The role of electrostriction in the generation of acoustic waves by optical forces in water.

We present semi-analytical solutions describing the spatiotemporal distributions of temperature and pressure inside low-absorbing dielectrics excited by tightly focused laser beams. These solutions are compared to measurements in water associated with variations of the local refractive index due to acoustic waves generated by electrostriction, heat deposition, and the Kerr effect at different temperatures. The experimental results exhibited an excellent agreement with the modeling predictions, with electrostriction being the dominant transient effect in the acoustic wave generation. Measurements at 4 . 0 ∘ C show that the thermoelastic contribution to the optical signal is significantly reduced due to the low thermal expansion coefficient of water at this temperature.

Unveiling bulk and surface radiation forces in a dielectric liquid.

Precise control over light-matter interactions is critical for many optical manipulation and material characterization methodologies, further playing a paramount role in a host of nanotechnology applications. Nonetheless, the fundamental aspects of interactions between electromagnetic fields and matter have yet to be established unequivocally in terms of an electromagnetic momentum density. Here, we use tightly focused pulsed laser beams to detect bulk and boundary optical forces in a dielectric fluid. From the optical convoluted signal, we decouple thermal and nonlinear optical effects from the radiation forces using a theoretical interpretation based on the Microscopic Ampère force density. It is shown, for the first time, that the time-dependent pressure distribution within the fluid chiefly originates from the electrostriction effects. Our results shed light on the contribution of optical forces to the surface displacements observed at the dielectric air-water interfaces, thus shedding light on the long-standing controversy surrounding the basic definition of electromagnetic momentum density in matter.

Advanced optoacoustic methods for multiscale imaging of in vivo dynamics.

Visualization of dynamic functional and molecular events in an unperturbed in vivo environment is essential for understanding the complex biology of living organisms and of disease state and progression. To this end, optoacoustic (photoacoustic) sensing and imaging have demonstrated the exclusive capacity to maintain excellent optical contrast and high resolution in deep-tissue observations, far beyond the penetration limits of modern microscopy. Yet, the time domain is paramount for the observation and study of complex biological interactions that may be invisible in single snapshots of living systems. This review focuses on the recent advances in optoacoustic imaging assisted by smart molecular labeling and dynamic contrast enhancement approaches that enable new types of multiscale dynamic observations not attainable with other bio-imaging modalities. A wealth of investigated new research topics and clinical applications is further discussed, including imaging of large-scale brain activity patterns, volumetric visualization of moving organs and contrast agent kinetics, molecular imaging using targeted and genetically expressed labels, as well as three-dimensional handheld diagnostics of human subjects.

Structural and functional 3D mapping of skin tumours with non-invasive multispectral optoacoustic tomography.

BACKGROUND: Recent advances in technology have enabled the development of various non-invasive skin imaging tools to aid real-time diagnosis of both benign and malignant skin tumours, minimizing the need for invasive skin biopsy. Multispectral optoacoustic tomography (MSOT) is a recently developed non-invasive imaging tool, which offers the unique capacity for high resolution three dimensional (3D) optical mapping of tissue by further delivering highly specific optical contrast from a depth of several millimetres to centimetres in living tissues. MSOT enables volumetric, spectroscopic differentiation of tissue, both in vivo and in real time, with and without the application of biomarker-specific probes, and is further able of providing spatial maps of skin chromophores, as well as underlying blood vasculature. METHODS: Three patients with suspicious skin tumours consented to have their lesions imaged with MSOT prior to excision. The histological findings and measurements were compared. RESULTS: We demonstrated the first in vivo clinical use of MSOT for 3D reconstruction of skin tumours in three patients with good histological correlation. CONCLUSION: Our findings confirm the potential benefit of the new imaging method in guiding surgical intervention to achieve a more precise excision with better clearance and lower relapse rates. It can also potentially help to shorten the duration of Mohs' micrographic surgery. Further large-scale studies are necessary to ensure correlation between MSOT and histology.

Multiscale edge detection and parametric shape modeling for boundary delineation in optoacoustic images.

In this article, we present a novel scheme for segmenting the image boundary (with the background) in optoacoustic small animal in vivo imaging systems. The method utilizes a multiscale edge detection algorithm to generate a binary edge map. A scale dependent morphological operation is employed to clean spurious edges. Thereafter, an ellipse is fitted to the edge map through constrained parametric transformations and iterative goodness of fit calculations. The method delimits the tissue edges through the curve fitting model, which has shown high levels of accuracy. Thus, this method enables segmentation of optoacoutic images with minimal human intervention, by eliminating need of scale selection for multiscale processing and seed point determination for contour mapping.

