of 7
Research Article
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
103
Highly efficient visible and near-IR photon pair
generation with thin-film lithium niobate
Nathan A. Harper,
1,
Emily Y. Hwang,
2,
Ryoto Sekine,
3
Luis Ledezma,
3
Christian
Perez,
4
Alireza Marandi,
2,3
AND
Scott K. Cushing
1,
1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
2
Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, California, 91125, USA
3
Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, USA
4
California State University, Los Angeles, California 90032, USA
These authors contributed equally to this work.
*
scushing@caltech.edu
Received 9 October 2023; revised 26 February 2024; accepted 27 February 2024; published 10 April 2024
Efficient on-chip entangled photon pair generation at telecom wavelengths is an integral aspect of emerging
quantum optical technologies, particularly for quantum communication and computing. However, moving to
shorter wavelengths enables the use of more accessible silicon detector technology, and opens up applications
in imaging and spectroscopy. Here, we present high brightness ((1.6
±
0.3)
×
10
9
pairs/s/mW/nm) visible–near-
IR photon pair generation in a periodically poled lithium niobate nanophotonic waveguide. The degenerate
spectrum of the photon pairs is centered at 811 nm with a bandwidth of 117 nm when pumped with a spectrally
multimode laser diode. The measured on-chip source efficiency of (2.3
±
0.5)
×
10
11
pairs/s/mW is on par with
source efficiencies at telecom wavelengths and is also orders of magnitude higher than the efficiencies of other
visible sources implemented in bulk crystal or diffused waveguide-based technologies. Further improvements in
the brightness and efficiencies are possible by pumping the device with a single-frequency laser, which would
also shrink the pair bandwidth. These results represent the shortest wavelength of photon pairs generated in a
nanophotonic waveguide reported to date by nearly an octave.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
https://doi.org/10.1364/OPTICAQ.507526
1. INTRODUCTION
Spontaneous parametric downconversion (SPDC) has been used
for decades to produce quantum entanglement in various pho-
tonic degrees of freedom, serving as a workhorse in emerging
quantum optical technologies. Compared with most nonlinear
processes, SPDC is relatively inefficient, requiring over one
million pump photons to produce one pair of entangled pho-
tons in even the highest performing crystals. However, recent
advances in nanophotonics, particularly in thin-film lithium nio-
bate (TFLN), have enabled significantly more efficient frequency
conversion and quantum state generation [1–3] through sub-
μ
m
interaction areas, high nonlinearities, and low material losses
[4,5]. By exploiting this platform, many recent demonstrations
of SPDC in TFLN [6–9] have achieved efficiencies three orders
of magnitude greater than that of bulk crystal-based sources [10]
and one order of magnitude greater than that of large diffused
waveguide-based sources [11]. To date, most TFLN-based pho-
ton pair sources are designed for SPDC at telecom wavelengths
because of the low losses in optical fibers at 1550 nm [12,13] and
back-compatibility for applications such as quantum communi-
cation [14], computing [15], and a globally connected quantum
network [16].
Although telecom photons are preferred for quantum informa-
tion applications, visible and near-infrared photons are generally
better suited for imaging and spectroscopy. Experiments at these
wavelengths can take advantage of multi-pixel detectors such
as electron-multiplying charge-coupled devices (EM-CCD) and
single-photon avalanche detector (SPAD) arrays, enabling the
measurements needed for imaging [17–19] and characteriza-
tion of high-dimensional entangled states [20,21]. Furthermore,
the electronic transitions of molecules and atoms become
accessible at near-IR wavelengths, allowing for fluorescence
lifetime measurements [22–24], compatibility with quantum
memories [25], and fundamental studies of few-photon nonlin-
earities [10,26,27]. More generally, near-IR and visible photons
can be detected with high quantum efficiency and low dark
noise using existing mature silicon technology at room tem-
perature, compared with near-IR–IR detectors which require
cryogenic cooling [28]. Despite these advantages, all demon-
strations of nanophotonic pair production have resided in the
telecom region, and the best near-IR and visible photon pair
sources are still large-area waveguides [29–32] and bulk periodi-
cally poled crystals [10,26,29,33–36]. To date, lithium niobate is
the only nanophotonic platform that exhibits high transparency
2837-6714/24/020103-07 Journal © 2024 Optica Publishing Group
104
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
Research Article
at near-UV pump wavelengths and supports a
χ
(
2
)
nonlinear-
ity. Thus, despite the known challenges, TFLN is uniquely
posed to address this wavelength range. Potential reasons for
the lack of visible TFLN devices stem from the difficulty in fab-
ricating visible nonlinear circuits on thin-film lithium niobate
due to factors such as the ultra-short poling periods required
for quasi-phase matching and losses from material absorption
[37–39] and scattering [40]. In spite of these difficulties, high-
performance visible devices in thin-film lithium niobate are
becoming increasingly common for classical applications such
as electro-optic modulation and second harmonic generation
[41–45].
