Optical phased array neural probes for beam-steering in brain tissue
Wesley D. Sacher
1
,
2
,
3
,
†
,
∗
, Fu-Der Chen
2
,
3
,
†
, Homeira Moradi-Chameh
4
, Xinyu Liu
1
, Ilan Felts Almog
2
, Thomas
Lordello
2
, Michael Chang
4
, Azadeh Naderian
4
, Trevor M. Fowler
1
, Eran Segev
1
, Tianyuan Xue
2
, Sara
Mahallati
4
, Taufik A. Valiante
4
,
5
,
6
, Laurent C. Moreaux
1
, Joyce K. S. Poon
2
,
3
, and Michael L. Roukes
1
1
Division of Physics, Mathematics, and Astronomy,
California Institute of Technology, Pasadena, California 91125, USA
2
Department of Electrical and Computer Engineering, University of Toronto,
10 King’s College Rd., Toronto, Ontario M5S 3G4, Canada
3
Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle, Germany
4
Krembil Research Institute, Division of Clinical and Computational Neuroscience,
University Health Network, Toronto, Ontario, Canada
5
Division of Neurosurgery, Department of Surgery,
Toronto Western Hospital, University of Toronto, Toronto, Ontario, Canada
6
Institute of Biomaterials and Biomedical Engineering,
University of Toronto, Toronto, Ontario, Canada
†
Equal contribution and
∗
Corresponding author: wesley.sacher@mpi-halle.mpg.de
Implantable silicon neural probes with integrated nanophotonic waveguides can deliver patterned
dynamic illumination into brain tissue at depth. Here, we introduce neural probes with integrated
optical phased arrays and demonstrate optical beam steering
in vitro
. Beam formation in brain
tissue was simulated and characterized. The probes were used for optogenetic stimulation and
calcium imaging.
Genetically encoded optogenetic actuators and fluores-
cence indicators have become powerful tools in the inter-
rogation of brain activity, since they enable the control
and imaging of neurons with high cell-type specificity and
single-cell spatial resolution [1–3]. Today’s optical sys-
tems for optogenetics and functional fluorescence imag-
ing, such as multi-photon microscopes and implantable
fiber optics, are typically built from bulk off-the-shelf
components and are physically large and complex [4].
Yet, advances in silicon (Si) integrated photonics have
led to the dense integration of nanoscale waveguides and
devices into millimeter-scale circuits that achieve com-
plex functions [5, 6]. Thus, Si photonic technology can
be leveraged to create nanophotonic tools that miniatur-
ize optical systems for neurobiology and deliver light into
brain tissues in ways that are not possible with bulk op-
tics. One approach is to realize implantable chip-scale
photonic devices that deliver and control patterned illu-
mination in brain tissues at depths inaccessible by free-
space optics, i.e., beyond the optical attenuation length.
Along these lines, nanophotonic waveguides with grat-
ing coupler (GC) light emitters [7–10] and micro-light-
emitting-diodes (
μ
LEDs) [11] have been integrated onto
implantable Si probes. In brain tissues, since light mostly
scatters forward [12], low-divergence beams can be emit-
ted from GCs over distances of 200-300
μ
m [7, 8]. Com-
pared to
μ
LEDs, nanophotonic waveguide-based probes
do not generate excess heat beyond that caused by the
light itself, can more precisely tailor the optical emission
profile, are compatible with wafer-scale foundry manufac-
turing [9, 13], and can achieve a high light source density.
Furthermore, as evidenced by the recent advancements
in Si photonic beam-forming [5, 14, 15], sophisticated
gratings and photonic circuit designs can enable precisely
patterned illumination with high spatial resolution.
Here, we report the first implantable Si neural probes
capable of optical beam-steering in tissue. The probes
use silicon nitride (SiN) optical phased arrays (OPAs)
as light emitters; the emitted beams were steered by
wavelength tuning. The OPAs were designed to oper-
ate at blue wavelengths for the excitation of the opsin
Channelrhodopsin-2 (ChR2) and the genetically encoded
calcium indicator GCaMP6. The probes were validated
in vitro
in mouse brain slices, demonstrating sufficient
power for optogenetic stimulation and functional imag-
ing as well as spatial control of the beam on the neuron
scale. A preliminary report of this work appeared in [16].
