I always enjoy hearing stories about how scientific technologies grow and develop. There’s something fascinating about the way yesterday’s barely-functional prototype morphs into tomorrow’s reliable tool – not least because it doesn’t always work out that way, and when it does, the pathway to success is usually anything but straightforward.
Consider the supercontinuum or “white-light” laser. This apparent contradiction in terms produces a veritable rainbow of spatially coherent light, typically by sending ultrafast pulses of laser light through a nonlinear optical fibre. It has several uses, but the combination of bright light and broad spectrum makes it particularly attractive for biomedical imaging and quality control.
In its early days, though, the supercontinuum laser was – well, a bit of a pain. According to Carsten Thomsen, who led the effort to commercialize the device for use outside specialist research laboratories, the earliest prototypes at his company NKT Photonics only lasted a few hours before the fibre degraded so much that the device became unusable. The reasons for these failures weren’t clear, and when Thomsen and his colleagues began to investigate, they initially weren’t even sure what to look for. At one point, they resorted to setting up a whole room full of supercontinuum lasers – racks and racks of them – and monitoring them as they quietly destroyed themselves. “We needed to understand the physics behind the failure,” Thomsen told audience members at an event organized by the European Photonics Industry Consortium (EPIC) during Photonics West. “Once we understood the physics, we could do improved lifetime testing.”
The answer, it transpired, was that the powerful laser light was creating defects in the glass within the optical fibres. The company solved this problem by purchasing a small materials-science firm and using its expertise to develop better fibre materials. By 2008, these efforts had pushed the laser’s lifetime up to 3000 hours, or about four months of continuous operation. That may not sound like much, but it proved good enough to attract the attention of a microscopy firm, Leica Microsystems. Leica wanted to use supercontinuum lasers to excite fluorophors in biological samples labelled with various dyes, and its involvement proved crucial to commercializing the device. In fact, Thomsen, who is now NKT Photonics’ vice president for ultrafast lasers, told me that his advice for entrepreneurs is always “find someone who will use your product even if it’s not perfect”. A decade or so later, he adds, NKT’s supercontinuum lasers can run continuously for two or three years without failing – although not necessarily at the higher powers that a few of the company’s more industrially-oriented customers would like to access.
I thought about the supercontinuum laser again during a later talk by Eric Aguilar, who is the chief executive and co-founder of a California-based start-up called Omnitron Sensors. He and his colleagues are working on a LiDAR (laser ranging and detection) system for autonomous cars in which the position of the ranging beam is dithered, or quickly shifted around, in a way that mimics the slight but rapid movement of an animal’s eyes. “Humans dynamically compensate for motion using their inner ear,” Aguilar told me. “We want to give this same property to LiDAR.”
Aguilar declined to give further details, and in the Q&A session after his talk, a sceptical audience member challenged him, asking, “How do I know you’re real?” While better LiDAR systems would certainly help autonomous vehicles get better at detecting and avoiding objects, it remains to be seen whether Aguilar’s bio-inspired idea will make it into a practical product. But then, that’s the beauty of stories. You never know how they’re going to end.
An accelerated corrosion process in materials proposed for use in the long-term storage of high-level nuclear waste has been discovered by researchers in the US. Xialei Guo and Gerald Frankel at Ohio State University and colleagues made their discovery by doing a laboratory recreation of the conditions of nuclear waste storage under proposed guidelines. Their findings suggest the need for better storage systems.
High-level nuclear waste is created by weapons programmes and nuclear reactors. It must be stored securely for hundreds of thousands of years until it is rendered safe by radioactive decay. Most countries plan to store their high-level waste deep underground, but there are currently no operational facilities – although one is under construction in Finland.
The current plan is to immobilize high-level waste in glass or ceramic using a process called vitrification. The hot glass or ceramic would be cast into stainless steel cannisters that would be sealed and buried deep within underground repositories surrounded by highly impermeable rocks. Regular safety inspections would check the material for any corrosion both within the immobilizing material and the steel. With these precautions in place, researchers had hoped that waste could be safely stored for thousands of years.
Critical oversight
Guo and colleagues believe that they have spotted a critical oversight in this plan that relates to corrosion that occurs at glass-steel and ceramic-steel interfaces if water is able to penetrate the cannister – something that is likely to happen at some point in time.
They point out that after a steel cannister is cast, the glass or ceramic it contains will contract as it cools down, creating a thin gap between the vitrified material and the inside wall. The presence of water will encourage the interior steel surface to corrode, causing an increase in the acidity within the gap and creating a better environment for further corrosion. This will then cause the glass or ceramic to release corrosive ions, causing further corrosion to occur.
The result, they argue, is a feedback loop that accelerates the deterioration of both materials at their interface – leading to the release of radioactive material into the surrounding environment
To study the effect, Guo and colleagues recreated the conditions of nuclear waste storage in the lab by pressing a slab of stainless steel against samples of different glasses and ceramic materials. When placed in environments simulating geological repositories, they observed that interactions between the two materials accelerated corrosion at their interfaces.
Frankel’s team believe that this previously unconsidered effect could shorten significantly the useful lifetime of nuclear waste storage packages. If they are correct, this highlights a pressing need for policymakers to rethink how waste will be stored. The team also hopes that their results could help researchers develop more compatible materials to minimize interface corrosion, enabling them to design much better systems for waste storage.
Precision monitoring of patient motion in real-time during radiotherapy could become much easier in the future thanks to millimetre wave (mmWave) technology. According to research from Washington University School of Medicine in St. Louis, mmWave technology can monitor unobstructed surface displacements with an accuracy of 0.1 mm, and better than 1 mm accuracy when monitoring displacements through a radiotherapy immobilization device or a hospital gown. Importantly, the technology is low cost and easy to use (Med. Phys. 10.1002/mp.13980).
