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Novel photovoltaics generate electrical power from thermal sources

A new type of photovoltaic device can generate useful amounts of electrical power from sources that radiate heat at moderate temperatures. So say researchers at Sandia National Laboratories in the US, who succeeded in recovering power densities between 27–61 μW/cm2 from thermal sources at 250–400°C. The new energy-harvesting technology might be used on waste heat from nuclear power plants or chemical manufacturing facilities. According to Paul Davids, who led the research effort, it could also aid the development of compact thermal power supplies for deep space probes and other remote applications.

Standard photovoltaic devices (such as solar photovoltaic cells) work by absorbing incident radiation across the bandgap of a semiconducting device. These devices usually feature p-n junctions designed so that light is absorbed within a region of the device known as the depletion width. When this region absorbs a photon, the resulting electron-hole pair is spatially separated by the region’s internal electric field, and this separation of charge induces an open circuit voltage across the device.

Devices of this type work well for energetic photons in the visible range of the spectrum – including those produced by our Sun, which has an effective black-body temperature of around 6000 °C. Objects at temperatures between 100 to 400 °C, however, emit light mainly in the thermal infrared part of the electromagnetic spectrum, with wavelengths between 7-12 μm. The waste heat radiated from many modern industrial processes falls into this range, so putting even a small percentage of it to good use could significantly reduce energy consumption.

Photon-assisted tunnelling

The problem, Davids says, is that as the photons’ energy decreases and approaches the thermal energy of the converter, standard photovoltaic converters become exceedingly inefficient. This drawback led his team to seek alternative ways of generating current.

“In our device, the photocurrent produced by the electron-hole pairs is not from photon absorption but from photon-assisted tunnelling,” he explains. “This tunnelling acts to shuttle charge into a periodic array of wells formed by interdigitated p-n junctions under our subwavelength metallic grating.” 

The photon-assisted tunnelling current is driven by infrared radiation confined in the tunnel barrier of the device, which the researchers fabricated from a silicon dioxide layer just 3-4 nm thick. This structure provides a predominantly one-way path for electrons to be separated from holes, leading to a large open circuit voltage across the device and efficient conversion of infrared radiation into electrical power.

Application areas

The devices developed by Davids and colleagues can be made using standard CMOS processes routinely employed in manufacturing advanced semiconductor chips. This means they could be fabricated in high volumes. In the nearer term, the researchers say they would like to use their energy-harvesting technology to develop power supplies for deep space probes, which cannot use photovoltaic cells because they are too far from the Sun. The Sandia devices could also provide enough power to be used as a primary source of electrical power generation, or as a supplement to standard thermoelectric methods, Davids says.

Another potential application lies in recovering electrical power from large cloud-computing data centres, which dissipate a lot of heat and must be cooled continuously to keep processors below 120 °C. “If we can recover electrical power from this waste heat, we could improve the energy efficiency of this growing part of the energy consumption market,” Davids tells Physics World.

The researchers, who report their work in Science, say they are now focusing on enhancing the conversion efficiency of their devices and simplifying their fabrication process. “As we continue to improve the efficiency of our technology and scale up to larger areas, we are confident that many more application areas will emerge,” Davids says. “There are also several exciting connections with recent advances in passive photonic cooling and structured light emitters, which when combined with our technology could open up other untapped avenues for power creation and reclamation.”

Coronavirus hits the conference calendar, physicists excel in ‘deep tech’ start-up challenge, remembering Freeman Dyson

The big physics news this week is the cancellation of the March Meeting of the American Physical Society because of concerns over the spread of the COVID-19 coronavirus.

In this episode of the Physics World Weekly podcast we hear from conference delegates who had travelled to Denver Colorado, only to find that the March Meeting had been cancelled. We chat about how attendees have organized alternative meetings – in person and in cyberspace – and ponder the future of the scientific conference in the Internet age.

We also hear from physicists who came tops in a “deep tech” start-up challenge that was held last month at Photonics West in San Francisco and remember the iconoclastic mathematical physicist Freeman Dyson, who died last week age 96.

