# Investigation of nanophotonic lithium niobate waveguides for on-chip
evanescent wave sensing

Nathan A. Harper  1,${\dagger}$ Emily Y. Hwang  2,${\dagger}$ Philip A.
Kocheril  1,${\dagger}$ and Scott K. Cushing1  1Division of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena, California
91125, USA
2Department of Applied Physics and Materials Science, California Institute of
Technology, Pasadena, California, 91125, USA
${\dagger}$These authors contributed equally to this work.
<EMAIL_ADDRESS>
††journal: opticajournal††articletype: Research Article

Thin-film lithium niobate is a promising photonic platform that could allow
for light generation, manipulation, and sample interaction all within the same
compact device. Integrated sample interaction geometries for on-chip sensors
have not yet been well explored in lithium niobate. We use a model thin-film
lithium niobate rib waveguide excited with a 406 nm laser diode to spatially
resolve the scattering and fluorescence distributions from a 60 nm dye-doped
polymer film. We determine that both the quasi-transverse-electric (TE) and
quasi-transverse-magnetic (TM) modes have propagation losses dominated by
scatter, and that the absorption due to the sample only accounts for
approximately 3% of the experiment for both modes. Although the TM mode has
better overlap with the sample, the even stronger propagation loss due to
scattering in the TM mode (32.5 $\pm$ 0.3 dB/cm) compared to the TE mode (23.0
$\pm$ 0.2 dB/cm) reduces how many molecules are excited with a given input
power, thus the TE mode could be more appropriate for sensing. Our findings
have important implications in compatibility with other integrated components
within a practical lithium niobate device and serve to guide future studies
into sample integration towards completely on-chip lithium niobate-based
sensors.

##  1 Introduction

Thin-film lithium niobate (TFLN) is a promising candidate for on-chip sensing
due to its strong nonlinearities and low material losses, which could allow
for light generation, manipulation, and sample interaction all within the same
compact device[1, 2]. Starting from a single off-chip laser diode or
amplifier, several different routes have been explored to generate light
spanning from the UV-A[3] to the mid-IR[4]. Lithium niobate’s strong quadratic
nonlinearity, sub-$\mu$m modal confinement, and ability for quasi-phase
matching enable highly efficient frequency conversion through second harmonic
generation[5], optical parametric amplification[6], optical parametric
oscillation [7, 8], and spontaneous parametric downconversion[9]. Nonlinear
frequency conversion in TFLN is highly tunable across one to two octaves from
the pump laser. Lithium niobate’s electro-optic effect can then be exploited
for fine frequency tuning with an external microwave source[10].
Alternatively, ultrafast laser pulses can be generated on-chip [11, 12, 13],
to produce white light with broad spectra from cascaded higher-order
interactions[14]. In addition to on-chip light sources, TFLN’s nonlinearities
are useful for producing compact spectrometers[15, 16] with integrated
detectors[17].

While frequency generation in TFLN is being widely explored, integrated sample
interaction geometries for on-chip sensors are comparatively less studied. On
other photonic material platforms, optical waveguides are commonly used for
miniaturized and modular platforms for molecular spectroscopy[18]. Since the
evanescent field of light coupled into a waveguide extends only a few hundred
nanometers past its surface, evanescent field sensing allows for highly
specific interrogation of molecules held to the surface of the waveguide[19,
20]. Evanescent field-stimulated fluorescence spectroscopy has found
applications in the biological sciences, facilitating specific detection of
pathogenic biomarkers at the single-molecule level in complex biological
samples [21]. Evanescent wave sensors have been successfully implemented with
multiple waveguide architectures, including fiber optic waveguides, planar
waveguides, and rib waveguides, using materials such as silicon, silicon
nitride, and glass[22, 23, 24, 25], but have yet to be demonstrated in TFLN to
the best of our knowledge. Evanescent wave sensing can be modeled with mode
overlap calculations between the waveguide mode and the sample to investigate
the efficiency of the sample excitation, depth of excitation, and field
intensities. However, practical devices need to account for additional sources
of loss that are not well captured by modeling, such as the scattering from
sub-wavelength imperfections in the waveguide surface, scatterers in the
sample, and how the refractive index contrast at the material interfaces might
impact both mechanisms. Rayleigh scattering from sub-wavelength defects scales
as $\lambda^{-4}$[26], and therefore increases for the near-UV and visible
wavelengths that are needed to excite many common fluorescent dyes. Impurity
ion (Cr3+, Cu+, Fe2+, Fe3+, Ni+2) resonances in the lithium niobate film can
also cause additional loss[27, 28].

