Optical control of scattering, absorption and lineshape in nanoparticles

We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response

Adaptive spatial combining for passive time-reversed communications

Passive time reversal has aroused considerable interest in underwater communications as a computationally inexpensive means of mitigating the intersymbol interference introduced by the channel using a receiver array. In this paper the basic technique is extended by adaptively weighting sensor contributions to partially compensate for degraded focusing due to mismatch between the assumed and actual medium impulse responses. Two algorithms are proposed, one of which restores constructive interference between sensors, and the other one minimizes the output residual as in widely used equalization schemes. These are compared with plain time reversal and variants that employ postequalization and channel tracking. They are shown to improve the residual error and temporal stability of basic time reversal with very little added complexity. Results are presented for data collected in a passive time-reversal experiment that was conducted during the MREA’04 sea trial. In that experiment a single acoustic projector generated a 2/4-PSK phase-shift keyed stream at 200/400 baud, modulated at 3.6 kHz, and received at a range of about 2 km on a sparse vertical array with eight hydrophones. The data were found to exhibit significant Doppler scaling, and a resampling-based preprocessing method is also proposed here to compensate for that scaling

Transcranial focusing of ultrasonic vortices by acoustic holograms

[EN] Acoustic vortex beams have great potential for contactless particle manipulation and torque-based biomedical applications. However, when focusing acoustic waves through aberration layers such as the human skull at ultrasonic frequencies results in strong phase aberrations which prevent the generation of sharp acoustic images. In the case of a wavefront containing phase dislocations, skull aberrations inhibit the focusing of acoustic vortex beams inside the cranial cavity. In this work, we demonstrate that phase-conjugated acoustic holograms can encode time-reversed fields simultaneously allowing the compensation of the aberrations of the skull and the generation of a focused vortex inside an ex-vivo human skull. The method is applied for single-element geometrically focused sources and results in a very simple and compact ultrasonic system. This work will pave the road to design low-cost particle trapping applications, clot manipulation, torque exertion in the brain and acoustic-radiation-force based biomedical applications.This research was supported by the Spanish Ministry of Science, Innovation, and Universities through "Juan de la Cierva-Incorporacion" Grants No. IJC2018-037897-I and No. PID2019-111436RB-C22, by the Agencia Valenciana de la Innovacio through Grants No. INNVAL10/19/016, No. INNVA1/2020/92, and No. INNCON/2020/009, and by the Generalitat Valenciana through Grant No. ACIF/2017/045. The action was cofinanced by the European Union through the Programa Operativo del Fondo Europeo de Desarrollo Regional (FEDER) of the Comunitat Valenciana, Grant No. IDIFEDER/2018/022.Jiménez-Gambín, S.; Jimenez, N.; Camarena Femenia, F. (2020). Transcranial focusing of ultrasonic vortices by acoustic holograms. Physical Review Applied. 14(5):1-10. https://doi.org/10.1103/PhysRevApplied.14.054070S11014

