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.”

The post Metasurface optical device boosts nonlinear frequency conversion appeared first on Physics World.

For a few years in the early 1990s, a lot of New York streets sparkled. Picture a Manhattan avenue at dusk, wet from a passing shower, the streetlights and headlights catching something in the road surface itself. Not the usual dull black, but a faint, restless glitter, as if the asphalt had been dusted with something. That shimmer was crushed bottles. The city had figured out how to grind its glass waste into the road.

The material had a name that told you exactly what it was: glassphalt. It went from a curiosity nobody trusted, to something people demanded outside their own buildings, and then it quietly disappeared. And it’s a small case study in why good recycling ideas so often don’t stick.

The glittering street, and why the city made it

Asphalt is mostly sand and gravel held together by a black binder. Glassphalt is what you get when crushed glass replaces part of that sand and gravel.

The Federal Highway Administration, in its guidelines on waste glass in pavement, says road-surface mixes with roughly 10 to 15 percent crushed glass perform fine. From 1989, the city’s transportation department started putting the glass content of its mix at about 10 percent.

The FHWA called the New York program the “most aggressive” glass-recycling paving effort in the country. From 1990 through 1995 the city used roughly 225,000 metric tons, about 250,000 tons, of glass in resurfacing. By 1991, crews had already laid glassphalt on nearly 900 lane-miles of Manhattan and Brooklyn streets.

What made the idea clever, as far as we can tell, is the kind of glass it used. Bottle recyclers want clean glass sorted by color. They won’t buy the mixed-color material that piles up at the bottom of a recycling stream. Glassphalt didn’t care about color. It could swallow exactly the low-value waste glass that nobody else wanted and turn it into road. The city reported saving about $500,000 a year by using crushed glass in place of some sand and gravel, and keeping 38,000 tons of glass out of landfills in the process.

From public suspicion to actual demand

People did not warm to it right away. A road that shimmers looks, at first glance, like something is wrong with it. But the turnaround, if the accounts from the time are right, was fairly complete.

Harold Watson, the transportation department’s assistant commissioner for asphalt operations, had become something of an in-house champion for the material. Watson said in 1991: “Now, when we do a road, if we don’t have glass on it, people come in and start complaining.”

That’s a nice story about public mood, not a measured fact, and it comes from a fan of the material, so we’d hold it loosely. Watson was clearly frustrated by a quieter resistance too. In his telling, “There’s a lot of people who can’t find anything wrong with glassphalt but won’t say anything good about it.” Read that as one supporter’s view rather than a neutral account, but it captures the odd spot the material was in: hard to fault, hard to fully endorse.

The Plaza, and a request for more sparkle

The clearest sign that the public had come around involved one very visible stretch of Fifth Avenue. After the city resurfaced the section in front of the Plaza Hotel with glassphalt, the hotel’s owner, Donald Trump, wanted the surrounding streets given the same treatment. A recycled-glass road surface had become something worth lobbying for outside a luxury hotel.

There’s something funny in that. The exact same crushed bottles a bottle plant would reject as worthless were, when spread across Fifth Avenue and lit up after rain, a selling point. The sparkle did real marketing work that the engineering never asked for.

Why the biggest glass-paving program in the country faded

None of the appeal saved it. The program’s central weakness was that its raw material came from curbside recycling collection, and collection is expensive.

During a budget crisis, Mayor David Dinkins floated suspending the recycling operation to save about $65 million a year, which would have cut off the glass supply. Commissioner Lucius Riccio said at the time: “That would be a shame. We’re doing a record amount of paving and we could use all the glass they could give us.” That tension is the whole story in miniature. The paving side wanted all the glass it could get, and the collection side that fed it was the first thing on the chopping block when money got tight.

What we take from a road paved with bottles

New York didn’t invent glassphalt. Baltimore had paved at least 17 streets with it from the mid-1970s through the mid-1980s, chasing the same sparkle.

What New York proved was that the idea could run at real scale, absorb hundreds of thousands of tons of the exact glass nobody else wanted, and win over a skeptical public along the way.

The sparkle was arguably the least interesting thing about it. It got the headlines, it got Trump lobbying, it made a wet avenue look faintly enchanted. But the real achievement was much plainer: a workable use for low-value mixed glass that would otherwise cost money to bury.

The reason it faded had nothing to do with whether the roads glittered, and everything to do with the fragile system behind it, the collection network and the raw cost of glass against dirt-cheap gravel. A recycling idea can be sound as engineering and still lose to logistics. That, more than the shine, is what a road made of bottles has to teach.

The post Beginning in 1990, New York City repaved streets with asphalt containing crushed recycled glass. By 1995, crews had used 250,000 tons of glass in resurfacing applications across the city, including a section of Fifth Avenue in front of the Plaza Hotel. appeared first on Space Daily.

On Tuesday, two survivors of child sex abuse accused the Justice Department of failing to notify dozens of victims found in Jeffrey Epstein’s massive collection of child sex images.

In a proposed class action, survivors explained that Epstein apparently started collecting child sex images in the 1990s, many of which the DOJ recovered after his arrest. In 2025, the FBI started reviewing “tens of thousands of videos” containing illegal child sexual abuse material (CSAM) and issued a memo confirming that “a large volume of images” seized depicted likely minors.

Some of these victims may have had contact with Epstein, as the disgraced financier kept a "modelbook" of nude photos of young girls roped into his ring. However, the FBI also identified hundreds of CSAM images in cache, which include approximately 15–20 images of known CSAM dating back to the 1990s. And so far, the DOJ has largely ignored those victims, who likely never met Epstein and have no way of knowing if he possessed, transported, or traded their images, survivors alleged.

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