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<reference>
Low-loss waveguides on Y-cut thin film
lithium niobate: towards acousto-optic
applications
LUTONG CAI*, ASHRAF MAHMOUD AND GIANLUCA PIAZZA
Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA,
15213, USA

Abstract: We investigate the dependence of photonic waveguide propagation loss on the
thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify
trade-offs between optical losses and electromechanical coupling of surface acoustic wave
(SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer
reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW
device. Optical racetrack resonators with different lengths were fabricated by argon plasma
etching to experimentally extract waveguide losses. By increasing the thickness of the oxide
layer from 1 µm to 2 µm, propagation loss of 2 µm (1 µm) wide waveguide was reduced from
1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm), and, resonators with quality
factor greater than 1 million were demonstrated. An oxide thickness of approximately 1.5 µm
is sufficient to significantly reduce propagation loss due to leakage into the substrate and
simultaneously attain good electromechanical coupling in acoustic devices. This work not
only provides insights on the design and realization of low-loss photonic waveguides in
lithium niobate, but most importantly offers experimental evidence on how the oxide
thickness directly impacts losses and guides its selection for the synthesis of highperformance acousto-optic devices in Y-cut lithium niobate on insulator.

1. Introduction
Lithium niobite (LN), a versatile optical material which possesses outstanding properties
including nonlinear optical, electro-optical, piezoelectric, and acousto-optical (AO) effects
and a wide transparency range (350 nm ~ 5.2 μm) [1], has wide application in
telecommunication networks, sensors, frequency conversions, quantum optics and
microelectromechanical systems (MEMS) [2-5]. The emergence of lithium niobite on
insulator (LNOI, thin film of LN bonded on low refractive index material) in the last two
decades has led to rapid growth of LN photonics since its high index contrast waveguide
structure enables dramatic reduction of the device footprint and enhancement of optical
effects with respect to implementations in the bulk material [6]. Substantial cutting-edge
research activities have flourished based on compact thin film LN photonic devices for a
variety of applications [7-15]. For AO applications, LN features high elasto-optic coefficients
like GaAs, but comes also with very strong piezoelectricity over many other materials (e.g.,
GaAs, ZnO, quartz…) [16], which results in acoustic devices that have a substantially higher
electromechanical coupling coefficient [17]
However, compared to more mature material platforms like silicon on insulator (SOI) and
other semiconductor materials, producing low loss waveguides in LNOI is extremely
challenging. Dry etched waveguides have exhibited high propagation loss due to rough
sidewalls or byproduct formed by chemical reaction [18,19]. Hybrid waveguides consisting of
other material loaded on top of the LN thin film avoid etch issues but confine only a portion
of the light in the active LN layer, hence they do not fully harness the material capabilities
[20]. Only recently, low-loss subwavelength photonic waveguides have been attained by

optimizing plasma etching conditions and coating over-cladding layers on X-cut and Z-cut
LNOI [21,22]. Despite the significant progress, none of these prior demonstrations directly
relates the achievable low loss in LNOI waveguides to the thickness of the buried oxide layer
(BOX), but rather focus on the etch method to define the waveguides. It is important to note
that, for AO devices, the buried oxide layer (BOX) has a direct impact on the
electromechanical coupling coefficient (kt2) and thicker films of oxide negatively impact the
performance of MHz devices such as those used for inertial sensing applications [13].
Therefore, it is well worth studying the dependence of the waveguide loss on the BOX
thickness on LNOI as that would also impact the design of high-coupling AO devices.
Differently from prior demonstrations, we study this dependence in Y-cut LN as it is one of
the most appropriate cuts for the implementation of acousto-optic (AO) devices. In fact, in
this cut, surface acoustic waves (SAW) can be efficiently excited by taking advantage of the
high kt2 of the film and result in resonators with high quality factor [17].
Both simulations and experiments were conducted to study the dependence of waveguide
losses on the thickness of the BOX layer. As predicted by simulations, the waveguide loss
extracted from racetrack (RT) resonators decreased with increasing the BOX thickness. Lowloss waveguides (0.33 dB/cm) and high Q resonators (>1,000,000) were fabricated by argon
plasma etching followed by RCA cleaning. Most importantly, the results show that a
thickness of 1.5 µm could be used as a good compromise between photonic and acoustic
performance.
2. Simulations
2.1 Electromechanical coupling coefficient (kt2) as a function of oxide thickness

