Enhanced all-optical cavity-tuning using graphene.

All-optical tuning of the resonance of an optical cavity is used to realise optical signal-processing including modulation, switching, and signal-routing. The tuning of optical resonance is dictated by the two primary effects induced by optical absorption: charge-carrier-generation and heat-generation. Since these two effects shift the resonance in opposite directions in a pure silicon-on-insulator (SOI) micro-ring resonator as well as in a graphene-on-SOI system, the efficiency and the dynamic range of all-optical resonance-tuning is limited. In this work, in a graphene-oxide-silicon waveguide system, we demonstrate an exceptional resonance-tuning-efficiency of 300 p m/m W (0.055 π/m W), with a large dynamic range of 1.2 n m (0.22 π) from linear resonance to optical bistability. The dynamics of the resonance-tuning indicates that the superior resonance-tuning is due to large linear-absorption-induced thermo-optic effect. Competing free-carrier dispersion is suppressed as a result of the large separation between graphene and the silicon core. This work reveals new ways to improve the performance of graphene-on-waveguide systems in all-optical cavity-tuning, low-frequency all-optical modulation, and switching.

Gate Tunable Cooperativity between Vibrational Modes.

Coupling between a mechanical resonator and optical cavities, microwave resonators, or other mechanical resonators have been used to observe interesting effects from sideband cooling to coherent manipulation of phonons. Here we demonstrate strong coupling between different vibrational modes of MoS2 drum resonators at room temperature. We observe intermodal as well as intramodal coupling. Cooperativity, a measure of coupling between the two modes, can be tuned by more than an order of magnitude by changing the direct current gate bias. The large measured cooperativity of about 900 at room temperature indicates that the phonon population can be coherently transferred between the modes for more than 500 cycles. This coherent oscillation is of great interest in studying quantum effects in macroscopic objects.

The effect of strain on effective Duffing nonlinearity in the CVD-MoS2 resonator.

We demonstrate all electrical measurements on NEMS devices fabricated using CVD grown monolayer MoS2. The as-grown monolayer film of MoS2 on top of the SiO2/Si wafer is processed to fabricate arrays and individual NEMS devices without the complex pick and transfer techniques associated with graphene. The electromechanical properties of the devices are on par with those fabricated using the exfoliation method. The frequency response of these devices is then used as a probe to estimate the linear thermal expansion coefficient of the material and evaluate the effect of strain on the effective Duffing nonlinearity in the devices.

Carbon-nanotube-on-waveguide thermo-optic tuners.

We demonstrate on-waveguide thermo-optic tuners based on solution-processed metallic carbon nanotubes (CNTs) on silicon-on-insulator (SOI) and silicon nitride (SiN) microring resonators operating around 1550 nm. On SOI microring resonators using planarized wire waveguides, a thermo-optic power efficiency of 29 mW/FSR and a thermal transient of 1.3 µs are achieved. The heater is shown to be more power-efficient than conventional metal heaters and has lower thermal transient than both metal heaters and graphene-based heaters. On SiN microring resonators using rib waveguides, improvement in power efficiency with an increase in coverage of CNTs is demonstrated, indicating localized heating using the CNTs; this is favorable for low thermal cross-talk. An optimal power efficiency of 142 mW/FSR and a thermal transient of 5.8 µs are achieved.

On-chip optical transduction scheme for graphene nano-electro-mechanical systems in silicon-photonic platform.

We present a scheme for on-chip optical transduction of strain and displacement of graphene-based nano-electro-mechanical systems (NEMS). A detailed numerical study on the feasibility of three silicon-photonic integrated circuit configurations is presented: the Mach-Zehnder interferometer (MZI), the micro-ring resonator, and the ring-loaded MZI. An index sensing based technique using an MZI loaded with a ring resonator with a moderate Q-factor of 2400 can yield a sensitivity of 28 fm/Hz and 6.5×10-6%/Hz for displacement and strain, respectively. Though any phase-sensitive integrated-photonic device could be used for optical transduction, here we show that optimal sensitivity is achievable by combining resonance with phase sensitivity.

Parametric amplification in MoS2 drum resonator.

Parametric amplification is widely used in diverse areas from optics to electronic circuits to enhance low level signals by varying relevant system parameters. Parametric amplification has also been performed in several micro-nano resonators including nano-electromechanical system (NEMS) resonators based on a two-dimensional (2D) material. Here, we report the enhancement of mechanical response in a MoS2 drum resonator using degenerate parametric amplification. We use parametric pumping to modulate the spring constant of the MoS2 resonator and achieve a 10 dB amplitude gain. We also demonstrate quality factor enhancement in the resonator with parametric amplification. We investigate the effect of cubic nonlinearity on parametric amplification and show that it limits the gain of the mechanical resonator. Amplifying ultra-small displacements at room temperature and understanding the limitations of the amplification in these devices is key for using these devices for practical applications.

Trans-tibial guide wire placement for femoral tunnel in single bundle anterior cruciate ligament reconstruction.

