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Magnesium as a Novel UV Plasmonic Material for Fluorescence Decay Rate Engineering in Free Solution Yunshan Wang,† Eric M. Peterson,‡ Joel M. Harris,‡ Kanagasundar Appusamy,§ Sivaraman Guruswamy,§ and Steve Blair*,† †

Department of Electrical and Computer Engineering, ‡Department of Chemistry, and §Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, United States S Supporting Information *

ABSTRACT: We report modification of the ultraviolet (UV) fluorescence decay rate of p-terphenyl dye molecules by magnesium (Mg) nanoapertures in free solution, in comparison with aluminum (Al). Mg nanoapertures exhibit a lifetime reduction of up to ∼7.2×, which to our knowledge is the largest lifetime reduction of a UV fluorescence dye reported so far in the literature. In comparison, Al nanoapertures exhibit a lifetime reduction of ∼5.3×, exceeding the previously reported value ∼3.5× due to smaller aperture size employed here. Simulation results reveal large plasmon resonance frequency shifts between Mg and Al nanoapertures, contributing to different lifetime reductions, where average lifetime reductions of Mg nanoapertures are greater than those of Al nanoapertures with diameters smaller than 40 nm. The dependence of count rate per molecule (CRM) on aperture size and aperture undercut is also reported, revealing that CRM increases with increasing undercut.

1. INTRODUCTION UV plasmonics has drawn increased attention in recent years, holding promise in enabling label-free sensing of biomolecules such as DNA, peptides, and proteins whose intrinsic fluorescence lie in the UV range.1 However, these biomolecules exhibit relatively small quantum yields (QY) and extinction cross sections.2,3 In order to realize label-free detection of biomolecules, significant enhancement needs to be achieved.4 Plasmonic structures have been reported to enhance native fluorescence of DNA and peptides.5,6 However, quantitative fluorescence analysis that can differentiate the contribution of radiative and excitation enhancement7,8 is needed for UV studies. In that direction, previously, we have studied lifetime modification of UV fluorescence molecules in Al nanoapertures;9 specifically, freely diffusing molecules instead of molecules immobilized on a substrate,5,6 which are more prone to photobleaching and quenching effects. Other than Al, metals such as magnesium, gallium (Ga), and indium (In) have been shown to support surface plasmons in the UV range as well.10−12 We are particularly interested in Mg since it possesses a higher localized surface plasmon figure-ofmerit than Al through portions of the UV spectrum.11,13 Mg has also recently been demonstrated as an active plasmonic material for hydrogen sensing.12,14 In addition, Mg has been alloyed with other metals such as Al and Ga to achieve tunable UV plasmonic resonance.13,15−17 The challenge of using Mg as a UV plasmonic material is that its oxide layer continues to grow into the bulk Mg when exposed to water and oxygen, which deteriorates its plasmonic properties significantly.13,18 © XXXX American Chemical Society

Various methods have been demonstrated to improve resistance of Mg to oxidization.13,16,19 We conduct experiments in 1-octanol, a nonaqueous solvent, to prevent Mg from being exposed to water and oxygen. We report here, for the first time, the application of Mg plasmonic structures to modify decay rate of UV dye molecules in free solution and achieved the highest reported lifetime reduction of UV dye molecules. To compare the performance of Mg and Al nanoapertures, we designed nanoapertures with the same geometric parameters and measured the lifetime change of freely diffusing pterphenyl. For Al nanoapertures, we observed greater lifetime reduction ∼5.3× instead of previously reported ∼3.5×9 due to smaller aperture sizes employed here. Lifetime reduction refers to the ratio of the lifetime of molecules in free solution to the lifetime of molecules inside the nanoapertures. Previous simulations suggested that Mg nanoapertures produce greater decay rate modifications compared to Al apertures of diameters smaller than 50 nm.20 More detailed simulations comparing Al and Mg nanoapertures are discussed in this paper. Experimental results compare the lifetime reduction and net enhancement from Mg and Al nanoapertures and are consistent with simulations. Received: February 27, 2017 Revised: April 28, 2017 Published: May 4, 2017 A

DOI: 10.1021/acs.jpcc.7b01934 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic of an array of nanoapertures in Mg or Al film. Inset shows a cross-section view of a single nanoaperture. (b) SEM image of an array of nanoapertures on a Mg film. Diameters of nanoapertures increase in the vertical direction, while undercuts increase in the horizontal direction. The inset shows a cross-section view of a single nanoaperture with scale bar 50 nm.

