Laser asteroid defense

Could lasers protect the Earth from wayward asteroids? A number of schemes have been proposed for pushing asteroids gradually to move their orbits away from the planet. Now, two California professors are proposing a bold scheme to build solar-powered space lasers powerful enough to evaporate a 500 m asteroid in about a year--or to make short work of a 17 m asteroid like the one that exploded near Chelyabinsk, Russia, on February 15.

Philip Lubin of the University of California (Santa Barbara, CA) and Gary Hughes of California Polytechnic State University (San Luis Obispo, CA) began planning the project they call DE-STAR--for Directed Energy Solar Targeting of Asteroids and exploRation--a year ago. On February 14, they issued a press release timed to the close approach by asteroid 2012 DA14. They were as stunned by the Russian explosion as everyone else.

Their bold proposal seeks to take advantage of the dramatic improvements in high-power diode lasers and solid-state lighting to build giant orbital phased arrays of lasers powered by electricity from huge solar panels. They envision starting with a desktop 1 m array called DE-STAR 0, then scaling up to a 10 m array called DE-STAR 1. They have proposed that NASA support a conceptual study of scaling up to a 10 km DE-STAR 4 array, powerful enough to vaporize a half-kilometer asteroid 150 million kilometers away. Even bigger versions could be used for laser propulsion; they estimate that a 1000 km DE-STAR 6 array could accelerate a 10 ton spacecraft close to the speed of light.

Future DE-STAR array samples composition of an asteroid as it propels an interplanetary spacecraft. (Courtesy of Philip Lubin)

The scheme may sound fantastic, but Lubin says it violates no laws of physics and requires no "technological miracles." It merely envisions continuing technological progress at the rate of the past 50 years, which took us from the feeble LEDs and diode lasers of 1963 to today's powerful emitters. They assume photovoltaic cells that can convert 70% of incident solar energy into electricity, and diodes which can convert 70% of the input electrical power into light.

Lubin doesn't think it will be easy. He worries about issues including the mass needed to build the giant array, and controlling output phase across the array with the precision needed to tightly focus the emission. But he predicts his assumptions will be considered "extraordinarily conservative and modest" in 30 to 50 years.

That remains to be seen, but space-based solar-powered diode arrays are worth investigating. They could go beyond asteroid defense to could help move asteroids, collect valuable materials from them, or provide power resources in space--as well as inspiring some fun science-fiction stories.

Bright future for silicon

The Wiley-VCH journal ChemPhysChem issued an embargoed press release embargoed early on the morning of November 21, 2012, heralding "a bright future for silicon." Just eight hours later, they lifted the embargo, citing "early reporting" of the research by Brian Korgel of the University of Texas (Austin, TX) and colleagues.

Embargo breaks often indicate hot stories, and the headline hinted at an important step toward the elusive goal of efficient light emission from silicon. Yet the next line was more muted: "Ordered nanocrystal arrays may provide a new platform to study and tailor the light-emitting properties of silicon." What is the real story?

Silicon is a wonderful material for electronics, but its photonic uses have been hobbled by an indirect bandgap that makes it very hard for electrons dropping into the valence band to release their energy as photons. That leaves silicon far behind III-V compounds like gallium arsenide for LEDs and diode lasers. Yet silicon is far ahead of other semiconductors in electronics, and companies like Intel (Santa Clara, CA) want to integrate photonics into their integrated circuits.

So far they have demonstrated "silicon lasers" by optically pumping Raman lines in silicon and III-V diode laser chips bonded to silicon. Both were important advances. But neither met the real goal--electrically powered emitters based on silicon that could be integrated into standard semiconductor chip production processes.

In their ChemPhysChem paper, Korgel and colleagues take a different approach, tapping the bright luminescence produced by silicon quantum dots. They write that their major achievement is devising a chemical technique that causes self-assembly of "the first colloidal Si nanocrystal superlattices." Self-assembly is essential because individual dots are too small to fabricate by conventional photolithography, and transmission electron microscope images show the dots are closely spaced in regular face-centered-cubic arrangements (see photo).

TEM image silicon nanocrystals in the 111-oriented (c) and 112-oriented (d) plans, with depictions of the crystalline structures shown in insets. (Courtesy Yixuan Yu et al., ChemPhysChem, Wiley-VCH Verlag GmbH & Co. KGaA, http://dx.doi.org/10.1002/cphc.201200738 [2012]. Reproduced with permission)

The authors say that covalent bonds with the hydrocarbon solvent make the silicon-nanocrystal superlattices stable to 350 degrees Celsius, higher than other similar superlattices. That's encouraging news, because self-organized nanocrystals are a promising fresh approach to structuring silicon to emit light more efficiently. But so far electrical excitation--sought for integrated optoelectronics--has far to go to match the efficiency of optical excitation of isolated silicon quantum dots. So Korgel is understandably optimistic about having "a new playground for understanding and manipulating the properties of silicon in new and unique ways," and is appropriately cautious in not claiming silicon lasers are just around the corner.

