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Home » Page 1107

Are gravitational waves being ‘redshifted’ away by the cosmological constant?

The theoretical framework underlying gravitational waves may have to be revamped to account for dark energy and the acceleration of the expansion of the universe. That’s the conclusion of researchers in the US, who say that while gravitational waves from nearby sources will be unaffected, next-generation detectors such as the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope – which aim to detect gravitational waves from billions of light-years away – may fall foul of the expansion of the universe. While such telescopes will still detect gravitational waves, the signal detected from more distant waves could be fairly different to what is currently expected, say the researchers.

Dark energy is best explained by a small but positive value for the cosmological constant, which describes the energy density of space. It was the famous factor that Albert Einstein discarded from his general theory of relativity, when it was found in 1929 that the universe was expanding. For the next 69 years, theorists assumed that the cosmological constant was equal to zero. However, in 1998, it was discovered that the expansion of the universe was accelerating, driven by the mysterious dark energy, and the cosmological constant was back in the running.

Because the value of the cosmological constant is very small (10–52 m2), it had been assumed that it would have a negligible effect on the mathematical descriptions of gravitational waves. However, Abhay Ashtekar and colleagues at the Institute for Gravitation and the Cosmos at Penn State University in the US believe that it throws a spanner into the works of our current gravitational-wave theories.

Gravitational kicks

“Even a tiny cosmological constant casts a long shadow on the theory of gravitational waves,” says Ashtekar. He told physicsworld.com that “the current theory, laid down some 50 years ago by Hermann Bondi, Rainer Sachs and Roger Penrose, makes such strong use of the assumption that the cosmological constant equals zero that it now has to be rebuilt, starting from its foundations.”

Ashtekar and team have already begun this process, deriving a new generalization for Einstein’s famous quadrupole formula, which describes the rate at which gravitational waves carry away energy from a system involving two or more massive objects, such as a binary black hole. The merger of two black holes into one can give the combined black hole a “kick” in one direction – the team’s work on modifying the quadrupole formula describes a more accurate version of these kicks.

Even a tiny cosmological constant casts a long shadow on the theory of gravitational waves
Abhay Ashtekar, Penn State University

However, rebuilding the overall theoretical framework of gravitational waves developed by Bondi, Sachs and Penrose in the 1960s is a far greater challenge. “For a cosmological constant greater than zero, we do not yet know what gravitational waves mean in full general relativity, nor do we have expressions for the energy and angular momentum that they carry,” says Ashtekar.

Going the distance

According to the Penn State trio, the effect of the cosmological constant is a cumulative one – the more distant a gravitational-wave-emitting object is, the more expanding space the gravitational waves have to cross to reach us, and therefore the greater the effect of the cosmological constant on them.

The current generation of ground-based detectors, such as Advanced LIGO, can only detect gravitational waves from objects up to about 800 million light-years away – according to Ashtekar, this is not far enough for the cosmological constant to have a noticeable effect. However, LISA and the Einstein Telescope should be able to detect gravitational waves from the other side of the visible universe, and the mathematical models for these signals will need revisiting.

“I’m intrigued by the claim that there might be some measurable consequences for very distant and long wavelength sources that future detectors might probe,” says Martin Hendry, at the University of Glasgow, who was not involved in the Penn work. He agrees with the team’s conclusion that current detectors looking at nearby sources won’t be affected by the cosmological constant.

Looking ahead

B S Sathyaprakash at Cardiff University, who was also not involved in the current work, suggests that “gravitational-wave signals that we detect from cosmological distances might carry the signature of dark energy,” and could be used to probe it. “It will be very interesting to see how our signals are modified by the inclusion of the corrections predicted by [Ashtekar’s] work, and this is what we look forward to doing in the coming years.”

The likes of LISA and the Einstein Telescope will not be operational before the 2030s, giving theorists the chance to rework the equations. Ashtekar says that it is “now clear that the reason the problem had remained open so long was because the inclusion of a cosmological constant, however small, requires a deep change in the basic conceptual structure and mathematical techniques needed to describe gravitational waves in full general relativity”.

The research is to be published in Physical Review Letters.