2. EXPERIMENT 2.1. Sample Fabrication. A Mg film with 90 nm thickness was deposited onto a coverslip with a 10 nm Al seed layer.21 A 100 nm Al thin film was sputtered (Denton discovery 18) onto 1 in. diameter UV fused silica coverslips (Esco Optics, Inc.) at a deposition rate 3.7 Å/s. The coverslip thickness is 160 μm. The dielectric constants of the Mg and Al films were measured using a Woollam variable angle spectroscopic ellipsometer (VASE) and are reported in section 4. Nanoapertures were milled into Mg and Al films by focused ion beam (FEI Helios) under iodine gas injection. Designed sizes of nanoapertures range from 40 to 90 nm and milling depth ranges from 20 to 200 nm into the fused silica substrate. As depicted in Figure 1a, a 2-D array of nanoapertures was fabricated with increasing diameter in one direction and increasing undercut in the perpendicular direction. The inset shows the cross-section view of a nanoaperture with undercut. Undercut is the depth of an aperture into the substrate. Each aperture is spaced ∼1 μm apart from each other to enable excitation of a single aperture at a time. A SEM image of the top view of an array of nanoapertures milled into a Mg film is shown in Figure 1b. The inset of Figure 1b shows a SEM image of the cross-section of a single nanoaperture. The scale bar in the inset is 50 nm. From the SEM images of the cross sections of single nanoapertures, we measured upper diameters, bottom diameters, and undercuts of nanoapertures and plotted these parameters against dose (milling time in FIB) in Figure S1. Bottom diameters and undercuts generally increase with increasing dose. It is also noteworthy that top diameters are larger than bottom diameters, and the difference can be as large as 50 nm. Therefore, the exact shape of each nanoaperture needs to be considered during numerical simulations, as will be discussed later. 2.2. Fluorescence Lifetime Measurement. The measurement setup is illustrated in Figure 2 and is described in detail in Figure S3. A pulsed laser source (266 nm) was focused with a Zeiss Ultrafluar 40× (N.A. 0.6) glycerine objective through the substrate. A drop of 1-octanol solvent containing 100 μM p-

Figure 2. A UV objective focuses light onto one nanoaperture and collects the emission of dye molecules freely diffusing in and out of the nanoaperture.

terphenyl dye was placed on top of the metal film. The expanded diagram shows dye molecules only within the nanoapertures being excited due to the fact that the thickness of Al or Mg film is much larger than their corresponding skin depth. Since the dye molecules can freely diffuse in and out of the nanoapertures, photoexcited molecules are constantly replenished by molecules from bulk solution. Therefore, the fluorescence intensity of a nanoaperture remains relatively constant, which ensures the accuracy of lifetime and fluorescence intensity measurements. Fluorescence emission is imaged onto a 30 μm confocal pinhole before passing through an emission bandpass filter (357 ± 22 nm). Finally, emission is collected by a PMT detector which is monitored by a timecorrelated single-photon counting (TCSPC) system. The lifetime of fluorescence emission is recovered through reconvolution of a fitted arrival-time histogram with the instrument response function (see Supporting Information9). Fluorescence count rate is recorded by the number of photons detected each second above the dark rate.

3. COMPUTATIONAL DETAILS 3.1. Fluorescence Model. Modification of fluorescence dynamics is briefly described here.22,23 The change in quantum yield can be expressed as B


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Figure 3. (a) Ideal cylindrical model of nanoapertures. (b) Conical aperture model with undercut region described by a parabolic function.