Crowdsourced AI challenge aims to improve mammography accuracy

Mammography screening is widely employed for early detection of breast cancer. But mammograms currently rely on subjective human interpretation and, as such, the screening process is not perfect. In the USA, for example, such screening leads to an estimated 10% false positives, which increases patient anxiety and can result in unnecessary interventions or treatments.

Advances in deep learning and increased computational power have recently renewed interest in the use of artificial intelligence (AI) to increase screening accuracy. With this aim, the Digital Mammography (DM) DREAM Challenge used a crowdsourced approach to develop and validate AI algorithms that may improve breast cancer detection. The goal: to assess whether such algorithms can match or improve interpretations of mammograms by radiologists (JAMA Netw. Open 10.1001/jamanetworkopen.2020.0265).

The DM DREAM Challenge – directed by IBM Research, Sage Bionetworks and the Kaiser Permanente Washington Research Institute – is the largest objective study of deep learning performance for automated mammography interpretation to date. “This DREAM Challenge allowed for the rigorous and appropriate assessment of tens of advanced deep learning algorithms in two independent databases,” explains Justin Guinney, president of the DREAM Challenges.

The challenge required participants to develop algorithms that input screening mammography data and output a score representing the likelihood that a woman will be diagnosed with breast cancer within the next 12 months. In a sub-challenge, the algorithms could also access images from previous screening examinations, as well as clinical and demographic risk factor information.

The data for the challenge were provided by Kaiser Permanente Washington (KPW) in the USA and Karolinska Institute (KI) in Sweden. The KPW data set, which included 144,231 screening exams from 85,580 women, of whom 1.1% were cancer positive, was split for use in algorithm training (70%) and evaluation (30%). The KI data set, used only for algorithm validation, comprised 166,578 exams from 68,008 women, of whom 1.1% were cancer positive.

To ensure the privacy of these data, both data sets were securely protected behind a firewall and not accessible to challenge participants. Instead, participants sent their algorithms to the organisers for automated training and testing behind the firewall.

Crowdsourced competition

The challenge was taken up by more than 1100 participants, making up 126 teams from 44 countries. In a first stage, the algorithms were trained and evaluated on the KPW data, with AUC (a measure of how well the algorithm’s continuous score separates positive from negative breast cancer status) used to evaluate and rank algorithm performance.

Interestingly, including clinical data and prior mammograms did not improve the algorithms’ performance. The DM DREAM team suggest that perhaps participants did not fully exploit this information and recommend that future algorithm development should focus on the use of a patient’s prior images.

DM DREAM Challenge workflow
A: Participants submitted models to be trained and evaluated behind a firewall in a secure cloud (grey box). B: The eight best models were combined into the CEM, trained using the KPW training set and evaluated in the KPW and KI evaluation datasets. C: A final ensemble method, CEM+R, incorporated radiologists’ interpretation. (Courtesy: CC-BY/JAMA Netw. Open 10.1001/jamanetworkopen.2020.0265)

The eight top-performing teams were invited to collaborate to further refine their AI algorithms, to evaluate whether an ensemble approach could improve overall performance. The output of this “community phase” was the challenge ensemble method (CEM), a weighted aggregation of algorithm predictions. This CEM model was also integrated with the radiologists’ assessment into a second ensemble model called CEM+R.

To compare CEM predictions with radiologists’ interpretation (recall/no recall), the competition determined CEM specificity when using the sensitivity of radiologists at each institution. For the KPW data set (with a radiologist sensitivity of 85.9%), the top-performing AI model, the CEM and the radiologists achieved specificities of 66.3%, 76.1% and 90.5%, respectively. While CEM remained inferior to the radiologists’ performance, CEM+R increased the specificity to 92%.

The challenge team repeated the assessment using the KI data. For these exams, each mammogram underwent double reading by two radiologists, so the first reader interpretation was used to mirror the KPW data set. At the sensitivity of first readers’ (77.1%), the specificities of the top model, the CEM, the radiologists and the CEM+R were 88%, 92.5%, 96.7% and 98.5%, respectively. Again, CEM+R provided the highest specificity. The team also compared the ensemble method with the double-reading results, observing that in this case, the CEM+R did not improve upon the consensus interpretations.

