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Abstract
We demonstrate reflection geometry optical trapping using double nanoholes in a metal film. Symmetry breaking of the double nanohole allows for selecting the scattered trapping laser light of orthogonal polarization to the incident beam. This orthogonal polarization light shows a few percent increase when the nanoparticle (e.g., a 20 nm polystyrene particle, or protein bovine serum albumin) is trapped. The reflection geometry simplifies the optical setup and frees up one side of the trap, which has great potential for adding microfluidics to the other side or working with opaque or highly scattering samples.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Conventional optical tweezers have provided insight into biophysics, as recognized by the 2018 Nobel prize [1,2]. These tweezers require tethering and/or fluorescence to analyze single proteins and thereby modify the protein of interest, and in turn, this impacts their biophysical properties [3–5]. Also, the trapping volume is approximately a million times larger than a protein itself and the typical protein is a few nanometers across so that many proteins (or nanoparticles) are trapped at once unless careful dilution and/or filling of the solution is used to prevent more particles from diffusing into the trapping volume [6].
Shaped nanoapertures in metal films have been used by many researchers worldwide to trap and analyze individual nanoparticles, including single proteins, quantum dots, and nanoflakes [7–23].
Most of these past works used transmission mode to detect changes in the laser intensity when a nanoparticle was trapped; however, there are a few cases where the reflection mode was used which simplified the optical setup by exciting and collecting from the same side [19,24]. In those past works, however, most of the reflected light was from the surrounding metal film and this added a lot of background signal and noise, so that the trapping was barely discernible.
To significantly improve the detection of nanoparticle trapping, here we introduce a polarization selective reflection scheme, where the reflected light of orthogonal polarization is detected. Usually, a planar gold film will not change the polarization of the reflected beam, but the double-nanohole (DNH) aperture is polarized and therefore breaks the symmetry of the system. As a result, the reflected beam is scattered into the orthogonal polarization if the DNH is not aligned along a high symmetry axis (e.g., if it is tilted at 45 degrees). As a result the light scattered off the DNH can be isolated from the usual reflection off the gold film by a polarizing beam splitter, and trapping can be seen clearly in the orthogonal polarization reflection path. This configuration opens up new possibilities by freeing up the other side of the trapping setup (e.g., for adding microfluidics), and by allowing for working in opaque, scattering or absorbing samples (e.g., crude oil samples [25], or blood serum [26]).
2. Experimental method
2.1 DNH fabrication
Colloidal lithography was used to create randomly distributed DNHs in a 70 nm thick gold film [27,28]. The film was sputtered on an indium tin oxide on glass substrate. The holes on the gold film are randomly distributed and have different orientations. Polystyrene nanospheres of diameter 800 nm in water with 0.01% w/w concentration were used in the drop coating method. The surface was oxygen plasma etched for 230 seconds at 30 W (Harrick, PDC-002) to reduce the aperture size and cusp separation prior to gold sputtering [27]. The resulting DNH is shown in Fig. 1(A) with hole diameter of 420 nm and cusp separation of 55 nm.
Fig. 1. A) SEM image of the double nanohole on the gold film. B) Sample structure. C) Optical setup schematic. LP: linear polarizer. PBS: polarizing beam splitter.
2.2 Sample preparation
The solution containing the nanoparticles to be trapped was placed in a micro well between the gold layer and a microscope cover slide. The laser transmits through the gold layer onto the solution and reflects back through an oil immersion 100 $\times$ objective lens as shown in Fig. 1(B).
Three solutions were used. Polystyrene particles with average diameter of 20 nm were in water with concentration of 0.02% w/v. Bovine Serum Albumin (BSA) was used with concentration of 150 $\mu$M in phosphate buffered saline. We also trapped hexagonal boron nitride nanoflakes (average thickness 3 to 4 layers and width 50-100 nm) in a solution of water and ethanol.
2.3 Optical setup
As shown in Fig. 1(C) the 785 nm laser beam (z-laser Z80M18S3) was polarized and transmitted through a polarizing beam splitter (PBS) that transmits the laser polarization. The laser beam goes through a microscope objective lens (100 $\times$ 1.40 NA) to reach the sample. The power of the laser before the objective lens is 10 mW. The reflected light from the sample goes through a polarizing beam splitter. The reflection has two orthogonal polarizations and the polarization perpendicular to the laser polarization will be reflected to go through the measurement devices to be detected at an avalanche photodiode (APD).
Figure 2 shows the CMOS image of the LED light transmitted through the gold film. A polarizer was used to highlight the locations of the DNHs that are sitting at an angle to the LED light polarization, as shown in Fig. 2(A). In Fig. 2(B) we can see all apertures without the polarizer in front of the LED, so there is no polarization selection. The LED light wavelength was 940 nm and was visible on the CMOS camera.
