Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part I: Materials, devices and selected applications.

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part I of this two part review, firstly, active and passive nanomaterials and nanostructures for acoustics are presented. Following that, representative applications of nanoacoustics including material property characterization, nanomaterial/nanostructure manipulation, and sensing, are discussed in detail. Finally, a summary is presented with point of views on the current challenges and potential solutions in this burgeoning field.

Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part II: Nanoacoustics for biomedical imaging and therapy.

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part II of this two-part review, this paper concentrates on nanoacoustics in biomedical imaging and therapy applications, including molecular ultrasound imaging, photoacoustic imaging, ultrasound-mediated drug delivery and therapy, and photoacoustic drug delivery and therapy. Firstly, the recent developments of nanosized ultrasound and photoacoustic contrast agents as well as their various imaging applications are examined. Secondly, different types of nanomaterials/nanostructures as nanocarriers for ultrasound and photoacoustic therapies are discussed. Finally, a discussion of challenges and future research directions are provided for nanoacoustics in medical imaging and therapy.

Supercontinuum-laser diffuse reflectance spectroscopy in conjunction with an extended Kubelka-Munk model-a methodology for determination of temperature-dependent quantum efficiency in highly scattering and fluorescent media.

Temperature-dependent diffuse reflectance measurements on Cr-doped α-alumina monoliths have been performed using supercontinuum-laser illumination and CO2-laser heating. These measurements have been interpreted using an extended Kubelka-Munk (K-M) model describing diffuse-light propagation in highly scattering and fluorescent media to assess the temperature dependence of fluorescence quantum efficiency. Analysis of experimental results has provided a qualitative understanding of the temperature-dependent conditions for model applicability and also suggests methods for using supercontinuum-laser diffuse reflectance spectroscopy for detection of unknown fluorescent dopants.

Experimental characterization and physics-based modeling of the temperature-dependent diffuse reflectance of plasma-sprayed Nd2Zr2O7 in the near to short-wave infrared.

Supercontinuum-laser illumination in conjunction with CO2-laser heating has been implemented to measure the near to short-wave infrared (970-1660 nm) diffuse reflectance of plasma-sprayed Nd2Zr2O7 as a function of temperature. Owing to the broadband nature of this experimental technique, the diffuse reflectance of plasma-sprayed Nd2Zr2O7 has been measured at many wavelengths and has been shown to decrease with increasing temperature. A physics-based model for diffuse reflectance predicated on the crystal/electronic band structure of highly scattering semiconductor materials has been constructed to interpret the results of these measurements. Baseline materials characterization has also been performed to assist in the development of crystal/electronic band structure-optical property relationships that could be useful for the design of next-generation environmental barrier coatings. This characterization has included ambient and non-ambient x-ray diffraction as well as room-temperature, integrating-sphere diffuse reflectance spectroscopy.

Palladium nanoparticle formation processes in fluoropolymers by thermal decomposition of organometallic precursors.

Palladium nanoparticles were synthesized directly in solid fluoropolymer films by thermal decomposition of a palladium acetylacetonate precursor molecularly infused in the fluoropolymer matrix. This chemical infusion synthesis technique was studied using transmission electron microscopy along with selective area electron diffraction to gain insight into the nucleation and growth of palladium nanoparticles. Formation of palladium nanoparticles can be correlated with defects in the polymer matrix as well as their associated free volume such that a relationship between average particle size and mean free volume fraction can be constructed. At low processing temperatures, the average particle radius increases monotonically with the processing time but more complicated variations occur for longer times. The growth of nanoparticles was interpreted using a modified diffusion-limited growth model. While nearly monodisperse nanoparticles dispersed throughout the polymer volume were obtained at low processing temperatures, surface percolation of nanoparticles was observed at relatively high temperatures owing to high precursor decomposition and diffusion rates.

Entanglement-Based Thermoplastic Shape Memory Polymeric Particles with Photothermal Actuation for Biomedical Applications.