The results show promise for deep learning to enhance the accuracy of mammography screening. While no single AI algorithm outperformed the radiologist benchmarks, the CEM+R model improved performance over single-radiologist interpretation, such as used in the USA. In the double-reading and consensus environment, as seen in Sweden for example, adding AI may not have as great an effect. However, that it’s likely that training an ensemble of AI algorithms and radiologists consensus assessments would improve accuracy.

The challenge team conclude that combining AI algorithms with radiologist interpretation could reduce mammography recall rates by 1.5%. With some 40 million women screened for breast cancer in the USA each year, this means more than half a million women annually would not have to undergo unnecessary diagnostic work-up.

Death by prime numbers

Prime Suspects: the Anatomy of Integers and Permutations
(Select pages from Prime Suspects: the Anatomy of Integers and Permutations by Jennifer and Andrew Grenville; illustrated by Robert J Lewis; © 2019 by Princeton University Press. Reprinted by permission.)

Two people are dead, and the police are baffled. Arnie Int, the 60-year-old lieutenant to the godfather of the Integer crime family, is found brutally murdered in a drainage tunnel. Later, the petite body of young ballet dancer Daisy Permutation is also found and brought to the morgue. Gruff and grizzled lead detective Jack von Neumann suspects a link between the two, and has brought in a consultant on the case – legendary mathematician and professor of forensic science C F Gauss.

So begins the unique graphic novel Prime Suspects: the Anatomy of Integers and Permutations, an imagined world “where detectives work closely with mathematicians”. A forensic crime drama, mixed in with number theory, as well as an exploration of student–mentor relationships, all in the graphic novel format, Prime Suspects was written by the Canada Research Chair in Number Theory at the University of Montreal mathematician Andrew Granville and writer, educator and director Jennifer Granville; with illustrations by Toronto-based graphic designer Robert J Lewis. Bringing in elements from film noir, TV police shows and famous movies, coupled with some amazing art work, subtle mathematical humour and corny science jokes, and what you have is a one-of-a-kind creation – indeed, Prime Suspects has it all from minus to plus infinity.

Early in the story, Gauss involves two of his most promising students – the snooty Sergei Langer and the hip, red-haired heroine Emmy Germain – while a pair of documentarians have arranged to record the team’s work. Eager to impress (and become the professor’s new research assistant), Langer and Germain both attempt to uncover the details of the murders. At the morgue, the team soon begins to find some surprising links between the two bodies, including peculiar cuts on the chest, and both hearts having been surgically removed.

While Langer barfs at the gruesome sight, Germain reaches into Int’s chest and pulls out a bloody clump of tissue with the distinct shape of “7309”; and so the book is off to the races. “Primes are the fundamental constituent parts of integers,” she tells the confused film producers, “their genetic code, if you like.” We also learn that the ballet dancer’s family business is the Alternating Group – a tongue-in-cheek description, though not as visceral as the primes. A bit corny, perhaps, but corny in the pursuit of larger truths.

A bit corny, perhaps, but corny in the pursuit of larger truths

As the story moves to Gauss’s luxury penthouse (don’t all legendary mathematicians have one?), the trio play billiards and discuss the similarities between primes, the fundamental constituents of integers, and cyclic permutations (or cycles), the fundamental constituent parts of [Daisy’s] permutation. Langer, ever anxious to show off his smarts, says “they’re about as similar as apples and iPhones”. In fact, as the rest of the book goes on to show, primes and cycles play similar roles in the study of integers and permutations, respectively. Even many of the equations describing their traits are similar; there’s a cyclic equivalent to the 15-year-old real mathematician Carl Friedrich Gauss, who found that the number of prime numbers among the first N integers is near log(N). (Physicists use the notation ln(N)).