Fig. 1. Simulated electromechanical coupling coefficient (kt2) for various TBOX and acoustic
wavelengths (Λ) on a YZ cut LNOI (see inset for wafer stack). The presence of oxide clearly
has a deleterious impact on the device kt2. It is interesting to note how the impact changes with
the specific wavelength. This is due to changes in penetration of the acoustic and electric fields
in the thin films of LN, oxide and the thick LN substrate. At larger acoustic wavelengths, the
oxide thickness is a small fraction of the acoustic devices. At intermediate wavelengths, the
oxide becomes a dominant part of the active SAW and its impact on k t2 is more dramatic. At
the smallest wavelength, the acoustic and electric fields are almost entirely confined in the thin
film of LN and the oxide has a lower impact on kt2.

kt2, broadly defined as the ratio of the conversion between electrical and acoustic energy in a
piezoelectric transducer, is an important figure of merit that determines the selection of a
piezoelectric material for acoustic applications. kt2 is also of paramount importance in AO
devices, since it ultimately determines the device size (inversely proportional to kt2) and the
effectiveness with which an acoustic wave interacts with light. kt2 of SAW devices in bulk LN
can be readily computed via numerical methods [17]. We extended the same methodology to
derive the kt2 of SAW devices for LNOI by using finite element methods in COMSOL
Multiphysics. We investigated the dependence of kt2 as a function of different BOX
thicknesses (TBOX) and acoustic wavelengths (Λ) for a Y-cut LNOI substrate. Y-cut LN was
selected because it is one of the preferred cuts for SAW applications given the larger kt2 and
lower acoustic losses that can be attained [17]. In this analysis, a z-propagating SAW wave
was simulated in a thin film of LN having a thickness of 500 nm, a variable oxide thickness
and a thick LN substrate (see Fig. 1 inset for material stack). The thickness of the top LN
layer was set at 500 nm as the preferred value for the making of photonic waveguides with a
well-confined quasi-TE00 mode around 1550 nm. It is important to note that the presence of a
thick LN substrate below the oxide is important for the effective excitation of large acoustic
wavelength devices. In fact, if such substrate was not present or substituted with silicon, then
large wavelength SAWs would not be effectively excited. The results of this analysis are
shown in Fig. 1 where kt2 is plotted versus TBOX for different Λ. Although TBOX has a different
impact on the kt2 depending on the specific wavelength (see Fig. 1 caption for further
explanation), it is clear that the presence of T BOX has a deleterious effect on the
electromechanical coupling of the SAW device. Therefore, depending on the specific
application, the oxide thickness should be carefully selected and should not generally be made
to exceed 2 µm except for particular acoustic wavelengths. In the following sections, we
detail the impact of this same oxide thickness on photonic losses for the same Y-cut LNOI, so
as to provide guidance in the design of high-performance AO devices.
2.2 Propagation losses as a function of oxide thickness
The schematic cross-section of a partially etched waveguide and the corresponding mode
profile are shown in Fig. 2(a) and (b), respectively. The waveguide cross-section is the y-x
crystal plane of the film and light propagates in the z-direction. We performed a full-vectorial
finite difference simulation using a commercial software, Lumerical, to compute the
propagation losses in this waveguide structure [23]. A perfectly matched layer (PML)
boundary was used to enclose the waveguide region and absorb any incident electromagnetic
field at its boundary. Both LN and BOX were defined as dielectric materials without intrinsic
loss, hence the only accountable losses in the simulations were coming from the optical field
decaying through the BOX and coupling to a radiation mode in the substrate (absorbed by the
PML in the simulation framework). The contour plot in Fig. 2(c) shows that loss can be
reduced by either increasing T BOX or the width of the rib (W). The rationale behind this
behavior can be explained by looking at the optical field exponential decay in the substrate
direction, ~ exp(-αL), where α is the decay rate and L is the decay length. As TBOX increases,
the decay length, L, increases, while as W increases a higher effective refractive index (neff)
waveguide is formed thus enhancing the decay rate, α. Losses of 0.01 dB/cm, 0.1 dB/cm, 1
dB/cm and 5 dB/cm are marked in the contour plot of Fig. 2(c) to roughly highlight what
minimum oxide thickness is required in order to achieve such level of losses for a given
waveguide width. We also calculated the dependence of losses on Tbox and W for bent
waveguides with a bending radius of 20 μm as shown in Fig. 2 (d). Clearly, because of the
additional radiation losses the bent waveguides exhibit higher losses but similar trends. For
large radius, e.g., the 100 μm we used in the devices reported in this work, the dependence of
losses on Tbox and W is practically the same as that for straight waveguides as illustrated in
Fig. 2(c).

