Frontiers in Optics 2007 (FiO), the 91st Annual Meeting of the Optical Society of America, will be held from Sept. 16-20 in San Jose, Calif., alongside Laser Science XXIII, the annual meeting of the American Physical Society Division of Laser Science.
FiO RESEARCH HIGHLIGHTS
Following are a few of the many technical highlights to be discussed at the meeting:
* Restoring Sight, One Pixel at a Time
* Near-Infrared LIDAR Helps Pilots
* Better, Stronger, Faster: High-Throughput Sperm Sorting
* Detecting Malaria with Light
* Gigantic Photoresponse Can Speed Up Optical Switches for Faster Internet Speeds
* Explaining a 21st Century Version of Young’s Experiment
Restoring Sight, One Pixel at a Time
Researchers at the University of Southern California (USC) are developing a tiny camera for prosthetic systems that can be implanted directly into the human eye and connected to the retina, the part of the eye that converts visual information into electric signals that travel to the brain. Such an implantable camera would represent an important milestone in the ultimate goal of providing limited vision to those rendered blind by certain diseases, via a fully implantable retinal prosthetic device.
In both retinitis pigmentosa and age-related macular degeneration — two of the most common causes of vision loss — the photoreceptor layer of the retina is destroyed, but the inner layers remain largely intact, still capable of responding to incoming signals and transmitting output signals to the brain’s visual cortex via the optic nerve. The discovery several years ago that direct electrical stimulation of retinal nerve cells in blind test subjects produced some sense of vision led to the development of the first retinal prosthesis.
Current retinal prostheses are designed to be used with an external (extraocular) camera mounted in a pair of glasses — awkward because subjects must move their heads in order to scan the environment. The miniaturized prototype being developed by the USC team would be directly implantable and would allow for natural eye and head movements.
In order to optimize the design constraints for their ultra-miniature camera, the group performed a series of studies to determine the minimum requirements for vision-related tasks like object recognition, face recognition, navigation, and mobility. They found that surprisingly few pixels were required to achieve good results for many of those tasks: approximately 625 pixels in total, compared to more than a million for a typical computer display. They also found that in many cases blurring images — both before and after they were converted into pixels — resulted in significantly improved object recognition and tracking — even better for moving objects than for static ones.
Taken together, these findings have made it possible to substantially relax the once extremely stringent design requirements of key components of the intraocular camera, thereby reducing the prototype intraocular camera’s size and weight from an object the size of a Tylenol tablet down to an object that is now about one-third the size of a Tic-Tac. Early prototypes have been highly successful in initial tests, although human FDA trials are still at least two years out. (Paper FThT1, “Intraocular Camera for Retinal Prostheses: Design Constraints Based on Visual Psychophysics”)
Near-Infrared LIDAR Helps Pilots
Airline pilots will have more advance warning of potentially hazardous atmospheric conditions — such as icing — using a new near-infrared Light Detection and Ranging (LIDAR) system developed by scientists at RL Associates in Chester, Pa. The system, now in a prototype testing phase, will also provide better images in foggy, rainy or extremely hazy conditions, making it easier for pilots to take off and land in those conditions, thereby potentially reducing flight delays.
Right now, other experimental systems use visible green light to detect the different types of particles in the atmosphere. Most commercial planes, however, don’t have this kind of system, and flights are grounded rather than risk a foggy landing or misidentifying clouds of icy particles. The RL Associates LIDAR system, which could be quickly commercially deployed, is slated for testing in approximately 18 months.
LIDAR exploits the same basic principle as radar, using light waves instead of radio waves. Lasers use light at wavelengths much smaller than radio waves, so they are much better at detecting very small objects. LIDAR already is frequently used in atmospheric physics — but not on commercial planes — to measure the densities of various particles in the middle and upper atmospheres. According to Mary Ludwig of RL Associates, the system uses a laser light beam that is polarized, or has its electric field pointing in a specific direction. The system beams the polarized infrared light out, and then records the amount of polarization that returns to the sensors. Rain and fog return a less polarized signal, and metal and people return a more polarized signal. The data is then processed to form an image of the ground, or could be translated into verbal commands if needed.