Figure 1 shows the OPA neural probes, which consisted
of 4 shanks, each
∼
18
μm
thick, 3 mm long, and 50
μm
wide, on a 250
μm
pitch, and a thicker base region. The
SiN waveguides were 200 nm thick. The probes were fab-
ricated on 100 mm diameter silicon-on-insulator wafers as
described in [7]. On each shank were 4 OPAs, with the
design shown in Fig. 1(b). A star coupler split the light
in the input waveguide into 16 delay line waveguides. The
delay lines were routed for a differential path-length, and
each terminated with a light-emitting grating. As the
input wavelength was tuned, the differential phase-shift
between the light-emitting gratings led to the angular
steering of the emission [17]. The delay lines consisted
of waveguides with an initial single-mode width of 240
nm following the star coupler for a length of 12
μ
m that
adiabatically widened to 400 nm to reduce phase error.
The differential length of the delay lines was 16
μ
m, cho-
sen so the free spectral range (FSR) (
∼
6 nm) matched
the wavelength tuning range of our external cavity diode
laser of 484.3 to 491 nm. The pitch and width of the
arrayed gratings in the OPA were 700 nm and 300 nm,
arXiv:2108.04933v1 [physics.optics] 10 Aug 2021
2
FIG. 1. OPA neural probes. (a) Schematic of the OPA neural probe connected to the scanning system (inset) optical micro-
graphs of an image fiber bundle facet with cores addressed by the scanning system. (b) Scanning electron micrograph (SEM)
of one of the shanks of an OPA neural probe (inset) SEM of a portion of the SiN gratings prior to top cladding deposition
during fabrication. (c) Photograph and micrographs of an OPA neural probe. (d) Illustration of the experimental apparatus
for testing the neural probes in brain slices. Abbreviations: microelectrode array (MEA), artificial cerebrospinal fluid (ACSF).
(e, f) Photographs of the packaged OPA neural probe emitting light in fluorescein; (e) has a smaller field of view and external
illumination applied for visibility of the shanks, (f) shows the emitted beams (zoomed out, no illumination). (b, c, e, f) are
from [16]; the fiber bundle inset of (a) is from [9].
respectively. The period of each grating was 440 nm;
in water, the steering plane [Fig. 1(a)] was angled at
∼
25
◦
from the normal of the probe. The OPAs were
designed for low crosstalk between the arrayed waveg-
uides. The array pitch was significantly larger than the
half-wavelength criterion for emission of a single grating
order, and typically 3 lobes were emitted from each OPA.
Optimization of the array pitch, increased SiN thickness
for higher optical confinement and lower crosstalk, and
apodization [15] may suppress these additional lobes. In
the following, we focus on one of the OPA neural probes
that was packaged and studied in detail.
The probe was passive to reduce tissue heating, and
the 16 OPAs on the probe were independently addressed
using the spatial addressing scheme in [9, 18]. As shown
in Fig. 1(a), a micro-electro-mechanical system (MEMS)
mirror deflected a laser beam into individual cores of an
image fiber bundle, which was attached to the probe base.
Each fiber core was aligned to an edge coupler on the
probe, which was connected to an OPA on the shank.
The switching time of the MEMS mirror was
∼
5 ms.
About
∼
10
μW
was emitted from each OPA.
Following the method in [9], to characterize the emit-
ted beam profiles in non-scattering media, the probe was
immersed in a fluorescein solution and the fluorescence
was imaged. The setup in Fig. 1(d) was used, but with
the chamber replaced with a container of fluorescein; the
probe was angled so the emitted beams were parallel to
the fluid surface. Top-down images of the emission pat-
tern from the probe at various wavelengths are shown in
Fig. 2(a). Over a wavelength tuning range from 484.3
to 491 nm, the beams were steered continuously
±
16
o
with narrow beams formed within a distance of 300
μm
(Fig. 2). The two side lobes were at angles of about
±
32
o
from the main beam. Figure 2(b) shows the beam profile
imaged from the side; the full-width-at-half-maximum
(FWHM) thickness was
<
19
μm
over a 300
μm
propa-
gation distance. The peak intensity of the main lobe was
7 to 17
×
larger than the background light intensity at
propagation distances of 50 to 300
μ
m, Fig. 2(a). A sig-
nificant component of the background was due to optical
scattering from the photonic circuit.
OPA beam formation in brain tissue was verified and
investigated in simulation and experiment. The simu-
FIG. 2. Characterization of the neural probe beam profiles
in fluorescein. (a) Top-down beam profiles at various wave-
lengths (
λ
). (b) Side profile of the beam at
λ
= 484
.
3 nm;
the top surface of the shanks is delineated by the dashed line.
The scale bars are (a) 50
μ
m, (b) 100
μ
m. (c) Top-down
measured (“Exp”) and simulated (“Sim” in water) FWHM
beam width versus propagation distance of the central lobe
at
λ
= 491 nm.