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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Researchers at the University of the Witwatersrand (Wits) in Johannesburg, South Africa, have succeeded in harnessing a relatively unexplored property of light – its topology – to robustly transfer information through the atmosphere. The work could advance the development of more reliable long-distance optical communications, including space-based ones, and connect remote and underserved communities worldwide.

Their work is detailed in two separate studies, one based on classical and the other on quantum optics. These studies show that the topological information can remain remarkably robust – even when the states of light carrying this information are subject to the severe distortions caused by atmospheric turbulence. Instead of relying entirely on increasingly sophisticated hardware or computational algorithms to correct atmospheric distortions before or after light transmission, the new concept provides a fundamentally different way of thinking about robust optical communication in complex environments, such as air or water, they say.

Both studies are based on the same fundamental idea: creating an optical topological structure known as a skyrmion in a light field and testing whether it remains robust when it passes through a distorting environment, explains Andrew Forbes, who is head of the Structured Light Lab at the Wits School of Physics. Skyrmions are quasiparticles with a two-dimensional, knot-like structure and were first observed in magnetic materials. They have, however, recently been seen in electromagnetic fields and the electric field of light waves. Here, the swirling structures are created by twisting the polarization or spin vectors of light in space so that every polarization is found exactly once, twice, three times and so on – the count being the skyrmion number. Skyrmions are topologically stable, which means that external perturbations do not affect them in any way.

Real-world test

In the first study, Forbes and colleagues used laser light with a wavelength of 532 nm shaped into various vectorial beams using a spatial light modulator and a modified Mach–Zehnder interferometer to generate optical skyrmions with skyrmion numbers of 1, 2 and 3. These numbers, which are always whole integers, describe how twisted the vortex-like structure of the skyrmion is. They then sent the laser light through a 270-m-long real-world free-space optical link located on Wits’ Braamfontein campus in the centre of Johannesburg.

The researchers measured the topology of the skyrmions with a Stokes polarimetry setup and found that the skyrmion number remains robust in a wide variety of atmospheric conditions, ranging from “calm, cool morning air to the highly erratic and intense distortions at midday” when it is hotter. “This result holds true even when the vectorial polarization of the underlying laser beam has been highly scrambled by atmospheric turbulence effects – and was therefore unrecognizable,” says Forbes.

In-built correlations

In the second work, the team created the topology through the in-built correlations between two photons entangled in their optical angular momentum (OAM). The OAM quantum states are not themselves stable in distorting environments and the information they encode is easily lost. The researchers investigated the robustness of a skyrmion with a skyrmion number of 1 in a controlled laboratory environment by studying photons in media with varying levels of turbulence.

The result? Despite local distortions, the skyrmions maintained topological numbers close to 1. This, they say, suggests the topological number is fundamentally decoupled from modal crosstalk induced by the turbulence and the spreading of an individual photon’s OAM.

Despite the very different physical systems, both experiments reveal a similar result, says Forbes: the conventional properties of optical fields can be strongly degraded by distortions, while the topological information encoded in those states remains remarkably robust. Together, the two works demonstrate that topology can be harnessed in both classical and quantum optical systems as a robust carrier of information in very different communication and information-processing scenarios.

Robust protection

Topology is a concept that appears throughout physics, from fundamental particle physics to magnetism and condensed matter systems. In many of these systems, explains Forbes, topology comes with a physical protection mechanism – such as an energy barrier – that explains why a topological structure is resistant to external disturbances. In optics, however, the situation is rather different. Optical topologies can be created and manipulated with relative ease, but their robustness is not necessarily guaranteed by any particular underlying physical protection mechanism. “We therefore wanted to address a fundamental question: how robust is optical topology when light encounters a real-world, highly distorting environment?

“The atmosphere provides a particularly interesting testbed because it is highly relevant to optical sensing and communications and represents an extreme case where the distortions vary rapidly both in space and time,” he tells Physics World. “Demonstrating robustness in this setting was therefore both a fundamental test of the dynamics and resilience of optical topologies and an indication of its potential practical value.”

