Optical Trap Loading of Dielectric Microparticles In Air

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.