Optical tweezers use focused laser to trap microobjects suspended in the medium to the focal point. They are becoming an indispensable tool in microbiology because of its ability to trap tiny biological particles so that single particle analysis is possible. However, it is still very difficult to trap particles such as DNA molecules that are smaller than the diffraction limit. Although trapping of those is possible by increasing the laser power inversely proportional to the cube of the particle diameter, such high power can cause permanent thermal damages. One of the current solutions to this problem is to intensify the local field by the use of the near-field enhancement coming from nanoplasmonic structures illuminated with lasers. Such solution allows one to use low powered laser and still be able to trap them. In this paper, we present the trapping of a single DNA molecule by the use of the strong field enhancement due to a sub-micrometer sized hole drilled on a gold plate by an e-beam milling process and the trapping is verified by the measurement of the scattering signal that comes from the trapped DNA.
Since the discovery of the trapping nature of laser beam, optical tweezers have been extensively employed to measure
various parameters at micro/nano level. Optical tweezers show exceptional sensitivity to weak forces making it one of
the most sensitive force measurement devices. In this work, we present a technique to measure the stiffness of a
biomaterial at different points. For this purpose, a microparticle stuck at the bottom of the dish is illuminated by the
trapping laser and respective QPD signal as a function of the distance between the focus of the laser and the center of the
microparticle is monitored. After this, microparticle is trapped and manipulated towards the target biomaterial and when
it touches the cell membrane, QPD signal shows variation. By comparing two different QPD signals and measuring the
trap stiffness, a technique is described to measure the stiffness of the biomaterial at a particular point. We believe that
this parameter can be used as a tool to identify and classify different biomaterials.
The introduction and subsequent expression of external DNA inside single living mammalian cell (transfection) can be achieved by photoporation with femtosecond laser. After photoporation, external DNA can be introduced by trapping and successive insertion of DNA coated nanoparticle in the cell using optical tweezers. To maximize the transfection efficiency, one of the major aspects is that the photoporated cell should not be damaged and cell membrane should heal itself immediately or after sometime while the cells are healed in the CO<sub>2</sub> incubator. Furthermore, the size of hole created as a result of photoporation should be more than the size of DNA coated nanoparticle to be inserted inside the cell. In this paper, an analysis has been done on single cell of important breast cancer cell lines named MCF-7 and MDAMB231. Size of holes created in cell membrane after photoporation has been measured and the required optimum energy with sustained cell life were determined. Using this analysis, most favorable conditions for maximum transfection efficiency can be determined.
The simulation of electromagnetic problems using the Finite-Difference Time-Domain method starts with the geometric
design of the devices and their surroundings with appropriate materials and boundary conditions. This design stage is
one of the most time consuming part in the Finite-Difference Time-Domain (FDTD) simulation of photonics devices.
Many FDTD solvers have their own way of providing the design environment which can be burdensome for a new user
to learn. In this work, geometric and material modeling features are developed on the freely available Google SketchUp,
allowing users who are fond of its environment to easily model photonics simulations. The design and implementation of
the modeling environment are discussed.
Transfection is the process of introducing DNA into cells so that the introduced DNA will function and produce proteins.
This technique is useful to study the function of various DNA sequences and in the future may lead to gene therapy for
curing genetic diseases. Currently, a number of techniques are available for both population and individual cells
transfection. Although individual cells transfection is less commonly used than the population transfection, it has
benefits because it allows controlled single cell analysis. In this paper, we present a new laser assisted transfection
method for individual cells. In this technique, two lasers are used to perform the transfection procedure and third laser is
used to detect the position of DNA coated nanoparticle which is inserted in the cell. This technique has relatively high
transfection efficiency and good post-transfection cell viability.
A dual-beam method for the near-axial rotation of dielectric nanorods was devised. The method uses two laser beams, where a focused Gaussian beam holds the object in the beam axis while a focused Laguerre-Gaussian beam rotates the object. The near-axial rotation of ZnO nanorods using this method was then experimentally demonstrated, and the radial offset distance of the rotating nanorod from the beam axis was quantified via a video tracking method.
Three-dimensional position of optically trapped dielectric particles can be detected by measuring the back-focal plane
interference pattern of incident and scattered fields. Time-domain surface current based near zone to far zone
transformation was implemented to compute the interference pattern by a spherical scatterer under a focused Gaussian beam. Computed results are compared with experimental data for validations.
