Peter Woolliams

Measurement techniques for piezoelectric nanogenerators

Electromechanical energy harvesting converts mechanical energy from the environment, such as vibration or human activity, into electrical energy that can be used to power a low power electronic device. Nanostructured piezoelectric energy harvesting devices, often termed nanogenerators, have rapidly increased in measured output over recent years. With these improvements nanogenerators have the potential to compete with more traditional micro- or macroscopic energy harvesting devices based on piezoelectric ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) or electrostatic, electret or electromagnetic kinetic energy harvesters. Power output from a nanogenerator is most commonly measured through open-circuit voltage and/or short-circuit current, where power may be estimated from the product of these values. Here we show that such measures do not provide a complete picture of the output of these devices, and can be misleading when attempting to compare alternative designs. In order to compare the power output from a nanogenerator, techniques must be improved in line with those used for more established technologies. We compare ZnO nanorod/poly(methyl methacrylate) (PMMA) and ZnO nanorod/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) devices, and show that despite an open-circuit voltage nearly three times lower the ZnO/PEDOT:PSS device generates 150 times more power on an optimum load. In addition, it is shown that the peak voltage and current output can be increased by straining the device more rapidly and therefore time-averaged power, or time-integrated measures of output such as total energy or total charge should be calculated. Finally, the internal impedance of the devices is characterised to develop an understanding of their behaviour and shows a much higher internal resistance but lower capacitive impedance for the ZnO/PMMA device. It is hoped that by following more rigorous testing procedures the performance of nanostructured piezoelectric devices can be compared more realistically to other energy harvesting technologies and improvements can be rapidly driven by a more complete understanding of their behaviour.

Spatially deconvolved optical coherence tomography

In this paper we present spatially mapped point-spread function (PSF) measurements of an optical coherence tomography (OCT) instrument and subsequent spatial deconvolution. The OCT B-scan image plane was divided into 2400 subimages, for which PSFs were determined from OCT measurements of a specially designed phantom. Each PSF was deconvolved from its corresponding subimage of the phantom using the Lucy-Richardson algorithm. Following deconvolution, all of the subimages were reassembled to form a final deconvolved image, from which the resolution improvement was quantitatively assessed. The lateral resolution was found to improve by 3.1 microm compared to an axial resolution enhancement of 4.5 microm. The spatial uniformity of both axial and lateral resolution was also observed to increase following deconvolution, demonstrating the advantage of deconvolving local PSFs from their associated subimages.

Optical coherence refractometry

We introduce a novel approach to refractometry using a low coherence interferometer at multiple angles of incidence. We show that for plane parallel samples it is possible to measure their phase refractive index rather than the group index that is usually measured by interferometric methods. This is a significant development because it enables bulk refractive index measurement of scattering and soft samples, not relying on surface measurements that can be prone to error. Our technique is also noncontact and compatible with in situ refractive index measurements. Here, we demonstrate this new technique on a pure silica test piece and a highly scattering resin slab, comparing the results with standard critical angle refractometry.

Femtosecond laser micro-inscription of optical coherence tomography resolution test artifacts.

Optical coherence tomography (OCT) systems are becoming more commonly used in biomedical imaging and, to enable continued uptake, a reliable method of characterizing their performance and validating their operation is required. This paper outlines the use of femtosecond laser subsurface micro-inscription techniques to fabricate an OCT test artifact for validating the resolution performance of a commercial OCT system. The key advantage of this approach is that by utilizing the nonlinear absorption a three dimensional grid of highly localized point and line defects can be written in clear fused silica substrates.

Elastographic contrast generation in optical coherence tomography from a localized shear stress

A technique for generating contrast in two-dimensional shear strain elastograms from a localized stress is presented. The technique involves generating a non-uniform, localized stress via a magnetically actuated implant. Its effectiveness is demonstrated using finite-element simulations and a phantom study provides experimental verification of this. The method is applied to a superficial cancerous lesion model represented as a stiff inclusion in normal tissue. The lesion was best distinguished from its surroundings using total shear strain elastograms, rather than individual strain components. In experimental phantom studies, the lesion was imaged using optical coherence tomography (OCT) and could still be distinguished in elastograms when not readily identifiable in standard OCT images.

