Associate Professor John Holdsworth

The results of previous researches indicated that there were problems with the mental model and students' conceptual understanding of the action-reaction law (Newton's third law, NTL). This research aimed to reveal the effect of a simple approach in teaching NTL. The research was conducted in the first-year of pre-service physics teachers at the Physics Education Department of Tadulako University. Research designs for three consecutive years were (1) one-group, pre-test, and post-test design, (2) a static group comparison (pre-test for the experimental group), and (3) a quasi-experimental. The approach used was an interactive demonstration that consisted of five phases, i.e: eliciting an intuitive argument, demonstrating a continuous force: pulling, demonstrating a continuous force: pushing, demonstrating impulsive force: collisions, and refining the concept with Elby's pair. Data were collected using a multiple-choice test developed in previous research. The results of the data analyses showed that the approach could improve students' understanding of the action-reaction law, supporting conceptual change by exhibiting N-gain in the moderate and high categories. The instructional design can be considered for implementation in learning in high schools, lecture on pre-service physics teachers and basic physics lecture, in general.

Conspicuous coloured displays from ultraviolet to bright red have been documented in many species throughout the animal kingdom. These colours often occur as sexual signals and can be incorporated into different types of integuments (e.g. scales, feathers, skin). Two main mechanisms are known to produce coloured integuments: pigmentation and tissue structure. Although pigmental and structural coloration are separate mechanisms and can occur independently, some coloured displays might emerge from a combination of both. Here, we demonstrate, using biochemical, optical and morphological methodologies, that the yellow coloration of the skin located around the eye of Common (Indian) Mynas (Acridotheres tristis) is produced by both light-reflecting nanostructures and light-absorbing carotenoid pigments. Our analysis confirms that nanostructured collagen in the avian dermis work in combination with carotenoid pigments to produce vivid integumentary colours. Identifying the mechanisms behind the production of a coloured signal provides a basis for predicting how a signal's function might be influenced by environmental factors such as fledgling nutrition.

This work investigated the photophysical pathways for light absorption, charge generation, and charge separation in donor-acceptor nanoparticle blends of poly(3-hexylthiophene) and indene-C60-bisadduct. Optical modeling combined with steady-state and time-resolved optoelectronic characterization revealed that the nanoparticle blends experience a photocurrent limited to 60% of a bulk solution mixture. This discrepancy resulted from imperfect free charge generation inside the nanoparticles. High-resolution transmission electron microscopy and chemically resolved X-ray mapping showed that enhanced miscibility of materials did improve the donor-acceptor blending at the center of the nanoparticles; however, a residual shell of almost pure donor still restricted energy generation from these nanoparticles.

We describe an extreme ultraviolet (XUV) interferometric technique that can resolve ~100 zeptoseconds (10-21 s) delay between high harmonic emissions from two successive sources separated spatially along the laser propagation in a single Gaussian beam focus. Several improvements on our earlier work have been implemented in the advanced interferometer. In this paper, we report on the design, characterization and optimization of the advanced Gouy phase interferometer. Temporal coherence for both atomic argon and molecular hydrogen gases has been observed for several harmonic orders. It has been shown that phase shift of XUV pulses mainly originates from the emission time delay due to the Gouy phase in the laser focus and the observed interference is independent of the generating medium. This interferometer can be a useful tool for measuring the relative phase shift between any two gas species and for studying ultrafast dynamics of their electronic and nuclear motion.

The phase segregation in P3HT:PCBM blend films has been investigated from an experimental and theoretical viewpoint. Optical microscopy, atomic force microscopy, scanning electron microscopy and X-ray diffraction show that thermal annealing of P3HT:PCBM blend films leads to the formation of PCBM aggregates. These aggregates are composed of dense randomly packed ~50 nm PCBM crystallites with an overall aggregate density of ~0.85 g cm-3. By applying the critical radius of nucleation for PCBM and the Stokes-Einstein equation for mobility of PCBM in a P3HT matrix, a model is developed which explains the formation of both crystallites and aggregates.