Optical Focusing and Imaging through Scattering Media

Optical techniques, which have been widely used in various fields including bio-medicine, remote sensing, astronomy, and industrial production, play an important role in modern life. Optical focusing and imaging, which correspond to the basic methods of utilizing light, are key to the implementation of optical techniques. In free space or a nearly transparent medium, optical imaging and focusing can be easily realized by using conventional optical elements, such as lenses and mirrors, due to the ballistic propagation of light in these media. However, in scattering media like biological tissue and fog, refractive index inhomogeneities cause diffusive propagation of light that increases with depth, which restricts the use of optical methods in thick, scattering media. Generally speaking, scattering media poses three challenges to optical focusing and imaging: wavefront aberrations, glare, and decorrelation. Wavefront aberrations can randomize light traveling through a scattering medium, disrupt the formation of focus, and break the conjugate relation in imaging. Glare caused by backscattering will largely impair the visibility of imaging, and decorrelation in dynamic media requires systems that counter the effect of scattering to operate faster than the decorrelation time. In this thesis, we explored solutions to the problem of scattering from different aspects. We presented Time Reversal by Analysis of Changing wavefronts from Kinetic targets (TRACK) technique to realize noninvasive optical focusing through a scattering medium. We showed that by taking the difference between time-varying scattering fields caused by a moving object and applying optical phase conjugation, light can be focused back to the location previously occupied by the object. To tackle the decorrelation of living tissue, we built up a fast digital optical phase conjugation (DOPC) system based on FPGA and DMD, which has a response time of 5.3 ms and was the fastest DOPC system in the world before 2017. We demonstrated that the system is fast enough to focus light through 2.3mm-thick living mouse skin. As for glare, inspired by noise canceling headphones, we invented an optical analogue termed coherence gated negation (CGN) technique. CGN can optically cancel out the glare in an active illumination imaging scenario to realize imaging through scattering media, like fog. In the experiment, we suppressed the glare by an order of magnitude and allowed improved imaging of a weak target. Finally, we demonstrated a method to image a moving target through scattering media noninvasively. Its principle roots are in the speckle-correlation-based imaging (SCI) invented by Ori Katz. We improved the technique and extended its application to bright field imaging of a moving target.</p

Detection Strategies and Intercept Metrics for Intra-Pulse Radar-Embedded Communications

This thesis presents various detection strategies and intercept metrics to evaluate and design an intra-pulse radar-embedded communication system. This system embeds covert communication symbols in masking interference provided by the reflections of a pulsed radar emission. This thesis considers the case where the communicating device is a transponder or tag present in an area that is illuminated by a radar. The radar is considered to be the communication receiver. As with any communication system, performance (as measured by reliability and data rate) should be maximized between the tag and radar. However, unlike conventional communication systems, the symbols here should also have a low-probability of intercept (LPI). This thesis examines the trade-offs associated with the design of a practical radar-embedded communication system. A diagonally-loaded decorrelating receiver is developed and enhanced with a second stage based on the Neyman-Pearson criterion. For a practical system, the communication symbols will likely encounter multipath. The tag may then use a pre-distortion strategy known as time-reversal to improve the signal-to-noise ratio at the radar receiver thereby enhancing communication performance. The development of several intercept metrics are shown and the logic behind the design evolutions are explained. A formal analysis of the processing gain by the desired receiver relative to the intercept receivers is given. Finally, simulations are shown for all cases, to validate the design metrics

Optical tracking of nerve activity using intrinsic changes in birefringence

Changes in birefringence (or dynamic birefringence) provide an arguably cleaner method of measuring IOS as compared to scattering methods. Other imaging methods have substantial limitations. Nerves inherently exhibit a static (rest condition) birefringence that is associated with the structural anisotropies of axonal protein filaments, membrane phospholipids and proteins, as well as surrounding tissues, which include Schwann cells and axon sheaths. The dynamic birefringence, or “crossed-polarized signal” (XPS), in neurons arises from activity in axons and occurs with a rapid momentary change, typically a decrease, in the birefringence when action potentials (APs) propagate along them. We improved the signal-to-noise to make detecting this signal an easier task, and present the XPS as a viable candidate for detecting AP activity in anisotropic nervous tissue. Our data collectively serves as a strong indication that there is a capacitive-charging-like effect directly inducing a gradual recovery (long tail) of the XPS to baseline, and also causing a smoothing of the XPS trace. A setup was constructed that successfully demonstrated the feasibility of tracking propagating compound APs in a peripheral nerve using the XPS. We made significant progress in the attempt to investigate birefringence of myelination. For the first time, the XPS in a myelinated tissue was detected, and it appears to be bipolar in nature. Further work in investigating the nature of this signal is needed, and is currently underway. Since changes in birefringence in neurons are associated instantaneously with electrophysiological phenomena, they are well-suited for fast imaging of propagating action potentials in neuronal tissue. In summary, imaging based on polarization sensing of changes in birefringence offers promise for an improved noninvasive method of detecting and tracking AP activity in myelinated and unmyelinated nerves and could be designed for pre-clinical and surgical applications

