University of Rochester

OPT253 Lab 3-4 Report

Characterizing Quantum Dots and ColorCenters in Nanodiamonds as Single

Emitters

Author:Nicholas CothardPeter Heuer

Professor:Dr. Svetlana Lukishova

November 20th 2013

List of Figures

1 Bandgap samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 A Diagram of a confocal microscope (Image from Wikipedia) . . . . . . . . . . . . 33 Picture of our Hanbury-Brown and Twiss interferometer . . . . . . . . . . . . . . . 44 Picture of TTL pulse as observed on oscilloscope . . . . . . . . . . . . . . . . . . . 45 Wiring diagram for calibrating time delay . . . . . . . . . . . . . . . . . . . . . . . 56 Wiring diagram for antibunching measurement . . . . . . . . . . . . . . . . . . . . 57 Screen capture of quantum dot raster scan . . . . . . . . . . . . . . . . . . . . . . . 68 Screen captures of antibunching curves . . . . . . . . . . . . . . . . . . . . . . . . . 69 Diagram of AFM (Image from Wikipedia) . . . . . . . . . . . . . . . . . . . . . . . 710 Test samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 AFM images of a quantum dot sample . . . . . . . . . . . . . . . . . . . . . . . . . 812 Wiring diagram for fluorescence lifetime measurement . . . . . . . . . . . . . . . . 813 Pulse from laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914 Quantum Dot Fluorescence Images from Confocal Microscope . . . . . . . . . . . . 915 Quantum Dot Fluorescence Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . 1016 Nanodiamond Sample Sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1017 Nanodiamond Fluorescence Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . 1118 Inverted image of the laser through the confocal microscope . . . . . . . . . . . . . 1119 532nm spectral line of the excitation laser . . . . . . . . . . . . . . . . . . . . . . . 1220 Spectrum of single emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1221 Quantum dots blinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1322 Screen capture showing a bleached signal flow . . . . . . . . . . . . . . . . . . . . . 13

List of Tables

1 Quantum Dot Fluorescence Lifetimes (ns) . . . . . . . . . . . . . . . . . . . . . . . 92 Nanodiamond Fluorescenec Lifetimes (ns) . . . . . . . . . . . . . . . . . . . . . . . 10

1 Abstract

Single photon sources utilizing single emitters are essential for the realization of quantum technol-ogy. We produced single photons (antibunched light) with a quantum dot source, and measuredthe fluorescence lifetimes of quantum dots and nanodiamond color centers to be approximately4ns and 3ns respectively. A spectrometer, atomic force microscope, and electron-multiplying CCDcamera were also used to further characterize the samples. This investigation suggests that nan-odiamond color centers are a superior single emitter because they are less susceptible to blinkingand bleaching phenomena that can destroy quantum dots.

2 Introduction

Modern quantum technology increasingly relies on the ability to produce a weak beam of spatiallyseparated photons, known as a single photon source. This is complicated by the fact that pho-tons are bosons, which statistically tend to clump together. Simply attenuating a laser beam tothe single photon level will create a much weaker beam, but the photons will naturally distributethemselves into groups rather than spread out evenly. This phenomenon is called photon bunching.

Photon bunching can be most easily circumvented by producing each photon in the beam witha single emitter, such as an individual atom or molecule. When a single emitter is stimulated, itemits a single photon. The emitter must then wait a characteristic fluorescence lifetime beforeemitting a second photon. If the fluorescence lifetime is long enough, the first photon will have

1

moved too far away to interact with the second photon by the time it is produced. The resultingbeam of evenly spatially spaced photons is said to be antibunched.

In order to stimulate only one emitter, a very dilute solution of single emitters is depositedonto a slide. As the solution is spread out, the single emitters will be left on the slide relativelyfar away from one another. A single emitter can then be stimulated with a highly focused laserbeam through a confocal microscope. A wide range of single emitters, from atoms and ions tomolecules, have been used to produce antibunched light. In this lab we studied the properties oftwo promising single emitters; colloidal quantum dots and nanodiamond color centers. Colloidalquantum dots are small pockets of semiconductor with a small band gap embedded within a largersemiconductor with a larger band gap. These two structures combine to create discrete energylevels, which then absorb and emit light much like a single atom. Color centers in nanodiamondsare common defects in the diamond lattice where two adjacent carbon atoms are missing andone has been replaced by a nitrogen atom. The resulting ’color center’ also has an energy levelstructure that makes it behave much like an atom, just like a quantum dot.

