Today’s article is entitled “Lightning Control By Lasers.”

The goal of these projects is to design essentially next-gen lightning rods that could shoot lasers at thunderclouds which would then cause a lightning strike at the laser source (which would be a grounding rod for the discharge).

To create a discharge like this, the laser utilizes short, intense pulses to ionize air (similar to what we use in our lab), which would then act as the conduit for the lightning to follow.

The laser they want to build is also pretty sick.

While it has the same wavelength (800nm) and pulse lengths (70fs – 2ps) as our lab system it’s power is over 100 times greater than the new half-million dollar system that was just installed (and is nearly 1000 times greater than the older laser system). Their laser will put 350 mJ into every pulse that it fires, giving it a peak power of 5 TW (or 5×10¹² Watts, equivalent to a billion microwave ovens running at once) at 10 Hz rep rate (one pulse every tenth of a second).

Basically, do not stand in the way of this laser.

And finally, because this needs a picture:

On the top is normal electrical discharge (what a small lightning strike would look like), and on the bottom is the laser guided lightning.

This just screams for an evil mad scientist and a plot to take over the world.

From: “Lightning control by lasers,” Nature Photonics, Vol. 3, no. 3, pp. 120-121 (2009).

Knowing someone like this gets you the knowledge, that apparently kept quiet by our government, that there is snow falling on Mars.

Now, to bring this to photonics, let’s look at the technique of LIDAR that was used to detect the falling atmospheric snow. Unfortunately the snow vaporizes before it reaches the ground, so there won’t be any snow-Martians.

LIDAR stands for “LIght Detection And Ranging” and relies on firing laser pulses into the sky and looking for bits that are reflected back by debris in the atmosphere. It’s analogous to when you shine a laser or flashlight into the fog and some of it gets scattered back at you.

The principle is fairly simple, and the only difficulty lies in collecting the scattered laser pulse. Luckily, signal-to-noise is greatly improved with the use of coherent laser pulses, so you at least know when you are seeing something.

That’s all for this week.

Next up, a group has developed a double-sided solar cell. Traditionally, solar cells can only generate power with light illuminating one side of the cell, but with this technology a greater amount of light could be generated. This technology relies more on photosensitive dyes as opposed to traditional silicon, and could potentially drop the cost of solar cells significantly.

Why is two sides better? Imagine putting a mirror on the bottom of the solar panel, and instantly doubling its efficiency! [Bifacial dye-sensitized solar cells based on an ionic liquid electrolyte – p 693-698 Seigo Ito, Shaik M. Zakeeruddin, Pascal Comte, Paul Liska, Daibin Kuang & Michael Grätzel doi:10.1038/nphoton.2008.224]

Finally, a group has developed an optical trap – a laser that’s used to capture atoms – using an airy function (which should appeal to the math nerds) as the shape of the beam. I can’t comment on whether this is any more successful than other optical traps, but it is cool to know that atoms can be finely controlled by light. [Optical trapping: Riding along an Airy beam – pp652 – 653 Demetrios N. Christodoulides doi:10.1038/nphoton.2008.211]

Apparently, Scotch tape, when unrolled at 3 cm/s will produce x-rays. UCLA researchers have observed x-rays being released from unrolling scotch tape under vacuum.

Although the Edmonton Journal article is a bit fuzzy on the science, the journal Nature describes the process as “tribouminescence” which occurs when a crystal is rubbed. Basically, as the tape is pulled away, charge gets built up on either the roll or the peeled off piece. As the charge accelerates back in the vacuum, it releases radiation bursts across the electromagnetic spectrum.

The researchers oddly go far enough to suggest there’s enough energy in unrolling tape that it could generate nuclear fusion – the holy grail of power generation. The tape does release enough x-rays, however, to image the bone of a (grad students?) finger.

They note that the radiation is only released in a vacuum. This is likely because under non-vacuum it would discharge immediately.

Industrial laser cutters are typically made from CO₂ or solid state Nd:YAG lasers, which are among the highest (continuous) output power lasers that are commercially available. Laser output powers are routinely up to 2 kW continuous. By focusing the laser onto the material meant to be cut. The intense electric field essentially turns solid material to plasma – or ionized gases. This means that lasers can cut materials that are traditionally extremely difficult to cut. The laser is typically controlled by a computer so the precision of laser cutting is typically higher than traditional means. And finally, since light does the cutting, there is no blade to be replaced.

