Experimental Investigation of the Grain Noise in Interferometric Detection of Ultrasonic Waves

Besides their other obvious advantages over conventional ultrasonic sensors, laser interferometers offer optical diffraction limited apertures that are far smaller than the acoustic wavelength in the specimen under inspection. This unique feature can be exploited for the purposes of super-resolution near-field acoustic microscopy, which detects the rapidly decaying evanescent vibrations produced by surface and near-surface scatterers such as small fatigue cracks, pores, anomalous grains, etc. However, higher resolution also means higher sensitivity to inherent microscopic material inhomoge-neities. In this paper, experimental results are presented for the incoherent material noise in 2024 aluminum and Ti-6Al-4V titanium alloys at two different nominal frequencies of 5 and 10 MHz. It is shown that the incoherent grain noise significantly increases as the illuminated spot size decreases. Above the acoustic wavelength, the observed phenomenon is mainly due to the increasing sensitivity of the receiver to propagating scattered waves generated in the interior of the specimen. Below the acoustic wavelength, the further increasing material noise is mainly due to evanescent vibrations caused by nearby scatterers.     

Laser excitation through fiber optics for NDE

This paper describes experiments designed to generate acoustic waves by using a laser pulse, transmitted through fiber optics, to thermally shock the surface of a steel specimen. The purpose of this effort was to explore the noncontacting generation of Rayleigh surface waves appropriate to the interrogation of structures for the detection of subcritical defects, with the ultimate goal of developing an efficient laser-based nondestructive evaluation technique utilizing flexible fiber optics.     

Imaging in diffuse media with pulsed-ultrasound-modulated light and the photorefractive effect

Acousto-optic imaging in diffuse media is a dual wave-sensing technique in which an acoustic field interacts with multiply scattered laser light. The acoustic field causes a phase modulation in the optical field emanating from the interaction region, and this phase-modulated optical field carries with it information about the local optomechanical properties of the media. We report on the use of a pulsed ultrasound transducer to modulate the optical field and the use of a photorefractive-crystal-based interferometry system to detect ultrasound-modulated light. The use of short pulses of focused ultrasound allows for a one-dimensional acousto-optic image to be obtained along the transducer axis from a single, time-averaged acousto-optic signal. The axial and lateral resolutions of the system are controlled by the spatial pulse length and width of the ultrasound beam, respectively. In addition, scanning the ultrasound transducer in one dimension yields two-dimensional images of optical inhomogeneities buried in turbid media.     

Optical piezoelectric transducer for nano-ultrasonics

Piezoelectric semiconductor strained layers can be treated as piezoelectric transducers to generate nanometer-wavelength and THz-frequency acoustic waves. The mechanism of nano-acoustic wave (NAW) generation in strained piezoelectric layers, induced by femtosecond optical pulses, can be modeled by a macroscopic elastic continuum theory. The optical absorption change of the strained layers modulated by NAW through quantum-confined Franz-Keldysh (QCFK) effects allows optical detection of the propagating NAW. Based on these piezoelectric-based optical principles, we have designed an optical piezoelectric transducer (OPT) to generate NAW. The optically generated NAW is then applied to one-dimensional (1-D) ultrasonic scan for thickness measurement, which is the first step toward multidimensional nano-ultrasonic imaging. By launching a NAW pulse and resolving the returned acoustic echo signal with femtosecond optical pulses, the thickness of the studied layer can be measured with <1 nm resolution. This nano-structured OPT technique will provide the key toward the realization of nano-ultrasonics, which is analogous to the typical ultrasonic techniques but in a nanometer scale.     

Probing thermomechanics at the nanoscale: impulsively excited pseudosurface acoustic waves in hypersonic phononic crystals

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system's initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system's excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.     

Thermal effects in scanning acoustic microscopy for fine resolutionapplications

A novel scanning acoustic microscopy technique for achieving high resolution acoustic images by employing thermal effects and image subtraction has been studied and demonstrated. Experiments were performed on a perspex block patterned with a machined grid on the reverse surface, and on a buried channel in similar material. If was found that using the image subtraction technique, short periods of sample heating can lead to a stronger pattern selectivity, because of the strong temperature dependency of the elastic parameters of the polymer. In previous SAM techniques improvement in signal has been achieved through the use of special liquids as acoustic coupling media between the acoustic lens and the sample. The reported technique retains water as the coupling medium and the acoustic impedance matching is performed by varying the elastic parameters of the sample itself through direct heating. The temperature increase in the sample decreases the velocity of propagation of acoustic waves in the solid, and brings the acoustic impedance close to that of water. A theoretical model, including expressions for the acoustic aberrations, depth dependence and acoustic impedance matching has been derived. Examples of the results obtained are presented     

