of 9
Research Article
Vol. 32, No. 16 /29 Jul 2024 /
Optics Express
27931
Subtleties of nanophotonic lithium niobate
waveguides for on-chip evanescent wave
sensing
N
ATHAN
A. H
ARPER
,
1,
E
MILY
Y. H
WANG
,
2,
P
HILIP
A. K
OCHERIL
,
1
T
ZE
K
ING
L
AM
,
3
AND
S
COTT
K. C
USHING
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 Physics, St. Catharine’s College, University of Cambridge, Cambridge CB2 1TN, UK
These authors contributed equally to this work.
*
scushing@caltech.edu
Abstract:
Thin-film lithium niobate (TFLN) is promising for optical sensing due to its high
nonlinearities, but its material properties present unique design challenges. We compare the
sensing performance of the fundamental modes on a TFLN waveguide with a fluorescent dye
sample. The TM mode has better overlap with the sample, with a 1.4
×
greater sample absorption
rate versus the TE mode. However, the TM mode also scatters at a 1.4
×
greater rate, yielding
less fluorescence overall. The TE mode is, therefore, more appropriate for sensing. Our findings
have important implications for TFLN-based sensor designs.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Thin-film lithium niobate (TFLN) is a promising candidate for on-chip sensing because its strong
nonlinearities and low material losses allow for light generation, manipulation, and sample
interaction within the same compact device [1,2]. Lithium niobate’s strong quadratic nonlinearity,
sub-
μ
m modal confinement, and ability for quasi-phase matching lead to efficient frequency
conversion through second harmonic generation [3], optical parametric amplification [4], optical
parametric oscillation [5,6], and spontaneous parametric downconversion [7,8]. TFLN has been
used to generate light spanning from the UV-A [9] to the mid-IR [10] in photonic circuits, with
efficiencies unmatched in any other platform. Lithium niobate’s electro-optic effect and versatile
fabrication allows for the efficient modulation of light with low voltages, high bandwidths,
compact footprints, and excellent insertion losses [11,12]. Electro-optic modulators on TFLN can
also be utilized as light sources, including frequency combs [13,14], ultrafast laser pulses [15],
and frequency shifters [16]. Efficient switches are useful for routing light to different sensing
regions on one chip, and the electro-optic effect has been harnessed to increase the resolution of
on-chip Fourier-transform interferometers [17]. With the addition of waveguided detectors [18],
the components necessary for a fully integrated optical sensor can all be fabricated on TFLN.
While frequency conversion sources and electro-optic modulation in TFLN are being widely
explored, integrated sample interaction geometries for on-chip sensors are less studied. 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 analyte interrogation [19–21].
Evanescent wave sensors have been implemented in materials such as silicon, silicon nitride,
and glass with multiple waveguide architectures, including fiber optic waveguides, planar
waveguides, slot waveguides, rib waveguides, and strip waveguides [22–25]. For example, TE
#529570
https://doi.org/10.1364/OE.529570
Journal © 2024
Received 13 May 2024; revised 27 Jun 2024; accepted 8 Jul 2024; published 18 Jul 2024