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Illuminating, beautiful and disturbing: a photographic record of the Trinity atomic test

One does not simply walk into the Trinity test site. Although the radioactivity from the world’s first atomic explosion has mostly dissipated over the past 81 years, the gates to this real-life Mordor are still guarded by military police, and the US Army continues to use it as a missile range.

Members of the public are allowed in just once a year, on the third Saturday in October, between 8 a.m. and 3.30 p.m. Apart from these tightly controlled “open house” events, the site where Manhattan Project participants gathered on 16 July 1945 to witness the dawn of the nuclear age is strictly off-limits.

For fans of atomic history who lack the time, money or inclination to visit this inhospitable corner of New Mexico during its annual open house, Trinity: an Illustrated History of the World’s First Atomic Test is the next best thing. In some respects, this coffee-table-sized book by Emily Seyl and her colleagues at the Los Alamos National Laboratory’s National Security Research Center (NSRC) may even be better.

As the book makes clear, there is not much to see at Trinity nowadays. A few dilapidated buildings; a commemorative obelisk; a fragment of a half-million-pound steel tank – these are all that remain of the once-bustling site where the first atomic device, known as the Gadget, was assembled and tested.

Trinity’s great achievement is to turn back the clock and make both the site and the individuals who worked there come alive. Using more than 200 photos from the test’s official archive and other sources, Seyl and her colleagues have created an important public record of a period that is fast receding from living memory and a place that is still veiled in secrecy. The result is a book that is by turns illuminating, beautiful and disturbing.

A fresh look at familiar history

Let’s start with the illuminating part. As an undergraduate intern at Los Alamos, I read Richard Rhodes’ seminal book The Making of the Atomic Bomb. Since then, I’ve read (and occasionally reviewed for this magazine) numerous other works of atomic history, from Ray Monk’s 800-page biography of the Manhattan Project’s scientific leader, J Robert Oppenheimer, to Denise Kiernan’s The Girls of Atomic City and Kate Brown’s Plutopia, both of which focus on less well-known figures.

Even so, once I got past Trinity’s introduction – which takes just six pages to cover Lise Meitner and Otto Frisch’s 1938 realizations about nuclear fission; the start of the Second World War; Enrico Fermi’s demonstration of the first nuclear chain reaction; and the launch of the Manhattan Project to weaponize these discoveries and thereby secure an Allied victory – I was into material new to me.

That steel tank, for example. Its name is Jumbo, it cost $12m in 1940s money, and it was originally designed to withstand the conventional explosives that imploded the Gadget’s plutonium core. If the explosives worked, but the chain reaction didn’t (a definite possibility – hence the need to test the Gadget’s design before deploying it in a bomb), the idea was that Jumbo would prevent plutonium from spraying all over the site.

So far, so sensible. In early 1944, Manhattan Project engineers duly drew up specifications for a containment vessel. An Ohio-based manufacturer, Babcock & Wilcox, agreed to build it, though for security reasons no-one told them what this mysterious “accumulator” was for. The finished Jumbo was then transported, under wraps, to New Mexico by heavy-duty freight train, offloaded onto a specially built carriage and wheeled to a location 800 yards from the intended ground zero.

Which is where it stayed. By the time it arrived in spring 1945, scientists working at the Manhattan Project’s nerve centre in Los Alamos, nine hours’ drive north of Trinity, had grown confident enough in the Gadget that Jumbo was deemed surplus to requirements, more likely to confuse diagnostic tests than to provide any useful safety function.

The fact that technicians had spent days hauling Jumbo across the desert is an excellent example of how the Manhattan Project combined herculean effort and ingenious engineering with tremendous cost and institutionally stupid bureaucracy

The fact that their technician colleagues had spent days hauling it across the desert is thus an excellent example of how the Manhattan Project combined herculean effort and ingenious engineering with tremendous cost and institutionally stupid bureaucracy. Despite this, I do not recall reading about Jumbo before. It’s to Seyl’s credit that she and her NSRC managers granted this potentially embarrassing episode so many pages of gorgeous black-and-white photos.

Because make no mistake, Trinity is gorgeous. I love the New Mexico desert, but you don’t need to be a fan of this austere landscape to appreciate these photographs of the people who toiled there and the machines they built. Though Oppenheimer and other well-known physicists appear in several photos, much of the book focuses on the contributions of relative unknowns such as Jack Aeby, a technician in the Army’s Special Engineer Detachment.

