Clean loading of silica nanoparticles with a radius as small as 50 nm is required for experiments in levitated optomechanics that operate in ultra-high vacuum. We present a cheap and simple experimental method for dry launching of silica nanoparticles by shaking from a polytetrafluoroethylene surface (PTFE). We report on the successful launching of single silica nanoparticles with a minimum radius of 43 nm, which is enabled by the low stiction to the launching surface. Nanoparticles with radii of 43 and 71.5 nm are launched with a high flux and small spread. The measured velocities are significantly smaller than 1 m/s. The demonstrated launching method allows for controlled loading of dry nanoparticles with radii as small as 43 nm into optical traps in (ultra-)high vacuum, although we anticipate that loading of smaller sizes is equally feasible.

Levitated particles and the manipulation of their center-of-mass motion in vacuum with optical tweezers, magnetic traps, or Paul traps recently emerged as promising candidate systems to address questions in macroscopic quantum physics, thermodynamics in the quantum regime, and for the search of new physics beyond the standard model.1 Optically levitated silica nanoparticles of around 100 nm in diameter have recently been prepared in their motional quantum ground state.2–5 Furthermore, various proposals consider nanoparticles of similar sizes in order to create macroscopic superpositions or for matter–wave interferometry.6,7 In addition, the absence of clamping allows for unique experimental possibilities, such as dynamic and non-linear potential landscapes,7,8 making levitated nanoparticles a valuable resource in technological applications, such as force sensing.9,10 However, most experiments with levitated nanoparticles suffer from decoherence induced by gas collisions. This is in large part due to the loading mechanism, which is typically based on spraying nanoparticles from an aqueous solution with an ultrasonic nebulizer.11 While this method is easy and cheap to implement, it contaminates the vacuum chamber and optical components within and requires ambient pressure conditions to be carried out.12 Therefore, current efforts push toward technological improvements to routinely operate in ultra-high vacuum (UHV).

An ideal loading strategy for the next generation of experiments in levitated optomechanics should be cost-efficient and adaptable to the variety of trapping techniques and experiments currently under development. Several loading mechanisms have been developed in this direction. Laser-induced acoustic desorption (LIAD) is able to load various particle sizes and materials into Paul traps at pressures down to 107mbar.13–15 Despite the large release velocity of particles, Paul traps create deep potential wells, thus allowing for trapping of fast particles in ultra-high vacuum by switching the trap on with precise timing.14 Particles of arbitrary sizes can be launched with an aerodynamic lens with velocities similar to LIAD.16 However, direct loading under similar conditions is difficult for the relatively shallow optical potentials without relying on the residual gas to slow down particles. Another strategy focuses on the transport of an externally trapped particle into the vacuum chamber, either by transporting the particle using a load-lock method17,18 or with an optical conveyor belt through a hollow core fiber.19,20 Although these methods were successful in handing over a nanoparticle into an optical tweezer, they come with additional optical components and lasers that add a significant overhead to experimental setups.

In the pioneering work on optical tweezers, microparticles with a radius of 10μm were shaken off glass substrates by using piezoelectric transducers (piezos).21 This method is cheap, simple, and versatile since it can, in principle, be used with any trap and in any environment. Although the particles are not transported deterministically into the final trap, the contamination of the environment is negligible in comparison to using a nebulizer. Recently, there has been a revived interest in launching small nanoparticles with this method.22–27 The main challenge is to overcome the strong stiction forces as the acceleration required to launch a nanoparticle scales with the inverse of the particle cross-section. In our experiment, we successfully release nanoparticles as small as 43 nm in radius from a polytetrafluoroethylene (PTFE) coated substrate. In Fig. 1, we compare our result to the above-mentioned loading mechanisms in terms of velocity and particle sizes, highlighting the potential of the method developed in this work. In the following, we present the experimental setup and provide a detailed procedure to prepare particles on the substrate. We then characterize the flux, spread, and velocity distribution of the launched nanoparticles. The analysis is based on images provided by scanning electron microscopy (SEM), which confirms the presence of single particles released from the launching substrate.