Determining which single emitters are right for which applications requires a thorough un-derstanding of their physical and optical properties. We prepared samples of quantum dots andnanodiamonds, suspended in toluene and chiral nematic liquid crystal respectively. We physicallyexamined these samples with an atomic force microscope, demonstrated that they produced an-tibunched light, and collected data on their fluorescence lifetimes and spectral properties. Wealso investigated several drawbacks of quantum dots as compared to nanodiamonds, such as theirtendency to blink on and off and eventually bleach.

3 Preparing Quantum Dot and Nanodiamond Samples

In Lab 3-4 we created two different types of single emitters. First, we created a sample from adiluted quantum dot solution. To do this we placed 10µL of the prepared diluted quntum dot solu-tion onto a microscope glass cover and placed it on a spin coating machine. The platform was spunat 3000rpm for approximately 30s while the slide was held onto the platform with suction. Thesample was dry and evenly distributed so it was ready to be looked at with the confocal microscope.

The second type of single emitter that we prepared were single emitters in a photonic bandgapcholesteric liquid crystal host. To prepare this, we used capillary tubes to deposit a drop of thechiral nematic liquid crystal blend onto the microscope slide. Then, we deposited a drop of thenanodiamond solution onto the slide next to the liquid crystal blend. We waited for the water inthe solution to evaporate and then mixed the liquid crystal blend into the quantum dot remains.We mixed for about five minutes and then placed a glass cover slip on top of the sample. Afterapplication of pressure on the slides, the sample turned red (Figure 1a) and blue depending onthe viewing angle.

Another method of creating a photonic bandgap host is to begin with a solid form of the liquidcrystal on a glass slide. When placed on a hotplate and heated to about 200◦C, the solid enters aliquid crystal state becomes transparent. Once it reaches a temperature about about 240◦C, theliquid crystal enters a fully liquid state and becomes colored again. Placing a slide on top of thesample and applying pressure, we saw a blue-green colored photonic bandgap material (Figure 1b).

2

(a) Red bandgap material from cholestericliquid crystal host

(b) Blue-green bandgap material from solidliquid crystal

Figure 1: Bandgap samples

4 Demonstrating Antibunching

To measure the anti-bunching properties of our quantum dot sample, we used a confocal micro-scope to control and drive the emissions of our single emitter. A confocal microscope uses an laserto excite our single emitter sample which to release signal photons of a different wavelength. Forthe antibunching measurements, we used a HeNe 633nm excitation laser which produced 800nmphotons from the single emitter.

Figure 2: A Diagram of a confocal microscope (Image from Wikipedia)

We measured the continuous wave HeNe laser power to be 0.52mW before the confocal micro-scope and so, we applied two orders of magnitude attenuation infront of the confocal microscopeinput. The excitation laser enters the microscope setup and reflects off the dichroic mirror. Adichroic mirror is one which reflects a specific range of wavelengths and transmits others. Thebeam passes through the objective and onto the sample which through the process of fluorescencelifetime, produces anti-bunched 800nm photons. The 800nm photons pass through the dichroicmirror and out of the confocal microscope.

To measure antibunching of the single emitters, we sent the single photons to a Hanbury-Brownand Twiss interferomter (Figure 3) to measure the coincidence timing of consecutive photons. If

3

the photons are completely antibunched, we expect that no photons will arrive at the same time.

Figure 3: Picture of our Hanbury-Brown and Twiss interferometer

The photons produced in the confocal microscope are sent into the interferomter which consistsof a beam splitter, two photodetectors, and a computer which can measure the time between twophotons arrival. In our setup, we used two APDs (avalanche photodiodes) to detect the photonsand send electrical signals to the TimeHarp computer card which measures the time between de-tector firings. One APD is connected to the start channel of the TimeHarp while the other APD isconnected to the stop channel. We need two APDs because each APD has a “deadtime” in whichit must rest itself after detecting a photon. During the deadtime, the detector may miss a photonso a second detector is used in attempt to capture any photons that may arrive during deadtime.