Hurdles and challenges

One of the major challenges was simply doing precision optical experiments outdoors, he adds. “We had to contend with the unpredictability of nature: changes in sunlight and temperature, wind, rain and even the thermal expansion of buildings, which can affect optical alignment, making it difficult to maintain stable measurements over long periods.”

Another hurdle to overcome was to find a way of measuring the optical topology itself. Existing methods to do this can be quite sensitive to experimental noise and imperfections, so the researchers say they had to carefully distinguish genuine changes in the optical channel from apparent changes introduced by the measurement process. Ultimately, this meant developing new ways to both obtain and process the experimental data so that they could reliably track the topological properties of the light despite the noise that was present.

Applications include free-space optical, satellite and space communications, and linking remote or underserved communities, where atmospheric turbulence and the lack of physical infrastructure can present significant challenges, says Forbes. “More broadly, topology could be useful beyond communication,” he adds. “Indeed, its underlying robustness could potentially be exploited for optical sensing, information processing and other applications where light has to propagate through complex or noisy environments.”

The quantum experiment adds another possibility, he notes. “We know that entanglement decays in real-world conditions that are not pristine.  Our work shows that despite this, the quantum information stored in the topology remains intact.  This may allow us to distribute information across a quantum network in a more reliable and robust manner.”

Looking ahead, the Wits researchers say they will now assess whether the robustness they have observed extends to other kinds of challenging environments, such as underwater and biological tissue, where scattering, birefringence and other phenomena can be much more prevalent than in the atmosphere.

“We also want to develop detectors and techniques for measuring optical topology,” says Forbes. “Improving how we characterize these structures will be important for moving from proof-of-principle experiments towards practical applications.

“Finally, we would like to actually exploit the robustness of the topology we have identified in high-speed communication experiments. This means implementing realistic communication protocols and investigating how much information can be reliably transmitted using a ‘topological alphabet’.”

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Researchers in France and Germany have created the first all-optical photonic time crystal. The nanostructure could be used to make ultrafast optical computers and amplifiers, frequency converters and perhaps even new types of lasers that work in the terahertz range.

Photonic crystals are nanostructured materials with a refractive index that varies on a length scale comparable to the wavelength of light, producing a phenomenon known as a photonic bandgap. This gap affects how photons propagate through the material in a way that resembles how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In the case of photonic crystals, light in certain wavelength ranges can pass through the material, but other wavelengths cannot.

Photonic time crystals (PhTCs) are similar to this, except their properties vary periodically in time, not space. Among other adjustments, this means that instead of photonic bandgaps, they host momentum bandgaps. These gaps are special in that light waves whose momenta fall within them grow exponentially in time.

Optical modulation at THz frequencies

PhTCs show much promise for applications like new types of tuneable lasers and ultrafast frequency converters, but controlling their photonic properties has proven very difficult. This is because these properties need to be modulated very strongly and on ultrashort timescales – on the order of the temporal period of the light itself.

Producing such extremely fast modulations is a key goal for scientists working in a branch of highly nonlinear optics known as time-domain photonics. Previously, their efforts have paid off in the microwave frequency range using PhTCs containing electrical circuits, but all-optical systems have proven elusive.

A team of physicists led by Yannis Laplace of the Ecole Polytechnique in France has now made a PhTC with properties that can be optically modulated at terahertz (THz, 10¹² Hz) frequencies. The new PhTC is made from a type of photonic crystal known as a plasmonic metamaterial: an artificially engineered nanostructure consisting of micron-sized cavities made of gold atop an insulating layer and a semiconductor material based on indium and antimony. The cavities trap photons between the gold and semiconductor layers, and surface plasmons – collective, coherent oscillations of conduction electrons that interact very strongly with light – then keep them on the surface of the semiconductor.

Working with scientists from the Collège de France and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany, the researchers applied pulses of THz light from the TELBE light source at the HZDR’s ELBE accelerator to their structure. They found that they could use this intense multi-cycle THz light field to modulate the material’s optical properties over timescales of just picoseconds.

