Name: optical trap loading of dielectric microparticles in air
Uploaded: 2017-02-05T13:00:00
Description: Watch this Scientific Journal Video about Optical Trap Loading of Dielectric Microparticles In Air at JoVE.com

All work performed under the support of the National Institute of Standards and Technology. Certain commercial equipment, instruments, or materials are identified to foster understanding of this protocol. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Caution: Please consult all relevant safety programs before the experiment. All the experimental procedures described in this protocol are performed in accordance with the NIST LASER safety program as well as other applicable regulations. Please be sure to select and wear proper personal protective equipment (PPE) such as laser protection glasses designed for the specific wavelength and power. Handling dry nano/microparticles may require additional respiratory protection.
1. Design and Fabricati...

<ol>
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The piezoelectric launcher is designed to optimize the dynamic performance of a selected PZT. Proper selection of PZT materials and management of ultrasonic vibrations are the key steps to yield a successful experiment. PZTs have different characteristics depending on the type of transducer (bulk or stacked) and component materials (hard or soft). A bulk type PZT made of a hard piezoelectric material is chosen for the following reasons. First, hard piezoelectric materials have lower dielectric losses and higher mechanica...

Ashkin reported the acceleration and trapping of microparticles by radiation pressure in 1970.1 His novel achievement promoted the development of optical trapping techniques as a primary tool for fundamental studies of physics and biophysics.2,3,4,5 To date, the application of optical trapping has focused mainly on liquid environments, and been used to study a very wide range of systems, from the behavior of colloids to the mechanical properties of single biomolecules.6,7,8 Application of optical trapping to gaseous media, however, requires resolving several new technical issues.
Recently, optical trapping in air/vacuum has been increasingly applied in fundamental research. Since optical levitation potentially provides nearly-complete isolation of a system from the surrounding environment, the optically levitated particle becomes an ideal laboratory for studying quantum ground states in small objects,4 measuring high-frequency gravitational waves,9 and searching for fractional charge.10 Moreover, the low viscosity of air/vacuum allows one to use inertia to measure the instantaneous velocity of a Brownian particle11 and to create ballistic motion over a wide range of motion beyond the linear spring-like regime.12 Therefore, detailed technical information and practices for optical traps in gaseous media have become more valuable to the broader research community.
New experimental techniques are required to load nano/microparticles into optical traps in gaseous media. A piezoelectric transducer (PZT), a device that converts electric energy into mechano-acoustic energy, has been used to deliver small particles into optical traps in air/vacuum5,12 since the first demonstration of optical levitation.1 Since then, several loading techniques have been proposed to load smaller particles using volatile aerosols generated by a commercial nebulizer13 or an acoustic wave generator.14 The floating aerosols with solid inclusions (particles) randomly pass near the focus and are trapped by chance. Once the aerosol is trapped, the solvent evaporates out and the particle remains in the optical trap. However, these methods are not well suited to identify desired particles from within a sample, load a selected particle and to track its changes if released from the trap. This protocol is intended to provide details to new practitioners on selective optical trap loading in air, including the experimental setup, fabrication of a PZT holder and sample enclosure, trap loading, and data acquisition associated with the analysis of particle motion in both the frequency and time domains. Protocols for trapping in liquid media have also been published.15,16
The overall experimental setup is developed on a commercial inverted optical microscope. Figure 1 shows a schematic diagram of the setup used to demonstrate steps of the selective optical trap loading: freeing the resting microparticles, lifting the chosen particle with the focused beam, measuring its motion, and placing it onto the substrate again. First, translational stages (transverse and vertical) are used to bring a selected microparticle on the substrate to the focus of a trapping laser (wavelength 1064 nm) focused by an objective lens (near-infrared corrected long-working distance objective: NA 0.4, magnification 20X, working distance 20 mm) through the transparent substrate. Then, a piezoelectric launcher (a mechanically pre-loaded ring-type PZT) generates ultrasonic vibrations to break the adhesion between microparticles and a substrate. Thus, any freed particle can be lifted by the single-beam gradient laser trap focused on the selected particle. Once the particle is trapped, it is translated to the center of the sample enclosure containing two parallel conducting plates for electrostatic excitation. Finally, a data acquisition (DAQ) system simultaneously records the particle motion, captured by a quadrant-cell photodetector (QPD), and the applied electric field. After finishing the measurement, the particle is controllably placed onto the substrate so that it can be trapped again in a reversible manner. This overall process can be repeated hundreds of times without particle loss to measure changes such as contact electrification occurring over several trapping cycles. Please refer to our recent article for details.12

