Ashwinkumar Sampathkumar

Multiplexed Optical Operation of Distributed Nanoelectromechanical Systems Arrays

We report a versatile all optical technique to excite and read-out a distributed nanoelectromechanical systems (NEMS) array. The NEMS array is driven by a distributed, intensity modulated optical pump through the photothermal effect. The ensuing vibrational response of the array is multiplexed onto a single probe beam in the form of a high frequency phase modulation. The phase modulation is optically down converted to a low frequency intensity modulation using an adaptive full-field interferometer, and subsequently detected using a CCD array. Rapid and single step mechanical characterization of ∼44 nominally identical high-frequency resonators is demonstrated. The technique may enable sensitivity improvements over single NEMS resonators by averaging signals coming from a multitude of devices in the array. In addition, the diffraction limited spatial resolution may allow for position-dependent read-out of NEMS sensor chips for sensing multiple analytes or spatially inhomogeneous forces.

Shock‐controlled bubble cloud dynamics and light emission.

Cavitation bubble collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye. In this talk we describe cavitation bubble cloud experiments carried out in spherical resonators at ambient and acoustic pressures up to 30 MPa. Key to our system is the ability to nucleate with temporal and spatial controls, which we achieve using dielectric breakdown in water from pulsed focused laser beams. Our observations show that the cloud dynamics are controlled by the repetitive emission of shock waves, which propagate outward from the inertial cloud collapse, reflect off of the sphere wall, and then converge on the resonator center. Shock convergence phenomena and light emission from compact cloud collapse will be discussed. [Work supported by the Impulse Devices, Inc.]

Optical nucleation of bubble clouds in a high pressure spherical resonator

An experimental setup for nucleating clouds of bubbles in a high-pressure spherical resonator is described. Using nanosecond laser pulses and multiple phase gratings, bubble clouds are optically nucleated in an acoustic field. Dynamics of the clouds are captured using a high-speed CCD camera. The images reveal cloud nucleation, growth, and collapse and the resulting emission of radially expanding shockwaves. These shockwaves are reflected at the interior surface of the resonator and then reconverge to the center of the resonator. As the shocks reconverge upon the center of the resonator, they renucleate and grow the bubble cloud. This process is repeated over many acoustic cycles and with each successive shock reconvergence, the bubble cloud becomes more organized and centralized so that subsequent collapses give rise to stronger, better defined shockwaves. After many acoustic cycles individual bubbles cannot be distinguished and the cloud is then referred to as a cluster. Sustainability of the process is ultimately limited by the detuning of the acoustic field inside the resonator. The nucleation parameter space is studied in terms of laser firing phase, laser energy, and acoustic power used.

Shock‐driven growth of bubble clouds.

Laser‐nucleated bubble clouds in a high‐pressure spherical resonator have previously been reported to develop into tight clusters after many acoustic cycles. The formation of these organized clusters is largely driven by the reconvergence of shocks from earlier cloud collapses. Highly temporally (40 Mfps) and spatially (3 μm/pixel) resolved images reveal that the expansion of the shock‐nucleated clusters is initially very fast (> 8 km/s), much faster than the shocks themselves. However, this explosive growth of the cluster does not begin until hundreds of nanoseconds after the shock passes, and so the cluster never surpasses the shock itself. The phase diagrams of the cluster and shocks are mapped out and the shock‐driven nucleation is discussed. [Work supported by the Impulse Devices, Inc.]

Pressure Dependence of Laser-Induced Dielectric Breakdown in Water

The effects of pressure on laser-induced breakdown events at superthreshold irradiances in water were investigated over a range of pressures from 0 to 1380 bar. Breakdown events were generated using 5-ns Nd:YAG laser pulses of wavelength 532-nm. Observations of breakdown events were made using imaging and single detector techniques. Applications for use as a static and/or acoustic pressure sensor were explored. Using imaging techniques over a range of static pressures from 0 to 275 bar, it was observed that the laser-induced breakdown threshold increased with increasing pressure. An ability to infer acoustic pressure via optical breakdown techniques using imaging was demonstrated. Single detector measurements over the same pressure range were observed to follow a similar trend.

