of 5
Research Article
Vol. 11, No. 3 / March 2024 /
Optica
371
In situ
tuning of optomechanical crystals with
nano-oxidation
Utku Hatipoglu,
1
,
2
,
Sameer Sonar,
1
,
2
,
David P. Lake,
1
,
2
Srujan Meesala,
1
,
2
AND
Oskar Painter
1
,
2
,
3
,
*
1
Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
2
Kavli Nanoscience Institute and Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA
3
AWS Center for Quantum Computing, Pasadena, California 91125, USA
These authors contributed equally to this work.
*opainter@caltech.edu
Received 19 December 2023; revised 7 February 2024; accepted 7 February 2024; published 8 March 2024
Optomechanical crystals are a promising device platform for quantum transduction and sensing. Precise targeting of
the optical and acoustic resonance frequencies of these devices is crucial for future advances on these fronts. However,
fabrication disorder in these wavelength-scale nanoscale devices typically leads to inhomogeneous resonance frequen-
cies. Here we achieve
in situ
, selective frequency tuning of optical and acoustic resonances in silicon optomechanical
crystals via electric field-induced nano-oxidation using an atomic-force microscope. Our method can achieve a tun-
ing range
>
2 nm (0.13%) for the optical resonance wavelength in the telecom C-band, and
>
60 MHz (1.2%) for the
acoustic resonance frequency at 5 GHz. The tuning resolution of 1.1 pm for the optical wavelength and 150 kHz for the
acoustic frequency allows us to spectrally align multiple optomechanical crystal resonators using a pattern generation
algorithm. Our results establish a method for precise post-fabrication tuning of optomechanical crystals. This technique
can enable coupled optomechanical resonator arrays, scalable resonant optomechanical circuits, and frequency match-
ing of microwave-optical quantum transducers.
© 2024 Optica Publishing Group under the terms of the Optica Open Access
Publishing Agreement
https://doi.org/10.1364/OPTICA.516479
1. INTRODUCTION
Optomechanical crystals (OMCs) provide a coherent interface
between optical photons and acoustic phonons [1]. This capability
is now being utilized in microwave-optical quantum transducers
[2–6] towards connecting gigahertz frequency superconducting
quantum processors [7] via low-loss optical communication chan-
nels. In parallel efforts, OMCs have enabled coherent control and
routing of phonons in chip-scale optomechanical circuits [8–10]
with demonstrations of nonreciprocal optical transmission [11]
and topologically protected phonon transport [12]. However,
scaling to multi-node quantum networks and more complex
optomechanical circuits is challenging due to variations in optical
and acoustic resonance frequencies across devices caused by fab-
rication imperfections. In particular, since critical feature sizes in
OMCs are affected by the precision limits of electron beam lithog-
raphy and reactive ion etching, the spread in resonance frequencies
across devices can be more than a hundred times the resonance
linewidths, thereby hindering frequency alignment.
Post-fabrication tuning of optical resonance frequencies in
chip-scale microcavities has been achieved using a variety of tech-
niques including laser-assisted thermal oxidation [13], strain
tuning [14,15], thermo-optic tuning [16,17], and gas condensa-
tion [18,19]. The atomic force microscope (AFM) nano-oxidation
technique has been used to realize polarization degenerate micro-
cavities [20] and to create low-loss microcavities from line defects
in a photonic crystal [21]. However, since OMCs co-localize opti-
cal and acoustic resonances in a wavelength-scale volume, selective
tuning of both optical and acoustic resonances without adding
significant scattering losses is a complex endeavor and an outstand-
ing technical challenge. Here we use AFM nano-oxidation tuning
to demonstrate such control over the resonance frequencies of
OMCs. In our approach, field-induced oxidation of the silicon
device surface with high spatial resolution allows us to tune the
acoustic resonance by modifying the local mass distribution and
elasticity, and the optical resonance by modifying the local refrac-
tive index. By using a pattern generation algorithm to guide the
nano-oxidation sequence in real-time, we achieve simultaneous
alignment of the optical and acoustic resonance frequencies of
multiple OMC cavities.
2. AFM NANO-OXIDATION SETUP
A simplified schematic of the experimental setup is shown
in Fig. 1(a). We perform nano-oxidation using a conductive
chromium/platinum coated silicon AFM tip with a radius
<
25 nm
[22] and track the optical and acoustic resonances in real-time by
2334-2536/24/030371-05 Journal © 2024 Optica Publishing Group
Research Article
Vol. 11, No. 3 / March 2024 /
Optica
372
(b)
(c)
(a)
Fig. 1.
