Stanford Photonics In 2013

Accelerating into X-rays, environment and life-sciences.

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Every September, the Stanford Photonics Research Center and their partners at Scottish Universities and in industry showcase the latest developments in laser applications at an on-campus symposium in Palo Alto, Calif. This year, highlights included remarkable developments in particle accelerators, pollution detection and bio-medicine and looked toward increased societal impact through the American National Photonics Initiative and the UK-based SU2 Program.

Laser accelerators
Stanford has long been a center for photonics, microelectronics and electron accelerator development, but accelerator technology has stayed mired in the vacuum-tube era rather than benefiting from Moore’s Law or super-high frequency. There is a reason for that: The Lawson-Woodward Theorem prevents freely propagating waves from providing unidirectional acceleration. To get an electron to high energy requires it to surf down the slope of an electric potential that always points in the direction it is going and moves at the same speed as the accelerating electron. That cannot happen in a freely propagating light wave, and so all accelerator technology relies microwaves confined to centimeter-sized cavities stacked one after another for meters and kilometers.

There are two ways around that difficulty: The laser wake-field accelerator uses a short laser pulse to ionize a plasma. An electron that happens to follow the laser pulse at high enough speed sees a plasma wave that can accelerate it further – up to 1GeV/cm – according to Prof. Chan Joshi of UCLA. Scattering another laser pulse off the accelerated relativistic electrons yields bright keV-to-MeV X-rays from a 5µm source. According to Don Umstadter of the University of Nebraska – Lincoln, such unique table-top sources could replace building-scale linacs for research and therapy.

The other way, shown by Edgar Peralta of Stanford, is to use the electric field near the surface of a phase-shifting dielectric grating, illuminated from behind by a powerful laser. If the grating-modified field always points forward as the electron moves parallel to the surface at near the speed of light, the electron is accelerated. The grating period is wavelength-scale, of course, and the field only extends a micron or so beyond the surface, requiring careful alignment, but potentially this lithographically fabricated micro-accelerator technology might shrink a linac to wafer dimensions!

Trace gas detection
How do you tell when water pumped from under a desert or melted from an ice core last fell as rain? Naturally occurring radioactive isotopes provide a useful clock, if you can find and detect those atoms. Most people are aware of 14C which is created by cosmic rays, but only lasts a few thousand years. If you need to date something around a million years old, you need 81Kr, which is really rare (6000 atoms per mole of air). Nevertheless it is possible to find, capture and detect fluorescence from individual atoms of that isotope using lasers, according to Zheng-Tian Lu of Argonne National Laboratories. It does take 100kg of water or ice to liberate enough gas for a proper measurement, though. Lu showed that water under the Egyptian desert has been there 300,000 years.

Methane and other pollutants aren’t that rare in the atmosphere, but telling the difference between biogenic methane and natural gas leaks means identifying the ratio of stable carbon isotopes in the source. Chris Rella of Picarro, Inc. described technology that did just that in the Greeley, Colorado gas field, which lies under cattle country and downwind from methane-producing municipal dumps. The trick was to use cavity ring-down spectroscopy (CRDS) and an “air core” (a long tube into which air is sucked and stored) while driving around in a GPS equipped van. The prompt signal from the CRDS drive-by scan identified the source location and a later probe of the air stored in the “core” allowed a leisurely determination of isotope ratios. It turned out that 78±13% of the methane detected was natural gas leaked by specific defects in the collection system, with nothing oozing out of the ground due to fracking and only 22% or so from other sources.

Large molecules like formaldehyde can also be detected using robust airborne laser sensing technology based on difference frequency generation (DFG), where two fiber-amplified diode lasers mix in a crystal to produce longer wavelength light. Dirk Richter of the University of Colorado described flights over Houston that revealed point sources of that smog-forming pollutant mapped to petrochemical plants using an automated multi-wavelength DFG system.

Medicine
Perhaps because the symposium took place on the Medical School Campus, there were far too many health-related applications to report them all. Daniel Palanker of Stanford reviewed optical technologies to restore sight for the blind, including retinal implants, in one plenary talk. Clinical application of novel tools such as Raman spectroscopy for colon cancer detection and optical sectioning for brain-tumor resection filled one session. Two others covered stem cell imaging with and without molecular labeling. Monochromatic X-ray sources facilitate phase-contrast imaging of biological systems, perhaps improving detection of cancers in real patients and perhaps not. Next year, SPRC will certainly report even more remarkable possibilities.