An all-optical photoacoustic microscopy system for remote, noncontact characterization of biological tissues

Conventional photoacoustic microscopy (PAM) employs light pulses to produce a photoacoustic (PA) effect and detects the resulting acoustic waves using an ultrasound transducer acoustically coupled to the target tissue. The resolution of conventional PAM is limited by the sensitivity and bandwidth of the ultrasound transducer. We have investigated an all-optical, “pump-probe” method employing interferometric detection of the acoustic signals that overcomes limitations of conventional PAM. This method does not require contact with the specimen and provides superior resolution. A 532-nm “pump” laser with a pulse duration of 5 ns excited the PA effect in tissue. Resulting acoustic waves produced surface displacements that were sensed interferometrically with a GHz bandwidth using a 532-nm CW “probe” laser using a Michelson interferometer. The pump and probe beams were coaxially focused using a 50× objective giving a diffraction-limited spot size of 0.5 µm. The phase-encoded probe beam was demodulated using a homodyne interferometer. The detected time-domain signal was time reversed using k-space wave-propagation methods to produce a spatial distribution of photoacoustic sources in the target tissue. Performance was assessed using 3D images of fixed, ex vivo, retina specimens. Apparatus design and imaging results for the all-optical PA system and possible applications will be discussed.

Laser nucleation of single bubbles and clouds in an acoustic resonator via pressure-dependent dielectric breakdown

Obtaining bubbles on demand at precise times and locations in a non-contact fashion can be useful in a variety of applications. Of special importance is the combination of laser nucleation with acoustics, so that bubbles are only just nucleated by the optics but grown to macroscopic size solely by the acoustics. We present theory and experiment for the non-thermal laser nucleation of bubbles in an acoustic field in the absence of significant absorbing/scattering particles. First we present theory and experiment for the threshold for dielectric breakdown in water, resolving the distinct minimum at 20 bar. Then, we present a method and results for nucleating single and multiple bubbles with temporal uncertainty of 5 ns, and spatial uncertainty of 1 mm. Results for bubble number and first cycle expansion are reported as functions of the timing of the nucleating laser pulse with respect to the acoustic field. [Work supported by Impulse Devices, Inc.]

Spatially and temporally resolved single bubble sonoluminescence and its entrainment in Rayleigh-Taylor jets

Previous investigations of the temporal and spatial evolution of single bubble sonoluminescence (SBSL) have shown events to last on the order of tens to hundreds of picoseconds with spatial extents of less than 1 um. Here we present observations of the temporal and spatial evolution of laser-nucleated SBSL events in a high-pressure spherical resonator. Using high-speed imaging, we observe large, long-lived SBSL events reaching diameters of up to 50 um and lasting on the order of 30 ns. Observations of events entrained in Rayleigh-Taylor jets resulting from instabilities in the final stages of the bubbles collapses will also be presented. We observe the light emitting region entrained in these jets to reach velocities in excess of 4500 m/s and to travel up to 100 um before being extinguished. The size and duration of events, and the velocity of those entrained in Rayleigh-Taylor jets, will be compared to the maximum radius and collapse velocity of the bubbles responsible for generating them to develop a better understanding of the dynamics leading to, and the mechanisms responsible for light emissions during highly energetic collapse events. [Work supported by Impulse Devices, Inc.]

High pressure phase transitions in the fluid region surrounding the collapse point of large single bubbles in water

Observations from imaging experiments will be presented which have shown persistent, long-lived spherical objects to form in the fluid region surrounding large, single bubbles in highly over-pressured water. Objects have been observed to form in a region of fluid where pressures are first predicted to exceed 0.8 GPa, and to extend radially inward to where fluid pressures are predicted to reach 6 GPa. These pressures bound those requisite for transitions in water to the crystalline phases of Ice-VI and Ice-VII, at 1.1 GPa and 2.1 GPa, respectively. The objects have been observed to behave in a fashion more consistent with a highly viscous fluid. They support and recover from large shape deformations, as well as support fluid flows within them. While water does have phases which are known to exhibit properties of highly viscous fluids, they have only been observed to form at or near cryogenic temperatures, typically via hyperquenching or quasi-static pressurization at low temperatures. Here, we present evidence for a high pressure liquid-liquid phase transition in water surrounding collapsing bubbles at room temperature. [Work supported by Impulse Devices, Inc.]

Spatially and temporally resolved sonoluminescence from compact bubble clouds.

Previous investigations of flash duration for single bubble sonoluminescence (SBSL) events in water have shown emission pulse widths on the order of 30–300 ps. The spatial extent of the light‐emitting region in SBSL has not been successfully resolved, but it is less than 1 μm. Here we report temporal and spatial observations of light emission from laser‐nucleated, compact bubble clouds at high static pressure. Employing high‐speed imaging and PMT monitoring, we observe events with durations on the order of 50 ns, whose spatial extent can reach 1 mm in radius. The evolution of event size, spatial uniformity, and intensity will be monitored and compared with parametric data (maximum radius of cloud, outgoing shock velocity, static pressure, and post nucleation time) to discover correlations between light emission and hydrodynamics. [Work supported by the Impulse Devices, Inc.]