Here, we extend TFLN-based SPDC sources to shorter wave-
lengths to produce high-brightness photon pairs in the visible
and near-IR. The device produces an on-chip efficiency of
(
2.3
±
0.5
) ×
10
11
pairs/s/mW, which corresponds to a per-photon con-
version efficiency of more than 1 photon pair converted in every
10,000 pump photons [
(
1.1
±
0.2
) ×
10
4
pairs/s/photon]. This
efficiency is nearly two orders of magnitude better than visible-
light diffused waveguide SPDC sources [31] and is on par with
the highest-performing TFLN sources in the telecom regime [8].
The SPDC from this device exhibits a broad spectrum centered
at 811 nm with a degenerate FWHM bandwidth of 117 nm and an
average brightness of
(
1.6
±
0.3
10
9
pairs/s/mW/nm, a num-
ber limited by at least an order of magnitude by the pump laser
linewidth (0.8 nm). Consistent with this bandwidth, we measure
an ultrashort coherence time of
40 fs for the entangled pho-
tons with an indistinguishability of 100
±
1
%
. These results are
the shortest wavelength entangled photons generated in TFLN
by nearly an octave to date. Our results therefore show that,
although pumped at wavelengths near what would usually be
considered its cutoff range, TFLN can equally be a platform for
visible–near-IR entangled photon applications as it is at telecom
wavelengths.
2. DEVICE DESIGN AND FABRICATION
The periodically poled lithium niobate waveguides [Fig. 1(a)]
were simulated in Lumerical MODE to determine the quasi-
phase-matching poling period. The guided modes at the design
pump wavelength (406 nm) and SPDC center wavelength (812
nm) were simulated using the bulk Sellmeier coefficients of
lithium niobate [46] and silicon dioxide [47] with the geometric
parameters shown in Fig. 1(c) and a 60
°
sidewall, which is
consistent with the fabrication process. To take advantage of
lithium niobate’s largest nonlinear tensor element (
d
33
=
28
pm/V) [48], only the fundamental quasi-TE modes of X-cut
lithium niobate were considered. An etch depth of 420 nm, a
top width of 1.5
μ
m, and a total LN thin film thickness of
600 nm were targeted for ease of optical coupling, fabrication,
elimination of slab-mode leakage, and near-2-
μ
m poling period
while providing high performance. For these parameters, the
effective refractive indices (
n
eff,pump
=
2.29,
n
eff,SPDC
=
2.09) result
in a quasi-phase-matching poling period of
Λ
=
λ
pump
/
n
eff
=
2.03
μ
m at the target pump wavelength of 406 nm [Fig. 1(d)].
The devices were fabricated from a 1 cm by 1 cm die of
a 5% MgO-doped X-cut thin-film lithium niobate on insu-
lator wafer (NanoLN), which consists of 600 nm of lithium
niobate bonded to 2
μ
m of silicon dioxide on a 0.4-mm sil-
icon substrate. An MgO-doped film was chosen to lower the
coercive field necessary for poling [49,50] and to reduce poten-
tial photorefractive effects from the violet laser diode [51].
Fig. 1.
(a) Schematic of the periodically poled lithium niobate
nanophotonic waveguide. (b) Second harmonic microscopy image
of the periodic poling. Note that the distance between the electrodes
is 15
μ
m while the waveguide top width is 1.5
μ
m, allowing for
multiple waveguides in the center of the poled region where the
duty cycle is
50%. (c) Mode profiles and waveguide geometry of
the fundamental quasi-TE modes at the designed pump and SPDC
center wavelengths. (d) Refractive indices and corresponding poling
periods for a range of SPDC wavelengths.
