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Portable Optical Tweezers Educational Kit
Sample Microscope Video Captures Taken Using the Optical Tweezers Educational Kit to Trap 1 µm and 3 µm Beads
Optical Tweezers Educational Kit
Educational Kit Details
Optical tweezers, also known as optical traps, move and manipulate small particles using only a beam of light. A focused laser beam is used to exert forces on electrically uncharged particles with sizes from 1 to 10 µm, allowing the particles to be trapped, moved, and manipulated. This optical tweezers lab kit is optimized for classroom and lab use. It features an easy-to-construct optical path and sample positioning stage, a visible laser source, and a camera system for easy demonstration. The educational kit is assembled on a 30 cm x 60 cm (1' x 2') aluminum optical breadboard (included) and can be easily moved for demonstration purposes without needing realignment.
A sample preparation kit, available separately below, provides additional accessories for preparing samples that can be manipulated with the optical tweezers demonstration kit. The sample preparation kit is optimized for use with the Educational Kit.
Alternate Optical Tweezer Options
We also offer a highly configurable modular optical tweezers system for research and advanced graduate laboratories. For a comparison of the capabilities of our educational and modular tweezers options, please see the Comparison tab. For advanced experimental setups, we offer a complete optical tweezers microscope system. This system is meant for users who desire a turnkey optical tweezers solution for inverted microscopes in a research laboratory setting.
Thorlabs Educational Products
Thorlabs' educational line of products aims to promote physics, optics, and photonics by covering many classic experiments, as well as emerging fields of research. Each kit includes all the necessary components and a manual that contains both detailed setup instructions and extensive teaching materials. These educational lab kits are being offered at the price of the included components, with the educational materials offered for free. Technical support from our educational team is available both before and after purchase.
Purchasing Note: Both English and German language manuals/teaching information are available for this product. The imperial educational kit contains the English manual and US-style power cord. The appropriate manual and power cord will be included in the metric kit based on your shipping location. The power supplies and other electronic devices in both the metric and the imperial kit accept voltages from 100 to 230 VAC. Please contact Tech Support if you need a different language, cord style, or power supply. As with all products on our website, taxes are not included in the price shown below.
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The Optical Tweezers Kit is mounted on a (1' x 2') 30 cm x 60 cm aluminum breadboard and can be moved without needing realignment. The screws and red outlines show the locations of mounted components.
Thorlabs educational optical tweezers kit is designed for classroom, lab, and other educational uses. It features a visible laser light source and an objective that does not require oil immersion. The CMOS viewing camera can be connected to a PC for demonstration use. The entire system is mounted on a 30 cm x 60 cm (1' x 2') aluminum breadboard and can be easily moved without needing realignment.
The EDU-OT2(/M) is an updated version of the EDU-OT1(/M) kit with several changes for improved assembly and performance. The laser segment updates include an SR9A-DB9 strain relief cable and LTN330-A variable collimator to simplify the mounting and collimation of the trapping laser. The laser segment also has CP09 removable cage plates to allow the beam expander to be removed for easier lens adjustment. The microscope system was updated with SM1CPL10 lens tube couplers and shorter cage rods to simplify assembly and adjustment, while a 70:30 (R:T) beamsplitter cube directs more laser power to the trap. For finer resolution on the Z-axis of the sample stage, the micrometer-driven MT1(/M) has been exchanged for the adjuster screw-driven MT1B(/M).
Laser and Microscope System
The EDU-OT2(/M) kit uses a L658P040 658 nm laser diode as the trap laser source. This 40 mW visible laser allows the spot to be easily observed through the microscope during operation for intuitive classroom demonstrations. The laser is focused through a Zeiss 63X, 0.8 NA objective, which also serves as the objective for the microscope. Sample illumination is accomplished using a previous generation MCWHL5 white LED, and the sample is viewed through a Thorlabs DCC1645C color CMOS camera. The laser, microscope, and optical path of the optical tweezers kit are shown below to the left.
Sample Positioning System
Samples are placed on the 3-axis sample positioning stage and moved around the static laser beam during experiments. The stage consists of two motorized MT1-Z8 MT1/M-Z8) 12 mm travel translation stages for X- and Y-axis travel, plus a manual MT1B (MT1B/M) stage for Z-axis translation. The motorized stages are controlled by KDC101 servo motor controllers with customizable velocity settings. The sample positioning stage is shown below and to the right.
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Microscope and Camera Assembly with Beam Paths (Sample Stage not Shown)
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Sample Positioning Stage
Vertical Adjuster Screw Provides 150 µm Travel per Revolution
Optical Tweezers Operation
Optical traps can be characterized by two essential forces: the scattering force and the gradient force. The scattering force can be attributed to the principle of radiation pressure. Since the incoming laser light is partly absorbed and/or reflected by the particles, a momentum transfer occurs, which makes the particles move away from the light source. Thus, the scattering force increases with the laser power.