In recent years the ability has been developed to employ laser cutters for very fine projects. Laser micromachining refers to the process of using lasers to etch devices on the micron scale. This opens the field for mass production of microfluidics (pictured), ink jet printer components, x-ray apertures, ruby-based orifices, and leak testing components.

Laser cutting is still a rather expensive technology, but don’t be surprised if in a decade or so your children’s high school shop class has a laser in it.

A photonic crystal is a physical array of a material with a periodic index of refraction. Typically the crystals are made in either 2 or 3 dimensions. The period of the material is typically comparable to the wavelength of the light meant to be controlled (so half a micron for visible). Photonic crystals are often used to control the direction and flow of light (as crystals can be designed to prevent the flow of certain wavelengths and gaps can be left to direct light through the crystal – like a channel).

The result of this group was to create a device which could be easily fabricated, and could efficiently couple light from the source to the crystal, from which it could be extracted to a fibre optic cable or put to other uses.

As can be seen in the image (from ref), the wire is situated in the middle of the device and is capable of generating two wavelenths of light. One frequency can make it through each crystal array on either side of the wire. This divides the light and provides the ability to extract one of two colours (or both) at the same time.

• Hong-Gyu Park, Carl J. Barrelet, Yonging Wu, Bozhi Tian, Fang Qian and Charled M. Lieber, “A wavelength-selective photonic-crystal waveguide coupled to a nanowire light source”, Nature Photonics ADVANCED ONLINE PUBLICATIONS, 7 September 2008.

The Maritimes of Canada (the provinces on the East coast) are not typically thought of in terms of groundbreaking research, yet at Dalhousie University Professor Kimberley Hall has been establishing an impressive optics lab over the past few years. And with a recent deal with Lockheed-Martin, Dr. Hall has her sights set high.

Dr. Hall’s research focuses on ultrafast spintronics, or using ultrafast lasers to probe the interactions of magnetic dipoles (the smallest unit of magnetism) with external forces. By understanding the processes involved with magnetic spin, and being able to control it, an entire new field of information processing could potentially be opened, based off of the spin states of electrons (spintronics), as opposed to the electric states (traditional electronics).

An advantage of spintronics is that spin states are inherently binary, or based on 1 and 0s (or in spin-terms, up and down). Whereas with electronics we typically relied on voltages, which could vary and make mistakes. Spintronics is an exciting field in the near future for physicists, and is something I’ll likely cover further in the future.

Dalhousie University is located in Halifax, Nova Scotia, and Dr. Hall is currently recruiting for new students and researchers.

First, in the online prepublication of Nature Photonics, there’s an article discussing on-chip near-field terahertz imaging. There’s a lot of buzz words there, so let’s figure out what’s going on.

Author’s Kawano and Ishibashi report the development of a semiconductor chip (like the processor in your computer) which features an integrated terahertz aperture, probe, and detector. The advantage is that with all of this integrated into one casing you firstly save space (and eventually cost), but also gain the ability to do high-resolution near-field imaging.

Typically one cannot take an image of an object that is smaller than the wavelength of the light being used. For example, you can’t use your digital camera to get a picture of something that’s half a micron (10^-6 scanning tunnelling microscope which uses a sharp point to generate maps of surfaces to around a nanometre accuracy.

So what’s cool about Kawano’s paper is that they’ve created a single chip which can generate Thz images with sub-milimeter resolution. Remember that THz-imaging is unique in that it can transmit through things opaque to visible light (like cloth, plastic, etc.) but can still detect metals and some other materials.

• Yukio Kawano & Koji Ishibashi, “An on-chip near-field terahertz probe and detector”, Nature Photonics ADVANCED ONLINE PUBLICATIONS, 10 August 2008.

Now, to go a bit more in depth on Paul Corkum group’s paper on imaging the wavefunction of Nitrogen.

Basically, the wavefunction (or orbital to chemists) is a mathematical construct. That is, it’s something physicists made up to fit the data. However, many made up constructs have proven so useful that it is likely they are accurate representations of the real world. The best example of this is the photon – the idea that light is comprised of particles of waves (or wave-packets). When two molecules wavefunctions interact, we have chemistry. Unfortunately this takes place on time scales of attoseconds (10^-18 s), which until recently have not been accessible to measure. Recent advances in laser technology have made this possible however.