FIG. 1.

State of the art launching mechanisms at low pressures (under ballistic conditions). Solid lines show the range of particle sizes that has been launched with a certain method, while the dashed lines present the potential for launching of smaller sizes. Laser-induced acoustic desorption (LIAD) is able to launch particles with small sizes,28 however, with velocities higher than 1 m/s (in the ballistic regime).13–15 Loading with hollow core fibers (HCF) allows for the precise control of the nanoparticle velocities and has been demonstrated for nanoparticles with a radius of 122.5 nm.19,29 Shaking off nanoparticles from a substrate with the piezoelectric transducer (piezo) has been reported for particles with a radius of 88.5 nm.22 In this work, we present the setup that is able to launch nanoparticles with a radius of 43 nm.

FIG. 1.

State of the art launching mechanisms at low pressures (under ballistic conditions). Solid lines show the range of particle sizes that has been launched with a certain method, while the dashed lines present the potential for launching of smaller sizes. Laser-induced acoustic desorption (LIAD) is able to launch particles with small sizes,28 however, with velocities higher than 1 m/s (in the ballistic regime).13–15 Loading with hollow core fibers (HCF) allows for the precise control of the nanoparticle velocities and has been demonstrated for nanoparticles with a radius of 122.5 nm.19,29 Shaking off nanoparticles from a substrate with the piezoelectric transducer (piezo) has been reported for particles with a radius of 88.5 nm.22 In this work, we present the setup that is able to launch nanoparticles with a radius of 43 nm.

Close modal

The use of piezos to launch silica particles from glass surfaces has previously been realized in several experiments.21,22,24–27 However, high piezo drives are required to overcome the van der Waals stiction force between the particles and the surface, and the smallest launched particle had a radius of 88.5 nm.22 The Derjaguin–Muller–Toporov (DMT) theory predicts the stiction force between two particles with radii R1 and R2,30,31

FDMT=4πReffγ,
(1)

where γ is the effective solid surface energy between the two particles, and Reff = R1R2/(R1 + R2). For a particle with the radius R1R deposited on a flat surface (R2 → ∞), it follows that the stiction force per mass (i.e., acceleration) increases as FDMT/mR−2. The effective surface energy of silica of γ = 0.014 J/m2 has been measured between two silica microparticles.30 The stiction force can effectively be reduced by using different materials with lower surface energies γ. In contrast to previous experiments, we use PTFE coated slides (coating thickness: 20 μm) instead of glass slides, as the surface energy of PTFE is, in general, expected to be lower than for silica. However, there is little known about the stiction force between PTFE and silica under experimental conditions similar to the ones in our system. We note that higher surface roughness of the launching substrate might lead to significantly lower stiction force as it decreases the contact surface between the particles and the substrate.32–36 

The launching setup is shown in Fig. 2(a). The mechanical design, based on previous studies,24 uses a clamped piezo to launch silica particles from a PTFE coated slide. One side of the PTFE coated slide is clamped to the piezo, while the particles are deposited on the other end by scraping them off a baked glass slide. Importantly, the process of baking a glass substrate with nanoparticles ensures that the stiction force dominates over any other attractive force, e.g., the capillary force.30,37 The piezo is driven with a sinusoidal signal with frequency ω, which is close to a piezo resonance such that the slide oscillates with maximum amplitude. Further information on the setup and the slide preparation can be found in the supplementary material.

FIG. 2.

(a) The piezoelectric launching setup. A sinusoidal drive signal from the function generator (FG) is amplified with the high voltage amplifier by a factor of 100 and applied to the piezo, which is clamped to the PTFE-coated slide. Nanoparticles with radius R are launched from the surface of the oscillating PTFE coated slide. (b) The acceleration provided by the piezo at the piezo resonance frequency of 235.5 kHz as a function of the driving voltage. The acceleration increases drastically as the voltage increases. (c) The blue and the green shaded regions below the highest measured acceleration (red dotted line) represent the particle sizes that can be launched from the glass- and the PTFE-coated slides, respectively. Acceleration required to launch nanoparticles from the glass slide as provided by the DMT model (blue solid line), crossing the maximum acceleration close to the launched nanoparticle size of R = 492.5 nm (blue dashed line). Particles as small as R = 43 nm were launched from the PTFE-coated slide (green dashed line).

