Optomechanics and nonlinear mechanics of suspended photonic crystal membranes

The recent demonstration of strong interactions between optical force and mechanical motion of an optomechanical structure has led to the triumphant result of mechanical ground-state cooling, where the quantum nature of a macroscopic object is revealed. Another intriguing demonstration of quantum physics on a macroscopic level is the measurement of the Casimir force which is a manifestation of the zero- point energy. An interesting aspect of the Casimir effect is that the anharmonicity of the Casimir potential becomes significant when the separation of microscale objects is in the sub-100nm regime. This regime is readily accessible by many of the realized gradient-force-based optomechanical structures. Hence, a new avenue of probing the Casimir effect on-chip all-optically has become available. We propose an integrated optomechanical platform, consisting of a suspended photonic crystal membrane evanescently coupled with a silicon-on-insulator substrate, for (i) measuring the Casimir force gradient and (ii) counteracting the attractive force by exerting a resonantly enhanced repulsive optical gradient force. This thesis first presents the full characterization of the optomechanical properties of the system in vacuo. The interplay of the optical gradient force (optomechanical coupling strength $g_{om}/2\pi=- 66GHz/nm$) and the photothermal force manifested in the optical spring effect and dynamic backaction is elucidated. Static displacement by the repulsive force of 1nm/mW is also demonstrated. In the second part of the thesis, the nonlinear mechanical signatures upon a strong coherent drive are reported. By resonantly driving the photonic crystal membrane with a piezo-actuator and an optical gradient force, we observed mechanical frequency mixing, mechanical bistability and non-trivial interactions of the Brownian peak with the driving signal. Finally we present our recent progress in establishing electro- static control of individual photonic crystal membranes to reduce and calibrate the electrostatic artifact which plagues Casimir measurements. The results discussed in this thesis point towards an auspicious future of a complete realization of a Casimir optomechanical structure and novel applications with nonlinearity afforded by the Casimir force and the optical gradient force.Engineering and Applied Science

Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces

Whether intentionally introduced to exert control over particles and macroscopic objects, such as for trapping or cooling, or whether arising from the quantum and thermal fluctuations of charges in otherwise neutral bodies, leading to unwanted stiction between nearby mechanical parts, electromagnetic interactions play a fundamental role in many naturally occurring processes and technologies. In this review, we survey recent progress in the understanding and experimental observation of optomechanical and quantum-fluctuation forces. Although both of these effects arise from exchange of electromagnetic momentum, their dramatically different origins, involving either real or virtual photons, lead to different physical manifestations and design principles. Specifically, we describe recent predictions and measurements of attractive and repulsive optomechanical forces, based on the bonding and antibonding interactions of evanescent waves, as well as predictions of modified and even repulsive Casimir forces between nanostructured bodies. Finally, we discuss the potential impact and interplay of these forces in emerging experimental regimes of micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical Systems" in Annalen der Physi

Control of buckling in large micromembranes using engineered support structures

In this paper we describe a general method to avoid stress-induced buckling of thin and large freestanding membranes. We show that using properly designed supports, in the form of microbeams, we can reduce the out-of-plane deflection of the membrane while maintaining its stiffness. As a proof of principle, we used a silicon-on-insulator (SOI) platform to fabricate 30 µm wide, 220 nm thick, free-standing Si membranes, supported by four 15 µm long and 3 µm wide microbeams. Using our approach, we are able to achieve an out-of-plane deformation of the membrane smaller than 50 nm in spite of 39 MPa of compressive internal stress. Our method is general, and can be applied to different material systems with compressive or tensile internal stress.United States. Defense Advanced Research Projects Agency. (Contract N66001-09-1-2070-DOD

Association between Physical Activity Knowledge and Levels of Physical Activity in Chinese Adults with Type 2 Diabetes

<div><p>Background</p><p>Physical activity (PA) is an important treatment regimen for diabetes. The purposes of this study were to evaluate people’s knowledge of how exercise influences wellbeing (termed “PA knowledge” or “knowledge of PA” in this paper) and the resulting association with levels of PA in Chinese adults with Type 2 diabetes, and to identify the valuable demographic and lifestyle factors that possibly influence the association between PA knowledge and level of PA.</p><p>Methods</p><p>Two hundred and fifty-eight adults with Type 2 diabetes completed an interviewer-administered survey at a diabetes clinic in Hong Kong. Data on demographics, lifestyle factors and diabetes-related medical indicators were obtained. A 20-item questionnaire was developed to measure PA-related knowledge (one point scored for each correct answer; aggregate score up to 20 points). level of PA was measured by the International Physical Activity Questionnaire.</p><p>Results</p><p>The proportions of correct answers to each question ranged from 19.4 to 90.7%. Compared with poorly educated participants, those with university education level and above had PA knowledge scores 1.7 points higher (14.3 <i>vs.</i> 12.6, <i>P<</i>0.05). Younger, female, and obese participants were more likely to have lower level of PA (all <i>P<</i>0.05). After adjustment for age, gender, (BMI) and education level, the odds of having a moderate-to-high level of PA was 19% greater with 1 unit increase in PA knowledge score [95% confidence interval (CI): 1.09–1.29; <i>P<</i>0.001], this association was strongest in participants with tertiary education level or above [odds ratio (OR): 1.35; 95% CI: 1.03–1.77; <i>P<</i>0.05].</p><p>Conclusions</p><p>PA knowledge was positively associated with level of PA. Education level significantly influenced the association between PA knowledge and level of PA, leading to the suggestion of vulnerable groups to target for PA improvement in the face of diabetes.</p></div

Physical activity level according to different demographic and lifestyle factors.