Three-dimensional optoacoustic tomography at video rate.

Using optoacoustic excitation, a complete volumetric tomographic data sets from the imaged object can in principle be generated with a single interrogating laser pulse. Thus, optoacoustic imaging intrinsically has the potential for fast three-dimensional imaging. We have developed a system capable of acquiring volumetric optoacoustic data in real time and showcase in this work the undocumented capacity to generate high resolution three-dimensional optoacoustic images at a rate of 10 Hz, currently mainly limited by the pulse repetition rate of the excitation laser.

Cavity-enhanced biosensing utilizing plasmon resonance modes.

Traditionally, surface plasmon resonance (SPR) biosensors utilize absorption of light radiation incident upon noble metal films above the total internal reflection angles. Herein we extend the SPR phenomenon to incorporate cavity plasmon resonance (CPR) excitation of metallic films at incidence angles below the critical angle. While SPR occurs for TM polarized light only and requires very specific excitation conditions, which could be disadvantageous in some practical designs, CPR does not require complicated evanescent field excitation above the critical total internal reflection angle and can be implemented for both transverse electric (TE) and transverse magnetic (TM) fields even under normal incidence (TEM). These and other unique features of CPR enable a more flexible design of highly efficient and sensitive biosensing devices.

Optimized microbolometers with higher sensitivity for visible and infrared imaging.

Optimal absorption method for improving the sensitivity of bolometric detection is explored. We show that, in addition to its role in conventional conducting-film detection, the application of plasmon resonance absorption offers highly promising characteristics for efficient far-field thermal detection and imaging. These characteristics include good frequency sensitivity, intrinsic spatial (angle) selectivity without focusing lenses, wide tunability over both infrared and visible light domains, high responsivity and miniaturization capabilities. In this context, we examine the well-known surface plasmon resonance (SPR) regime, but also report on a new type of plasmon resonance excitation, the cavity plasmon resonance (CPR), which offers more flexibility over wide ranges of wavelengths, bandwidths, and device dimensions. Both CPR and SPR occur in metallic films, which are characterized by high thermal diffusivity essential for fast bolometric response.

Broadband absorption spectroscopy via excitation of lossy resonance modes in thin films.

It is shown that broadband absorption spectroscopy utilizing thin lossy film configurations can be optimally facilitated when applied to metallic and insulating materials. For metallic films, the zero-order highly lossy resonance mode, characterized by ultra wideband absorption behavior under normal incidence, can be shifted, under parallel-polarization oblique incidence, toward a narrow band light-wavelength surface plasmon resonance condition. Higher order low-loss modes, however, occur in thin insulating films, exhibiting Debye relaxation behavior, typical for many aqueous solutions and biological substances. They can be excited in a highly scalable and sensitive manner in various frequency bands, between light and radio frequencies.

Optimization of bulk and surface biosensing in plane stratified configurations.

Various biochemical and tissue sensors utilize layered configurations to accurately measure the physical parameters (e.g. refractive index) of the suspension or tissue under investigation. The most sensitive techniques, like these based on well-known Surface Plasmon Resonance (SPR) phenomenon, are usually limited to surface sensing at the infrared and light wavelength bands. We present a new type of broadband absorption biosensing method, which utilizes Lossy Resonance Modes (LRM) in thin lossy film configurations and can be facilitated in a highly scalable and sensitive manner for both bulk and surface sensing in various frequency bands, from radio to light frequencies.

Optimal dispersion relations for enhanced electromagnetic power deposition in dissipative slabs.

Broadband analysis of a prototype model associated with electromagnetic waves absorption in a lossy dielectric slab renders a closed-form theoretical prediction of an infinite number of optimal absorption paths in the complex refractive index domain. While for thin slabs (in terms of incident wavelength range) each path corresponds to a lossy Fabry-Perot-type resonance modes of order m=0,1,2,... and provides at least 50% absorption of the incident wave power even for ultrathin slabs, for optimal thick slabs, the fraction of absorbed power, asymptotically estimated via Lambert W function, increases up to 100% for consecutive continuation of the m=1 normal mode only.

Increased acoustic and electromagnetic energy deposition in a layered tissue model.

By analyzing and optimizing acoustic and electromagnetic waves absorption in a simplified layered model of hyperthermic configuration, it is shown that the commonly used attenuation metrics are not always proper means for determining the generally optimal parameters of typical thermal therapy problem with finite extent target The conditions, parameters and bounds for optimal (maximal) incident power absorption for the layered model have been found analytically and explicitly and are presented in terms of basic wave propagation characteristics, thus, also providing the necessary data for optimal synthesis of absorbing tissues and materials.