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Applicability of optical trapping tools for nanomanipulation is limited by the available laser power and trap efficiency. We utilized the strong confinement of light in a slot-graphite photonic crystal to develop high-efficiency parallel trapping over a large area. The stiffness is 35 times higher than our previously demonstrated on-chip, near field traps. We demonstrate the ability to trap both dielectric and metallic particles of sub-micron size. We find that the growth kinetics of nanoparticle arrays on the slot-graphite template depends on particle size. This difference is exploited to selectively trap one type of particle out of a binary colloidal mixture, creating an efficient optical sieve. This technique has rich potential for analysis, diagnostics, and enrichment and sorting of microscopic entities.
© 2016 Optical Society of America
1. Introduction
Optical trapping serves as a powerful tool for the manipulation of matter on the nanoscale [1–4]. The ability to immobilize multiple nano objects on a substrate will play a key role in future integrated analytical platforms [4–6]. At a given laser power, the applicability of trapping techniques is limited by the trap efficiency [7], often quantified by trap stiffness. Developing highly efficient and versatile optical trapping systems will facilitate new experiments and applications in broad areas from physics to biology [8–11].
To enhance the trapping performance, several past approaches have engineered the trapped objects [7, 12, 13] or taken advantage of their special properties, such as plasmonic resonances [14–16]. However, such approaches limit the application range. A more flexible approach is to improve the efficiency of the trap itself. The optical near fields of plasmonic and integrated photonic structures provide strong optical gradients, resulting in highly efficient traps [17–27]. Among these, all-dielectric designs provide absorption-free operation with negligible heating.
In our earlier work, we realized an array of near-field traps using the structured light fields above a 2D silicon photonic crystal [24]. In order to further enhance the trapping performance, we theoretically proposed to include slots within each unit cell of the photonic crystal lattice [28]. Due to the boundary conditions on the electric field, the narrow slot in the dielectric strongly enhances the electric field intensity [29], creating a highly efficient optical trap. Previously, we have fabricated such a device, known as a slot-graphite photonic crystal, and characterized its optical mode [30].
Here, we experimentally demonstrate the optical trapping of nanoparticles in our design. The trap stiffness is 35 times higher than our previously demonstrated photonic crystal traps [24], based on simple square lattice. We demonstrate trapping of dielectric as well as metallic nanoparticles of different sizes using the same photonic crystal device. We also study the growth kinetics of nanoparticle clusters in the photonic crystal trap. The difference in kinetics with respect to the particle size is used to selectively trap one type of particles out of a mixture, creating an efficient optical sieve. We expect this capability to be of widespread interest for microfluidic, lab-on-chip applications.
2. Device design
The template is made of silicon, and the pattern of holes and slots is arranged in a modified graphite lattice [30]. The slot-graphite photonic crystal is designed to support a guided resonance mode around 1550nm, where the silicon absorption is negligible. When the incident light wavelength is tuned to excite the mode, the optical field is confined within the slot and the local electromagnetic field intensity is enhanced [28]. The electric-field gradient just above the slab surface attracts the nanoparticles towards the traps [23, 28]. A schematic view of optical trapping in the slot-graphite template is shown in Fig. 1.
Fig. 1 Schematic view of optical trapping using a slot-graphite photonic crystal. Incident light from below excites a guided-resonance mode of the photonic-crystal slab, giving rise to optical forces on nanoparticles in colloidal solution.
Figure 2(a) shows the normalized electric field intensity (|E|2) in the z = 0 plane for y-polarized incident light, calculated using the 3D finite-difference time domain (FDTD) method (Lumerical). The photonic crystal has a lattice constant (a) of 820nm, air hole radius (r/a) of 0.155, slot width (wx) of 550nm, and slot height (wy) of 90nm. Particles are expected to trap in the high intensity slot region.
Fig. 2 Slot-graphite photonic crystal device. (a) Mode profile (|E|2) on resonance for y-polarized incident light. The circles represent the positions of the holes, and rectangles represent the positions of slots. b) SEM image of the device used in the experiment. The scale bar represents 1 µm. (c) Measured transmission spectrum of the device.
We fabricated the slot-graphite photonic crystal on a silicon-on-insulator wafer (SOITEC) with a 250nm thick silicon layer on top of a 3µm silica layer [24, 30]. The SEM image of the photonic crystal device is shown in Fig. 2(b). The transmission spectrum of the fabricated device was characterized in cross polarization mode at normal incidence [31] and is shown in Fig. 2(c). The guided resonance mode is strongly confined to the slab and appears as a peak in the transmission spectrum [31]. The resonant wavelength and quality factor are determined to be 1559nm and 1217, respectively, by fitting the experimental spectrum to a Fano function.
