Toward Computational Electron Ghost Imaging

Lucas Clark Burnette Author

07/27/2021 Added

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Working towards developing a method to effectively construct an array of nanotip fibers used for computational electron ghost imaging.

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[00:00:01.032]Hello, my name is Lucas Clark Burnette.
[00:00:03.490]I am from Minnesota State University in Mooorhead,
[00:00:06.778]and I've spent my summer research working
[00:00:08.360]toward computational electron ghost imaging.
[00:00:12.230]I figured it would be best to start with a more simple
[00:00:14.330]explanation of what ghost imaging is,
[00:00:16.230]and then work my way towards what my research was on.
[00:00:20.290]So very briefly, we have to discuss classical ghost imaging.
[00:00:24.340]First, a laser passes through a beam splitter.
[00:00:26.970]This results in two highly correlated beams of light.
[00:00:30.320]We have two detectors here,
[00:00:31.700]a bucket detector and a pixelated detector.
[00:00:34.460]The bucket detector can only tell when photons are fitted,
[00:00:37.460]so there is no spatial information being recorded.
[00:00:40.870]On the other hand,
[00:00:41.910]the pixelated detector gives us the spatial information
[00:00:44.710]since it can record where the photons sit.
[00:00:47.810]The sample objects sits in front of the bucket detector
[00:00:50.480]and the pixelated detector is programmed to only record
[00:00:53.230]a pixel when the light strikes both the bucket detector
[00:00:56.040]and itself.
[00:00:57.230]Since both the beams are correlated in position
[00:00:59.890]by sweeping across the whole sample,
[00:01:01.770]a silhouette of the sample object is created.
[00:01:06.440]Next, we have computational ghost imaging.
[00:01:09.250]In this set up, the beam splitter is replaced by a mirror.
[00:01:12.640]Instead of splitting into two separate beams,
[00:01:14.840]the laser shines on a spatial light modulator
[00:01:17.220]or SLM for short.
[00:01:19.220]The SLM encodes the beam with spatial information,
[00:01:21.950]so we don't have to rely on the pixilated detector
[00:01:24.390]to give us the spatial data.
[00:01:27.580]For computational electron ghost imaging,
[00:01:30.090]instead of a laser beam,
[00:01:31.370]we wanna try to use an electron beam.
[00:01:33.800]So naturally the first question that arises is,
[00:01:36.270]how do we get an electron source?
[00:01:38.760]Unfortunately, for me, my mentor, Dr. Botolan
[00:01:41.800]had already conducted research in the past,
[00:01:43.700]to manufacturer these nanotip fibers that you see here
[00:01:46.690]in the gray with the lasers pointing at them.
[00:01:51.260]In the past, Dr. Botolan and his colleagues
[00:01:53.470]use these fibers to observe photo field electron emission.
[00:01:57.350]They have a whole paper on the topic,
[00:01:58.730]so I'm not gonna get into the physical principles,
[00:02:01.330]but basically you can shine a laser into these fibers
[00:02:03.870]and they emit electrons from their tips.
[00:02:08.620]So,
[00:02:09.740]we know that we have a source of electrons
[00:02:11.380]to use for ghost imaging.
[00:02:13.000]Now, why would we want to do it?
[00:02:17.900]First off, the spatial resolution of classical ghost imaging
[00:02:21.040]is on the order of millimeters.
[00:02:22.890]By focusing an electron beam,
[00:02:24.370]the spatial resolution of about one nanometer
[00:02:26.890]can be achieved.
[00:02:28.400]This will allow us to image things that are many times
[00:02:30.660]smaller than what could normally be done
[00:02:32.350]with regular ghosts imaging.
[00:02:34.870]The second advantage of electron ghost imaging,
[00:02:37.630]which can actually be applied to other forms
[00:02:39.820]of ghost imaging too,
[00:02:41.080]is the option to replace imaging cameras with a simple
[00:02:43.700]bucket detector.
[00:02:46.350]Another huge advantage of electron ghost imaging,
[00:02:48.890]is the reduced exposure time.
[00:02:51.100]This could have a pretty big impact in the field
[00:02:53.070]of nanomedicine imaging.
[00:02:55.230]Right now, it is hard to image certain types
[00:02:57.270]of biological tissues of traditional electron microscopy,
[00:03:00.870]because the contrast is too low.
[00:03:03.560]So let's say we wanna improve the contrast,
[00:03:05.850]so we turn up the power on our electron microscope,
[00:03:08.160]a little bit.
[00:03:09.830]From experience,
[00:03:12.300]we found out that doing this will actually destroy
[00:03:14.950]any living cells that we were trying to look at.
[00:03:17.370]So if we had a way to take an image of the cell
[00:03:20.010]with shorter exposure time and still get the high contrast
[00:03:23.270]that we need, we would have opportunities
[00:03:26.000]to make advancements in our nanomedicine.
[00:03:30.560]So, that's our background on ghost imaging.
[00:03:33.360]Now let's get to where my research fits into the puzzle.
[00:03:37.180]Earlier, I mentioned the nanotip fibers
[00:03:39.250]were going to be used as the source of electrons.
[00:03:42.090]My job was to figure out how to create
