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Liquid Lenses

August 2, 2008
A liquid lens uses one or more fluids to create an infinitely-variable lens without any moving parts by controlling the meniscus (the surface of the liquid.) There are two primary types, transmissive and reflective. These are not to be confused with liquid-formed lenses that are created by placing a drop of plastic or epoxy on a surface, which is then allowed to harden into a lens shape.Reflective liquid lenses are actually variable mirrors, and are used in reflector telescopes in place of traditional glass mirrors. When a container of fluid (in this case, mercury) is rotated, centripetal force creates a smooth reflective concavity that is ideally suited for telescope applications. Normally, such a smooth curved surface has to be meticulously ground and polished into glass in an extremely expensive and tricky process (remember the Hubble Space Telescope mirror fiasco?) A reflective liquid lens would never suffer from that problem, as a simple change in rotation speed would change the curve of the meniscus to the proper shape. Scientists at the University of British Columbia (UBC) have built a 236-inch (6-meter) Liquid Mirror Telescope (LMT). The world's 13th largest telescope, its reflective surface is made of a flat container of mercury spinning at about 5 RPM. The telescope costs only about $1 million, a significant reduction from the roughly $100 million cost of what a conventional telescope with a regular solid glass mirror of the same size would require. Transmissive liquid lenses use two immiscible fluids, each with a different refractive index, to create variable-focus lenses of high optical quality as small as 10 µm (microns). The two fluids, one an electrically conducting aqueous solution and one a nonconducting oil, are contained in a short tube with transparent end caps. The interior of the tube and one of the caps is coated with a hydrophobic material, which causes the aqueous solution to form a hemispherical lens-shaped mass at the opposite end of the tube. The shape of the lens is adjusted by applying a ac voltage across the coating to decrease its water repellency in a process called electrowetting. Electrowetting adjusts the liquid's surface tension, changing the radius of curvature in the meniscus and thereby the focal length of the lens. Extremely shock and vibration resistant, such a lens is capable of seamless transition from convex (convergent) to concave (divergent) lens shapes with switching times measured in milliseconds. In addition, the boundary between the two fluids forms an extremely smooth and regular surface, making liquid lenses of a quality suitable for endoscopic medical imaging and other space-constrained high-resolution applications like microcameras and fiber-optic telecommmunications systems. The aforementioned liquid-formed lenses are a cool technology as well, and used mostly on image sensors. Tiny drops of epoxy are placed on each pixel, which then form individual lenses to increase light-capturing ability. They are also used on novelty items to create a magnifying effect.
   LabCast 
MIT-based website with videos about the latest science and technology trends.
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Attosecond sources and Attosecond science

July 18, 2008
A recent revolution in laser technology has opened the door to the generation of flashes of light that can freeze the ultrafast motion of electrons inside atoms and molecules. Our emerging capability of reproducibly generating and measuring attosecond-duration bursts of light marks the beginning of a new era in exploring motion in the microcosm: the era of attoscience. Attoseconds (10-18 seconds, which is 1/1000000000000000000 of a second) constitute the natural scale for the motion of electrons on the atomic scale. This motion comes now under scrutiny in real-time studies. These advances begun a decade ago when physicists used intense femtosecond laser pulses to ionize a rare gas (such as neon), and found that new electromagnetic waves were generated in form of "high-order harmonics" at odd multiples of the original optical pulse frequency. It is the interplay between constructive and destructive interference in the superposition of these monochromatic light waves of equally spaced frequencies that gives rise to temporal beating, the underpinning process of attosecond pulse generation.

The physical explanation for the generation of these short pulses of light can be summarized as follows. An intense femtosecond pulse of light will ionize a rare gas, but will do so in a single optical cycle (which is approximately 2 x 10-15 seconds). The free electron will then gain energy as it is accelerated by the same laser light. The force exerted on the electron by the laser light will change direction as the electrical field of the laser oscillates and smash it back in to the nuclei. This re-combination process will release a photon which is an odd harmonic of the original laser light and will be much shorter in time. Recently a research team lead by Ferenc Krausz of the Max Planck Institute of Quantum Optics in Garching, Germany, along with researchers in Austria and the Netherlands have been able to generate pulses as short as 250 attoseconds. This has allowed them to see one of the most intriguing features of quantum mechanics called ‘tunneling’. Tunneling is when an electron can escape from the electrical forces that bind him to the nuclei even when he does not posses enough energy to over come the barrier. Even though predicted many years ago theoretically, these have never been observed directly. This is due to the fact that theses tunneling events occur on an Attosecond time scale. The Krausz group now report on watching these tunneling events in their lab and thus validating once again the theory of quantum mechanics.

FIG. 1 The waveform of few-cycle pulse of red laser light (half oscillation period: 1.25 femtosecond = 1250 attoseconds) has been directly sampled by an extreme ultraviolet burst of 250 attosecond duration, providing evidence for the technical capability of controlling and measuring processes on the attosecond scale.
   Physorg 
Place to watch videos covering space, science, and technology.
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Light Filaments

July 4, 2008
Since the availability of ultra short laser oscillators and the development of the Chirped Pulse Amplification (CPA) scheme in mid 1980s various research fields have opened to scientists. One of these fields is the propagation of high intensities through the atmosphere. Initially it was believed that ultra-short laser pulses are not suited for long range atmospheric propagation. For example, considering the linear propagation of a 30 fsec (one femtosecond is 1/000000000000000 of a second) pulse with beam waist of 5 mm, will yield after 1 km of propagation a peak intensity that is reduced by a factor of 1000. This is due to the combined effect of diffraction and group velocity dispersion (GVD). In 1995 researchers from the University of Michigan showed that for an NIR femtosecond pulse, that the opposite trend is observed; that is, the peak intensity of the laser pulse increases with the propagation distance.To explain this we should remember that since the peak power of the pulses generated is high (P > 10 ^ 10 W) the atmospheric medium should be considered as a nonlinear medium. Which results in nonlinear focusing (Kerr effect), which is an outcome of an intensity dependant term which is added to the refractive index.

While the beam focuses, the intensities reached (I ~ 1013 W/cm2) are sufficiently high to ionize the air molecules. One of the effects of the plasma produced is an additional negative term to the index of refraction which arrests the self-focusing.

Here denotes the electron density, and is the critical density, which above it the entire laser electric field is reflected.The combination of these two effects gives rise to long localized light channels, called filaments. Of course, the exact evolution of laser pulse while propagating is much more complex then the naďve picture presented above. The interplay between spatial, temporal and spectral effects results in multifaceted dynamics which will be presented in the following paragraphs.The ability of these intense filaments to propagate over large distances in the atmosphere sparked the imagination for a variety of possible applications. Such as remote sensing, generation of artificial “stars” (for space telescopes calibration), remote EMP generation and lightening control (by utilizing the plasma channel left in the wake of a light filament).
   Free Science Lectures 
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About "Ed Stevens" (The online video guide expert who writes this blog)
Mr. Stevens holds a PHD in physics and is interested in new forms of media he is an avid user of the internet and bleeding edge technologies - Ed is also a good friend of mine
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