Wen Jun Toe

Characterization of semiconductor nanowires using optical tweezers.

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.

Resolving stable axial trapping points of nanowires in an optical tweezers using photoluminescence mapping.

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.

Nonconservative dynamics of optically trapped high-aspect-ratio nanowires.

We investigate the dynamics of high-aspect-ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the dynamics reveals a broad spectral resonance of the order of kHz with peak properties that are strongly dependent on the input trapping power. A dynamical model incorporating linear restoring optical forces, a nonconservative asymmetric coupling between translational and rotational degrees of freedom, viscous drag, and white noise provides an excellent fit to experimental observations. A persistent low-frequency cyclical motion around the equilibrium trapping position, with a frequency distinct from the spectral resonance, is observed from the time series data.

Intrinsic heating in optically trapped Au nanoparticles measured by dark-field spectroscopy

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.

Fishnet metamaterials with incorporated titanium absorption layer

Some metamaterial applications require the use of high-power lasers, but the incoming radiation may damage the metamaterials. In addition to that, the presence of an absorptive material placed close to metamaterial surface can lead to quick heating of the surrounding area, resulting in serious thermal damage or melting of the fabricated pattern. We study the impact of a titanium absorptive layer on top of a conventional fishnet structure and we show that due to increased absorption the melting power is reduced by nearly 50% and thermal damage leads to the formation of microbumps on the exposed surface.

Characterisation of Au nanorod dynamics in optical tweezers via localised surface plasmon resonance spectroscopy

We present a study of the trapping properties of Au nanorods of different aspect ratios in an optical tweezers and comparison with other characterization techniques like transmission electron microscope (TEM) imaging and dynamic light scattering (DLS). This study provides information on the dynamics and orientation of Au nanorods inside an optical trap based on a time study of their localised surface plasmon resonance (LSPR) features. The results indicate that the orientation of the Au nanorods trapped in our optical tweezers varies with time and LSPR spectra can provide information on the angle of the nanorod with respect to the direction of propagation of the trapping laser.

Anomalous dynamic behaviour of optically trapped high aspect ratio nanowires

In the following study we investigate the dynamics of high aspect ratio nanowires held in a single gradient force optical trap in an overdamped environment. Power spectrum analysis performed on the stochastic trajectory of the optically trapped nanowires indicate that the motion of these nanowires shows characteristics of underdamped motion, where a broad resonance peak is present in the power spectrum of amplitude fluctuations under certain conditions. The resonance occurs when the nanowires are trapped at a height of 50 μm from the cover slip of the sample chamber. The emergence of a resonance peak in the power spectrum could be attributed to the non-conservative motion of nanowires being nonspherical, thus creating a bias towards cyclic motion as examined theoretically by Simpson and Hanna [12].

Using Spectroscopic Techniques to Interrogate Trapping Dynamics of Nanoscale Objects

We explore the use of optical trapping combined with spectroscopy to interrogate the properties of individual nanoparticles. We focus on how physical properties influence trap stiffness and asymmetry, rotational motion and stable axial trapping points.

PL mapping and optimized optical trapping of nanowires SLM beam shaping

We report a novel method for using a spatial light modulator (SLM) to spatially map the luminescent properties of single trapped semiconductor nanowires by dynamic optical tweezers. Being able to control the axial position of the trapping focus with respect to the excitation source, the composition along the long axis of the nanowire can be probed. We also explore the feasibility of tailoring trapping beam shape to enhance the axial trap stiffness for long nanowires (> 5 μm).

Dark field optical tweezers for studying nanoparticle dynamics

We report a method of characterising physical and optical properties of nanoparticles using optical tweezers combined with dark field microscopy. The technique uses measurements from Brownian dynamics of the trapped nanoparticles to determine localised surface plasmon resonance (LSPR) spectroscopy to determine nanoparticle size information.