescence , Quantum Dots , Surface Acoustic Waves
The foundation for a triggered photon source was realized by convolving the energy bandgap of a quantum dot with a surface acoustic wave. The devices consisted of an InP substrate on which InAs/InP quantum dots were grown. It was then coated with a layer of piezoelectric ZnO by radio-frequency magnetic sputtering. Modulation of the device was enabled through aluminum interdigitated transducers that were deposited on the sample, which excited surface acoustic waves. The expected resonance of the interdigitated transducers was around 200MHz. However, resonances at 200MHz and 300MHz were recorded, due a Sezawa mode excitation. The preferential excitation of modes was likely due to variations in the ZnO film thickness. The target quantum dot emission was around 1550nm, matching with the C-band used in fibre optic communication channels. The largest wavelength measured for the ground state energy emissions from these dots was 1580nm, though typical lowest energy emission peaks were in the range of 1300-1400nm. Unidirectional Stark shifts in the photoluminescence emission of the quantum dots were observed as surface acoustic waves were applied. This quantum confined Stark effect is thought to be due the polarization of the InP/InAs due to the electric field in the ZnO layer, providing a second order effect. The electrical field from the ZnO layer potentially contributes a linear effect. The modulation of the quantum dot energy is due to the strain field but due to the electrical coupling form the ZnO layer, exact determination of the strain field's contribution is not possible. The emission modulation effect is quadratically dependent on both applied SAW power and inital emission energy. Convolution of the quantum dot emission with the surface acoustic wave-induced bandgap modulation was also observed, resulting in a split emission peak. A splitting of 4.97meV was observed using a linear surface acoustic wave power density of at least 1.69W/m and a laser excitation density of 3.17uW/um^2.
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