Fig. 2. (a) Schematic cross-section of the Y-cut LNOI rib waveguide. PML boundary condition
was applied in the simulation to represent radiation losses into the substrate. (b) E intensity
distribution of the fundamental TE-like mode of the waveguide with W = 1 μm and
Tetch = 300 nm. Calculated dependence of loss on W and TBOX for (c) straight waveguide and
(d) bent waveguide (radius = 20 μm).

3. Experimental demonstration of low loss waveguides in Y-cut lithium
niobate
Methods such as the cut-back and Fabry-Perot (FP) interference [24,25] are commonly used
to extract waveguide losses. The cut-back method needs consistent coupling efficiency from
the fiber to the waveguide to accurately extract loss, while the FP method needs a perfectly
polished end-face to avoid the impact of coupling losses on the extraction process. An
alternative way to extract loss that can cancel out the role of input/output coupling efficiency
is measuring the losses (extracted from the measured Q factor) of optical racetrack (RT)
resonators with different dimensions. Waveguide losses can then be extrapolated from the
propagation loss of the straight waveguide forming the RT [21]. In the following two
sections, we will discuss the details on the fabrication and optical characterization of RT
resonators and the extraction of waveguide losses in Y-cut LNOI substrates.
3.1 Fabrication
The fabrication flow is shown in Fig. 3(a). We used three Y-cut LNOI samples with 1 μm,
1.5 μm and 2 μm thick BOX respectively, to explore the dependence of waveguide loss on
BOX thickness. First, photonic patterns including grating couplers (GC), feeding waveguides
(“U” shaped) and RT resonators, were patterned on CSAR 62 positive resist by electron-beam
lithography. Among all the techniques to produce photonic devices on LNOI, physical
etching using Ar plasma features high anisotropic etching profile. Therefore, we etched the
LNOI film by inductively coupled plasma reactive ion etching (ICP-RIE) using the following
parameters: Ar flow of 30 sccm, bias power of 100 W, ICP power of 600 W and pressure of
5 mT. The etching depth (Tetch) was about 300 nm. This recipe exhibited a selectivity to
CSAR 62 resist of 1:1.

Fig. 3. (a) Fabrication flow of LNOI photonic devices. The LNOI wafer was purchased from NGK
Insulators, LTD. (b) SEM pictures of GC and waveguide before RCA cleaning. The redeposition of
LN along the sidewalls of the patterned features is clearly visible. (c) Microscope picture of one of
the fabricated RT resonators (upper right) and SEM pictures of various photonic components after
RCA cleaning. The waveguide is 2 μm wide and the coupling region (lower right) between the
waveguides features a gap of 200 nm. The sidewall is very smooth after RCA cleaning. θ is the
angle between the propagation direction of the straight waveguide and the crystalline z-axis (θ = 0°
for the RT shown in the microscope picture).