The modulation transfer function of an optical coherence tomography imaging system in turbid media

In this paper we describe measurements of the contrast transfer function, modulation transfer function and point-spread function of an optical coherence tomography (OCT) imaging system through scattering layers having a dimension-less scattering depth over the range 0.2-6.9. The results were found to be insensitive to scattering density, indicating that these measurement parameters alone do not well characterize the practical imaging ability of an OCT instrument. Attenuation and increased noise floor due to optical scattering were found to be the primary imaging limit and the effect of multiple scattering on OCT resolution was negligible.

Passivation of Zinc Oxide Nanowires for Improved Piezoelectric Energy Harvesting Devices

This paper evaluates the improvement in performance of ZnO nanowires energy harvesters using p-type copper thiocyanate (CuSCN) passivation. Two types of p-n junction based devices: ZnO/PEDOT:PSS (poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate)) and ZnO/CuSCN/PEDOT:PSS were fabricated. It was observed that, the passivation of nanowires using CuSCN improved the performance four times, yielding a peak power density of 303 μWcm−2 for a load of 3.54 kΩ. The results were supported by impedance analysis of each device and it was observed that the piezoelectric voltage output of the device depends on its RC time constant.

Voltage control of the magnetic coercive field: Multiferroic coupling or artifact?

The ability to dynamically tune the coercive field of magnetic thin films is a powerful tool for applications, including in magnetic recording disk technologies. Recently, a number of papers have reported the electrical voltage control of the coercive field of various magnetic thin films in multiferroic composites. Theoretically, this is possible in magneto-electric (ME) multiferroics due to the piezoferroelectric component that can be electrically activated to dynamically modify the properties of the magnetic component of the composite via a direct or strain mediated ME coupling. In this paper we fabricated and examined such structures and we determined that the magnetic coercive field reduction is most likely due to a heating effect. We concluded that this effect is probably an artifact that cannot be attributed to a multiferroic coupling.

Measurement of the Three-Dimensional Point-Spread Function in an Optical Coherence Tomography Imaging System

Two significant figures of merit for optical coherence tomography (OCT) systems are the axial and transverse resolutions. Transverse resolution has been defined using the Rayleigh Criterion or from Gaussian beam optics. The axial resolution is generally defined in terms of the coherence length of a Gaussian shaped source. Whilst these definitions provide a useful mathematical reference they are somewhat abstracted from the three dimensional resolution that is encountered under practical imaging conditions. Therefore, we have developed a three-dimensional resolution target and measurement methodology that can be used to calibrate the three-dimensional resolution of OCT systems.

Variations in optical coherence tomography resolution and uniformity: a multi-system performance comparison

Point spread function (PSF) phantoms based on unstructured distributions of sub-resolution particles in a transparent matrix have been demonstrated as a useful tool for evaluating resolution and its spatial variation across image volumes in optical coherence tomography (OCT) systems. Measurements based on PSF phantoms have the potential to become a standard test method for consistent, objective and quantitative inter-comparison of OCT system performance. Towards this end, we have evaluated three PSF phantoms and investigated their ability to compare the performance of four OCT systems. The phantoms are based on 260-nm-diameter gold nanoshells, 400-nm-diameter iron oxide particles and 1.5-micron-diameter silica particles. The OCT systems included spectral-domain and swept source systems in free-beam geometries as well as a time-domain system in both free-beam and fiberoptic probe geometries. Results indicated that iron oxide particles and gold nanoshells were most effective for measuring spatial variations in the magnitude and shape of PSFs across the image volume. The intensity of individual particles was also used to evaluate spatial variations in signal intensity uniformity. Significant system-to-system differences in resolution and signal intensity and their spatial variation were readily quantified. The phantoms proved useful for identification and characterization of irregularities such as astigmatism. Our multi-system results provide evidence of the practical utility of PSF-phantom-based test methods for quantitative inter-comparison of OCT system resolution and signal uniformity.