This organization existed to divert soldiers with technical training away from battlefields and into atomic grunt work, and Aeby, an amateur photographer, asked his group leader (the physicist Emilio Segrè) if he could document the explosion with his personal camera. Due to technical problems with the official Kodak cameras, Aeby’s snapshot is now the only decent colour image we have of the Trinity test.

Man wearing a hat sat on the ground next to a large round metal object
What was he thinking? Donald Hornig with the Gadget just before the first atomic test. (Courtesy: Los Alamos National Laboratory)

My favourite photo in the book is of a chemist called Donald Hornig. As the inventor of the X unit – a heavy-duty switch that triggered the Gadget’s detonators – Hornig had the unenviable job of arming this switch a few hours before the test, with a summer lightning storm flickering all around him. He thus became the last person to see the Gadget intact, and his expression as he slumps next to it is hard to read. Is he smiling? Proud? Or merely exhausted and wishing the photographer would leave him alone?

The final pages of Trinity depict events that followed the test, including the atomic bombings of Hiroshima and Nagasaki. Only five photos are known to exist of the immediate aftermath in Hiroshima, and the book reproduces three of them. Taken by Yoshito Matsushige, a staff photographer at the regional newspaper Chugoku Shimbun, they are far more disturbing for what they don’t show than for what they do. Although Matsushige later explained that he felt a professional duty to document the bombing, he found himself unable to press the shutter on most of what he witnessed. It was simply too horrible.

Beauty and horror make an unsettling combination. Despite Trinity’s quality, it is hard to see it appealing to a broad audience; in one of the book’s two forewords, Jim Eckles, who used to work in the missile range’s public affairs office, suggests it will “make a great memento for visitors to the site”. This unambitious statement is probably correct, but I hope that some physicists – weaponeers, disarmament campaigners and everyone in between – will also find space for it on their shelves. Our atomic age may yet end with a whimper, but it began with a bang, and it is worth remembering that bang in all its complicated majesty.

  • 2026 University of Chicago Press $39.00/£32.00hb 344pp

Proton–oxygen collisions put cosmic-ray models to the test

Physicists at CERN have recreated the collisions that trigger cosmic-ray air showers in Earth’s atmosphere. Results from the ATLAS experiment on the Large Hadron Collider (LHC) show that none of the computer models used to simulate showers gets every detail right. Their measurements of particle production in proton–oxygen collisions are more than ten times more precise than the differences between the models – which could be improved by these latest results. This will be a step toward understanding the most energetic particles in the universe.

Cosmic rays are energetic particles (mostly protons) from outer space. They strike Earth’s atmosphere at nearly the speed of light, setting off cascades of secondary particles.

“Cosmic-ray air showers are sprays of particles raining down from high in the sky,” explains Jesse Liu of New York University, the lead author of a paper that describes the study. He adds that cosmic rays come from “extreme corners of our universe,” with exploding stars and supermassive black holes in distant galaxies as likely sources.

Still, the origin of the most energetic cosmic rays remains “one of the biggest mysteries in astrophysics,” says Karl-Heinz Kampert of the University of Wuppertal in Germany, who was not involved in the study. Such particles carry a hundred billion-billion electron volts or more – compared to the million-million electron volts achieved by the LHC. At the highest energies, only about one such particle hits each square kilometer of Earth’s surface per century. “The only way to measure them in sufficient numbers is to detect the extensive air showers they produce,” he says.

Difficult calculations

Interpreting these showers relies on computer simulations. But, the role of the strong nuclear force, which governs the underlying collisions, is notoriously difficult to calculate. The simulations therefore use models informed by accelerator data, yet “existing models widely disagree with one another on how these showers form,” says Liu.

The most energetic cosmic rays collide at energies far beyond the reach of the LHC, so “we must extrapolate the known properties of particle interactions at man-made accelerators to those occurring in the upper atmosphere,” Kampert explains. Moreover, the LHC’s beams had previously consisted only of protons or heavy nuclei such as lead, rather than the light nitrogen and oxygen nuclei found in air. “For more than a decade, the cosmic-ray community has been working to convince the CERN and particle physics communities of the importance of such measurements,” he says.

