LAB 3: Confocal Microscope Imaging of single-emitter fluorescence. LAB 4: Hanbury Brown and Twiss setup. Photon antibunching. Roshita Ramkhalawon

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1 LAB 3: Confocal Microscope Imaging of single-emitter fluorescence LAB 4: Hanbury Brown and Twiss setup. Photon antibunching Roshita Ramkhalawon PHY 434 Department of Physics & Astronomy University of Rochester Rochester Abstract As applications for single photon sources become more widespread, the need for dependable single photon sources becomes more acute. In this lab, we learn how to produce single photons by making use of different types of single emitters, such as the color centers of nanodiamonds and colloidal quantum dots. We also use a Hanbury Brown-Twiss setup to prove that the single emitters being used in this lab exhibit antibunching. INTRODUCTION AND THEORETICAL BACKGROUND The advent of applications such as secure quantum cryptography and linear optical quantum computing has placed stringent demands on single photon sources. An attenuated laser beam is no longer a satisfactory replacement for these applications which require a source which consistently and reliably produces single photons. In contrast, an attenuated laser beam predominantly produces almost no photons and moreover, the probability of getting two bunched photons is never zero. As the quest for single-photon sources continues, many different types of single emitters have been discovered and are currently being researched. Amongst those emitters are single molecules, color centers of nanodiamonds and colloidal quantum dots. In this lab, we are going to explore the properties of these different single emitters. Single emitters reliably produce single photons because of the fluorescence lifetime - the time taken for an excited electron to fall to its ground state and emit a photon. Two or more photons cannot be produced simultaneously because of this process. (see Fig 1) In order to prove that the single emitters definitely exhibit antibunching, we carry out the Hanbury Brown-Twiss experiment which shows that there is a correlation between the outputs of the detectors situated after the beam-splitter.

2 Classically, the correlations between the intensities of the transmitted beam I T and the transmitted beam I R are given by the second order temporal coherence g (2) T,R (τ), where τ is the time delay between the two intensity measurements. g (2) T,R (τ) is defined below for classical fields. g (2) T. R = < < I T I T ( t + τ ) I ( t + τ ) > < R ( t) > I R ( t) > It can be shown that for simultaneous measurements of classical fields, g (2) T,R = g (2) () 1 For a non-classical field, for example a field containing exactly one photon, we evaluate the quantum-mechanical operator for a single photon field. g (2) T. R = < < : I I T T ( t + τ ) I ( t + τ ) > < R ( t) : > I R ( t) >. We find that g (2) ()=, which violates the classical inequality. This phenomenon is known as antibunching. In this experiment, we measure interphoton time one APD is connected to the start and the second is connected to the stop electronics. The time interval between pairs of photons is measured and a histogram is plotted, showing the number of second photons arriving at a definite time interval after the first photons. In case of antibunching, the histogram has a minimum at τ =, indicating the presence of a single emitter. PROCEDURE

3 Fig 1. Inner workings of the confocal microscope Fig 2. Experimental Setup 1. Four samples were prepared in this lab in order to explore three different types of single emitters. The first sample consisted of nanodiamond crystals in index matching oil. The second sample contained colloidal quantum dots in cholesteric photonic bandgap liquid crystal host. The third sample used CdSeTe spin-coated quantum dots

4 in newly prepared 1 nm solution. Lastly, the fourth sample was spin-coated using 1-6 molar solution of DiI Dye molecules in toluene. 2. After preparation of the sample, a sliver of the sample is placed on a small square glass slide. An oil drop is added onto the sample holder of the confocal microscope and then, the glass slide is mounted on it and held on by a pair of small magnets. 3. The solid state laser is turned on so that it illuminates the sample. We can view the sample through wide-field microscopy which is equivalent to using the EM-CCD camera to see the sample in real-time. In doing this, we find the fluorescence more easily. Once this is done, we focus the laser onto the sample and switch to confocal microscopy to scan only one specific area of that sample. Forw. or APD1 6. Backw.or APD position (nm) Fig 3. APD scanning signal for the nanodiamond crystals shown on Fig The sample is scanned closely to detect any fluorescing single emitters. We usually scan a x µm area of the sample and then choose a suitable 5x5 µm area to zoom onto. 5. Scanning of the sample as seen through the confocal microscope is done through Raster scanning in this procedure, the sample is scanned through one line at a time. Any horizontal stripes seen in the scanning pictures can be attributed to the blinking of the single emitters. While the raster scanning is occurring line by line, the single emitters turn on and off, hence creating thin black stripes in the picture of the scan.