 

The pioneers of weather forecasting

Late in the autumn of 1703, a colossal storm swept across southern England, causing more than 8000 deaths and severely damaging buildings, forests and ships. The “Great Storm” came with no warning; indeed, there would be no storm warnings or weather forecasts for another century and a half. Progress was slow, in part, because weather observation – unlike astronomy – was a local phenomenon. While anyone, anywhere on Earth could see the entire vast expanse of the heavens, they could only see the weather to their own horizon. Hence, the only records were spot measurements contained in ship’s logs, diaries and personal experience.

The Weather Experiment tells the story of how this situation began to change. Author Peter Moore takes us on a compelling journey through the early history of weather forecasting, bringing to life the personalities, lives and achievements of the men who put in place the building blocks required for forecasts to be possible. Nowadays, when satellite images of the Earth appear on every TV weather forecast and meteorologists have established conventions for recording and classifying weather, it is hard to comprehend just how visionary these pioneers had to be. For them, the idea that one could learn something useful from mapping what the weather was doing at a single moment, on a continental or global scale, was little short of revolutionary.

In order to progress beyond isolated observations and records, people with an interest in weather needed a way to exchange information quickly. Descriptions of the weather also needed to be standardized so that comparisons could be made. (Even now, speedy communication of weather observations is essential and the ever-increasing volumes of data from satellites and remote sensing instruments pose challenges to modern technology.) Moore’s book is therefore not just about the weather, but also about the development of early communication methods such as the telegraph and Morse code, which made it possible to exchange observations and to collect data for mapping or tabulation.

In 1853 a group of maritime nations agreed (following an American suggestion) that sailors on their ships would log their position, air pressure, temperature, wind force and direction at set times each day. The man selected to co-ordinate the UK’s weather-recording effort was Robert FitzRoy, and his appointment marked the beginning of what has become the Meteorological Office (Met Office). FitzRoy’s job was not to predict weather, though. Instead, he was charged with facilitating and gathering the various weather observations, with the aim of producing climatological wind charts to reduce the cost of shipping.

FitzRoy, however, believed strongly that advance warning of storms could save lives and property at sea. As captain of the HMS Beagle in 1829 (during the voyage before his famous circumnavigation with Charles Darwin), he had lost two seamen in a storm while surveying the South American coast. Shortly before the squall hit, he had noticed a dip in barometric pressure, and in its wake he began taking barometric readings every three hours. Unusually for a commander, FitzRoy had always kept daily records of pressure and temperature, and he carried this enthusiasm into his new role. Three years into his meteorological work, he had established a ship-based weather observing network, issued wind charts and designed a new type of barometer. He had also published a weather guide booklet called “Barometer and Weather Manual”, based on his mandate to instruct ship’s captains on methods of distinguishing between cyclones and ordinary gales.

In the summer of 1859 FitzRoy persuaded the British Association to pass a resolution calling on the government to fund a storm-warning system. Before any system could be put in place, however, the steam clipper Royal Charter sank off the island of Anglesey with the loss of more than 450 lives. Galvanized by the disaster, in June 1860 the Board of Trade approved FitzRoy’s plan for a storm-warning service. His plan included 13 coastal telegraph stations supplied with meteorological equipment that would transmit readings of temperature, pressure, wind direction and information about the state of the weather to London at nine o’clock each morning. A map would then be produced, and if a storm was noticed, a warning message would be sent back down the line. To transmit the message to ships, a series of cones was hoisted aloft at major ports. In 1861 this service was expanded to newspapers, which began printing interpretations or predictions of weather for the next two days.

In many ways, FitzRoy’s system was far ahead of its time and, then as now, the forecasts service was not without its detractors. Sadly (and frustratingly), there is apparently no record of the methods FitzRoy used to produce his controversial storm warnings. In any case, after his suicide in 1865, the service was stopped, and although the Board of Trade re-introduced a storm intelligence service several years later, that still left a gap to be filled in the future.