MgO-doped LN has been demonstrated to lower propagation
losses [38], but this doping may also result in irregular poled
domains due to leakage current in the charged domain walls
[50]. To quasi-phase match the SPDC sources, poling elec-
trodes (7 mm long) were first fabricated by performing a metal
lift-off through electron beam lithography with bilayer poly-
methyl methacrylate (PMMA) resist followed by electron beam
evaporation of titanium (15 nm) and gold (55 nm). The elec-
trodes were poled with a 490-V and 70-
μ
s square wave, and
the poled domain formation was monitored with second har-
monic microscopy [52] [Fig. 1(b)]. After poling, waveguides
were defined through an aligned electron beam lithography step
with hydrogen silesquioxane (HSQ) resist followed by argon
inductive coupled plasma reactive ion etching to achieve an etch
depth of 420 nm, verified through atomic force microscopy.
The chip facets were manually polished to increase coupling
efficiency, resulting in a final waveguide length of approxi-
mately 8 mm. A broadband oscillator was used to verify the
phase-matching wavelength through second harmonic genera-
tion (Supplement 1 Section S2) and was found to match with the
computationally predicted second harmonic wavelength within
±
3 nm. This small discrepancy was likely due to fabrication
tolerances, particularly in the etch depth and film thickness.
3. DEVICE CHARACTERIZATION
The spectrum, generation rate, and coherence properties of the
entangled photon pairs produced from the fabricated device
are characterized, as shown in Fig. 2. In these experiments,
the room-temperature periodically poled waveguide is pumped
Research Article
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
105
Fig. 2.
Schematic for coupling and detection of the photon pairs.
I, isolator; ND, variable neutral density filter; HWP, half-wave
plate; L, aspheric lens; LPF, long-pass filter; emICCD, electron-
multiplying intensified charge-coupled device; BS, beamsplitter; D,
single-photon avalanche detector; TT, time-tagger; M, mirror. (a)
Optical setup to couple into and out of the TFLN waveguide. (b)
Characterization scheme for measuring the photon pair spectra. (c)
Characterization scheme for coincidence counting. (d) Optical setup
for the Michelson interferometer.
with a free-running laser diode (Coherent OBIS LX 405 nm)
to produce entangled pairs [Fig. 2(a)]. An antireflection-coated
aspheric lens (NA
=
0.58, Thorlabs C140TMD-A) couples the
free-space pump beam to the fundamental TE mode of the wave-
guide. The photon pairs produced in the fundamental TE mode
are collected off-chip and collimated using a similar aspheric
lens (NA
=
0.58, Thorlabs C140TMD-B).
The spectra of the entangled photon pairs are measured to
assess the phase-matching properties and tunability of the device
(Fig. 3). Pairs collected from the waveguide are transmitted
to a grating spectrometer and measured using an electron-
multiplying intensified camera [Fig. 2(b)]. To tune the SPDC
emission, the center wavelength of the pump wavelength is var-
ied from 405 to 406.4 nm by changing the drive current of
the laser diode. Variable neutral-density filters are used to keep
the pump power for all the collected spectra at a consistent 10
μ
W. Each spectrum was divided by the spectrometer wavelength
response and then normalized to the maximum count rate to infer
the relative intensity. Three distinct phase-matching regions are
explored [Fig. 3(a)]: (1) at long pump wavelengths, the phase-
matching condition is not satisfied and SPDC emission is not
Fig. 3.
(a) Measured and (b) theoretical SPDC spectra as a function of pump wavelength. Note that the arbitrary units for intensity could
also be interpreted as counts per max counts due to the normalization of the spectra. (c) Lineouts of the measured and theoretical SPDC
spectra at a pump wavelength of 405.7 nm, corresponding to the dashed horizontal lines in panels (a) and (b). Note that the frequency
bandwidth per nm and the downconversion efficiency are wavelength dependent (Supplement 1 Eq. (S1)), resulting in higher intensities for
shorter wavelengths. The theoretical spectra in panels (b) and (c) are calculated with the laser linewidth taken into account, described further
in Supplement 1 Section S6.