That changed in July 2025, when the LHC was reconfigured “to make protons acting as cosmic rays collide with oxygen nuclei playing the role of Earth’s atmosphere,” says Liu. The ATLAS detector recorded the charged particles these collisions produced.

Key parameter

ATLAS physicists studied “how many particles are created during collisions of protons and oxygen nuclei, alongside what energies and angles these particles fly out at,” Liu explains. They also determined the cross section, which “tells us how often these collisions occur,” he adds. Kampert calls it “a key parameter,” because it determines the average depth at which cosmic rays first interact in the atmosphere. This depth affects “essentially all air-shower measurements that are needed to infer the mass” of the incoming particle.

The measured cross section lies at the low end of model predictions, agreeing with only two of the seven models tested. The team also used it to infer the cross section for protons colliding with air, a quantity previously measured at such high energies only by cosmic-ray observatories. The result agrees with earlier air-shower measurements at similar energies.

“Our measurements are more than ten times more precise than the differences between the existing computer models,” says Liu. “Our data show that no model correctly describes the number of particles created in these collisions.” Some models misjudge the frequency of rare, particle-rich collisions by a factor of ten. A model called Angantyr best reproduces the particles’ energies and directions, but no model describes all the results consistently.

Blind spots and next steps

Kampert points out that the measurement has a blind spot because multipurpose detectors like CERN’s ATLAS and CMS are optimized to track particles flying out at relatively large angles to the beam. He explains, “most of the collision energy remains unseen by ATLAS and CMS and escapes close to the beam pipe, at very small angles. This unseen region most strongly influences the features of the air showers.” Specialized experiments, such as LHCf at CERN, are designed to fill this gap.

The next step, says Liu, is “to apply our data to improve the computer models of cosmic-ray showers.” Physicists will then check whether this eases the “muon puzzle” — facilities such as the Pierre Auger Observatory in Argentina see more muons, heavier cousins of electrons, in air showers than simulations predict. “Our proton–oxygen data are an important step toward resolving these puzzles and help decipher cosmic mysteries, namely what high-energy cosmic rays are made of and where they come from,” Liu adds.

“Model builders have been eagerly awaiting this data and have started improving their models based on it and earlier accelerator data,” says Kampert. Because Auger results suggest that the most energetic cosmic rays are mostly heavier nuclei such as carbon, nitrogen, oxygen and silicon, he calls data from oxygen–oxygen or nitrogen–nitrogen collisions “another important step forward.”

Earlier this year, he and colleagues outlined in Nature Reviews Physics how such data could feed into a common tuning of the models, an undertaking he calls “a great community effort involving researchers from different fields of science.”

“I hope [our work] strengthens the cross-disciplinary connections between particle physics and high-energy astrophysics, with much to learn from each other,” says Liu.

The research is described in Physical Review Letters.

In situ and operando optical microscopy and spectroscopy: investigating perovskite optoelectronic devices at work

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The performance of optoelectronic materials and devices relies on carrier transport, recombination, ion migration and phase segregation. They unfold on length scales from nanometers to micrometers and timescales from picoseconds to hours. Conventional characterization averages over this heterogeneity and rarely captures these events in operational devices.

Optical microscopy and spectroscopy can recover this hidden picture. They convert local material function directly into optical contrast, and that contrast can be resolved across the length and time scales where the function happens.

In this webinar, Dr Sudipta Seth will talk about advanced optical microscopy and spectroscopy that interrogate halide perovskite materials and corresponding optoelectronic devices under relevant in situ and operando conditions. This approach uncovers the local fields, transport bottlenecks, and defect chemistry that set solar cell performance. He will show how these local optical signatures translate into a mechanistic understanding of device operation, and how that understanding feeds back into the design of better probes, interfaces, defects and devices.

The talk is aimed equally at materials scientists curious about what happens inside their samples and at microscopists and spectroscopists looking for new problems suited to their tools.

Certificate of attendance will be awarded upon completion of the webinar.

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Sudipta Seth is a research associate at KU Leuven, Belgium, where he conducts advanced research at the intersection of materials chemistry, optoelectronic devices and spectroscopy through the development of innovative microscopy methodologies. He leads the nanoscale device spectroscopy research line at Hofkens Laboratory. He completed his PhD at the University of Hyderabad and subsequently worked as a postdoctoral fellow at Lund University and as a visiting junior fellow at the Tokyo Institute of Technology. His work integrates single-particle spectroscopy, super-resolution and nanoscale microscopy and ultrafast spectroscopy to investigate fundamental photophysics in semiconductor materials and optoelectronic devices. He has received several academic fellowships, including INSPIRE-SHE (India), Wenner-Gren Postdoctoral Fellowship (Sweden), FWO Research Stay Abroad (Belgium), and Marie Sklodowska-Curie Postdoctoral Fellowship (European Commission).