Microassembly has been identified as one of critical techniques in innovating the promising era of micro/nano
technology. Several works have been investigated to fabricate various micro-devices such as micro-sensors and microactuators.
Assembly plays an important role for fabricating micro-devices. However, there are only few studies in the
assembly of microparts. In this paper, we present manipulation and assembly of three-dimensional microparts produced
by two-photon polymerization where optical trapping technique was used to manipulate microparts. We show exemplary
microassembly formed by assembling two microparts, a movable female part and a male part fixed on a glass substrate.
Optical trapping of nanorods has attracted many researchers due to many potential applications of nanorods in sensor
technologies. It is well known that nanorods align with the propagation axis or the polarization direction of a laser beam.
However, there are only few studies about the axial rotation of nanorods. In this study, we present a method for the
measurement of the rotational frequency of nanorods.
Janus particles are composed of two fused hemispheres of different substances. Such an anisotropic structure results in different trapping characteristics under a focused laser beam. In this paper, we show the axial and transversal trapping forces of Janus particles under a focused laser beam and discuss the conditions that would result in stable trapping. We also show the performed experiments and the applied numerical simulations using the Finite Difference Time Domain method.
This paper reports the measurement of elastic constants between two DNA strands that are simultaneously hybridized by
a third target-DNA linker. Two probe-DNA strands that are immobilized on fluorescent beads and a target-DNA linker
formed a hybridized assembly through the Watson-Crick based pairing. Elastic constants of the resulting assembly were
measured using a force calibrated dual optical trap. This study can be used to detect the existence of a target-DNA linker
with a specified nucleotide sequence, indicating its potential use in DNA biosensors.
Optical tweezers are widely used for manipulating microscopic objects. Compared to other contact type microscopic
manipulators such as micro-grippers that exhibit firm gripping, optical tweezers inherently possess loose gripping. For
example, if a user tries to move target objects too fast such that the drag force of the viscous fluid exceeds the trapping
force, target objects will escape from the effective trapping region. When this happens in a standard user interface
environment with only video feedback, the user would sense this with a visual cue and slow down or slightly reverse the
movement of the trap. In this study we enrich the user interface by adding a haptic cue that is a sense of forces and
torques so that the user will sense the drag force and torque that is proportional to the gap distance and angle between the
line trap and the nanowire. We present some preliminary results of putting haptic cue for manipulating nanowires.
We investigate that components held in multiple optical traps can be manipulated and assembled together using snap-fit assembly. There are several works on manipulating microscopic objects with optical tweezers and assembling them. However, these techniques cannot sustain the assembled structure after turning off the laser source. In contrast, our technique utilizes snap-fit assembly so that assembled components do not detach. With this approach, components and sub-assemblies can be readily controlled in real-time and assembled into a permanent assembly. Our method can be used for constructing micrometer scale devices.
Optical tweezers is a promising manipulation tool for objects in the range of micrometers to nanometers. Although there are many reported works on manipulating objects made of different materials and objects of irregular shapes, it is more suitable for non-opaque materials and objects that are symmetrical. Furthermore, there are potential damages on the objects arising from immense heat that is produced by the laser beam. These problems can be alleviated by trapping objects (micro-handles) and using them collectively as a gripper to indirectly hold and manipulate a target object. Holding denotes equilibrium of forces exerted by the tools on a target object. However, there still is a problem with this approach. When the trapping volume is larger than the size of a tool, target objects get pulled towards the center of the trapping volume. This breaks the force equilibrium and gripping thus fails. In this paper, we report a new design of tools that can overcome this problem. The tool is a slender object with one end acting as a probe while the other end is spherical so that trapping is easy. The length of the tool is designed to be larger than the radius of the trapping volume. Thus the target object is never pulled towards the trapping center. A group of multiple identical tools will surround and push a target object at the probe tips resulting in a stable grasp.
There are several new tools for manipulating microscopic objects. Among them, optical tweezers (OT) has two distinguishing advantages. Firstly, OT can easily release an object without the need of a complicated detaching scheme. Secondly, it is anticipated to manipulate an object with six degrees of freedom. OT is realized by tightly focusing a laser beam on microscopic objects. Grabbing and releasing is easily done by turning a laser beam on and off. For doing a dexterous manipulation on an object, a complicated potential trap must be calculated and applied. We foresee that such calculation method will be developed in the near future. One of the candidates for implementing the calculated trap is scanning optical tweezers (SOT). SOT can be built by using actuators with a scanning frequency in the order of a hundred Hertz. We need fast scanners to stably trap an object. In this study, we present our design of such SOT. The SOT uses piezo-actuated tilt mirror and objective positioner to scan full three-dimensional workspace.