Spatial manipulation of nanoacoustic waves with nanoscale spot sizes

Coherent acoustic phonons are generated at terahertz frequencies when semiconductor quantum-well nanostructures are illuminated by femtosecond laser pulses. These phonons-also known as nanoacoustic waves-typically have wavelengths of tens of nanometres, which could prove useful in applications such as non-invasive ultrasonic imaging and sound amplification by the stimulated emission of radiation. However, optical diffraction effects mean that the nanoacoustic waves are produced with spot sizes on the micrometre scale. Near-field optical techniques can produce waves with smaller spot sizes, but they only work near surfaces. Here, we show that a far-field optical technique--which suffers no such restrictions--can be used to spatially manipulate the phonon generation process so that nanoacoustic waves are emitted with lateral dimensions that are much smaller than the laser wavelength. We demonstrate that nanoacoustic waves with wavelengths and spot sizes of the order of 10 nm and 100 nm, respectively, can be generated and detected.     

Partially coherent lensfree tomographic microscopy [Invited

Optical sectioning of biological specimens provides detailed volumetric information regarding their internal structure. To provide a complementary approach to existing three-dimensional (3D) microscopy modalities, we have recently demonstrated lensfree optical tomography that offers high-throughput imaging within a compact and simple platform. In this approach, in-line holograms of objects at different angles of partially coherent illumination are recorded using a digital sensor-array, which enables computing pixel super-resolved tomographic images of the specimen. This imaging modality, which forms the focus of this review, offers micrometer-scale 3D resolution over large imaging volumes of, for example, 10-15 mm(3), and can be assembled in light weight and compact architectures. Therefore, lensfree optical tomography might be particularly useful for lab-on-a-chip applications as well as for microscopy needs in resource-limited settings.     

Determination of the optical absorption coefficient via analysis oflaser-generated plate waves

A technique has been demonstrated which measures the optical absorption coefficient in weakly to moderately absorbing homogeneous isotropic elastic plates. This technique involves generating plate waves with a pulsed Nd:YAG laser and then detecting the disturbances in the acoustic far field where the majority of the acoustic energy has been coupled into the first symmetric, s_o, and antisymmetric, a _o, plate modes. The optical absorption coefficient is determined by comparing the experimental and theoretical expression for the ratio of the integrated power spectra of the s_o and a_o modes     

Application of the Schlieren pulsed method for the observation of simple and multiple scattering of ultrasonic waves

The Schlieren pulsed method uses short-term lighting triggered by an acoustic pulse. This allows for an observation of elastic deformation fields in pulsed regime and for an evaluation of the evolution of the pulse in the interior of homogenous and heterogeneous media. In this paper we apply the Schlieren pulsed method to determine the conditions of the change from simple to multiple scattering. An ultrasound transducer put in water emits a wide-band pulse. The pulse goes through a region of parallel cylindrical wires, uniformly spaced and perpendicular to the acoustic beam. Varying the width of the zone of the scatterer cylinders, it is possible to optically observe the transmitted acoustic field and to calculate the transmission coefficient. By studying the behavior of this coefficient in function of the medium width, we obtained the zone in which the transition between simple and multiple scattering happens.     

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70?cm(-1) were clearly resolved. Single laser-based tunable excitation may greatly simplify CRS measurements and extend the practicality of CRS microscopy.     

AFM observation of surface acoustic waves emitted from single symmetric SAW transducers

We report the first experimental observation of surface acoustic waves (SAWs) launched from a single symmetric SAW transducer, employing scanning acoustic force microscopy (SAFM). SAFM is a simple technique for the imaging of complex interdigital transducer (IDT) radiation patterns with nanometer lateral resolution. We demonstrate submicron lateral resolution and high sensitivity by investigating a single excitation element on a weakly coupling substrate (GaAs), visualizing the launched wave and second-order effects     

Photoacoustic holographic imaging of absorbers embedded in silicone

Light absorbing objects embedded in silicone have been imaged using photoacoustic digital holography. The photoacoustic waves were generated using a pulsed Nd:YAG laser, λ=1064 nm, and pulse length=12 ns. When the waves reached the silicone surface, they were measured optically along a line using a scanning laser vibrometer. The acoustic waves were then digitally reconstructed using a holographic algorithm. The laser vibrometer is proven to be sensitive enough to measure the surface velocity due to photoacoustic waves generated from laser pulses with a fluence allowed for human tissue. It is also shown that combining digital holographic reconstructions for different acoustic wavelengths provides images with suppressed noise and improved depth resolution. The objects are imaged at a depth of 16.5 mm with a depth resolution of 0.5 mm.     