FIG. 2.

(a) The piezoelectric launching setup. A sinusoidal drive signal from the function generator (FG) is amplified with the high voltage amplifier by a factor of 100 and applied to the piezo, which is clamped to the PTFE-coated slide. Nanoparticles with radius R are launched from the surface of the oscillating PTFE coated slide. (b) The acceleration provided by the piezo at the piezo resonance frequency of 235.5 kHz as a function of the driving voltage. The acceleration increases drastically as the voltage increases. (c) The blue and the green shaded regions below the highest measured acceleration (red dotted line) represent the particle sizes that can be launched from the glass- and the PTFE-coated slides, respectively. Acceleration required to launch nanoparticles from the glass slide as provided by the DMT model (blue solid line), crossing the maximum acceleration close to the launched nanoparticle size of R = 492.5 nm (blue dashed line). Particles as small as R = 43 nm were launched from the PTFE-coated slide (green dashed line).

Close modal

The acceleration transferred to nanoparticles has to overcome the van der Waals force such that a = FDMT/mR−2. This expression should be compared to the maximum acceleration provided by driving the piezo, apiezo = ω2δd, where δd is the substrate displacement. We choose the piezo resonance frequency at 235.5 kHz, where the measured displacements (see the supplementary material) correspond to accelerations of up to 6×107m/s2 [Fig. 2(b)]. This sets the limit for the minimum particle radius that can be launched for a given substrate material and roughness [red dotted line, Fig. 2(c)]. Given the maximally achieved accelerations, following from the simple DMT model, we should be able to launch particles from the glass slide down to R ∼ 630 nm. In practice, we are able to launch a small number of particles with a radius of 492.5 nm and no particles with a radius of 377.5 nm, which approximately fits to the estimate. On the other hand, we are able to launch particles with a radius of 43 nm from the PTFE surface. We conclude that the acceleration required to launch particles from the PTFE surface is more than two orders of magnitude smaller than for the glass slide. We note that nanoparticles with smaller radii were unavailable at the time of our measurements.

Using this method, we have successfully trapped 71.5 nm particles in an optical trap (waist of 0.7μm) in the diffusive regime at pressures around 100 mbar (see the supplementary material). For the remainder of the text, we focus on the characterization of the launching method, namely on the flux, standard deviation of the position spread, and launching velocities. We conduct all launching efforts at a pressure of 103 mbar, where the nanoparticles move in the ballistic regime (see the supplementary material). Throughout our work, we use silica nanoparticles with the nominal radii of (43 ± 3) and (71.5 ± 2) nm (Microparticles GmbH). Note that these sizes are of particular interest as they correspond to particles that have recently been prepared in the motional quantum ground state.2–4 

In order to have a high repetition rate of experiments in levitated optomechanics, it is imperative to trap a nanoparticle in a reasonable time. Successful trapping of a nanoparticle depends strongly on the spread of launched particles and their flux, i.e., the number of particles passing through the trapping potential in a given time frame. In our consideration, we focus on launching into an optical tweezer as it is presently the most common trapping method in experiments. The trap volume of an optical tweezer is given by ∼λ3, where λ is the laser wavelength. In our experiment, we use λ = 1064 nm such that the flux numbers are reported as a number of particles passing through an area of 1μm2/s at a distance of 12 mm from the launching substrate.

The setup to characterize the flux and the spread of the particles launched from the PTFE coated slide is shown in Fig. 3(a). We place a long rectangular slit with a width of 150 μm at a distance of 2 mm below the slide. A fraction of the launched particles and clusters travel through the slit and fall for ∼12 mm until they are deposited on a glass collection slide. The travel distance is long enough such that the particles acquire a velocity of 0.5m/s due to free fall. We image the slide with a scanning electron microscope (SEM) with a magnification chosen such that a single particle covers 10 pixels in each image. The total scanned area corresponds to the particles that have traversed the slit through an area of (200 × 150) μm2 around its center. The images are then processed such that single particles and clusters are isolated from the background (see the supplementary material); however, only the single particles are of interest to us for future experiments. The ratio of clusters to the total number of launched objects is 10%–15% and depends on the particle radius.