The electron-beam resist left on the sample was removed by oxygen plasma cleaning right
after etching. The final step included cleaning the etched patterns by RCA (NH 4OH, H2O2
and H2O mixed by the volume ratio of 1:1:5) at 60°C for 30 min. This RCA step is critical as
it removes the organic residues and insoluble particles left behind by the etch step by
changing their zeta potentials [26]. From the SEM images of the etching profile before and
after RCA cleaning (Fig. 3(b) and (c)), it was obvious that the RCA cleaning effectively
stripped the byproducts of the LN etch process and any residual resist that was stuck to the
waveguide sidewalls.
3.2 Characterization
To characterize RT resonators, we coupled light in/out of the device by GCs shown in Fig.
3(c). The insertion loss of each GC was measured to be 7 dB at the wavelength of 1550 nm
around which we characterized the RT resonators. By sweeping the wavelength of the tunable
laser, the transmission spectra of RT resonators were recorded by a power-meter.
The RT resonators had fixed bend radius (R = 100 μm) but varying straight waveguide
lengths (L = 300 μm, 700 μm, 2000 μm and 4000 μm. Total length of RT being Ltot =
2πR+2L) and widths (W = 2 μm and 1 μm). First, we measured the transmission spectra of
these RTs with different lengths and then extracted their intrinsic losses (loss in a single-pass
trip besides coupling loss), α, by fitting the experimental curves to a Lorentzian function. The
transmission of RT resonators (L = 4000 μm) fabricated on a 2 μm thick BOX (TBOX = 2 μm)
with W = 2 μm and 1 μm are shown in Fig. 4(a) and (b), respectively. The unloaded Q (QU)
for these devices are higher than 1,000,000 for the case in which W = 2 μm. By comparing α
from RTs with different L (or Ltot), we could eliminate the impact of bent waveguide loss and
get the straight waveguide losses by simply looking at the slope of the fitted curve plotting
loss vs. RT length (see Fig. 4(e) and (f)). The propagation loss of the straight waveguide with

W = 2 μm (1 μm) was as low as 0.37 ± 0.02 dB/cm (0.77 ± 0.07 dB/cm), which was among
the lowest propagation loss reported so far for dry-etched LNOI waveguides with similar
dimensions and without any over-cladding oxide layer [27]. The bending loss (100 μm radius)
for W = 2 μm (1 μm) is extracted to be 1.1 ± 0.1 dB/cm (1.55 ± 0.3 dB/cm). Using an overcladding layer could further reduce the scattering loss due to the lower index contrast between
LN and the surroundings but requires additional steps (e.g. lithography and etching) to open
windows to enable electrical contacts to external signal sources if such waveguides are to be
used in practical applications for electro-optic (EO) or acousto-optic (AO) modulators.

Fig. 4. (a) and (b): Measured (black dots) and fitted (red curves) transmission of RT resonator
(L = 4000 μm) with W = 2 μm and W = 1 μm, respectively. (c) and (d): Loaded and unloaded
Q factors in RT resonator as a function of the total length (Ltot = 2πR+2L) for 2 μm and 1 μm
wide straight waveguides respectively. (e) and (f): Intrinsic loss in RT resonator as a function
of the total length. Average propagation losses of 0.37 dB/cm and 0.77 dB/cm for 2 μm and
1 μm wide straight waveguides were extracted by linearly fitting the experimental data. The
variations in the value of the average propagation losses are set equal to one standard deviation
of the losses extracted from several resonances around 1550 nm.

