Optical Trap Loading of Dielectric Microparticles In Air

Summary

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

Abstract

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.

Introduction

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/vacuum^5,12 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

Protocol

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 Fabrication of a PZT Holder and a Sample Enclosure

1. Design a PZT holder and a sample enclosure
   NOTE: Particular design values vary depending on the selection of a PZT.
   1. Open the computer-aided design (CAD) software package. Draw a two-dimensional (2D) sketch of a holder for a given PZT dimension. Develop the 2D sketch to volumetric features using combinations of Extrude/Extrude-cut.
   2. Click Sketch, draw a rectangle and extrude it to make a rectangular cube.
   3. Sketch a disk on the top surface of the cube to define a circularly recessed feature to cover and hold the ring-type PZT.
   4. Define a central hole to have an optical access for both real-time imaging and trapping.
   5. Define a circular guide along the rim of the central hole to insert a flat metallic (copper) ring to concentrate the ultrasonic power toward the center area as shown in Figure 2a.
   6. Create two bore holes for M6 screws on the PZT holder to be assembled with a bottom plate (purchased, 4 mm thick bottom aluminum plate with a hole in the center), as shown in Figure 2c and 2d.
   7. In a similar manner, design a rectangular frame of the sample enclosure. Click Sketch, and draw a rectangle, extrude the rectangle to make it a rectangular box.
   8. Draw a smaller rectangle on the top surface of the rectangular box and extrude-cut the rectangle to make it as a rectangular tube.
   9. Draw a smaller rectangle on the side wall of the tube and Extrude-cut to transform it into the frame of sample enclosure box.
   10. Convert these three-dimensional (3D) models into a stereolithography (STL) file format for a 3D printing process (Figure 2b).
2. 3D printing of the designed objects
   1. Open the design file ("-.STL") from the 3D printer operating software. Lay the object flat 0/.and center the object on (0, 0, 0) by clicking the object to select it and using the alignment functions: "Move", "On Platform", and "Center". Orient the PZT holder to face the delicate features upward. The recessed surface will be faced upward.
   2. In the menu go to the "Settings" and the "Quality" tab. Set the printing values as following, Infill: 100%, Number of shells: 2, and Layer height: 0.2 mm.
   3. Preview the objects to check the total print time and make sure the layered objects will be printed as desired. Export the 3D print file in a ".x3g" format and save it to use in the 3D printer.
   4. Turn on the 3D printer and warm it up until the temperature of the extrusion nozzle reaches an operating temperature, 230 °C. Load the design file from a memory card or network drive.
   5. During the warm up, place the Build platform with blue painter's tape to help objects adhere securely. As a thermoplastic material for the printing job, use a polylactic acid (PLA) filament for both objects.
   6. Print the designed objects. Once the printing job is finished, turn off the printer after it has cooled down.
   7. Detach the printed object from the platform using a chisel. Straighten up the printed objects. If the orientation is appropriately chosen, the PZT holder can be directly used without further post-processing.
   8. For the sample enclosure, prepare one pair of indium tin oxide (ITO) coated coverslips and three glass coverslips to cover the frame. Use a diamond cutter to fit the coverslip to the enclosure.
   9. Wire the two parallel conducting plates using a fast drying silver paint to supply voltage across two plates. Glue these five windows onto the sample enclosure using an instant adhesive glue.
      NOTE: The one pair of ITO coated coverslips are installed on the sample enclosure in parallel (facing each other) to provide uniform electric field and to generate ballistic motion of the naturally charged particle along the electric field. The three conventional coverslip cover the rest of sample enclosure surfaces (top and two other sides) to protect the trapped particle from the external flow of air