BACKGROUND: Femoral tunnel location is of critical importance for successful outcome of ACL reconstruction. The aim was to study the femoral tunnel created by placing free hand guide wire through tibial tunnel, using the toggle of the guide wire in the tibial tunnel to improve femoral tunnel location. MATERIALS AND METHODS: 30 cases of a single bundle quadrupled hamstring graft anterior cruciate ligament reconstruction by trans-tibial free hand femoral tunnel creation is studied in this prospective study. The side to side play of the guide wire in the tibial tunnel was used to improve the tunnel location on femoral wall. The coronal angle of the femoral tunnel was measured on the anteroposterior radiograph. The femoral tunnel location on the lateral radiograph of the knee was recorded according to Amis method. Lysholm scoring was done preoperative and at each follow up. Assessment of laxity was done by Rolimeter (Aircast(™)) and pivot shift test. RESULTS: The mean coronal angle of the femoral tunnel in postoperative radiograph was 47°. In lateral radiograph, the femoral tunnel was found to be >60% posterior on Blumensaat line in 67% cases (n = 20) and in the 33% cases (n = 10) it was anterior. The mean Lysholm score improved from 74.6 preoperative to 93.17 postoperative with no objective evidence of laxity. CONCLUSION: The free hand trans-tibial creation of the femoral tunnel leads to satisfactory coronal obliquity, but it is difficult to recreate anatomic femoral tunnel by this method as the tunnel is consistently anterior in the sagittal plane.

Single-protein nanomechanical mass spectrometry in real time.

Nanoelectromechanical systems (NEMS) resonators can detect mass with exceptional sensitivity. Previously, mass spectra from several hundred adsorption events were assembled in NEMS-based mass spectrometry using statistical analysis. Here, we report the first realization of single-molecule NEMS-based mass spectrometry in real time. As each molecule in the sample adsorbs on the resonator, its mass and position of adsorption are determined by continuously tracking two driven vibrational modes of the device. We demonstrate the potential of multimode NEMS-based mass spectrometry by analysing IgM antibody complexes in real time. NEMS-based mass spectrometry is a unique and promising new form of mass spectrometry: it can resolve neutral species, provide a resolving power that increases markedly for very large masses, and allow the acquisition of spectra, molecule-by-molecule, in real time.

Towards single-molecule nanomechanical mass spectrometry.

Mass spectrometry provides rapid and quantitative identification of protein species with relatively low sample consumption. The trend towards biological analysis at increasingly smaller scales, ultimately down to the volume of an individual cell, continues, and mass spectrometry with a sensitivity of a few to single molecules will be necessary. Nanoelectromechanical systems provide unparalleled mass sensitivity, which is now sufficient for the detection of individual molecular species in real time. Here, we report the first demonstration of mass spectrometry based on single biological molecule detection with a nanoelectromechanical system. In our nanoelectromechanical-mass spectrometry system, nanoparticles and protein species are introduced by electrospray injection from the fluid phase in ambient conditions into vacuum, and are subsequently delivered to the nanoelectromechanical system detector by hexapole ion optics. Precipitous frequency shifts, proportional to the mass, are recorded in real time as analytes adsorb, one by one, onto a phase-locked, ultrahigh-frequency nanoelectromechanical resonator. These first nanoelectromechanical system-mass spectrometry spectra, obtained with modest mass sensitivity from only several hundred mass adsorption events, presage the future capabilities of this approach. We also outline the substantial improvements that are feasible in the near term, some of which are unique to nanoelectromechanical system based-mass spectrometry.

GABAA receptor modulation in dorsal root ganglia in vivo affects chronic pain after nerve injury.

Neuropathic pain (NPP) due to sensory nerve injury is, in part, the result of peripheral sensitization leading to a long-lasting increase in synaptic plasticity in the spinal dorsal horn. Thus, activation of GABA-mediated inhibitory inputs from sensory neurons could be beneficial in the alleviation of NPP symptoms. Dorsal root ganglia (DRG) conduct painful stimulation from the periphery to the spinal cord. Long-lasting down-regulation in GABA tone or sensitivity in DRG neurons has been reported in animals with neuropathy. To determine the function of GABA in DRG in the development of NPP, we examined how the acute pharmacological GABA(A)-receptor modulation of L5 DRG in vivo affects the development of NPP in rats with crush injury to the sciatic nerve. Direct application of muscimol and gaboxadol, GABA(A) agonists, to L5 DRG immediately after injury induced dose-dependent alleviation, whereas bicuculline and picrotoxin, GABA(A) antagonists, worsened NPP postaxonal injury. The pain-alleviating effects of muscimol and gaboxadol were blocked by bicuculline. Muscimol, applied at the time of injury, caused complete and long-lasting abolishment of NPP development. However, when muscimol was applied after NPP had already developed, its pain-alleviating effect, although significant, was short-lived. Using a fluorescent tracer, sodium fluorescein, we confirmed that local DRG application results in minimal spread into the corresponding dorsal horn of the ipsilateral spinal cord. GABA(A) receptors in DRG are important in the development of NPP after peripheral nerve injury, making timely exogenous GABAergic manipulation at the DRG level a potentially useful therapeutic modality.

A patient with congenital complete heart block undergoing multiple exposures to general anaesthesia.

We present a patient with congenital complete heart block who underwent multiple anaesthetic exposures for eye examination and bilateral cataract surgery. The diagnosis was made during the first general anaesthetic. Various complications encountered during the multiple exposures are discussed.