The second, more important force is the gradient force. If the laser beam acts on particles with a higher refractive index than the aqueous medium in which they are dispersed, they travel in the direction of maximal light intensity, allowing the particles to be trapped in the laser focus. If the laser is tightly focused, the gradient force can exceed the scattering force so that the particles can be trapped and moved in all three spatial dimensions.
For experimental purposes, microscopic glass or plastic beads (about 1 to 10 µm) or various other objects are dispersed in liquid (water, alcohol) on a glass slide. The particles can then be moved and manipulated by trapping them in the focused laser beam and moving the slide, which is attached to a positioning stage. The objective, CMOS camera, and an additional tube lens compose a microscope, which allows for the observation of the trapping procedure on the PC monitor. Various experiments can be performed with this setup, including trapping of particles with varying laser powers (up to 40 mW), evaluation of the effective viscosity of the dispersion via Brownian motion, determination of the optical trapping forces and their harmonic potential, and statistical analysis of the probability of the presence of the particles in the trap.
Laser Safety Information
The class 3B laser diode used in this kit emits up to 42 mW of optical power, which can cause damage to the eyes if viewed directly. The laser driver is equipped with a key switch and safety interlock, which should be used appropriately to avoid injury. Additionally, we recommend wearing appropriate laser safety glasses when using this kit. See the Laser Safety tab for details.
The Optical Tweezers Kit Used to Trap and Manipulate Various Particles Including 1 µm and 3 µm Beads and Large Starch and Fat Particles
Several experiments that students can undertake as part of a lab course are outlined below. In addition to these exercises, the manual contains instructions for more activities such as adjusting the setup, finding the correct focus plane for the camera and laser, and arranging trapped particles within a sample.
Samples for the optical tweezers kit are simple to prepare. A sample containing 1 µm or 3 µm glass beads is useful, as these are well-suited for getting to know the operation and handling of the optical tweezers. Alternatively, an emulsion of cream in water will also produce particles that can be captured with the optical tweezers kit.
The following materials are necessary to create the sample:
First, place a drop of the solution with the particles in the watch glass dish and combine with sufficient distilled water. Place this mixture in a well on the microscope slide using a pipette. Put a cover glass over the sample so that there are no air bubbles between the glass and the sample.
The samples can either be prepared before each experiment or they can be sealed between the slide and the cover glass with a UV adhesive. We recommend allowing students to prepare new samples as an educational exercise.
The OTKBTK sample preparation kit, available below, contains additional accessories that can be used to prepare samples for this optical tweezers kit.
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After the laser is turned off, the particle of fat moves back to the surface of the emulsion.
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A particle of cream is held at the focus of the laser underneath the surface of the emulsion.
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The laser focus pushes down one of the cream particles floating at the surface of the emulsion.
Manipulation of a Cream Particle within a Cream Water Emulsion
Particles of dairy cream in a cream water emulsion are an appropriate size to be trapped by the optical tweezers in this kit. A sample can be created by mixing a drop of dairy cream with enough water to create a solution that is slightly milky in appearance. If one attempts to trap the cream particles with the laser, they will disappear from the focus and can no longer be clearly seen on the monitor (see the image to the right). The observation can be explained by the composition of the cream/water emulsion. Cream consists primarily of fat, which collects on the surface when mixed with water. The cream particles are therefore located on the surface of the water. However, the laser focus is located at a deeper level: when the cream particles are trapped, they are pulled down into the emulsion. This effect can be observed when the particle at which the laser is directed is tracked by adjusting the height of the stage as the particle moves deeper into the solution.
After the cream particle located in the optical trap has been brought into focus and can clearly be seen on the monitor, the laser can be switched off and the particle observed. Since the cream particle is not held in place by an optical trap after the laser is switched off, it will move upward once again to the surface of the water. Again, the motion of the particle can be tracked by adjusting the height of the stage.
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A sketch illustrating Brownian motion.
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The mean squared displacement for different sizes of glass beads is shown in the graph above.
Brownian motion is the random motion (translation and rotation) of microscopic free particles suspended in a fluid resulting from their collisions with the atoms or molecules of the fluid. Under the microscope, the paths of particles are seen as short, straight lines (see the figure to the right). The Brownian motion can be observed in experiments using the optical tweezers. The glass beads are located in a medium that consists of molecules that are constantly moving in all directions. Because of this, the molecules repeatedly bump into the beads, which causes a vibrating motion of the beads that can be observed under the optical tweezers. The higher the temperature, the more the molecules move.