Imaging with millimetre waves, which have a frequency range of 30–300 GHz, could overcome limitations of existing techniques for measuring patient motion, such as optical tracking of a patient’s skin surface, fluoroscopy and CT imaging, and real-time position management systems. Optical imaging cannot penetrate clothing or thermoplastic immobilization devices, while CT and fluoroscopy expose patients to additional radiation.
The mmWave device developed by the team can monitor breathing and cardiac motion simultaneously, and can decompose a chest displacement profile into breathing and cardiac waveforms. As such, the technology could also potentially replace respirometers and other breathing monitoring devices that provide signals for respiratory-gated treatments or four-dimensional CT scans.
Joshua Olick-Gibson and colleagues investigated the feasibility of using mmWave technology to track surface motion of a patient during radiation treatment. They conducted measurements with and without obstructions, which included a hospital gown, an alpha cradle, and an immobilization face mask worn at various displacements and heights by a healthy volunteer.
The researchers affixed a mmWave device with two transmitters and four receivers on the inside top bore of a non-rotating linear accelerator, pointed downwards towards a reflective brass plate placed on the treatment couch. The device transmitted chirp signals at 77–81 GHz. The surface is detected by fast Fourier transform of the reflected chirp signal within a rough range bin; fine displacements within that range bin were calculated through phase extraction and phase demodulation.
The researchers measured displacements of 10, 7.5, 5.0, 2.5 and 1.0 mm at table heights of 100, 150 and 200 mm below the machine isocentre. They also examined submillimetre displacements at a height of 200 mm. To obtain breathing and cardiac waveforms, the researchers sent the measurements through two separate bandpass filters and compared them with measurements from a Vernier Respiration Monitor Belt and an electrocardiogram.
The mmWave device could dynamically track both horizontal and vertical table displacements and was able to measure displacements as small as 0.1 mm, the precision limit of the couch displacement. Obstruction by the gown caused the measured values to decrease slightly, with a difference of 0.27 mm from the actual displacement (10 mm), compared with 0.05 mm for unobstructed measurements. The cradle obstruction caused a 1.04 mm error at a displacement of 10 mm, the largest decrease from the unobstructed measurements.
The authors were impressed with the mmWave device’s ability to record the volunteer’s facial movements within the immobilization mask. However, they were unable to quantify the movements to verify its accuracy. They note that this will require a study comparing cone-beam CT images of the patient in the same setup.
Other planned research includes measuring the true sensitivity of the mmWave device with respect to sub-0.1 mm displacement, as well as studies using four sensors in the gantry to measure motion in the cross-plane.
“We need to make significant algorithmic improvements to increase the mmWave device’s accuracy on uneven and diffuse surfaces such as skin,” Olick-Gibson, the project’s primary research engineer, tells Physics World. “We have initiated an implementation of a focusing lens and are currently testing its efficacy. We are also exploring optimization and component decomposition algorithms to improve the distance accuracy of the system beyond the current range resolution.” He estimates that this will take approximately a year.
After completing this development work, the team hopes to conduct multi-volunteer clinical trials of the device to demonstrate its efficacy in a clinical environment with a statistically significant sample of subjects. “Ultimately, we hope that our approach using mmWaves will be implemented as a continuous patient motion monitor during radiation therapy,” says Olick-Gibson. “We believe this tool will assist therapists and physicians in delivering more precise radiation therapy that accounts for intra-treatment movement.”
Complex space The new foyer and café at Bristol Old Vic has been acoustically designed so that small groups can enjoy intimate conversations (left). At the back of the foyer, the auditorium wall (right) has acoustic qualities that allow this area to be used as a performance space. (Courtesy: Fred Hawarth)
In the historic city centre of Bristol in the UK, down a cobbled street lined with mismatched buildings, is the oldest continuously running theatre in the English-speaking world – the Bristol Old Vic.
Built in 1766, and originally called the Theatre Royal, the building underwent a multi-million-pound refurbishment to mark its 250th anniversary. The work required detailed and careful design to ensure that the great Georgian auditorium – renovated in the first phase of the project – can serve the acoustic needs of a wide range of live theatre, music and dance.
Just as complex were the acoustic requirements of the rest of the building, which was renovated in phase two. This second stage included additional performance spaces and offices, as well as a foyer that accommodates a café bar and a further potential performance space. All these different functions have specific and often distinct acoustic requirements, which can be at odds with a host of other technical, cultural and aesthetic demands.
Someone who helps overcome these kinds of hurdles to achieve the ideal acoustic set-up is Bob Essert. Having studied both engineering and music, in 2002 he set up Sound Space Vision (SSV)– a London-based company of acousticians and architectural consultants.
One of SSV’s current projects is a £48.8m renovation of another Bristol auditorium: the city’s Colston Hall, which lies just down the road from Bristol Old Vic and is due to reopen in 2021. As an 1800-seat concert venue, the scale of Colston Hall offers plenty of space for the artists who have performed there since it first opened in 1867, from full-scale symphony orchestras to the Beatles. It has what is often described as a “shoebox” geometry – long with high ceilings that give plenty of space in front of the musicians for a rich sound around the audience, and less space for the sound to get lost behind the performance area (see Levitt Bernstein Architects’ rendering below). The shoebox design is a classic format that some say produces the best acoustics, with nine out of the world’s top 10 concert halls having this shape according to a 2016 survey by Business Insider.
Shoebox geometry Colston Hall render by Levitt Bernstein Architects.
While Essert says the biggest determinant of acoustics is scale, geometry comes second on his list of factors, followed by the materials used. “All three play a part,” he says. A vastness of length, height and general scale in a performance space is not, however, always desirable. Essert points to the hall at the Yehundi Menuhin School in Surrey, UK, as an example where SSV aimed for more compact dimensions that could seat 300 people in a space crafted specifically for solo and chamber performances. “The further away the boundaries of the room are from the listener and to a certain extent the performers, the weaker the sound is,” says Essert.