Using the same method reported above for extracting propagation losses, we measured the
dependence of waveguide losses on different thicknesses of the BOX layer (TBOX = 1 μm,
1.5 μm). The measurement results are summarized in Table 1. It is clear that waveguide loss
decreases with increasing T BOX independently of the width of the waveguide as predicted by
simulations. Note that, for small T BOX, i.e. TBOX = 1 μm, the leakage loss dominates over all
other loss sources in the waveguides because the simulated value closely matches the
experimental data. As TBOX increases, the energy leakage into the substrate becomes
negligible and other loss factors like scattering loss become dominant. In particular, for the

cases of TBOX = 2 μm and 1.5 μm, losses displayed in the table can be almost exclusively
attributed to scattering losses since the leakage obtained from simulation is much less than the
measured loss. This is a very important finding as it points out that TBOX of ~ 1.5 µm is
sufficient to ensure low propagation loss without significantly impacting the
electromechanical performance of SAW devices.
Since the design of high-performance EO or AO devices requires rotation of the
waveguide with respect to the X-axis of the Y-cut crystal [17], we also measured the loss of
the waveguide with TBOX = 2 μm for other two propagation directions (θ = 45° and 90°). We
summarize these results in Table 1. For the case of W = 1 μm, it appears that the losses
increase as θ increases from 0° to 90°. We speculate that since the value of neff lowers as θ
increases due to the birefringence of LN, then the smaller waveguides tend to suffer more
from scatting losses as light experiences greater interaction with the waveguide sidewall.
Table 1. Summary of the waveguide propagation losses (Unit: dB/cm)
TBOX = 1μm

TBOX = 1.5μm

θ = 0°

θ = 0°

θ = 0°

TBOX = 2μm
θ = 45°

θ = 90°

W = 1μm

3 ± 0.5

0.94 ± 0.01

0.77 ± 0.07

0.83 ± 0.08

1.09 ± 0.07

W = 2μm

1.85 ± 0.4

0.59 ± 0.03

0.37 ± 0.02

0.23 ± 0.01

0.34 ± 0.02

4. Conclusions
In conclusion, we experimentally investigated the dependence of waveguide propagation loss
on BOX thickness, waveguide width and in-plane orientation for Y-cut LNOI. As predicted
by FDTD simulations, the measured loss decreased with increasing T BOX and reached values
< 0.4 dB/cm when TBOX was selected to be 2 μm. By comparing the simulated and measured
losses, we can conclude that leakage losses are dominant when T BOX is small, while, other
loss sources (i.e. scattering loss) are more relevant when T BOX is large. Effectively, this work
shows that for oxide thicknesses exceeding 1.5 µm, propagation losses due to substrate
leakage can be dramatically reduced. Since, as evidenced by COMSOL simulations, the kt2 of
SAW devices tends to deteriorate as T BOX increases, 1.5 µm of oxide could be considered as a
good compromise for the making of high-performance AO devices. Ultimately, the selection
of the oxide thickness will depend on the specific application and acoustic frequency for
which the AO device is used. Nonetheless, this work reports important experimental insights
on how propagation losses are affected by BOX thickness in Y-cut LNOI and offers general
guidelines in the selection of the BOX thickness for high performance AO devices such as the
inertial sensor reported in [13].
Funding
This material is based upon work supported by the DARPA PRIGM-AIMS program under
Award No. N66001-16-1-4025. Any opinions, findings, and conclusions or recommendations
expressed in this publication are those of the authors and do not necessarily reflect the views of
DARPA.
Acknowledgments
Authors acknowledge S. Hiramatsu and Dr. Yuji Hori from NGK Insulators, LTD. for
providing the LNOI wafers.
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</reference>

<statements>
1. Representative recipe (Desiatov/Loncar): Ar, ICP 600 W, bias 100 W, 5 mTorr, ~300 nm etch. [arXiv] Zhang et al. (Optica 4, 1536, 2017) achieved Q up to 10^7 and 2.7 dB/m with Ar etching.
</statements>

Begin the assessment now. Output only the JSON list, without any conversational text or explanations.