As a student of physics who received their career-worst grade in undergraduate topology class (in my defence, I did join the class two weeks late), the properties of numbers and sequences come more easily to me than the more abstract properties of permutations. We all understand prime numbers. A permutation of, for example, the four integers (1 4 5 8) is (4 5 8 1); another is (5 8 1 2), and so on. M items can be permuted in M! (M factorial) ways, and every permutation can be broken up into “cycles”.

These cycles (a bit too involved to include in this review; see Wikipedia’s entry on “permutations”) all containing M or fewer items, uniquely represent a permutation much as primes uniquely represent an integer. Exactly one in every M permutations on M letters is a cycle. Phew.

Just when you start to fear the book is going over your head, and that you’ve missed something and can’t keep up with the maths, you thumb through the rest of the book to discover a chapter near the end called “The mathematics of prime suspects”. There, the mathematics of both primes and cycles is laid out in traditional mathematical garb, lingo and rigour. It felt like arriving home after a foreign trip.

It was then I realized the graphical storyline wasn’t meant to teach the details of the maths so much as to wet my whistle and motivate me to learn more. Which it did, not just via the later, textual chapters but by going online to Wikipedia and YouTube and reading a few papers (some by one of the authors, Andrew Granville). As it emerges, the central question here is: why are the anatomies of integers and permutations so similar?

Later in the book a third murder victim, Polly Nomial, is found whose body has been spread over Finite Fields…. I won’t give away the murderer or the story’s conclusion, but I will give away its success: it entertains while provoking one’s curiosity. Every page has something witty on a background sign or screen, and the book is chock full of homages to famous mathematicians, right down to the character names. With its beautiful and expressive artwork Prime Suspects is truly “mathematics as you’ve never seen it before”.

Wave power, terahertz physics and bumper careers advice for graduates

Physics World March 2020 cover

After this winter’s storms caused massive waves to batter the UK coastline, it’s easy for us to see that the sea contains vast amounts of power and energy, but is also highly destructive.

In our cover feature of the latest edition of Physics World magazine, Stephen Ornes investigates why the wave-energy industry is struggling to harness the power of the world’s oceans. Both financial and technical hurdles stand in the way, but researchers and pioneers are persevering with the hope that this clean, renewable source could be the answer to our energy problems.

Elsewhere in the issue, we’re excited to launch of a new series of interviews providing careers advice for physics graduates. Tushna Commissariat asks 10 of today’s top physicists three questions to find out about their day-to-day jobs and what they wish they knew when they started their careers. The March magazine, which is now out in print and digital formats, also features an interview with Nobel laureate Steven Chu, a look at how terahertz science is exploring space, and a mind-bending article on time crystals.

Remember that if you’re a member of the Institute of Physics, you can read the whole of Physics World magazine every month via our digital apps for iOS, Android and Web browsers. Let us know what you think about the issue on Twitter, Facebook or by e-mailing us at pwld@ioppublishing.org.

For the record, here’s a run-down of the full issue.

  • Joe McEntee reports from the 2020 Physics in Food Manufacturing conference in Leeds
  • Nobel laureate Steven Chu from Stanford University talks about his successes as energy secretary and tells Richard Blaustein how the US can collaborate in a competitive environment
  • Paul Ewart argues that compulsory retirement can be detrimental to physics
  • James McKenzie looks at why businesses must take action on climate change
  • Robert P Crease reviews the Hayden Planetarium’s new space show
  • Huge technical and financial hurdles face anyone seeking to harness the vast power of the world’s oceans. But as Stephen Ornes explains, for a devout band of researchers and hi-tech business pioneers, the dream of “blue energy” lives on
  • The terahertz range has been barely exploited compared to the rest of the electromagnetic spectrum. Sidney Perkowitz discusses the astronomical applications that have opened up with advances in terahertz detection
  • Peter Hannaford and Kryzsztof Sacha look at how time crystals could have similar applications to condensed-matter devices
  • David Appell reviews Prime Suspects: the Anatomy of Integers and Permutations by Andrew Granville and Jennifer Granville
  • Andrew Robinson reviews Einstein in Bohemia by Michael Gordin
  • Ask me anything: 10 top physicists share their careers advice for physics graduates, including Helen Margolis (NPL), Sadik Hafizovic (Zurich Instruments), Chao-Yang Lu (UST China) and Cather Simpson (Engender Technologies)
  • Caitlin Duffy interrogates the lack of experiments around the claims of biodynamic wine growing