In simple terms you can think of sound waves attenuating and losing intensity as they travel across the dimensions of the room. As Essert emphasizes, how loud a performance sounds is a key factor for making the audience feel enveloped and immersed in the experience, and as a result designing specifically for solo performers means ideally designing a smaller space. So how can a solo be heard in a space designed to accommodate a full symphony orchestra, and give a feeling of intimacy in a hall that seats 1800?
Reflections on sound design
Ultimately the impact of a production on the audience is dominated by the artistry of the performers on stage. However, an effect that can help a performance to sound intimate and enveloping, even in a huge hall, is reflected sound. Because sound moves at a finite speed – 343 m/s in dry air at 20 °C – any reflections from the boundaries of the room will reach someone in the audience with a delay of several milliseconds compared with the sound that has travelled directly from the performers. You may not consciously hear the delay, but Essert points out that as the brain assembles audio input, this delay – and crucially, the amplitude and direction of arrival – affects the experience.
Soft furnishings as opposed to hard walls will dampen these reflections as demonstrated back in 1895 by US physicist Wallace Clement Sabine, who is widely acknowledged as the founder of architectural acoustics. During an assignment to improve the acoustics of the Fogg lecture hall at Harvard University, he armed himself with an organ pipe and a stopwatch and embarked on a series of experiments, determining by ear how long a sound took to decay as he, for example, changed the number of cushions in the room. Sabine soon established that it was the area of cushions (or any absorbing material) that was linearly related to reverberation time.
The advent of the oscilloscope in the 1960s moved acoustics technology up a gear, making it possible to directly image sound input and analyse the delays from these reflections. Researchers then began to find out more about the role of the direction of sound. For instance, reflections from the sides can make audiences feel more immersed in the experience, just by being surrounded by the sound.
An appreciation of the role of reflections drew attention to the way sound is fed from one surface to another, and affected the design of performance spaces. The basic shoebox geometry is still popular with architects as it has been since the construction of medieval churches, effectively the concert halls of their day. But in the early 1980s – following research in the 1960s and 1970s by Michael Barron and Harold Marshall in the UK and research groups in Göttingen and Berlin – Essert and other acousticians began shaping geometries to guide sound. By engineering the direction in which they reflected the sound, they could bring more sound in from the side. Examples of this architecture include Christchurch Town Hall in New Zealand, the Royal Concert Hall in Nottingham, UK, and the Meyerson Symphony Center in Dallas, US.
Levels of sound
Colston Hall has already seen several renovations and reconstructions (figure 1), the most recent being in 1951 led by Philip Hope Bagenal, the UK’s most prolific concert-hall acoustician of that period. The 1936 renovation had been focused towards cinema – which was then the market-leading use for halls of that nature – resulting in an emphasis on sight lines, audience capacity and cinema sound. But, having survived the Blitz, the concert hall fell victim to a fire started by a cigarette in 1945, and in the 1951 rebuild, Bagenal and architect J Nelson Meredith restored the interior to prioritize classical music performances. Most notably, Bagenal and other acousticians in the UK back then felt that British concert halls lacked definition. The British musical life and taste had been coloured by the sound of town halls around the country, explains Essert – “tall, flat floor spaces that produced a muddy sound”.
1 Many-phase makeover Bristol’s Colston Hall has been refurbished several times, including in 1936 (top left) and 1951 (top right). For the current project, Sound Space Vision took spatial sound measurements of the space (bottom left) and created an acoustic computer model of the proposed design (bottom right). (Courtesy: Sound Space Vision)
Bagenal endorsed a stepped rectangular plan for Colston Hall and introduced materials that would absorb bass “to avoid boom”. In particular, he added a canopy over the stage to project the clarity of string instruments. Although the oscilloscope was not yet established in 1951 so not available to aid design, it had been realized that canopies can reflect sound back to musicians so they can hear themselves.
One of the issues now being addressed by SSV’s renovations at Colston Hall is a literal shortcoming of this canopy. Following extensions to the stage to accommodate larger orchestras, the canopy no longer covers the string section who sit at the front of the stage. In addition, it also turns up at the leading edge, directing the sound out to the audience and making it even harder for the string musicians to hear themselves. Among the renovations SSV is helping implement will be an extended and reshaped canopy with more rigging in it to meet more extensive technical requirements.
Not all reflections are helpful either. The balconies at Colston Hall previously extended over 14 rows of the auditorium, creating a “dead zone” for hundreds of seats: multiple reflections from the bottom of the balcony attenuated a lot of the sound, leaving it dry and weak by the time it reached the seats at the back of the tier under the balcony. The renovation project will include dividing the balcony from one deep structure into two shallower ones so that there are no seats so deep under one low ceiling.
Symbiotic solutions
Back at the Bristol Old Vic, reflections again came in handy to meet the multipurpose needs of the new foyer. It has been cleverly designed so that people can enjoy a quiet conversation over a coffee without being deafened by the sound of everyone else’s chatter. However, with a pressure to maximize revenue from the building, the same space also needs to provide a more vibrant atmosphere and is even designed to accommodate gigs, where audiences do want to be immersed in sound. Vangelis Koufoudakis – an acoustician from the design firm Charcoalblue who worked on the Bristol Old Vic refurbishment – admits that trying to meet multipurpose requirements like this can be problematic. “You can end up with something like a sofa bed – it’s not a great sofa and it’s not a great bed.” Fortunately, architects and acousticians on the project were able to “dig out” a unique solution 250 years in the making.