2. Optical Trap Loading of a Selected Microparticle

1. Sample preparation
   1. Store the microparticles in an evacuated desiccator to reduce contact with moisture in the air before the experiment.
   2. Pour out a small portion of microparticles onto a glass slide and immediately put the manufacture's bottle back in the desiccator.
   3. Pick up some of the microparticles with a glass capillary tube. Scatter the particles over the substrate by gently tapping on the capillary while holding the capillary over the coverslip.
   4. Verify the quantity and distribution of deposited particles on the substrate using a dark-field microscope.
      Note: In the sample preparation step, the particle is just scattered on a coverslip and imaged with an optical microscope to verify overall arrangement before inserting them (a coverslip with scattered microparticles) between the PZT and PZT holder. Since the surface adhesion is strong enough to hold individual microparticles on the substrate, the adhered particles are firmly fixed unless significant external force is applied.
2. Piezoelectric launcher assembly
   1. Obtain all the components of the piezoelectric launcher: the flat bottom plate, insulating film, the PZT, the glass coverslip, a copper ring, the PZT holder, two M6 screws, and the sample enclosure.
   2. Apply a thin film (or tape) on the bottom plate to insulate the PZT. The glass coverslip isolates the top of the stack.
   3. Assemble the stack by centering the PZT on top of the flat plate now insulated with tape, followed by the coverslip, the copper ring, and the PZT holder. Screw the stack together maintaining the centering of the PZT to avoid shorting the PZT to the holder if the holder is conducting as shown in Figure 2c and 2d. The copper ring provides an evenly distributed mechanical preload on the stack for plastic PZT holders.
   4. Finally, glue the sample enclosure onto the stack and mount the assembly on an XYZ translational stage in the microscope.
3. Configuration of the PZT launcher
   NOTE: Driving the PZT with a high voltage signal has potential electrical hazards. Please consult with safety personnel before the experiment. All the electrical connections should be secured before the experiment. Turn off the amplifier and disconnect PZT leads whenever possible.
   1. Connect the PZT leads to the voltage amplifier and connect the function generator to an input port of the voltage amplifier.
   2. Turn on the function generator and configure it to generate continuous square waves with an output voltage of 1 V. Do not generate the voltage signal until all the connections are verified and secured.
   3. Turn on the voltage amplifier and generate the square wave of output voltage 1 V by enabling the output.
   4. Connect the monitoring output port (output voltage 200 V) of the amplifier to an oscilloscope. Configure the amplifier to have gain of 200 V/V by turning the gain knob on the front panel. Verify that the monitoring output voltage has an amplitude of 1 V as measured by the oscilloscope.
   5. Once the function generator and the amplifier are configured, find the resonant frequency of the PZT launcher by scanning the modulation frequency of the driving signal while the real-time video microscope images adhered particles. Repeat the scanning until the microparticle motion is a maximum. Use this frequency (64 kHz here) to release particles.
      NOTE: The modulation frequency is manually changed (scanned) from zero to 150 kHz to find the resonant frequency.
   6. Configure the function generator to generate a square wave with a specified number of cycles in burst mode. Press the "Burst" button on the front panel and select "N Cycle Burst".
   7. Choose the burst count by pressing "# Cycles" soft key and set the count to 10 or 20.
   8. Configure the square waveform to generate voltage signals with an amplitude of 600 V (three times the voltage used for continuous excitation) at the resonant frequency of 64 kHz which has found from the previous step. Verify that the pulsing signal releases the target particle in a repeatable manner by ensuring particles move after each pulse.
4. Selective optical trap loading
   NOTE: The PZT launcher assembly is installed on a manual linear translation xy stage. The particles can be translated relative to the fixed beam focus by moving the translational stage.
   1. Remove the laser line filter to identify the focus of the trapping beam by rotating the microscope turret (Figure 3a). Move the motorized focusing block back and forth vertically around the best focus of the visible image to optimize focus.
   2. Once the focus position is verified, put the filter back to give a clear real-time video without interference from the trapping beam.
   3. Translate the sample to place a selected particle at the focus position of the trapping laser. Focus on the particle to image the center of a selected particle, which places the nominal trapping position below the particle center by about one half radius while leaving the levitation position above the particle.
   4. Adjust the power supply connected to the electro-optic modulator (EOM) driver to set the optical trapping power. The optimal power depends on particle size and material. The optical power was found through repeated trials to determine the power sufficient to levitate the particle without ejecting it from the beam. Here, use an optical power of 140 mW at the back focal plane of the objective to trap the 20 µm diameter polystyrene (PS) particles.
   5. After the center of the selected particle is aligned, actuate the piezoelectric launcher with several pulses. The change of the particle image from a static focused image to a moving blurred image indicates successful loading to the levitation position.
   6. Translate the levitated particle vertically about a millimeter above the substrate by moving the objective lens up to prevent possible surface interactions. Then reduce the optical power to transition the levitated particle (Figure 3b) into the nominal trapping position (Figure 3c) which is more stable.
      NOTE: The optical power of trapping laser can be modulated by an electro-optic modulator (EOM). The EOM regulates the output power with a bias voltage supplied through a digital power supply. One can observe the transition from the levitation to trapping position through the CCD while slowly reduces the optical power.
   7. For the position measurement, as depicted in Figure 3c to 3d, carefully move the center of the PZT holder to the optical axis and then move the objective lens up (vertically) to translate the particle into the middle of sample enclosure (9 mm above the substrate) where the fringe electric field is minimized.
   8. After performing the measurement as described below, place the particle on the substrate by moving the objective down until the particle touches the substrate. Since most of the particles are applied near the corners, the trapped particle can be easily recognized and re-trapped when it is placed in the central area. This enables reversible trap loading to measure changes occurring beyond a single trapping event such as contact interactions of the particle and substrate.