What Dr. Corkum’s group does is take an ultrafast (femtosecond 10^-15 s) laser source, and mix pulses together in such a way that even shorter pulses are created. I can’t fully explain it here, since I don’t fully understand what’s going on in this situation. An analogy can be made when you think of beat frequencies of sound. This happens when you mix two (or more) low frequencies together and can generate higher frequencies (or inversely shorter pulses).

To image the wavefunctions it requires three steps: aligning the molecules relative to the lab, selecting an ionization, and copying the state to a laser pulse.

The first two steps are necessary so that they know what they’re looking at. The method of imaging they use is “computed tomography” which takes one-dimensional projections of an image and creates cross-sections and generates a 3-D image from the information. This requires an object that can be fixed in a reference frame, and rotated. This is apparently not as big of a deal for lasers to do with single atoms, so steps one and two are essentially done.

They then use the attosecond pulses to image slices of the atom – which yields the frequency information about the highest occupied molecular orbital (the valence band, or where all the chemistry occurs) of the molecule. Their process also retains the phase of the wave going through the molecule – which is key to generating the absolute wavefunction, and not just the probability cloud (or the wavefunction squared, which is what is usually detected – this shows where the electrons are “most likely to be”).

If this hasn’t blown your mind yet (it should), they finish the paper by proposing that they should theoretically be able to watch electrons move. What they suggest is that they will be able to watch chemistry happen, i.e. watch a molecule interact with another.

• J. Itantani, J. Levesque, D. Zeidler, Hiromichi Niikura, H. Pepin, J. C. Kieffer, P. B. Corkum & D. M. Villeneuve, “Tomographic imaging of molecular orbitals”, Nature 432, 867-871 (2004).

This ends edition 2 of Photonics Briefs

[tags]photonics, lasers, wavefunctions, quantum mechanics, semiconductors, terahertz[/tags]

• Duncan Graham-Rowe & Rachel Won, “Lasers for engine ignition”, Nature Photonics 2, 515-517 (2008).

Next up, there’s a feature interview with Nasr Hafz, who works at the Advanced Photonics Research Institute in Korea. There he and his colleagues use lasers to accelerate electrons over the span of a centimetre to speeds which used to take several meters of space. The basic idea is that a laser beam enters a plasma (an electrically charged gas) which causes the negative and positive charges to split apart – as the pulse leaves the plasma the electrons accelerate across the gap and a couple are extracted for relativistic electron experiments. The key advantage here is that a typical lab room can perform experiments that used to require entire buildings. Their group is already achieving 1 GeV energies. To put this in perspective, many synchrotrons are able to reach around this level (the new LHC is 14 TeV, or 14,000 times more energetic). This puts a lot of power in a very small space.

• Interview with Nasr Hafz, Nature Photonics, 2, 580 (2008).

Finally, the technology focus (on spectroscopy) features an interview with Ferenc Krausz from the Max-Planck Institute of Quantum Optics and the Ludwig-Maximilians-University of Munich in Germany on attosecond laser technology (which was first discovered in Canada at the National Research Council in Ottawa). Attosecond lasers are the next frontier after femtosecond lasers, and are now the shortest pulses in existence. An attosecond is 10^-18 seconds or one billionth of one billionth of a second. On this time scale some really cool physics can be observed. This is the time scale of the orbit of an electron around an atom. This is also about the time it takes an light to travel the length of a few atoms or small molecules. So far the coolest thing I’ve heard accomplished with attosecond spectroscopy was the imaging of the wavefunction of a nitrogen molecule (usually we can only talk about the probability or magnitude squared of the wavefunction, but Paul Corkum’s group in Ottawa claims to have actually measure the wavefunction). This is a really exciting field in photonics, and a lot cool stuff is likely to come out in the near future from these groups.

• “Attosecond Science”, Nature Photonics, 2, 548 (2008).

So watch next Sunday for my next photonics briefs. Also, feel free to suggest any updates I should include in this feature.

[tags]science, photonics, attosecond, laser ignition, particle accelerator[/tags]