Moore’s book concentrates mainly on the 19th century, so it does not cover the development of the equations we now use to understand the weather and to make predictions based on numerical models. Instead, it focuses more on the history of observational meteorology, discussing James Glaisher’s dramatic balloon ascents of 1862–1866 and John Constable’s paintings of skies as well as FitzRoy’s efforts. It also covers certain personal aspects of the debate over various theories for the cause of storms, and whether air was rotating around storms (as suggested by William Redfield) or inwards and rising upwards (as suggested by James Espy). These competing theories were reconciled in 1856, after the deaths of both men, when a third meteorologist, William Ferrel, recognized the role played by the Earth’s rotation and the Coriolis force in moving air around the atmosphere.

After more than 30 years working for the Meteorological Office and with the international numerical weather prediction community, I found it interesting to see how the thoughts and actions of the 19th-century pioneers influenced the practice of modern weather forecasting and measurement. A good example is the practice of producing synoptic analyses and global forecasts at 0000UTC and 1200UTC. Nowadays, with weather satellites, ground-based remote sensing and automatic weather stations there is no need to be tied to these synoptic times, but the practice continues. As we celebrate 50 years of numerical weather prediction it is stimulating to read about the lives of the pioneers of meteorology and weather forecasting and to see how far we have progressed since then.

  • 2015 Chatto & Windus/Farrar, Straus and Giroux £20.00/$30.00hb 416pp

Is the solar system's planetary count back up to nine?

 

By Tushna Commissariat

In August 2006 the distant world that is Pluto was “downgraded” from planet to “dwarf planet” by the International Astronomical Union. Now, 10 years on, it seems that a new planet may be joining the ranks of the other more familiar eight, thanks to two researchers from the California Institute of Technology in the US, who have uncovered evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system.

Although it has not been observed directly just yet, Konstantin Batygin and Mike Brown discovered the planet’s existence via mathematical modelling and computer simulations. The newly found “Planet 9” is about 10 times as massive as Earth and its orbit is nearly 20 times farther from the Sun on average than Neptune, placing it firmly within the Kuiper belt. If the duo’s calculations are correct, it means that it takes Planet 9 anywhere between 10,000 and 20,000 years to complete one orbit, making it a long year indeed. The research is presented in The Astronomical Journal (which is published by IOP Publishing, which also publishes Physics World).

(more…)

Underground ring lasers will put general relativity to the test

Physicists have started work on a new experiment that aims to measure a very subtle prediction of Einstein’s general theory of relativity known as the Lense–Thirring effect. Describing the distortion of space–time by a spinning mass, the effect has previously been measured by NASA’s LAGEOS satellites to an accuracy of 10%. Now, the Gyroscopes in General Relativity (GINGER) experiment will try to measure the effect under Italy’s Gran Sasso mountain to an accuracy of about 1%.

Einstein’s general theory of relativity says that the motion of a body placed in the gravitational field of another large, spinning object, such as the Earth, will be modified in two ways. One, known as geodetic precession, is due to the curvature of space–time around the Earth. The Lense–Thirring effect, on the other hand, involves the rotating Earth twisting space–time around itself as it turns. First predicted in 1918, the effect is also known as “frame dragging”.

Angela Di Virgilio of Italy’s National Institute of Nuclear Physics in Pisa and colleagues plan to use ring-laser gyroscopes to measure the effect at slightly below the Earth’s surface. These devices use highly reflective mirrors to direct a pair of laser beams in opposite directions around a fixed and very stable loop. The ring works as a resonating cavity for both beams, but each resonates at a slightly different frequency. The beam that tends to travel with the Earth as it rotates takes slightly longer to complete its journey than does the opposing beam. This means that it travels for slightly longer through the resonant cavity. Because the beams have a spread of frequencies, the cavity-length difference translates into two slightly different selected frequencies, and hence a series of beats, which reveal the Earth’s rotation rate.

Tiny modulation

Frame dragging appears as a tiny difference between the rotation rate of the Earth as measured by GINGER and the rotation rate of the Earth as measured by an observer who is not on Earth and in a frame that is fixed to the stars. The latter can be determined very accurately using very long baseline interferometers. What makes the measurement challenging is just how tiny the difference is. The Earth rotates through about 10–5 radians each second but the Lense–Thirring effect leads to a change of only 10–14 radians per second. In other words, the researchers would be looking for variations of one part in a billion.