A design of a microscopy system tailored for optical tweezers with a capability of an automatic focusing is presented. In this design, we utilize lenses, motorized mechanical stage, lamp and a digital camera to magnify and see a micrometer sized spheres floating in a thin film of water. The system can automatically translate the stage that holds the specimen to obtain the best focused image. The best focused image is "sharp." Mathematically, the best focused image shows the maximum amount of high frequency terms from the images obtained by translating the stage. The metric that calculates how one image is focused is called the Focus Measure (FM). Unfortunately, low frequency components also increase this FM. And an optical imaging system is a low pass filtering system. Thus the primary concern is to lower the low frequency components in an image. The electric signals from each pixel of a CCD include noises that are inherent in each pixel. The result of this is an FM profile that has multiple local maxima. This is the most critical reason why an Automatic Focusing System (AFS) yields incorrect focusing results. Available techniques have been tested and from this experience, the most appropriate Focus Measure Filter (FMF) that has the sharpest FM despite the low frequency terms and noises has been selected. Furthermore, a maximum search algorithm that is immune to local maxima in an FM profile is discussed. Using this FMF and search algorithm, an Automatic focusing system (AFS) tailored for optical tweezers is presented. The system is implemented on personal computers equipped with Pentium 4 processors.
In order to realize the flexibility optical trapping offers as a nanoassembly tool, we need to develop natural and intuitive interfaces to assemble large quantities of nanocomponents quickly and cheaply. We propose a system to create such an interface that is scalable, inter-changeable and modular. Several prototypes are described, starting with simple interfaces that control a single trap in the optical tweezers instrument using a 3-dimensional Phantom haptic device. A networkbased approach is adopted early on, and a modular prototype is then described in detail. In such a design, individual modules developed on different platforms work independently and communicate with each other through a common language interface using the Neutral Messaging Language (NML) communication protocol. A natural user interface is implemented that can be used to create and manipulate traps interactively like in a CAD program. Modules such as image processing and automatic assembly are also added to help simplify routine assembly tasks. Drawing on lessons learned from the prototypes, a new system specification is formulated to better integrate the modules. Finally, conclusions are drawn on the overall viability and future of network-based systems for nanoassembly using optical tweezers.
There are increased needs for manipulating microscopic objects. One of enabling technologies is an instrument called optical tweezers (OT) that uses a focused laser beam to trap and move microscopic objects. OT has been shown effective for directly manipulating spherical, cylindrical or axis-symmetrical shapes. For other forms of shapes that do not show any symmetry, there have been works on using micrometer sized balls as a handle to indirectly manipulate the objects. Direct manipulation is difficult because complex trapping potential needs to be calculated to stably trap non-symmetrical shapes. User interfaces for these "indirect" systems use a computer mouse to design a layout of balls for surrounding (holding) an object and a trajectory that describes how these balls as a whole moves. The contained object pushed by these surrounding balls then moves accordingly. In this study, we introduce an intuitive user interface system for manipulating these balls. Using virtual reality gloves, each finger tip position of an operator is used to position control these balls. This user interface system enables the operator to intuitively grasp, move and release irregular formed shapes.
Optical tweezers (OT) uses a focused laser beam to trap and move microscopic objects. The design of OT consists of laying out stationary and moving (or rotating) lenses and mirrors so that two design constraints are met at the objective back aperture (OBA): 1) the laser beam has to be pivoted around the center, and 2) the beam has to keep the same degree of overfilling. While these constraints are met, the objectives are to maximize the divergence/convergence angle and the beam rotation angle at the OBA. They are each accomplished by moving a lens or rotating a mirror, respectively. There are few known designs that give (claimed) good performances while satisfying above constraints. However, these designs are improvised inventions with no attempts in optimization. In this paper, we propose a new method for designing an optimized OT that achieves the best performance with given pool of optical elements. Our method, (Topology Optimization of the Optical Tweezers Setup) first divides the layout space into finite lattices and then distributes lenses and mirrors to appropriate lattices. Subsequently, whether the attempted configuration conforms to the constraints is tested. If the test is successful, the layout and its performance are recorded. At the end, the best performing layout is found. In this paper, we primarily concentrate on optimizing the positions of lens components. In the future, this approach will be generalized for more complicated configurations that include mirror components.