PET tracks down causes of fever in children

Fever, an elevated body temperature of about 38.0–38.3 °C or above, is one of the most common symptoms in children presenting at the hospital. Fever can arise due to a variety of conditions – most commonly infection, followed by autoimmune disease and then malignancy. But in around half of infants up to three years old, and 10–20% of all children, no definitive cause is found, despite extensive diagnostic and laboratory tests. This condition referred to as fever of unknown origin (FUO).

In adults with FUO, PET/CT with the tracer 18F-FDG has been used to diagnose focuses of infection or inflammation. In children with FUO, however, studies are scarce and its usefulness remains unestablished. Now a team from University Medical Center Groningen (UMCG) has examined a large group of children with FUO to determine the value of FDG-PET/CT in finding the cause of fever (Eur. J. Nucl. Med. Mol. Imaging 10.1007/s00259-020-04707-z).

“Fever can be caused by many different diseases, some of which could be fatal if left untreated,” explains first author Jordy Pijl. “If the cause of fever is unknown, it can be very difficult to start the right treatment, leading to increased morbidity or even mortality of patients.”

Data mining

Pijl and colleagues searched UMCG’s electronic patient database for patients aged up to 18 years who had undergone FDG-PET/CT for fever evaluation between 2010 and 2019. For the study, they examined the scans of 101 patients with FUO (fever for eight or more days) and nine patients with fever without source (FWS, fever without a clear source for less than eight days).

In 68 of the 110 patients (62%), a definite cause of fever was found, with FDG-PET/CT identifying the cause in 53 of these cases. Common diagnoses included endocarditis, systemic juvenile idiopathic arthritis and inflammatory bowel disorder. In 42 patients (38%), no cause of fever was found on FDG-PET/CT or any other diagnostics and the children remained as FUO.

In one example case (shown in the above image), a 9-year-old boy presented at the hospital with anorexia, weight loss, fatigue and intermittent fever up to 39.5 °C. He had experienced these symptoms episodically over the past 18 months, but had no definite diagnosis. FDG-PET/CT showed extensive FDG uptake throughout the entire colon, suggesting inflammatory bowel disorder; intestinal biopsy established the final diagnosis of Crohn’s disease.

Based on the reference standard (the final diagnosis at the patient’s discharge), 53 FDG-PET/CT results were true positive, 10 were false positive, 38 were true negative and nine were false negative. These findings correspond to a sensitivity of 85.5%, specificity of 79.2%, positive predictive value of 84.1% and negative predictive value of 80.9%.

The researchers point out that, as the false negatives illustrate, it’s important to remember that not all cases of FUO can be diagnosed with FDG-PET/CT. They suggest precautions that could help avoid unnecessary false negatives, including patients sticking to low-carbohydrate diets, especially when a focus of fever is suspected in tissues with high metabolic activity, and minimizing corticosteroid dose, especially when vasculitis is suspected.

In 58 out of the patients (53%), FDG-PET/CT led to treatment modifications, including a change in antibiotics, starting immunosuppressive therapy and starting treatment with a non-steroidal anti-inflammatory drug.

Associated factors

The researchers also used multivariate logistic regression to look for clinical parameters associated with FDG-PET/CT performance. They found that the level of C-reactive protein was positively associated with FDG-PET/CT determining a true positive focus of fever, while leukocyte count was negatively associated with finding a true positive. No other factors were significantly associated with outcome, making it a challenge to identify FUO patients who may likely benefit from FDG-PET/CT investigations.