Quiz of the week: how long could a lunar city survive?

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Microbes may survive at the Moon’s south pole

The Moon’s polar regions may be less hostile to microbes than previously thought. This is the new finding from researchers at NASA and the University of Maryland in the US who say that future crewed missions to the lunar south pole will have to be carefully thought out to limit the unintended transfer of Earth life forms.

High ultraviolet radiation and temperatures make the Moon’s surface a harsh place and not somewhere bacteria and other micro-organisms can easily survive. This is particularly true for the equatorial regions, where all previous crewed missions have been sent. Things might be quite different at the lunar poles, however, since sunlight strikes these high-latitude regions at a low angle because of the Moon’s minimal tilt to the Sun of 1.5°. The result is long permanent shadows where it is cooler. The moonscape is also as rugged here as elsewhere with mountains and hills, which also cast shadows, and deep craters protected from the Sun’s heat and ultraviolet flux. These conditions allow for water ice too.

Because of these unique properties, future crewed missions, such as NASA’s Artemis IV, are scheduled to go to the Moon’s south pole, which has also been identified as a possible future location for a human outpost. The polar regions have already been observed by spacecraft such as ESA’s SMART-1, India’s Chandrayaan, Japan’s Kaguya and NASA’s Lunar Reconnaissance Orbiter. These provided a wealth of information that will be used to prepare future missions.

The problem is that no matter how stringent the sterilization procedures are prior to take-off, the astronauts on these missions will take microbes from Earth with them. Indeed, humans carry millions of microbes on their skin and even more in their bodies.

Landing sites for Artemis IV

To investigate whether microbes could survive in the polar regions, the researchers, led by Prabal Saxena, who works at NASA’s Goddard Space Flight Center, and Stefano Bertone at the University of Maryland, modelled how bacteria and fungi commonly found on human skin would behave in the environment of three candidate landing sites for the upcoming Artemis IV mission. The researchers began by making highly resolved spatial maps of the topography in these regions that included ruggedness, slopes and shadowing effects. They also modelled the UV exposure and temperature in these regions to identify specific locations where micro-organisms could survive for at least 24 h.

They obtained seasonal temperature data on length scales of 240 m from measurements by Lunar Reconnaissance Orbiter’s (LRO) Diviner. As for estimated regional UV (≤320 nm) fluxes, these were gleaned from averaged illumination maps with a pixel scale of 60 m based on topography derived from LRO Lunar Orbiter Laser Altimeter (LOLA) measurements.

“Since the temperatures at the lunar poles are rarely high enough to kill bacteria and low temperatures can preserve microbes on Earth, UV radiation is generally likely to be more dangerous in these regions, says Saxena. “We therefore analysed UV fluxes at finer spatial scales using a technique called ray tracing, and for some regions, included optical effects such as reflection and refraction as sunlight hits the Moon’s surface, leveraging updated LOLA-based topography maps on a scale of 5 m/pixel.”

Understanding how sunlight behaves at the poles is crucial, he adds. “Since the Moon has a very small axial tilt, the Sun appears to hover just above the horizon here, so even small hills and rocks can prevent light from reaching lower-lying ground, creating small, shadowed areas protected from UV rays.”

Up to seven days survival

The simulations revealed that the bacteria and fungi studied could survive in certain niches for up to or potentially beyond seven days. One particular fungus, the Aspergillus, is particularly resistant to UV radiation and could potentially survive in 15 to 30% of the areas assessed, even those that receive some sunlight during the lunar winter. The reconstructions also showed that all five microbes in this study, Bacillus, Deinococcus, Staphylococcus, Aspergillus and Fusarium could possibly survive in certain areas of the De Gerlache Rim’s permanently shaded regions, including when scattered UV light is incorporated into the simulations.

Survival does not mean growth (which requires active metabolism and potential reproduction), however, stresses Saxena. Indeed, surviving microbes are in a dormant, so-called cryptobiotic state and would only grow if the conditions were amenable to life – which is not the case for the Moon as we currently understand it.

