Acceleration Sensing, Feedback Cooling, and Nonlinear Dynamics with Nanoscale Cavity-Optomechanical Devices

Light has long been used for the precise measurement of moving bodies, but the burgeoning field of optomechanics is concerned with the interaction of light and matter in a regime where the typically weak radiation pressure force of light is able to push back on the moving object. This field began with the realization in the late 1960's that the momentum imparted by a recoiling photon on a mirror would place fundamental limits on the smallest measurable displacement of that mirror. This coupling between the frequency of light and the motion of a mechanical object does much more than simply add noise, however. It has been used to cool objects to their quantum ground state, demonstrate electromagnetically-induced-transparency, and modify the damping and spring constant of the resonator. Amazingly, these radiation pressure effects have now been demonstrated in systems ranging 18 orders of magnitude in mass (kg to fg).

In this work we will focus on three diverse experiments in three different optomechanical devices which span the fields of inertial sensors, closed-loop feedback, and nonlinear dynamics. The mechanical elements presented cover 6 orders of magnitude in mass (ng to fg), but they all employ nano-scale photonic crystals to trap light and resonantly enhance the light-matter interaction. In the first experiment we take advantage of the sub-femtometer displacement resolution of our photonic crystals to demonstrate a sensitive chip-scale optical accelerometer with a kHz-frequency mechanical resonator. This sensor has a noise density of approximately 10 micro-g/rt-Hz over a useable bandwidth of approximately 20 kHz and we demonstrate at least 50 dB of linear dynamic sensor range. We also discuss methods to further improve performance of this device by a factor of 10.

In the second experiment, we used a closed-loop measurement and feedback system to damp and cool a room-temperature MHz-frequency mechanical oscillator from a phonon occupation of 6.5 million down to just 66. At the time of the experiment, this represented a world-record result for the laser cooling of a macroscopic mechanical element without the aid of cryogenic pre-cooling. Furthermore, this closed-loop damping yields a high-resolution force sensor with a practical bandwidth of 200 kHZ and the method has applications to other optomechanical sensors.

The final experiment contains results from a GHz-frequency mechanical resonator in a regime where the nonlinearity of the radiation-pressure interaction dominates the system dynamics. In this device we show self-oscillations of the mechanical element that are driven by multi-photon-phonon scattering. Control of the system allows us to initialize the mechanical oscillator into a stable high-amplitude attractor which would otherwise be inaccessible. To provide context, we begin this work by first presenting an intuitive overview of optomechanical systems and then providing an extended discussion of the principles underlying the design and fabrication of our optomechanical devices.

Formation of jets by implusive acceleration of a curved free surface

The sudden axial acceleration of a column of liquid bounded at one end by a concave free surface has been found, experimentally, to produce a jet which issues from the free surface with a speed several times that imparted to the column.

Theoretical approximations to such flows, valid for small time, are formulated subject to the assumption that the fluid is inviscid and incompressible. In a special two-dimensional case, it is found that, for vanishingly small time, the velocity at the point on the free surface from which the jet emanates is π/2 times the velocity imparted to the column. The solutions to several problems in two and three dimensions assuming that the initial curvature of the free surface is small, lead to values for this ratio dependent upon the curvature—the initial velocity in the case of axial symmetry exceeding that of the analogous two-dimensional problem by approximately 25%.

Experiments conducted upon the phenomenon give values systematically in excess of those predicted by the theory, although theory and experiment are in qualitative agreement with respect to the displacement of the free surface. It is suggested that the discrepancy is attributable to effects of finite curvature having been imperfectly accounted for in the axially-symmetric analysis.

Photographic materials on pp. 115, 120, and 121 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Restauração de imagens de microscopia de força atômica com uso da regularização de Tikhonov via processamento em GPU

A Restauração de Imagens é uma técnica que possui aplicações em várias áreas, por exemplo, medicina, biologia, eletrônica, e outras, onde um dos objetivos da restauração de imagens é melhorar o aspecto final de imagens de amostras que por algum motivo apresentam imperfeições ou borramentos. As imagens obtidas pelo Microscópio de Força Atômica apresentam borramentos causados pela interação de forças entre a ponteira do microscópio e a amostra em estudo. Além disso apresentam ruídos aditivos causados pelo ambiente. Neste trabalho é proposta uma forma de paralelização em GPU de um algoritmo de natureza serial que tem por fim a Restauração de Imagens de Microscopia de Força Atômica baseado na Regularização de Tikhonov.

DistributedCL: middleware de processamento distribuído em GPU com interface da API OpenCL.