Making solid-state lighting fun

Solid-state lighting is a clean, green new market for optical technology, but it's hard to get very excited about white LEDs that merely replace older incandescent and fluorescent bulbs. Now, Philips is trying to make solid-state lighting fun with wirelessly controlled color-tunable bulbs called "Hue".

A Hue bulb screws into a standard light socket and contains red, green, and blue LEDs. A smartphone or iPad app controls the bulb's output through a wireless controller and a wireless receiver in the bulb. The app matches the LED outputs colors selected from a rainbow palette in the app, or from the user's favorite photos. Users can pick bright disco colors, shades of white from candlelight to sunlight, or anything in between.

A $200 starter set including the controller and three bulbs sounds like an impulse buy at the Apple Store -- and that's exactly where Philips is selling it, as a fun gadget. A single 600-lumen Hue bulb will set you back $60, more than triple the price of a Philips Ambient bulb that emits a pleasant white light. But playing with colored lights is much more fun, as Philips shows in a video.

The Hue isn't just a party light. You can set it to emit shades of white from a bright "energize" tone to start the morning to a warm "relax" shade to unwind in the evening. You can set each bulb to turn on and off when you want it. So it's an all-purpose adjustable light ready to put into any socket in the house, without costly rewiring.

Philips is first to market, but company is coming. LiFx (San Francisco, CA) in September sought support on Kickstarter to develop their own smart bulb, and was surprised to receive $1.3 million in pledges when they had sought only $100,000. They have demonstrated a bench version and now are designing a production prototype, which will include a white LED as well as the RGB emitters.

So far press attention has focused on controls and tunable colors, but I wonder what the green sources are. Philips is using a "lime green" LED from its LumiLEDs division because it gives better color rendering than standard green LEDs, but won't disclose the wavelength or composition. Is it a hard-to-make green LED, a phosphor-LED hybrid, or something else?  If anybody out there has a spectrophotometer and a Hue at hand, it would be interesting to see a spectrum.

iPhone sets a Philips Hue bulb to "relax" for a calming evening. (Courtesy of Philips Lighting)

Light guides light up 3M solid-state bulbs

3M has added a new twist to solid-state lighting--embedding light guides in the outer shell of the bulb to redistribute light emission evenly across its surface like the venerable frosted-glass incandescent bulb.

Solid-state lighting has been widely touted for its outstanding energy efficiency. LED bulbs now in hardware stores draw 13 W of electric power, emit as much visible light as 60 W incandescents, and have lifetimes of 25,000 hours, far beyond 1000-hour incandescents. But high prices and some subtle but significant problems are slowing their adoption.

The 3M bulb is aimed at one of those subtle problems. LEDs emit directionally from a small area. Hot filaments and fluorescent tubes are omnidirectional, and although filaments are small, frosted incandescent bulbs scatter the light so it seems to radiate from entire surface. Directionality is good news for applications that want light concentrated in one direction, such as street lighting outdoors and downlighting in homes and offices. But it can be a problem in light fixtures in the line of sight, especially when the light comes from a small area. An example is a non-name solid-state lamp I bought earlier this year from a big-box hardware store. Light comes from a small zone where blue LEDs and yellow phosphor are mounted, not from the bulb's frosted surface, producing an unpleasant glare.

Deep inside, the 3M bulb contains similar blue LEDs with yellow phosphors to generate directional white light. But instead of shining directly into the room, the light is coupled into light guides embedded in the bulb. Total internal reflection guides the light around the bulb to areas where the light is scattered out the surface and into the room, as shown in the figure. That reduces brightness to an acceptable level, making the bulb much more presentable in a light fixture.

The light guide in the 3M LED bulb carries light from the LED source to diffusing areas on the bulb surface. (Courtesy of 3M)

The bulb, shown in the photo below, can't be mistaken for an incandescent. It needs slits to dissipate heat, a cooling problem that it shares with other LED bulbs, and requires heat sinks that add to its environmental impact. But the design is an innovative step in the right direction, making LED lamps an attractive piece of decor rather than an efficient eyesore.

3M's Advanced LED light distributes light like an incandescent bulb. (Courtesy of 3M)