Laboratory 3: Confocal Microscopy Imaging of Single Emitter Fluorescence and Hanbury Brown, and Twiss Setup for Photon Antibunching

The purpose of this experiment was to study single photon sources and confirm that they exhibit photon antibunching. The quantum dot fluorescence was imaged using an EM-CCD camera in conjunction with the confocal microscope. CdSeTe quantum dots, which emit at 790nm when excited with a 633nm He-Ne laser, were studied. The quantum dot fluorescence was enhanced using a photonic band gap material as a micro cavity. The micro cavity was created using cholesteric liquid crystals. A Hanbury Brown, and Twiss setup, that utilized avalanche photodiode detectors, was used to collect antibunching histograms and confirm the photon antibunching properties of these quantum dots. The photon antibunching histogram that was expected for these quantum dots was achieved. Using the histogram’s shape, it was possible to extrapolate the fluorescence lifetime. The fluorescence lifetime of CdSeTe quantum dot in a cholesteric liquid crystal host was found to be 37.5 ns. Theory and Background A single photon source is a light source that emits single photons separated in time. Single photons have practical applications in the fields of quantum cryptography and quantum computing. Single photons are valuable to cryptography because a single photon cannot be split. A signal sent on a single photon level cannot be intercepted without the sender or receiver noticing.There is also interest in using single photons as qubits in the field of quantum computing. Examples of single photon sources include single atoms, single dye molecules, single quantum dots, and single color center nanodiamond. A single emitter that is excited by an external field emits only one photon at a time. After the emitter absorbs a photon it takes a certain amount of time to emit another photon of longer wavelength. The average amount of time it takes a single emitter to complete this process is known as the fluorescence lifetime. Since single emitters can only emit one photon at a time, they exhibit antibunching. Photon antibunching means that the photons are fairly equally spaced in time. Other sources of light cannot exhibit antibunching. While an attenuated laser beam may have an average photon spacing that is quite large, the laser will still produce pairs and triplets of photons. In terms of a mathematical definition of antibunching, the second order correlation function is given by: 〈 〉 〈 〉〈 〉 Where the joint photo-detection probability is g (2) , at two different times t and t+τ. Coherent light has a g (2) (τ) of 1, including τ=0. “Bunched” light has g (2) (0)>1 and g (2) (0)>g (2) (τ). However, antibunched light has a g (2) (0)<1 and a g (2) max(τ)=1 [1]. Mandel and Kimble were the first to _______________________ *Electronic Address: [email protected]