Here, we spatially resolve both the scattering and fluorescence distributions
in a model LN waveguide fluorescence sensor, which consists of a TFLN X-cut
rib waveguide cladded with a <100 nm dye-doped polymer film and excited with a
406 nm diode laser. Our method allows us to not only measure the overall
device loss but also separately quantify the desired losses (sample
absorption) and undesired losses (waveguide losses and sample scattering).
Theoretically, the fundamental quasi-transverse-electric (TE) and quasi-
transverse-magnetic (TM) modes of the rib waveguides have large differences in
desired sample absorption, with the TM mode expected to have a nearly two-fold
larger absorption cross section. Experimentally, the TM mode exhibits
significantly stronger propagation losses than the TE mode in this device
(32.5 $\pm$ 0.3 dB/cm compared to 23.0 $\pm$ 0.2 dB/cm), but unexpectedly
generates fewer fluorescent photons from the sample. By comparing the ratios
of the intensity of the light scattered from the waveguide compared to the
fluorescence from the sample, we determine that both modes have propagation
losses dominated by scatter, and that the absorption due to the sample only
accounts for approximately 3% of the experiment in both cases. While we
confirm that the TM mode has better overlap with the sample, the even stronger
loss due to scattering reduces how many molecules are excited with a given
input power, thus the TE mode could be more appropriate for sensing. Our
findings have important implications in compatibility with other integrated
components within a practical TFLN device, as the TE mode within an X-cut TFLN
film accesses the strongest optical and electro-optic nonlinearities, and
serve to guide future studies into sample integration within complete TFLN
devices.

Figure 1:  (a) Schematic of the lithium niobate nanophotonic waveguides and
PMMA/C153 film for evanescent wave sensing. (b) Waveguide geometry and
calculated electric field profiles profiles of the fundamental TE and TM modes
using Lumerical. Lineouts of the simulated electric field in the (c) $y$
direction in the center of the waveguide (at $x=0~{}\mu$m) and (d) $x$
direction in the PMMA/C153 film (at $y=200$ nm).

##  2 Device Design, Fabrication, and Characterization

To investigate the effect of the dye-doped films on the waveguide mode
propagation, the system was first simulated in Lumerical MODE (Figure 1b-d).
The confinement factors[29] of the fundamental TE and TM modes in the
C153/PMMA layer were calculated. For a 406 nm pump wavelength, waveguides with
film thicknesses >400 nm were found to have minimal confinement factors in
both the TE and TM modes. For these relatively thicker lithium niobate films,
the modal area is smaller than the waveguide cross section. Therefore, the
mode is well confined within the waveguide structure and does not extend into
the sample film, making them unsuitable for sensing. As the waveguide cross
sections decrease in area (film thicknesses <400 nm), a greater portion of the
mode extends outside of the waveguide and into the dye-polymer layer. For the
waveguide geometry shown in Figure 1b, the confinement factors of the
fundamental TE and TM modes respectively are 0.0073 and 0.013. This
discrepancy arises from the difference in the electric field profile between
the two fundamental waveguide modes (Figure 1c-d). Due to the polarization of
the modes and the aspect ratio of the waveguides, the electric field of the TM
mode has a discontinuity at the material interfaces. Thus, the TE and TM modes
will have different interaction intensities with the C153 dye-doped PMMA.
Incorporating the dye loss in the simulation material parameters, the
theoretical propagation loss due to the presence of the dye molecules is 4.4
dB/cm for the TE mode and 7.2 dB/cm for the TM mode.

The devices were fabricated from a 8 mm by 12 mm die of a 5% MgO-doped X-cut
thin-film lithium niobate on insulator wafer (NanoLN), which consists of 350
nm of lithium niobate bonded to 2 $\mu$m of silicon dioxide on a 0.4 mm
silicon substrate. Waveguides were patterned through an electron beam
lithography exposure with hydrogen silesoxquiane (HSQ) resist followed by
argon inductive coupled plasma reactive ion etching. The final device geometry
consists of a lithium niobate photonic waveguide with a $1.5~{}\mu$m top width
and $\sim$160 nm etch depth (Figure 1b). The chip facets were manually
polished to increase coupling efficiency, resulting in a final waveguide
length of approximately 5 mm.

