Light Can Be Reflected Not Only In Space But Also In Time

Anna Demming reports via Scientific American: [A]lthough so far there's no way to unscramble an egg, in certain carefully controlled scenarios within relatively simple systems, researchers have managed to turn back time. The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray's path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a "time reflection." Time reflections have fascinated theorists for decades but have proved devilishly tricky to pull off in practice because rapidly and sufficiently changing a material's optical properties is no small task. Now, however, researchers at the City University of New York have demonstrated a breakthrough: the creation of light-based time reflections. To do so, physicist Andrea Alu and his colleagues devised a "metamaterial" with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through. Metamaterials have properties determined by their structures; many are composed of arrays of microscopic rods or rings that can be tuned to interact with and manipulate light in ways that no natural material can. Bringing their power to bear on time reflections, Alu says, revealed some surprises. "Now we are realizing that [time reflections] can be much richer than we thought because of the way that we implement them," he adds. [...] The device Alu and his collaborators developed is essentially a waveguide that channels microwave-frequency light. A densely spaced array of switches along the waveguide connects it to capacitor circuits, which can dynamically add or remove material for the light to encounter. This can radically shift the waveguide's effective properties, such as how easily it allows light to pass through. "We are not changing the material; we are adding or subtracting material," Alu says. "That is why the process can be so fast." Time reflections come with a range of counterintuitive effects that have been theoretically predicted but never demonstrated with light. For instance, what is at the beginning of the original signal will be at the end of the reflected signal -- a situation akin to looking at yourself in a mirror and seeing the back of your head. In addition, whereas a standard reflection alters how light traverses space, a time reflection alters light's temporal components -- that is, its frequencies. As a result, in a time-reflected view, the back of your head is also a different color. Alu and his colleagues observed both of these effects in the team's device. Together they hold promise for fueling further advances in signal processing and communications -- two domains that are vital for the function of, say, your smartphone, which relies on effects such as shifting frequencies. Just a few months after developing the device, Alu and his colleagues observed more surprising behavior when they tried creating a time reflection in that waveguide while shooting two beams of light at each other inside it. Normally colliding beams of light behave as waves, producing interference patterns where their overlapping peaks and troughs add up or cancel out like ripples on water (in "constructive" or "destructive" interference, respectively). But light can, in fact, act as a pointlike projectile, a photon, as well as a wavelike oscillating field -- that is, it has "wave-particle duality." Generally a particular scenario will distinctly elicit just one behavior or the other, however. For instance, colliding beams of light don't bounce off each other like billiard balls! But according to Alu and his team's experiments, when a time reflection occurs, it seems that they do. The researchers achieved this curious effect by controlling whether the colliding waves were interfering constructively or destructively -- whether they were adding or subtracting from each other -- when the time reflection occurred. By controlling the specific instant when the time reflection took place, the scientists demonstrated that the two waves bounce off each other with the same wave amplitudes that they started with, like colliding billiard balls. Alternatively they could end up with less energy, like recoiling spongy balls, or even gain energy, as would be the case for balls at either end of a stretched spring. "We can make these interactions energy-conserving, energy-supplying or energy-suppressing," Alu says, highlighting how time reflections could provide a new control knob for applications that involve energy conversion and pulse shaping, in which the shape of a wave is changed to optimize a pulse's signal.

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