Han Du

On-chip integrated optofluidic complex refractive index sensing using silicon photonic crystal nanobeam cavities

Complex refractive index sensing is proposed and experimentally demonstrated in optofluidic sensors based on silicon photonic crystal nanobeam cavities. The sensitivities are 58 and 139 nm/RIU, respectively, for the real part (n) and the imaginary part (κ) of the complex refractive index, and the corresponding detection limits are 1.8×10(-5) RIU for n and 4.1×10(-6) RIU for κ. Moreover, the capability of the complex refractive index sensing method to detect the concentration composition of the ternary mixture is demonstrated without the surface immobilization of functional groups, which is impossible to realize with the conventional refractive index sensing scheme.

Mechanically-Tunable Photonic Devices with On-Chip Integrated MEMS/NEMS Actuators

This article reviews mechanically-tunable photonic devices with on-chip integrated MEMS/NEMS actuators. With related reports mostly published within the last decade, this review focuses on the tuning mechanisms of various passive silicon photonic devices, including tunable waveguides, couplers, ring/disk resonators, and photonic crystal cavities, and their results are selectively elaborated upon and compared. Applications of the mechanisms are also discussed. Future development of mechanically-tunable photonics is considered and one possible approach is based on plasmonics, which can confine light energy in the nano-scale space. Optomechanics is another innovation, derived from the strong coupling of optical and mechanical degrees of freedom. State-of-the-art studies of mechanically-tunable plasmonics and on-chip optomechanics are also selectively reviewed.

Tuning the quality factor of split nanobeam cavity by nanoelectromechanical systems.

A split nanobeam cavity is theoretically designed and experimentally demonstrated. Compared with the traditional photonic crystal nanobeam cavities, it has an air-slot in its center. Through the longitudinal and lateral movement of half part of the cavity, the resonance wavelength and quality factor are tuned. Instead of achieving a cavity with a large tunable wavelength range, the proposed split nanobeam cavity demonstrates a considerable quality factor change but the resonance wavelength is hardly varied. Using a nanoelectromechanical system (NEMS) comb-drive actuator to control the longitudinal and lateral movement of the split nanobeam cavity, the experimentally-measured change of quality factor agrees well with the simulated value. Meanwhile, the variation range of resonance wavelength is smaller than the full width at half maximum of the resonance. The proposed structure may have potential application in Q-switched lasers.

Precise control of coupling strength in photonic molecules over a wide range using nanoelectromechanical systems.

Photonic molecules have a range of promising applications including quantum information processing, where precise control of coupling strength is critical. Here, by laterally shifting the center-to-center offset of coupled photonic crystal nanobeam cavities, we demonstrate a method to precisely and dynamically control the coupling strength of photonic molecules through integrated nanoelectromechanical systems with a precision of a few GHz over a range of several THz without modifying the nature of their constituent resonators. Furthermore, the coupling strength can be tuned continuously from negative (strong coupling regime) to zero (weak coupling regime) and further to positive (strong coupling regime) and vice versa. Our work opens a door to the optimization of the coupling strength of photonic molecules in situ for the study of cavity quantum electrodynamics and the development of efficient quantum information devices.

Magnetic field sensor based on coupled photonic crystal nanobeam cavities

We report the design, fabrication, and characterization of a resonant Lorentz force magnetic field sensor based on dual-coupled photonic crystal nanobeam cavities. Compared with microelectromechanical systems (MEMS) Lorentz force magnetometers, the proposed magnetic field sensor has an ultra-small footprint (less than 70 μm × 40 μm) and a wider operation bandwidth (of 160 Hz). The sensing mechanism is based on the resonance wavelength shift of a selected supermode of the coupled cavities, which is caused by the Lorentz force-induced relative displacement of the cavity nanobeams, and thus the optical transmission variation. The sensitivity and resolution of the device demonstrated experimentally are 22.9 mV/T and 48.1 μT/Hz1/2, respectively. The results can be further improved by optimizing the initial offset of the two nanobeams.

Lateral shearing optical gradient force in coupled nanobeam photonic crystal cavities

We report the experimental observation of lateral shearing optical gradient forces in nanoelectromechanical systems (NEMS) controlled dual-coupled photonic crystal (PhC) nanobeam cavities. With an on-chip integrated NEMS actuator, the coupled cavities can be mechanically reconfigured in the lateral direction while maintaining a constant coupling gap. Shearing optical gradient forces are generated when the two cavity centers are laterally displaced. In our experiments, positive and negative lateral shearing optical forces of 0.42 nN and 0.29 nN are observed with different pumping modes. This study may broaden the potential applications of the optical gradient force in nanophotonic devices and benefit the future nanooptoelectromechanical systems.

Mode competition and hopping in optomechanical nano-oscillators

We investigate the inter-mode nonlinear interaction in the multi-mode optomechanical nano-oscillator which consists of coupled silicon nanocantilevers, where the integrated photonic crystal nanocavities provide the coupling between the optical and mechanical modes. Due to the self-saturation and cross-saturation of the mechanical gain, the inter-mode competition is observed, which leads to the bistable operation of the optomechanical nano-oscillator: only one of the mechanical modes can oscillate at any one time, and the oscillation of one mode extremely suppresses that of the other with a side mode suppression ratio (SMSR) up to 40 dB. In the meantime, mode hopping, i.e., the optomechanical oscillation switches from one mode to the other, is also observed and found to be able to be provoked by excitation laser fluctuations.

Polarization insensitive metamaterial perfect absorber at visible frequencies

A novel metamaterial absorber with broadband and polarization-insensitive response is proposed. Simulation and experimental results indicates high absorption efficiency in the visible range. This metamaterial absorber offers potential application in development of photovoltaics and thermal detection.

Lateral shearing tuning of dual coupled photonic crystal nanobeam cavities

This paper reports a novel tunable photonic device. The device is based on dual coupled photonic crystal (PhC) nanobeam cavities. Different from other studies, where the cavities are mechanically tuned by varying the coupling gap, here the cavities are tuned by lateral shearing while maintaining a constant coupling gap. In experiments, both electrostatic force and optical gradient force are used to realize the device tuning.

A Micromachined Tunable Bistable Mechanism

A novel micromachined tunable bistable mechanism is developed and demonstrated on a microelectromechanical systems prototype device. The proposed design, modeling, and characterization of the prototype device, which is electrostatically driven and fabricated by silicon-on-insulator multi-user MEMS processes (SOIMUMPs), are presented. The mechanism comprises noninterdigitated comb drives, which are specially designed to generate a nonlinear electrostatic force that results in adjustable bistability when balanced with a linear mechanical restoring force. The finite element method (FEM) simulation is utilized to analyze the bistability and its tunability. In experimentation, the distance between the two stable states is able to be tuned from 6.5 to 8 μm by just varying the locking voltage.