To achieve this sensitivity, and to track the Earth’s rotation in three dimensions to distinguish between the frame-dragging and geodetic precession, the GINGER design calls for at least three ring lasers, each consisting of a square loop with sides 6 m long, mounted at right angles to one another. Its location in the Gran Sasso National Laboratory, some 1400 m under the Earth’s surface, is also crucial, because it would help to isolate the experiment from wind, rain and other environmental “noise”. Indeed, The world’s best ring-laser gyroscope currently operating in Wettzell, Germany, falls short of the required 10–9 sensitivity by about a factor of 10, as a result of local movements in the Earth’s crust.

To find out whether Gran Sasso provides the necessary isolation, Di Virgilio and colleagues have set up a test apparatus called GINGERino. Installed in the innermost part of the lab, it consists of a square ring laser with 3.6 m-long sides that is mounted on a block of granite secured to the underlying bedrock via a concrete slab. The experiment has so far provided two sets of data, taken in spring and autumn last year, which the researchers say give a “good indication that the underground environment is good and stable”.

Damaged mirrors

However, Di Virgilio says that “there is room for improvement”, and that GINGER still faces significant technical challenges. Crucial to ring-laser experiments is being able to take data over extended periods of time to reduce error. But unwanted noise, partly due to damaged mirrors, has so far limited this “integration time” to just a few tens of seconds. The researchers aim to boost this to around an hour by upgrading the mirrors. They also want to install air-tight doors to limit variations in air pressure within the tunnel housing the experiment. Ultimately, Di Virgilio says that if GINGER is built at Gran Sasso, they will need further funding of €5m, so it can start taking data by around 2019.

GINGER will have to compete against the Laser Relativity Satellite (LARES) – an ongoing mission of the Italian Space Agency – which is aiming at an uncertainty of a few per cent. Di Virgilio points out that LARES, like LAGEOS, requires a separate space mission to map how Earth’s gravitational field varies in space, whereas GINGER does not need such a map.

Heiner Igel, a geophysicist at the University of Munich, says that the biggest challenge facing GINGER physicists is stabilizing the ring’s geometry. “Being under a mountain certainly helps, but the stabilization will still be necessary,” he says.

However, Clifford Will, an astrophysicist at the University of Florida, doubts that GINGER could observe the Lense–Thirring effect, no matter how stable the ring is. He argues that it will be very hard to know the ring’s shape, latitude and orientation all to one part in a billion. “This kind of absolute metrology is very hard to achieve,” he says. “So while there may be compelling geophysical reasons for developing such an instrument, I’m very sceptical that any test of general relativity can be done”.

The GINGERino results have been uploaded to the arXiv server.

‘Green-pea’ galaxies may have kick-started universal reionization

Nascent dwarf galaxies actively forming stars were most likely the reason why the universe reionized, several hundred million years after the Big Bang. That is the finding of an international team of astronomers, which has detected photons from a compact galaxy that have enough energy to ionize neutral hydrogen. The researchers’ discovery improves our understanding of how the first generation of galaxies reionized neutral gas after the cosmic “dark ages”.

Most matter in today’s universe lies in the vast swathes of diffuse gas that exists between galaxies, known as the intergalactic medium (IGM). Made up mainly of hydrogen, this gas has changed phase through the history of the universe, especially during its infancy. The early universe was so hot and dense that protons and electrons could not combine to form neutral hydrogen, which was instead ionized. But as the universe expanded, it cooled, and some 380,000 years after the Big Bang, neutral atoms began to form and space became transparent to light.

Universal phase shifts

The gas remained neutral for the next few hundred million years – a dark age that is largely invisible to astronomers because the atoms in the gas absorbed most short-wavelength light. This period prevailed until some very dense regions began to collapse, thanks to gravity. These resulting dense structures that formed within the neutral medium ultimately provided the energetic radiation that ionized all of the neutral hydrogen in the universe. This “epoch of reionization” was the universe’s final major phase transition, and triggered the clumpy, structured universe that we see today.

