In this research, we use tapered lens fiber to couple light into the waveguides both for transporting the particle and exciting LSPR mode in the bowtie. However, the alignment between waveguide and fiber is very critical because they are very different in mode size.
Any misalignment will cause severe scattering at the junction and lower the coupling efficiency. In this case the waveguide and bowtie will have insufficient optical power to propel and trap nanometric particles. In practical application the way to do coupling must be stable and easy to be accomplished with high efficiency. Therefore we have to discard the butt-coupling method. Instead a grating coupling must be one of the potential candidates for two reasons [35]. First, the fiber and waveguide are aligned nearly perpendicular to each other as shown in Fig. 4-1. This would eliminate the influences caused by the waves scattered directly from the butt-coupled input fiber. Second, with the grating coupler, there is no need to cleave the sample for coupling. This feature enables wafer scale testing and increases the amount of available devices on the sample. Also this will largely ease the optical route design on a sample chip.
Fig. 4-1 Coupling principle between fiber and photonic wires by means of a grating (Adapted from reference [35]).
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Except the coupling issue, the information extracted from the trapped particles, such as size or refractive index, is not provided by the bowtie itself. For further sensing and detection we can combine the Raman spectroscopy with our measurement system as shown in Fig. 4-2 to acquire the detailed information [36]. One can see the particles on a waveguide are moved along the waveguide and then individually addressed by a focused laser beam to obtain their characteristic Raman signature within 1 second acquisition time.
Fig. 4-2 Setup for waveguide propulsion and Raman spectroscopy (Adapted from reference [36]).
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Appendix A
Fig. A-1 plots the dependence of optical force on bow angle when it reduces from 90° to 20°. In this investigation the particle is at coordinate of (0, 0, 50 nm). We find the peak of Fz first blueshifts and then redshifts as the bow angle reduces. This phenomena has been observed in previous study [37]. Although the bowtie structure is simple, the coupling between optical waveguide and SPR mode is quite complex. Also the coupling between SPR mode and the PS particle will affect the resonance. However, the force will reach its maximum at 1.55 μm when the bow angle is 90°.
Fig. A-1 (a) Simulated spectra of Fz exerted upon the particle trapped by the bowtie structure with bowtie angle varied from 20° to 90°, r = 0, D = 300 nm, t = 30 nm, and g = 5 nm. (b) Summarizing peak wavelength and peak force with bowtie angle.
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Appendix B
Addition to trapping utility, we evaluate the wavelength shift caused by trapping of single particle and bulk medium sensing. In order to have highest influence of field distribution, we increase the gap size to 30 nm for being accessible for 20 nm particle. Fig. B-1 shows the resonance peak shifts 1 nm after trapping of a single nanoparticle in the gap, and the result is two times of that in the previous study utilizing slotted PhC cavity [25]. Moreover, we simulate the index change of bulk media from 1.35 to 1.45 as show in Fig. B-2 and find the bowtie structure has good sensitivity about 500 nm/RIU.
Fig. B-1 (a) Simulated spectra of 20 nm PS particle detection in the gap (0, 0, 15 nm). (b) Simulated transmission spectra with 20 nm PS particle detection in the slot center. (Adapted from reference [25])
Fig. B-2 Simulated spectra of bulk media sensing with n varied from 1.35 to 1.45, α = 90°, D = 300 nm, t = 30 nm, r = 0 nm and g = 30 nm.
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Appendix C
In this work, we simulate transmission extinction and force acting on particle as function of incident wavelength (Fig. C-1 (a)). The particle is at coordinate of (0, 0, 35 nm). The extinction spectrum is integrated at bottom of silicon nitride (z = -750 nm) which is at far field but force spectrum is integrated particle surface at near field (z = 35 nm). And there is a 100 nm wavelength difference between extinction peak and force peak. Therefore, we show extinction spectrum depending on different surface from z = -750 nm to 30 nm (Fig. C-1 (c)). We find that the extinction peak will shift to 1.55 μm when the integration surface closes to particle.
Fig. C-1 (a) Simulated extinction spectra and Fz acting on the PS particle as a function of incident wavelength with α = 90°, D = 350 nm, t = 30 nm, r = 10 nm and g = 30 nm. (b) Illustration of particle position in cross-section view. (c) Simulated extinction spectra as a function of incident wavelength for z = -750, 0, 15 and 30 nm surface.
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