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Home » Page 1856

Random laser feels the heat

Conventional lasers are based on a mirrored cavity filled with an optically active material. The substance – excited by an electrical current or another laser – emits light, which the mirrors reflect back into the cavity, stimulating more light emission. This amplification step is crucial for the production of intense radiation with a single wavelength.

But the laser developed by Wiersma and Cavalieri is a ‘random’ laser – so-called because amplification is achieved by the random diffusion of light in a laser material. As light scatters through the structure, it excites other atoms and makes them emit light. This is a ‘distributed cavity’, in which the material itself plays the role of the reflecting cavity, and there are no mirrors.

In a random laser, the degree to which light can scatter through the material determines the level of gain in the laser. If the light is strongly scattered, it is trapped for longer and can stimulate greater emission. This leads to a higher gain and greater light intensity.

Wiersma and Cavalieri realised that they could adjust the amount of gain by controlling how strongly the laser material scatters light. To do this, they filled a porous glass structure with laser dye dissolved in a liquid crystal. Liquid crystals are chain-like molecules that align to a different degree depending on their temperature. As the degree of alignment changes, so does the diffusion coefficient of the overall structure.

“The laser is made of compressed powdered glass, and it looks white because light scatters through it at random”, explains Wiersma. “The space between the many tiny glass grains is filled with the liquid crystal and the laser dye”.

At temperatures well below 35 degrees centigrade, Wiersma and Cavalieri found that the laser emitted extremely intense light with a very narrow range of wavelengths, around 610 nanometres. But as they increased the temperature, the intensity gradually fell and the range of wavelengths became broader. Laser action ceases at 42 degrees, which corresponds to a phase transition in the liquid crystal. At this point, the intensity dropped sharply and emission spectrum became very flat and broad.

The range of temperatures over which the laser is sensitive can be adjusted by changing the combination of glass and liquid crystal. According to Wiersma and Cavalieri, this could make it suitable for a wide variety of applications.

A new twist for stellar magnetism

Star-spots – like sunspots – are dark patches on the surface of a star caused by intense local magnetic fields. These fields suppress the convection of heat to the surface, which leads to cooler patches that radiate less light. Collier Cameron, of the University of St Andrews in the UK, and Francois, of the Observatoire Midi-Pyrénées in France, monitored the rotation of these spots on a star known as AB Doradus, which is 50 light years from Earth.

Collier Cameron and Francois found that star-spots at the equator of AB Doradus completed a circuit of the star every twelve hours – much more quickly than those at its poles. But over an eight-year period, they noticed that the equatorial spots slowed down and the polar spots speeded up. The researchers point out that they have not observed the star for long enough to know whether this effect is cyclic, but the rate of change suggests that the process takes a decade or two.

According to the Applegate effect, star-spot motion is linked to the rotation of the star, which determines the degree to which the star is gravitationally ‘flattened’. Collier Cameron and Francois have now confirmed that the variation in star-spot speed on AB Doradus could significantly affect its gravitational field – and they believe that this effect could explain the highly irregular orbits of the stars in some closely bound binary systems.

Collier Cameron and Francois are also optimistic that their studies of AB Doradus will help astronomers to understand the magnetic processes that give rise to the 11-year cycle of activity in the Sun. The researchers found in an earlier study that – similar to the Sun – star-spots at the equator of AB Doradus rotate faster than those at the poles. But the sunspots at the Sun’s equator rotate fifty times more slowly than similar spots on AB Doradus.

‘Microlens’ could bring dark matter into focus

Astronomical bodies such as galaxies can often be detected – even if they are invisible – because they behave as lenses: their gravitational fields bend the light emitted by background stars towards Earth as they pass in front of the stars. When smaller objects – such as white dwarfs or neutron stars – create the same effect, it is known as microlensing.

In 1993, astronomers at the Mount Stromlo Observatory in Australia observed a number of stars in the neighbouring Large Magellanic Cloud galaxy. The light from each star was bent towards Earth by a gravitational lens – an effect that was detected as a rise and a fall in the intensity of the star’s light as the lens passed in front of it.

In 1999, Nelson and colleagues used the Hubble Space Telescope to re-examine one of these stars at very high resolution. They found a very dim red object close to the line-of-sight of the star – in exactly the same place that the gravitational lens would be now, according to calculations based on the earlier intensity observations.

Astronomers can calculate the mass of a gravitational lens from the degree to which it bends light, the relative motion of the Earth and the distant star, and the motion of the object passing in front of the background star. Together with spectra gathered at the Very Large Telescope, Nelson and colleagues deduced that the object is a white dwarf star typical of those found in the Milky Way.