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			<td height="26" style="height:27px;width:319px;">ScotchBlue Painter&#39;s Tape Original</td>
			<td style="width:177px;">3M</td>
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			<td height="26" style="height:27px;width:319px;">Scotch 810 Magic Tape</td>
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			<td height="26" style="height:27px;width:319px;">Function/Arbitrary Waveform generator</td>
			<td style="width:177px;">Agilent</td>
			<td style="width:216px;">HP33250A</td>
			<td></td>
		</tr>
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			<td height="26" style="height:27px;width:319px;">Power supply/Digital voltage supplier</td>
			<td style="width:177px;">Agilent</td>
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			<td></td>
		</tr>
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			<td height="26" style="height:27px;width:319px;">Ring-type piezoelectric transducer</td>
			<td style="width:177px;">American Piezo Company</td>
			<td style="width:216px;">item91</td>
			<td></td>
		</tr>
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			<td height="26" style="height:27px;width:319px;">Electro-optic modulator</td>
			<td style="width:177px;">Con-Optics</td>
			<td style="width:216px;">350&#8722;80-LA</td>
			<td></td>
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		<tr height="26">
			<td height="26" style="height:27px;width:319px;">Amplifier for Electro-optic modulator</td>
			<td style="width:177px;">Con-Optics</td>
			<td style="width:216px;">302RM</td>
			<td></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:27px;width:319px;">Mitutoyo NIR infinity Corrected Objective</td>
			<td style="width:177px;">Edmund optics</td>
			<td style="width:216px;">46-404</td>
			<td>Manufactured by Mitutoyo and Distributed by Edmund optics</td>
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			<td height="26" style="height:27px;width:319px;">LOCTITE SUPER GLUE LONGNECK BOTTLE</td>
			<td style="width:177px;">Loctite</td>
			<td style="width:216px;">230992</td>
			<td></td>
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		<tr height="26">
			<td height="26" style="height:27px;">3D printer</td>
			<td style="width:177px;">MakerBot</td>
			<td style="width:216px;">Replicator 2</td>
			<td></td>
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			<td height="21" style="height:21px;width:319px;">Polylactic acid (PLA) filament</td>
			<td style="width:177px;">MakerBot</td>
			<td style="width:216px;">True Red PLA Small Spool</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Data Acquisition system</td>
			<td style="width:177px;">National Instruments</td>
			<td style="width:216px;">780114-01</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Quadrant-cell photodetector</td>
			<td style="width:177px;">Newport</td>
			<td style="width:216px;">2031</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Translational stage</td>
			<td style="width:177px;">Newport</td>
			<td style="width:216px;">562-XYZ</td>
			<td></td>
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		<tr height="21">
			<td height="21" style="height:21px;">Inverted optical microscope</td>
			<td style="width:177px;">Nikon Instruments</td>
			<td style="width:216px;">EclipsTE2000</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Fluorescence filter (green)</td>
			<td style="width:177px;">Nikon Instruments</td>
			<td style="width:216px;">G-2B</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Flea3/CCD camera</td>
			<td style="width:177px;">Point Grey</td>
			<td style="width:216px;">FL3-U3-13S2M-CS</td>
			<td>Trapping laser</td>
		</tr>
		<tr height="46">
			<td height="46" style="height:46px;width:319px;">Diode pumped neodymium yttrium vanadate(Nd:YVO4)</td>
			<td style="width:177px;">Spectra Physics</td>
			<td style="width:216px;">J20I-8S-12K/ BL-106C</td>
			<td></td>
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			<td height="21" style="height:21px;width:319px;">Indium tin oxide (ITO) Coated coverslips</td>
			<td style="width:177px;">SPI supplies</td>
			<td style="width:216px;">06463B-AB</td>
			<td>Polystyrene microparticles</td>
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			<td style="width:177px;">Tedpella</td>
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			<td height="21" style="height:21px;width:319px;">Dri-Cal size standards</td>
			<td style="width:177px;">Thermo Scientific</td>
			<td style="width:216px;">DC-20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">Optical Fiber</td>
			<td style="width:177px;">Thorlabs</td>
			<td style="width:216px;">P1&#8722;1064PM-FC-5</td>
			<td>bottom plate</td>
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			<td height="21" style="height:21px;width:319px;">Aluminium plate&#160;</td>
			<td style="width:177px;">Thorlabs</td>
			<td style="width:216px;">CP4S</td>
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		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:319px;">High voltage power amplifier</td>
			<td style="width:177px;">TREK</td>
			<td style="width:216px;">PZD700A M/S</td>
			<td></td>
		</tr>
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</table>