The kinetics and thermodynamics of PCBM phase segregation and aggregation in P3HT:PCBM blends has been studied. We develop a thermodynamic model for PCBM phase segregation in P3HT:PCBM blends which explains the formation of nanoscale crystallites which subsequently diffuse and coalesce into larger PCBM aggregates. We show that the formation of nanoscale crystallites during the film making process prevents spinodal decomposition of the P3HT:PCBM blends even at PCBM weight fractions above the spinodal decomposition boundary for the system. Finally, we demonstrate that the observed aggregate morphology can be understood in terms of a kinetic model based on the diffusional flux lines of PCBM crystallite which, in turn, govern the evolution of the macroscopic growth front.

The diffusion of PCBM in P3HT:PCBM blend films has been investigated using multi-wavelength scanning absorption microscopy (MWSAM). By studying the depletion of PCBM in the vicinity of the phase segregated PCBM-rich regions that form upon thermal annealing, we are able to measure the diffusion constant and activation energy for PCBM diffusion in P3HT:PCBM blend films. The measured kinetic parameters are consistent with the diffusion of nanoscale PCBM crystallites rather than molecular PCBM. We show that the presence of two distinct diffusion processes in these blend materials provides an explanation for the large differences that have been reported for PCBM diffusion in P3HT:PCBM blends. This insight allows us to develop a unified model for PCBM mass transport in these materials.

The far-field spatial distributions of higher order electromagnetic mode supercontinua were resolved spectrally and recorded. The supercontinua were created by precise control and direction of input pump energy offset axially from the photonic crystal fiber core. By processing the measured spectra, the spatial mode shape at each wavelength was determined. Discrete spectral features are associated with symmetrical spatial patterns arising from the host fiber geometry and suggest the electromagnetic mode pairing between the longer wavelength solitons and associated visible dispersive waves. Clear differences between supercontinua generated in fundamental and higher order electromagnetic modes exist. These data should inform theoretical studies as the solitons and the dispersive wave generated by fission may be matched by spatial orientation of the electromagnetic mode that both occupy. ©2013 Optical Society of America.

This article describes a position sensitive interferometer with closed-loop control of the reference mirror. A calibrated nanopositioner is used to lock the interferometer phase to the most sensitive point in the interferogram. In this configuration, large low-frequency movements of the sensor mirror can be detected from the control signal applied to the nanopositioner and high-frequency short-range signals can be measured directly from the photodiode. It is demonstrated that these two signals are complementary and can be summed to find the total displacement. The resulting interferometer has a number of desirable characteristics, it is optically simple, does not require polarization or modulation to detect the direction of motion, does not require fringe-counting or interpolation electronics, and has an effectively unlimited bandwidth. Experimental results demonstrate the frequency response analysis of a high-speed positioning stage. The proposed instrument is ideal for measuring the frequency response of nanopositioners, electro-optical components, MEMs devices, Ultrasonic devices, and sensors such as surface acoustic wave detectors.

The performances of the Michelson (MI) and Mach-Zehnder interferometers (MZI) are compared for laser linewidth measurement with the delayed self-heterodyne technique. Voigt fitting is used to extract the Lorentzian and Gaussian components for the MZI and MI with faraday mirrors and also standard gold mirrors. The MI with Faraday mirrors provides a measurement with significantly lower error, owing to the elimination of noise associated with random polarisation drifts. © 2011 AOS.

The performances of the Michelson (MI) and Mach-Zehnder interferometers (MZI) are compared for laser linewidth measurement with the delayed self-heterodyne technique. Voigt fitting is used to extract the Lorentzian and Gaussian components for the MZI and MI with faraday mirrors and also standard gold mirrors. The MI with Faraday mirrors provides a measurement with significantly lower error, owing to the elimination of noise associated with random polarisation drifts. © 2011 IEEE.

Polarization lidars transmit polarized laser pulses and collect both parallel, PI/, and cross-polarized, PI backscatter. The degree of depolarization, D = PI// PI, depends on rehctive index, size, phase and number density of the scattering medium. Lidar studies have shown values of D ranging from 0.02 to 0.04 in high number density of spherical liquid aerosols, due to multiple scattering, and values ranging h m 0.4 to 0.7 in ice crystals. In some special cases, D may be zero and greater or equal to one in oriented crystals and complex crystals respectively (G.P. Gobbi, 1998). In the absence of aerosols and particles, Rayleigh scattering by air molecules generates a small amount of depolarization of about 0.014. Polarization lidars have proven to be an effective diagnostic tool i n the assessment of cloud and aerosol properties,.