3. Measurements
The trapping signals can be seen in Fig. 3. Figure 3(A) shows blocking of the laser beam (with polystyrene particle already trapped to release particle) at 61.7 seconds, followed by unblocking of the laser at 62.5 s, followed by a $1.84\%$ change in the trapping signal at 63.5 s. Figure 3(B) shows blocking of the laser beam at 30 s while BSA particle is trapped, to release it. Then by unblocking the laser at 30.8 s a BSA particle was trapped again at 32.2 s with an increase in the signal by $2.9\%$. We also trapped hBN nanoflakes using a similar setup (results are shown in the Appendix because the setup for those runs used a 60$\times$ microscope objective).
We attempted Raman with present setup and characteristic peaks of polystyrene at 3054 cm$^{-1}$ could be observed (see Appendix); however, since the signal was quite poor, further optimization is required for that part of the setup and we do not present those results in this work.
4. Simulations
4.1 FDTD setup
Finite difference time domain (FDTD) simulations were used to determine the polarization-dependent reflection for tilted DNHs. A total field scattered field source was used with dimensions 700 nm by 700 nm by 100 nm, encompassing the double nanohole in a 70 nm thick gold film. This source was used to simulate plane-wave excitation but avoid aperturing effects (even though a focused beam was used it is approximately constant over the region of the aperture). The mesh type was ‘auto non-uniform’, the accuracy was set on 5 and mesh refinement was ‘staircase’. For the gold film, Johnson and Christy [29] data was used and glass and water data were from Palik [30]. Perfectly matched layer boundaries were used with dimensions 1.2 $\mu$m, 1.2 $\mu$m and 2 $\mu$m to ensure no reflection at the boundaries. The near-field distribution was monitored (as shown in Fig. 4, and the reflected intensity of each polarization was recorded.
4.2 Degree of polarization
To show the cross polarized scattering from the DNH, degree of polarization for different polarization angles of incident beam had been calculated from simulation results. Figure 5 shows the 1 minus the degree of polarization. The degree of polarization is defined as Eq. (1):
where $I_{\mathrm {max,min}}$ are the laser intensities at the maximum and minimum polarizations. The maximum follows the incident laser polarization.Fig. 5. 1 minus the degree of polarization for different polarization angles of the laser with respect to the long axis of the DNH fitted with a sinusoidal function.
From this figure, we see that 8.1% of the light is coupled into the orthogonal polarization at $45^\circ$ polarization. $1- V$ tells us how much we scatter into the other polarization from the DNH.
5. Discussion
Using the reflection geometry gives a simpler optical setup because we do not have to have a collinear collection objective on the other side of the sample. The reflection setup uses the same objective for excitation and collection. This also frees up space on the other side of the sample that can be used to add microfluidics and it can reduce the cost due to having less optical elements.
While a few past works have looked at reflection, they did not exploit the polarization of the nanoaperture to isolate the reflected signal [24]. By doing this, we get an improved signal to noise ratio and stable trapping.
It is possible in the reflection geometry to work on highly scattering or opaque samples. These samples include crude oil to measure asphaltene properties [25], blood serum [26].
Adding microfluidics is easier if you have more space on the other side of the sample because it provides space for connecting tubing [31]). Several works have shown the benefit of combining DNHs with nanopores [10,15]. Those approaches can also benefit from the reflection geometry shown here.
In the future we aim to improve the Raman measurements by replacing the poor quality fiber-probe Raman we used with a free-space filter setup as has been done in the past [19,32]. With this we expect to be able to measure the reflected Raman signal and identify the trapped nanoparticle.
6. Conclusion
By making use of symmetry breaking at the nanoscale from the double nanohole aperture, we show polarization selected optical trapping and detection of single polystyrene nanospheres and bovine serum albumin particles. The simplified optical setup makes it easier to add other devices to one side of the trap. It also creates the possibility of studying non-transparent solutions. The polarization selective nature of the new method is useful in working with high scattering materials.
Appendix
To do Raman spectroscopy, a Raman probe was used to split the laser beam and the Raman signal. Preliminary Raman data is shown here for the reflection setup detected with the spectrometer (Ocean Optics QE65000). However, improvements in the setup is required to obtain convincing results. Figure 6 shows the Raman shift spectrum from a 20 nm polystyrene nanosphere. The integration time was 3 minutes.
In order to increase the integration time to 3 minutes, a Raman peak at a long wavelength should be chosen to avoid spectrometer saturation. The main Raman Peak for polystyrene is at 1001 cm-1 which corresponds to 852 nm and is close to the laser wavelength. Another Raman peak for polystyrene is at 3054 cm-1 which is further away from the laser wavelength. The intensity is lower than the main peak and in order to detect it, more accuracy in measurement and longer integration times are required. Because this peak is further away from the laser wavelength of 785 nm, the saturation of the spectrometer around laser peak will not affect the measurement.
Figure 7 shows blocking of the laser beam at 8.2 s while an hBN nanoflake is trapped, to release it. Then by unblocking the laser at 8.4 s an hBN nanoflake was trapped again at 9.2 s with an increase in the signal by $4.8\%$.
Funding
Natural Sciences and Engineering Research Council of Canada (RGPIN-2017-03830).
Acknowledgments
The authors acknowledge useful discussions with Ghazal Hajisalem and Michael Dobinson.
Disclosures
The authors declare no conflicts of interest.
Data availability
Simulation scripts will be provided on reasonable request.
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