While researchers know that the reionization occurred gradually about 400 million years after the Big Bang, we still do not understand what caused it to occur. Current models show that there were simply not enough galaxies at the time to provide the radiation to cause a universal phase transition. Another snag is that photons and radiation have to escape the galaxies to ionize the IGM. However, star-forming galaxies would be so full of neutral hydrogen that they would absorb much of the ionizing photons, meaning that even for a galaxy producing lots of radiation, only a little would escape its grip.

Bright spark in the dark

Although photons from the reionization epoch will never reach today’s telescopes, it may be possible to detect similar photons from closer galaxies coming from when the universe was about two billion – not 400 million – years old. But detecting even this radiation is challenging, because it is often masked by light from other galaxies along our line of sight. Also, it is difficult to work out if these closer galaxies differ from early ones.

After 20 years of effort, one such galaxy has now been found by a team led by Trinh Thuan from the University of Virginia in the US. The researchers have made the first observations of a nearby dwarf galaxy emitting lots of ionizing photons into the IGM. Named J0925+1403, it is a “green-pea galaxy” – a newly discovered, rare kind of galaxy that appears green to light sensors and is round and compact, like a pea. These compact galaxies are thought to be actively producing stars, and can host massive stellar explosions or winds strong enough to eject ionizing photons.

Using data from the Sloan Digital Sky Survey, the researchers identified about 5000 compact galaxies emitting very intense UV radiation, and ultimately selected five galaxies for further observation. Using an ultraviolet spectrometer aboard the Hubble Space Telescope (HST), the researchers studied J0925+1403 and found it to be an ideal candidate – the galaxy’s emission lines suggested that the gas around it was unusually highly ionized.

Perfect pea

“This galaxy appears to be an excellent local analogue of the numerous dwarf galaxies thought to be responsible for the reionization of the early universe,” says Thuan. The team found that J0925+1403, which lies about three billion light-years from Earth, is ejecting ionizing photons with an intensity never seen before – about 8%, compared with the 1–3% usually seen escaping other nearby galaxies. The total radiation escaping the galaxy is enough to ionize a mass of IGM gas that is almost 40 times greater than the galaxy’s stellar mass itself.

The team’s discovery suggests that such dwarf galaxies could have played a key role in cosmic reionization. However, more work needs to be done to fully understand what happened in the early universe. The current study looked at just one galaxy, whereas a large population of them would be needed to kick off the reionization epoch.

Same as before?

Writing for Nature News and Views, astronomer Dawn Erb from the University of Wisconsin Milwaukee in the US, who was not involved in the current work, points out that it is also unknown “whether this galaxy is similar to those that reionized the universe; its small size, high-ionization state and relatively low degree of enrichment by elements heavier than helium generally match the expected properties of such objects, but none of these properties have been measured for the earliest galaxies”. She adds that the team’s observation “emphasizes the need for additional, larger studies” to further our understanding of ionizing radiation in nearby galaxies today, as well as during earlier epochs.

However, Thuan is optimistic about progress. “As we make additional observations using Hubble, we expect to gain a much better understanding of the way photons are ejected from this type of galaxy, and the specific galaxy types driving cosmic reionization,” he says. Thuan adds that these observations are the precursors for even better ones that can be made once the James Webb Space Telescope is launched in 2018.

The work is published in Nature.

Transforming light

Transforming light is a film about the 2015 Day of the Dead celebrations in Mexico City and how a multimedia lightshow blended old traditions with new technologies to spectacular effect. The film was produced by Jorge Benjamín Ruiz Gutiérrez and his team of film-makers at the National Autonomous University of Mexico (UNAM).

El Día de los Muertos (the Day of the Dead) is an annual celebration where Mexicans – and many others around the world – remember their deceased family and friends. It is a vibrant and colourful time of the year, featuring a diverse mix of arts and crafts, such as the orange “flowers of the dead” and the beautifully decorated models of Catrina, the “lady of the dead”. Festivities build up to the big day itself, 2 November, coinciding with All Souls’ Day in the Catholic calendar.

