The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

Tweezers with a twist

In this paper we summarize the hardware and software issues of impedance-based structural health moni- toring based on piezoelectric materials. The basic concept of the method is to use high-frequency structural excitations to monitor the local area of a structure for changes in structural impedance that would indicate imminent damage. A brief overview of research work on experimental and theoretical stud- ies on various structures is considered and several research papers on these topics are cited. This paper concludes with a discussion of future research areas and path forward. Piezoelectric materials acting in the "direct" manner pro- duce an electrical charge when stressed mechanically. Con- versely, a mechanical strain is produced when an electrical field is applied. The direct piezoelectric effect has often been used in sensors such as piezoelectric accelerometers. With the converse effect, piezoelectric materials apply local- ized strains and directly influence the dynamic response of the structural elements when either embedded or surface bonded into a structure. Piezoelectric materials have been widely used in structural dynamics applications because they are lightweight, robust, inexpensive, and come in a variety of forms ranging from thin rectangular patches to complex shapes being used in microelectromechanical systems (MEMS) fabrications. The applications of piezoelectric mate- rials in structural dynamics are too numerous to mention and are detailed in the literature (Niezrecki et al., 2001; Chopra, 2002). The purpose of this paper is to explore the importance and effectiveness of impedance-based structural health mon- itoring from both hardware and software standpoints. Imped- ance-based structural health monitoring techniques have been developed as a promising tool for real-time structural dam- age assessment, and are considered as a new non-destructive evaluation (NDE) method. A key aspect of impedance-based structural health monitoring is the use of piezoceramic (PZT) materials as collocated sensors and actuators. The basis of this active sensing technology is the energy transfer between the actuator and its host mechanical system. It has been shown that the electrical impedance of the PZT material can be directly related to the mechanical impedance of a host structural component where the PZT patch is attached. Uti- lizing the same material for both actuation and sensing not only reduces the number of sensors and actuators, but also reduces the electrical wiring and associated hardware. Fur- thermore, the size and weight of the PZT patch are negligible compared to those of the host structures so that its attach- ment to the structure introduces no impact on dynamic char- acteristics of the structure. A typical deployment of a PZT on a structure being monitored is shown in Figure 1. The first part of this paper (Sections 2 and 3) deals with the theoretical background and design considerations of the impedance-based structural health monitoring. The signal processing of the impedance method is outlined in Section 4. In Section 5, experimental studies using the impedance approaches are summarized and related previous works are listed. Section 6 presents a brief comparison of the imped- ance method with other NDE approaches and, finally, sev- eral future issues are outlined in Section 7. 2. Theoretical Background

Overview of Piezoelectric Impedance-Based Health Monitoring and Path Forward

Optical forces can be used to manipulate biological and colloidal material in a non-contact manner. This forms the foundation of a wealth of exciting science, particularly in the fields of physics, biology and soft condensed matter. Although the standard Gaussian single-beam trap remains a very powerful tool, shaping the phase and amplitude of a light field provides unusual light patterns that add a major new dimension to research into particle manipulation. This Review summarizes the impact and emerging applications of shaped light in the field of optical manipulation.

Shaping the future of manipulation

Optical trapping is an established field for movement of micron-size objects and cells. However, trapping of metal nanoparticles, nanowires, nanorods and molecules has received little attention. Nanoparticles are more challenging to optically trap and they offer ample new phenomena to explore, for example the plasmon resonance. Resonance and size effects have an impact upon trapping forces that causes nanoparticle trapping to differ from micromanipulation of larger micron-sized objects. There are numerous theoretical approaches to calculate optical forces exerted on trapped nanoparticles. Their combination and comparison gives the reader deeper understanding of the physical processes in an optical trap. A close look into the key experiments to date demonstrates the feasibility of trapping and provides a grasp of the enormous possibilities that remain to be explored. When constructing a single-beam optical trap, particular emphasis has to be placed on the choice of imaging for the trapping and confinement of nanoparticles.

https://www.spiedigitallibrary.org/journals/Journal-of-Nanophotonics/volume-2/issue-1/021875/Optical-manipulation-of-nanoparticles-a-review/10.1117/1.2992045.pdf

Optical manipulation of nanoparticles: a review

In any optical system, distortions to a propagating wavefront reduce the spatial coherence of a light field, making it increasingly difficult to obtain the theoretical diffraction-limited spot size. Such aberrations are severely detrimental to optimal performance in imaging, nanosurgery, nanofabrication and micromanipulation, as well as other techniques within modern microscopy. We present a generic method based on complex modulation for true in situ wavefront correction that allows compensation of all aberrations along the entire optical train. The power of the method is demonstrated for the field of micromanipulation, which is very sensitive to wavefront distortions. We present direct trapping with optimally focused laser light carrying power of a fraction of a milliwatt as well as the first trapping through highly turbid and diffusive media. This opens up new perspectives for optical micromanipulation in colloidal and biological physics and may be useful for various forms of advanced imaging.

