Zhu Diao

Nanophotonic detection of side-coupled nanomechanical cantilevers

A silicon nanophotonic Mach-Zehnder interferometer (MZI) is used to detect the mechanical resonance of a cantilever external to a nanophotonic waveguide. Small cantilever devices, below the cut-off for waveguide supported modes, are fabricated ∼140 nm away from one MZI arm. Cantilever resonant frequencies up to 60 MHz are measured with mechanical quality factors around 20 000 and signal to noise ratios up to 1000. Phase-locked loop frequency stability measurements indicate a mass sensitivity of 2 zg in an example cantilever of 0.5 pg mass. An interferometric transduction mechanism is confirmed, and the system is shown to work effectively in all-optical operation.

Torque-mixing magnetic resonance spectroscopy

Mechanically detected spin resonances The interaction of spins in a sample with a magnetic field can generate forces that can be sensed with cantilever probes. Losby et al. measured the resonance signals at room temperature with a micromechanical torque magnetometer. The difference between two applied radio-frequency signals corresponded to the mechanical frequency of the resonator. This approach revealed the vortex core dynamics of the ferri-toferro–magnetic transition in a micrometer-sized yttrium-iron-garnet single-crystal disk. Science, this issue p. 798 Electronic spin resonances of a magnetic single crystal are measured with a mechanical torque sensor. A universal, torque-mixing method for magnetic resonance spectroscopy is presented. In analogy to resonance detection by magnetic induction, the transverse component of a precessing dipole moment can be measured in sensitive broadband spectroscopy, here using a resonant mechanical torque sensor. Unlike induction, the torque amplitude allows equilibrium magnetic properties to be monitored simultaneously with the spin dynamics. Comprehensive electron spin resonance spectra of a single-crystal, mesoscopic yttrium iron garnet disk at room temperature reveal assisted switching between magnetization states and mode-dependent spin resonance interactions with nanoscale surface imperfections. The rich detail allows analysis of even complex three-dimensional spin textures. The flexibility of microelectromechanical and optomechanical devices combined with broad generality and capabilities of torque-mixing magnetic resonance spectroscopy offers great opportunities for development of integrated devices.

Thermo-mechanical sensitivity calibration of nanotorsional magnetometers

We report on the fabrication of sensitive nanotorsional resonators, which can be utilized as magnetometers for investigating the magnetization dynamics in small magnetic elements. The thermo-mechanical noise is calibrated with the resonator displacement in order to determine the ultimate mechanical torque sensitivity of the magnetometer.

Stiction-free fabrication of lithographic nanostructures on resist-supported nanomechanical resonators

The authors report a highly flexible process for nanostructure lithography to incorporate specific functions in micro- and nanomechanical devices. The unique step involves electron beam patterning on top of released, resist-supported, surface micromachined structures, hence avoiding hydrofluoric acid etching of sensitive materials during the device release. The authors demonstrate the process by creating large arrays of nanomechanical torque magnetometers on silicon-on-insulator substrates. The fabricated devices show a thermomechanical noise-limited magnetic moment sensitivity in the range of 5 × 106 μB at room temperature and can be utilized to study both magnetostatics and dynamics in nanomagnets across a wide temperature range. The fabrication process can be generalized for the deposition and patterning of a wide range of materials on micro-/nanomechanical resonators.

Confocal Scanner for Highly Sensitive Photonic Transduction of Nanomechanical Resonators

We show that a simple confocal laser scanning system can be used to couple light through grating couplers into nanophotonic circuits. The coupling efficiency is better than 15% per coupler. Our technique avoids using multi-axis fibre stages and is especially advantageous when the nanophotonic circuit is kept in vacuum, e.g., for nanomechanical resonator displacement transduction. This was demonstrated by recording the resonant response of a nanomechanical doubly clamped beam embedded in a race-track optical cavity. The nanophotonic transduction offers an increase of two orders of magnitude in transduction responsivity compared with conventional free-space optical interferometry.

