Nanomechanical Analog of a Laser: Amplification of Mechanical Oscillations by Stimulated Zeeman Transitions

We propose a magnetomechanical device that exhibits many properties of a laser. The device is formed by a nanocantilever and dynamically polarized paramagnetic nuclei of a solid sample in a strong external magnetic field. The corresponding quantum oscillator and effective two-level systems are coupled by the magnetostatic dipole-dipole interaction between a permanent magnet on the cantilever tip and the magnetic moments of the spins, so that the entire system is effectively described by the Jaynes-Cummings model. We consider the possibility of observing transient and cw lasing in this system, and show how these processes can be used to improve the sensitivity of magnetic resonance force microscopy.Comment: REVTeX version 4: 4 pages, 2 figures. Submitted to Phys. Rev. Lett. This version incorporates suggestions of John Sidles and PRL referee

High-Frequency Nanomechanical Resonators for Sensor Applications

This thesis describes new ways of operating high-frequency nanomechanical resonators and using them for sensor applications. The first part of the thesis is devoted to the techniques of detecting, actuating, and tuning the resonance motion of nanomechanical resonators. First, I consider piezoresistive detection using integrated thin-film piezoresistors made of doped semiconductors or metals. I describe the piezoresistive downmixing technique, which typically results in better performance than the conventional DC biasing technique. I then proceed to the possible ways of actuating the motion of nanomechanical resonators. After describing the challenges of applying the piezoshaker actuation technique to high-frequency resonators, I consider two alternatives: permanent-magnet magnetomotive actuation and Joule-heat-driven thermoelastic actuation. I demonstrate that the combination of thermoelastic actuation and piezoresistive detection can be used to efficiently detect multiple modes of nanomechanical resonators. Finally, I consider two ways of tuning the frequency of nanomechanical resonators: electrostatic tuning and absorptive tuning. The second part of the thesis is devoted to applications of nanoscale resonators to spin sensing, studies of dissipation of mechanical motion, and gas sensing. I consider possible ways of observing the coupling between mechanical motion and spins, describe our experimental results, and explore the analogy between coupled the spin--resonator system and the quantum-optical model of a laser. I then describe the results of quality-factor measurements in vacuum and air for doubly clamped beams and other resonator geometries. Finally, I describe a way to build better gas sensors by using arrays of nanomechanical resonators and present the preliminary gas-sensing data.</p

Smart-Cut Layer Transfer of Single-Crystal SiC Using Spin-on-Glass

The authors demonstrate “smart-cut”-type layer transfer of single-crystal silicon carbide (SiC) by using spin-on-glass (SoG) as an adhesion layer. Using SoG as an adhesion layer is desirable because it can planarize the surface, facilitate an initial low temperature bond, and withstand the thermal stresses at high temperature where layer splitting occurs (800–900 °C). With SoG, the bonding of wafers with a relatively large surface roughness of 7.5–12.5 Å rms can be achieved. This compares favorably to direct (fusion) wafer bonding, which usually requires extremely low roughness (\u3c2 Å rms), typically achieved using chemical mechanical polishing (CMP) after implantation. The higher roughness tolerance of the SoG layer transfer removes the need for the CMP step, making the process more reliable and affordable for expensive materials like SiC. To demonstrate the reliability of the smart-cut layer transfer using SoG, we successfully fabricated a number of suspended MEMS structures using this technology

Numerical and Experimental Study on the Addition of Surface Roughness to Micro-Propellers

Micro aerial vehicles are making a large impact in applications such as search-and-rescue, package delivery, and recreation. Unfortunately, these diminutive drones are currently constrained to carrying small payloads, in large part because they use propellers optimized for larger aircraft and inviscid flow regimes. Fully realizing the potential of emerging microflyers requires next-generation propellers that are specifically designed for low-Reynolds number conditions and that include new features advantageous in highly viscous flows. One aspect that has received limited attention in the literature is the addition of roughness to propeller blades as a method of reducing drag and increasing thrust. To investigate this possibility, we used large eddy simulation to conduct a numerical investigation of smooth and rough propellers. Our results indicate that roughness produces a 2% increase in thrust and a 5% decrease in power relative to a baseline smooth propeller operating at the same Reynolds number of Rec = 6500, held constant by rotational speed. We corroborated our numerical findings using thrust-stand-based experiments of 3D-printed propellers identical to those of the numerical simulations. Our study confirms that surface roughness is an additional parameter within the design space for micro-propellers that will lead to unprecedented drone efficiencies and payloads.Comment: 23 Pages, 9 Figure

A Model for Emission Yield from Planar Photocathodes Based on Photon-Enhanced Thermionic Emission or Negative-Electron-Affinity Photoemission

A general model is presented for electron emission yield from planar photocathodes that accounts for arbitrary cathode thickness and finite recombination velocities at both front and back surfaces. This treatment is applicable to negative electron affinity emitters as well as positive electron affinity cathodes, which have been predicted to be useful for energy conversion. The emission model is based on a simple one-dimensional steady-state diffusion treatment. The resulting relation for electron yield is used to model emission from thin-film cathodes with material parameters similar to GaAs. Cathode thickness and recombination at the emissive surface are found to strongly affect emission yield from cathodes, yet the magnitude of the effect greatly depends upon the emission mechanism. A predictable optimal film thickness is found from a balance between optical absorption, surface recombination, and emission rate

Minimizing the Ground Effect for Photophoretically Levitating Disks

Photophoretic levitation is a propulsion mechanism in which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in vacuum chambers in close proximity to the chamber floor and walls. We report here experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which microflyers lift off, can greatly increase the photophoretic lift forces relative to their free-space (mid-air) values. To characterize this so-called "ground effect" during vacuum chamber tests, we introduced a new miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimized a microflyer's extraneous interactions with underlying surfaces. We compared our new launchpads to previously used wire-mesh launchpads for simple levitating mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, wire-mesh launchpads increased the photophoretic lift force by up to sixfold. A significant ground effect was also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface was less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars.Comment: 7 pages, 7 figures, including the Supplemental Materia

In-plane nanoelectromechanical resonators based on silicon nanowire piezoresistive detection

We report an actuation/detection scheme with a top-down nano-electromechanical system for frequency shift-based sensing applications with outstanding performance. It relies on electrostatic actuation and piezoresistive nanowire gauges for in-plane motion transduction. The process fabrication is fully CMOS compatible. The results show a very large dynamic range (DR) of more than 100dB and an unprecedented signal to background ratio (SBR) of 69dB providing an improvement of two orders of magnitude in the detection efficiency presented in the state of the art in NEMS field. Such a dynamic range results from both negligible 1/f-noise and very low Johnson noise compared to the thermomechanical noise. This simple low-power detection scheme paves the way for new class of robust mass resonant sensor