When the APDs absorb a photon, they emit a TTL (Transistor-Transistor Logic) pulse to theTimeHarp card. The TTL pulse is a step function pulse (Figure 4) which begins the time inte-gration of the TimeHarp card. When a pulse hits the start channel of the card, capacitors in thecard begin charging. When a pulse hits the stop channel of the card, the capacitor is released andtime between start and stop pulses can be determined by the amount of charge that collected inthe capacitor. From this, we can measure the time between consecutive photons. The TimeHarpcard requires that the stop signal must be inverted with respect to the start signal, and that bothsignals be attenuatedl.

Figure 4: Picture of TTL pulse as observed on oscilloscope

4

Using the start and stop pulses, we can build a histogram of time differences between con-secutive photons. If our photons are anti-bunched, we would expect to see no data points for∆t = 0 but in our setup, we used an electronic time delay to delay the pulse of the stop channelso we expect to see no data points at ∆t = tdelay. To measure the value of tdelay, we connected asingle APD to both the start and the stop channels with an inverter in the stop channel to satisfythe TimeHarp card (Figure 5). The electrical pulses from the APDs were divided between thechannels and the coincidence time measured between start and stop was the time that the stopsignal was delayed. This was measured to be 57.34ns.

Figure 5: Wiring diagram for calibrating time delay

Figure 6: Wiring diagram for antibunching measurement

To observe anti-bunching, both APDs were connected to the TimeHarp (Figure 6), the exci-tation laser was turned on, and a raster scan of the sample was created. To do this, we used aPiezo translation stage which moved the sample in the x and y directions over the microscopeobjective. Doing this, we were able to form an image of our quantum dot samples using a rasterscan method. The raster scan image takes over a minute to complete so the scan is not only aspatial scan, it is also a temporal scan. Figure 7 is an example of a raster scan of the quantumdot sample. It is clear that the sample changes over time since we can see that some dots seem todissappear in some rows. This is due to a phenomenon known as blinking which will be discussedlater in this report.

5

Figure 7: Screen capture of quantum dot raster scan

With the raster scan completed, we were able to move the Piezo stage to a specific spot onthe sample and collect single photons from a single quantum dot. To do so, we used LabViewsoftware which controlled the Piezo stage to move a single emitter over the microscope objective.We then began collecting time differences from the TimeHarp card. Figure 8 show the data fromtwo trials. Figure 8a shows collection from a quatum dot that appears to have died before we couldcollect data. The low amounts of photons and the uniform distribution over time do not show anysigns of anti-bunching. This may have been the result of a phenomenon known as bleaching whichwill be dicussed later in the report. Figure 8b has a dip at approximately ∆t = 52ns. Very fewphotons arrived for this value of ∆t and this is approximately the delay time that we measured.The absence of photons around ∆t = 52ns suggests that the photons are antibunched and oursample is a sample of true single emitters.

(a) Flat curve that does not demonstrateantibunching

(b) ”V” curve demonstrating antibunchingaround ∆t = 52ns

Figure 8: Screen captures of antibunching curves

6

5 Atomic Force Microscopy of Sample Surface

Once the samples were prepared they were examined with an atomic force microscope. The re-sulting images allowed us to study how evenly and densely the single emitters were distributedover the substrate.

In its simplest form, an atomic force microscope works by dragging a needle with an atomicallysharp point along the surface of the sample. As the needle moves along the sample, it moves upand down, maintaining a constant force between the tip and the sample. This change is measuredby bouncing a laser beam off of the top of the needle and recording its deflection with a photosensor. Hookes law then allows us to calculate the force on the needle to infer its distance fromthe sample. More complicated modes vibrate the tip of the needle near its resonance frequency asit moves over the sample. When the needle is closer to the surface, the forces of the atoms in thesample dampen the tips oscillation. This change in frequency can be used to infer the height ofthe tip above the sample.

We utilized a combination of these techniques, known as tapping mode. The tip is vibratedabove the sample with relatively high amplitude such that the tip ’taps’ the surface. Changes inthe inter-atomic forces felt by the needle at different locations on the sample can be used to inferits height. This method has the advantage of minimally disturbing the sample, so that we can besure not to alter the position of the single emitters on the slide while performing the measurement.

Figure 9: Diagram of AFM (Image from Wikipedia)

We first tested the AFM by examining the surface of a CD-ROM (Figure 10a) and a calibrationsample (Figure 10c). We then observed a sample of quantum dots, zooming in to image a singledot (Figure 11c).