observed; (2) from 405.6 to 405.9 nm, the degenerate wave-
lengths are phase matched; and (3) at short pump wavelengths,
the spectrum splits and nondegenerate emission extending to the
cutoff wavelength of the filter is observed. The dip in intensity
in Fig. 3(a) around 755 nm is likely due to on-chip loss, poten-
tially from a mode-crossing [53], and is investigated in further
detail with a transmission measurement (Supplement 1 Section
S2). Due to the linewidth of the laser used in the experiment
(0.8-nm FWHM), the spectra are considerably broadened com-
pared with the spectra expected from a single-frequency pump
laser (Supplement 1 Figure S4). Nevertheless, experiment and
theory reach qualitative agreement by accounting for the pump
linewidth [Fig. 3(b)], described further in Supplement 1 Section
S6. Further spectral broadening can also arise from propagation
loss at the pump or SPDC wavelengths [54] or index variations
[55–57]. For all subsequent experiments, a laser center wave-
length of 405.7 nm is used for degenerate phase matching. The
resulting spectrum [Fig. 3(c)] is centered at 811.4
±
0.7 nm with
a FWHM bandwidth of 117 nm (53 THz) that accounts for 85%
of the overall flux.
The pair generation efficiency of the device is measured
through coincidence counting between two SPADs [Fig. 2(c)].
In this measurement, the pairs from the chip were split at a
50:50 broadband beamsplitter (Thorlabs BS014) and coupled
to multimode optical fibers (Thorlabs M122L01) connected to
the detectors. Coincidence detection events between the SPADs
(Laser Components Count) were recorded with a time-tagger
(Picoquant PicoHarp 300). Figure 4(a) shows a representative
raw coincidence histogram recorded at 4.3 nW of on-chip pump
power. The temporal correlation in this graph (3.4 ns FWHM) is
given by the response time of the SPADs and not the entangled
photon correlations (see Fig. 5 later in the text). The coinci-
dence counts are corrected by background subtraction of the
number of counts in a 9.5-ns window at the histogram peak
from the number of counts in another 9.5-ns window in a back-
ground region far from the peak. Sweeping the laser power with
a neutral density filter yields the curves in Figs. 4(b) and 4(c),
which are linearly fit to determine the pair generation efficiency.
Additional experimental details and the raw data from these
measurements are given in Section S4 and Table S1. To account
for the wavelength dependence of the SPAD quantum efficiency
(Supplement 1 Section S3), all wavelengths in the spectrum are
integrated over to calculate the average detection efficiency for
single photons (
η
1
=
0.52) as well as the average joint pair detec-
tion probability (
η
12
=
0.27). Including a factor of 2 due to the
probability of splitting pairs at the beamsplitter yields Eq. (1)
106
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
Research Article
for the measured efficiency of the source:
E
=
m
1
m
2
m
c
η
12
2
η
2
1
.
(1)
Here,
E
is the pair generation efficiency,
m
1
and
m
2
are the sin-
gles rates at the two detectors, and
m
c
is the rate of coincidences,
Fig. 4.
(a) Raw coincidence histogram, including accidentals, at
an input on-chip pump power of 4.3 nW. Measured (b) coincidence
counts with accidentals subtracted and (c) singles counts while
sweeping the input power. The vertical axes are measured in coin-
cidence or singles counts per second (cps). The fitted slopes (lines)
produce the pair generation efficiency of our device. Note that error
bars are included in panels (b) and (c), but are smaller than the data
markers. Additional experimental details and the raw data can be
found in Supplement 1 Section S4 and Table S1.
Fig. 5.
(a) Measured and (b) simulated two-photon Michelson interferogram for the device. (c) Corresponding singles counts out of the
interferometer, demonstrating single-photon interference within the coherence length of the source. (d) Coincidence counts and (e) singles
counts far from the coherence length of the source. All measured coincidence counts [panels (a) and (d)] do not have accidentals subtracted.
Note that error bars are included in panels (d) and (e), but are smaller than the data markers. Details of the simulated two-photon Michelson
interferogram in panel (b) and the sinusoidal fit in panel (d) can be found in Supplement 1 Section S7.