Even nanomechanical modes transduced by integrated photonics

We demonstrate the actuation and detection of even flexural vibrational modes of a doubly clamped nanomechanical resonator using an integrated photonics transduction scheme. The doubly clamped beam is formed by releasing a straight section of an optical racetrack resonator from the underlying silicon dioxide layer, and a step is fabricated in the substrate beneath the beam. The step causes uneven force and responsivity distribution along the device length, permitting excitation and detection of even modes of vibration. This is achieved while retaining transduction capability for odd modes. The devices are actuated via optical force applied with a pump laser. The displacement sensitivities of the first through third modes, as obtained from the thermomechanical noise floor, are 228 fm Hz−1/2, 153 fm Hz−1/2, and 112 fm Hz−1/2, respectively. The excitation efficiency for these modes is compared and modeled based on integration of the uneven forces over the mode shapes. While the excitation efficiency for the ...

Wavelength-division multiplexing of nano-optomechanical doubly clamped beam systems.

Wavelength-division multiplexing is demonstrated for a set of two doubly clamped beams. Using a single input/output waveguide in a nanophotonic detection system, the two mechanical beams are independently addressable using different wavelength channels as determined by their respective racetrack resonator detection cavities. The two cavities slightly overlap, which also enables the mechanical frequency of both beams to be detected simultaneously with a single wavelength. Finally, to physically map which wavelength channel corresponds to which specific device, a heating laser is targeted individually on each beam to create a reversible mechanical frequency shift. This multiplexing method would allow for the simpler detection of large arrays of nanomechanical devices in a sensor system.

Single laser modulated drive and detection of a nano-optomechanical cantilever

Nano-optomechanical systems (NOMS) offer many advantages in transducing nanomechanical motion. These include very high displacement sensitivities along with very large frequency detection bandwidths due to their optical nature. It follows from this that NOMS are a promising avenue for on-chip nanomechanical mass sensing. Nanomechanical beams are becoming smaller to increase their mass sensitivity and nanophotonic detection is well suited to transduce these smaller devices.

Nanomechanical AC Susceptometry of an Individual Mesoscopic Ferrimagnet

Abstract A novel method for simultaneous detection of both DC and time-dependent magnetic signatures in individual mesoscopic structures has emerged from early studies in spin mechanics. Multifrequency nanomechanical detection of AC susceptibility and its harmonics highlights reversible nonlinearities in the magnetization response of a single yttrium iron garnet (YIG) element, separating them from hysteretic jumps in the DC magnetization.

Integrated silicon photonics transduction of even nanomechanical modes in a doubly clamped beam

Nanomechanical devices have significant potential as mass sensors because small changes in vibrational resonance frequency can be translated to mass of an analyte for single-molecule detection. By observing multiple vibrational modes simultaneously, the mass and position of the adsorbed analyte can be determined more accurately. Additionally, by using optomechanical actuation and detection of nanomechanical devices, the displacement sensitivity can be improved significantly, which is highly beneficial for mass sensing applications. In particular, optomechanical transduction can be achieved using integrated photonics, which is a compact approach that greatly simplifies the optomechanical measurement. However, even vibrational modes are difficult to detect with integrated photonics, since with even modes there is a zero effective index shift over the vibrating beam. In this work, we demonstrate the measurement of even nanomechanical modes. We fabricated a doubly clamped beam by releasing a straight section of an optical racetrack resonator from the silicon dioxide underneath. By performing this process twice, a step is fabricated in the substrate beneath the beam. The step permits the excitation and detection of even modes of vibration due to a nonzero effective index shift. Transduction of odd modes is retained. The displacement sensitivities of the first through third modes are obtained from measurements of the thermomechanical noise floor, and are 228 fm Hz-1/2, 153 fm Hz-1/2, and 112 fm Hz-1/2, respectively. The devices are actuated by the optical force, by modulating a pump laser with an electro-optic modulator. With the addition of the driving force, up to the fifth vibrational mode is observed. The driving force for the first through third modes is compared and modeled based on integration over the mode shapes. Since the step length is approximately 38% of the beam length, the optical force on each mode is approximately 0.4 pN μm-1mW-1 for an applied optical power of 0.07 mW.