Reconfigurable Optothermal Microparticle Trap in Air-Filled Hollow-Core Photonic Crystal Fiber

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient-driven flow of air along the core surface. Known as “thermal creep flow,” this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

Figure 1

(a) Optical setup: MO—microscope objective; NA—numerical aperture; YAG—yttrium aluminum garnet.. (b) Measured near-field intensity. (c) Schematic of how fiber heating causes speed variations of the microparticle in the heated region. Heating element is not drawn to scale (it is $\sim 50$ times longer than the outer fiber diameter). (d) Scanning electron micrograph of HC-PCF structure, core radius $R=6\mu \mathrm{m}$.

Figure 2

(a) Schematic of thermally induced air flow pattern in the hollow PCF core. (b) Doppler velocimetry trace of an $a=3.2\mu \mathrm{m}$ particle moving through the heating element (position indicated on top) at an optical drive power of 72 mW. (c) Threshold temperature difference $\Delta T_{\mathrm{TH}}$, above which the particle is unable to pass through the heated region, as a function of optical power. The right-hand axis shows the calculated axial component of the optical force ($\mathrm{\text{slope}}=0.6\mathrm{pN}/\mathrm{mW}$).

Figure 3

(a) Schematic and (b) movie image sequence of an $a=3\mu \mathrm{m}$ particle propelled along the fiber by 50 mW of laser power (MOVIE 1 in Supplemental Material [22]). It stops abruptly when approaching the black mark (length 0.5 mm). The dashed red curve indicates the numerically computed trajectory.

Figure 4

(a) Schematic of experimental configuration with counter-propagating beams (wavelength 982 and 1064 nm), power imbalance $\Delta P$. (b) Doppler velocimetry (symbols) and numerical speed traces (lines) of an $a=3.2\mu \mathrm{m}$ particle propelled through partially absorbing mark (length 1 mm) at three values of $\Delta P$.