One of the spiritual elements is the belief that for this one day of the year, your deceased loved ones return to be with you again. Light plays an important role in this as it is believed to help guide the souls from the darkness into the light. In 2015 in Mexico City, the festivities were given a modern twist thanks to a new video-mapping technique used to project light onto traditional structures such as altars and decorative skulls. Transforming light documents the event, which brought together science, technology and art in a stunning visual performance. The results were enjoyed by many.

This is the final film in a series we have commissioned for the International Year of Light (IYL 2015), with each film telling local stories involving light and its applications and how they can affect people’s lives. The first film in the series follows an amateur astronomer seeking out a patch of dark sky amid the dazzling lights of New York City. The second is a film about the role of light in regulating sleep cycles. The third looks at how LED lights are helping students in a remote Indian community to study after sunset, and the fourth profiles some of the New York photographers who are eschewing digital techniques in favour of more traditional analogue methods. You can see all those films along with others on the theme of light in this video showcase.

‘Choreographic crystals’ have all the right moves

A new type of crystal defined in terms of the relative motions of its constituents has been proposed by three physicists in Canada. The researchers came up with the idea of “choreographic crystals” while thinking about how to use several satellites to detect gravitational waves. They realized that the mathematics describing the synchronized orbits of four satellites can be generalized to describe the choreographed motions of lots of objects. The trio speculates that such crystals could exist in real materials and have devised a way to identify such systems, if they exist.

The new concept of “choreographic crystals” has been developed by Latham Boyle, Jun Yong Khoo and Kendrick Smith of the Perimeter Institute for Theoretical Physics (PI) in Waterloo, Ontario. The idea took shape several years ago when Boyle was looking at how the upcoming Laser Interferometer Space Antenna (LISA) mission will use three satellites orbiting the Sun to detect gravitational waves. The satellites are set in a triangular configuration and will monitor each other’s locations using laser beams to detect tiny changes in their positions that occur when a gravitational wave passes through the solar system.

Hidden symmetry

LISA will shed light on the origin and polarization of the gravitational waves it detects, but Boyle suspected that adding a fourth satellite would give even more useful information. However, he quickly realized that a spatially symmetric orbital configuration involving four satellites did not exist. Undeterred, Boyle realized that his focus on spatial symmetry was limiting his options – and that a symmetric orbit of four satellites can occur when plotted in terms of the satellites’ velocity and position.

“It turns out there’s this really, really symmetric orbit for four satellites,” explains Boyle, who describes the configuration as a “dynamic tetrahedron”. His interest piqued, Boyle wondered if the concept could be generalized to include more satellites, and joined forces with his PI colleagues Smith and Khoo to find out.

Smith quickly developed a way of finding all possible symmetric satellite swarms with “choreographic order”. Unlike conventional crystals, which are unchanged by spatial operations such as translation, reflection or rotation, these dynamic crystals are unchanged by combinations of changes in space and time. A certain rotation of a dynamic crystal, for example, puts the system into the same state as it would be in after evolving for a certain time.

Into the blue

The trio also came up with a way of illustrating choreographic order. The above figure shows two examples of 2D choreographic crystals with high degrees of symmetry. In the crystal on the left, each element is at the centre of a blue hexagon and can follow a trajectory first through a yellow hexagon, then through a pink hexagon and then back to a blue hexagon – the movement occurring in one of three possible directions shown by the arrows. In the crystal on the right, each element can also move from blue to yellow and on to pink – but only in one direction, as shown by the arrows.

While the team does not know if such choreographic order exists on a microscopic level in materials, it says it should be possible to see evidence for the phenomenon in diffraction experiments, where the positions of the diffraction peaks would be shifted. Boyle describes such shifts as a “very clear-cut signal to look for”. However, he warns, “I don’t know if these things will be out there in nature or not, but it makes sense to look for them.”

The choreographic crystals are described in Physical Review Letters.

