Confocal Microscope Imaging of Single-Emitter Fluorescence and Photon Antibunching By Dilyana Mihaylova Abstract The purpose of this lab is to study different types of single emitters including quantum dots, single walled carbon nanotubes and gold nanoparticles and nanodiamonds. We used a set up with a confocal microscope, two avalanche photo diodes, EMCCD camera and a spectrometer. We took confocal scans and sometimes antibunching measurements. Background Single photon sources can be prepared from single emitters which due to their molecular structure when excited emit photons which exhibit antibunching characteristics. When photons are antibunched they are separated in time and space. Single emitters are used instead of an attenuated laser beam because a laser would sometimes emit photons that are bunched together. In order to look for antibunching we used two avalanche photo diodes (APDs) in a Hanbury Brown and Twiss set up. We know that photon antibunching occurs when the second order correlation function g (2) (0)<1, where g (2) (τ) is defined as: g (2) (τ) = n1 ( t) n2 ( t + τ ) n ( t) n ( t + τ ) 1 2 For this lab we used CdSeTe quantum dots, single walled carbon nanotubes, gold nanoparticles and NV-color centers in nano diamonds. Quantum dots are nanoparticles made out of thousands of molecules of semi conducting material. Single walled carbon nanotubes are constructed by wrapping one atom thick layer of graphene into a cylinder. Nitrogen vacancy (NV) color center in a nanodiamond is formed by a missing carbon atom adjacent to a substitutional nitrogen impurity in the diamond lattice. When excited, NV color centers in nano diamonds behave as single-photon emitters. The applications of single photon sources include secure quantum communication, quantum metrology and quantum computing.
Experimental Set up Fig 1.a Fig 1.b Photographic image (Fig 1.a) and a diagram of the experimental setup (Fig 1.b). The photographic image shows the 532nm laser, confocal microscope, spectrometer and EMCCD camera. We observed the different emitters using a confocal microscope. The advantage of a confocal microscope is that it allows the user to focus on a precise spot and thus on a single emitter. The samples are placed on thin glass slides, either by spin coating or by simply putting a few drops of the solution on the slide. The slide is placed on a stage with a nano drive which allows for a specific emitter to be selected to be observed. The confocal microscope has several output ports. One port is connected to an EMCCD camera which is used for taking images of the sample and for alignment. A second port goes to a spectrometer for fluorescence measurements which are also recorded with the EMCCD camera. A third port goes to a Hanbury Brown and Twiss set up with two APDs and a 50/50 beam splitter. We are using two APDs instead of one because of a dead time each APD goes through once it detects a signal. The signal from the APDs goes through a delay system and Time Harp 2000. The emitters are excited with 532nm solid state pulsed laser. The light of the laser goes through filters and diaphragms before entering the confocal microscope.
Experimental Procedures Due to the high sensitivity of the equipment all measurements were taken in darkness. We used a 532nm solid state pulsed laser as our excitation source. When taking confocal scans it is important to make sure that there are enough neutral density filters in front of the confocal microscope which can be removed as needed. We used an oil immersed objective in order to increase the numerical aperture of the objective. We looked at several different examples of single emitters. We prepared the single walled carbon nano tube sample by putting a drop of CNT sample #1 dated (5/17/12) which was prepared by Prof. Krauss s group. The nanodiamonds and gold nanoparticles samples were similiarly prepared by putting a drop of the solution on a glass slide. The quantum dots sample was prepared by putting a 1μl drop of 800nm 10nM quantum dots solution and spin coating it for about 60seconds. This method of preparations allows for an even distribution of the solution on the slide. Results: Fig 2 A confocal scan of a carbon nanotube fluorescence
Fig 3. Confocal scan of nano diamonds fluoresence. The bright spot is probably a cluster of emitters. Fig 4 Spectum of single walled carbon nano tubes.
Fig 5 Spectrum of nanodiamonds with 5s exposure Fig 6 Spectrum of gold nanoparticles (right) with corresponding image from EMCCD camera (left).
Fig 7.a Fig 7.b Fig 7.c Fig 7 Confocal fluorescence microscope scan of quantum dots (Fig 7.a) with corresponding time series for a selected emitter (Fig 7.b) and antibunching measurements (Fig 7.b). No antibunching was observed.
Conclusion As a part of this lab we carried out confocal microscope fluorescence imaging and spectral measurements of quantum dots, single walled carbon nanotubes, nanodiamonds and gold nano-particles. We also took anti bunching measurements of a sample of quantum dots. No antibunching was observed.