UCLA

Single photon counting has led to many innovations in the manner we observe physical properties of materials, molecules and biological substrates. This comes with the ability to also generate photon streams that record emissive events on the timescale of picoseconds. Using time correlated single photon counting (TCSPC) the events of light stimulation can be recorded with high precision. I leverage the use of TCSPC as a platform to develop complimentary methods to study and manipulate this collected photon stream. Within this I develop methodologies to probe systems across the spectrum from the UV-visible to the short-wave infrared or SWIR (1-2 �m). In Chapter 1 I describe the basis of using light to stimulate photophysical events, covering definitions and theory. There I will also introduce single photon counting along with the ability to resolve spectral signatures through interferometry. In Chapter 2, I cover the implementation of Mach-Zehnder interferometry to simultaneously take broadband spectra while retaining the timing information of photon events. We call this technique Decay Associated Fourier Spectroscopy (DAFS), a method that utilizes interferometry to associate photophysical lifetimes with spectral signatures. I demonstrate DAFS’ capability by recovering Forster resonance energy transfer (FRET) dynamics for a mixture of an organic dye and quantum dots. I then demonstrate the broadband resolution of DAFS by mixing a visible and SWIR quantum dot and recovering the spectra and lifetime data simultaneously over an octave spanning window. In Chapter 3, I introduce a follow-up method to the aforementioned DAFS. Taking advantage of the spectral coherence of emitted photon streams, I introduce interferometric or Fourier filtering to separate signals and term this method “Spectrally-selective Time-resolved Emission Filtering” or STEF. I demonstrate STEF in the context of several systems including laser scatter and emitter, mixed and emitters, and imaging biological systems. STEF allows me to bypass the use of conventional filters in the study of complex chromophore mixtures. In Chapter 4, I then develop another method that utilizes precision timing of multiple pulses to shelf and study short-time events such as fluorescence and long-time events such as phosphorescence simultaneously. Here we term the method fluorescent optical cycling or FOC. Using FOC, I can use pulse trains of on and off times to collect the “total photon economy” or all photon events to understand all radiative, nonradiative, and other energy dissipation pathways. Along with a simple kinetic model, I use FOC to measure these properties in dual emitting platinum complexes, highly phosphorescent anti-boron clusters, and SWIR-emitting singlet oxygen. To finish, I show that FOC and DAFS can be used in tandem to achieve spectral specificity. I will then finish with a perspective on preliminary efforts to establish measurement schemes that could complement the above novel methods. Within Chapter 5, I describe and demonstrate our ability to collect SWIR Fluorescent lifetime imaging microscopy (SW-FLIM), Fluorescent Correlation Spectroscopy (FCS), and dip probe studies. Alongside this, I will discuss experimental plans, to combine these techniques with DAFS, STEF, and FOC while offering perspective on future experiments.