In situ wavefront correction and its application to micromanipulation

Holographic or diffractive optical components are widely implemented using spatial light modulators within optical tweezers to form multiple, and/or modified traps. We show that by further modifying the hologram design to account for residual aberrations, the fidelity of the focused beams can be significantly improved, quantified by a spot sharpness metric. However, the impact this improvement has on the quality of the optical trap depends upon the particle size. For particle diameters on the order of 1 microm, aberration correction can improve the trap performance metric, which is the ratio of the mean square displacement of a corrected trap to an uncorrected trap, in excess of 25%, but for larger particles the trap performance is not unduly affected by the aberrations typically encountered in commercial spatial light modulators.

/pdf/aberration-correction-in-holographic-optical-tweezers-5amad2q0ib.pdf

Aberration correction in holographic optical tweezers.

An adaptive filter is used to estimate the feedthrough capacitance of a piezoelectric sensoriactuator in this work. The mechanical response of the piezostructure is resolved from the electrical response of the piezoelectric device through standard adaptive signal processing tech niques. Two common adaptive algorithms are reviewed for the given application: the LMS and the RLS. For spectrally white inputs the adaptation of the digital compensator yields a filter output which is proportional to the electrical response of the piezoelectric device. Thus, the remaining elec trical signal consists of the charge due to the mechanical response of the piezostructure. The adap tive filter converges to a combination of the feedthrough capacitance and the real portion of the mechanical piezostructure response, and the filter error is the quadrature component of the mechanical response. Preliminary results from the theoretical analysis and numerical simulations indicate that, under certain conditions, adaptive signal ...

Adaptive Compensation of Piezoelectric Sensoriactuators

Vibration-induced jitter degrades the pointing and imaging performance of precision optical systems. Practical active jitter reduction is achieved by maintaining beam alignment with mirror-positioning control systems. In the presence of time-varying or uncertain disturbances, jitter control systems using fixed-gain feedback control loops cannot operate without significant limitations on their performance. A feedback control technique called Q-parameterization can adapt to time-varying disturbances by adjusting its parameters in real time to maintain optimal performance. Adaptive feedback jitter control using Q-parameterization is experimentally verified on an optical testbed, increasing jitter reduction compared to an H2-optimal fixed-gain controller.

Adaptive feedback control of optical jitter using Q-parameterization

Phase compensation for feedback control of enclosed sound fields

We report the development of a novel nanolithographic system that combines the design capabilities of computer-aided design/computer-aided manufacturing (CAD/CAM) software with the nanolithographic abilities of the atomic force microscope (AFM). The AFM is a powerful tool for research at the nanoscale and can be used to perform a variety of serial nanolithographic techniques. A custom-built three-axis AFM system, designed to execute nanolithography, has been constructed and interfaced with a CAD/CAM design environment. This technique utilizes the CAD/CAM software to create, in a virtual design environment, the desired nanoscale patterns. Then, using a G-code interpreter and software algorithms to control the three-dimensional motion of the system, the design is replicated automatically by using conventional nanolithographic procedures. In this report, AFM-based anodization lithography on a silicon wafer and subsequent AFM imaging is used to confirm the successful automatic replication of the desired nanoscale patterns. Note to Practitioners-The impetus for this research was based on the desire to create a custom nanolithographic platform that could be changed and manipulated as per the users specifications and operated easily by anyone. Commercial atomic force microscopes (AFMs) that are used for nanolithography and other studies have their own specific software that enables little or no change to the workings of the system, rendering prototyping of new research techniques to be difficult. These closed AFM instruments can be difficult to operate and are user unfriendly. This report delineates our construction of an AFM system and how we have incorporated very familiar software environments and common computer-aided-design programming language to conduct nanolithography. A brief verification of how our system performs is included; we have deposited and imaged a series of surface features in a direct write fashion on a silicon surface using a common lithographic technique that exists in the research environment.

/pdf/automated-cad-cam-based-nanolithography-using-a-custom-4b3y5bm9d4.pdf

Daniel G. Cole

Papers

Aberration correction in holographic optical tweezers.

Adaptive Compensation of Piezoelectric Sensoriactuators

Adaptive feedback control of optical jitter using Q-parameterization

Phase compensation for feedback control of enclosed sound fields

Automated CAD/CAM-based nanolithography using a custom atomic force microscope