Airborne particle generation for optical tweezers

Optical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force to physically hold and move micro-particles.¹ The most advanced of these devices are only able to work in liquid media.² While this is appropriate for many biological applications, particle handling in the air is of great interest in other fields. Micro- and nanoengineering, for example, often deal with materials like carbon nanotubes or microspheres that can be grouped together into a desired shape provided there is a means of handling these micro-materials in the air. The problem with optical tweezers is that the forces they generate are not strong enough to overcome the sticking due to humidity of microscale particles on surfaces, making particle-handling unfeasible.³

This issue can be solved by assuring the devices can pick up, hold and position particles in the air without surface contact. Our approach is to generate airborne elements using a vibrating membrane device that separates the particles on a membrane surface and lifts them.⁴ After they are lifted, their movement should be vertical upwards with zero velocity at the so-called dead point. Here, the moving direction changes from upwards to downwards, and the particles can be trapped by the optical tweezers (see Figure 1).

We generated the required membrane vibration by thermo-mechanical actuation. We started by converting aluminum-nitride (AlN) thin films into self-supporting, optically transparent membranes that were mechanically and thermally stable and very flexible. The AlN was deposited with a nanocrystalline fiber texture, and the film tensile stress was adjusted. On top of the transparent membranes, we integrated resistive heater elements that made use of different geometries for varying the resulting heated spot on the membrane (see Figure 2).

Figure 1. Working principle of the thermo-mechanically actuated aluminum-nitride membrane with the three states of operation.

Figure 2. Thermal information of two heated membranes with different geometries. These images show the heated spot on the membrane surface that causes the thermal-induced deflection. P_Hstands for heating power, and the scale on the right shows the temperature.

Figure 3. View of the membrane surface with part of the resistive heater element (in black). A change of particle distribution (in yellow) before and after actuation is clearly visible.

Circular AlN membranes with diameters up to 3mm based on 500nm thin films allowed us to achieve thermally induced deflection up to 30μm. Using a white light interference microscope while applying a static heating power, we verified that the deflection was stable for multiple heating steps. We then investigated the thermal switching mode (in the case where the heating is driven by a square-pulse excitation). In this case, the heating power had a fast on-and-off state, implying that membrane elements were deflected and undeflected with different repetition rates in sudden bursts, generating the required vibration of the surface. Optical inspection showed that particles were separated and lifted from the membrane due to this vibration (see Figure 3).

Further, we performed experiments on particle lifting height with 10μm silica beads. Our work revealed that these beads can be lifted up to tenths of micrometers. We also investigated the thermal cut-off (the repetition rate from which the maximum thermal induced deflection decreases by 3dB) for the frequency range from 1Hz to 1kHz, and we found it to be 10–12Hz for different membranes with various heater geometries. However, even at a repetition rate of 20Hz of the square waveform, we were able to achieve successful particle lifting.

We have shown that our AIN membranes can be heated to successfully separate and lift particles from their surfaces. We achieved particle lifting up to repetition rates that were slightly above the thermal cut-off up to a height of a few tenths of micrometers for 10μm silica beads. In our future work, we will integrate a membrane device into an optical tweezers system to verify that these grippers are capable of picking up the lifted particles. We will then perform trapping experiments with different sized silica beads, and we will investigate the particle transport process and the re-deposition experimentally.

The authors thank the German Research Foundation for funding this work as part of the Collaborative Research Center 622 ‘Nanopositioning and nanomeasuring machines’ at the Ilmenau University of Technology.