fϕ =

frad fτ

describe the actual nanoapertures in experiments and will be used to model results in the Experiment section. Three-dimensional electromagnetic simulation is performed using Lumerical FDTD Solutions. Symmetric boundaries are used along the y direction and antisymmetric boundaries are used along the x direction at the center of the aperture due to the x-polarized excitation and dipole orientation. Perfectly matched layers (PML) are used at the other boundaries. The sampling size is 2 × 2 × 2 nm3. In order to calculate excitation enhancement, a plane wave with unit amplitude (1 V/m) is introduced inside the substrate, which normally illuminates the nanoaperture from the bottom. Average enhancement is calculated by integrating the total intensity within a volume within the aperture and dividing by the integrated intensity within the same volume but in the absence of the Al/Mg layer. For the emission calculations, analysis of the FDTD results uses the fact that every rate constant is proportional to its corresponding power.27 An electric dipole with unit amplitude (1 V/m) is positioned at the center of the active region. The radiative emission (krad), for example, is calculated as the transmission through monitors around the structure, while the spontaneous emission (krad + knr) is calculated as the transmission through monitors around just the dipole. Here we used frequency-domain field monitors that collect the field profile in the frequency domain across a spatial region that is defined by the monitor. The radiative enhancement can be calculated by dividing the radiative emission in the presence the ′ ) to the radiative emission in the absence of nanoaperture (krad the nanoaperture (krad). The lifetime change of the ideal dipole ζ28 is calculated by dividing the spontaneous emission from the dipole within the nanoaperture (k′rad + k′nr + knr) to the dipole emission in the absence of the nanoaperture (krad + knr); the predicted lifetime change of a real molecule is then obtained from ζ by using the molecule’s native QY as a correction factor (see eq 2),9,20 which takes into account nonradiative losses. Calculations are performed for the x dipole orientation only, due to the symmetry of the aperture and the fact that the z orientation makes a negligible contribution to far-field emission.

(1)

where f rad = krad ′ /krad is the ratio of the radiative rate with the presence of metallic structure (k′rad) and without the structure (krad). The ratio of modified and native spontaneous emission rates fτ =

k′ + k′nr + k nr τ = rad = 1 + ϕ0(ζ − 1) τ′ k rad + k nr

(2)

represents the reduction in lifetime of the molecule24 and is experimentally measurable; here ϕ0 is the native quantum yield, and ζ is the change of lifetime of an ideal dipole emitter. Net fluorescence enhancement (NE) is expressed as NE = fκ fI

frad fτ

(3)

where f I is the excitation enhancement and fκ is change in collection efficiency (which we assume to be 1). 3.2. Simulation Model. The fluorescence simulation models considered in this paper are illustrated in Figure 3. The material properties used in the simulation were taken from ellipsometry data measured on the films used in fluorescence measurements, as will be shown in section 4. The nanoapertures are supported by a semi-infinite silica (SiO2) substrate and covered by 1-octanol. 1-Octanol, a nonaqueous solvent, is chosen instead of aqueous solvent to avoid the degradative reaction between Mg oxide and water. For reference, the refractive index of 1-octanol is 1.46 measured at 325 nm.25 The thicknesses of the native oxide layer along the exposed surfaces and the interfacial oxide layer at the substrate are assumed to be 4 nm, which is based on X-ray photoelectron spectroscopy (XPS) measurements in our previous studies.11,15 Initially, the model assumes nanoapertures represented by perfect cylinders throughout the metal film and the undercut region into the substrate. This model was adopted by our previous studies.9,20 Although this model qualitatively captures the trend of lifetime reduction versus diameter and undercut, it fails to agree with experimental data quantitatively. Figure 3b shows a conical aperture26 where the region within the metal film has a tapered profile, while the undercut region within the substrate has a parabolic shape. Cross sections of nanoapertures are shown in Figure S2. This model was found to better

4. RESULTS AND DISCUSSION 4.1. Experimental Results. The optical constants and measurement procedures of Mg and Al film can be found in our previous publications.11,15 Both films were kept in argon environment during sample transfer but exposed to ambient environment during ellipsometry measurements. Figures 4a and C


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Figure 4. (a) Real part of the permittivity of Al and Mg from Palik and measured data. (b) Imaginary part of the permittivity of Al and Mg from Palik29 and measured data. (c) Figure of merit from which bounds on the absorption, scattering, and spontaneous emission rate enhancement can be obtained.