The spot size of our laser when passed through our microscope objective can be calculated as:

x = 0.61λ

NA= 240nm (1)

We can see from the image in Figure 11a that the single emitters on the sample are sufficientlyseparated to be targeted individually by a beam of this size.

7

(a) CD-ROM Surface.(b) Calibration Sample

Surface. (c) Test sample.

Figure 10: Test samples

(a) Initial image of sample. (b) Zoomed in on smaller area. (c) Single dot.

Figure 11: AFM images of a quantum dot sample

6 Measuring Fluorescence Lifetimes

Figure 12: Wiring diagram for fluorescence lifetime measurement

In order to measure the fluorescence lifetime of single emitters, we first performed a raster scanwith the confocal microscope to locate a single emitter to examine. Once we found an emitter, wepositioned it in the focus of the confocal microscope with the translation stage.We then excited thesingle emitter with a picosecond scale laser pulse. The resulting fluorescence photon was capturedby an avalanche photo diode (APD), sending a TTL logic pulse (Figure 4) to start a timer on aTimeHarp computer DAQ card. When the next laser pulse was emitted, the laser sent anotherpulse (Figure 13) to the computer, stopping the timer.

8

Figure 13: Pulse from laser

Over many cycles, a histogram of the time between the laser pulses and the photon detectionswas created. These curves (Figure 15) show that the probability of a fluorescence photon beingemitted decays exponentially. We measured the time constant (the fluorescence lifetime) of thesample by fitting these curves with decaying exponential function in IgorPro.

Data was collected for both quantum dots in toluene and nanodiamonds in chiral nematicliquid crystals at different sites and with varying pump beam intensitys.

(a) Site 2 (Poor data) (b) Site 3 (c) Site 4

Figure 14: Quantum Dot Fluorescence Images from Confocal Microscope

Three quantum dot sites (Fig. 14a, Fig. 14b, and Fig. 14c) were tested. Two sites (Fig. 14band Fig. 14c) demonstrated exponential decay, with lifetimes of about 4ns (Fig. 15b and Fig. 15c).However, the first site exhibited a linear decay (see Fig. 15a). Since the sample in question wastaken on the very edge of a quantum dot cluster (see Fig. 14a), it is possible that no quantumdots were actually within the sampling region.

76.5µW 35.2µWSite 2 Bad data Bad dataSite 3 4.34 5.95Site 4 3.50 4.56

Table 1: Quantum Dot Fluorescence Lifetimes (ns)

Five nanodiamond sites were examined (Figure 16). When fit with an exponential curve, allexhibited lifetimes ranging from 3ns to 3.5ns.

9

(a) Site 2 (Poor data) (b) Site 4, 76.5µW (c) Site 4, 35.2µW

Figure 15: Quantum Dot Fluorescence Curves

(a) Site 1 (b) Site 2 (c) Site 3

(d) Site 4 (e) Site 5

Figure 16: Nanodiamond Sample Sites

76.5µW 35.2µWSite 1 3.29 3.53Site 2 3.20 3.43Site 3 3.12 3.50Site 4 3.40 3.57Site 5 3.09 3.47

Table 2: Nanodiamond Fluorescenec Lifetimes (ns)

In all cases, data was taken with an excitation beam intensity of 76.5 µW , and with an attenu-ated beam of (0.46) × 76.5µW = 35.2µW . For all the samples tested, attenuating the pump laserby 0.46x increased the fluorescence lifetime by approximately 10 percent (see tables 1 and 2).

10

(a) Site 1, 76.5µW (b) Site 1, 35.2µW

Figure 17: Nanodiamond Fluorescence Curves

7 Spectra of Single Emitter Samples

In order to further characterize our single emitter sample, we attempted to collect its spectrum.For this analysis to be conclusive, more investigation is required but unfortunately we were unableto collect the appropriate measurments due to time constraints.

From the viewing port on the confocal microscope, it is possible to see the interference paternof the excitation laser. Figure 18 is an image taken with a cooled CCD camera of the interferencepatern due to abberations and internal reflection. We were able to focus the confocal microscopeby hand using this image. The microscope was in focus when the image was most crisp.