The large-scale motion of galaxies suggests that they contain much more mass than astronomers can detect with telescopes. Astronomers have long struggled to understand what makes up this ‘dark matter’, which accounts for around 90% of the matter in the universe. One suggestion is ‘massive compact dark halo objects’ – or MACHOs – which include white dwarfs. Current estimates suggest that 8 – 50% of dark matter could be made up of MACHOs, but Nelson and co-workers are optimistic that their discovery will improve that estimate.

Solar cell edges towards endless energy

Photocatalysis is the use of energy from absorbed light to initiate chemical reactions. Semiconductors are useful in such reactions because they can be designed so that their electronic characteristics change when they absorb radiation. But their energy bandgaps are often large, and this means that only photons with short-wavelengths and high-energies – such as ultraviolet photons – can promote electrons from the valence band to the conduction band.

To create a suitable material, Zou and co-workers added nickel to the semiconductor indium tantalum oxide. This reduced its energy bandgap from 2.6 to 2.3 electronvolts, which means that visible photons carry enough energy to make electrons jump the bandgap. They immersed this semiconductor in water and illuminated it with an arc lamp. As the semiconductor absorbs energy from the photons, electrons jump from the valence band to the conduction band, leaving positive holes in the valence band.

Provided the conduction band is at a higher energy than the ‘reduction potential’ of hydrogen, the ‘promoted’ electrons drift to the surface of the semiconductor where they combine with hydrogen ions in the water to make hydrogen gas. To balance this reaction, the valence band must be at a lower energy than the ‘oxidation potential’ of oxygen – this allows the positive holes to surface and accept electrons from oxygen ions in the water, creating oxygen gas.

The new semiconductor is also resilient – existing semiconductors that use visible light either corrode or become inert when they come into contact with water. Zou and colleagues point out that although their set-up is only 0.66% efficient, they are confident that this will improve when they increase the surface area of the semiconductor, and adjust its layout.

Ice crystals pull together

When fallen snow is compressed by further snowfall, individual ice crystals bond with each other to form a porous mass – a process known as ‘sintering’. The bonds that join the crystalline grains determine the overall structural properties of the snow – such as strength, viscosity and creep – as well as its thermal, optical and electromagnetic characteristics. This process is complex in snow, because thermal variations cause water vapour to diffuse through the structure, which then re-freezes. This phenomenon depends on vapour pressure.

Adams and co-workers collected fresh snow and examined it at sub-zero temperatures with a scanning electron microscope after it had rested in a freezer for several months. They noticed that an elevated ridge had developed around the perimeter of nearly all the interfaces between grains of snow. This ridge – dubbed a ‘grain boundary ridge’ – is evidence for a process known as ‘grain boundary diffusion’, in which mass redistributes itself from the centre of the interface towards its edges.

The ridge could be critical to the sintering process because the vapour pressure above a surface is related to its shape. This means that ice tends to migrate – as water vapour – from sharper features to more rounded regions. According to Adams and colleagues, the abundance of these ridges should greatly increase the rate that mass redistributes itself in packed snow.

According to Adams and co-workers, the existence of the ridges suggests that the role of grain boundary diffusion has been greatly underestimated in ice, and throws into question the whole sintering process. “It is also reasonable to assume that this mass-transport mechanism exists in all crystalline materials”, they write.

A knowledge of the structure of snow is important for the mechanical removal of drifts from snow-bound communities, and for skiing conditions. Snow also plays an important role in the environment because it reflects light and heat from the Sun back into the Earth’s atmosphere.

A history of the electron with no holes

A full review by Per Dahl, formerly at the Lawrence Berkeley National Laboratory, appears in the December issue of Physics World.

Somewhat for convenience, the simplistic answer to the question “who discovered the electron, and when?” is usually given as “J J Thomson, in 1897”.

Of course, there were many other physicists studying the electron at the turn of the 20th century, as we are amply reminded in Histories of the Electron: The Birth of Microphysics. The volume addresses the “discovery” of the electron, and the microphysical world that it ushered in. It also tackles a host of associated issues, from the technical to the epistemological. To sort it all out, the editors – Jed Buchwald and Andrew Warwick – have enlisted no fewer than 19 expert contributors, and we are not disappointed in the work at hand.

Somewhat heavy in places for historians and general readers with less than a first-rate background in physics or philosophy, the volume is nevertheless crammed with an impressive amount of material on the discovery of the electron and its intellectual impact on human affairs.