3. Experiment
The photonic crystal device is mounted in a shallow PDMS microfluidic chamber. Polystyrene nanoparticles with 520nm diameter (Thermo Scientific) are injected into the chamber using a syringe pump. When the incident laser is tuned to the wavelength of the guided-resonance mode, nanoparticles are attracted toward the slab. We trapped a few nanoparticles on the slab at an incident power of 30mW and determined the trap stiffness from the variance in particle position [24, 32]. The measured variance was corrected for motion blur and tracking error [33]. The experiment was repeated 15 times to collect the statistics. The distribution of trap stiffness is shown in Fig. 3.
Fig. 3 Trap stiffness for incident y-polarized light. (a) Histogram of stiffness values in the direction perpendicular to the polarization of the incident light. (b) Stiffness in the direction parallel to the incident polarization.
The mean stiffness in the x and y direction were found to be 52 pN/nm/W and 79 pN/nm/W respectively. The stiffness in the y direction is higher than that in the x direction. This is expected as the particles are mechanically confined in the y direction, due to their ability to partially sink into the slots. In addition, the field confinement is stronger in the y direction than in x. The stiffness is an order of magnitude greater than our previously demonstrated square lattice photonic crystal design [24].
We carried out the trapping experiment for varied particle size and composition. Within the illumination area, multiple particles were trapped. Some empty sites were observed, possibly due to the inhomogeneties in slot dimension. We were able to trap dielectric particles with diameters ranging from 300nm to 780nm and metallic nanoparticles with diameters ranging from 250nm to 400nm. The ability to trap varied particle sizes and composition suggests the near ‘universal’ character of slot-graphite traps. Figure 4 shows the optical microscope images of a typical experiment. In these images, we used the maximum available optical power in our setup (180mW) to trap as many particles as possible.
Fig. 4 Trapping in slot-graphite lattice. Assembly of (a) 520nm polystyrene particles (b) 300nm gold nanoparticles.
To investigate the kinetics of particle trapping in the slot graphite optical lattice, we measured the number of trapped particles as a function of time. Polystyrene particles with diameters of 380nm and 520nm are trapped separately with the same trapping power and particle concentration. The results are shown in Fig. 5(a). The trapping process reaches a dynamic equilibrium in which the number of trapped particles does not vary significantly. We observe that the total number of trapped particles in equilibrium is similar for the two particle sizes, while the initial cluster growth rate is higher for smaller particles.
Fig. 5 Instantaneous number of trapped particles for a colloidal solution containing equal concentrations of (a) 380nm or 520nm polystyrene alone. (b) 380nm and 520nm polystyrene particles together.
We next carried out trapping experiments in a colloidal mixture containing equal concentrations of 380nm and 520nm polystyrene particles. Figure 5(b) shows the instantaneous number of trapped particles as a function of time. In equilibrium, almost all of the trapped particles (94.3%) had a diameter of 380nm. The system thus shows a tendency to selectively trap the smaller size particles.
In a colloidal mixture containing two types of nanoparticles, each species competes for the available trapping sites in the lattice. The trapping sites are mutually exclusive: it is impossible for two different particles to trap in the same site at the same time. The higher diffusivity of smaller particles increases the probability of finding a smaller particle in the vicinity of a trapping site than a larger one. Thus, the smaller particles occupy the available sites at a faster rate. In addition, the smaller particles experience less drag, reducing the probability of trapped particles being dislodged by the flow.
We calculated the depth of the optical potential at the equilibrium trapping position of trapped 380nm and 520nm polystyrene particles. The nanoparticles can partially sink into the slots as they are trapped, as shown in Fig. 6(a).
Fig. 6 (a) Cross sectional view of the sinking of a nanoparticle into the slot. P is the lowest point of the particle. (b) Distance from the surface of the photonic crystal to the bottom of the particle (point P) as the particle moves along the y direction for two different particle diameters. (c) Optical potential experienced by the particles at the corresponding positions.
The contact height of nanoparticles as they traverse along the shorter dimension of the slot is given in Fig. 6(b). Smaller particles sink more into the slots. The transverse optical potential experienced by the particles, taking the sinking effect into consideration is shown in Fig. 6(c).The 380nm particles experience a deeper potential than the 520nm particles at the equilibrium trapping position. This can provide additional stabilization to the smaller particles and prevent their replacement in the trapping sites by the larger particles. This is in good agreement with the experimental observations
A detailed study of the interplay between diffusion, drag and optical potential depth is the subject of ongoing work. In preliminary studies, we have used numerical simulations of the Langevin equations to study broad trends. Our results indicate that trap selectivity can be increased by increasing the flow speed of the injected particle solution, which increases the differential drag between different sized particles. Meanwhile, the effect of differential diffusivity on trap selectivity increases with chamber height.
4. Summary
In summary, we present a stiff, highly versatile near field optical trapping system which can be used to optically assemble a large number of submicron nanoparticles with different sizes and compositions. We demonstrate that the slot-graphite photonic crystal can selectively trap one type of particles out of a mixture, acting as an efficient optical sieve.