Nano-oxidation setup schematic and OMC cavity mode profiles. (a) Simplified schematic of the AFM nano-oxidation setup. The AFM tip is
used to perform nano-oxidation while measuring the optical and acoustic resonances of the device in real-time with a dimpled optical fiber. The AFM tip
is operated in tapping mode over a grounded silicon sample and biased with a square wave voltage, allowing for electrochemical formation of an oxide layer
on the silicon surface. The inset illustrates the raster scan used to generate the oxide layer. (b) Electric field intensity profile of the optical mode. The optics-
focused oxidation region is outlined in green. (c) Displacement magnitude profile of the acoustic breathing mode. The acoustics-focused oxidation regions
are outlined in green.
performing optomechanical spectroscopy via a dimpled optical
fiber coupled to the device [23]. The AFM is operated in tapping
mode while a voltage bias is applied to the conductive tip, and
the silicon-on-insulator (SOI) chip with a silicon device layer
resistivity of 5 k

-cm is grounded. When the voltage-biased AFM
tip is brought close to the silicon surface, a strong electric field
triggers an electrochemical reaction between ions in the native
water meniscus and the silicon surface, resulting in local oxidation
of silicon. Importantly, this reaction can proceed even when the
silicon surface is covered with native oxide, because the strong
local electric field allows oxyanions (OH
, O
) to diffuse through
the native oxide [24]. In our experiments, we maintain the AFM
setup enclosure at ambient conditions with a relative humidity of
40
±
5% and temperature of 23
±
1
C, and apply a square wave
voltage oscillating between
±
10 V at a frequency of 20 Hz to the
AFM tip.
The OMC cavities in this work were fabricated on an SOI
chip with a 220 nm thick silicon device layer. The cavities were
patterned via electron beam lithography followed by reactive ion
etching, and finally suspended by removing the buried oxide layer
with an anhydrous vapor hydrofluoric acid etch. The devices were
designed to support a fundamental TE-like optical resonance at a
wavelength of
1550 nm and a breathing acoustic resonance at
a frequency of
5.1 GHz [Figs. 1(b) and 1(c)] [25]. An on-chip
adiabatic waveguide coupler [26] allows for optical coupling to
the OMC cavity using a dimpled optical fiber. The optical spec-
trum of the OMC cavity was probed in reflection mode, and the
resonance frequencies were recorded before and after each step of
the nano-oxidation sequence. To measure the acoustic spectrum
of the device, we routed the optical signal reflected from the device
to a high-speed photodetector and measured the power spectral
density (PSD) of the photodetector electrical output on a spectrum
analyzer. The result is proportional to the PSD of thermal dis-
placement fluctuations of the acoustic mode, which are transduced
onto the optical signal via the optomechanical interaction in the
device. In these measurements, the laser was blue-detuned with
respect to the optical resonance frequency by a detuning close to
the acoustic resonance frequency and operated at low power to
minimize optomechanical back-action on the acoustic mode [27].
3. NANO-OXIDATION CHARACTERIZATION
From the simulated mode profiles shown in Figs. 1(b) and 1(c),
we identified two strategic oxidation regions for selective tuning
of the optical and acoustic modes. Nano-oxidation in the region
shown with the green rectangle in Fig. 1(b) is expected to induce
a relatively large change in the optical resonance due to a high
concentration of electric field energy. Here the impact on the
acoustic resonance is expected to be minimal due to two reasons.
First, the displacement amplitude is small in this region. Second,
even though the stress amplitude is large in this region, the Young’s
modulus of the oxide layer when weighted by its thickness is similar
to that of the original silicon. In contrast, nano-oxidation on the
regions shown with the green rectangles in Fig. 1(c) leads to a large
acoustic resonance shift and a small optical wavelength shift due to
the high concentration of motional mass and low concentration
of electric field energy. In Section D of Supplement 1, we show
the results of finite element method (FEM) simulations where we
investigated the optical and acoustic frequency shifts due to cre-
ation of an oxide pixel at an arbitrary location on the OMC surface.
This allowed us to identify candidate regions for fine tuning of
optical and acoustic resonance frequencies with maximal selectiv-
ity. For coarse frequency tuning, we opted to use larger, rectangular
oxide patterns instead of oxide pixels to maximize the tuning range
at the expense of selectivity.
Prior to experiments on OMC devices, we characterized the
effect of the AFM tapping amplitude and scanning velocity on
nano-oxidation using a bare silicon chip. We observed that lower
tapping amplitude and slower scanning velocities result in wider
and thicker oxide lines. We measured the three-dimensional
profiles of field-induced oxide lines and pixels using AFM mea-
surements as described in Section B of Supplement 1. Based on the
amount of frequency tuning required, we operated the AFM in two
distinct modes, which we refer to as “mild” and “aggressive” tuning
modes. In the mild tuning mode, the tapping amplitude and scan
speed were set to 15 nm and 100 nm/s, respectively, resulting in
oxide thickness of approximately 1.2 nm. In the aggressive tuning
mode, the tapping amplitude and scan speed were set to 5 nm and
50 nm/s, respectively, leading to oxide thickness of approximately
2.5 nm. In the mild tuning mode, nano-oxidation was found to
generate single pixels with a lateral size of approximately 25 nm.