Use a sample with the 3 µm glass spheres. You must first switch the laser off so that you can observe only Brownian motion. For evaluation, a video sequence with a duration of 2 minutes or more must be recorded. During this period, about 5 particles, which do not touch each other, should be in the image. A similar video should be recorded with the 1 µm spheres. The videos can be evaluated with the aid of image analysis software, which provides the x and y position of a particle over time (see the manual for a recommendation for a free software package).
We recommend evaluating and graphing the data obtained with the aid of a table calculation program. First, the mean squared displacement of the beads must be determined. This can be calculated from the positions of the beads (xi (ti ),yi (ti )) at different times, ti, which can be measured from the video:
The mean position value up to each time tn can be calculated by averaging all of the measured position values over time. To eliminate statistically possible deviations of individual particles, the mean value should also be averaged over M particles:
We recommend using at least 5 particles for this calculation. The values obtained for the average squared displacement, <r ²>(t n), with respect to time for three sizes of glass beads are plotted in the graph to the right. Note that the slope of the lines decreases with increasing diameter of the beads, meaning that larger beads move less. This result can easily be explained by Brownian motion: the 1 µm spheres can be more easily set into motion by impact with the water molecules than larger spheres. Therefore, a 1 µm bead travels more in a certain time interval than a larger bead.
Maximum Holding Force of the Optical Trap
Frictional forces from the surrounding liquid will act on the individual glass beads moving through the solution with velocity, v, and inhibit their motion. This force is proportional to the bead size and the viscosity of the fluid:
Here, R describes the radius of the bead. The viscosity, ηeff, describes how "thick" the combination of water and beads is, which means it is different for each sample. It can be calculated from the mean squared displacement of particles in the fluid, which was experimentally determined in the Brownian Motion experiment described above. The slope, m, of the line describing the beam squared displacement is related to the viscosity by the following equation:
where ηeff denotes the effective viscosity, R is the radius of the bead, and T is the temperature of the sample in Kelvin, and k B is the Boltzmann constant, which has an approximate value of 1.38 x 1023 J/K. The effective viscosity should be on the order of 10-3 N s/m².
If the bead is in the optical trap, two forces act on it. First, the frictional force, F R, which is caused by the suspension in which the glass bead is located, and the holding force, F H, of the optical trap. The maximum holding force is defined as the force needed to maintain a speed v max at which the bead can just be held by the trap. This is the case where the maximum holding force and frictional force are in balance:
For the EDU-OT2(/M), the holding force will typically be on the order of several pN, dependent on the contrast in refractive index between the trapped particle and the surrounding liquid.
Portable Optical Tweezers Kit Components
Thorlabs' Optical Tweezers Educational Kits are available in imperial and metric versions. In cases where the metric and imperial kits contain parts with different item numbers, metric part numbers and measurements are indicated by parentheses unless otherwise noted.
Imperial Kit: Included Hardware and Screws
Metric Kit: Included Hardware and Screws
We recommend operating these optical tweezers using the ThorCam™ and Kinesis® software packages. A guide to software installation and settings can be found in the manual.
Thorlabs offers three different optical tweezers options: our Microscope System for users who desire an out-of the box research solution, our open-architecture Modular System designed for research and advanced teaching labs, and the Educational Kit featured on this page designed for introducing the basics of optical trapping. While our modular system and the demonstration kit look similar there are many key distinguishing features that are summarized in the table below.
Laser Safety and Classification
Safe practices and proper usage of safety equipment should be taken into consideration when operating lasers. The eye is susceptible to injury, even from very low levels of laser light. Thorlabs offers a range of laser safety accessories that can be used to reduce the risk of accidents or injuries. Laser emission in the visible and near infrared spectral ranges has the greatest potential for retinal injury, as the cornea and lens are transparent to those wavelengths, and the lens can focus the laser energy onto the retina.
Safe Practices and Light Safety Accessories
Lasers are categorized into different classes according to their ability to cause eye and other damage. The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. The IEC document 60825-1 outlines the safety of laser products. A description of each class of laser is given below:
The Portable Optical Tweezers Educational Kit was developed in cooperation with Antje Bergmann and Daniela Rappa from the Karlsruhe Institute of Technology.
Do you have ideas for an experiment that you would like to see implemented in an educational kit? Contact us at email@example.com; we'd love to hear from you.
The OTKBTK is designed for use with our OTKB Modular Optical Tweezers and our EDU-OT2 Educational Discovery Kit. It allows users to quickly prepare a sample and test for optical trapping once they have completed construction. Included with the kit are the following: