DENSE MICROPLASMAS
Sonoluminescence originates in a plasma that is so dense with free charges that it blocks visible light. The video shows a strong laser pulse hitting the plasma which forms inside of a collapsed 100 μm diameter bubble. The photograph illuminated with green laser light shows our apparatus for studying how a 1 μm diameter sonoluminescence plasma blocks light.
A new phase of matter leads to Sonoluminescence
In order to behave like a Planck blackbody, the charge density must be greater than 1021 cm-3. How does this happen? We believe that the free electrons cooperate with the atoms and ions to lower the energy barrier for ionizing atoms. The figure from a Physics Today Search and Discovery article shows the formation of the blackbody as the bubble implodes. The spectral line of the excited state of xenon at 823 nm (the infrared of the spectrum) is unable to propagate out of the heavily charged system as the compression reaches its maximum.
Bird’s-eye view of a bubble collapse. (a) A streak photograph shows the time evolution of an acoustically driven xenon bubble, which emits a 1-µs flash of sonoluminescence (SL) as it nears its minimum radius of about 50 µm. (Image courtesy of Brian Kappus.) (b)These emission spectra were collected from a similar bubble 400 ns before, 200 ns before, and 400 ns after it reached its minimum radius—roughly at the stages indicated by the blue, green, and red arrows in panel a. The disappearance and reappearance of the peak at 823 nm suggests that the bubble goes from transparent to opaque and back. The black curves are blackbody fits. (Adapted from ref. 6.) Citation: Phys. Today 65, 4, 18 (2012); http://dx.doi.org/10.1063/PT.3.1507. Analysis of the spectrum of Sonoluminescence led the Suslick lab to similar charge densities (Inertially Confined Plasma in a Sonoluminescing Bubble).
Shake Tube
Images of nanosecond duration demonstrate how Sonoluminescence created inside phosphoric acid can block intense laser pulses (from right to left). A dense plasma is created at the moment of bubble collapse (top) and is heated by a focused laser pulse (bottom).
Cone Bubble
Backlit strobe photograph of “Conical Sonoluminescence” at the moment of light emission. An accelerating column of ethylene glycol collapses a large volume of low-pressure xenon gas into a conically-shaped hollowed-out polycarbonate housing. The rising fluid blocks a He:Ne laser which is used to time the strobe relative to the plasma emission, and highlights the fluid in red. Plasma is formed at the apex of the cone during maximum compression and produces a bright flash of light, whose flashwidth can range from 100 nanoseconds to 10 milliseconds!
Dancing Sulfuric Acid Bubble
Long exposure of xenon sonoluminescence in concentrated sulfuric acid. An objective (left) focuses a high-intensity laser pulse for initial bubble seeding. The long camera exposure results in a swirling mass of light that traces the bubble’s path within the fluid.
This dense plasma is universal. Not only does it form in many different Sonoluminescence configurations, it can even be created OUTSIDE a liquid.
Quiz: Which of these flashes of light below comes from Sonoluminescence?
The correct answer is that the middle image comes from Sonoluminescence. The other are images from spark discharges (left) and laser breakdown (right) in high-pressure gases. However, all these images originate from dense microplasmas whose properties are much alike.
Creating Sonoluminescence outside a liquid
Blackbody microplasmas can be created using femtosecond laser breakdown and spark discharges in high-pressure gases. This platform allows us to explore the unique plasma of Sonoluminescence “outside” its liquid confines. We are uncovering new properties of dense plasmas.
Experimental realization of Sonoluminescence “outside” a liquid. A femtosecond laser is focused into the center of a chamber filled with high-pressure gases.
Femtosecond laser pulses are focused inside a high-pressure gas chamber, generating plasmas of ~15,000 K. Like Sonoluminescence, these microplasmas emit blackbody radiation.
High-Power Optical Switch
Just like Sonoluminescence, these microplasma formed by a spark can also block visible light and in fact can be used to protect equipment from unfriendly probes.
Nanosecond framing camera images demonstrate how an intense laser pulse (from left to right) is absorbed into the surface of a dense microplasma generated by a spark discharge. In this arrangement, the laser pulse is completely absorbed by the plasma and prevents wholesale damage to a camera imaging the laser.
High-voltage across micron-sized tungsten needles create nanosecond spark discharges. In high-pressure gases, these discharges become blackbodies and therefore opaque to visible light. This platform allows one to block intense laser pulses on command, and may provide protection to sensitive imagers.