In the world of acoustics, we love irregular shapes because they stop sound focusing or other unwanted acoustic artefacts
Vangelis Koufoudakis
In the case of the foyer, the architects were keen to provide an open space that connected the theatre to the street and city beyond. Most of the walls of the café-bar area are sound-absorbing. Irregular angles as opposed to parallel walls avoid strange resonances and the room makes liberal use of wood wool – recycled timber and wood filings that absorb sound and convert it to heat. The ceiling of the foyer is a structural diagonal grid formed by glued laminated timber – “glulam” beams. The diagonals form irregular angles that trace back to historical room geometries in the rest of the building. “In the world of acoustics, we love irregular shapes because they stop sound focusing or other unwanted acoustic artefacts,” says Koufoudakis. As a result of these and other acoustic tricks of the trade, the vast open-plan foyer – which you might expect to sound clanging and echoey – provides the perfect acoustics for a quiet tête-à-tête. How then to allow for a more vibrant atmosphere in the same space at different times?
By unearthing the building’s original stone wall to the Georgian auditorium at the far end of the café-bar area, the project team was able to exploit it as an acoustically reflecting backdrop for a performance space directly in front. The wall itself is broken and pockmarked from the passage of time, which means that it reflects a diffused sound with no strange high-frequency resonances. “It’s an amazing architectural surface that reveals the historic scars of the theatre,” says Tom Gibson of Haworth Tompkins and the project architect for phase two of the refurbishment. The thermal mass of the rugged masonry surface also helps regulate the temperature in the café bar.
Level-headed design
The foyer benefits too from another architectural quirk that turned out to be a blessing in disguise. Various add-ons and renovations over the centuries since the theatre was first constructed have led to different ground levels. The project team did not want to disturb the 1970s basement slab or foundations as this could have been expensive and an archaeological risk. “Basically, the old city wall used to run through the foyer and we were worried we might find some historic skeletons,” says Gibson. One of the design challenges was therefore to resolve the difference between the historic floor levels, 1970s floor levels and the newly proposed levels. The solution has been to ramp the new foyer down to street level to provide universal access for the first time in the theatre’s history, while the upper ground floor level creates a convenient raised stage area in front of the original auditorium wall.
2 Centuries in the making These 3D Nolli models show Bristol Old Vic before (a) and after (b) its 2016–2018 redevelopment. The original theatre building was deliberately set back a distance from the street and over its 254-year history there have been many different entrances. In the 1970s an adjacent building called Coopers’ Hall was used for this purpose. The new purpose-built foyer has allowed Coopers’ Hall to be refurbished as an event space and a small studio theatre. (Courtesy: Haworth Tompkins)
The architects have also been able to exploit the various ground levels throughout the site to ventilate the venue’s studio theatre. This relatively small room was moved from the basement and ground floor in front of the auditorium to the basement and ground floor in the Coopers’ Hall section, an adjacent building that served as the theatre’s entrance in the 1970s design (figure 2). The move led to a non-compliant head height in the basement directly under the foyer next to the street and created space constraints that made it difficult to install traditional mechanical ventilators, which need a lot of room. “There was in any case an intent from the project team to naturally ventilate the new studio theatre to save energy and associated costs,” adds Gibson. The basement spaces (with non-compliant head height once the new foyer ground floor level was designed) provided an opportunity to build in a new natural ventilation “labyrinth”. It draws in air from the roof of the foyer through a masonry maze, which chills and quietens the noisy outside air. The result: cool air enters the studio theatre with minimal acoustic disturbance.
In fine shape
Not all architectural statements come from a fortunate alignment of pragmatic technical requirements, however. The Berliner Philharmonie in Germany is widely considered a milestone in the history of concert-hall design, and made a landmark departure from the basic shoebox geometry that had dominated for so long. It was constructed between 1960 and 1963 to replace the former home of the Berlin Philharmonic orchestra, which had been bombed in the Second World War. “People always gather in circles when listening to music informally,” said the architect Hans Scharoun, an observation that led him to design the concert hall with the audience seated around the orchestra on the slopes of a large bowl, like vineyard terraces. This bold design inspired a number of architects who also wanted to make “a statement building” and the vineyard geometry has been widely adopted over the past 15 years.
Sounds different The Berliner Philharmonie was built in 1960–1963 with a design that resembles a bowl or a vineyard. The width is double that of a typical shoebox design. (Courtesy: Shutterstock/posztos)
However, the vineyard geometry has been less popular with acousticians. When the audience is spread out so far in such a wide room, the sound intensity and the subjective intensity of the music are reduced for all. As a result, extending the surround form to a 2000-seat hall with no balconies reduces the intensity and immersion in sound that was intended by a musical composers. And because the audience encircles the stage, people sitting behind the orchestra will hear things differently to those in front, and instruments such as the trombone may sound bright on axis but quieter elsewhere. “You may be effectively getting a French horn concerto because you’re only two feet away from them,” says Essert.
That’s why Essert feels the shoebox-like geometry is getting a revival. There has also been interest in the psychoacoustics of tall narrow concert halls to stop audiences from feeling “boxed in”. The new ceiling at Colston Hall , for example, will have a slight pitch at the sides, mitigating the negative focusing effects of the previously concave ceiling. Convex curves spread the sound in a helpful way and deviate from a pure cuboid, feeling less “boxy”.
Multitasking
Another challenge in venues like Colston Hall is to cater for amplified and non-amplified music in the same space. While acoustics optimized for an orchestra will ideally enrich the sound, designs for amplified music aim for clarity of sound with little reverberation so that what the audience hears is almost exactly what is coming from the speakers. Digital engineering can adjust levels for an amplified performance in an idealized neutral space to a degree, but it cannot fully replace what a room with richer acoustics would do for a live classical performance. Working with constraints of building budgets, retractable panels made from glass-fibre board or even just curtains may be incorporated to absorb reverberation for amplified music and introduce some acoustic versatility.