5 Fig 4. Different scanning area of nanodiamond crystals in phase matching oil (i) x µm scan (ii) 5 x 5 µm scan 6. Once we have a 5x5 µm scan of the sample, we select a point which seems to be a probable single emitter to test for antibunching behavior. The microscope output is to the APDs and the nanodrive mechanical system is turned on. The APDs, the computer with TimeHarp and the computer that records the APDs output signal are turned on. This point is scanned in time and the time-harp file and histogram of the photon counts are both saved. 7. Some calibration of the electronics being used is necessary before taking the measurements. There is a delay introduced by the time correlated single photon counting card that needs to be corrected for. This can be done by sending the same signal from one detector to start and stop channels using cables of equal length. By doing this, we find out that the zero interphoton time is 64.68ns.

6 8. We also find out the duration of the laser pulse by sending the laser output through an oscilloscope and plotting a graph of the pulse. 9. We can measure the fluorescence lifetime of the single emitters by sending the laser output to stop and the first APD to start. We then collect data of the pulse and fit it to N t /τ = N o e where τ is the fluorescence lifetime. RESULTS AND ANALYSIS Calibration We find out the point for which interphoton time is zero by sending the same signal through cables of equal length. It is found to be ns. The zero point can be altered by changing the delay time. We also use an oscilloscope to confirm the time duration of the laser pulse.

7 Each laser pulse above can be calculated to be 13 ns. Imaging and scanning of emitters Fig 5. Fluorescing single emitters as seen from the CCD camera in wide-field microscopy mode

8 We first obtain very small antibunching with nanodiamonds as is shown. The selected emitter is marked with the crosshair and is scanned for antibunching Fig 6. Very small antibunching with nanodiamonds We obtain no antibunching with the sample of colloidal quantum dots in cholesteric photonic bandgap liquid. Fig 7. No antibunching histogram with quantum dots in cholesteric photonic bandgap liquid

9 APD1 APD time (ms) 4. Fig x3.5 µm colloidal quantum dots in cholesteric photonic band gap liquid crystal host and its associated TimeHarp trace. No antibunching obtained for this emitter. Note the horizontal stripes indicating blinking APD1 APD time (ms). Fig x 12.5 µm sample of CdTeSe spin-coated colloidal quantum dots in newly prepared 1 nm solution and its associated TimeHarp trace which shows the blinking of the selected quantum dot. Fig 1. Antibunching obtained with CdTeSe spin-coated colloidal quantum dots in 1 nm solution

10 Fig 11. DiI dye in toluene solution of concentration 1-6 M Calculation of fluorescence lifetime We obtain the data below from connecting the laser to stop and one APD to start. The pulse below is for a delay of 19ns for the sample of DiI dye in toluene solution. Fig 12. Fluorescence lifetime of DiI dye molecules in toluene

11 The pulse obtained from TimeHarp is fitted in Excel to the equation N the fit gives us 1/τ =.13. Hence τ = 1/.13 = 76.9 ns. N e t /τ = o. In this case, We hence obtain the fluorescence lifetime of the DiI dye molecules to be 76.9 ns. CONCLUSION We prepared four different samples of emitters and carried out fluorescence confocal microscopy with each of those samples in order to image single emitters. We show antibunching of photons by using a Hanbury Brown Twiss setup. It was extremely difficult to obtain antibunching with some of the samples used since sample preparation is essential to the final results. The solution being used must be of the right concentration and have to be properly mixed. Solutions that have not been used for a while are more likely to form clusters of emitters. Also, spin coating of the sample is also useful in reducing clusters. In the case of clusters of emitters, antibunching cannot be demonstrated because the laser excites more than one emitter at a time and several photons are released simultaneously. We obtain antibunching specifically with CdTeSe spin-coated colloidal quantum dots in 1 nm solution, as as shown in Fig 1. The histogram of antibunching in Fig 1 has a peak on the right which is not part of the antibunching effect this peak is an artifact of the inverter being used and is present on all the antibunching histograms collected in the lab. We also observe other phenomenons characteristic of single emitters such as blinking and photobleaching. Photobleaching in particular complicates the observation of single emitters, since they are destroyed by the light exposure necessary to image them. However, single emitters still remain of the most reliable single photon sources available nowadays and it might be possible to develop them further so that more single photon applications become widespread