While diagnostic tests such as a chest X-ray or urinalysis are quick, easy to perform and relatively cheap, and should thus be considered first, FDG-PET/CT provides a valuable diagnostic tool for evaluating children with FUO, the authors conclude. “FDG-PET/CT is a rapidly developing technique that can provide a quick full-body evaluation with less and less radiation, so in the future, it will likely climb up the diagnostic ladder of fever,” says Pijl.

“Now that we have looked into children with all causes of fever, we would like to focus on evaluating the use of FDG-PET/CT for specific causes of fever,” he tells Physics World. “Also, we are planning studies to evaluate the newest PET/CT scanners that can perform a scan in less time and with less radiation, thereby further establishing the role of FDG-PET/CT in diagnosing patients – and especially children – with fever or other symptoms of diseases.”

Superconductor transition switches single-molecule magnet

A superconductor can switch the magnetic moment of a single-molecule magnet placed on top of it. This novel phenomenon, discovered by researchers in Italy, occurs because of quantum tunnelling of magnetic spins, and might be exploited in future quantum information technologies.

Single-molecule magnets are paramagnetic materials that can switch their magnetization between two states – “spin up” and “spin down”, for example. At low temperatures, these molecular complexes retain their magnetic state even in the absence of a magnetic field because reversing the magnetization would require them to overcome an energy barrier. This magnetic “memory” effect could be exploited in spintronics and quantum computing applications since the spins can act as stable quantum bits, or qubits.

According to study lead author Giulia Serrano, the combination of molecular magnets and superconductors is currently a hot research topic. Among other findings, researchers have discovered that monolayers of paramagnetic molecules can influence the temperature at which an adjacent layer of material becomes superconducting (that is, conducting electricity with no resistance). This change in the superconducting transition temperature Tc occurs because the paramagnetic monolayers create local states in the bandgap of the superconductor.

Influence of the superconducting transition

Serrano and colleagues in Roberta Sessoli’s group at the University of Florence have now found that this interaction also works in the opposite sense: a material undergoing a superconducting transition can influence the spin dynamics of nearby single-molecule magnets. In their experiments, the researchers studied clusters of four iron atoms (Fe4) incorporated into the structure of a complex molecule containing ligands derived from a trialcohol. The geometry of this molecule keeps the iron atoms in a propeller-like arrangement that protects the high spin of the Fe4 magnetic core at low temperatures.

The team did their experiments in a ultrahigh vacuum chamber, where they used a thermal sublimation technique to deposit the Fe4 clusters onto the surface of lead (111). This material, a type-I superconductor, changes from a metal to a superconductor at a Tc of 7.2 K., but Serrano explains that superconductivity is only established if the applied magnetic field is lower than the critical field Hc. For lead, Hc is around 800 oersteds.

The researchers then analysed the magnetism of the Fe4 using synchrotron light and a technique called X-ray magnetic circular dichroism (XMCD). They found that at Hc, the lead superconductor switches the magnetization state of the Fe4 by “activating” the resonant quantum tunnelling of its magnetic spins. Quantum tunnelling is the process by which quantum particles can penetrate energy barriers that would be insurmountable to classical objects.

A new magnetization switching mechanism

Serrano and colleagues say this phenomenon is a new magnetization switching mechanism – a hypothesis they backed up by observing magnetic hysteresis loops, which show how the magnetic flux density, B, of a material changes as a function of an applied magnetic field, H.

As lead undergoes its superconducting transition, an increasing fraction of it enters a so-called Meissner state, which occurs when a material placed in a magnetic field expels magnetic flux from its interior as it becomes a superconductor. This state has the effect of locally cancelling the external magnetic field of the molecular magnet and “unblocking” its magnetization state.

“Single-molecule magnets in contact with these superconducting lead regions thus switch their ‘blocked’ magnetisation state to a resonant quantum tunnelling regime by the activation of the quantum tunnelling process,” Serrano tells Physics World.

As the lead transitions to the superconducting state, the number of switching events increases as more regions of the material become superconducting. This can be seen as a gradual decrease of the magnetization value in the hysteresis loop of the single-molecule magnet, she says.

