Near-Field optical Angular orientation for Manipulating Biological Materials

Near-field optical manipulation techniques has demonstrated abilities to trap, transport, and handle microand nanoscale materials. To show further capabilities of our near-field optical devices, we demonstrate angular orientation of biological materials with a photonic crystal resonator using. A single microtubule is rotated by optical torque resulted from polarization in electric field, changing its angular orientation parallel to the direction of the electric field on a silicon nitride photonic crystal (PhC) resonator. Simultaneous application of optical force and torque extends the functionality of our near-field PhC resonators to be more powerful tool in biophysics study, and microand nanoscale physics. Summary of Research: The near-field photonics have shown better capabilities [1] for which traditional optical tweezers have accomplished within the limit of diffraction. Yet despite the potential significance in the optical manipulation, the angular orientation based on near-field optics has not been demonstrated to date. The control of the angular orientation can provide an additional capability to the near-field optical manipulation techniques. Furthermore, in biophysical study, the orientation control especially over elongated objects can be potentially significant in manipulation in that most biomolecules such as DNA, virus, and bacteria are the rod-like shapes [2]. Here we demonstrate that our resonators can perform the orientation for biological materials. In other words, we exhibited the angular orientation of a single microtubule, in which optical torque induced by dipole moment on the microtubule aligns the microtubule parallel to electric field of linearly polarized electromagnetic wave. The single microtubule is subject to the linearly polarized electric field (transverse electric mode) on the silicon nitride photonic crystal resonator we developed [3]. The rotation was described in exponential form as f (θ, t) = f (θ0, 0) ⋅ e -t/τ, where τ is the time constant of the orientation [4]. Figures 1, 2, and 3 illustrate that a single microtubule is oriented by the optical torque. When the microtubule moved on the resonator, it started to interact with the electric field, partially polarized as long as a width of the resonator, wres = 600 nm. At t0 = 0 the orientation angle of the microtubule is π/2 with respect to the horizontal direction across the resonator located vertically. To minimize the interaction between the translational motion and the rotational motion, the hydrodynamic flow was relaxed prior to observing the orientation. The microtubule was moved to the resonator as close using slow flow (u = 1 μL/hr) from left to right, so that it can interact with the resonator at the beginning. At the end the microtubule was oriented parallel to the polarized direction of the electric field. The time constant of the orientation, τ, was determined by measurements of orientation angles as a function of time, θ(t). Figure 4 shows the measured orientation angles of the microtubule with respect to time during the orientation. For the quantitative orientation measurement, a software ImageJ with a plugin, OrientationJ Measure, was used to measure the orientation angles. When the microtubule is rotating under the optical torque, the orientation angles approaches exponentially to the final orientation angle θ = 0 with respect to time as modeled [5] in f (t, θ) = f (0, θ0) ⋅ e -t/τ. The fitting to e-t/τ resulted in the time constant of the orientation τ = 2.603 s.