One of SSV’s projects that took these versatility requirements to a new level was the Xiqu Centre in Hong Kong, where the space has to cater not only for amplified and non-amplified Western music but various traditions of Chinese operas from Beijing, Shanghai, Guangdong and Hong Kong as well. Optimizing this concert venue meant providing the means to balance the sound of the singers with respect to the orchestra, and to emulate the open-air acoustics these traditions were fostered on. The room’s finishes and the audio system in the Xiqu Centre were developed hand in hand.
Unusual needs The Xiqu Centre in Hong Kong has unusual acoustic demands. It stages a wide range of musical styles, so the auditorium was designed with complex shapes, gaps and insulation to absorb or scatter sound, including motorized curtains that can be adjusted as needed. (Courtesy: Sound Space Vision)
The situation gets further complicated, however, as acousticians are no longer catering for audiences expecting great live orchestral sound. Today’s concert-goers instead expect to hear something that sounds like what they hear on their sound systems at home. The problem is that these recordings are generated by engineers who locate microphones at carefully identified positions around the hall or recording studio and then electronically mix the levels and add channels so that you can hear the clarity of the solo and have the resonance of the room at the same time. “You can’t actually get that sound,” says Essert. “But our ears have been attuned to it.” One approach to deliver simultaneous clarity, resonance and envelopment with architecture is to build a room within a room.
The idea emerged during Essert’s projects with Russell Johnson from Artec Consultants in New York, where he found himself repeatedly faced with the problem of devising multipurpose design solutions. In the 1980s Artec introduced a “reverberation chamber” to certain concert halls, such as the Meyerson Symphony Center in Dallas, US, and Symphony Hall in Birmingham, UK. Essentially this couples the inner concert hall that the audience sees to a secondary space, often using concrete doors on heavy pivots. That secondary space will usually have a volume of another several thousand cubic metres and can be a “hard” or “soft” space depending on the use of curtains. This allows it to act as a net absorber or net reverberation generator, but the initial time decay of the room – the first 10–20 dB decay of sound after it arrives – is generated by the geometry of the inner room. The idea was developed further by Artec in Singapore, Los Angeles, Reykjavik and Budapest, and also influenced the design team working on the Paris Philharmonie. Essert used the same principles on the Sage Gateshead in the UK, partially coupling the main space with another above a movable ceiling.
While acoustic design is based on the physics of sound, it hinges on a legion of other structural and technical considerations that multiply as venues take on additional functions to assist their revenue streams. And when it comes to renovating historic venues, engineering solutions must not only be sensitive to the building’s history, but also comply with planning constraints and meet the wide-ranging expectations of audiences. Pulling off that tricky combination is no mean feat. But by ensuring that all contributing factors come together – room geometry, sight lines, comfort, architectural features, building materials and so on – architects and acousticians can provide an experience that, be it rap or rhapsody, coffee or cabaret, leaves every visiting artist, customer and audience member content.
A new 3D camera made from a stack of transparent graphene photodetectors can capture and focus on objects that are different distances away from the camera lens. The device, made by researchers at the University of Michigan in the US, might find use in applications as diverse as biological imaging, driverless cars and robotics.
Most of today’s optical imaging systems use a flat optical detector to record the intensity of light reflected from an object at each pixel. However, since these systems detect light in only one plane, all the information concerning the direction of the light rays is lost. This means that the recorded images are simple 2D projections of the actual 3D object being imaged.
The images acquired thus have a finite field depth – that is, objects at only a particular distance from the camera are in sharp focus, and items in front or behind that distance are out of focus and appear blurred. What is more, because of the 2D projection, it is impossible to determine how far away each object is from the camera.
An ideal imaging system would overcome this problem by producing a complete representation of the 3D scene with an infinite depth of field in a single exposure. Such a device could be made of different detector arrays, each stacked along the path of the different incoming light rays. By then detecting multiple focal planes of data all at the same time, algorithms could be used to reconstruct a complete (“light field”) image of the scene in three dimensions, with each object in the scene being in focus. Furthermore, the distance to each object could be determined. Such range detection, as it is known, would be extremely useful for applications like driverless cars and robots.
A stack of transparent sensors made from graphene
A team of researchers led by Zhaohui Zhong, Jeffrey Fessler and Theodore Norris of the Department of Electrical Engineering and Computer Science at the University of Michigan have now succeeded in building such a photodetecting device. Their new 3D camera uses a stack of transparent sensors made from graphene (a sheet of carbon atoms just one atom thick that has excellent optical transmittance in the visible and near‐infrared range) to simultaneously capture images that are focused to different distances from the camera lens.
The researchers fabricated their photodetector on a transparent glass substrate (rather than the silicon chips usually employed in such devices) using graphene as the light-sensing layer, the conducting channel layer, the gate layer and even the interconnects (which are usually made of metal). Using graphene for all the different functional components of the device in this way allows for sensitive light detection and a transparency of around 95%.
To prove that their design works, the researchers built a single-pixel focal stack light field camera comprising a 100 mm focal length front imaging lens and two single-pixel graphene photodetectors separated by 2 mm. The test object was a point source formed by an illuminated 30-micron-sized pinhole. The point source, the centre of the imaging lens and the two photodetectors were all aligned along the same optical (z) axis.
Range detection
When the point source is far from the lens, the image produced is completely out of focus on both graphene detectors. However, when the researchers then move the point source towards the lens, it becomes perfectly focused on the front detector while remaining out of focus on the back detector. “As we continue to move the point source towards the imaging lens, the intensity of the image decreases on the front detector and increases on the back detector,” the researchers explain. “At some point, the test object is sharply imaged on the back detector while staying out of focus on the front one.”
How larger sensor arrays might capture a scene. Courtesy: Catharine June
By knowing the lens focal length and the positions of the detectors, the researchers say they can determine the distance of the object from the lens. Likewise, knowing the light intensity profile along the z axis allows them to calculate the 3D depth information of the scene from the data using image processing algorithms.