“Cells may also be dead,” he tells Physics World, “but even dead cells may persist in the environment as another potential source of contamination.”

The work could apply beyond the Moon, he says. Indeed, a number of different airless bodies such as Mercury, Ceres, some asteroids and comets and exoplanets that may have similar properties could also have potential survivable niches for microbial life. Understanding how these niches may exist, how life may be transferred to them and how human exploration may leave its mark will all serve as a key testbed for future potential human exploration of Mars, which likely has far more habitable environments.

“We need to understand what was there before us, because when we search for signs of life beyond our planet on these bodies, we will want to make sure it’s not stuff we brought,” says Saxena.

The research is described in Science Advances.

Tiny biodegradable stars enhance drug delivery to the skin

Delivering drugs directly to the skin provides a targeted means of treating dermatological conditions while minimizing side effects. The approach is limited, however, by the stratum corneum – the skin’s outer layer – which acts as a barrier that can restrict the effectiveness of most topically applied drugs. To overcome this, a team at Georgia Tech has developed biodegradable “STAR particles” that use microscopic needles to painlessly puncture the skin and increase its absorption of medication.

“Very few drugs can be absorbed effectively into the skin, which means that many drugs in dermatology are given by mouth or injection,” explains team leader Mark Prausnitz. “This exposes the whole body to the drug, often causing side effects and reducing drug efficacy. STAR particles painlessly make micropores in the skin that allow drug that is rubbed on the skin to be absorbed. This targets drug delivery exactly to the site where it is needed.”

Previously, the researchers fabricated the STAR particles from a ceramic material (titania), which is safe to use on the skin, but not biodegradable, raising possible environmental concerns. In their latest work, reported in Advanced Healthcare Materials, they designed polymer STAR particles that dissolve or biodegrade after use, reducing their potential environmental impact.

STARs effectively create micropores

Prausnitz and colleagues created STAR particles from three polymers: water-soluble poly(vinyl alcohol) (PVA), enzyme-degradable cellulose acetate (CA) and hydrolysable polylactic acid (PLA). All three materials have previously been used to create microneedle patches, which enhance drug delivery but can generally only be used on small areas of skin.

“For dermatological conditions like eczema and psoriasis, patients need to treat skin with variable and sometimes large areas,” says Prausnitz. “STAR particles provide the power of a microneedle patch to increase skin permeability with the flexibility to apply them over large and variable areas by simply rubbing a gel or cream containing STAR particles on the skin.”

The researchers fabricated the polymer STAR particles using femtosecond laser micromachining to create star-shaped structures with sharp, well-defined microneedle tips and a tapered profile. They tested the ability of the various particles to puncture pig skin samples.

While PVA STAR particles cannot be applied using water-based vehicles, when suspended in non-aqueous formulations such as isopropyl palmitate (a non-toxic ingredient widely used in dermatology), they successfully punctured the skin. The STAR particles remained intact after use and the formulations worked equally well after storage for one week.

Similarly, CA and PLA STAR particles in water formulations demonstrated consistent skin-puncturing ability, even after a week’s storage, with no visible damage to the particles after application. The researchers note that titania STAR particles in water (examined as a control) generated more pores than the polymer particles, due to their higher hardness.

Drug delivery demonstration

The team next investigated how the STAR particles could enhance the delivery of three drugs – tacrolimus, methotrexate and copper tripeptide-1 – into pig skin samples.

Tacrolimus is used to treat inflammatory skin conditions and is soluble in non-aqueous solvents. After topical drug delivery, rubbing the skin with PVA STAR particles in isopropyl palmitate for 10 s or 30 s increased drug levels within the skin 1.7- or 3.2-fold, respectively, compared with control samples.

For CA STAR particles, the researchers examined methotrexate, which is commonly used for psoriasis treatment but cannot be delivered topically due to its very low skin permeability. Treatment with CA STAR particles for 10 or 30 s increased intradermal delivery of methotrexate 5.4- or 25.2-fold, respectively.

Finally, they used PLA STAR particles to deliver copper tripeptide-1, a skincare ingredient employed for anti-aging, wound healing and skin regeneration applications. The particles enhanced intradermal drug delivery 12.1- or 37-fold, after 10 or 30 s application, respectively.