The PMMA/C153 films were prepared after the waveguide fabrication. To form the
films, $1~{}\mu$L of a 10 mM stock solution of Coumarin 153 (Millipore Sigma
546186) in dimethyl sulfoxide (Millipore Sigma 276855) was added to
$99~{}\mu$L of a PMMA (Millipore Sigma 182230) solution (1% in toluene;
Millipore Sigma 244511). $20~{}\mu$L of the PMMA/C153 solution was then
transferred to the LN chip through spin coating at 300 rpm for 10 s followed
by 2000 rpm for 60 s (MicroNano Tools BSC-100). This spin coating resulted in
a film thickness of $\sim$60 nm, confirmed through profilometry on a separate
device and in good agreement with previous measurements of PMMA films [30].
Assuming minimal evaporation of the dimethyl sulfoxide, the C153 concentration
is approximately 5 mM in the film. A confocal fluorescence micrograph verified
that the film was relatively homogeneous across the sample, though strong
variations in the fluorescence at the waveguide sidewalls and surface were
observed.

Figure 2:  (a) Optical setup for waveguide coupling and fluorescence
detection. I, isolator; ND, variable neutral density filter; HWP, half-wave
plate; L, lens; LPF, long-pass filter. (b) Optical image of the lithium
niobate chip and lensed setup.

The fluorescence and scatter from the PMMA-C153 film on the waveguide are
characterized as shown in Figure 2a. In these experiments, the room-
temperature waveguide is pumped with a free-running laser diode (Coherent OBIS
LX 406 nm). An antireflection-coated aspheric lens (NA = 0.58, Thorlabs
C140TMD-A) couples the free-space pump beam to the fundamental TE or TM mode
of the waveguide depending on the polarization of the free-space beam, which
is tuned with a half-wave plate. The total scatter from the top of the
waveguides is collected using a camera lens (Edmund Optics 54691, $f=$75 mm,
operated at $f/4$) and camera (Teledyne FLIR BFLY-U3-23S6M-C), with a
collection efficiency of $\sim$ 0.025%. To distinguish the fluorescence, a
long pass filter (Thorlabs FELH0500) with a cutoff wavelength of 500 nm
filters out the 406 nm pump scatter, while rejecting only 10% of the generated
fluorescence. A second aspheric lens collects the output of the waveguide into
a power meter to monitor the stability of the coupling. Figure 2b shows an
image of the device and the coupling lenses used in this setup. All images
were acquired for 5 minutes over many exposures and were background corrected
using images recorded with the laser shuttered. Alignment into the waveguide
and image acquisition were performed with 5 nW and 100 nW of free-space power,
respectively, to avoid photobleaching the sample.

Due to the presence of an edge bead resulting from the spin-coating process, a
4.3 mm section in the center of the waveguide with uniform film thickness is
used for data analysis. The images are further processed by summing the
intensity of 24 pixels in the vertical direction about the waveguide at each
horizontal pixel. The counts due to fluorescence are scaled by a factor of
1/0.9 from the intensities recorded with the filter in place to account for
loss caused by the passband location relative to the C153 emission spectrum.
The counts due to scatter are determined by subtracting the counts due to
fluorescence from the counts recorded without the filter in place. A
lithographically defined feature on the device is used to infer the
measurement dimensions from the camera images.

##  3 Results and Discussion

Figures 3a and 3b show the intensities measured across the images of the
C153/PMMA-cladded waveguide when the TE and TM modes are respectively excited.
All traces fit well with a single exponential of the form $Ae^{-\alpha x}$,
where $x$ is length, $\alpha$ is the propagation loss of the device with units
of length-1, and $A$ is a scaling term. Since the fluorescence and scattering
rates should be proportional to the intensity of light within the waveguide at
each position, the loss coefficient measured from fluorescence should match
that of the scatter. This proportionality holds for the TE measurements, but
not for the TM mode. This discrepancy could potentially be explained by
nonuniformity in the loss mechanism along the waveguide or by the excitation
of higher-order modes, which are expected to have different ratios of
scattering and absorption rates, or by underestimates in the error of the fit.
While the noise in the time domain of the scattering intensity is lower than
that of the fluorescence (see Supplementary Material), the spatial variance in
the scattering data of Figure 3 is clearly higher than that of the
fluorescence, particularly in the TE mode. This could be the case due to two
factors: 1) fabrication imperfections along the waveguide that cause
scattering are unevenly distributed on the 10-100 $\mu$m scale and 2) the
coherence length of the laser (approximately 65 $\mu$m from the laser’s
measured 0.8 nm spectral bandwidth) yields a speckle pattern due to
interference of the different scattering sites. Because the fluorescence
intensity yields a better fit, the loss coefficient measured via fluorescence
is chosen for all subsequent results.

