Kristina Davitt

290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles

We demonstrate a compact system, incorporating a 32-element linear array of ultraviolet (290 nm and 340 nm) light-emitting diodes (LEDs) and a multi-anode photomultiplier tube, to the in-flight fluorescence detection of aerosolized particles, here containing the biological molecules tryptophan and NADH. This system illustrates substantial advances in the growth and fabrication of new semiconductor UV light emitting devices and an evolution in packaging details for LEDs tailored to the bio-aerosol warning problem. Optical engineering strategies are employed which take advantage of the size and versatility of light-emitting diodes to develop a truly compact fluorescence detector.

Heteroepitaxy of AlGaN on bulk AlN substrates for deep ultraviolet light emitting diodes

The authors report the growth of AlGaN epilayers and deep ultraviolet (UV) light emitting diodes (LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaN heteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaN LEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.

Equation of state of water under negative pressure

We report on the simultaneous measurements of the speed of sound and the density in liquid water under negative pressure. Application of a focused acoustic wave to the bulk liquid is able to generate negative pressures before nucleation of the vapor phase occurs. A method for time-resolved Brillouin scattering is developed to measure the speed of sound during the passage of a 1 MHz ultrasonic wave. This is coupled with a fiber optic probe hydrophone which allows the determination of the density. Together, these methods give an ambient temperature equation of state of metastable liquid water down to the acoustic cavitation threshold. Empirical equations of state of water are based on experimental data at positive pressure; the validity of their extrapolation to negative pressures had been tested only indirectly or with very weakly metastable liquid. We provide thermodynamic data that prove the fidelity of recent equations of state down to -26 MPa. However, this raises questions regarding the nature of the cavitation threshold observed in acoustic experiments, which is far less negative than expected.

Water at the cavitation limit: Density of the metastable liquid and size of the critical bubble

The ability of a liquid to sustain mechanical tension is a spectacular manifestation of the cohesion of matter. Water is a paradigmatic example, because of its high cohesion due to hydrogen bonds. The knowledge of its limit of rupture by cavitation can bring valuable information about its structure. Up to now, this limit has been obscured by the diversity of experimental results based on different physical measures of the degree of metastability of the liquid. We have built a fiber optic probe hydrophone to provide the missing data on the density of the liquid at the acoustic cavitation limit. Our measurements between 0 and 50°C allow a clear-cut comparison with another successful method where tension is produced in micron-sized inclusions of water in quartz. We also extend previous acoustic measurements of the limiting pressure to 190°C, and we consider a simple modification of classical nucleation theory to describe our data. Applying the nucleation theorem gives the first experimental value for the size of the critical bubble, which lies in the nanometer range. The results suggest the existence of either a stabilizing impurity in the inclusion experiments, or an ubiquitous impurity essential to the physics of water.

Stability of angular confinement and rotational acceleration of a diatomic molecule in an optical centrifuge

Modern femtosecondtechnology can be used to create laser pulses that induce controlled spinning of anisotropic molecules to very high angular momentum states (“optical centrifuge”). In this paper we extend our previous study [M. Spanner and M. Ivanov, J. Chem. Phys. 114, 3456 (2001)] and focus on the stability of angular trapping and forced rotational acceleration of a diatomic molecule in an optical centrifuge. The effects of laser intensity modulations and rovibrational coupling are analyzed in detail, classically and quantum mechanically. The numerical simulations show excellent qualitative agreement between the quantum and classical systems. Forced rotations of the classical system can exhibit chaotic behavior, which becomes rather unique when the accelerating rotation of the angular trapping potential combines with efficient rovibrational coupling. In this regime the Lyapunov exponent becomes time-dependent and the trajectories separate as exp ( λF(t) ) .

Fiber optic probe hydrophone for the study of acoustic cavitation in water

We use focused ultrasound bursts to submit a liquid to mechanical tension. When the pressure in the sound wave reaches a sufficiently low value, vapor bubbles are nucleated in the bulk liquid. According to nucleation theory, increasing the ultrasound frequency increases the cavitation threshold by a calculable amount. To check this, we have built a fiber optic probe hydrophone based on one originally proposed by Staudenraus and Eisenmenger [Ultrasonics 31, 267 (1993)]. We have adapted the pressure calibration and data analysis of this tool to make it appropriate for precise measurements of tension in liquids. We are able to resolve the fractional change in the pressure threshold for cavitation in water that results from a twofold increase in the frequency. This provides a test of nucleation theory in general.

Application of light-emitting diodes for aerosol fluorescence detection

We demonstrate a proof-of-concept optical spectroscopic system for bioaerosol-particle fluorescence detection, in which a pulsed high-power laser is replaced by a highly compact linear array of sequentially fired light from blue light-emitting diodes. The results suggest that low-cost, compact optical aerosol detection may be feasible with the contemporary emergence of efficient UV light-emitting diodes.

Spectroscopic Sorting of Aerosols by a Compact Sensor Employing UV LEDs

A compact sensor for physically sorting bioaerosols based on fluorescence spectra from single particles excited using arrays of ultraviolet light emitting diodes (UV LEDs) is presented. The optical system integrates electronics for real-time processing of spectral data and a miniaturized aerodynamic deflector for particle separation. Fluorescent polystyrene latex spheres are used to demonstrate fluorescence collection on-the-fly, operation of a real-time spectral algorithm, and physical separation of individual particles. This sensor illustrates the utility of recently developed UV LEDs, in conjunction with novel optical design and custom electronics, to shrink the size of aerosol fluorescence detection systems.

A microscale photovoltaic neurostimulator for fiber optic delivery of functional electrical stimulation

Recent advances in functional electrical stimulation (FES) show significant promise for restoring voluntary movement in patients with paralysis or other severe motor impairments. Current approaches for implantable FES systems involve multisite stimulation, posing research issues related to their physical size, power and signal delivery, surgical and safety challenges. To explore a different means for delivering the stimulus to a distant muscle nerve site, we have elicited in vitro FES response using a high efficiency microcrystal photovoltaic device as a neurostimulator, integrated with a biocompatible glass optical fiber which forms a lossless, interference-free lightwave conduit for signal and energy transport. As a proof of concept demonstration, a sciatic nerve of a frog is stimulated by the microcrystal device connected to a multimode optical fiber (core diameter of 62.5 microm), which converts optical activation pulses ( approximately 100 micros) from an infrared semiconductor laser source (at 852 nm wavelength) into an FES signal.

Defects at the Nanoscale Impact Contact Line Motion at all Scales.

The contact angle of a liquid drop moving on a real solid surface depends on the speed and direction of motion of the three-phase contact line. Many experiments have demonstrated that pinning on surface defects, thermal activation and viscous dissipation impact contact line dynamics, but so far, efforts have failed to disentangle the role of each of these dissipation channels. Here, we propose a unifying multiscale approach that provides a single quantitative framework. We use this approach to successfully account for the dynamics measured in a classic dip-coating experiment performed over an unprecedentedly wide range of velocity. We show that the full contact line dynamics up to the liquid film entrainment threshold can be parametrized by the size, amplitude and density of nanometer-scale defects. This leads us to reinterpret the contact angle hysteresis as a dynamical crossover rather than a depinning transition.