Safety considerations

As the STAR particles are designed to create micropores in skin, it’s possible that they could unintentionally end up in other parts of the body, where the microneedles could cause damage. While this risk is likely small (as the STAR particles require forceful application to be effective), the water-soluble PVA STAR particles eliminate any potential risk as they rapidly dissolve upon contact with wet tissues.

The CA and PLA STAR particles will likely retain their structure immediately after use, but will eventually become blunt and weak upon enzymatic degradation or hydrolysis. Likewise, any potential environmental consequences of non-degradable STAR particles are reduced or eliminated with the water-soluble and biodegradable materials.

The team concludes that the biodegradable STAR particles address environmental and safety concerns while enhancing drug delivery to the skin. The approach paves the way for improved patient outcomes and broader applications, enabling delivery of hydrophilic drugs and larger molecules that are usually blocked by the stratum corneum barrier. Prausnitz tells Physics World that the research is primarily targeted at treatment of dermatological diseases that can spread over large areas of skin – such as psoriasis, eczema, vitiligo and allergic rashes – as well as cosmetic applications.

“We have licensed the STAR particle technology to a company that is preparing for a clinical trial next year using STAR particles to deliver siRNA as a novel eczema treatment,” he says. “At Georgia Tech, we are focused on advanced materials, manufacturing and formulations for the next generation of STAR particles.”

Spectacular image of the Shark Nebula wins Royal Observatory Greenwich prize

Kuwaiti photographer Ali Alobaidly has beaten thousands of amateur and professional photographers from around the world to bag the 2026 Royal Observatory Greenwich’s ZWO Astronomy Photographer of the Year.

The image – The Quiet Predator – captures the Shark Nebula, which lies almost 650 light years away in the constellation Cepheus. The nebula is made of interstellar dust dense enough to block starlight with the blue-coloured areas in the image being “reflection nebulae” – clouds of interstellar dust that do not emit their own visible light but rather scatters and reflects light from nearby stars.

The image was taken in August 2025 with an Askar 140 APO telescope from the Jahra Governorate, Kuwait.

Sam Wen, founder and chief executive office of the astrophotography firm ZWO, says the image “is a stunning blend of technical excellence and artistic vision”.

“Showcasing the intricate structures of a rarely photographed nebula, the image combines remarkable detail with a striking composition and colour palette, inviting viewers to explore the hidden beauty of the cosmos,” adds Wen.

Alobaidly says it is a “tremendous honour” to receive the award.

“The juxtaposition of an oceanic predator prowling the skies above the Kuwaiti desert is something I find deeply poetic, and it is what first drew me to this remarkable region of the sky,” adds Alobaidly. “To me, it represents perfectly my love for both the Universe and the deserts of my ancestors.”

Alobaidly now hopes to continue using astrophotography for outreach and scientific research.

As well as winning the £10,000 top prize, the image will go on display along with other selected pictures from the competition at an exhibition at the National Maritime Museum observatory on 18 September.

The award – now in its 18th year – is run by the Royal Observatory Greenwich in association with ZWO and BBC Sky at Night Magazine.

Creating better batteries for electricity grids

This episode of the Physics World Weekly podcast features Lin Ma, who is at the University of North Carolina at Chapel Hill. His research group tackles energy and environmental challenges using sustainable materials and electrochemical techniques.

In this episode I chat with Ma about the growing importance of energy storage in electricity grids. Much of this is driven by the growth of wind and solar generation, which can both be intermittent in nature.

Ma talks about the batteries that are currently used in grids and explains what improvements are needed as more renewable energy comes on line. We chat about the different battery technologies that can be used for grid storage and Lin explains why sodium-ion batteries offer advantages over lithium-based devices when it comes to large-scale deployment. Lin also talks about some of the safety issues surrounding large battery facilities and how artificial intelligence can be used to improve many aspects of battery technology.

This podcast is presented in partnership with the Electrochemical Society. The mission of the ECS is to advance theory and practice at the forefront of electrochemical and solid state science and technology, and allied subjects.

Uncertainty and the random universe: a slide puzzle

Image courtesy: Shutterstock/svekloid

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Metasurface optical device boosts nonlinear frequency conversion

By combining a band-structure engineered multi-quantum-well heterostructure with a nanostructure known as a metasurface, physicists in Austria and the US have created a device that can efficiently transform light into different frequencies – including those at technologically important near-infrared wavelengths. Their technique, which boosts nonlinear frequency conversion, could help in the development of smaller and more efficient components for telecommunications, quantum information transfer and other photonics technologies.

