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The light fantastic

Lasers are found everywhere, but what does the future hold for the technology?

Just 50 years ago, the laser was invented by Theodore Maiman at the Hughes Research Laboratories in Malibu. At the time, it was greeted as an 'invention looking for a solution' yet, today, lasers are everywhere –barcode readers; cd and dvd players; eye surgery, cardiology and medical diagnostics; optical communications; forensics; military systems; and in fusion energy research. And that is a brief summary!

In fact, the laser has one of the most wide ranging application portfolios of any technological device. Much of that is due to the electronics industry. By creating semiconductor lasers, it made the laser easier to use and more cost effective. Such has been its success, you might think the laser was reaching some sort of application plateau, but the opposite looks to be the case.

"Radically new laser systems continue to emerge," says Dr Thomas Giallorenzi, Science Adviser to the Optical Society of America. "One example is the quantum cascade laser. Unlike the conventional semiconductor laser, which generates a single photon for each electron generated, this creates numerous photons for each electron through a cascade process. In the 15 years since its first laboratory demonstration – a very short time span for science – the quantum cascade laser proved the star of the 2009 Annual Conference on Lasers and Electro-Optics. It has many potential capabilities, including remote atmospheric sensing of gases and defeating infrared guided anti aircraft missiles."

A combination of lasers and nanotechnology is proving to be a powerful weapon in medicine, as Giallorenzi explains.

"By directing gold nanoparticles on to cancerous tumors using monoclonal antibody technology, then illuminating them with laser light, it is possible to destroy malignant cells selectively, while normal cells remain intact. In the future, this nanoscale technology applied to human cells could lead to targeted disease therapies that are far more selective, less toxic and more effective than many treatments used today."

A wholly new kind of laser does not come along very often, but that is what physicist Jeremy Baumberg at the University of Cambridge is working on. These are based on 'polaritons' and potential applications could include lasers that use far less power, sensors with extremely high sensitivity and, potentially, quantum computers. Polaritons also exhibit the extraordinary quantum based phenomenon of superfluidity, a phase of matter somewhat similar to superconductivity, but in which the viscosity of a fluid vanishes, rather than electrical resistance.

Creating polaritons is complex. The basic idea is to pack electrons into a quantum well – a sandwich of semiconducting materials less than a micron thick. Energy is then added as light or voltage; some electrons absorb it and jump to a higher energy level, creating holes, as found in semiconductors. An electron-hole pairing is an exciton and is usually not long lasting. The energised electron goes back into the hole, releasing its extra energy as a photon.

However, if the quantum well is placed between a pair of polished mirrors to create a cavity about 1µm long, the photon is reflected back into the system to create another exciton, which in turn emits its energy as another photon. By controlling the distance between the mirrors, the photons being emitted by many excitons build in intensity and resonate with each other, creating more excitons and, hence, more resonant light waves. The result is that energy cycles between light and matter so quickly – in trillionths of a second – that quantum physics says it impossible to say in which of the two states it is stored. This is a polariton.

The link with lasers is that, unlike electrons, which repulse each other, polaritons readily form into a coherent quantum state. This means they should in principle be excellent at generating coherent laser light – potentially much better than electrons. Thus, a very small amount of energy should be enough to start a resonant transition between light and matter and the resulting polaritons will enter the same quantum state, emitting coherent laser light when they decay.

Polaritons hold the prospect of achieving extraordinary things. At the Swiss Federal Polytechnic Institute in Lausanne, Benoît Deveaud-Plédran thinks they will help to create an exotic form of matter called a Bose-Einstein Condensate (BEC). Neither solid, liquid nor gas, BECs exhibit the same strange quantum behaviour as if they were a single atom, but on a much larger scale, making that behaviour much easier to observe. Unfortunately, BECs usually only form at just above absolute zero, making them very difficult to study, let alone use. Polaritons could make it much easier.

"Already, there has been clear observation of Bose Einstein condensation of polaritons," says Deveaud-Plédran. "This is a very important achievement as it will allow us to study such condensates at room temperature or even above. Superfluidity and other very interesting effects have already been observed."

Another potential application of polariton condensates is the creation of entangled states in a solid material like a semiconductor, which could then be used to perform quantum computing. Polaritons should also be an excellent room temperature source of identical photons, an ideal resource for developing encoding schemes for use in quantum cryptography.
Lasers have several features that are exploited in different ways – coherence, power and speed. For the last, the term 'ultrafast' was coined in 1982 to describe dynamical processes observed with lasers that occur on sub picosecond (10-12) or femtosecond (10–15) timescales. Today, processes on attosecond (10–18) scales are now accessible, with laser pulses as short as 80as demonstrated at the University of Vienna in 2004.