<p>Physical activity level according to different demographic and lifestyle factors.</p

Odds ratios (ORs) for level of PA per unit increase in PA knowledge score by logistic regression analysis.

<p>Odds ratios (ORs) for level of PA per unit increase in PA knowledge score by logistic regression analysis.</p

Descriptive statistic of participants (<i>n</i> = 258).

<p>Descriptive statistic of participants (<i>n</i> = 258).</p

Proportions of different answers in each physical activity knowledge question.

<p>Proportions of different answers in each physical activity knowledge question.</p

Bonding, antibonding and tunable optical forces in asymmetric membranes

We demonstrate that tunable attractive (bonding) and repulsive (anti-bonding) forces can arise in highly asymmetric structures coupled to external radiation, a consequence of the bonding/anti-bonding level repulsion of guided-wave resonances that was first predicted in symmetric systems. Our focus is a geometry consisting of a photonic-crystal (holey) membrane suspended above an unpatterned layered substrate, supporting planar waveguide modes that can couple via the periodic modulation of the holey membrane. Asymmetric geometries have a clear advantage in ease of fabrication and experimental characterization compared to symmetric double-membrane structures. We show that the asymmetry can also lead to unusual behavior in the force magnitudes of a bonding/antibonding pair as the membrane separation changes, including nonmonotonic dependences on the separation. We propose a computational method that obtains the entire force spectrum via a single time-domain simulation, by Fourier-transforming the response to a short pulse and thereby obtaining the frequency-dependent stress tensor. We point out that by operating with two, instead of a single frequency, these evanescent forces can be exploited to tune the spring constant of the membrane without changing its equilibrium separation.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract No. W911NF-07-D-0004)United States. Army Research Office. Institute for Soldier Nanotechnologies (Contract No. W911NF-07-D-0004)United States. Defense Advanced Research Projects Agency (contract N66001-09-1-2070-DOD

Sustained Inhibition of VEGF and TNF-α Achieves Multi-Ocular Protection and Prevents Formation of Blood Vessels after Severe Ocular Trauma

Purpose: This study aimed to develop a clinically feasible and practical therapy for multi-ocular protection following ocular injury by using a thermosensitive drug delivery system (DDS) for sustained delivery of TNF-α and VEGF inhibitors to the eye. Methods: A thermosensitive, biodegradable hydrogel DDS (PLGA-PEG-PLGA triblock polymer) loaded with 0.7 mg of adalimumab and 1.4 mg of aflibercept was injected subconjunctivally into Dutch-belted pigmented rabbits after corneal alkali injury. Control rabbits received 2 mg of IgG-loaded DDS or 1.4 mg of aflibercept-loaded DDS. Animals were followed for 3 months and assessed for tolerability and prevention of corneal neovascularization (NV), improvement of corneal re-epithelialization, inhibition of retinal ganglion cell (RGC) and optic nerve axon loss, and inhibition of immune cell infiltration into the cornea. Drug-release kinetics was assessed in vivo using an aqueous humor protein analysis. Results: A single subconjunctival administration of dual anti-TNF-α/anti-VEGF DDS achieved a sustained 3-month delivery of antibodies to the anterior chamber, iris, ciliary body, and retina. Administration after corneal alkali burn suppressed CD45+ immune cell infiltration into the cornea, completely inhibited cornea NV for 3 months, accelerated corneal re-epithelialization and wound healing, and prevented RGC and optic nerve axon loss at 3 months. In contrast, anti-VEGF alone or IgG DDS treatment led to persistent corneal epithelial defect (combined: + immune cells into the cornea (combined: 28 ± 20; anti-VEGF: 730 ± 178; anti-IgG: 360 ± 186, cells/section), and significant loss of RGCs (combined: 2.7%; anti-VEGF: 63%; IgG: 45%) and optic nerve axons at 3 months. The aqueous humor protein analysis showed first-order release kinetics without adverse effects at the injection site. Conclusions: Concomitant inhibition of TNF-α and VEGF prevents corneal neovascularization and ameliorates subsequent irreversible damage to the retina and optic nerve after severe ocular injury. A single subconjunctival administration of this therapy, using a biodegradable, slow-release thermosensitive DDS, achieved the sustained elution of therapeutic levels of antibodies to all ocular tissues for 3 months. This therapeutic approach has the potential to dramatically improve the outcomes of severe ocular injuries in patients and improve the therapeutic outcomes in patients with retinal vascular diseases