Este trabalho apresenta a proposta de um middleware, chamado DistributedCL, que torna transparente o processamento paralelo em GPUs distribuídas. Com o suporte do middleware DistributedCL uma aplicação, preparada para utilizar a API OpenCL, pode executar de forma distribuída, utilizando GPUs remotas, de forma transparente e sem necessidade de alteração ou nova compilação do seu código. A arquitetura proposta para o middleware DistributedCL é modular, com camadas bem definidas e um protótipo foi construído de acordo com a arquitetura, onde foram empregados vários pontos de otimização, incluindo o envio de dados em lotes, comunicação assíncrona via rede e chamada assíncrona da API OpenCL. O protótipo do middleware DistributedCL foi avaliado com o uso de benchmarks disponíveis e também foi desenvolvido o benchmark CLBench, para avaliação de acordo com a quantidade dos dados. O desempenho do protótipo se mostrou bom, superior às propostas semelhantes, tendo alguns resultados próximos do ideal, sendo o tamanho dos dados para transmissão através da rede o maior fator limitante.

GPU-accelerated optimization of fuel treatments for mitigating wildfire hazard

Fuel treatment is considered a suitable way to mitigate the hazard related to potential wildfires on a landscape. However, designing an optimal spatial layout of treatment units represents a difficult optimization problem. In fact, budget constraints, the probabilistic nature of fire spread and interactions among the different area units composing the whole treatment, give rise to challenging search spaces on typical landscapes. In this paper we formulate such optimization problem with the objective of minimizing the extension of land characterized by high fire hazard. Then, we propose a computational approach that leads to a spatially-optimized treatment layout exploiting Tabu Search and General-Purpose computing on Graphics Processing Units (GPGPU). Using an application example, we also show that the proposed methodology can provide high-quality design solutions in low computing time. © 2013 The Authors. Published by Elsevier B.V.

Investigation on acceleration response of fiber optic mandrel hydrophone

The acceleration response of fiber optic mandrel hydrophone is studied based on the theory of elastic dynamics in this paper. For the first time the influence of the optical fiber on the acceleration response is taken into consideration. And the derivation of the radial deformation of the mandrel is presented in details. The acceleration response is evaluated both theoretically and experimentally and the results are in good agreement.

基于GPU的多片元效果的高效绘制

现代GPU的图形绘制管线一般基于扫描线转换算法。模型中的三角面片经过光栅化后投影到屏幕上，在投影区域的各像素位置分别生成一个对应的基本处理单位，称之为片元。在光栅化过程中，当场景中的物体互相遮挡时，相应像素位置会产生多个片元与之对应。一般图形应用只需处理视点所能直接看到的表面，即每个像素只需要保留离视点最近或最远的片元信息，但是有一些绘制效果需要同时处理同一像素位置对应的多个片元，这些特殊的效果通常称为多片元效果，包括顺序独立的透明现象、半透明现象、体绘制以及折射等。然而，现有GPU只针对不透明物体的绘制进行了硬件层面的优化，使得光栅化后每个象素位置只保留最近或最远的片元，其余都在生成最终图像时被抛弃，所以目前多片元效果的绘制需要重复光栅化整个场景多遍，才可以收集到每个像素对应的所有片元。当场景规模较小时算法性能较高，但对于大型复杂场景来说，模型的多次顶点变换将成为绘制瓶颈，导致算法效率下降，因而难以在交互式应用中广泛使用。 针对这个问题，本文提出一种基于桶排序的高效深度剥离算法，将GPU上的多绘制目标缓存作为桶数组，采用桶排序原理以及最大/最小融合模式来收集投影到同一个像素上的多个片元并按深度排序，最后在后处理中再对场景进行延迟着色以绘制多片元效果。当发生桶内片元冲突时可以采用多遍绘制或者自适应划分的方式来降低片元冲突概率，以进一步提高绘制的准确性。针对透明现象的绘制，提出一种基于桶内动态融合的改进算法，采用并发读写的方法逐一融合落入同一个桶内的所有片元，并在后处理中按从前向后的顺序融合各个桶内的颜色值。由于同时发生桶内片元冲突和读写冲突的概率非常小，因而可以进一步提高绘制结果的准确性。实验结果表明，基于桶排序的深度剥离算法可以高效地处理大型场景多片元效果的绘制，同时生成与真实结果非常相近的绘制效果。 针对传统图形管线的不足之处，本文进一步设计并实现了CUDA渲染器：第一个可以在当前图形硬件上运行的全线可编程的图形管线系统，并基于该框架设计了两种新的透明现象的高效单遍绘制策略：第一种策略称之为多级深度测试策略，该策略利用了CUDA 的原子操作符atomicMin，可以在单遍绘制中动态收集所有片元并排序；第二种策略称之为固定数组缓存策略，该策略利用CUDA 的原子操作符atomicInc，可以在单遍绘制中按光栅化顺序收集所有的片元并在后处理中排序。实验结果表明，基于CUDA渲染器的这两种片元收集策略可以在单遍场景遍历中高效地绘制多片元效果，同时生成与真实结果非常相近的绘制效果。 未来的工作方向在于进一步完善基于桶排序的深度剥离算法，设计更加完善的深度区间划分方式，使得桶数组可以与片元一一对应，以完全消除桶内片元冲突。此外，可以进一步完善CUDA渲染器，使其可以更高效地处理遮挡剔除以及反走样等其它经典图形问题。