The result is 'attosecond spectroscopy', which could make it possible to observe charge transfer in photovoltaic cells and transistors directly, allowing researchers to understand precisely how the electrons move around such devices. This knowledge could help to create more efficient photovoltaic cells and faster switching transistors.

In 2007, attosecond pulses were used to probe one of the fastest distinct events yet recorded, the individual elementary steps of photoemission – the process by which electrons are emitted from a material by light – namely excitation, transport and emission.

Other applications for ultrafast lasers are for micromachining parts, as no material can withstand the intensity of a femtosecond laser pulse.
Because a small amount of energy is used, only a tiny amount of material is removed with each pulse, enabling precise cutting. This is what enables laser eye surgery – a process discovered through an unfortunate laser eye injury. Ultrafast laser pulses deliver the exact amount of energy needed to break protein bonds in the eye without affecting the surrounding tissue.
Even attosecond lasers may not be the ultimate, with hopes that zeptosecond (10–21) pulses may be achieved. The original 'ultrafast' devices would then be made to look positively sluggish.

In terms of power, lasers may well prove crucial in achieving the dream of fusion, which if it succeeds will solve the world's energy needs forever. A central project here is the High Power Laser Energy Research facility, or HiPER. Led by the UK and involving a 10 nation consortium of researchers and funding bodies, HiPER's goal is to demonstrate the performance of all the component technologies for power plant scale operation within the next 10 years.

To do this, HiPER aims to draw on innovations taking place elsewhere in laser science, including the technology used in the welding and machining industry and several ongoing high power laser research projects. One example is the Extreme Light Infrastructure project, which aims to create laser pulses with peak powers of up to a few hundred petawatts (about 1017W) using the same type of diode pumped laser technology that HiPER will require.

In the last 25 years, lasers have transformed astronomy. To counter blurring from atmospheric turbulence, astronomers use adaptive optics, which measure snapshots of the turbulence and then correct for the resulting optical distortion using a deformable mirror. The turbulence is changing constantly, so corrections have to be made hundreds of times a second. Early adaptive optics systems used light from a bright star to measure the turbulence, but most objects of astronomical interest do not have bright stars close enough, limiting the use of adaptive optics.

In the 1980s, astronomers realised that they could use a laser to make an artificial star as a substitute, which could be pointed anywhere in the sky. Laser guided star adaptive optics systems are now widely used, with every major 8 to 10m telescope having its own laser beacon. The results are dramatic, giving today's best ground based telescopes better spatial resolution at infrared observing wavelengths than the Hubble Space Telescope, thanks to their larger mirrors.

The future will be even better, with proposed giant telescopes all planning to use multiple laser guide stars at the same time. This will mean astronomers will be able to correct for turbulence in the entire 3d column of air above the telescope. These multiple laser systems will draw on the techniques of tomography used in medical imaging CAT scans to reconstruct the turbulence profile, enabling adaptive optics correction over much wider fields of view than is possible today.

Another advance in viewing is using lasers to create a remote sensing system, based on terahertz wave techniques, that could transform the detection of hidden explosives, chemical, biological agents and drugs. In July, a team at Rensselaer Polytechnic Institute in the US announced the development of system that can see through clothing and packaging materials and which can identify immediately the THz 'fingerprints' of hidden materials.

The system uses laser induced fluorescence, which involves focusing two laser beams together in the air to create a plasma that interacts with a generated THz wave. The plasma fluorescence carries information from a target material to a detector, where it is compared with the spectrum of materials stored in the THz library, enabling identification of a target material. It can show what compound or compounds are hidden, so in a chemical spill, for example, it could identify its precise composition.
Potential developments? How about gamma ray lasers; potentially hugely powerful devices? Comparing today's lasers with gamma ray systems would be like the difference between chemical and nuclear explosions. An entirely new material, positronium, has already been created that could lead to gamma ray lasers. Positronium comprises an electron and a positron bound together in an 'exotic atom'.

Finally, lasers have just played a part in what is potentially one of the most significant measurements ever made – a revised size for the radius of the proton, one of the building blocks of matter. At the Paul Scherrer Institute in Switzerland, a new laser based system discovered the proton measures 0.8418fm (10-15m), not as was previously thought 0.8768fm. It may not sound a lot, but if lasers have led to a change in a fundamental constant, it could be their greatest achievement yet.

Author
David Boothroyd

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