decreasing undercut, especially for small diameters, while for Al nanoapertures, lifetime reduction increases at a slower rate. This might be caused by different nanoaperture geometries as shown in Figure S1. For Mg nanoapertures, both top and bottom diameters decrease with reducing undercut, while for Al nanoapertures, the top diameters show relatively constant to increasing trends with reduced undercut. Thus, we can expect a faster change of lifetime reduction with respect to undercut for Mg nanoapertures since smaller diameters causes larger lifetime reduction based on our previous studies.9 The dashed lines are fitting curves using the function a/(b + x), where x is the undercut and a and b are fitting parameters. Those fitting curves are to guide the eyes through the data points. 4.2. Simulation Results. To examine the enhancement factors of Mg and Al nanoapertures at the excitation (λ ∼ 270 nm) and emission (λ ∼ 340 nm) wavelengths of the pterphenyl dye, Figure 6 plots the rate-enhancement distributions along the depth of nanoapertures with the cylindrical geometry, assuming a 150 nm undercut. The cylindrical shape was chosen in this case to simplify the comparison between Mg and Al nanoapertures. Figures 6a and 6d plot excitation enhancement within the Mg and Al nanoapertures versus monitor position along the nanoaperture. Excitation enhancement was calculated by averaging the electric field intensity within a 10 nm thick monitor and normalized by the averaged intensity within the same monitor without the metal film but with the substrate and solvent. The positions with negative numbers are located in the undercut region. For Mg nanoapertures, the resonance peaks are within the center of the nanoaperture about 20−50 nm above the metal− substrate interface. As the diameter of the apertures becomes larger, the resonance peaks also shift toward the middle of the nanoaperture, with the greatest enhancement for 72 nm diameter. The shift of the resonance peaks from the metal− dielectric interface to the center of the nanoaperture is a result of waveguide modes at the excitation wavelength. This is consistent with prior simulation results.20 As for Al nanoapertures, maximum excitation enhancements occurs near 0 nmthe position of the metal−substrate interfaceand the enhancement diminishes away from this interface. This is a signature of localized surface plasmon excitation at the bottom interface of the nanoapertures. As the diameter of the apertures becomes larger, the resonance peak reduces and spatially shifts toward the middle of the nanoaperture. In addition to the resonance peak of the nanoaperture, in both cases there is a strong resonance peak within the undercut region, which is caused by standing waves

4b show the real and imaginary parts of the permittivities of Al and Mg films. Those optical constants are bulk optical constants by assuming a 4 nm surface oxide layer ellipsometry model. Solid lines are from the ellipsometry data, while dashed lines are from the Palik handbook.29 A relevant figure of merit that bounds on the absorption, scattering, and spontaneous emission rate enhancement is |χ|2/ Im{χ},30 where χ is electric susceptibility. Figure 4c calculates this figure of merit using both the Palik and measured material data. From the Palik data, Mg is a better material for wavelengths longer than 360 nm. From the measured data, Mg and Al are nearly equivalent, with Mg being slightly better for wavelengths longer than 650 nm. Figures 5a and 5b show the measured fluorescence lifetime reduction for p-terphenyl versus undercut and diameter of Mg

Figure 5. (a) Experimentally measured lifetime reduction by Mg nanoapertures plotted versus undercut. (b) Experimentally measured lifetime reduction by Al nanoapertures plotted versus undercut. Each symbol represent a different diameter.

and Al nanoapertures. The legends show designed diameters. For both Mg and Al nanoapertures, the lifetime reduction increases with reduced aperture diameter and undercut. Excitation intensities were kept below the saturation intensity. Lifetime reduction was calculated by dividing lifetime values in free solution by the lifetime values observed inside nanoapertures. For Al nanoapertures, each data point was averaged over measurements on three separate apertures milled under the same FIB settings, ostensibly with the same diameter and undercut. The standard deviation is used for the error bars. For Mg nanoapertures, only one measurement was taken. The greatest lifetime reduction for Mg nanoaperture is ∼7.2×, while for Al nanoapertures it is ∼5.3×. The measured lifetime reduction from Mg nanoapertures exceeds that from Al nanoapertures for diameters less than 60 nm. The slopes of the lifetime reduction curves are different for Mg and Al nanoaperturesMg lifetime reduction increases rapidly with D


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Figure 6. Simulated enhancements at 270 nm excitation and 340 nm dipole emission versus monitor position within Mg nanoapertures with different diameters: (a) excitation enhancement, (b) radiative enhancement, and (c) lifetime reduction. Simulated enhancements at 270 nm excitation and 340 nm dipole emission versus monitor position of Al nanoapertures with different diameters: (d) excitation enhancement, (e) radiative enhancement, and (f) lifetime reduction.