The beginning of a novel research field

Serrano says that the team’s observations open new perspectives for using such hybrid systems in quantum information technologies. As well as being exploited as qubits with a magnetization that can be switched quickly, single-molecule magnets could also be used as local sensors for probing the superconducting state, she adds.

According to the researchers, who report their work in Nature Materials, the new result heralds the beginning of a novel research field aimed at better understanding how single-molecule magnets – and magnetic molecules in general – interact with various kinds of superconductors. They suggest that superconductors with complex domain structures, such as vortex states, would be particularly interesting to study.

Coronavirus concerns halt APS March Meeting – delegate reactions

News started to filter out late on Saturday 29 February that the March Meeting of the American Physical Society (APS) had been cancelled due to “rapidly escalating health concerns” over coronavirus COVID-19. But this news came too late for many delegates who were already on their way to Denver, including many flying in from overseas. In this short video, physicists who had made the trip – including students and industry scientists – speak about the impact of the cancellation on them. Physicists are an enterprising bunch, so despite the frustration, some delegates have turned a bad situation into an opportunity, including Itamar Sivan, chief executive of Quantum Machines.

Terahertz microscope produces highly accurate ‘ghost’ images

A new type of microscope that can detect terahertz (THz) electromagnetic waves with unprecedented accuracy could be used to reconstruct detailed images that are inaccessible through standard methods. The device, which was developed by researchers at the University of Sussex in the UK, relies on a technique called nonlinear ghost imaging and could find applications in areas such as the life sciences, quality control in manufacturing and airport security.

THz radiation lies between microwaves and infrared radiation on the electromagnetic spectrum. Like X-rays, it easily passes through materials that are opaque to visible light, but its lower energy means that it does so without damaging living tissues. It is thus safe to use on even the most fragile biological samples. Images produced using THz radiation are also hyperspectral, meaning that each pixel in the image contains the electromagnetic signature of the corresponding area of the imaged object. This property enables researchers to visualize the molecular composition of objects and so distinguish between different materials.

Until now, however, microscopes capable of capturing images that preserve the fine details revealed by THz waves were not considered possible. This, explains project leader Marco Peccianti, is because the details you want to see are typically much smaller than the THz wavelength, and the more you focus on them, the more their electromagnetic signature is altered. “The main challenge in THz cameras today is not just about collecting an image but about preserving the object’s spectral fingerprint, which can easily be corrupted by the technique employed,” says Peccianti, who leads the Emergent Photonics (EPic) laboratory at Sussex.

Passing visible light patterns through a thin nonlinear crystal

To overcome this problem, Peccianti and colleagues developed a camera based on visible-light patterns generated by a laser. These patterns change in time, and the researchers pass them through a thin nonlinear crystal that converts them into patterns of THz light. The THz patterns are then projected onto the object being imaged, which – as in an optical microscope – is located very close to the camera.

When this series of known THz light patterns shines onto the object, a single-pixel field detector measures the intensity of the resulting scattered light. By processing multiple signals of varying light intensity from the detector, the researchers are able to reconstruct an image of the object without actually imaging the light scattered directly from it – a computational technique known as ghost imaging.

“The way our technique images the object is based on the fact that each light pattern is actually a THz pulse in time (with an associated shape in space),” Peccianti tells Physics World. “Just as in ultrasound imaging, we receive a wave that contains full information about the object. Our technique captures all the object information and ‘de-shuffles’ its content to produce a hyperspectral image with hitherto inaccessible fidelity.”

Potential applications for the camera include electromagnetic biopsy of skin tumours and quality control in manufacturing. It could also be used to perform chemical analyses of samples in a non-destructive way, such as in airport security, Peccianti says.

The researchers, who report their work in Optica, say they are now applying machine-learning techniques to speed up the image reconstruction process in their system.

Optical gyroscope on a chip can detect Earth’s rotation

A gyroscope sensitive enough to detect Earth’s rotation has been created using a chip-based optical cavity. It was developed by Kerry Vahala and colleagues at Caltech in the US and its performance is currently on par with some commercial chip-based devices that use other measurement techniques. The team says that their design could be improved so that its sensitivity is tens or hundreds of times better than other chip-based gyroscopes – and with further effort, the technology could be adapted to create commercial devices.