Figure 3:  Measured fluorescence and scatter in the waveguide ROI when coupled
into the (a) TE and (b) TM modes.

A waveguide with identical dimensions on the same die but without the
PMMA/C153 cladding shows reduced loss compared to the samples shown in Figure
3. The loss coefficients of these experiments are summarized in Table 1, and
plots of the spatially resolved scattering intensity and exponential fits for
the bare waveguide are provided in Supplementary section X. Compared to the
bare waveguide, the TE and TM modes of the cladded waveguide exhibit excess
propagation losses of $6.9\pm 0.5$ dB/cm and $14.4\pm 0.8$ dB/cm respectively.
However, it is not clear from these results if this loss is due to absorption
from the fluorophore or from additional scattering of the PMMA film. We
attempt to quantify this distribution by comparing the intensities of the
fluorescence and the scatter. We assume that the total propagation loss of the
waveguide $\alpha_{\text{total}}$, as measured in Figure 3, is composed of a
component $\alpha_{\text{scatter}}$ due to scattering and a component
$\alpha_{\text{absorption}}$ due to absorption from the C153, and follows
Beer’s Law:

$I(x)=I_{0}\,\textrm{exp}\left[-\alpha_{\text{total}}x\right]=I_{0}\,\textrm{exp}\left[-(\alpha_{\text{scatter}}+\alpha_{\text{absorption}})x\right]$
(1)

At any given point on the waveguide, the amount of light lost to scattering
and fluorescence is proportional to the loss coefficients.

$-\frac{1}{I(x)}\left(\frac{\textrm{d}I(x)}{\textrm{d}x}\right)=\alpha_{\text{scatter}}+\alpha_{\text{absorption}}$
(2)

Integrating over a length of the waveguide, the ratio in the total amount of
photons lost to scattering versus molecular absorption is equal to the ratio
in their respective propagation coefficients. This relationship is valid as
long as the loss coefficients due to scattering and absorption are constant
along the waveguide, which we show to be the case for the TE mode, and to be
approximately valid for the TM mode. Over the 4.3 mm section of the waveguide
used for loss measurements, the ratios of total counts due to scatter versus
total counts due to fluorescence were $15.82\pm 0.01$ and $15.73\pm 0.04$ for
the TE and TM modes, respectively. To infer the ratio in losses from the ratio
in counts, the following assumptions are used: 1) The collection efficiency of
the system is the same for any photon radiated from the waveguide. 2) The
camera quantum efficiency was 30% at 406 nm and $\sim$70% over the emission
range of C153 (500-600 nm), as reported on the camera datasheet. 3)
Fluorescence from the dye has a 90% chance of transmitting through the filter
based on the filter attenuation. 4) The quantum yield of Coumarin 153 in PMMA
is 90%, the same as its quantum yield in nonpolar solvents such as cyclohexane
[31]. 5) The PMMA film is thin enough that absorption of scatter and
fluorescence reabsorption are negligible. With these assumptions, the
probability that a photon lost to molecular absorption generates a count
through fluorescence is roughly 2.1 times as high as the probability that a
photon lost to scatter generates a count, largely a result of the spectral
response of the camera sensor. The ratio in the scatter to absorption
propagation losses for the TE and TM mode are therefore $33.22\pm 0.02$ and
$33.03\pm 0.08$, respectively. The individual coefficients consistent with
these ratios and the measured total propagation loss are displayed in Table 1.

Experiment |  $\alpha_{\text{total}}$ (dB/cm) |  $\alpha_{\text{scatter}}$ (dB/cm) |  $\alpha_{\text{absorption}}$ (dB/cm)
---|---|---|---
C153/PMMA, TE | 23.0 $\pm$ 0.2 | 22.4 $\pm$ 0.2 | 0.67 $\pm$ 0.01
C153/PMMA, TM | 32.5 $\pm$ 0.3 | 31.5 $\pm$ 0.3 | 0.95 $\pm$ 0.01
Bare Waveguide, TE | 16.1 $\pm$ 0.5 | - | -
Bare Waveguide, TM | 18.1 $\pm$ 0.7 | - | -
Table 1: Summary of measured and inferred propagation losses caused by
scattering and C153 absorption.