Figure 7. (a) Simulated lifetime reduction, (b) average radiative enhancement, and (c) average net enhancement of Mg nanoapertures with cylindrical shaped holes versus undercut for different diameters. (d) Simulated lifetime reduction, (e) average radiative enhancement, and (f) average net enhancement of Al nanoapertures with cylindrical shaped holes versus undercut for different diameters. Scattered points are simulated data, while dashed lines are fitted curves.

interface. However, for Mg nanoapertures, the greatest enhancement occurs for the smallest diameter of 30 nm while for Al nanoapertures, the greatest radiative enhancement occurs for the 52 nm diameter aperture. For the lifetime reduction of an ideal dipole (here, ϕ0 = 1), maxima are located at the top and bottom interfaces as in Figure 6c,f. The lifetime reduction from Mg exceeds that from Al at the smaller

inside the undercut region formed by reflections at the substrate−liquid and aperture interfaces.31 Figures 6b and 6e plot the radiative enhancements of Mg and Al nanoapertures for emission at 340 nm. For emission calculations, a dipole with unit amplitude moved along the height of nanoapertures. Both Mg and Al nanoapertures show maximum radiative enhancement near the metal−substrate E


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Figure 8. (a) Experimentally measured lifetime reduction by Mg nanoapertures plotted versus undercut. Each symbol represent a different diameter. (b) Simulated lifetime reduction of p-terphenyl dye by Mg nanoapertures. (c) Simulated average radiative rate enhancement by Mg nanoapertures. (d) Experimentally measured lifetime reduction by Al nanoapertures plotted versus undercut. (e) Simulated lifetime reduction of p-terphenyl dye by Al nanoapertures. Dashed lines are fitted curves using the function a/(b + x). (f) Simulated average radiative rate enhancement by Al nanoapertures. Dashed lines are fitted curves using a third-order polynomial.

diameters due to ohmic losses, as their radiative enhancements are comparable. For diameters greater than 50 nm, Al nanoapertures show comparable or slightly greater lifetime reduction than Mg. To compare with experimental results, average lifetime reduction and count rate enhancement across the depth of the apertures are shown in Figure 7a,c,d,f. Average lifetime reduction is calculated by averaging the lifetime reduction of each dipole position across the undercut and aperture region, weighted by the net enhancement at that position. Net enhancement (NE) is calculated by eq 3. Similarly, average radiative enhancement is calculated by averaging the radiative enhancement of each dipole position weighted by the net enhancement. Although radiative enhancement cannot be measured directly in experiments, the calculation provides insight into how radiative enhancement contributes to lifetime reduction. Average net enhancement is averaged across the depth monitors with no weighting. From Figures 7a and 7d, we can see that for nanoapertures with diameters greater than 40 nm, Al nanoapertures show greater lifetime reduction compared with Mg nanoapertures. For diameters smaller than 40 nm, Mg shows greater lifetime reduction. This is due to the fact that for Mg nanoapertures maximum net enhancement is vertically offset from where the maximum lifetime reduction occurs. Therefore, only for very small nanoapertures does Mg show greater lifetime reduction on average due to its greater lifetime reduction at the metal−substrate interface. In Figures 7a and 7d, lifetime reductions are fitted using the function a/(b + x), where x is the undercut and a and b are fitting parameters, and plotted as dashed lines. Figures 7b and 7e show that for Mg nanoapertures the highest average radiative enhancement occurs for diameters ∼32 nm, while for Al nanoapertures, greatest average radiative enhancement occurs for ∼52 nm diameter. Dashed lines are third-order polynomial fitting of the simulated points. The greatest average radiative enhancements

