topological photonic effects

A sponge slowly watered from a watering can gets soaked all the way through. But if you dump the same bucket of water on it all at once, the middle gets wet while the edges stay almost dry. That's roughly how a material with nonlinear absorption behaves: the more intense the flux, the more it gets stuck at the point of impact.

A stack of playing cards lies still. You can slide the top card with your finger: that's crude intervention. Or you can blow so that the whole stack momentarily changes shape, but the cards don't scatter. That's roughly how light, in a new experiment, rearranges a crystal of tin selenide (SnSe). Usually, to change the structure of a solid, you heat it or pass a current through it.

Two pebbles drop into water at a distance. The waves from them run toward each other and at certain spots they cancel out — the water becomes smooth, even though each stone alone would create ripples. Much the same happens with a pair of photons (light particles) born in a single crystal and flying off in different directions.

A weak laser beam falls onto a tiny structure — a hybrid cavity made of metal and glass. Inside is trapped a single light-emitting molecule. Normally, such light only slightly changes color as it scatters off thermal vibrations of atoms.

Usually, light does not turn an insulator into a conductor: that requires powerful laser pulses that break the material's structure. But in twisted bilayer graphene at the magic angle, things are different.

A beam of light enters frosted glass and fades — nothing unusual. But physicists have long known how to make light pass through without losing a single photon. This trick is called electromagnetically induced transparency: two light waves cancel each other at the right point, and absorption disappears. Now a similar feat has been pulled off with atoms.

Imagine a multilayered shell where metal and insulator layers alternate like pages in a book. Such a structure can make an object hidden inside invisible to an observer. Light bends around it as water smoothly flows around a smooth stone: no ripple, no splash — no reflection, no shadow.

Inflation is the ultra-fast expansion of the universe in its first moments. It was driven by a field resembling a ball rolling down a hill. But there could have been a second, almost imperceptible ball nearby. The authors of this work considered exactly that scenario: a light spectator field (the axion), frozen at the top of its potential.

Light from an ordinary bulb seems utterly chaotic: its waves oscillate out of sync, like a crowd where everyone walks in their own direction. Such light is called incoherent. For a long time, it was believed that elegant topological structures (knots, rings, links) could only be created in coherent laser light, where all waves march in step.

A magnetic field can hold a star against complete collapse: that's the idea behind a new kind of black hole. Instead of the usual point of infinite density, inside there is a field that prevents space from crunching down to zero. It's like a spring: the harder you squeeze, the more it pushes back.

Light, discovered by Maxwell, behaves differently in different media. In a new study, researchers simulated a two-dimensional crystal that absorbs light and has magnetic properties. In it, light waves behave like passengers on a one-way escalator: they relentlessly move toward edges and corners, where they accumulate.

A candle flame flickers in a draft but doesn't go out — the tongue of fire briefly changes shape and then returns. Optical skyrmions behave much the same way — tiny vortices in a light beam, reminiscent of swirling eddies. Until recently, they were created only in the perfect, smooth light of a laser.

An ordinary light sensor answers the question "how much light" but not "where did it come from." To do that, you need lenses, mirrors, or entire arrays. In a new study, researchers propose a sensor that determines the direction of a light beam on its own, without any optics.

A gamma-ray burst is a brief but extremely powerful flash of light, born from the merger of neutron stars or the collapse of a massive star. Inside such a flash, plasma flows race at enormous speeds and collide with each other.

Light usually pushes an atom straight ahead, like wind pushing a sail. But if the beam is twisted into a corkscrew, the picture changes: the atom can fly off to the side, as if caught by a spinning wheel. In the new work, physicists calculated that such a vortex photon can transfer almost all of its rotation to the atom — up to 100 percent.

An ordinary lens always refracts light the same way. Its properties are frozen in glass forever. But now there's a surface that changes the game on the fly. Scientists made an ultrathin film with resonant elements and infused it with liquid crystals — the same stuff used in phone screens.

A laser beam twisted into a spiral looks like a drain vortex in a sink: light moves not only forward but also in circles. Such vortex beams already help move tiny particles and transmit more data. But once they pass through an inhomogeneous medium, the vortex falls apart, like water from a leaky bucket.

A swimmer cutting through water at the speed of the waves running on it finds themselves in a strange position: the waves they themselves generate can neither overtake them nor fall behind. They pile up at the swimmer's bow and freeze. A similar phenomenon occurs when a light source moves inside a medium at a speed matching the speed of energy transfer of the waves.

The tear film on the eye is an ultrathin layer of fluid that protects the cornea and keeps vision sharp. It’s usually pictured as a perfectly smooth surface, but the real cornea, even a healthy one, is covered in microscopic bumps.

A water droplet on a leaf disappears faster under the sun than heat calculations would predict. A few years ago, a hypothesis emerged: light itself, without heating, could knock water molecules off the surface. This was dubbed the photomolecular effect. If it exists, clouds, climate, and even everyday clothes drying could work differently.

Two layers of a magnetic material, rotated relative to each other by fractions of a degree, like two slightly offset screens on a window. In their normal state, they don't produce anything special, but at a certain twist angle, an unexpected effect emerges.

The tiny green alga Chlamydomonas spends its life rowing with two flagella toward light, like a swimmer heading for shore. It senses light with a photoreceptor protein, and behind it lies a red spot — the 'eyespot'—which casts a shadow and helps the cell figure out where the light is coming from.

A glowing dot the size of a molecule. Run a current through it, and it flashes like a tiny lightbulb. That's what the experiment with dye molecules on a thin salt film looks like. When current enters a molecule, one electron hands it energy. The molecule immediately gives that energy back, as light. It's like a swing: give it one push, and it rocks and then stills.

Light from a single bulb usually spreads in all directions, and catching it in equal portions across several channels is quite a puzzle. In the new work, light is split into nine identical beams without a single lens or mirror. Instead, there's a glass plate with etched waveguide paths, along which light naturally flows in the desired way.

The popping lid of a jam jar is the hero of this story. Press it, and with a loud click it flips from a convex to a concave shape. Exactly the same happens with a hinged beam inside each cell of a new mechanical lattice. Scientists assembled these snapping beams into a hexagonal grid, like honeycomb, but made of elastic plates. When one beam is sharply compressed, it snaps and pushes its neighbor.

Quantum light is not just a stream of particles but an intricate pattern woven from space and time. A single photon can be stretched, squeezed, smeared across different directions.

A diamond speck smaller than a hundredth of a millimeter glows red. Inside it is a crystal defect: two neighboring carbon atoms, where one is missing and a nitrogen atom sits nearby. Such a pair captures light and re-emits it, like a tiny beacon that doesn't fade at room temperature. This speck was brought near a pathway on the chip's surface.

A crystal without a center of symmetry is like a staircase where all the steps are tilted in the same direction: electrons, given a kick by light, roll predominantly in one direction. This is how a direct current is born — the photogalvanic effect.

Modern computer algorithms design photonic integrated circuits — light chips capable of separating signals by frequency (color) for spectroscopy. Unlike traditional engineering, light paths in these chips are not straight waveguides but chaotic, almost organic mazes.

Ordinary light scatters in all directions, but physicists turned it into something like a conveyor belt — a one-way stream that doesn't turn back. To do this, they sent two counter-propagating beams through a cloud of Rydberg atoms, bloated to the size of a tiny dust speck.

Two voices in a molecular duet can sound in unison or cancel each other into silence. In physics, such silence is called a dark state: a quantum excitation that does not interact with light and therefore lives unusually long.