Researchers Create Gold Aluminum, Black Platinum, Blue Silver Using Tabletop Laser

[n2doc]

ScienceDaily (Feb. 1, 2008) — Using a tabletop laser, University of Rochester optical scientists have turned pure aluminum, gold. And blue. And gray. And many other colors. And it works for every metal tested, including platinum, titanium, tungsten, silver, and gold.


Chunlei Guo, the researcher who a year ago used intense laser light to alter the properties of a variety of metals to render them pitch black, has pushed the same process further in a paper in today's Applied Physics Letters. He now believes it's possible to alter the properties of any metal to turn it any color—even multi-colored iridescence like a butterfly's wings.

Since the process changes the intrinsic surface properties of the metal itself and is not just a coating, the color won't fade or peel, says Guo, associate professor of optics at the Institute of Optics at the University of Rochester. He suggests the possibilities are endless—a cycle factory using a single laser to produce bicycles of different colors; etching a full-color photograph of a family into the refrigerator door; or proposing with a gold engagement ring that matches your fiancée's blue eyes.

"Since the discovery of the black metal we've been determined to get full control on getting metals to reflect only a certain color and absorb the rest, and now we finally can make a metal reflect almost any color we wish," says Guo. "When we first found the process that produced a gold color, we couldn't believe it. We worked in the lab until midnight trying to figure out what other colors we could make."

Guo and his assistant, Anatoliy Vorobeyv, use an incredibly brief but incredibly intense laser burst that changes the surface of a metal, forming nanoscale and microscale structures that selectively reflect a certain color to give the appearance of a specific color or combinations of colors.
The metal-coloring research follows up on Guo's breakthrough "black metal" discovery in late 2006, when his research team was able to create nanostructures on metal surfaces that absorbed virtually all light, making something as simple as regular aluminum into one of the darkest materials ever created.

Guo's black metal, with its very high absorption properties, is ideal for any application where capturing light is desirable. The potential applications range from making better solar energy collectors, to more advanced stealth technology, he says. The ultra-brief/ultra-intense light Guo uses is produced by a femtosecond laser, which produces pulses lasting only a few quadrillionths of a second. A femtosecond is to a second what a second is to about 32 million years. During its brief burst, Guo's laser unleashes as much power as the entire electric grid of North America does, all focused onto a spot the size of a needlepoint.

more:

http://www.sciencedaily.com/releases/2008/02/080201090845.htm


Gold Aluminum, Blue Titanium, Gold Platinum. (Credit: Richard Baker, University of Rochester)

[Occulus]

1. WOW

:wow:

Now that's cool.

[FiveGoodMen]

2. "Guo's laser unleashes as much power as the entire electric grid of North America does"

Where does that power come from and what does that sudden demand do to the rest of the grid?

[caraher]

3. It's all in the duty cycle

The peak power is enormous, but the fraction of time during which it's on is even smaller. To take some typical values for that kind of laser you might get a peak power of a terawatt (1 billion watts) but the total energy of 1 pulse is it's average power times its duration, which might be 50 femtoseconds (or 50 millionths of a billionth of a second). Multiply those numbers and you get 50 millionths of a joule.

The average optical power is the energy of one pulse times the number of pulses per second. The maximum pulse rate for these lasers is around 100 million times per second, so even if you had 50 microjoule pulses the average optical power would be 5 kW. And that isn't possible because you need to selectively amplify pulses to reach that 50 microjoule power. Most of the pulses are simply thrown away. Thus, the repetition rate for amplified pulses is many orders of magnitude smaller (1000 amplified pulses per second is pretty common, though some lasers can reach maybe a few hundreds of kHz). An especially powerful amplified ultrafast laser might put out an average of 5 watts. But duration of all the short pulses in one second of operation for, say, 50 femtosecond pulses at amplified at 1 kHz, would be 50 trillionths of a second each second.

The grid is disturbed by these lasers by about the amount of turning on a hair dryer.

[FiveGoodMen]

5. Meaning they must have some serious capacitors to build up the juice between firings

Yes?

[caraher]

7. They can...

but the way it works, probably not to the extent you might think, because the transformation of energy from electrical to optical takes place over time scales long compared to the optical pulse.

It's basically a two-stage process. First you make a train of femtosecond pulses in a laser cavity that itself draws its energy from a continuous, typically 5 watt laser (these days, a diode pumped solid state laser, but an argon ion laser works too). The shortness of the pulse comes from something known as Kerr lens modelocking (once called "magic" mode locking before the theory was worked out) and not from having a short-pulsed energy source. It's a nonlinear optical effect that involves getting a number of slightly different frequencies of light to circulate in lockstep within the laser cavity.

The second part is amplification, and the way that works is you take a high-powered pulsed laser (this one usually does have those big capacitors) and dump energy from that into the same material as the original lasing medium (almost always titanium-doped sapphire). This pulsed laser might have a pulse duration of a few nanoseconds, and the energy it deposits in the amplifier crystal would typically hang around for maybe a few microseconds before it's lost to spontaneous emission. (But of course, you want that energy to go into the femtosecond pulse and not fluorescence!) You send one of your ultrafast pulses through the crystal several times, and on each pass it absorbs some of that energy.

One detail I've omitted is the challenge of making an intense pulse without destroying the amplifier crystal. The trick is to take your femtosecond pulse and "stretch" it out temporally so it's much longer, but doing so in a way that let's you recompress it after you've added energy. The typically way of doing this is by making light of different frequencies take different amounts of time to reach the amplifier. This is called "chirping" the pulse (imagine what a sound whose frequency increases or decreases with time sounds like - this is the "chirp"). One way to do this is with a diffraction grating and system of lenses such that different colors travel different distances; other techniques involve optical fibers with carefully engineered properties. After the pulse soaks up the energy you undo the original chirping (by similar means), which leaves you with a femtosecond pulse once again.

[FiveGoodMen]

8. Thanks! Now THAT's a detailed answer.

A little bit over my head, but much appreciated.

[Occulus]

11. Gosh, that was a great couple of posts

How is it you know so much about this subject? I take it you've worked with/studied high-powered lasers at some point.

Could you tell us how you came by that bit of knowledge above?

[krispos42]

9. It can run off of a wall socket

To alter an area of metal the size of a dime currently takes 30 minutes or more, but the researchers are working on refining the technique. Fortunately, despite the incredible intensity involved, the femtosecond laser can be powered by a simple wall outlet, meaning that when the process is refined, implementing it should be relatively simple.

Energy equals power times time. this uses a wad of power in a very brief period of time, so the actual energy amount is small. It's like swinging an axe. All that mass takes time to swing, but is delivered onto a tiny area in a tiny fraction of a second.

[Random_Australian]

4. Science - it works, bitches!

Nothing more needs saying. Science rules.

[Nihil]

6. Personally ...

... I think paint does a better job ...

[krispos42]

10. Hmmmm...

So in the future, we could have things made out of steel, with an ultra-thin coating of gold on them to resist corrosion, then use the laser to "paint" the gold whatever color we want?

No rust, no peeling, and no fading? Just the occasional wax job?

Sign me up!