Buy the book

Histories of the Electron: The Birth of Microphysics: Amazon UK/Amazon US

A marriage made for the nanoworld

Light is beautiful. It can probe matter in a multitude of ways, but there are limitations when we try to use it to investigate the revolution in nanotechnology that is currently taking place. The first problem with light in the nanometre domain is diffraction. Light cannot be focused to a point smaller than half its wavelength – this is the famous Rayleigh criterion of optical resolution. The second problem is out-of-focus light. In essence, light that passes through a lens illuminates the regions before and after the focal point, as well as the focal spot itself.

An approach known as spectral confinement may be capable of restricting light to within a few nanometres along certain directions. Spectral confinement occurs when we consider how the electrons and atoms in a molecular or a solid-state system interact with the large electric field that is produced by a laser.

The combination of near-field optics (with its spatial restrictions on light) and nonlinear optics (with its spectral confinement of light), is a marriage made in heaven. And in the quest to probe optical properties at the resolutions associated with the nanoworld, the combined technique is the star on the horizon.

Now Jeffrey Guest and co-workers at the University of Michigan and the Naval Research Laboratory in Washington have taken a step on this road by employing both near-field optics and nonlinear optics to probe the nanoworld of a quantum-dot system (J Guest et al. 2001 Science 293 2224).

In the December issue of Physics World, Aaron Lewis of The Hebrew University of Jerusalem, Israel, shows how the new approach opens a window on the analysis of nanoscale semiconductor systems.

Are physicists useful?

“But the greatest error of all is mistaking the ultimate end of knowledge. For some men covet knowledge out of a natural curiosity and inquisitive temper; some to entertain the mind with variety and delight; some for ornament and reputation; some for victory and contention; many for lucre and a livelihood; and but few for employing the Divine gift of reason to the use and benefit of mankind.” Francis Bacon: On the Dignity and Advancement of Learning (1605).

Are physicists useful to industry? To clear the decks, it might be pertinent to distinguish between two sorts of physics. There is one sort that might possibly be useful, and another sort – glorious though it may be – that certainly is not. Some 100 years ago, this distinction would have made no sense. Physics was a unity. The results of research into electromagnetism, relativity and quantum processes affected the whole. Out of it was born an understanding of materials, of plasmas, of the atom and its nucleus, and of some esoteric elements of space travel.

One has only to look at present-day information technology (which emerged from semiconductor physics), nuclear technology (atomic and plasma physics) and medical physics to see three of the powerful spin-offs from modern physics. We should also not forget aerospace engineering (fluid mechanics), motor engineering (thermodynamics), and telescopes and cameras (optics), which are all the products of classical physics. All of this – modern plus classical – is “mainstream physics”, and physicists who specialize in mainstream physics are surely going to be useful in industry, aren’t they? At least there is a prima-facie case.

Big science: esoteric and extravagant?

But what about high-energy physics and cosmology? A different case has to be made for the usefulness of physicists in these fields. Undoubtedly fascinating though they are, these topics seem to dwell in a different realm that is esoterically mathematical. In high-energy physics there are quarks, there are gluons, and there is a mathematical structure called “quantum chromodynamics” (or QCD) to describe them. Whatever is discovered in this realm may be fundamental, and even elegant and delightful, but it is almost certainly commercially useless.

Of course, there have been some notable spin-offs from accelerator technology (see Particles for profits Physics World April pp44-45). Hospitals can now buy cyclotrons to make radioisotopes for nuclear medicine. Medical physicists routinely use positron emission tomography, while the defence industry is even using QCD software to determine the best locations to place antennae on ships. And who could imagine a world without the CERN-inspired Web?

But these advances are themselves not high-energy physics and are not why high-energy physics is pursued. Discounting serendipitous spin-offs, particle physics is economically useless. With the cost of experiments rocketing into the stratosphere, the subject is in crisis. And without experiment, the investigation can no longer be physics. Some experiments can be done in astrophysics that bear on cosmology, and – insofar as that is the case – cosmology is physics.

Since space and time, after Einstein, have both condensed into a malleable space-time continuum with properties determined by what Newton knew as gravity, an esoteric activity has focused on the mathematical peculiarities of general relativity. This theory shares with quantum theory the problem that neither appears to be complete.

Theorists in these fields revel in time-travel scenarios in general relativity and in many-worlds extravagances in quantum theory. Cosmologists, latching on to the statistical nature of the world as revealed by quantum theory, invent ensembles of universes. Whatever the status of all this – much of it is applied mathematics bordering on mathematical theology and is certainly distinct from physics as an empirical science – it is impossible to see any material benefit of the kind that Francis Bacon had in mind.

So physicists specializing in these fields are not immediately useful even though their creations are fascinating. Nevertheless, any exposure to physics of whatever sort cannot be bad. There is nothing more intellectually wonderful than physics, and familiarity with the work of genius creates an awareness of the highest standards. So industry need not write off high-energy physicists and cosmologists even if they write off high-energy physics and cosmology.