Towards new types of lasers in the THz range

“This study builds on previous work in my group related to the development and study of tuneable plasmonic metamaterials in the THz range as a means to create functional devices to control light-matter interactions in this range,” Laplace says. “One of the central questions in our work was to determine if the temporal modulation of the metamaterial would be strong and fast enough.

“By developing a theoretical model in collaboration with our colleague Marco Schiró and his team at the Collège de France that reproduced the experimental observations remarkably well, we showed that the system presented the characteristic spectroscopic signatures expected for a PhTC and provided an explanation for the behaviour of the photons therein.”

The researchers, who detail their work in Nature, also found that the ultrafast temporal modulation reduced the dissipation of photons within the metamaterial by half and they are now looking to reduce these optical losses even further.

“The THz frequency range lies at the junction between electronic and photonic technologies and is historically less developed than these two,” Laplace explains. “With the THz PhTCs that are now achievable, we hope we can soon develop devices like THz amplifiers, frequency converters and maybe new types of THz lasers in this range, hence contributing to the closing of the so-called ‘THz gap,’” he tells Physics World.

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As the world of quantum computing grows and researchers work towards scaling up quantum systems, energy consumption is becoming a major concern. Beyond the small scale and high level of control in a laboratory environment, the lasers that power quantum systems result in a large energy cost that may act as a bottleneck for the integration of quantum technology.

Publishing their findings in Optica, researchers have demonstrated that sunlight can be used to generate pairs of entangled photons, providing a more energy efficient alternative to laser-pumped systems. This collaborative project combined theoretical research led by Robert Boyd at the University of Ottawa with technology developed by Hanieh Fattahi’s research group at the Max Plank Institute for the Science of Light in Erlangen, Germany.

Overcoming assumptions

Optical quantum computers use pairs of entangled photons as qubits, with the nonlinear optical processes used to produce them typically relying on lasers. This is largely due to two assumptions: firstly, that high optical coherence is required, meaning that the light waves maintain a steady phase relationship; and secondly, that lasers are the only way to achieve the necessary optical power density for these methods.

Sunlight, while naturally abundant, is largely incoherent and is difficult to collect in concentrations comparable to that of lasers. As such, this natural reservoir of optical input has been largely overlooked as a direct resource for nonlinear optical processes.

The researchers behind this latest study are challenging these assumptions with a new system that uses sunlight to fuel a process called spontaneous parametric down-conversion (SPDC). In this process, a pump beam – usually a laser but replaced here with solar light – interacts with a nonlinear crystal, splitting individual photons into entangled pairs.

Previous work from Boyd’s team demonstrated that incoherent light from an LED can produce photons with entangled polarization states. By achieving entanglement powered by sunlight, this new research further overturns both assumptions about the necessary properties of the optical input.

Harnessing the Sun

To harness the plentiful yet highly dispersed sunlight, Fattahi’s team had to develop a device that focuses sunlight down to a point approximately 2 mm wide, to be coupled to a fibre as thin as a human hair. To achieve this, they used a Fresnel lens and spectral filter to collect sunlight, and a glass cone-shaped concentrator to funnel the light to a point, where it is coupled to the fibre. The fibre then directs the sunlight into the nonlinear crystal to induce SPDC.

Fattahi described the “significant practical challenges” involved with implementing such a system. Along with having to continuously track the Sun, the outdoor nature of the experiment left it vulnerable to both environmental effects and background light. Eventually compensated for by enclosing the system in a tent, the background light was initially offset by students beginning measurements at around 3 a.m. to make the most of the natural darkness.

Overcoming both theoretical and practical concerns, the system was able to successfully produce entangled photons. The states produced violate Bell’s inequality (as the photon correlations cannot be modelled classically) with an S value of 2.54, where S = 2 is the threshold for indication of quantum behaviour, and a 94% fidelity to the target entangled Bell state.

Significantly, when normalized in terms of the spectral bandwidth of the pump, sunlight-powered SPDC demonstrates a photon pair generation rate comparable to that of the laser-pumped process. This method eliminates the need for electrical-to-optical energy conversion, providing a viable path towards increasing energy efficiency.

