Optically Trapping Large Metallic Particles in Air Using a 'Boat' Trap with Direct-Drawn Sidewalls

Our work describes the construction and operation of a reconfigurable boat-shaped optical trap for capturing and manipulating particles in air. Existing trapping methods compromise between trap localization and capture area. This protocol introduces a boat trap, achieving both simultaneously.

This protocol can be applied in standard laboratory environments under ambient air conditions while following Class 4 laser safety protocols. After preparing the carbon-coated glass microspheres, set up the optical system using a kinematic beam elevator and a ruler to set the beam height to 23 centimeters. Reduce the laser power to less than 50 milliwatts.

Use a polarizer to determine whether the polarization is vertically oriented. Adjust the microscope to beam height facing in the negative Z direction at a distance greater than 100 centimeters from the output of the beam elevator. Place a Fourier transform lens 10 centimeters in the negative Z direction, with the convex surface facing in the positive Z direction.

Then place the remaining three Fourier transform lenses 20 centimeters apart with alternating convexities. Position the acousto-optic modulators 10 centimeters in the negative Z direction from the lenses closest to and third closest to the laser. Ensure that the acousto-optic modulator closest to the laser is vertically oriented.

Position the acousto-optic modulator two 10 centimeters from the lens third closest to the laser. Align it horizontally and rotate it approximately two degrees away from the optical axis. For each channel of the arbitrary waveform generator, connect a voltage controlled oscillator, an amplifier, and one acousto-optic modulator.

Then connect the amplifiers to the acousto-optic modulators. Using the arbitrary waveform generator, set the voltage supplied to both voltage controlled oscillators to five volts, corresponding to a frequency of 100 megahertz. Then decrease the tilt of acousto-optic modulator two to zero.

Gradually increase the angle until the first order beam reaches its maximum brightness. Align both beams exiting the first acousto-optic modulator to enter the second modulator. Of the four beams exiting the second acousto-optic modulator, allow only the top right beam to pass through the aperture.

Next, place a beam splitter to direct a portion of the beam to an observation surface. With the arbitrary waveform generator amplitudes set to zero, observe the resulting beam at the observation surface appearing as a dot. Set the vertical channel offset to five volts, amplitude to three volts, and frequency to four kilohertz.

Observe the beam forming a vertical line at the observation surface. Then adjust the horizontal channel offset to five volts, the amplitude to three volts, and the frequency to four kilohertz. The beam should form a diagonal line at the observation surface.

Tune the vertical channel to eight kilohertz and a 90 degree phase offset, resulting in a Lissajous pattern on the observation surface. Then press the Align Phase button to convert the Lissajous pattern to a parabolic shape. Adjust the vertical channel phase offset to refine the parabolic shape and observe the refined parabolic pattern at the observation surface.

Place a telecentric lens with a 45 millimeter focal length on a linear stage, positioning it 55 millimeters in front of the objective in the positive Z direction. Then place a dichroic filter designed to reflect 532 nanometer light in front of the lens. Connect the camera to a personal computer using a Universal Serial Bus cable with the associated imaging software.

Then tilt the dichroic filter slightly between five and 10 degrees until the parabolic trap shape appears. Illuminate the beam splitter with a fiber light source and adjust the fiber light to center its illumination around the parabolic trap. Using a laboratory spatula, place approximately one gram of particles inside a 25 millimeter long, 12 millimeter diameter glass cylinder mounted to allow rotation along the cylinder axis.

Place the loaded glass cylinder centered at 15 millimeters away from the objective, corresponding to the objective working distance. After rotating and tapping the cylinder, observe the camera display showing default or no trapped particles, particles falling, and trapped particles. After installing and configuring the software, launch Visual Studio 2022 and open the project file.

Press the green Run button to start the program. Observe the video feed and terminal appearing on the screen. Ensure that the green trapping parabola is visible on the video feed.

If it is not visible, adjust power, filtering, or alignment until the trapping region appears. Press the Windows, Shift and S key simultaneously to capture the screen. Click and drag the mouse pointer from the top left to the bottom right corner of the video feed.

Click the pop-up to open the screenshot editor. Click the Edit in MS Paint button within the editor. Then place the cursor in the top left corner of the parabola in the MS Paint window.

Record the coordinates shown at the bottom left corner as X minimum and Y minimum. Then place the cursor in the bottom right corner of the parabola in the MS Paint window. Record the coordinates shown at the bottom left corner as X maximum and Y maximum.

Once done, close the video feed by pressing the Escape key. Enter X minimum, X maximum, Y minimum, and Y maximum on line 280 in the is_trapped function of main. cpp within the project.

For test frequency randomization, launch Visual Studio 2022 and open the project file if it is not already open. Open Windows PowerShell or Command Prompt. Run the command pythonc import random and print a random sample from the range zero to 101.

Then copy the generated list of test numbers. Edit line 443 of main. cpp by pasting the generated list into the test number sequence and save the project in Visual Studio 2022.

Increase laser power to the level needed for the experiment. Run the experiment by clicking the green run button and tapping and rotating the tube of particles. After the experiment has finished, open the results.

txt file in a text editor. Observe that each program run generates new results under the heading, Start of a new batch of results, in the format, test number, result, followed by trap time in nanoseconds. For data visualization, copy the test numbers and trap times from the Comma-Separated Values file into example.xlsx.

After applying the required filters, observe the additional columns and a scatter plot that populates with trap time versus trap draw frequency. Carbon-coated microspheres produce dark shadows when backlit. Glass microspheres, particles with dimensions on the order of 100 micrometers, settled approximately to the bottom of the parabolic trap.

When a particle was successfully trapped, the automated particle detection software displayed a red T.When no trapped particle was found, the software displayed a green N.Trapping rates and lifetimes may increase or decrease as a function of environmental variables such as temperature or humidity. Solid gold particles on the order of one micrometer were not observable along the optical axis, but their light scatter was observed orthogonal to the axis. Video recordings confirmed that particles maintained their position for extended durations exceeding several minutes.

This protocol allows a researcher to draw and quickly modify a boat trap to trap and move a wide variety of particles in the air. Future studies can integrate Field Programmable Gate Array control to enable programmable manipulation of trap shape, weighting, and cross-sectional location within a 2D plane.