Expertise and engineering

Given the possibility and will to retrain erstwhile devotees of what might be termed “hyperphysics”, the difference between particle physicists and cosmologists on the one hand and mainstream physicists on the other becomes unimportant. So we come back to the question: are physicists useful to industry? Surely the answer is, of course! The burgeoning fields of optical communications and display technology, to mention but two, certainly need the solid-state and quantum-theoretic expertise that physicists possess.

However, expertise is not enough. It is one thing to carry out theoretical or experimental work to elucidate the physics, it is really quite another to make a device work. To get a device to operate efficiently and economically is engineering. That is to say, it is the deployment of not only knowledge (physics) but also of judgement and intuition, which are essential in the inevitable absence of certain knowledge.

Device engineering is not academic physics. All practical engineering structures – transistors or whatever – are nearly always fiendishly complicated. The basic physics is not usually in doubt. The relevant fundamental equations are in place, but applying them to the problem in hand and obtaining solutions that can be used in practice is rarely straightforward.

Delicate choices of approximations and idealization have to be made. As often as not, it is necessary to resort to intense numerical computation, so theoretical physicists must become number crunchers. Where some details of the physics are unclear, experimental work can attempt to clarify the situation – for example, by illuminating the role of some impurity that it would be nice if it was not there in the first place.

Experimental physics here has to do with the contingent rather than the fundamental. It is rare to encounter new physics – so theorists must develop the art of idealization, and experimentalists must embrace contingency. Being a physicist is just the beginning. Applying physics is the challenge. Physics is simple, engineering physics is not.

Wanted: soft skills

So are physicists useful? Well, they could be. But it does need a phase change. In addition to hard technical competence, there must be an array of “soft” skills. What these are has been defined in the recent report Employers’ Views of Postgraduate Physicists, which the Institute for Employment Studies carried out for the Engineering and Physical Sciences Research Council (see Physics and work Physics World July p15). Creativity is highly desirable, but it is no use inventing something that nobody wants or can afford, so there must be business awareness. Inventing something or getting something useful to work or spotting a possible modification is no good if nobody knows about it, so there must be communication skills along with other desirable skills – flexibility, working in a team, leadership and so on.

Apart from communication skills, very little of this can be usefully taught, so universities should not waste time trying. What can be done will be best left to the employer and osmosis in the workplace. As Wittgenstein once remarked: “What a lot of things a man must do in order for us to say he thinks.” What a lot of things a physicist must do for us to say he is useful.

* Editor’s note: there is still time to enter the Physics World survey What’s your philosophy? (see October p18). Responses can be submitted on line on the Brookhaven National Laboratory’s site at www.bnl.gov/bnlweb/physq

Beefing up the strong force

Protons and neutrons are composite objects that consist of quarks bound by the strong force. Free quarks, and the gluons that hold them together, are not observed in nature because the coupling strength between quarks becomes stronger as the distance between them increases. The theory describing this strong force is called quantum chromodynamics.

In the December issue of Physics World, Gerrit Schierholz of DESY in Germany describes recent calculations by Tony Thomas and co-workers at the University of Adelaide in Australia, and the Jefferson Lab and the Massachusetts Institute of Technology, both in the US, that offer a possible solution to a long-standing discrepancy between lattice QCD and experiments (W Detmold et al. 2001 Phys. Rev. Lett. 87 172001).

Collisions feel the cold

For many years atomic-collision experiments basically involved measuring the deflection of fast projectiles that had passed through gaseous targets, or measuring the amount of light given off by atoms and molecules as they were bombarded with different projectiles. In recent years, however, the atomic-physics community has aspired to much more: we want to prepare a target of non-interacting atoms or molecules in a particular quantum state, strike it with projectiles of perfectly known speed, direction and internal state, and then record the time, frequency, speed, energy, direction, spin and internal state of every fragment that emerges from the collisions.

It is as if we have switched from three-ball billiards to 15-ball pool, and want to record and interpret not just the single impact of the cue ball on another ball but also the “break” shot that scatters all 15 balls over the entire table in a unique way. A further step in this direction has recently been taken by two teams of physicists that, for the first time, have combined the latest laser cooling and trapping techniques with a well established method for studying collisions.

This breakthrough has allowed the teams – one based at the KVI laboratory in Gröningen in the Netherlands, the other at Kansas State University in the US – to map out the complete kinematic details for each ion-atom encounter in collision experiments at keV energies.

In the December issue of Physics World, Keith B MacAdam of the University of Kentucky, USA, investigates the new technique.

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