# Morning session

Cheap Optical Transducers (CHOTs) for in-situ application and ultrasonic testing
Victoriya Ageeva
Applied Optics, Faculty of Engineering, University of Nottingham
Cheap optical transducers (CHOTs) are patterns on the surface of a component activated by lasers to generate and detect ultrasound. Excited optically, with minimal surface impact, and fully customisable, CHOTs provide a simple alternative to conventional piezoelectric transducers, offering wireless, remote operation. Of particular interest is application of CHOTs for in-situ ultrasonic inspection of hard-to reach and complex-geometry components such as those of aero-engines. Endoscopic CHOT pulser uses optical fibres to provide flexible instrumentation for testing of components with complex access paths. It is complemented by the development of the alternative CHOT fabrication methods to allow application of sensors in-situ, onto large and curved parts, as well as those already in service, challenging for laboratory-limited methods. The g-CHOTs (used for generation of surface acoustic wave) produced on film carriers to allow transfer of the transducer and application onto components in-situ are presented, including fabrication and functionality test results.

Analysis of the fundamental torsional guided wave field within a hollow cylinder using a 3D-FFT
Keith Thornicroft[1,2], Alex Haig[2], Cristinel Mares[1] and Peter Mudge[2]
[1] School of Engineering and Design, Brunel University. [2] TWI Ltd, Granta Park, Great Abington, Cambridge, CB21 6AL.
The field of guided wave testing (GWT) is a relatively new development within the non-destructive evaluation sector. The market for this technology has been driven by pipeline operators who have a need to screen for corrosion and degradation of their pipeline. In order to improve the detection capability of the guided wave tooling is it important to be able to optimise the output of the transducer array. This paper describes an experimental procedure which was implemented to quantify the output of a circumferentially distributed transducer array. Using a scanning laser vibrometer it was possible to sample an area of the pipe away from the transducer array and measure the displacement behaviour at each discrete point. With vibrometry information gained from both circumferential and axial positions over a range of frequencies, a 3D-FFT technique was developed that reveals circumferential mode order and axial wavenumber both of which are dependent upon frequency. The resultant plot shows the contribution of wave modes generated by the circumferentially distributed transducer array as received by the vibrometer.

Spatially resolved acoustic spectroscopy: a laser ultrasonic technique for materials characterisation
Wenqi Li, Richard Smith, Jethro Coulson, Matt Clark, Mike Somekh and Steve Sharples
Applied Optics, Faculty of Engineering, University of Nottingham
Material characteristics such as strength, stiffness and fracture resistance are strongly related to the underlying microstructure. In order to predict the mechanical behaviour of industrial materials such as titanium, nickel and their alloys, detailed knowledge about their texture is required. A robust measurement tool is introduced which can be used to determine the crystallographic orientation of a material.

Recent progress in laser ultrasonic multi-dectectors receivers for non destructing evaluation
M Messaoudi, B Pouet, S Breugnot
Bossa Nova Technologies, 11922 Jefferson Boulevard, Culver City, CA 90230
New laser interferometric schemes were recently introduced and improved in order to fully take advantage of possibilities offered by laser-based ultrasonic (LBU) inspection and to broaden their integration into industrial inspection systems and into laboratories setups. We will present the recent advances on a first LBU receiver based on multi-channel random-quadrature (MCRQ) detection. The MCRQ receiver is fiberized and well suited for industrial application such as thickness control on moving sample using zero group velocity Lamb wave resonance. The second receiver is based on two-wave mixing in photorefractive crystal and it has been improved to be able to measure simultaneously in-plane and outof-plane displacements. Thermoelastic generation on aluminium sample experiment enhances difference between both components, which opens up new prospects for ultrasound studies in the lab. Keyword Speckle, ultrasound, quadrature, two-wave mixing, interferometer, laser.

SKED: Speckle Knife Edge Detector
Samuel O Achamfuo-Yeboah, R A Light and S D Sharples
Applied Optics, Faculty of Engineering, University of Nottingham
The optical detection of ultrasound from optically rough surfaces is severely limited using a conventional setup because the detected light is speckled. We present a CMOS integrated circuit that can detect ultrasound in the presence of speckle. The detector circuit is based on the simple knife edge detector. It is self-adapting and is fast, inexpensive, compact and robust. The detector is implemented as a widefield camera with 32x32 smart pixels. We present the theory of its operation and discuss results validating the concept and operation of the device. We will also present results that show that it can work with optically rough surface finishes that have roughness (Ra) up to 2$\mu$m.