Nonlinear frequency conversion is an optical process in which a material is used to generate light of a different frequency to that incident on the material. It is now routinely employed in a wide range of photonics devices that rely on frequency mixing, ultrafast signal processing, broadband and pulsed light sources, and high-speed light modulation. It is also used to generate entangled photon pairs for quantum communication. One common type of frequency conversion, and the one used in this new study, is second-harmonic generation. Here, two input photons are combined to produce one photon with twice the energy.

Efficient nonlinear devices require light with high electric field strengths and materials that respond to these intense electromagnetic fields by producing a light polarization that is not directly proportional to the applied field. “Only a handful of such nonlinear crystals exist and these have complicated structures with weak nonlinearities, which means they require high optical powers to make photons interact with each other,” says Marcus Ossiander from the Institute of Experimental Physics at TU Graz in Austria, one of the authors of the new study published in Nature Nanotechnology. “While, new such crystals emerge from time to time, their structure is fixed, which dictates which light wavelengths they work at and how efficiently.”

Asymmetrically coupled multi-quantum wells

Recently, a group of researchers at the University of Texas at Austin in the US, led by Seth Bank, also an author of this latest study, succeeded in creating an optical nonlinearity not via a new crystal structure but by growing a semiconductor metamaterial made from nanoscale layers of gallium arsenide and aluminium gallium arsenide. These layers contain multiple quantum wells that are asymmetrically coupled to each other.

The quantum wells confine electrons in one dimension while allowing them to move freely in the other two dimensions, something that has the effect of restricting their energy levels into discrete, quantum states. Since they are asymmetrically coupled, the electrons end up mainly moving in one direction when exposed to light. This so-called “one-way street leads to increased nonlinear electron oscillations, allowing light waves to interact with each other extremely efficiently and exceed naturally occurring nonlinearities.

Ossiander and his colleagues, including Federico Capasso at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), have now added a second, two-dimensional metamaterial atop the first one. This metasurface, which consists of titanium dioxide pillars, each several hundred nanometres in size, precisely creates the particular light polarization required to deflect incoming light so that it scatters along the one-way street. This enhances the interactions between light waves even further and so increases the intensity of light in the structure.

An extra lap

Researchers have known for a while now that metasurfaces are extremely good at resonantly trapping light and increasing the interaction of light with matter. Usually, the process is straightforward, explains first author Pernille Undrum Fathi, who works in Capasso’s group: you implement a resonant metasurface, and it enhances, for example, the nonlinear effects that you are aiming for.

“The new material sent us for an extra lap, however, because, during our experiments, we discovered that the designable second-order nonlinear susceptibility tensor element (the required incident polarization) is different to that of conventional materials,” she says. “Therefore, when we shone light at exactly normal incidence, the metasurface–material combination generated light waves whose polarizations cancelled each other out completely, so annihilating any enhanced optical nonlinearity.”

However, the researchers found that tilting the sample by just 0.3° broke this symmetry and solved this challenge. Indeed, they were able to increase the effective nonlinear conversion of the light to three orders of magnitude higher than that for a non-patterned heterostructure. The effect is also higher than previously reported values for comparable devices at near‑infrared wavelengths.

“Nonlinear optics are ubiquitous in modern science and technology, with examples including the generation of light at new frequencies (where lasers might be unavailable), all-optical signal processing and the generation of entangled photon pairs for quantum communication,” Ossiander tells Physics World. “Enhancing the fundamental processes enabling this technology will allow for much more efficient, compact devices and enable new measurements that would previously be impossible.”

Metasurfaces offer an excellent route to control and enhance the interaction of light with matter, and the team was intrigued to find out what could be made possible when combining these materials with nanostructures, adds Fathi.

“Working with new materials of course comes with additional uncertainty, and we spent a lot of time learning about and understanding the mechanisms of these new structures and how the particular nonlinear tensor elements of this material interact with the resonant modes introduced by the metasurface,” she says.

“The material we studied is very interesting but also complex,” adds Ossiander, noting that the reseachers believe they can learn how to make even better use of the designer nonlinearity in it by further investigating its fundamental properties. “Indeed, there is a new publication in Optica by Bank’s group on how to improve the multi-quantum-well material even more.”

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