happen when the undercut is from 25 to 100 nm. Overall, Al nanoapertures show greater average radiative enhancement than Mg nanoapertures. Figures 7c and 7f show that for both Mg and Al nanoapertures net enhancement increases with undercut, but the dependence on diameter is different. For Mg nanoapertures, net enhancement increases with diameter, while for Al nanoapertures with small undercut, net enhancement increases with diameter, and for large undercut, Al net enhancement decreases with increasing diameter. Figures 8b and 8e plot simulated lifetime reduction of a nonideal dipole (here, ϕ0 = 0.88) side by side with corresponding experimental data. Measured nanoaperture geometries show significant deviation from designed diameters, and the actual aperture shape is not cylindrical. These simulations therefore use the conical hole geometry illustrated in Figure 3b and use the corresponding geometrical parameters of each nanoapertures from Figure S1. For the conical nanoaperture model, the diameter of each monitor varies according to the size of the nanoaperture at the position of the monitor, so that the monitor lies just within the nanoaperture. As before, lifetimes are calculated for each 10 nm thick monitor within the nanoaperture and undercut region. The averaged lifetime is the lifetime averaged across the depth, weighted by volume and net enhancement of each slice volume. The simulated lifetime results match very well with experimental data, except for the smallest undercuts and diameters where simulated results are higher than experimental data. This could be due to detection limits of the measurement system. For small apertures with small undercuts, the number of photons collected is comparable to the dark counts of the PMT. Nevertheless, both the simulations and experimental results show the trend of greater lifetime reduction with decreasing diameter and undercut. In Figure 8a,b,d,e, dashed lines are fitted curves using the function a/(b + x), where x is the undercut and a and b are fitting parameters. F


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The Journal of Physical Chemistry C Notes

Figures 8c and 8f plot simulated average radiative enhancement that uses the conical hole geometry and the corresponding geometrical parameters as in Figures 8b and 8e. The averaged radiative enhancement is the radiative enhancement averaged across the depth, weighted by volume and net enhancement of each slice volume. From Figure 8c, it shows that the highest average radiative enhancement for each Mg aperture is reached at the smallest aperture diameter, and with increasing aperture diameter, maximum radiative enhancement is found at increasing undercut. This is consistent with the trend shown in simulations of cylindrical aperture, as in Figure 7b. On the other hand, the highest average radiative enhancement for each Al aperture size occurs when undercut is less than 50 nm and reaches a maximum for the smallest aperture diameter. This is different from the trend shown in Figure 7e, as the actual geometries of apertures deviate from designed geometries. The dashed lines are fitting curves using a third-order polynomial function.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Yunshan Wang, Kanagasundar Appusamy, Sivaraman Guruswamy, and Steve Blair received funding from NSF MRSEC Grant DMR-1121252 to support this work. Eric Peterson and Joel Harris received funding from the Chemistry Division of NSF(Award No. CHE-1608949). This work made use of University of Utah USTAR shared facilities support, in part, by the MRSEC Program of NSF under Award No. DMR-1121252. We thank the Surface Analysis and Nano-Scale Imaging Lab at Utah Nanofab to the access of the ellipsometry and FIB and help from the supporting staff: Brian R. Van Devener, Paulo Perez, and Randy C. Polson. The support and resources from the Center for High Performance Computing at the University of Utah are gratefully acknowledged.

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5. CONCLUSIONS In summary, we reported UV fluorescence intensity and lifetime modification of diffusing p-terphenyl dye molecules by Mg and Al nanoapertures. Lifetime reduction of ∼7.2× was observed for Mg nanoapertures, and ∼5.3× was observed for Al. The dependence of count rate per molecule (CRM) on aperture size and undercut was also investigated (additional experimental results are shown in the Supporting Information). FDTD simulations took into account the conical shape of the nanoapertures and undercut region, along with the native oxide layer, and compared well with experimental results. Both experiments and simulations show that Mg nanoapertures produce greater change of lifetime for small nanoapertures compared with Al, which suggests that Mg could be more promising for UV applications where modification of upper state lifetime is important, such as improving modulation speed of light-emitting diodes.32 On the other hand, Al nanoapertures show greater average radiative enhancement than Mg, which suggests that Al is better material for UV fluorescence enhancement applications. Net enhancement for both Mg and Al nanoapertures increase with undercut, revealing that radiative rate enhancement dominates over nonradiative rate at the undercut region of nanoaperture, while nonradiative rate is dominant within the metallic region of nanoaperture; nevertheless, realizable net enhancements are of order 1. New nanoaperture designs need to be explored in order to achieve greater net enhancement while maintaining large background suppression.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01934. Fabrication SEM images, system setup details, selection of photo arrival histogram, count rate measurement results, and selected simulated near-field images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.B.). ORCID

Yunshan Wang: 0000-0002-8102-1588 Joel M. Harris: 0000-0002-7081-8188 G


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