Triggering shape-memory functionality under clinical hyperthermia temperatures could enable the control and actuation of shape-memory systems in clinical practice. For this purpose, we developed light-inducible shape-memory microparticles composed of a poly(d,l-lactic acid) (PDLLA) matrix encapsulating gold nanoparticles (Au@PDLLA hybrid microparticles). This shape-memory polymeric system for the first time demonstrates the capability of maintaining an anisotropic shape at body temperature with triggered shape-memory effect back to a spherical shape at a narrow temperature range above body temperature with a proper shape recovery speed (37 < T < 45 °C). We applied a modified film-stretching processing method with carefully controlled stretching temperature to enable shape memory and anisotropy in these micron-sized particles. Accordingly, we achieved purely entanglement-based shape-memory response without chemical cross-links in the miniaturized shape-memory system. Furthermore, these shape-memory microparticles exhibited light-induced spatiotemporal control of their shape recovery using a laser to trigger the photothermal heating of doped gold nanoparticles. This shape-memory system is composed of biocompatible components and exhibits spatiotemporal controllability of its properties, demonstrating a potential for various biomedical applications, such as tuning macrophage phagocytosis as demonstrated in this study.

Temperature-dependent diffuse reflectance spectroscopy of plasma-sprayed Cr-doped α-alumina using supercontinuum laser illumination and CO2 laser heating.

This study presents results for the high-temperature (up to 1550 K) optical properties of polycrystalline Cr-doped α-alumina materials. Diffuse reflectance spectra in the wavelength range of 510-840 nm are presented as a function of temperature to illustrate changes to the optical behavior of these materials including a previously unreported thermally activated splitting of the U-band absorption (A24→T24) in octahedrally coordinated Cr3+. Measurements were made using a unique laser-based approach for high-temperature solid-state spectroscopy, involving front-side supercontinuum laser illumination and back-side CO2 laser heating. This approach required development of samples that could withstand related thermal stresses, and measurements were made on plasma-sprayed, Cr-doped α-alumina monoliths. Measured spectra are interpreted, in part, using published optical spectra for ruby; agreement between results here with those obtained using more traditional methods serves to validate the measurement methods used for this work.

Laser-induced, in situ, nanoparticle shell synthesis in polymer matrix nanocomposites.

This work investigates processes involved in the patterning and production of structured nanoparticles in polymer matrix nanocomposites using femtosecond laser irradiation. An in situ, chemical vapor deposition process was used to synthesize silver nanoparticles in the bulk of an optically transparent polytetrafluoroethylene-co-hexafluoropropylene polymer matrix. The strong optical absorption at the surface plasmon resonance frequency was used to selectively irradiate and photothermally heat the material near particles using femtosecond laser pulses. Having species for chemical vapor deposition in the near-particle environment allows for localized decomposition of the precursor species via unimolecular reactions. Decomposition products can subsequently participate in the production of a variety of core-shell nanostructures. The overall process is demonstrated using femtosecond, photothermal heating of silver nanoparticles to decompose tungsten hexacarbonyl in the polymer matrix leading to the formation of tungsten oxide shells surrounding the silver. For this system, a 40 nm red shift of the surface plasmon resonance was measured. Control of the spatial and temporal characteristics of the excitation source allows for synthesis of nanocomposites with a high degree of control over the location, composition and size of nanoparticles in a polymer matrix resulting in patterned materials with continuously variable properties.

High-spatial-resolution sub-surface imaging using a laser-based acoustic microscopy technique.

Scanning acoustic microscopy techniques operating at frequencies in the gigahertz range are suitable for the elastic characterization and interior imaging of solid media with micrometer-scale spatial resolution. Acoustic wave propagation at these frequencies is strongly limited by energy losses, particularly from attenuation in the coupling media used to transmit ultrasound to a specimen, leading to a decrease in the depth in a specimen that can be interrogated. In this work, a laser-based acoustic microscopy technique is presented that uses a pulsed laser source for the generation of broadband acoustic waves and an optical interferometer for detection. The use of a 900-ps microchip pulsed laser facilitates the generation of acoustic waves with frequencies extending up to 1 GHz which allows for the resolution of micrometer-scale features in a specimen. Furthermore, the combination of optical generation and detection approaches eliminates the use of an ultrasonic coupling medium, and allows for elastic characterization and interior imaging at penetration depths on the order of several hundred micrometers. Experimental results illustrating the use of the laser-based acoustic microscopy technique for imaging micrometer-scale subsurface geometrical features in a 70-µm-thick single-crystal silicon wafer with a (100) orientation are presented.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy.