[00:03:43.740]a nice array of these fibers.
[00:03:46.270]For this, a larger array of fibers would mean
[00:03:48.790]the resolution of our images would also be higher.
[00:03:51.950]I found the easiest way to stack these fibers
[00:03:53.970]was in this triangular pattern that you can see here.
[00:04:03.640]All right,
[00:04:04.500]so now I have my list of issues that I ran into
[00:04:07.070]and the solutions I came up with.
[00:04:09.410]And this list is not comprehensive,
[00:04:11.790]but I thought it would be
[00:04:13.664]good to hit some of the main points.
[00:04:17.460]So right off the bat,
[00:04:18.370]the first issue is that the fibers are very small
[00:04:21.090]and very fragile.
[00:04:22.430]The whole fiber is about 20 centimeters long
[00:04:25.130]with a diameter of 125 microns.
[00:04:28.470]And then the tips are about two millimeters long
[00:04:31.350]and tapered to a point of about 50 nanometers.
[00:04:35.840]So if you touch the tip, it's almost guaranteed to break.
[00:04:40.160]To lower the risk of breakage,
[00:04:41.470]I designed a fiber clamp that mounts onto a 3D micrometer.
[00:04:45.640]This allowed me to grab the fiber where it's less fragile,
[00:04:49.740]meaning back from the tip and make very fine adjustments
[00:04:52.980]that would have been impossible to do by hand.
[00:04:57.730]Issue number two, is the fact that each fiber
[00:04:59.890]needed a separate voltage in order to emit electrons.
[00:05:04.125]To avoid messing up the symmetry of the array
[00:05:07.020]a smaller 25 micron tungsten wire was adhered to the fiber
[00:05:11.250]with a conductive silver paste.
[00:05:14.320]Issue number three is the fact that the fibers need to be
[00:05:16.840]in a uniform array, ideally as close as possible.
[00:05:20.560]Since each fiber is coated in a thin film of gold
[00:05:23.070]at the tip, this means the fibers
[00:05:24.970]must be electrically insulated from each other.
[00:05:28.260]To maintain uniformity and adhesive with low expansion
[00:05:31.410]needed to be used.
[00:05:33.230]I found that coating the wire in thin film of nail polish
[00:05:36.280]worked as a good insulator.
[00:05:38.750]However, this leads me into my fourth issue,
[00:05:41.550]which is the fact that this array of fibers,
[00:05:43.330]needs to be put in a vacuum.
[00:05:45.700]Unfortunately, I never had time to test the nail polish
[00:05:49.020]in a vacuum, so this issue still remains unresolved.
[00:05:52.980]From what I've read, nail polish is an uncommon material
[00:05:56.610]to put a vacuum system.
[00:05:58.100]So I think this would be something to experimentally test
[00:06:01.580]in the future.
[00:06:03.910]And then this brings us to the final issue that I had,
[00:06:06.360]which was with the adhesive,
[00:06:08.160]possibly reacting with the cladding of the fiber,
[00:06:10.730]and then just messing up the electron emission.
[00:06:15.350]To test this, I basically did trial and error
[00:06:17.900]with a bunch of different adhesives,
[00:06:19.970]including the silver paste, tire seal,
[00:06:22.270]liquid electrical tape, and nail polish.
[00:06:24.930]To test if the power transmission was effective,
[00:06:27.090]I measured the power through a fiber
[00:06:28.860]before anything was changed.
[00:06:31.120]Then I put on whatever adhesive I was testing
[00:06:33.890]and gave it 24 hours for any possible reactions
[00:06:37.020]to take place and then measured power again, the next day.
[00:06:41.450]I kind of gave it away, I gave away the answer already,
[00:06:43.960]but the nail polish ended up being the best material,
[00:06:47.050]since it was low reactive and it had the right viscosity
[00:06:49.830]that made it easy to apply to the fibers.
[00:06:55.920]To conclude my research,
[00:06:57.130]I found that the best method to create an array
[00:06:59.930]was to assemble each fiber with its supply voltage
[00:07:02.570]and insulate each fiber before putting them together.
[00:07:06.490]On average, the diameter of the component increased from 125
[00:07:10.115]to 350 microns.
[00:07:12.300]This value is largely dependent on the amount of insulation
[00:07:15.110]each fiber had,
[00:07:16.480]which was pretty inconsistent due to the polish
[00:07:18.520]being applied by hand.
[00:07:20.600]Moving forward, I think it would be best to investigate
[00:07:23.070]how the array of fibers will hold up in a vacuum
[00:07:26.390]as well as how the nail polish out gases in the vacuum.
[00:07:29.860]Additionally, I think it could be beneficial
[00:07:32.090]to explore ways to revise the current method of insulating
[00:07:35.040]the fibers or exploring alternative methods altogether.
[00:07:42.340]Well, that about concludes my research for the summer,
[00:07:45.180]for my acknowledgements.
[00:07:46.320]I would like to give a special thanks to Sam Keramati,
[00:07:49.000]Rachel Corzine, Will Sippel
[00:07:51.030]and the rest of the physics department at UNL
[00:07:53.070]for their support and assistance this summer.

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Toward Computational Electron Ghost Imaging

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