FIG. 3.

Measurement of the flux and spread of the launched nanoparticles. (a) The particles are launched along gravity onto the collection slide. A 150 μm wide slit selects a subset of the launched particles emulating a point source and is placed 12 mm above the collection slide. Inset: SEM image showing single 43 nm particles and clusters launched onto the collection slide. Zoom in: SEM image of a single 43 nm nanoparticle on the collection slide. (b) The position dependence of the launched particles is obtained by counting nanoparticles from the SEM images along the width of the slit. The total number of launched particles and the spread is lower for the 43 nm particles, which is expected due to the higher van der Waals force to the substrate.

FIG. 3.

Measurement of the flux and spread of the launched nanoparticles. (a) The particles are launched along gravity onto the collection slide. A 150 μm wide slit selects a subset of the launched particles emulating a point source and is placed 12 mm above the collection slide. Inset: SEM image showing single 43 nm particles and clusters launched onto the collection slide. Zoom in: SEM image of a single 43 nm nanoparticle on the collection slide. (b) The position dependence of the launched particles is obtained by counting nanoparticles from the SEM images along the width of the slit. The total number of launched particles and the spread is lower for the 43 nm particles, which is expected due to the higher van der Waals force to the substrate.

Close modal

The total single particle count is 4.56 × 106 for the 71.5 nm particles and 2.3 × 106 for the 43 nm particles in 2 h of launching, giving a total flux for the given scanned area of (0.021 ± 0.005) and (0.011 ± 0.005) μm−2 s−1, respectively. The higher count of the larger particles is expected as they require smaller accelerations to be launched. Although the flux is decreased for the smaller size, the high count suggests that launching of even smaller nanoparticles is feasible. However, this remains to be shown in the future studies as particles with smaller radii were unavailable in the course of this work. We plot the particle count as a function of the distance on the collection slide in Fig. 3(b). We assume a Gaussian distribution of the collected nanoparticles and extract the spread of (1.5 ± 0.1) mm [(1.8 ± 0.1) mm] for the 43 nm (71.5 nm) nanoparticles at a distance of 12 mm. The spread is larger for the larger particle size. We attribute this again to the smaller force required to launch the larger particles, which allows for a larger velocity component parallel to the launching slide.

The trap depth of the optical tweezer is typically well below 10 000 K. In order to increase chances of trapping a nanoparticle flying through the trap volume, it is desirable to launch nanoparticles with energies smaller or on the order of the trap depth. We characterize the launching velocities with the setup presented in Fig. 4. The launching assembly is now mounted vertically in order to release the nanoparticles in the horizontal direction. We mount a 150 μm wide slit parallel to the launching substrate. The slit is placed at a height of h = 11 mm above a 5.0 cm long glass slide such that single 71.5 nm particles and clusters that pass through the slit follow a parabolic trajectory and land at various distances from the base of the slit. We calculate the initial launching velocity for each nanoparticle from the measured traveled distance.

FIG. 4.

Measurement of the launching velocities. (a) Nanoparticles with a radius of 71.5 nm are released along the horizontal axis and pass through a 150 μm wide slit. The center of the slit is placed h = 11 mm above a horizontally mounted collection slide, which is long enough to capture all particles that are launched with a velocity smaller than 1 m/s. (b) We capture SEM images of the slide along its length and count the number of particles as a function of their distance from the slit. Insets: Three SEM photos are shown here from different sections of the slide as an example. The number of launched single nanoparticles and clusters clearly decreases the further away the image taken is from the slit. (c) Cumulative probability of the launched nanoparticles as a function of the launched velocity.

FIG. 4.