all in units of counts/s/mW. A derivation of Eq. (1) can be
found in Supplement 1 Section S5. Accounting for the 10.2-
dB transmission loss of the pump laser into the waveguide,
a pair generation efficiency of
(
2.3
±
0.5
10
11
pairs/s/mW
is measured, which is equivalent to a per-pump-photon effi-
ciency of
(
1.1
±
0.2
10
4
pairs/s/photon. Over the 117-nm
FWHM bandwidth of the spectrum, this efficiency translates to
an average brightness of
(
1.6
±
0.3
10
9
pairs/s/nm/mW. The
uncertainties here and throughout this work are reported as one
standard deviation, derived from the standard error in the fits
of Figs. 4(b) and 4(c) and the uncertainty in the detector quan-
tum efficiency. The ratio of singles counts to coincidence counts
suggests that the transmission of the SPDC from the waveguide
to each of the two detectors is 8.4 dB and 8.0 dB, respectively,
which includes losses out of the waveguide and of the free-
space optics. Our theoretical efficiency (Supplement 1 Section
S6), including the FWHM of the pump laser, is 2.66
×
10
11
pairs/s/mW, in close agreement with our experimental results.
Finally, the two-photon interference is measured [Fig. 2(d)] to
demonstrate the non-classical behavior of the produced photon
pairs. Figure 5 shows the measured two-photon interferogram
[Fig. 5(a)] obtained from the device without subtracting acci-
dentals, as well as the one-photon interferogram [Fig. 5(c)]
for comparison. Due to the aforementioned temporal resolu-
tion of the SPADs used here, the four unique paths through
the interferometer are indistinguishable and combine to yield
the interference pattern. The important features of the interfer-
ogram are as follows. (1) Near the zero time delay, photons are
delayed within the coherence length of the source and exhibit
both one- and two-photon interference. A visibility of 100
±
1%
is measured within the coherence length, with an uncertainty
derived assuming Poissonian statistics. This near-perfect visi-
bility indicates good mode overlap in the interferometer and
indistinguishability of the photons within the pair. (2) Far from
the coherence length of the source (delays greater than
±
20
fs), interference between two photons taking different paths
disappears, which explains why the single-photon interference
[Fig. 5(c)] disappears in this region, shown in more detail in
Fig. 5(e). Notably, interference between pairs of photons that
travel together through the interferometer persists with a fringe
period at half the pair wavelength [Fig. 5(d)]. This feature sug-
gests quantum interference due to the energy-time entanglement
of the pairs, and would not be observed if the light was gener-
ated from a coherent or a thermal source with a similar spectrum
[58]. A fringe visibility of 43
±
3
%
is observed in this region
Research Article
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
107
far from the coherence length, which is close to the theoretical
maximum of 50% for this experiment due to the temporal res-
olution of the detectors. The uncertainty in visibility is given
from the standard error in the fit of Fig. 5(d), the details of
which are outlined in Supplement 1 Section S7. The qualitative
agreement between the measured and theoretical two-photon
interferogram [Figs. 5(a) and 5(b)] suggests that SPDC and
genuine energy-time entanglement are being produced.
4. DISCUSSION
Compared with the state-of-the-art for visible photon pair
sources, the device presented in this work exhibits substantially
improved brightness and efficiency due to the small effective
area of the nanophotonic waveguide. The device’s performance
against reported literature devices spanning from the visible
to IR is plotted in Fig. 6 using efficiency, brightness, and
wavelength as the figures of merit. Our visible–near-IR device
demonstrates improved efficiency and brightness even over other
TFLN sources at the better-explored telecom band [6,7,9] and
has comparable performance to the most efficient telecom TFLN
source (Ref. [8]) to date. At short wavelengths, the phase-
matching bandwidth decreases due to group velocity dispersion
in lithium niobate, but the efficiency remains high because the
downconversion spectral power density scales with the inverse
Fig. 6.
Comparison of relevant literature SPDC sources to
this work with respect to (a) efficiency and (b) brightness
against the SPDC wavelength. Horizontal error bars represent
the reported bandwidth of the sources. Data for the efficiency,
brightness, center wavelength, and bandwidth are taken from
Refs. [10,26,29,33,35,36,60,61] for bulk crystal sources, Refs.
[11,29–32,62–67] for large waveguide sources (including microma-
chined and diffused waveguides), and Refs. [6–9] for TFLN sources.
Note that Refs. [11,64] are not included in panel (b) since the bright-
ness and bandwidth were not reported. The shaded region represents
the typical telecommunication wavelength window.
fifth power of the SPDC wavelength (
λ
5
s
) [29]. Thus, in addi-
tion to the benefits of higher-energy photon pairs, decreasing
the SPDC wavelength to access higher efficiencies could enable
single-photon nonlinearities when integrated with a resonator
[59]. The high efficiency of this device has significant implica-
tions for practical uses of entangled photons, including allowing
the use of low-power laser diodes for pair generation, reduc-
ing integration times, allowing high signal-to-noise coincidence
measurements at low (nW) laser powers even with losses present,
and reducing fluorescence and stray light.