While their current graphene sensors do not have high enough resolution to depict actual images at present, the researchers have simulated how larger sensor arrays arranged in their set-up might capture a scene (see figure above). They say they have also developed the algorithms required to perform such image reconstructions from transparent focal stack data.
As well as applications in autonomous driving and robotics, the new photodetectors, which are detailed in Nature Photonics, might also be ideal for biological imaging in cases where it is important to image 3D volumes, the Michigan team tell Physics World. “There are many applications in optical ranging and object identification that may not require the full computational reconstruction of 3D objects and we are currently pursuing this line of work,” they add.
Acoustics for audiences: creating better-sounding concert venues is the cover feature this month.
There’s something for everyone in the latest edition of your favourite physics magazine, which is now out in print and digital formats.
The cover feature is fascinating, examining the challenges of creating concert halls and other public venues that have the best possible acoustics for audiences.
That’s a problem that’s been around for years, but how do you deal with venues of historical significance that need to be sensitively refurbished, within a fixed budget while also bringing in new revenue streams, for example via bands or shows in the foyer or otherwise under-used spaces?
Elsewhere in the issue find out about the mystery of the “cosmic cold spot”, new Young’s double slits experiments using the electrons ionized from atoms by lasers, as well as the need to ditch the word “quantum supremacy”.
• Towards a quantum advantage – Leonie Mueck, Carmen Palacios-Berraquero and Divya M Persaud argue that the term “quantum supremacy” should be replaced in favour of one that is more responsible
• Flying carbon-free – Greta Thunberg crossed the Atlantic on a zero-emission yacht, but how realistic is it to de-carbonize air travel? James McKenzie thinks he has the answer
• Conspiracy theories – As we enter the third decade of the 21st century, why – asks Robert P Crease – do conspiracy theories still abound?
• Sound designs – The refurbishment of public buildings is often more complex than meets the eye. Anna Demming speaks to acousticians and architects about the acoustic considerations behind their designs for public spaces, and some of the tricks to tackle the conflicting demands on these venues
• Double slits with single atoms – Thomas Young’s double-slit experiment is famous for demonstrating the principle of interference. Andrew Murray explains why it’s now possible to carry out an equivalent experiment using lasers that have excited individual rubidium atoms
• The enduring enigma of the cosmic cold spot – Syed Faisal ur Rahman delves into the various explanations for the strange “cold spot” in the cosmic microwave background, the ancient light of the Big Bang, that bathes the universe
• A critical mass of secrets – Margaret Harris reviews Trinity: the Treachery and Pursuit of the Most Dangerous Spy in History by Frank Close
• An uncertain growth – Susan Curtis reviews Growth: From Microorganisms to Megacities by Vaclav Smil
• More questions than answers – Qamar Scott reviews Extraterrestrial Languages by Daniel Oberhaus
• Quantum computing from the ground up – Trapped-ion computing pioneer Chris Monroe describes how decades of experience in academic and government research led him to start his own
quantum computing firm
• Once a physicist – meet Julie Bellingham is an award-winning garden designer based in Oxfordshire, UK
• Getting the film physics right – Sidney Perkowitz on a new breed of independent movie makers
Big science doesn’t get any bigger than the study and detection of gravitational waves, the elusive ripples in spacetime that have their origin in cataclysmic cosmic events – such as collisions between neutron stars or black holes – hundreds of millions of light years from Earth. At the heart of this endeavour is the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), two widely separated observatories in the US – one in Hanford, Washington, the other in Livingston, Louisiana – which registered the first detection of gravitational waves in 2016, a breakthrough deemed so fundamental it was acknowledged with the Nobel Prize in Physics the following year.
From 2011 through 2017, Gabriela González, professor of physics and astronomy at Louisiana State University (LSU) in Baton Rouge, served three successive terms as spokesperson and scientific lead for the LIGO Scientific Collaboration. This international research effort involves more than 1300 scientists and engineers from over 100 scientific institutions – all working as one to exploit the LIGO observatories in the emerging field of gravitational-wave astronomy and astrophysics.
González’s central role in the LIGO success story is evidenced by a string of high-profile awards, among them the US National Academy of Sciences Award for Scientific Discovery, the Bruno Rossi Prize from the High Energy Astrophysics Division of the American Astronomical Society, and the Special Breakthrough Prize in Fundamental Physics. Here she tells Physics World why an award closer to home has perhaps meant more than most after she was named the 2019 Southeastern Conference (SEC) Professor of the Year.
What did it mean to you when you were chosen as the SEC Professor of the Year for 2019?
The SEC award has touched me very much. There’s a previous winner of this award – Isiah Warner, professor of environmental and analytical chemistry at LSU – who has done so much great work in supporting minorities in science and higher education. I’ve tried to emulate Isiah’s approach in my own career, so it was very special when I learned that I was receiving the 2019 award.
Being the SEC Professor of the Year is also important because it recognizes my teaching and mentoring work with students here at LSU – not just the scientific achievement with LIGO. More broadly, the SEC Faculty Achievement Awards (see box, below) show that the SEC institutions value academic excellence in teaching and research just as much as their well-known sporting successes.
Our young scientists are the future: we need students who are open to new ideas and who can learn to innovate and think independently.
Gabriela González, LSU
How do you approach the teaching and training of the early-career scientists in your LSU team?
Our young scientists are the future: we need students who are open to new ideas and who can learn to innovate and think independently. That’s one of the reasons I’m a big believer in institutional collaboration. My graduate students and postdocs spend their time not just at LSU but at the LIGO observatories in Livingston and Hanford as well as other institutions in the US and internationally. Their research projects always involve collaboration – that’s something I actively encourage. I also aim to foster a spirit of independence, though sometimes my students probably think they get too much independence and would like a little more time from me.
The SEC award also recognizes your contribution to science outreach and education. Tell us more.
