We have devised an air-suspended nano-optomechanical structure that is capable of precisely detecting in-plane motion up to a precision of subnanometer level. To achieve the detection, we utilized a basic silicon photonic building block (i.e., a nanowire waveguide directional coupler) in conjunction with nanoelectromechanical systems (NEMS). We numerically optimized the design and experimentally demonstrated a displacement sensitivity of 8.83×10−5 V/nm−1 with a low noise-level resolution of 0.172 nm/√Hz in a 1 Hz bandwidth centered at 950 Hz. As the waveguide coupler design does not segmentize nanowire waveguides, we eliminated the undesirable insertion losses and coupling losses irrelevant to the measurand. Furthermore the design is simple, ultracompact, and can be easily integrated with on-chip photonic systems, which may be beneficial for applications that require a compact displacement sensor with high accuracy and precision.
Microoptoelectromechanical systems (MOEMS) are promising choices in achieving compact and yet precise sensors
with intensity / phase modulation techniques or resonance shift based techniques. Particularly in the area of displacement
sensing, MOEMS device may potentially offer a high accuracy and yet compact solution for highly sensitive portable
sensors. We propose a novel approach of a hybrid device consisting of nano-mechanical structures and nanowire silicon
photonics to achieve a new displacement sensing mechanism that does not require segmentization or intersecting of
waveguides. In this work, we demonstrate that by optimizing a relatively broadband air-suspended nanowire waveguide
directional coupler design that is integrated using silicon photonics structures, we can achieve sufficient attenuation
without the need of waveguide segmentization thus effectively reducing the undesired insertion and coupling losses.
First, we numerically design and optimize, utilizing a 3D FDTD numerical method, a nanowire waveguide directional
coupler that is capable of achieving a -13 dB extinction ratio at submicron displacements. Next, we fabricate the
proposed nanowire photonic waveguide directional coupler utilizing a simple and monolithic fabrication approach. The
nano-mechanical structures are then characterized and calibrated in-situ under a scanning electron microscopy (SEM).
The optical sensitivities of the waveguide directional couplers are then characterized on a vibration isolation optical table
under low noise conditions. The noise spectrum densities are also characterized by driving the structures under an AC
actuating voltages to understand the minimum detectable nano-displacements.
KEYWORDS: Near field, Waveguides, Silicon, Near field scanning optical microscopy, Photonic crystals, Optical design, Resonators, Control systems, Nanophotonics, Atomic force microscopy
Research interest for silicon nanophotonics is a topic of heavy interest currently due to the requirements for high density
communications of integrated devices with small footprints in the semiconductor industry. Silicon photonic crystals
(PhC) are nanoscale subwavelength periodic structures that possess the capability to induce strong interaction between
light and matter. PhC nanocavities utilizes the photonic bandgap effect to trap certain frequencies of light within a small
confined region for a diverse range of applications such as enhancement and suppression of spontaneous emission,
efficient and compact lasers, add/drop multiplexers, optical filters and sensing etc. In this paper, we describe a
mechanically-perturbative near-field probe with a special design shape to achieve low-loss and precise resonance control
of PhC nanocavities. One-dimensional (1D) PhC are chosen for our study due to the ease of integrating with low-loss
SOI waveguide technology and easy integration with nanomechanical structures. Sub-micron microelectromechanical
systems (MEMS/NEMS) technology is introduced as an ideal integration platform with such near-field probe designs
due to its capabilities to accurately control fine displacements without the need of bulky equipment such as atomic force
microscopy (AFM), scanning near field microscope (SNOM) or highly sensitive piezo-controlled micromanipulator
stages. We propose that such near-field probe designs are capable of achieving large resonance spectral shift of up to few
nm with high re-configurability, highly accurate actuation displacements, low power consumption, and portability. In
this work, we propose an approach utilizing numerical methods to study and characterize the electromagnetic interaction
between PhC nanocavities and nanomechanically displaced near-field nano-probes.
A compact and low power control of photonic crystal nanocavity resonance was devised, simulated, and experimentally validated utilizing a hybrid integration of a microelectromechanical systems driven nanoprobe. The experimental results demonstrated a reversible resonance tuning up to 5.4 nm with minimal Q-factor degradation.
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