anisotropic light propagation

A swing that rocks on a sawtooth hill behaves strangely: without kicks, it gets stuck in the first dip, but with rhythmic pushes it can move uphill, downhill, or stay frozen in place. That is exactly what physicists saw when they placed a single dust grain in a trap with a sawtooth bottom and started pushing it alternately with laser beams.

A water droplet on a fluoropolymer film initially behaves as expected: it sits and evaporates. But once it shrinks to a quarter of its initial size, it suddenly takes off and, within fractions of a second, covers a distance several times its own radius. Then it crawls slowly but persistently, leaving behind an invisible trail of electric charges. That trail is what drives the motion.

A wave packet is like a traveler who either loses steam and spreads out along the way, holds a steady pace, or gathers into a fist. In a 50-meter flume with water 5 meters deep, scientists tracked how a bell-shaped group of waves behaves when its height is changed but not its width.

A crowd at the entrance to a narrow corridor behaves differently than in a spacious hall: some people get pressed against the walls, others are pushed forward. Roughly the same happens when an elongated particle finds itself in a fluid teeming with self-propelled dumbbells. Until now, it was unclear why such a particle moves faster along its long axis than across it, and how the crowd density affects its speed.

The night sky looks serene, but it is actually pierced by an invisible rain of charged particles: cosmic rays. Scientists have long tried to figure out where the most energetic ones are born. Until recently, it was thought that our Galaxy supplies particles up to about 4 EeV, the so-called "ankle" in the spectrum.

Light born when a fast electron enters a material is usually assumed to be the same in all directions. But in crystals where optical properties depend on direction, that's not the case. Run your hand over velvet: one way it's smooth and shiny, the other — dark and rough. Similarly, a crystal responds differently to light depending on how it's oriented.

The crystal is like a city with different streets: on some, cars drive fast and far, on others, they crawl and get stuck. In the layered material CrPS₄, magnetic waves that carry spin (a particle's intrinsic angular momentum) behave the same way.

Pulsar PSR J1740+1000 is a city-sized star spinning faster than a kitchen blender. After a supernova explosion, it left behind a dense core that accelerates particles to near-light speeds. These particles fly into space, and previously it was thought they scatter in all directions, like splashes from a stone thrown into water.

Toss a handful of sand onto a table — the grains scatter equally in all directions. But if a magnet is hidden under the table and the sand is iron, the trajectories get trickier: an invisible structure overlays the chaos. That's roughly how molecular motion works inside a living cell.

In 1,433 pairs of galaxies scattered across the sky, astronomers noticed an unusual pattern. Neighbors located 'above' or 'below' a massive stellar system contain up to a quarter more oxygen than those lying in the plane of its disk.

A crystal that looks like a stack of pancakes was placed on a smooth substrate of magnesium oxide. The result was a film only a few hundred atoms thick. When they ran a current through it and switched on a magnet, the resistance jumped by nearly a quarter—22.5% in a 9-tesla field. Inside the film, electricity is carried by two streams of particles.

Shine light on an FePS3 crystal, and it responds with its own glow, like a neon sign, but much fainter and in the infrared range. This glow is not random: it depends on how the crystal is oriented. The scientists took samples from a thick chunk down to a single-atom layer and observed four emission bands — at 1.24 electronvolts, 1.79, 2.3, and 2.56.

A thin film for a solar cell or display is like a dense crowd in a square: each person faces a different direction, and whether light passes through the crowd or gets stuck depends on which way everyone is looking. In organic electronics, molecules play the role of passersby. Until now, instruments could only measure the crowd's overall direction — the average head tilt and its spread.

Lymphatic vessels are one-way streets. Through them, fluid flows only forward thanks to tiny flap valves. These valves open in just one direction, creating asymmetry, directionality, without which the flow would stop. In the lab, scientists modeled a vessel and sent compression waves along it, like a parade of gentle nudges.

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.

Inside a barium titanate crystal, there's a built-in electric field, like a permanent magnet, but for charges. When a flash of light hits it, electrons are knocked loose and start darting around, colliding with atoms. But the crystal isn't the same in all directions: it has an axis along which the field is strongest.

Ordinary window glass may seem frozen forever, but inside, atoms never stay still. In amorphous germanium telluride (GeTe)—a material used in rewritable discs and non-volatile memory — physicists have for the first time directly tracked how atomic bonds stretch and bend in trillionths of a second.

A river flows in one direction, but a wave on its surface travels in the opposite direction. The same splitting happens in hyperbolic materials: the energy of a light wave and its fronts diverge, preventing a sharp lens from being created.

Felt looks like a tangled ball of wool, but under X-rays you can see hidden trails inside. When a water drop lands on such material, it doesn't just spread across the surface — some of the liquid dives deep, like water into blotting paper. Scientists captured this process for the first time with an X-ray camera snapping hundreds of frames per second. The secret lies in the needles.

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.

In a thin layer of a crystal, electrons behave differently than in an ordinary piece of metal. They are cramped, collide with each other more often, and feel the shape of the lattice more strongly. Usually, such states are created artificially by stacking layers of different materials. But sometimes nature does it all by itself.

Toss a handful of pebbles into a stream: some settle on the bottom, others get swept around. That's roughly how waves behave in a disordered medium — light in frosted glass, sound in porous material. Usually, disorder slows things down: the wave gets stuck, spreads out, loses direction. But in a new study, physicists built an acoustic lattice where things work differently.

Light passing through a window pane refracts the same in all directions. But in thin layered crystals, it's a different story: light traveling along the layers sees the material differently than light traveling across them. It's as if a sheet of paper were transparent but transmitted light differently along and across its fibers.

Toss a handful of peas into a pot of semolina porridge — the peas quickly sink, pushing the grains aside. But if you make the porridge very thick and sticky, the peas get stuck. That's roughly how light lithium atoms behaved in a cloud of heavy potassium atoms at temperatures just a few millionths of a degree above absolute zero.

Light from distant galaxies passes through telescope lenses like through a stack of window panes. Each pane is coated with a thin film to reduce glare. But this film acts as a polarizing filter: one component of the light wave passes a bit more freely, the other a bit worse. As a result, the wave rotates slightly, and the telescope sees a different picture than what is in the sky.

Water in a puddle after rain swirls differently than in a deep lake. A thin layer of fluid feels the bottom, and that changes the nature of its vortices. Scientists created such a layer in the lab: they poured in an electrolyte, ran a current and a magnetic field, and the fluid began to move.

Thirty nanometers is the distance over which a phonon sprinter hasn't yet had time to accelerate. In thin aluminum nitride films, thermal resistance remains almost unchanged up to this thickness: heat transport proceeds in the ballistic regime, where phonons (quanta of crystal lattice vibrations) fly…