The future of photon production

The development of quantum technology is vital for the future of secure communication, ultraprecise sensing and high-performance computation. Taking steps to develop a sunlight-driven method of photon pair production will potentially allow scaling of this technology without the major energy cost.

Minimizing energy consumption and system complexity would be a huge benefit for systems operating in demanding conditions, such as polar deserts. This research is particularly striking when considering space-based quantum technology. Sun-synchronous orbits could allow near constant access to a naturally occurring, reliable pump-source.

Fattahi and her team intend to develop a field-deployable version of this system, with a focus on greater integration which should “significantly improve mechanical stability, efficiency and overall practicality”. There is also potential to expand the portion of the solar spectrum used in this experiment. Fattahi described the “exciting possibility” of using different spectral regions to drive multichannel entangled-photon generation, increasing efficiency and capacity.

Research into new, energy-efficient photon production has the potential to remove a major barrier for the integration of quantum technologies and contribute to a brighter future for quantum computing.

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Producing pairs of correlated or entangled photons usually requires a complex laser system. Now, however, researchers at China’s Xiamen University have shown that these crucial ingredients in quantum optics experiments can come from a far more basic source: sunlight. The discovery could simplify optical systems that rely on a process known as spontaneous parametric down-conversion (SPDC), allowing the technology to be deployed in space and other locations with limited or no access to electricity.

SPDC occurs when a short-wavelength photon passes through a nonlinear crystal and gets converted into twin photons with a longer wavelength. Traditionally, the initiating (pump) photon in this paired-photon source comes from a coherent laser. However, recent research suggested that fully coherent light sources might not be strictly necessary, and that partially coherent sources might also be able to drive SPDC.

A Sun-tracking system

In the new work, researchers led by Wuhong Zhang and Lixiang Chen took this idea even further. Could sunlight – which is inherently incoherent – also drive SPDC?

In principle, Zhang and Chen suspected the answer was “yes”. After all, a ray of sunlight, like a laser beam, is just a stream of photons. The problem is that unlike photons from a laser, the brightness and incidence angle of solar photons is constantly changing. This makes it difficult to collect enough pump photons to produce correlated photon pairs at high rates.

To overcome this problem, the researchers installed a Sun-tracking system on the roof of their laboratory building. This was essentially a telescope mount that moves with the Sun to collect light continuously throughout the day. They then worked out how to efficiently couple this collected sunlight into a multi-mode fibre and transmit it into their laboratory. There, they used the light to pump a nonlinear crystal made of periodically poled potassium titanyl phosphate (PPKTP). Within this crystal, the SPDC process converted pump photons into correlated photon pairs, demonstrating that it is indeed possible to use sunlight to generate photon pairs with strong position correlations.

The team faced numerous challenges along the way. The low spatial coherence and temporal instability of sunlight needed to be mitigated, and the coupling into the fibre needed to be very efficient. However, Chen notes that sunlight does have one advantage over traditional laser sources. “Sunlight is inherently broadband in its spectrum, so it can precisely provide any favourable wavelength,” he says. “This means it could be adapted to diverse application scenarios.”

Towards laser-free and electricity-independent SPDC light sources

According to Zhang, the team’s work shows that laser-free and electricity-independent SPDC light sources are possible. Potential applications could include correlation-enhanced sensing in remote areas and space-based quantum key distribution and teleportation, and he tells Physics World that one of the team’s next goals is to test the system in outdoor environments.

Chen adds that the system, which is described in Advanced Photonics, could also become a platform for fundamental studies of how the coherence of light affects the photon-splitting process in SPDC. He and his colleagues are now focusing on improving the efficiency of their sunlight collection system; optimizing the design of their nonlinear crystal so that it is better adapted to the Sun’s broadband spectrum; and implementing advanced image reconstruction techniques such as compressed sensing. “We think that introducing AI technologies such as artificial neural networks and deep learning into such imaging systems will be a key priority in going forward,” Chen says. “They will inform us on how to exploit sunlight more efficiently to implement various advanced quantum information protocols.”

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