We report on the optical trapping characteristics of InP nanowires with dimensions of 30 (±6) nm in diameter and 2-15 μm in length. We describe a method for calibrating the absolute position of individual nanowires relative to the trapping center using synchronous high-speed position sensing and acousto-optic beam switching. Through brownian dynamics we investigate effects of the laser power and polarization on trap stability, as well as length dependence and the effect of simultaneous trapping multiple nanowires.

Axially resolved microphotoluminescence mapping of semiconductor nanowires held in an optical tweezers reveals important new experimental information regarding equilibrium trapping points and trapping stability of high aspect ratio nanostructures. In this study, holographic optical tweezers are used to scan trapped InP nanowires along the beam direction with respect to a fixed excitation source and the luminescent properties are recorded. It is observed that nanowires with lengths on the range of 3-15 μm are stably trapped near the tip of the wire with the long segment positioned below the focus in an inverted trapping configuration. Through the use of trap multiplexing we investigate the possibility of improving the axial stability of the trapped nanowires. Our results have important implication for applications of optically assisted nanowire assembly and optical tweezers based scanning probes microscopy.

We investigate the dynamics of high aspect ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the Brownian dynamics reveals a broad spectral resonance of the order of a kHz with peak properties that are strongly dependent on the input trapping power. Modelling of the dynamical equations of motion of the trapped nanowire that incorporate non-conservative effects through asymmetric coupling between translational and rotational degrees of freedom provides excellent agreement with the experimental observations. An associated observation of persistent cyclical motion around the equilibrium trapping position using winding analysis provides further evidence for the influence of non-conservative forces.

Assessing the degree of heating present when a metal nanoparticle is trapped in an optical tweezers is critical for its appropriate use in biological applications as a nanoscale force sensor. Heating is necessarily present for trapped plasmonic particles because of the non-negligible extinction which contributes to an enhanced polarisability. We present a robust method for characterising the degree of heating of trapped metallic nanoparticles, using the intrinsic temperature dependence of the localised surface plasmon resonance (LSPR) to infer the temperature of the surrounding fluid at different incident laser powers. These particle specific measurements can be used to infer the rate of heating and local temperature of trapped nanoparticles. Our measurements suggest a considerable amount of a variability in the degree of heating, on the range of 414-673 K/W, for different 100 nm diameter Au nanoparticles, and we associated this with variations in the axial trapping position.

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