optical trap loading of dielectric microparticles in air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

The PZT launcher is designed using a CAD software package. Here, we use a simple sandwich structure for the preloading (a PZT clamped with two plates), as shown in Figure 2. The PZT holder and the sample enclosure can be fabricated from a variety of materials and methods. For a quick demonstration, we choose 3D printing with thermoplastic as illustrated in Figure 2d. Based on the fabricated components, optical trap loading is shown in Figure 3</st...

Watch this Scientific Journal Video about Optical Trap Loading of Dielectric Microparticles In Air at JoVE.com

Optical Trap Loading of Dielectric Microparticles In Air

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...

The goal of this video protocol is to demonstrate how a piezoelectric launcher is constructed and used to overcome particle adhesion and ease particle release from the substrate to achieve micro particle levitation in air. The advantage of launching from a surface is that the particle may be selected based on size, shape, optical properties, or orientation. This last can be useful for trapping asymmetric particles.Repeated trapping and landing of particle can provide a new tool to study questions in surface chemistry, such as charged transfer during the particle's repeat interaction. Returning the particle to the surface also allows subsequent measurements of the same particle by other methods, such as scanning electron microscopy, and also allows repeated cycles of trapping and landing. First, 3D print a rectangular frame in a holder for piezoelectric transducer.Fit the narrow sides of the frame with indium tin oxide coated glass, and the wide sides and top with conventional glass to construct the transparent sample enclosure. Next, pour a small portion of 20 micrometer diameter dielectric micro particles onto a glass slide. Immediately return the container to a desiccator.Using a glass capillary tube, pick up some of the micro particles from the slide. Scatter the particles onto a cover slip by gently tapping the capillary tube. Use a microscope to verify the quantity and distribution of dispersed micro particles on the substrate.Next, begin piezo-launcher assembly by applying insulating film to an aluminum plate. Center a ring type piezoelectric transducer on the plate, ensuring that is separated from the plate by the insulating film. Place the micro particle dispersed cover slip on the transducer, and snap a copper ring into the 3D printed holder.Secure the transducer in place on the plate with the 3D printed holder and two M6 screws. Lightly glue the sample enclosure onto the holder with instant adhesive. Mount the completed launcher assembly on an XYZ translational stage of an inverted optical microscope.Connect the piezoelectric transducer leads to the voltage amplifier, and the amplifier to the function generator, ensuring that all connections are secure and properly grounded. Connect the amplifier monitoring output port to an oscilloscope to monitor the transducer-driving voltage signal. Configure the function generator to send a continuous square wave of zero to one volts to the amplifier.Set the amplifier gain to 200 volts per volt. Set up the microscope to image the micro particles in real time. Then manually scan the modulation frequency of the driving signal from 0 to 150 kilohertz while monitoring the micro particle motion.The piezoelectric assembly is designed to apply mechanical pre-load to the transducer, to amplify the resonance amplitude and reduce failure. Frequencies can help to determine the resonance frequency of transducer by monitoring the particle on the surface. The frequency at which micro particle motion is maximized with no adhered particles is the launcher resonance frequency.Repeat the scan as needed at lower voltages to precisely determine the resonance frequency. Set the wave form to generate a voltage signal at the resonance frequency with a 600 volt amplitude. Configure the function generator for a square wave with 10 to 20 cycles in burst mode.Check that the particles are released after each pulse. If the particles are not released, increase the amplitude or number of cycles. Remove the laser lined filter from the microscope, to locate the focus of the trapping beam on the CCD.Optimize the focus by adjusting the vertical position of