Figure 18: Inverted image of the laser through the confocal microscope

With the laser focused, we switched the port of the confocal microscope to send the image intothe spectrometer. Using a difraction grating spectrometer (which separates a light source into itsindividual wavelengths) the spectrum of the image was sent to an EM-CCD. Figure 19 shows thespectrum as observed by the CCD and the analyzed spectrum which was created by calibratedsoftware that decomposes the CCD image into a spectrum. The spectral line of the excitationlaser is clear to see at 531nm (actually 532nm).

11

(a) Inverted CCD image of laser spectrum (b) Spectrum of excitation laser

Figure 19: 532nm spectral line of the excitation laser

Placing the interference filters into the confocal microscope, we obtain the spectrum in Fig-ure 20. The 532nm line is still visible but the other peaks represent the spectral lines of the sample.We cannot be certain which lines belong to the single emitter. To be certain, it would be necessaryto capture the spectrum of a blank slide without the single emitter sample. Such a spectrum couldbe subtracted from a spectrum of the single emitters. This would reveal the spectral lines of thesingle emitter sample. If we had more time for this lab, we would have collected such a spectrumbut unfortunately, we did not.

(a) Inverted CCD image of single emitterspectrum (b) Spectrum of our single emitters

Figure 20: Spectrum of single emitters

8 Disadvantages of Quantum Dots

In Figure 7, we saw qunatum dots turn on and off in time. This phenomenon is known as blinkingand is a random event where a single emitter swtiches to an off state where it does not emitphotons. Eventually, the emitter will turn back on. This causes a problem because it prevents usfrom collecting data. Blinking can be seen in videos taken of the quantum dot sample. Figure 21shows this by presenting two frames from the video, one second apart. Notice that new dots haveappeared while some dots have disappeared.

Another phenomenon can be seen in Figure 22 which shows a screen capture of intensity vs.

12

(a) t= 0 (b) t 1s

Figure 21: Quantum dots blinking

time from the LabView software used to control data collection. The plot shows the number ofAPD counts per millisecond. The phenomenon known as bleaching can be seen where the signalabruptly drops. Bleaching occurs because of the fluorescent nature of our sample. The laser usedto excite the sample eventually destorys the fluorescent molecules and “bleaches”.

Figure 22: Screen capture showing a bleached signal flow

These phenomena are problematic for single photon sources because they limit the ability tocreate a constant, controlled beam of antibunched photons. These are problems that will needto be overcome if single photon sources, such as the quantum dots that we test here, are to beused for quantum cryptography in which controlled antibunched photon sources are a requirement.

Blinking and bleaching were not observed with the nanodiamond sample. For this reason, nan-odiamonds are a much better candidate for single emitters to be used in quantum cryptography.

9 Conclusion

In this lab we succesfully created single emitter sources and, with the use of a confocal microscope,we created an antibunched photon source using quantum dots. By examining the time betweenconsecutive photons that were emitted from an excited quantum dot, we successfully demonstratedantibunching of our sample (Figure 8b). An atomic force microscope was used to determine thephysical size of the quantum dots which was found to be on the order of 200nm. The fluorescencelifetime of both samples was measured to be on the order of 4ns and 3ns for the quantum dotand nanodiamond samples respectively. We attempted to capture the spectrum of our samplesbut due to time constraints, this measurement was inconclusive. With respect to the applicationof quantum cryptography which has been discussed thouroughly in this course, it was found thatnanodiamonds are superior candidates for efficient single emitters because quantum dots are un-reliable due to the blinking and bleaching phenomena.

Nick contributed to this report by writing the sample preparation, antibunching, spectra,

13

disadvantages, and conclusion sections of this paper. Peter contributed by writing the abstract,introduction, AFM, and fluorescence lifetime sections of this paper as well as calculating thefluorescent lifetimes of all of our samples. The images and figures were collected and compiled byboth of us, and together we typeset the report in LATEX.

10 References

AFM Diagram: http://en.wikipedia.org/wiki/File:Atomic_force_microscope_block_diagram.svg

Confocal Microscope Diagram: http://en.wikipedia.org/wiki/File:Atomic_force_microscope_block_diagram.svg

Lab 3-4: Single Photon Source, by Svetlana Lukishova. http://www.optics.rochester.edu/workgroups/lukishova/QuantumOpticsLab/homepage/opt253_labs_3_4_manual_08.pdf

14