Female astronomers through the ages, science-inspired phone cases and the return of the incandescent light bulb

Portraits of 21 leading female astronomers
Women and the RAS: portraits of 21 leading astronomers. (Courtesy: Maria Platt-Evans)

By Hamish Johnston

The first documented female astronomer in Britain was Margaret Flamsteed (1670–1739), who worked with her husband John at the Royal Observatory in Greenwich. That’s according to astronomer Mandy Bailey of the UK’s Royal Astronomical Society, who has written an article entitled “Women and the RAS: 100 years of Fellowship”. As the title suggests, this year is the centenary of the first women becoming fellows of the RAS.

To celebrate the centenary, the RAS commissioned Maria Platt-Evans to photograph 21 leading female fellows. The portraits appear above and are also presented in the slide show “Women of the Royal Astronomical Society”, which includes short biographies.

(more…)

New type of sound wave boosts drug delivery

A new class of sound wave that can dramatically improve the delivery of inhaled drugs and vaccines has been identified by researchers at RMIT University in Melbourne, Australia. Surface-reflected bulk waves (SRBWs) are a hybrid of bulk waves and surface waves. The team has already demonstrated that they can slash the time required to administer inhaled therapeutics through a nebulizer.

Nebulizers break medical suspensions and solutions into vapour that can be inhaled. They deliver drugs to patients with life-threatening or debilitating lung conditions such as cancer, asthma and cystic fibrosis.

In ultrasonic nebulizers, a piezoelectric chip converts electricity into ultrasonic vibrations that vaporize the liquid. These devices have considerable potential because they generate small aerosols that are good for delivery deep into the lung and could be used for pulmonary delivery of other therapeutics, such as vaccines. But their usefulness is limited because they produce vapours at very slow rates.

Stress and heat

The problem is that they rely on surface acoustic waves (SAWs), which travel across the surface of the piezoelectric material, to vaporize the liquid. The nebulization rate can be increased by boosting the intensity to increase the amplitude of the SAWs – but this causes the devices to fail. “A chip with surface acoustic wave alone cannot sustain enough vibrations and heat without failure because the stress and heat – due to vibrations – are constrained within the top layer of the chip,” explains Amgad Rezk, who led the research at RMIT.

To create more powerful waves, Rezk and his colleagues harnessed the power of the bulk waves that leak from the surface waves through the piezoelectric material. These acoustic waves, which cause the entire substrate to vibrate as one entity, are normally suppressed in nebulizers, but the researchers found they could be exploited in combination with SAWs to create a hybrid wave that can drive extremely efficient nebulization.

These SRBWs are created when a bulk wave propagates through the device and creates a travelling surface wave on the other side, before reflecting back and constructively recombining with the initial SAW. The surface waves continue to leak, creating multiple reflections that further strengthen the SRBW.

Key parameter

The key parameter for creating the different waves is the ratio between the substrate thickness and the acoustic wavelength, which is controlled by altering the resonant frequency of the device. When the frequency is relatively high, the wave is confined to the surface, while a low frequency generates bulk waves through the thickness of the substrate. Bulk waves are known to eventually converge into surface waves, and the researchers found that when they used a frequency that lies between the two, the hybrid SRBWs formed.

They used SRBWs in a new device named a HYbriD Resonance Acoustic, or HYDRA. It is able to produce a more powerful, higher-frequency wave than other ultrasonic nebulizers because the energy isn’t confined to the surface. “The heat distribution is more uniform throughout the chip and therefore much higher nebulization rates are accessible,” says Rezk. While nebulization rates with SAWs are between 0.2 and 0.4 ml per minute, the researchers achieved rates of almost 8 ml/min with the HYDRA device. They claim this could cut the time required for inhaling some vaccines from 30 minutes to as little as 30 seconds.

Rezk says that it opens up the possibility of a “handheld, portable, quiet nebulizer” for patients, which delivers a “controlled dosage and drop size reaching deep into the lungs, without wasting medicine in the throat and mouth”.

Interesting future

Andreas Winkler, an acoustofluidics researcher at the Leibniz Institute for Solid State and Material Research in Germany, says the research is interesting with regards to possible future applications of microacoustic fluid atomisers.

He cautions, however, that one major issue with the application of acoustic fluid atomization is “the size of the droplets produced”. “While it is known for SAW-based devices that the produced droplet size is in the regime required for deep lung inhalation and can be influenced by parameters like power and wavelength, bulk wave devices are much more restricted,” he explains. “Unfortunately, no results of droplet size measurement were published, so it is not proven that the droplet size can be controlled and that it lies in the size range required for inhalation.”