Measurement of the launching velocities. (a) Nanoparticles with a radius of 71.5 nm are released along the horizontal axis and pass through a 150 μm wide slit. The center of the slit is placed h = 11 mm above a horizontally mounted collection slide, which is long enough to capture all particles that are launched with a velocity smaller than 1 m/s. (b) We capture SEM images of the slide along its length and count the number of particles as a function of their distance from the slit. Insets: Three SEM photos are shown here from different sections of the slide as an example. The number of launched single nanoparticles and clusters clearly decreases the further away the image taken is from the slit. (c) Cumulative probability of the launched nanoparticles as a function of the launched velocity.

Close modal

The collection slide is imaged in steps along its length with the SEM. The image resolution is again chosen such that each particle is resolved by about 10 pixels. The insets in Fig. 4 show the SEM images at three different distances along the collection slide, which clearly demonstrate that the particles and clusters are more densely concentrated in the vicinity of the launching assembly. The area scanned by the SEM in each image corresponds to particles passing through a (150 × 120) μm2 area of the slit. We count the particles as a function of the distance along the collection slide (see the supplementary material) and calculate the velocity distribution histogram. The total particle count is 9.46 × 105 in an hour of launching time. The total flux is then (0.015 ± 0.005) μm−2 s−1, which is consistent with the flux obtained in previous measurements. We observe that 30% of the collected nanoparticles had a velocity smaller than 0.2 m/s, allowing for direct cooling and trapping in an existing optical cavity.2,38 Around 17% of nanoparticles have a velocity smaller than 0.07 m/s—corresponding to the temperature of T = mv2/kB ∼ 1000 K in a harmonic potential—which could be captured in the optical tweezer with the help of methods developed for LIAD and hollow core fibers.14,29 We note that the probability of launching slower nanoparticles might be increased by decreasing the piezo drive voltage, although this might result in a smaller total flux. This remains to be tested in the future studies.

We present a cheap and simple method that launches dry nanoparticles off a PTFE coated substrate driven with a piezo in vacuum. We demonstrate successful launching of silica nanoparticles with a radius as small as 43 nm, although from the measured particle flux we expect that launching of smaller particles is feasible. We show that particles with radii of 71.5 and 43 nm are launched with spreads of around ±2 mm at a free fall distance of 12 mm. Furthermore, 50% of the launched particles have a launching velocity smaller than 0.3m/s. Based on our measurements, a single particle would pass through the trap volume of an optical tweezer placed at this distance within 1–2 min, depending on the size of the particle. The launching setup has a small footprint and can be easily implemented in any optical, magnetic, or electric trap, thus enabling the next generation of experiments with levitated nanoparticles.

The supplementary material contains details about the experimental setup and preparation of launching slides, validity of the ballistic approximation, post-processing of the SEM images, and optical trapping of launched nanoparticles.

Note added in proof. We recently became aware of a similar work.39 

The authors are grateful for insightful discussions with M. Arndt, R. Quidant, and M. Frimmer. We thank S. Puchegger, S. Loyer, and B. Bräuer for their help with the SEM and AFM measurements. This project was supported, in part, by the European Research Council (ERC 6 CoG QLev4G and ERC Synergy QXtreme), the ERA-NET program QuantERA under the grants QuaSeRT and TheBlinQC (via the EC, the Austrian ministries BMDW and BMBWF, and research promotion agency FFG), the Austrian Science Fund (FWF) (Grant No. I 5111-N), and the European Union’s Horizon 2020 Research and Innovation Program under Grant No. 863132 (iQLev).

The authors have no conflicts to disclose.

Ayub Khodaee: Data curation (equal); Formal analysis (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal). Kahan Dare: Formal analysis (equal); Investigation (equal); Writing – review & editing (equal). Aisling Johnson: Conceptualization (equal); Methodology (equal); Supervision (equal); Writing – original draft (equal). Uroš Delić: Conceptualization (equal); Methodology (equal); Supervision (equal); Writing – original draft (equal). Markus Aspelmeyer: Funding acquisition (lead); Project administration (equal); Resources (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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