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications.

Femtosecond and nanosecond laser-induced breakdown spectroscopy of trinitrotoluene.

Femtosecond and nanosecond laser-induced breakdown spectroscopy (LIBS) were used to study trinitrotoluene (TNT) deposited on aluminum substrates. Over the detection wavelength range of 200-785 nm, we have observed emission from CN and C(2) molecules as the marker for the explosive with femtosecond LIBS. In contrast, the signal for nanosecond LIBS of TNT is dominated by emission from the elemental constituents of the explosive. Aluminum emission lines from the substrate are also observed with both femtosecond and nanosecond excitation and indicate the role played by the substrate in the interaction.

Effects of surface roughness on reflection spectra obtained by terahertz time-domain spectroscopy.

We present an analytical model that shows that reflection from a rough surface causes a Gaussian frequency roll-off for the spectral magnitude of a terahertz wave and reduces the signal-to-noise ratio of terahertz time-domain spectroscopy. The parameter that determines the width of the frequency roll-off is the standard deviation of the surface height distribution. Measurements of terahertz waves reflected from copper powder samples provide experimental evidence for this effect.

Modeling interband transitions in silver nanoparticle-fluoropolymer composites.

The interband transition contributions to the optical properties of silver nanoparticles in fluoropolymer matrices are investigated. For the materials in this study, nanoparticle synthesis within the existing polymer matrix is accomplished using an infusion process that consists of diffusing an organometallic precursor gas into the free volume of the fluoropolymer and decomposing the precursor followed by metal nanoparticle nucleation and growth. The resulting polymer matrix nanocomposite has optical properties that are dominated by the response of the nanoparticles owing to the broadbanded transparency of the fluoropolymer matrix. The optical properties of these composites are compared to Maxwell-Garnett and Mie theory with results indicating that interband transitions excited in the silver nanoparticles affect the optical absorption over a range of frequencies including the surface plasmon resonance. It is shown that calculations of the optical absorption spectrum using published data for the silver dielectric function do not accurately describe the measured material response and that a classical model for bound and free electron behavior can best be used to represent the dielectric function of silver.

Narrowband generation of ultrafast acoustic and thermal transients in thin films for enhanced detectability.

A technique for the narrowband generation of ultrafast acoustic and thermal transients in thin films is demonstrated; this technique allows for enhanced detectability of these transients. The approach pursued uses a reduced-bandwidth, optical pulse train for excitation that is constructed from a series of time-delayed pulses derived from a single-laser pulse. The underlying physical limitations of this approach are considered in order to assess conditions under which successful bandwidth reduction can be realized. Results in an aluminum thin film on a tungsten-carbide substrate show successful generation and detection of a narrowband acoustic signal centered at 32.34 GHz.

Hybrid laser/broadband EMAT ultrasonic system for characterizing cracks in metals.

Detection and characterization of defects in metal parts in industrial and commercial settings has typically been carried out by nondestructive ultrasonic inspection systems. Correct measurement of crack size is critical for lifetime prediction inspections. Normally, measurements are made based on far-field ultrasonic diffraction models and time-of-flight reflection signals making accurate measurements for parts less than approximately 25 mm in thickness impossible. In this work a hybrid noncontacting laser generation/broadband electromagnetic acoustic transducer (EMAT) detection system is used to characterize ideal cracks in aluminum in which the far-field condition for ultrasonic diffraction cannot be met. Time domain signals show that diffracted energy is measured in the geometrical shadow zone of the crack. Fourier transform methods are used to show that the frequency content of the diffracted signals is different than those from the waves that do not interact with the crack. Crack size measurements are made by using the frequency content of the ultrasonic signal rather than time-of-flight information.