Quantified visualisation of elastic wave phenomena using refracto-vibrometry.
Rob Malkin
University of Bristol
This presentation will discuss time resolved quantitative evaluation of elastic stress waves in solid media by utilising an adaptation of the well-established laser Doppler vibrometry method. We show that the introduction of elastic stress waves in a transparent medium gives rise to detectable and quantifiable changes in the refractive index, which is proportional to stress. The method is tested for mechanical excitation at a range of frequencies in an acrylic bar. This refractometric quantification can measure internal strains as low as 1x10$^{-11}$. Additionally, finite element analysis is used to gauge the validity of the results. We also discuss the effect of signal integration along the laser path length. In the presented work an acrylic bar is used, this method however should be applicable to any transparent solid.

Laser-based ultrasonic characterisation of Ge membranes
Oksana Trushkevych, Vishal A Shah, Maksym Myronov, John E Halpin, Stephen D Rhead, Martin J Prest, David R Leadley, and Rachel S Edwards
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
Piezoelectric transducers and laser interferometry are used to study vibrations of a 700 nm thick and 965$\times$965 µm square single crystal Ge membrane, in air and in vacuum. Ge on Si substrate is a basis for many optoelectronic devices, including sensors, photonic modulators, solar cells,and heterojunction bipolar transistors. Mechanically decoupling the Ge film from the Si substrate, producing a Ge membrane, can lead to a more rapid and higher sensitivity response in sensors, and allows adding new functionality to integrated systems. It is important to study the robustness to shock and the elastic behaviour of the membrane before building devices, and laser interferometry is an ideal method for these measurements. Resonance modes up to 3:2 of the membrane are studied in detail both in the frequency and the spatial domain, and residual stress of the membrane, as well as the quality factors of modes at various pressures, are extracted.

# Afternoon sessions

Resonant excitation of Brillouin scattering with a single nanoparticle as an opto-acoustic transducer
Yannick Guillet[1], Feng Xu[1], Salvatore Minissale[1], Serge Ravaine[2] and Bertrand Audoin[1]
[1] Université de Bordeaux, CNRS, UMR 5295, Talence F-33405 [2] Centre de Recherche Paul Pascal, CNRS, UPR 8641, Pessac F-33600, France
We demonstrate the detection of the GHz coherent phonons generated by an optically excited single nanoparticle embedded inside a polymer thin film. We use a picosecond ultrasonics setup relying on a common femtosecond pump-probe scheme. We detect not only the breathing mode of the nanoparticle but also the coherent phonons propagating close to the nanoparticle. The latter are detected through the photo-elastic interaction. We demonstrate that the detected Brillouin scattering magnitude is enhanced when the fundamental breathing mode frequency of the nanoparticle nearly matches the Brillouin frequency of the surrounding medium at the probe laser wavelength.

Picosecond ultrasonics for single-cell biology
Thomas Dehoux[1,2], Maroun Abi Ghanem[1,2], Atef Gadalla[1,2], Omar F. Zouani[3,4], Marie-Christine Durrieu [3,4], Bertrand Audoin[1,2] [1] Université de Bordeaux, CNRS, UMR 5295, Talence F-33405, [2] CNRS, I2M, UMR 5295, F-33400 Talence, France, [3] Université de Bordeaux, CBMN, UMR CNRS 5248, F-33607 Pessac, France, [4]Université de Bordeaux, LOMA, CNRS UMR 5798, F-33400 Talence, France

In this paper we use GHz acoustic phonons to probe the mechanical properties of single cells. We culture cells on top of a biocompatible Ti metal film. Low-energy femtosecond laser pulses are focused at the bottom of the film to a micron spot to allow single-cell investigation. The subsequent ultrafast thermal expansion launches a longitudinal acoustic pulse in Ti, with a broad spectrum extending up to 200 GHz. The acoustic pulse is transmitted to the cell owing to the cell-Ti intimate contact.

The phonon propagation in the cell is measured remotely with an ultrafast laser probe through Brillouin light scattering. This yields a direct measurement of the local stiffness and viscosity of cells. Simultaneously, the acoustic reflection coefficient at the Ti-cell interface is measured through the transient optical reflectance changes. This innovative technique offers a unique mean to investigate quantitatively cell-biomaterial interactions without fluorescent labels or mechanical contact to the cell.

Optically excited and optically probed high frequency acoustic transducers
Richard Smith, Fernando Perez, Leo Marques and Matt Clark
Applied Optics, Faculty of Engineering, University of Nottingham
The development of nanometre sized ultrasonic transducers is important for both industrial and biological applications. The small size can be important in its own right or necessary in order to generate acoustic waves with nanometric wavelengths. There are a number of potential applications of these small size transducers; ranging from sub optical wavelength acoustic imaging to embedded sensors.