3. Data Acquisition

1. Align the condenser and the focusing lens to maximize the QPD "SUM" signal with a particle in the trap.
2. Align the focusing lens to nominally zero the X and Y channels of the QPD, as shown in Figure 4c.
3. Repeat the adjustment of the condenser and the focusing lens until the Fourier transformed position signals (or power spectrum density (PSD) plots) of the X and Y channels superimpose to show balanced sensitivity. Properly aligned QPD signals (X and Y) show almost identical behavior, as shown in Figure 4b.
4. Once the QPD alignment is verified, connect the voltage amplifier to the two ITO plates. Connect the voltage monitoring output signal of the amplifier to the DAQ system to record the step excitation signal and the induced particle trajectory synchronously.
5. Supply a continuous square wave of 400 V to generate an electric field (Figure 4d) that moves the particle transversely to the optical axis by about 500 nm (Figure 4e). Measure the step response of the trapped particle using the QPD.
6. Average multiple periods as necessary to reduce the effects of Brownian motion. The induced motion can be used to measure the optical force over a wider range of motion than that of thermal fluctuations.^12,17 Figure 4d and 4e shows averaged signals of applied voltage and the induced particle trajectory over 50 iterations of step excitation.

Results

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

Discussion

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...

Materials

Name	Company	Catalog Number	Comments
ScotchBlue Painter's Tape Original	3M	3M2090
Scotch 810 Magic Tape	3M	3M810
Function/Arbitrary Waveform generator	Agilent	HP33250A
Power supply/Digital voltage supplier	Agilent	E3634A
Ring-type piezoelectric transducer	American Piezo Company	item91
Electro-optic modulator	Con-Optics	350−80-LA
Amplifier for Electro-optic modulator	Con-Optics	302RM
Mitutoyo NIR infinity Corrected Objective	Edmund optics	46-404	Manufactured by Mitutoyo and Distributed by Edmund optics
LOCTITE SUPER GLUE LONGNECK BOTTLE	Loctite	230992
3D printer	MakerBot	Replicator 2
Polylactic acid (PLA) filament	MakerBot	True Red PLA Small Spool
Data Acquisition system	National Instruments	780114-01
Quadrant-cell photodetector	Newport	2031
Translational stage	Newport	562-XYZ
Inverted optical microscope	Nikon Instruments	EclipsTE2000
Fluorescence filter (green)	Nikon Instruments	G-2B
Flea3/CCD camera	Point Grey	FL3-U3-13S2M-CS	Trapping laser
Diode pumped neodymium yttrium vanadate(Nd:YVO4)	Spectra Physics	J20I-8S-12K/ BL-106C
Indium tin oxide (ITO) Coated coverslips	SPI supplies	06463B-AB	Polystyrene microparticles
Fast Drying Silver Paint	Tedpella	16040-30
Dri-Cal size standards	Thermo Scientific	DC-20
Optical Fiber	Thorlabs	P1−1064PM-FC-5	bottom plate
Aluminium plate	Thorlabs	CP4S
High voltage power amplifier	TREK	PZD700A M/S