A high-frequency, 2-D array element using thermoelastic expansion in PDMS

Optical generation of ultrasound is a promising alternative to piezoelectricity for high-frequency arrays. An array element is defined by the size and location of a laser beam focused on a suitable surface. Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated an increase in conversion efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. Radiation pattern measurements with an 85 MHz spherically focused transducer indicated an array element size of 20 /spl mu/m. These measurements lacked the spatial resolution required to reveal fine details in the radiated acoustic field. Here we report radiation pattern measurements with a 5-/spl mu/m spatial sampling, showing that the radiated acoustic field is degraded by leaky Rayleigh waves launched from the PDMS/glass interface. We demonstrate that replacing the glass with a clear PDMS substrate eliminates the leaky Rayleigh waves, producing a broad and smooth radiation pattern suitable for a two-dimensional (2-D) phased array operating at frequencies greater than 50 MHz.     

Characterization of heterogeneous matrix composites using scanning acoustic microscopy

Acoustic microscopy was used to examine the morphology of multi-phase matrices and composites. The acoustic microscopy imaging could easily resolve the rubber domains dispersed within a thermosetting or thermoplastic continuous phase. However, because the thermoplastic and thermosetting phase domains had comparable elastic moduli, the resolution between them was not always clear. Rayleigh wave distortion of imaging remained as one of the serious limitations that needed to be overcome in order for this technique to be widely utilized in heterogeneous/anisotropic media. In its present form, the acoustic imaging technique can be used to augment other existing analytical tools in order to generate more detailed morphological information that is useful in understanding structure-property relationships for multi-phase toughened matrices used in advanced composites.     

Automated, portable, low-cost bright-field and fluorescence microscope with autofocus and autoscanning capabilities

Optical microscopy is a simple, yet essential, imaging technology. Conventional laboratory-grade optical microscopes are bulky and costly, confining their use to within laboratory settings and restricting their accessibility in regions of limited resources. With the aim of overcoming these limitations, we have realized a portable, low-cost, and highly automated optical microscope that integrates mass-manufactured components, including light-emitting diodes, a web camera, optical disk drives, and a microcontroller. Our implementation is capable of bright-field and fluorescence imaging with micrometer-scale resolution and controlled mechanical actuation of both the lens and sample. We interface the lighting, image capture, and mechanical actuators of the microscope into a single software environment, enabling automation of common microscope operations, such as image focusing and large-area sample visualization. Combination of mechanical actuation and software automation into a compact, low-cost microscope system is an important initial step toward the goal of making optical microscopy universally accessible, portable, and easy to use.     

Uncovering the hidden content of layered documents by means of photoacoustic imaging

We introduce a novel diagnostic approach for the uncovering of hidden content in layered documents, based on a photoacoustic imaging set‐up with optimised acoustic detection parameters. By exploiting the laser‐induced ultrasound following the intense absorption of pulsed optical radiation by printed ink, this traditional biomedical imaging method is proven capable of revealing text characters buried under several layers of stacked paper sheets at high spatial resolution and excellent contrast levels. Such a remarkable performance benefits from the effective combination of strong light scattering by paper fibrils and the virtually unobstructed propagation of the generated ultrasonic waves through multiple layers, enabling thus the accurate recording of text, even in double‐sided prints, with minimum shadowing artefacts. The imaging effectiveness, the implementation simplicity, the robustness, and the relatively low‐cost features offered by the proposed photoacoustic modality are anticipated to contribute in its wide adoption by the preservation of cultural heritage community, as a powerful diagnostic tool for the in‐depth investigation of complex multilayered objects.     

Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.     

In situ generation of surface plasmon polaritons using a near-infrared laser diode

We demonstrate a semiconductor laser-based approach which enables plasmonic active devices in the telecom wavelength range. We show that optimized laser structures based on tensile-strained InGaAlAs quantum wells-coupled to integrated metallic patternings-enable surface plasmon generation in an electrically driven compact device. Experimental evidence of surface plasmon generation is obtained with the slit-doublet experiment in the near-field, using near-field scanning optical microscopy measurements.     

Remote laser generation of narrow-band surface waves through optical fibers

This paper demonstrates the use of a fiberoptic bundle for flexible, compact, remote, and noncontact laser generation of surface ultrasonic waves in materials. The bundle is able to deliver Nd:YAG pulsed light with a 60% delivery efficiency up to an average energy of 55 mJ/pulse for a pulse duration on the order of 10 ns and a pulse repetition rate of 20 Hz without signs of fiber damage. Details of the bundle construction and surface preparation are given, and pulsed light delivery tests performed with single tapered fibers are discussed. The high-power light delivery capabilities of the bundle are demonstrated for the generation of narrow-band surface waves in a Carbon/PEEK composite laminate by a spatial modulation technique that employs a periodic transmission mask. Single laser pulse ultrasonic tonebursts are clearly detectable using a small aperture piezoelectric transducer while ensuring thermoelastic generation conditions. The theory of narrow-band generation of surface acoustic waves is improved by accounting for the strength nonuniformity of the illumination sources. In addition, the effect of the number of illumination sources on the bandwidth of the generated surface wave is assessed experimentally, and excellent agreement is shown with the theoretical results predicted by the improved model.