Me and my group at LSU are part of the team that helps run the LIGO Science Education Center in Livingston. We have a lot of high-school students and teachers visiting on field trips, while our “Science Saturdays” see LIGO open its doors to the public. There’s also a wider programme of engagement involving talks and presentations within the local community.
What about financial support for this work?
We recently received a $1m grant from the Patrick F Taylor Foundation, a charity that promotes educational initiatives across Louisiana. The funding means we can use the LIGO programme as a vehicle for science outreach to middle schools and high schools that are more remote or that may lack resources.
Our aim is to show students that internationally recognized research can be carried out at institutions in Louisiana by scientists who have gone to college here in Louisiana. It’s in the early days of development but we hope, in the long term, to increase the number of students who enrol and succeed in science.
You grew up in Argentina. What impact has your success had there?
It has meant a lot. It seems I’ve become something of a scientific celebrity back home and I’m often invited to give public talks. I get so much great feedback from children there – perhaps because they can relate to my back-story as a girl who grew up in Argentina. It makes me very happy that children – and especially girls – can see that science is an exciting and attractive career as a result of my efforts.
How has life changed for you since the discovery of gravitational waves?
Because of LIGO’s success, my career has developed in a lot of new directions – and especially since I completed my final term as the collaboration’s spokesperson. Last year, for example, I joined the steering committee of the Astro2020 Decadal Survey, a partnership between the National Academies and the astronomical community to identify funding priorities in astronomy and astrophysics. I also became editor-in-chief of the journal Classical and Quantum Gravity [published by IOP Publishing, publisher of Physics World] and joined the National Academies Board on Higher Education and Workforce.
I end up travelling a lot. I love it – but it’s not a normal life!
It just means more
“There’s a recognition among SEC leaders that the quality of our faculty is worth highlighting in its own right,” says Torie A Johnson, SEC associate commissioner for academic relations. (Courtesy: SEC)
The Southeastern Conference (SEC) Professor of the Year Award is presented annually to one SEC faculty member whose record of teaching and research places him or her among the elite in higher education. Winners are selected by the SEC provosts from among recipients of the SEC’s Faculty Achievement Awards, a foundational programme that recognizes academic excellence across the Conference’s 14 member universities.
“There’s tremendous value in recognizing professors who are making laudable contributions to higher education – both as researchers and educators,” explains Torie A Johnson, SEC associate commissioner for academic relations, an initiative designed to promote the academic achievements of the SEC universities. “These awards are part of a broader communication strategy and the compelling narrative we want to share about our faculty, students and administrators.”
While the SEC awards provide visibility and recognition to high-performing faculty members, the hope is that they also raise the academic awareness of SEC institutions nationally and internationally – by encouraging research collaborations with SEC scientists, for example, or promoting undergraduate and postgraduate recruitment into SEC institutions.
That recognition of academic endeavour is doubly important for SEC universities which, by and large, are better known in the US for their sporting success. (Last month, LSU reinforced that reputation when its American football team, the LSU Tigers, won the College Football Playoff National Championship.)
With this in mind, the SEC has for several years used a simple statement – “It just means more” – to underpin its marketing and positioning to the American public. Put another way: the SEC is not just about success in sports, it’s also highly accomplished in terms of academic research and undergraduate education.
“So, ‘it just means more’ connects to the faculty awards,” adds Johnson, “in the sense that we want everyone to know about the SEC through the work of Professor González and other outstanding SEC researchers and teachers like her.”
Senior physicists descended on Washington, DC last week with political uncertainty at an all-time high with the UK about to leave the European Union and the US still gripped by the impeachment trial of President Trump. The American Physical Society (APS) annual leadership meeting, which was held from 29 January to 1 February, saw leading scientists come together to discuss a range of topics with a focus around this year’s theme of international cooperation and competition.
The opening day saw panel discussions that tackled travel restrictions, obstacles facing collaboration as well as how financial uncertainty affects research funding. In a keynote address, Nobel laureate Steven Chu from Stanford University discussed both collaboration and competition. He touched on the uncertain support for basic research and the difficulties that arise when working with researchers from China where results can sometimes be published prematurely with some Chinese scientists even being under surveillance from the state.
Chu, who served as energy secretary from 2009 to 2013, also highlighted the increased difficulties for foreign students to study at US universities and continue working in the country as well as how clamping down on the ability of scientists to travel impacts science, in which immigrants have played a large role. For example, he showed that 31% of US Nobel prize winners in physics were immigrants. APS chief executive Kate Kirby explained that the APS is currently lobbying the US government to help ease restrictions on the F1 visa that would make it easier for students to stay in the country once they get their degree.
Yet there were also some bright spots for collaboration. David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), described how it has benefited from contributions by hundreds of individuals from outside the US who are a vital part of the LIGO Scientific Collaboration. “I am very encouraged by this meeting because it affirms the importance of international collaboration and how important it is for scientists from all over the world to get together to talk and exchange ideas,” Reitze noted. “Barriers to communication and exchange of information will compromise our ability to do the science.”
The meeting also saw Philip Bucksbaum from Stanford University officially become APS president, succeeding Nobel laureate David Gross from the Kavli Institute for Theoretical Physics in Santa Barbara, who held the position last year.
A glass sphere about 150 nm in diameter has been cooled to its motional quantum ground state using optical tweezers. The feat was performed by physicists in Austria and could bring us closer to testing the macroscopic limits of quantum mechanics. The cooling technique could also lead to experimental investigations of quantum gravity.
In recent decades, researchers have achieved great success in cooling large ensembles of atoms to create exotic systems such as the Bose-Einstein condensate (BEC) – which bagged its creators the 2001 Nobel Prize for Physics. A BEC is an ensemble of ultracold non-interacting atoms that have condensed into a single macroscopic quantum state. Now, physicists want to take this concept much further by cooling solids that comprise much greater numbers of atoms that interact very strongly.