A Unified Multi-Functional Dynamic Spectrum Access Framework: Tutorial, Theory and Multi-GHz Wideband Testbed

Dynamic spectrum access is a must-have ingredient for future sensors that are ideally cognitive. The goal of this paper is a tutorial treatment of wideband cognitive radio and radar—a convergence of (1) algorithms survey, (2) hardware platforms survey, (3) challenges for multi-function (radar/communications) multi-GHz front end, (4) compressed sensing for multi-GHz waveforms—revolutionary A/D, (5) machine learning for cognitive radio/radar, (6) quickest detection, and (7) overlay/underlay cognitive radio waveforms. One focus of this paper is to address the multi-GHz front end, which is the challenge for the next-generation cognitive sensors. The unifying theme of this paper is to spell out the convergence for cognitive radio, radar, and anti-jamming. Moore’s law drives the system functions into digital parts. From a system viewpoint, this paper gives the first comprehensive treatment for the functions and the challenges of this multi-function (wideband) system. This paper brings together the inter-disciplinary knowledge

Ultrasound-mediated Optical Imaging and Focusing in Scattering Media

Because of its non-ionizing and molecular sensing nature, light has been an attractive tool in biomedicine. Scanning an optical focus allows not only high-resolution imaging but also manipulation and therapy. However, due to multiple photon scattering events, conventional optical focusing using an ordinary lens is limited to shallow depths of one transport mean free path (lt\u27), which corresponds to approximately 1 mm in human tissue. To overcome this limitation, ultrasonic modulation (or encoding) of diffuse light inside scattering media has enabled us to develop both deep-tissue optical imaging and focusing techniques, namely, ultrasound-modulated optical tomography (UOT) and time-reversed ultrasonically encoded (TRUE) optical focusing. While UOT measures the power of the encoded light to obtain an image, TRUE focusing generates a time-reversed (or phase-conjugated) copy of the encoded light, using a phase-conjugate mirror to focus light inside scattering media beyond 1 lt\u27. However, despite extensive progress in both UOT and TRUE focusing, the low signal-to-noise ratio in encoded-light detection remains a challenge to meeting both the speed and depth requirements for in vivo applications. This dissertation describes technological advancements of both UOT and TRUE focusing, in terms of their signal detection sensitivities, operational depths, and operational speeds. The first part of this dissertation describes sensitivity improvements of encoded-light detection in UOT, achieved by using a large area (∼5 cm × 5 cm) photorefractive polymer. The photorefractive polymer allowed us to improve the detection etendue by more than 10 times that of previous detection schemes. It has enabled us to resolve absorbing objects embedded inside diffused media thicker than 80 lt\u27, using moderate light power and short ultrasound pulses. The second part of this dissertation describes energy enhancement and fluorescent excitation using TRUE focusing in turbid media, using photorefractive materials as the phase-conjugate mirrors. By using a large-area photorefractive polymer as the phase-conjugate mirror, we boosted the focused optical energy by ~40 times over the output of a previously used photorefractive Bi12SiO20 crystal. Furthermore, using both a photorefractive polymer and a Bi12SiO20 crystal as the phase-conjugate mirrors, we show direct visualization and dynamic control of TRUE focus, and demonstrate fluorescence imaging in a thick turbid medium. The last part of this dissertation describes improvements in the scanning speed of a TRUE focus, using digital phase-conjugate mirrors in both transmission and reflection modes. By employing a multiplex recording of ultrasonically encoded wavefronts in transmission mode, we have accelerated the generation of multiple TRUE foci, using frequency sweeping of both ultrasound and light. With this technique, we obtained a 2-D image of a fluorescent target centered inside a turbid sample having a thickness of 2.4 lt\u27. Also, by gradually moving the focal position in reflection mode, we show that the TRUE focal intensity is improved, and can be continuously scanned to image fluorescent targets in a shorter time