Compared to the TE mode, the strong decay of intensity as a function of length
for the TM mode suggests that the TM mode experiences higher loss, potentially
due to enhanced absorption of the fluorophore. While the TM mode does excite
the molecules more efficiently compared to the TE mode, it is important to
note that the counts generated by the TM mode are actually lower than the TE
mode, as can be seen in Figure 3. Due to increased scattering, fewer photons
are available along the length of the waveguide to excite the sample. While we
cannot directly measure the flux of photons within the waveguide due to
unknown end-fire coupling losses and losses from the edge bead, this
consideration is important because the signal-to-noise ratio of a shot-noise
limited sensor scales with the square root of the signal intensity.

Compared to the theoretically calculated propagation losses of 4.4 dB/cm and
7.2 dB/cm, we find that our experiments confirm the relative absorption
strength of the TM mode compared to the TE mode, but the absolute propagation
losses are much smaller. We attribute this discrepancy mainly to uncertainty
in the film parameters, such as the dye concentration, potential dye
aggregation, and the film profile in the vicinity of the waveguide, all of
which impact the theoretical propagation loss. The measured propagation loss
could be underestimated if the assumed dye quantum yield of 90% is too high,
which could be the case due to the PMMA microstructure around the dye,
quenching of the dye (such as self-quenching by homo-Förster resonance energy
transfer), or degradation of the dye from photobleaching. If the dye profile
is not homogeneously 60 nm thick in the vicinity of the waveguide, as
suggested by confocal fluorescence microscopy, the theoretically calculated
propagation losses decrease to 1.1 dB/cm and 1.6 dB/cm for the TE and TM modes
respectively. Decreases in the dye cross-section by up to 50% have also been
observed in the Coumarin family[32], further lowering the theoretically
calculated sample losses to within the levels that are experimentally
observed.

The main limitations of the device demonstrated here are the large
contribution of scatter to the overall loss and the uncertainty of the dye-
doped polymer film. Future work towards improving TFLN-based evanescent field
sensors can focus on decreasing the waveguide scattering loss imparted by the
fabrication process and incorporating a microfluidic system to analyze liquid
samples. Propagation losses down to 0.002 dB/cm have been demonstrated in TFLN
with methods such as post-fabrication annealing[33], ion beam milling[34], and
redeposition-free etching[35]. Compared to the losses in Table 1, such
improvements in the scattering loss will increase the fluorescence to scatter
ratio from the 3% demonstrated here to upwards of 30,000%. A liquid sample
will also eliminate much of the uncertainties associated with the polymer film
used here, as discussed above. Switching the phase of the sample has the added
benefit of being able to easily switch concentrations or dyes but at the cost
of requiring microfluidics, resulting in a more complicated overall
instrument.

##  4 Conclusion

TFLN rib waveguides are analyzed for their light-matter interaction strength.
The thin-film thickness is found to be the primary variable for increasing the
sample interaction. While the fundamental TM mode is predicted to exhibit a
two fold stronger sample interaction, increased scattering losses from the
waveguide for the TM mode outweigh this impact, resulting in fewer collected
fluorescence photons overall. We demonstrate the importance of quantifying
different loss mechanisms on the waveguide when measuring a LN sensor’s
efficacy, which we accomplish by comparing the spatially-resolved intensities
of scatter and fluorescence without requiring any knowledge of the actual on-
chip pump intensities. These measurements are important not only for our
specific sample of a dye-doped PMMA film, but for any sample which might
perturb the waveguiding properties of the device. Further improvements in the
sensor performance can be achieved with improved fabrication to decrease the
scattering loss and the implementation of a microfluidic system to analyze
liquid samples. Along with TFLN-based light sources and spectrometers, this
work represents a step toward fully integrated spectrometers.

Funding U.S. Department of Energy (DE-SC0022089)

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. The authors thank Ryoto Sekine and Alireza
Marandi for providing the TFLN dies and Giada Spigolon for her helpful
discussions in fluorescence imaging. N.A.H. was supported by the Department of
Defense (DoD) through the National Defense Science and Engineering Graduate
(NDSEG) Fellowship Program. E.Y.H. and P.A.K. were supported by the National
Science Foundation Graduate Research Fellowship Program under Grant no.
DGE-1745301. P.A.K. is grateful for financial support from a Hertz Fellowship.
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.

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