the motorized focusing block. Replace the filter, and translate the sample so a selected particle is at the trapping laser focus position. Focus on the particle to image the particle center.Actuate the piezoelectric launcher with several voltage pulses to launch the particle, adjusting the power supply of the electro object modulator as needed. When the particle blurs slightly and begins Brownian motion, trapping has occurred. The focus of trapping beam is critical for successful trap loading.Placing the pin focus few micron below the sample plane helps to increase the trapping efficiency. If the particle only moves slightly and remains in focus on the substrate, increase the trapping power. If the particle flies away to a new location on the surface, decrease the power.Once the particle is successfully loaded, move the objective lens up to translate the levitated particle about one millimeter above the glass cover slip to avoid surface interactions. Then reduce the optical power to move the levitated particle into the more stable nominal trapping position. Move the launcher so the transducer-holder is centered on the optical axis.Move the objective lens to vertically translate the particle higher into the sample enclosure. To begin the measurements, adjust the condenser and focusing lens until the power spectrum density plots of the X and Y channels are well matched. Then connect a high voltage source to the sample enclosure.To measure the trap properties and particle charge, apply an electrostatic field to generate ballistic motion of the particle. Record the step excitation signal in the induced particle trajectory. Average the data for multiple periods as needed to reduce the effects of Brownian motion.After performing the measurement, move the objective lens down to position the particle onto the center of the glass cover slip, to keep it separate from the other particles. The piezoelectric launcher assembly consists of a transparent enclosure to protect the levitated particle from air currents, and a piezoelectric transducer that vibrates the substrate to release the particle. The enclosure bottom is open so that the substrate can be placed directly on the transducer.The micro particles can either be trapped in the levitation position above focus or in the trapping position near focus where optical forces stabilize the particle in all directions. In the levitation position, gravity counteracts the upward radiation pressure so optical forces only stabilize the particle transversely. For small displacements, such as Brownian motion, the optical force can be treated as a linear spring.The resonance frequency can be determined from the power spectral density plot. The trap's stiffness and optical force in the linear regime can then be calculated from the resonance frequency, known mass, and measured displacement. Forces over wider ranges of displacement can be determined from the induced ballistic motion in the applied electric field.Analysis of ballistic displacement by the transient response method gives the resonance frequency, damping, and steady state displacement. Non-linear forces can be determined with the parametric force method. This approach levitating microparticles enables precision measurements using particles selected for size, shape, or material properties, and also allows subsequent measurements on the same particles.Using this method, launching and landing a particle can be repeated over hundred of cycles. This will also allow sensitive studies of particle surface interaction, such as contact electrification with single electron sensitivity.

Research

JoVE Journal

Engineering

Construction and Testing of Coin Cells of Lithium Ion Batteries

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime.  Lithium ion ...

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Fabrication of VB2/Air Cells for Electrochemical Testing

Fabrication of VB2/Air Cells for Electrochemical Testing

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

The Preparation of Electrohydrodynamic Bridges from Polar Dielectric Liquids

Horizontal and vertical liquid bridges are simple and powerful tools for exploring the interaction of high intensity electric fields (8-20 kV/cm) and ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Fabrication Process of Silicone-based Dielectric Elastomer Actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an ...