Selective assembly is of great interest in trace analysis, optical diagnostics, enrichment, and sorting of microscopic entities and molecules [34–37]. While size selective trapping of microscale objects has been demonstrated in microfluidic systems [38–41], optical trapping approaches eliminate the need for fluid flow [42], and are thus expected to enable a different application range. Previously, surface plasmon based optical tweezers have been used for selective optical manipulation [43]. Our traps offer an alternative dielectric solution. Ultimately, we can envisage an arrayed, on-chip device for selective capture and parallel detection of biomolecules attached to nanoparticles [44]
Appendix Methods
Device Fabrication: The device was fabricated on a silicon-on-insulator wafer (SOITEC) with a 250nm-thick silicon layer on top of a 3μm-thick silica layer and 600µm-thick silicon handle layer. The backside of the wafer was polished to a mirror finish with an average surface roughness less than 10 nm. A 220nm layer of Si3N4 was deposited on the backside using PECVD to reduce the reflection. The sample was spin-coated with PMMA-A4 950K. A 50μm diameter slot-graphite pattern was exposed using Raith 150 e-beam system with an acceleration voltage of 30 kV and beam aperture of 10µm. The pattern was transferred from the resist to the silicon layer by ICP-RIE etching using a modified Bosch process with gas mixture of SF6 and C4F8.
Microfluidic channel preparation: A thin, open top PDMS microfluidic chamber (1 mm × 4 mm area, ~1 µm thickness) was fabricated on a glass slide using standard photolithography methods. Tert-Butyl alcohol was used as a solvent for the PDMS. The photonic-crystal sample was mounted on a circular glass slide with a 2 mm circular hole at the center and inserted in a rotary stage. The chamber was pressed firmly on to the sample and sealed inside the rotary stage. Nanoparticles were injected into the chamber through microfluidic tubes using a syringe pump. A constant, low velocity, laminar flow was maintained in the chamber throughout the course of the experiment. Nanoparticle solutions were prepared in heavy water (D2O, Sigma Aldrich) to minimize the thermophoresis effects [45] resulting from water absorption around 1550nm. At this wavelength the absorption coefficient of D2O is 27 times smaller than H2O [46].
Experimental Setup: A Santec TSL-550 tunable laser (1500-1620nm) was used for the optical trapping. An erbium-doped fiber amplifier combined with polarization-control optics were used to control the power and polarization of the beam. An aspherical lens (f = 11 mm and NA = 0.25, Thorlabs C220 TME-C) was incorporated to collimate the beam from the laser through a single-mode fiber (mode diameter 10.4 ± 0.8 µm). An achromatic doublet (f = 30 mm, Thorlabs AC254) was used to refocus the beam to the back side of the sample. There are 3643 slots within the beam diameter, which was measured to be 26µm using the knife-edge method.
Particle Detection: The experiments were recorded using a 20X objective and a CMOS camera with a fixed exposure time of 33ms at 30 frames per second. Typical videos were 1000 or more frames in length. The coarse position of the particles were detected using MATLAB algorithms written by Blair and Dufresne (http://physics.georgetown.edu/matlab/), which are modified versions of IDL routines written by Crocker and Grier [47]. The difference in intensity of the highest brightness pixel for each particle is used to distinguish between particles of different sizes in the case of mixed particle trapping.
Stiffness analysis: The trap stiffness was calibrated from the variance in position of the trapped 520nm polystyrene particles. 520nm particles were used for the stiffness measurement, as they are clearly visible in the trapping videos and have reasonably high diffusivity. It is difficult to detect the accurate position of the particles when they are together in a cluster. In order to overcome this, a few isolated particles were trapped by turning down the power. The particle positions were detected with subpixel accuracy using the radial symmetry method [48]. The experiment was repeated, and position data was collected for 62 particles over 1000 frames. The variances in particle position were measured and corrected for motion blur due to the finite integration time of the camera and detection error [33]. The stiffness values were normalized to the local power at each trapping site. The maximum power per trap is estimated to be ~10.1 µW.
Potential calculation: The optical forces tend to pull particles toward the photonic crystal slab, causing the particles to sink slightly into the slots. We calculated the transverse optical force on dielectric particles (n = 1.59) of size 380nm and 520 nm, along the contact path as they traverse along the shorter dimension of the slots, by numerical integration of the electromagnetic force density over the particle volume [49]. We performed a line integration of the optical force along the contact path to calculate the transverse optical potential as
where r is the position vector of the particle.Acknowledgment
This work was funded by Army Research Office PECASE Award under Grant 56801-MS-PCS. The authors thank Ruobai Feng for helping with the experiment. Computation for work described in this paper was supported by the University of Southern California Center for High-Performance Computing and Communications. Electron microscopy imaging was supported by CEMMA (Center for Electron Microscopy and Microanalysis), University of Southern California.
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