Spectral dynamics of single quantum dots : a study using photon-correlation Fourier spectroscopy for submillisecond time resolution at low temperature and in solution

Abstract

Conventional single-molecule fluorescence spectroscopy is limited in temporal resolution by the need to collect enough photons to measure a spectrum, in frequency resolution by the dispersing power of the spectrometer, and by environmental conditions by the need to immobilize the chromophore on a substrate. In this thesis, we use the recently developed technique of photon-correlation Fourier spectroscopy (PCFS) to circumvent each of these limitations. PCFS combines the high temporal resolution of photon correlation measurements with the high frequency resolution of Fourier spectroscopy. The experimental setup consists of a Michelson interferometer where the two outputs are detected with avalanche photodiodes and cross-correlated with a hardware autocorrelator card. The interferometer maps spectral changes into intensity changes which can be measured with high temporal resolution by the autocorrelator. The distribution of spectral changes between photons with a given temporal separation determines the degree of correlation in the interferogram. By measuring the intensity correlation at different interferometer positions while dithering one mirror, a time dependent spectral correlation function is obtained. From this, we learn about the temporal evolution of the emission line shape at timescales approaching the lifetime of the emitter. In this body of work, we both apply PCFS to study low temperature colloidal quantum dots and extend the technique to extract spectral lineshapes and dynamics of single quantum dots freely diffusing in solution. In solution, spectral correlations originating from the same quantum dot are statistically enhanced and separable from the ensemble using intensity fluctuations from diffusion. We are able to use spectral correlations from many diffusing chromophores to determine the average single chromophore spectral correlation. This thesis begins with a review of spectral dynamics in quantum dots in Chapter 1. Chapters 2 and 3 describe the theoretical and experimental implementation of PCFS. Chapters 4 and 5 cover numerical simulations and experimental demonstrations of the extension of PCFS to single quantum dots obscured by an ensemble in solution. Finally, chapter 6 applies PCFS to single quantum dots at liquid helium temperatures.