The system can better detect different types of particles in the atmosphere, such as ice, supercooled liquid or just regular water vapor. It can also identify the difference between water vapor and other kinds of substances, such as metal or the human body. Ludwig says the RL Associates system is the first of its kind to use near-infrared. The system also employs a “range-gated detector” that is only turned on for very short periods of time when the return signal is expected. This leads to a vastly improved signal-to-noise ratio, resulting in better images, particularly in obscuring conditions such as fog or haze. (FThG4, “Near-Infrared LIDAR System for Hazard Detection and Mitigation Onboard Aircraft”)
Better, Stronger, Faster: High-Throughput Sperm Sorting
Researchers at the University of California, Irvine (UCI) and San Diego (UCSD), have developed a rapid new sorting technique for sperm using a laser trap that can separate stronger, faster sperm from slower sperm. Faster sperm are more likely to successfully fertilize an egg, so the technique could improve the chances of conception via in vitro fertilization by ensuring that only the fastest, strongest sperm are used. The technique could find wide application in animal husbandry and human fertility treatments.
UCI scientist Bing Shao and her colleagues at UCSD have developed a new laser-based technique that enables not only analysis of swimming speed, but on-the-spot sorting of more desirable faster from slower sperm. Shao’s team used special cone-shaped lenses called “axicons,” which, when combined with a standard lens and a laser, form a ring-shaped focus (an annular laser trap). Such an arrangement has been used for laser machining as well as for trapping atoms. The trap acts as a kind of “speed bump” for swimming sperm, depending on the power of the laser used: slower, weaker sperm below the threshold of the laser power being used will be slowed down, redirected, or stopped altogether in the trap, while faster, stronger sperm are hardly affected at all because their energies are above the critical threshold. The researchers used both human sperm and gorilla sperm in their experiments, the latter as a control, since gorilla sperm are slower and weaker than human sperm.
Since X sperm generally are heavier and swim slower, while Y sperm are lighter and swim faster, it is also possible to use this new technique to separate sperm carrying the gene for a female child from sperm carrying the gene for a male child to assist with gender selection. (Paper FWP4, “Annular Laser Trap: A Tool for High-Throughput Sperm Sorting and Analysis”)
Detecting Malaria with Light
It is now possible to analyze large tissue samples for signs of malaria with much greater detail and accuracy. To do this, scientists at the University of Waterloo in Ontario, Canada and Spain’s University of Murcia used a Macroscope — a patented technology developed by Biomedical Photometrics Inc., which enables imaging of much larger tissue samples at a very high resolution — in this case tissue infected with malaria. Using their new patented method and the Macroscope, the researchers measured tell-tale changes in the polarization of light reflecting off a sample of infected tissue.
The malaria parasite changes the polarization of light and this has been exploited to measure population density in blood samples using polarimetry. Melanie Campbell, a researcher at the University of Waterloo and immediate past president of the Canadian Association of Physicists, and her colleagues have extended this approach to analyzing tissue samples. They looked at both infected and normal tissue in their experiments, and used a confocal laser scanning Macroscope to measure changes in polarization and highlight the levels of malaria parasites in the tissue samples. By using the Macroscope to image larger tissue samples at higher resolutions, the severity of infection by the malaria parasite may be accurately quantified.
The technique allows large areas to be imaged in a single scan as opposed to the smaller field available with a traditional microscope. This avoids time-consuming “stitching” of a large number of smaller images and increases data accuracy. Not only could this new approach improve the assessment of the severity of cases of malaria, but it could be extended to assessing different tissues infected with other kinds of biological abnormalities — possibly including proteins associated with Alzheimer’s disease — that also interact with polarized light. (Paper FThK1, “Confocal Polarimetry Measurements of Tissue Infected with Malaria”)
Gigantic Photoresponse Can Speed Up Optical Switches for Faster Internet Speeds New research shows that an ultrafast, ultralarge change in reflectivity can be brought about with femtolasers, those that deliver pulses just quadrillionths of a second in length. Dramatic reflectivity changes will be useful in bringing about direct ultrafast optical-to-optical switches for quicker Internet data transfer, faster computers and other applications. In a recent experiment, femtosecond laser pulses falling on an organic salt target momentarily changed the material from an insulator (a bad reflector of light) to a semi-metal (a good reflector of light). The change in reflectivity this large — more than 100% — has never been achieved before in a photonic material; photo-induced changes are usually more like a few percent. Researchers found that the laser pulse required doesn’t even have to be particularly intense to cause the change.
