STANFORD, CA—“The laser field has seen no boundaries, only horizons.” With these words from Gerard Mourou, the Stanford Photonics Research Center launched the first official event of LaserFest, the 50th anniversary of the laser (see www.laserfocusworld.com/articles/364295), by featuring ‘extreme’ lasers at its annual SPRC Symposium. The SPRC 2009 Annual Symposium was held at Stanford University from September 14–16.
Mourou, now a professor at the Ecole Polytechnique in France, described how we are entering a new era in high peak power lasers. The 1960s began the Coulombic epoch, where lasers could excite atoms to higher energy states, but with the electrons still bound to the atoms. The 1990s saw the Relativistic epoch, where lasers can excite particles to relativistic levels. He said that we are about to enter the Nonlinear QED epoch, where lasers may be able to examine the vacuum itself by scattering off of the virtual particles generated in the vacuum (which is essentially the “ether” of yore).
An alternative to particle accelerators
Such new lasers offer an alternative to giant particle accelerators, which are becoming enormously large and expensive. A particle accelerator uses a “momentum paradigm,” meaning that it imparts high momentum to particles and then observes them in a small volume. An extreme ultrafast laser operates in an “amplitude paradigm,” said Mourou. It is good for low-mass particles, perhaps even dark energy, by observing them with low momentum in a larger volume.
The high intensities needed for such experiments may be finally achieved with the ultrafast laser being built at ELI, the Extreme Light Infrastructure (www.
extreme-light-infrastructure.eu). The ELI is the effort of twelve European partners, led by Mourou. He is also the inventor of the chirped pulse amplification (CPA) technique in ultrafast lasers and a pioneer of femtosecond ophthalmology.
The statistics on the ELI laser, once completed, are full of exponents. The goal is to achieve single-shot pulses with peak powers of 1018 W, or 1 exowatt, using 10 kJ pulses of 10 fs duration. When considering the spot size, the project aims to achieve 1023 W/cm2, or perhaps even as high as 1026 W/cm2. This is enough to impart energies of 1 TeV or more to electrons. By comparison, the vacuum breaks down at 1 PeV—‘only’ about 1 million times greater.
Another extreme: laser-based fusion
At another extreme, the National Ignition Facility (NIF) at Livermore, CA will be the most energetic laser system in the world when it is fully operational. In fact, each of the parallel solid-state lasers of the facility is the largest in the world. All 198 of them are combined and focused to a target the size of a peppercorn inside a can the size of a pencil eraser.
Principal associate director Ed Moses presented the NIF operation, saying that it is not so much the biggest laser on earth, but “the smallest laser that can achieve fusion.” He pointed out that projects of this kind take about 20 years to complete. In NIF’s case, it took 10 years to get from concept to groundbreaking, and 10 more years to build the facility itself.
Once again, the statistics on the laser system are mind-boggling. Each lamp-pumped solid-state laser will produce 20 kJ pulses in the infrared, which will be frequency shifted and combined to create a 1.8 MJ pulse in the ultraviolet. The pulses will be about 20 ns duration, and can be repeated about once every four hours. The combined energy will heat the target to about 100 million K, or more, about the scale of “a hand grenade or two.” The heating is so rapid that the plasma is inertially confined, thereby imploding the pellet, creating a pressure of about 100 billion atmospheres on the deuterium, according to the plan.
The NIF has three missions. First, it provides a way to verify theoretical models of the aging nuclear weapons arsenal, while still abiding by a nuclear test ban. Second, it can advance the study of fundamental physics as it recreates the conditions of the sun’s interior. And third, it offers a path to clean fusion energy. If successful toward the fusion energy goal, the next step would be to build a prototype reactor dedicated to power generation, with a steady state repetition rate and more efficient diode-pumped lasers. The other contender in fusion technology, magnetic confinement, is many years behind the laser technology, Moses said.
The lab is also interested in hybrid fusion-fission reactors that could use the neutrons generated in the fusion process to generate a subsequent fission process. Such a reactor may be able to use spent fuel. The vision is that the byproducts would not be radioactive and could be buried in a common landfill.
NIF officials are now eager to explain the vision to the broader public, after working 20 years to get the facility to this point, and it’s hard not be impressed with the effort NIF has brought the project to this point. There is also no question that these extreme laser projects bring substantial funding both to scientists and the commercial suppliers of key components (see www.laserfocusworld.com/articles/
357970). Many in the audience nonetheless seemed quietly skeptical that we are any closer to an actual fusion reactor, or that the hybrid fusion-fission reactor was feasible, perhaps because they have heard so much about laser fusion over the decades, with no real progress. In any case, with its multiple missions, the NIF may be able to declare success without actually hitting a home run.
Extreme lasers in x-rays and biology
In yet another presentation of extreme lasers, John Galayda presented the brightest laser to emit hard x-rays. The laser, called the Linac Coherent Light Source, is at the SLAC National Accelerator Laboratory, next to the Stanford campus. Galayda heads the effort, and pointed out that it took them 14 years to get from concept to groundbreaking, and four more years of construction. The laser turned on at the first try this year, and has already exceeded its design goals, producing 2 mJ pulses of 80 fs duration. It is tunable between 0.15 and 1.5 nm, in the hard x-ray range. Researchers interested in using the facility are encouraged to submit proposals, as described at lcls.slac.stanford.edu.
In a striking presentation of the practical use of extreme lasers and subwavelength imaging to examine cell biology, professor Steven Block showed how his Stanford lab and other scientists have used optical tweezers to study a family of proteins, appropriately called kinesins, that act as molecular motors within cells. The proteins move molecular cargo within cells along the fibrous cell structure. There are neuron cells, for example, that extend the length of our leg, or even a giraffe leg. The tweezers can manipulate the cargo and measure the mechanical forces of the kinesin, which helps uncover the nature of the protein and its role within the cell.
The SPRC Symposium was well attended throughout the sessions, which covered Stanford research topics ranging from slow light sensors and optical interconnects to nanomanufacturing, photovoltaics, and more. Student talks were sandwiched between the many invited speakers. One intriguing student topic explored thermal insulators better than vacuum through the use of photonic crystals that block thermal radiation.
—Tom Hausken, director of components research,Strategies Unlimited
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