Detection of coherent single-pass amplification of sub-Terahertz acoustic waves
C L Poyser, A V Akimov, R P Campion and A J Kent
School of Physics and Astronomy, University of Nottingham
We describe the use of an AlGaAs p-i-n photodiode to monitor the output of a single pass acoustic amplification, SASER, device. In the current scheme, the p-i-n detector is fabricated on one side of a 150um GaAs substrate, and two GaAs/AlAs superlattices (SLs), the lower of which can be placed under an electrical bias, are grown on the other side. The lower, SASER, SL was grown to specifications which have been previously shown, using an incoherent bolometric detection technique, to provide phonon amplification [1]. Femtosecond optical pumping of the top SL generates a quasi-monochromatic sub-Terahertz acoustic wave which propagates through the gain SL and the substrate to the p-i-n diode. This is gated by a time-delayed femtosecond pulse providing high resolution coherent detection. Evidence of coherent amplification in the SASER device was observed.

Brillouin imaging of cultured cells using picosecond ultrasonics
Fernando Perez-Cota, Richard Smith, Kevin Webb and Matt Clark
Applied Optics, Faculty of Engineering, University of Nottingham
Ultrasound is a well-established method for the mechanical characterisation and imaging of biological tissue. At high frequencies ultrasound has the potential to offer higher than optical resolution images which could provide useful mechanical information for cell biology studies. This has inspired research on using high frequency ultrasound on tissue and cells. Picosecond laser ultrasound, uses pulsed lasers to detect Brillouin oscillations and has reached important achievements in recent years; measurements on individual living cells [1] as well as characterisation of mechanical properties on fixed cells[2]. The work we present will discuss an alternative transducer arrangement [3] applied to this technique, which is used to reduce the laser light exposure to the cells. This combined with an ASOPS instrument to reduce acquisition time, allows the measurement of 2D images of the Brillouin frequency of mammalian fixed cells to be performed in a reasonable time frame.

Extending the limited aperture of all-optical planar sensor arrays used for photoacoustic imaging system.
R Ellwood, E Z Zhang, P C Beard, and B T Cox
Department of Medical Physics & Bioengineering, University College London
In recent years, a range of ultrasonic sensor configurations have been developed to record photoacoustic (PA) signals. The Fabry-Perot (FP) polymer film sensor benefits from being able to detect using small element sizes (10’s$\mu$m) with a low noise equivalent pressure ($\sim$0.21kPa). The small element size gives benefits in terms of lateral resolution in the final image. FP sensor arrays are typically designed to be planar, for simplicity of manufacture and interrogation. However, planar sensors have a limited view of the acoustic field, so some of the waves emitted from the PA source are not recorded. The lack of a complete data set results in artifacts in the reconstructed image. Removal of these artefacts is important for image quality, and in particular for quantitative multiwavelength photoacoustic imaging, as the artefacts will change with wavelength.
Quantitative ultrasound-modulated optical tomography: forward models, reconstruction, and challenges
Samuel Powell
Dept. of Computer Science, University College London, London, WC1E 6BT
Ultrasound-modulated optical tomography (UOT) is a hybrid technique which aims to recover images of the optical absorption and scattering coefficients of biological media by combining the contrast offered by near infra-red light with the spatial resolution of focussed or time-gated ultrasound fields.

Theoretical and experimental evaluation of fluorophore-labeled microbubble system
Qimei Zhang[1], Melissa L Mather[1,2], Bowen Tian[3] and Stephen P Morgan[1]
[1] Electrical Systems and Optics Research Division, Faculty of Engineering, University of Nottingham, UK [2] Institute of Biophysics, Imaging and Optical Sciences, Faculty of Engineering, University of Nottingham, UK [3] School of Pharmacy, University of Nottingham
Ultrasound modulated fluorescence tomography (USMFT) has potential to be a useful technique to obtain fluorescence images with optical contrast and US resolution in deep tissue. However, the very low modulation depth due to the intrinsic incoherent properties of fluorescence leads to low signal-to-noise ratios (SNR) and poor image contrast. To enhance the modulation depth, microbubbles (MBs), which are used regularly in diagnostic ultrasound (US) as contrast agents, can be used. At an appropriate concentration of fluorophore the MBs self-quench. With application of US the oscillation of microbubbles changes the intermolecular spacing of the fluorophores on the MB surface and this leads to a higher modulation of fluorescence signal. This enables localised fluorescence measurements in thick tissue to be performed. Here we focus specifically on the characteristics of fluorophore-labelled MBs and identify theoretically the optimised MB size, fluorophore concentration and US pressure to obtain the highest modulation depth. Some initial experimental results using a light scattering approach will also be presented.