optoelectronic device

A thin film of neodymium nickelate, fifteen nanometers thick, sits on a neodymium gallate substrate. Below one hundred eighty kelvin it behaves as an insulator: electrons are locked inside their atomic cells and cannot move freely.

In a dark room, you can hold a glowing sphere without touching it: walls of light push it away but do not burn it. That is roughly how the trap built in Rio de Janeiro for yeast cells works. A conventional optical tweezer, that is, a focused laser beam, acts like tweezers made of light, but the light itself pierces the cell from side to side.

Light in an optical fiber is like a stream: it flows inside a glass thread and doesn't spill out. A thick thread carries many streams at once, a thin one only one. Astronomers often need to pour light from a thick fiber into several thin ones to split it into colors.

A regular laser beam travels straight, like the light from a flashlight. But light can be twisted into a spiral, like the thread of a corkscrew. Such a beam carries orbital angular momentum: it doesn't just illuminate, it also spins anything it encounters.

A light beam usually travels in a straight line, but you can make it spin around its axis, like water swirling into a vortex as it drains. This 'twisted' light carries more information and is needed for ultra-fast communication and quantum computing.

Electricians working with high voltage are like divers: they need to see what's happening deep underwater without going down themselves. What's needed is a sensor that stays away from the dangerous current and relays information to the surface. Usually, light is used: a beam passes through a transparent crystal, and the electric field slightly rotates it.

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.

Organic light-emitting materials are like a busy intersection without traffic lights. Molecules receive energy but, instead of honestly giving it back as light, they often turn into dead ends: they form dark pairs (exciplexes — short-lived complexes that do not emit) or collide with each other, losing charge.

Passive sensors communicate by reflecting radio waves, like an echo. The energy savings are clear, but many reflections turn into a jumbled mess. It's like a choir where each singer repeats their own part without a conductor. A new approach assigns each sensor a specific rhythm — a digital code.

A solid electrolyte is not just a crystal lattice: it’s a bustling metropolis for ions. Each act of conduction resembles a trip through the city: there are wide highways with low energy barriers, gridlocks in bottlenecks, and tricky detours where an ion can get stuck.

A switch that knows which way to let current flow. That's how a Josephson diode works — a superconducting bridge where current goes willingly in one direction but meets resistance in the other. In the new study, such a bridge was made from graphene — a sheet of carbon just one atom thick.

An ordinary diode in electronics works like a one-way street: current flows in only one direction. In superconductors, where electricity flows without resistance, such behavior has long been elusive — current is usually symmetric. The authors of the new work found a way to create a perfect superconducting diode using light.

An ordinary photometry detector can't tell a left photon from a right one. But let it start moving, and everything changes. Imagine a rower moving between two flutists playing the same note. His ear is perfectly tuned to that note and ignores all others. As he moves, the pitch from the flutist ahead sounds higher, and from the one behind sounds lower.

Light leaves a trace even when there is almost none of it. A single photon, a tiny bundle of energy, strikes a silicon wafer and knocks out a handful of electrons. The trouble is that the handful is too small to notice. You need an amplifier. Usually, the signal is amplified by applying a voltage to metal contacts on the detector's surface.

A conventional laser is like an orchestra where musicians must warm up before playing in unison. You first need to pump enough atoms with energy so they 'flip' and are ready to emit light: this is called population inversion. Only then does the beam flash. A polariton laser plays by different rules: its 'musicians'—a mix of light and matter — join in almost without preparation.

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 flashlight shines into the eye, and the retina responds with electrical impulses, like an echo, but not of sound, of light. This echo can be recorded, and it reveals not only how the eye is working but also how the brain is. After all, the retina is a piece of the brain that has been pushed out to the periphery.

A copper wire hit by a laser is like a kitchen where the chef turns on the stove and watches the stopwatch at the same time. Copper atoms start to dance, losing the order of the crystal lattice, while electrons, like waiters in the dining room, instantly feel the change. But how much disorder do they need to notice?

Blue pixels in screens are like light bulbs that burn out fast. Conventional organic LEDs for blue are unstable: their light dims over time, and the color shifts. Chemists found a replacement — the rare metal europium.

An ordinary flash drive stores bits in electron traps, but you can only read them through a separate wire and amplifier. In the new development, the same piece of the semimetal tellurium plays both the role of storage and the light bulb.

Light can carry information faster than any electronics. But usually, between a computer and a laser sits a digital-to-analog converter — a device that turns zeros and ones into electrical signals. It's the bottleneck. The new transmitter does away with this middleman.