“In a solid, you can cram the same number of atoms you have in an ultracold gas into a volume that’s a billion times smaller,” explains quantum physicist Marcus Aspelmeyer of the University of Vienna, “That provides a natural way of generating large superposition states in which as many atoms as possible are all in one place or another, which is almost impossible in an ultracold gas.”
Experimental challenges
Studying the quantum properties of macroscopic solids has already been done by connecting an object of interest to a nanomechanical oscillator. The object is brought into resonance with the oscillator and the system is cooled by carefully drawing out phonons – which are quanta of vibrational energy. However, there are many experimental challenges associated with this technique, which involves cooling the apparatus to extremely low temperatures.
Another approach is to confine the oscillating object using light in an optical cavity (optical tweezers) and then reduce its vibrations by laser cooling. This minimizes interactions between the object and the outside world – which means that the apparatus does not need to be cooled. Groups including Aspelmeyer’s have attempted this in the past, using one laser to trap the object and a second laser to cool it. However, such schemes have failed to cool large solid objects to their ground states.
In the new work, Aspelmeyer and colleagues used a simpler scheme called cavity cooling by coherent scattering. It was developed in 2001 by team member Vladan Vuletić, who is currently at the Massachusetts Institute of Technology. The technique positions an object at a node of a standing wave field created by a single laser in an optical cavity. The benefit of this set-up is that photons cannot elastically scatter off the object.
Stokes versus anti-Stokes
There are, however, two much weaker inelastic scattering processes that can occur. One is Stokes scattering, which transfers energy from a photon to the object resulting in a lower-energy photon. The second process is called anti-Stokes scattering and involves energy being lost by the object, thereby creating a higher-energy photon. By judiciously selecting the trapping frequency, the researchers suppressed the Stokes scattering while maximizing the anti-Stokes scattering, thereby removing energy from the object and cooling it.
“When we applied this cooling method to our system, it immediately increased the cooling rate by a factor of ten or larger,” says Uroš Delić of University of Vienna, who is lead author on a paper describing the work. “It allowed for three-dimensional cooling rather than just one and basically solved all the problems we were having with our previous setup,” he adds.
After much laboratory work, they successfully cooled a glass nanoparticle containing about 100 million atoms to its motional ground state. Whereas schemes that use mechanical contact for cooling invariably require cryogenic cooling, the current experiment was performed using equipment at room temperature.
The researchers now hope to utilize an isolated glass sphere in its motional ground state to perform manipulations that would otherwise be difficult or impossible. “With laser light, we can arbitrarily change the potential landscape,” says Aspelmeyer, “If a particle in a harmonic ground state sees a non-linear potential, we will automatically generate a non-classical state. These are things that you fundamentally cannot do with clamped solid-state oscillators.” Ultimately, the researchers would like to study objects large and dense enough to generate a detectable gravitational field.
James Millen of King’s College London is enthusiastic about the results. “This is the first step towards doing something like the quantum double slit experiment with something truly massive,” he says. “That would tell you whether or not quantum mechanics works on this mass scale. There are all sorts of reasons people think it might not and all sorts of theoretical modifications to quantum mechanics that would be ruled out if you could do that experiment. However, you cannot do any experiment testing quantum mechanics until you’ve done this first step of cooling the particles down to their motional ground state.”
“There isn’t one valley of death. There are many.”
Bruce Tromberg’s words drew murmurs of recognition from a crowd at the Photonics West conference in San Francisco, US, where he opened a packed Saturday afternoon of talks on entrepreneurship in healthcare photonics. As the director of the US National Institute of Biomedical Imaging and Bioengineering (NIBIB), Tromberg is familiar with the effort required to transform scientific discoveries into clinical products. Many promising ideas never make it, which is why the phrase “valley of death” gets bandied about; it’s a catch-all term for a whole range of problems (scientific, engineering, regulatory, financial, clinical, and so on) that can derail a breakthrough on its path from lab to clinic.
Bridging this valley of death – or, rather, valleys of death – is a challenge for organizations like the NIBIB, Tromberg explained, because so many different issues contribute to the problem. In his talk, he highlighted the need for continued development of novel materials and physics simulations of biological systems. Beyond that, though, he also identified a need for more engineering work to translate those developments into practice. “The kinds of things we aspire to really can’t be done without a heavy lift from the engineering community,” he concluded.
The panel discussion that followed Tromberg’s talk offered further insights into how the valley(s) of death can be bridged. All the panellists had founded start-up companies in biomedical photonics, and I was interested to see a couple of familiar names among them. Photonics West brings thousands of industry experts and academic researchers to San Francisco each year, and for the past several years it has hosted a competition for start-ups alongside the usual round of talks and poster sessions. Saturday’s panel featured a few previous winners of this competition, including Brittany Berry-Pusey, who took top honours with her company Avenda in the 2019 contest; and Ryan Shelton, who won with PhotoniCare in 2018.
On the surface, these two companies are very different. Avenda is using machine learning to help surgeons distinguish between tumours and healthy tissue in patients with prostate cancer, while PhotoniCare has developed an optical coherence tomography instrument for diagnosing ear infections. Nevertheless, the advice they offered on overcoming the “valley of death” was striking in its similarity. After Berry-Pusey pointed out that it is easier for a scientist like her to pick up business skills than it would be for a businessperson to learn the technical details, Shelton chimed in to say that his strategy for developing business skills was “basically to buy coffee for anyone [around me] who looked like they might be knowledgeable”.
My favourite advice piece of advice, though, came from the panellist Zach Helft, who co-founded a company called C Light Technologies that develops eye-tracking systems for neurology diagnostics. After explaining that many scientist-entrepreneurs get so enamoured of their technology that they lose perspective, he advised the audience to take a step back and ask “So what?” If you can answer this “so what?” question five times, at successively deeper levels, Helft explained, you may have the proper distance from your work to go out and try to sell it to investors – and eventually to customers.
