3-1 Fabrication
The waveguides are made of ITO glass substrate covered by silicon nitride strips. The ITO layer is 200nm in thickness. Our waveguide structures are fabricated from silicon nitride film. At first, silicon nitride film is deposited on the ITO glass substrate by using plasma-enhanced chemical vapor deposition (PECVD). The film thickness is 300nm. Then a 240 nm polymethylmethacrylate (PMMA) layer is spun on the sample by a spin coater. The strip pattern is defined on the PMMA layer by using electron beam lithography (EBL) with three different widths (w = 0.5, 1, and 2μm). The first process flow is illustrated in Fig. 3-1.
Fig.3-1. The first part of fabrication process
Then the pattern defined on PMMA is transferred to silicon nitride layer by using reactive ion etching (RIE). The residual PMMA layer can be removed by O2 plasma. The second process flow is illustrated in Fig. 3-2. The SEM image of finished optical silicon nitride waveguide structures after these steps are shown in Fig.3-3.
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Fig.3-2. The second part of fabrication process
Fig.3-3. The top- and tilted-view SEM pictures of these structures (a) w=0.5μm (b) w=1μm (c) w=2μm
Because the patterns are defined by EBL in the 600×600μm2 field size, the total length of each waveguides is limited around 550μm. It is necessary to make an entrance for coupling light at one end of the waveguide. Generally, we used to cleave the substrate to complete this step, but there are a lot of difficulties for cleaving the glass substrate because there is no crystallographic orientation on it, and it is too hard to be cleaved. So we use the process of mechanical polishing to make a waveguide entrance for light coupling. After the waveguide
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structures are defined on silicon nitride layer, the process of mechanical polishing has two steps. The polishing reagent is water. The cross-section of waveguide entrance is shown in Fig.3-4.
Fig.3-4. The cross-section of entrance of 2μm wide waveguide.
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3-2. Measurement setup
The measurement setup is schematically illustrated in Fig. 3-5. A cell made up of
~0.11 mm thick double-sided tape and a cover slip is glue on the sample surface for forming a chamber, which contains the particles dispersed in de-ionized water. Thus the waveguides are covered by water surrounding. We use polystyrene micro-spheres (with 1 and 2 µm in diameter, n = 1.59) with standard deviation smaller than 1% suspended in de-ionized water and add 1%-by-volume Triton X-100 non-ionic surfactant to the particle solution to prevent aggregation of the nanoparticles and to minimize adhesion between micro-particles and the surface of the devices.
Fig.3-5. The diagram of the experimental setup
The continuous laser source is given by HP 8168 tunable laser with erbium doped fiber amplifier, and it operates at the range from 1480nm to 1580nm. The fiber is mounted on micro-positioning stages. Light is injected into the waveguides through a fiber with tapered tip, which is directly immersed in water. We use continuous laser source with 1500nm and 1550nm in wavelength to do the measurements. The output powers of 1500nm and 1550nm at the tapered end of fiber are 57mW and 55.6mW, respectively.
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The measurement setup is shown in Fig. 3-6. An optical microscope with illumination from above and a microscopic objective lens with 20 times magnification were used to observe the motion of particles. A CCD camera was mounted on top of the microscope and the images were captured on a computer.
Fig.3-6. The experimental setup
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3-3. Measurements and discussion
3-3-1. Particle transportation velocity
All particle transportation velocities are measured at 100 μm away from the input ports of the waveguide. Fig.3-7 (a)-(c) shows the 2μm particles move as time evolves on 0.5, 1, and 2μm wide waveguides, respectively, when the continuous-wave laser source operated at 1500nm. Fig (d)-(f) shows the same system under condition of 1550 nm in wavelength.
Fig.3-7. Motion of 2μm particles (indicated by red arrows) as time evolves on (a) 0.5, (b) 1, and (c) 2μm wide waveguides in 1500nm waveguide. (d) (e) (f) shows the same system which under condition of 1550 nm in wavelength.
We average the velocities among more than five samples for each experiment. In order to understand the contribution of optical force by optical waveguides, we measured the velocity without any structure on the substrate as the background velocity (Vb) caused by the wave scattered from the fiber tip. The background velocity is about 37μm/s for 2μm
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polystyrene particles. The average velocity (Vav) and the difference (Vab-Vb) between average velocity and background velocity are shown in Table.3-1.