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.

In a molecule resembling chlorophyll and built from carbon, a metal atom at the center acts as a toggle. Shifting it by fractions of a nanometer can make the molecule glow, quench, or boost its brightness a hundredfold.

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.

The crystal bismuth selenide (Bi₂Se₃) is symmetric by itself, like a neatly stacked sandwich: flip it over, and nothing changes. But twist one layer slightly relative to the other, and the symmetry breaks. The material begins to 'feel' which side the light comes from and responds with an electric current.

A steady laser beam is like a taut string: if it vibrates even a little, the music turns into cacophony. In a quantum computer, the laser holds atoms in a trap, and any frequency jitter ruins the computation. That's why laser setups used to occupy entire tables, and tuning took weeks.

Radio waves surround us, but our eyes can't see them. This new sensor, the size of a fingernail, turns them into a colored patch on a tiny screen. Inside the chip is a layer of liquid crystals — the same kind used in phone displays. Among them are embedded thin ferromagnetic nanowires. When a radio wave arrives, the nanowires heat up quickly.

A semiconductor diode normally conducts current in only one direction, like a spring-loaded door: it opens easily one way but pushes back the other. But if you dope the crystal heavily, the door starts behaving strangely: under reverse bias, the current doesn't just flow: it shoots up.

Light from a star travels for millions of years, yet in a telescope it still appears as a point: it's just too far away. To see a star's surface, you need an instrument the size of a city. In Chile, such a giant is already under construction: ELT with a 39-meter mirror made of 798 hexagonal segments.

Light is not just a beam but a wave with an electric field that changes at a speed unimaginable to ordinary clocks. Until now, this field could only be measured at a single point, like taking temperature with a single thermometer. Now researchers have created a method that shows the entire field at once, like a thermal imager showing a heat map.

Modern X-ray light sources are like an orchestra without a conductor: electrons in the beam fly every which way, and the radiation comes out blurry. The new idea is to make electrons line up in neat rows with a spacing of a few nanometers. Then the light they emit becomes coherent, like a laser beam.

A laser beam hits the skin, and tissues respond with a barely audible ultrasonic echo. From that echo, doctors can see blood vessels and tumors without making a single incision. But before the picture appears on the screen, the signal has to be cleaned of noise, just as a cook skims the foam off a broth to make the soup clear.

A thermos of liquid xenon cooled to minus 100 degrees Celsius. Inside are sensitive sensors that catch rare signals from dark matter particles. To bring these signals out, copper cables are usually used. But over the long haul, the signal weakens, and adjacent cables start 'cross-talking' — that's called crosstalk.

In the ocean, sometimes a huge solitary wave appears out of nowhere and then vanishes. Such rogue waves are rare but dangerous. Similar rare and intense spikes occur in other systems, from financial markets to lasers. The problem is that they are hard to predict, let alone trigger on demand.

On a lab bench sits a laser trap — an invisible hand that holds a glass bead the size of a bacterium in midair. Next to it is a mirrored cavity where light bounces between two reflective walls. Scientists made the bead and the light exchange quantum entanglement: their states are now linked so that measuring one instantly reveals the other.

A printer that prints not with ink but with metal and ceramic is now a reality. Scientists took an ordinary drop of water containing dissolved metal salts and made it deposit copper, zinc oxide, or nickel hydroxide onto a surface — all with precision down to millionths of a meter. The secret lies in electricity.

Light doesn't always travel in a straight line. Its wave can be twisted into a spiral, like a stream of water from a rapidly rotating hose. This twist is called orbital angular momentum, and it can hide information: each turn acts as a separate communication channel. The problem was that only bulky setups of lenses and mirrors could recognize these twists.

A laser beam passes through a glass cell containing rubidium vapor, and by how the atoms absorb light, you can determine their temperature. This is like measuring the speed of cars by the blur in a photograph: the faster the atoms move, the wider the absorption line becomes.

In the air of factory floors and laboratories, invisible solvent molecules drift. One of them, acetonitrile, has a faint smell, but at high concentrations it can harm health. Typical sensors for it are bulky and expensive, but here we have a thin crystalline film that responds on its own to the presence of vapor.

Every breath you take and every muscle twitch runs on a stream of electrons flowing through proteins inside your cells. These proteins aren't static: they constantly change shape, like a spring that compresses and releases. But until now, no one has seen exactly how these movements open or close the path for electrons.