Infrared laser quasi-elastic light scattering: A low-intensity infrared laser could form the core of a non-invasive screening technique for amyloid-beta plaques in the human lens. Such plaques, an early sign of Alzheimer’s disease, first appear around the periphery of the lens and are hidden by the iris.
A window into Alzheimer’s disease
Lee E. Goldstein, assistant professor of psychiatry at Harvard Medical School, turned the audience’s attention to a looming public health problem: the growth of Alzheimer’s disease among a population that is otherwise living longer than ever.
Alzheimer’s disease, first identified 100 years ago, is the result of a sticky, neurotoxic peptide called amyloid-beta that invades the brain and destroys its cells. Unfortunately, by the time the clinical symptoms of the disease manifest themselves, the illness has been progressing for a decade or more, and the damage is irreversible.
Two years ago, Goldstein and colleagues were the first to identify amyloid-beta plaques inside the lenses of the eyes of Alzheimer’s patients. “We had a great deal of trouble getting this published, because no one believed it,” he said. “It’s a brain disease—why would it be present in the lens?” But it was the first evidence that Alzheimer’s is indeed a systemic disease and not just a neurological disorder.
Not only do these lens plaques look different from age-related cataracts, they also present an optically accessible diagnostic window. Goldstein and his team, which includes laser physicist Anca Mocofanescu, are working to develop screening techniques for detecting amyloid-beta aggregates as small as 30 nm in patients’ eyes. Quasi-elastic infrared light scattering is a non-invasive technique that holds much promise for identifying the peptide so that physicians can start treating Alzheimer’s patients at a much earlier stage of the disease.
Integrating silicon photonics into computer chips could remove a big bottleneck in the data streams within computers. However, because of silicon’s indirect bandgap, it has been extremely difficult to make a light source that is compatible with low-cost CMOS manufacturing techniques.
Finding ways to bypass that indirect bandgap has become a major goal in photonics research. In light of potentially groundbreaking research that appeared in Optics Express last fall, FiO planners scheduled a last-minute talk by Alexander Fang of Intel Corp., one of the developers of a newly announced hybrid silicon laser (Opt. Express, 14, 9203, and OPN November 2006, p. 8). Fang and colleagues devised an electrically pumped laser combining silicon and indium phosphide.
Cary Gunn, vice president for technology of Luxtera Inc. of Carlsbad, Calif., said his company is already making processors with external light sources for optical communications. Luxtera uses Freescale Laboratories’ 0.13-µm silicon-on-insulator manufacturing process, which was also used for the Motorola Corp. PowerPC chips that formerly powered Apple Macintosh computers. “We’ve lowered the cost of the optics so much that the packaging is the dominant cost factor,” Gunn said.
Luxtera’s products include one chip with two independent 10-Gbit/s transceivers and another with a single 40-Gbit/s wavelength division multiplexed transceiver. The company is not pursuing the silicon laser, but rather has embedded other optical components into CMOS chips. Its products are expected to hit the market in early 2007.
FiO and its companion American Physical Society conference, Laser Science XXII (LS), kicked off with two plenary lectures on femtosecond and attosecond physics—two of the hottest topics in optics.
The field of femtosecond optics is already more than 30 years old, with the first sub-picosecond pulses coming in the early 1970s, recalled Erich P. Ippen, a past OSA president (2000) and a physics and engineering professor at the Massachusetts Institute of Technology. The advent of compact solid-state lasers in the late 1980s led to additional breakthroughs in the discipline.
Applications of ultrafast phenomena have exploded into other fields, especially biology and chemistry, Ippen said in his acceptance speech for the Frederic Ives Medal/Jarus W. Quinn Endowment. In 1999, Ahmed Zewail of Caltech won the Nobel Prize in chemistry for studying chemical-reaction transition states with femtosecond spectroscopy.
Even shorter wavelengths are coming in the future, from attosecond pulses to femtosecond X-rays, Ippen said. X-ray free-electron lasers—fourth-generation light sources—will bring new insights into molecular structural dynamics.
In the LS plenary talk, the winner of the APS’s Arthur L. Schawlow Prize, Paul B. Corkum of the National Research Council of Canada, called the attosecond world a blend of optical science and collision physics. Attosecond physics is becoming a rich technology that can be applied to many subjects such as atomic and molecular dynamics, Corkum said.
From a classical perspective, a strong electric field drives an electron from an atom. The electron can collide with the ion from which it left and it can do one of three things: knock an electron free, Coulomb-deflect or elastically scatter, or recombine to the level from which it left. In the last case, it emits a burst of light, which is the attosecond pulse. From the perspective of quantum mechanics, one portion of the wave stays behind while another travels along another path, making an interferometer.
“Optical biopsy” of the living retina: In vivo three-dimensional ultrahigh resolution OCT of a normal human retina at different views (a, b) with simultaneous fly through B-scans of the whole volume (upper left corner). Virtual C-scans system (c-f) enables arbitrary horizontal removal of different retinal layers revealing morphologic information inside the scanned volume.
Adaptive optics and other types of imaging
Researchers from the University of California reported that they had developed a scanning laser ophthalmoscope that could be used in a patient care setting. The device uses a micro-electro-mechanical (MEMS) deformable mirror in its adaptive optics (AO) system, according to Yuhua Zhang of UC-Berkeley. The ophthalmoscope, which the team also described in an Optics Letters article, provides increased brightness and improved contrast over non-AO systems and could improve the diagnosis of retinal disease.
In a related talk, Donald T. Miller of Indiana University showed that two kinds of AO cameras can image retinal photoreceptors. In addition to their conventional flood illumination camera, Miller’s team built a device using ultra-high spectral-domain optical coherence tomography (OCT).
A group led by Wolfgang Drexler of Cardiff University (Wales) used ultra-high-resolution OCT to perform an “optical biopsy” of the living retina. The researchers’ three-dimensional scanning achieved a resolution of 3 µm.
Of course, AO is most famous for its use in astronomy, and Lawrence Livermore National Laboratory is developing a pulsed 589-nm laser system that would create a sodium guide star at an altitude of roughly 100 km. The preferred upgrade architecture includes six 50-W pulsed lasers with dynamic refocusing, according to Livermore scientist Deanna M. Pennington.
As telescope apertures grow, elongation of the laser spot becomes a significant issue because it causes inaccuracies in closing the AO loop. Spot elongation can be mitigated by tracking laser pulses in the 10-km-thick sodium layer of the upper atmosphere. Custom CCDs with elongated pixels are required to track the pulse propagation through the sodium layer, and it appears to be practical to make such CCDs, Pennington said.
Livermore’s laser will be installed at Lick Observatory in late 2007 for a visible-light demonstration with an AO-corrected laser uplink, according to Pennington.