This “gigantic photoresponse” work began as a Tokyo Institute of Technology – Kyoto University collaboration but now also includes the U.S.’ Lawrence Berkeley Laboratory and the U.K.’s Oxford University. The new advance is that the change in reflectivity can be brought about in tens of femtoseconds rather than 150 femtoseconds. The new results will be reported at the meeting by Jiro Itatani, who has a joint appointment at Lawrence Berkeley Laboratory and the Japan Science and Technology Agency. (Paper FWA2, “Ultrafast Gigantic Photo-Response in Organic Salt (EDO-TTF)2PF6 Initiated by 20-fs Laser Pulses”)
Explaining a 21st Century Version of Young’s Experiment
When light strikes a metallic array of tiny openings, smaller than the wavelength of the light itself, interesting entities known as plasmons may be created. An electromagnetic phenomenon like light itself, the plasmons are waves of electrons that move on the surface of a material like ripples on a pond, but they can oscillate back and forth at the frequency of the incoming light. Like water ripples on a pond surface, plasmons travel in the plane of the metal but with a wavelength smaller, sometimes considerably smaller, than the original light.
Just as light can interact with plasmons, these plasmons traveling between the openings, or “apertures,” can be reconstituted as light at the apertures. The overall effect is that “large” light can pass through tiny holes.
Scientists are now running experiments to find how the plasmons appear and reform into light by passing light through apertures in various ways. One way is to do the plasmon version of a common high school physics lab experiment: passing waves through two slits, and watching how they interact on the other side. In a high school lab, the waves would be made of water; in the latest experiments, physicists examined the intermediary step in which the plasmons are created near the aperture, pass through, and then reform into a light wave on the other side. This kind of test results in interference patterns from which the coherence altering influence of surface plasmons can be deduced.
C.H. Gan of the University of North Carolina (UNC), Charlotte will report on some new theoretical predictions about the coherence properties of light transmitted through the slits. The theoretical predictions were done by computer simulations of the plasmons’ action. The detailed simulations, done with collaborators Greg Gbur of UNC Charlotte and T.D. Visser of the Free University of Amsterdam, show how surface plasmons traveling between the apertures result in a correlation of the light fields emitted from the apertures. Gan shows how this effect can be tuned (such as by varying the size or spacing of the slits). This tunability in turn has the potential to be exploited in new, potentially high-resolution, high-quality forms of coherence-related imaging. (Paper FTuS3, “Surface Plasmons in Young’s Experiment Modulate the Spatial Coherence of Light”)
PLENARY SESSION: NANOPHOTONICS AND OPTICAL FREQUENCY COMBS
At the plenary and awards session, two speakers will present topics that span numerous realms in cutting-edge optics. Nobel Laureate John L. Hall contributed significantly to the development of the laser, helping to take it from a laboratory curiosity to one of the fundamental tools of modern science. In his talk, “The Optical Frequency Comb: A Remarkable Tool with Many Uses,” he will describe a recent measurement tool that can verify assumptions involving miniscule distances within atoms yet also potentially help detect Earth-like planets outside our solar system. Dr. Hall is a senior fellow emeritus of the National Institute of Standards and Technology (NIST) and an adjoint fellow of JILA (formerly the Joint Institute for Laboratory Astrophysics).
The second talk, “Nanophotonics: From Photonic Crystals to Plasmonics,” will be presented by Eli Yablonovitch of University of California, Berkeley. The natural world is filled with crystals, structures made of building blocks arranged in a repeating pattern that interact with electron waves. In his talk, Yablonovitch will start by discussing photonic crystals, artificial, multidimensional, periodic structures that are intended for electromagnetic waves. Such nanophotonic structures are now being designed and used in electronic chips, silicon-on-insulator structures that can reduce current leakage and power consumption in state-of-the-art computer chips. Miniaturizing the structures further will take us toward nanoplasmonics, metallic-wired electrical circuits running at optical frequencies.
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Source: Colleen Morrison
Optical Society of America
