Sound controls the light 04.02.2015

At the beginning of the last century, the Soviet physicist Leonid Mandelstam theoretically showed that sound vibrations in a transparent substance can scatter light passing through this substance. Sound waves cause local changes in the density of the medium and, as a result, change the refractive index. As a result of such scattering, part of the light energy is lost. Independently of Mandelstam, the American physicist Leon Brillouin arrived at the same results. As a result, the interaction of sound and light in transparent media was called the Mandelstam-Brillouin effect.

However, we do not notice that loud music scatters the light from a light bulb, as, for example, the light of car headlights scatters in the fog. The effect will become noticeable only if instead of an ordinary light bulb we take a source of monochromatic radiation - a laser. The fact is that the laser beam is an electromagnetic radiation with one wavelength, which determines its "color". The red beam has one wavelength, the green beam has another.

Now let's take a fiber optic data line. The principle of its operation is that information is transmitted by changing the intensity of a light beam propagating along a transparent glass thread. A single fiber optic strand can be used simultaneously to transmit data over hundreds of channels, simply by using light beams of different wavelengths. Each channel corresponds to a specific laser wavelength. It is quite similar with the transmission of data over radio waves, except for one thing: if we increase the power of the radio transmitter, then the signal power and the range of its reception increase. If we increase the laser power to transmit a signal over an optical fiber, the transmission deteriorates - more and more of the signal will begin to be lost due to Mandelstam-Brillouin scattering. Therefore, there is a threshold signal power, which does not make sense to exceed, otherwise the transmitted light will simply be reflected back.

What did physicists from the University of Illinois do? On a thin fiber optic strand, they fixed a small glass sphere. This design is called a ring optical resonator. A laser beam from a fiber optic filament enters the resonator and, due to multiple internal reflection, remains in it, as in a trap. The key point in the experiment was the second laser beam, with a frequency that differs from the original one by a certain amount. The difference in the frequencies of the laser beams corresponded to the frequency of the acoustic vibrations of the sphere material. This made the optical fiber and resonator system transparent to the first beam.

What is most surprising, such a system turned out to be transparent to rays from only one side. It turned out to be a kind of optical turnstile - the light passes from one side, and cannot pass from the other. Such an interesting property arises due to the complex interaction of two light rays and acoustic waves in a material - the Mandelstam-Brillouin scattering effect. Only in this case, instead of preventing the passage of the beam through the fiber, he, on the contrary, provided him with a free corridor.

The discovery of such properties will make it possible to create miniature optical isolators and circulators, which are needed for fiber optic systems and, in the future, for quantum computers. Now these devices are based on the magneto-optical Faraday effect, and magnetic fields and materials are used to transmit light in only one direction. The discovery made will just help to get rid of unnecessary magnetic fields. In addition, it can be used to change the group velocity of a light beam - what physicists call "fast" and "slow" light, it is needed to store quantum information.