Optical gyroscopes work on a physical principle called the Sagnac effect. Under laboratory conditions, optical gyroscopes are among the most sensitive devices available to measure rotation. The Rotational Motions in Seismology gyroscope in Germany, for example, can measure minute variations in Earth’s rotation rate.

“Imagine a ring with a clockwise and a counter-clockwise laser beam,” says Valhala. “If the ring is not rotating, then the round-trip time for the beams to come back to some fixed point on the perimeter is identical. But as you rotate the ring, then the round-trip time becomes different, and the difference is proportional to the rotation rate.”

“Hockey puck” of optical fibres

The effect can be enhanced – and thus the sensitivity of the gyroscope increased – by increasing the distance travelled by the light. This is often achieved by sending the light down a long coil of fibre-optic cable, producing a device that looks, says Vahala, “like a hockey puck”. Producing such a device on an integrated-circuit chip would be difficult or impossible according to Daniel Blumenthal of University of California, Santa Barbara – who is not a member of Valhala’s team.

A possible solution lies in an alternative type of optical gyroscope called a ring laser gyroscope. Instead of using a long coil of optical fibre, ring laser gyroscopes effectively send light waves in both directions repeatedly around the same circular laser cavity. “Rather than having to go around a physical length of 200 m or longer, you’re recycling the light and amplifying it through the lasing action,” explains Vahala. “It’s that recycling action through many round trips that gives ring laser gyroscopes their sensitivity.”

In principle, one could simply send laser light with identical frequencies in both directions around the same cavity. At zero rotation, there would be no beat frequency between the interfering waves, and the beat frequency would grow as the rotation rate increased.

Locked out

Unfortunately, in ring laser gyroscopes, the clockwise and anticlockwise frequencies tend to lock together, suppressing the Sagnac effect at low rotation rates. “Different techniques have been developed in commercial ring-laser gyroscopes over the decades, including mechanically rocking the device back and forth to unlock the frequencies and allow the measurement to proceed,” explains Valhala.

Now, Vahala’s team has developed a more elegant solution, injecting different clockwise and anticlockwise pump laser signals into the cavity. These create the ring-laser signals via a process called stimulated Brillouin scattering. This prevents the two laser signals from locking together and provides a non-zero beat frequency between the signals. Any additional rotation will change this beat frequency.

The researchers applied this technique in an optical gyroscope with a 36 mm ring laser cavity on a silicon chip. They set the device on an optical table and tilted it first North and then South, detecting a frequency difference equivalent to that expected from the rotation of the Earth.

Blumenthal comments, “People, including my group, have put laser gyros on chips before. But the hard part, which this paper overcomes, is breaking this lock-up around the zero point.” He adds, “Measuring Earth’s rotation rate is considered your standard of the best you can do at zero movement because if you can measure that you can measure any other rotation and then subtract out the Earth’s rotation rate.”

Important first step

Blumenthal points out that only the resonant cavity is on a chip – with the pump laser, detectors and other components external to the chip in a stabilized enclosure. However, creating the cavity on a chip is a crucial first step towards a fully on-chip technology. Vahala’s team concur that the technology is not ready for commercialization and are working on ways forward.

However, Vahala says that the group’s chief priority is increasing sensitivity. “There are little MEMS gyroscopes based on the Coriolis effect in cellphones, video games – anywhere you need a very low-cost way of measuring the orientation of something – and these are by far the most commercially successful gyroscopes ever,” he says.

“[Our] gyroscope is competitive with some of these MEMS devices – but why would anyone want to pay for the development of an optical gyroscope on a chip to obtain the same level of performance as a MEMS device? These Sagnac devices have the potential to offer much better performance and, at somewhere between a factor of ten and a factor of a hundred improvement, these devices would probably start to create their own separate application space.”

The laser cavity and Earth-rotation measurements are described in papers in Nature and Nature Photonics respectively.

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