λ=1500nm λ=1550nm
Table.3-1. The measured velocities of 2μm particles
We noticed that the relation between waveguide width and the measured particle transport velocity itself conflicts with the theoretical prediction. Because there is no coupling structure designed at the input ports of these waveguides, the power ratios coupled into each waveguide are different.
domain Diameter
Cladding 125μm
Fiber core 8μm
Tip 2μm (radius of curvature)
Table.3-2. Simulation and geometric parameters of taper fiber.
In order to estimate the coupling efficiency, we construct a three-dimensional coupling model in FEM simulation which consists of a fiber tapered by 30° at the tip, as manufacturer specified, and a waveguide structure on glass substrate. The distance between the fiber tip and waveguide is assumed to be 3μm. The geometric parameters of the single mode taper fiber specified by the manufacturer are list in Table.3-2. In order to prevent the simulation out of the computational memory, here we did not introduce the 200nm ITO layer between the
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waveguide and substrate. The efficiencies are estimated to be 0.006, 0.012, and 0.028 for the cases of 0.5, 1, and 2μm wide waveguides, respectively.
For fair comparison, we normalize the velocities by the estimated powers coupled into the waveguides. Here we used the wavelength at 1500nm for instance. The 2μm particle transport velocities when per watt power coupled into 0.5, 1, and 2μm wide waveguides are shown in Table.3-3.
width (μm) V
av-V
b
(cm/s/W)
0.5 9.65
1 5.99
2 2.38
Table.3-3. The 2μm particle transport velocities when per watt power coupled into waveguides
The theoretical velocities are calculated by eqs. (2-15) is 1.2cm/s, 0.32cm/s, and 0.29cm/s, respectively. These differences may result from three causes. First, the existence of ITO layer fundamentally affects the boundary mode in simulations. Second, the distance between the waveguide and the fiber is variable in each experiment. And finally, there may be some deviations between the approximated hydrodynamic model and the practical waveguide system. Although the measured velocities are faster than those predicted theoretically, the result coincides with the predicted relation that a narrower waveguide can transport particles more efficiently.
Fig.3-8 shows the 1μm particles move as time evolves on 0.5, 1, and 2μm wide waveguides, respectively, with launching wavelength at 1500nm and 1550nm. The 1μm particles average velocity (Vav) and the difference (Vab-Vb) between average velocity and
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background velocity are listed in Table.3-4. In many observations, we find 1μm particles is easier to be disturbed than 2μm particles, because the size-dependent downward trapping force becomes weaker. It is obvious that the 1 μm particles has velocities much slower than 2 μm particles. This is because the evanescent field and the particle cannot interact significantly.
Thus the transported velocity of 1 μm particles degrades severely. It is almost that the wave directly scattered from the fiber tip transports the 1 μm particles. Therefore, the difference (Vab-Vb) between average velocity and background velocity may close to zero or even smaller that zero.
Fig.3-8. Motion of 1μm particles (indicated by red arrows) as time evolves on (a) 0.5, (b)1, and (c) 2μm wide waveguides in 1500nm waveguide. (d) (e) (f) shows the same system which under condition of 1550 nm in wavelength.
λ=1500nm λ=1550nm
Table.3-4. The measured velocities of 1μm particles.
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3-3-2. Sorting application
Here we mix 2μm with 1μm particles in water surrounding. The motions of particles are shown in Fig. 3-9. Due to that the optical forces depends on the size of particle, the mixed particles are separated by the difference of transporting velocity on the waveguide. Moreover, due to the weaker trapping force, the 1μm particle flow away from the waveguide and slow down after a few tens of micrometers. This function can be used to classify the particles by their size.
Fig.3-9. Motion of mix of 1μm and 2μm particles (indicated by red and blue arrows) as time evolves 1 μm wide waveguides in 1500nm waveguide.
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3-4. Summary
Due to the enhancement caused by ITO layer, our waveguide under a lower input power can transport particles much faster than that done by waveguide of the same width on silica film (as compared in [11]). With properly designed coupling structure, silicon nitride waveguide on ITO layer would be potential for developing efficient transport system on integration chip. Moreover, the propulsion efficiency of different size particles on silicon nitride waveguides open the way to sorting application, the classification of mixed particles by the size can be realized.
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