Although the efficiency and brightness of the device compares
well with literature, these values can be further improved by nar-
rowing the bandwidth of the pump laser, as discussed further in
Supplement 1 Section S6. A single-frequency pump is estimated
to shrink the phase-matching bandwidth from 117 nm to 35 nm,
increasing the brightness by a comparable factor. The SPDC
process is also most efficient near degeneracy because the group
velocity of the signal and idler are equal to first order, so the effi-
ciency could increase by a factor of 5. Furthermore, compared
with SPDC produced with a monochromatic pump, the radia-
tion produced here with a multimode pump will exhibit reduced
time-energy entanglement (and greater separability) due to the
uncertainty in the energy of the pump photons. The coherence
properties of the pump laser are also transferred to the SPDC
state, which can reduce the N00N state interference visibility at
delays longer than the coherence length of the pump laser. Note
that this negative effect is not observed here since the path length
differences in Fig. 5 cover more than the full coherence length of
the SPDC source but less than the coherence length of the pump
laser [68,69], indicating that the pump laser coherence does not
impair the visibilities measured in this work. Conversely, one
benefit of using a multifrequency pump is that the device sen-
sitivity to the laser wavelength is reduced, yielding a higher
stability average response with greater bandwidth. Another ben-
efit of the multifrequency pump is in time-bin entanglement,
which can be generated from revivals in the biphoton interfer-
ence at integer multiples of the laser cavity length [70]. For
these benefits, as well as for the greater accessibility of mul-
timode lasers, a multimode pump laser was used in this work
to represent an alternate and more applied use of photon pair
sources. Furthermore, despite the aforementioned drawbacks of
a multimode pump, the device was still able to achieve high effi-
ciencies on par with the current state-of-the-art telecom TFLN
pair sources.
5. CONCLUSION
Efficient photon pair generation has been demonstrated with an
integrated thin-film lithium niobate waveguide at visible and
NIR wavelengths (720–900 nm). An on-chip SPDC efficiency
of
(
2.3
±
0.5
10
11
pairs/s/mW, which is on par with reported
TFLN literature at the better-explored telecom wavelengths even
with a multimode pump laser, has been produced near the usu-
ally associated cut-off wavelengths for the pump (406 nm) in
TFLN. The photon pair spectra has an average brightness of
(
1.6
±
0.3
10
9
pairs/s/mW/nm, centered at 811 nm with a
117-nm bandwidth. To date, these results are the shortest wave-
length photon pairs generated in a thin-film platform by nearly
an octave. The work opens up opportunities to exploit the quan-
tum advantage of integrated entangled photon circuits beyond
telecom to imaging and spectroscopy applications in the visible
and NIR.
108
Vol. 2, No. 2 / 25 April 2024 /
Optica Quantum
Research Article
Funding.
Air Force Office of Scientific Research (FA9550-20-1-0040);
Army Research Office (W911NF-23-1-0048); U.S. Department of Energy
(DE-SC0020151); National Science Foundation (DGE-1745301, EECS
1846273).
Acknowledgments.
The authors gratefully acknowledge the critical
support and infrastructure provided for this work by The Kavli Nanoscience
Institute (KNI) and the Beckman Biological Imaging Facility at Caltech.
This work was additionally supported by the KNI-Wheatley Scholar in
Nanoscience and the Rothenberg Innovation Initiative. N.A.H. was sup-
ported by the Department of Defense (DoD) through the National Defense
Science and Engineering Graduate (NDSEG) Fellowship Program. E.Y.H.
was supported by the National Science Foundation Graduate Research Fel-
lowship Program under Grant no. DGE-1745301. Any opinion, findings,
and conclusions or recommendations expressed in this material are those
of the authors(s) and do not necessarily reflect the views of the National
Science Foundation.
Disclosures.
The authors declare no conflicts of interest.
Data availability.
Data underlying the results presented in this paper are
not publicly available at this time but may be obtained from the authors upon
reasonable request.
Supplemental document.
See Supplement 1 for supporting content.
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