The research is described in Advanced Materials.

Plasmons call the tune in new graphene-based terahertz laser

A new type of semiconductor laser has been created using the unique electronic properties of graphene. Designed in the UK by researchers at the University of Manchester, the prototype operates in the terahertz band and can be easily tuned to output radiation at specific wavelengths. The team says that its research could lead to the development of compact devices for a variety of different applications, from security scanning to medical imaging.

Terahertz radiation falls between the infrared and microwave regions of the electromagnetic spectrum and has the very useful property of passing through clothing, packaging and other common materials. As a result, terahertz radiation shows great promise in a wide range of applications, including security and medical scanning, drug and explosives detection as well as wireless communications. However, it has proven to be very difficult to create practical terahertz sources and detectors – so applications have so far been limited.

Coherent terahertz radiation can be created using quantum-cascade lasers, which were invented in 1994. These devices contain multiple quantum wells with energy bands that are split into subbands and minibands. When a bias voltage is applied to the laser, a periodic cascade of intersubband transitions is established. The population inversion necessary for terahertz lasing is then achieved through electrical injection.

‘Squeezing’ light

Terahertz light has wavelengths in the 100 μm – 1 mm range, so it must be “squeezed” into quantum-cascade lasers, which tend to be based on micron-sized structures. This is done using plasmons, which are bound states of electrons and photons that have much shorter wavelengths than the photons themselves. The plasmons are generated in metallic structures and the wavelength of the plasmons is an intrinsic property of the metal. This means that the output wavelength of a terahertz plasmon laser is fixed and cannot be retuned without changing the design.

“If you want to tune a laser, then preferably you want to do it electronically,” explains Subhasish Chakraborty at the University of Manchester.

To achieve electronic tuning, Chakraborty and colleagues utilized a unique property of graphene, which is a sheet of carbon just one atom thick. It turns out that the plasmon wavelength of graphene can be changed by placing it in an electric field – something that can be done by creating a gate structure similar to that used in a field-effect transistor.

They produced a laser containing a carefully designed, aperiodic series of quantum wells of various thicknesses made from gallium arsenide and aluminium gallium arsenide, on top of which they deposited a thin gold waveguide (see figure). Above this, they deposited an atomically thin layer of graphene. They made a series of subwavelength slits in the gold waveguide, forcing electrons to tunnel between the quantum wells in the form of plasmons in the graphene. Finally, they deposited a polymer electrolyte, which allowed them to apply a gate potential to the graphene and thereby control the plasmonic wavelengths it supports. This allowed the researchers to turn specific laser modes on and off, channelling the light energy into one or a few coherent modes and suppressing others.

Sharp modes

The team produced four prototype devices, all of which showed similar – although not identical – variation of the laser wavelength when the electrical potential was scanned from zero to 1 V. When zero potential was applied to the graphene, a large number of lasing modes were supported. When the potential was raised, however, the light emission was concentrated into a few specific, sharp modes defined by the positions of the slits.

Chakraborty stresses that the design is, at present, “a proof of concept”, and that more work needs to be done, notably to allow the researchers to control the voltage applied to each slit individually for maximum control of the lasing frequencies. Beyond that, he says, “we want to use a different gating technology consistent with the microelectronics industry”. He sees several potential applications in testing products for faults, airport scanners for concealed weapons or explosives and medical imaging. Further into the future, he believes that the rapid modulation of frequency possible with their devices could allow data transmission at terabit rates down fibre-optic cables.

Graphene plasmon expert Ortwin Hess of Imperial College London told physicsworld.com that in addition to the many potential practical applications of the laser, the theory underlying the research is fascinating. “I would love to see some direct dynamics,” he says. “How coherent is it? What kind of coherence statistics does it have? There’s a lot to explore once one has something like this…It really shows that, in graphene, plasmons play an important role, and it’s the coherent properties and also apparently the excitation of those that are very important.”

The research is described in Science.

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