Researchers from Lomonosov Moscow State University, in collaboration with colleagues from the United States and Germany, have experimentally demonstrated the effect of ultrafast optical switching in metamaterials. The experiment was carried out using femtosecond pulsed lasers and a nanostructured gallium arsenide semiconductor medium. The discovered effect can find application in fast calculations as one of the methods of ultrafast control of light at nanoscale.
The problem of controlling light at nanoscale and the problem of constructing optical images with a resolution higher than the wavelength are of interest to researchers both from a fundamental point of view (overcoming the diffraction limit) and from an applied point of view (superresolution, creation of optical chips; see Photonic integrated circuit). The field of knowledge devoted to these problems is called nanophotonics.
The theoretical possibility of many interesting optical effects at subwavelength scales has been shown long ago. Even Lord Rayleigh noticed that the scattering of light on small particles of water and dust suspended in the atmosphere gives a blue color of the sky. After Gustav Mee created the theory of light scattering by micro- and nanoparticles, it became possible to build metamaterials and metasurfaces – artificial nanostructured media with ordered particles that allow you to create a given optical response, that is, the ability to transform (reflect, refract, focus) light in a given way.
Since then, it has been shown more than once that materials structured on the nanometer scale have optical properties that differ qualitatively from the properties of the material from which they are made. By controlling the phase of the light wave at submicroscopes, it is possible to observe such effects as a negative refractive index (when the angle of refraction ceases to obey Snell’s law and increases in a denser medium), superfocusing (when it is possible to overcome the diffraction limit of focusing), and others. In particular, the “Veselago superlens” – a flat nanostructured medium – allows one to overcome the diffraction limit and focus light to a region less than half its wavelength. The superlens model was built theoretically by Soviet scientist Viktor Veselago in 1967. Later the idea was taken up by the English theorist John Pendry (see John Pendry, David Smith “In Search of a Superlens”). However, a truly active progress in nanophotonics began in recent years due to a technological breakthrough in the possibilities of fabricating nanostructures of a given geometry and chemical composition with good accuracy.
Such materials are sensitive to the properties of the environment: they can change color depending on its composition or temperature. Therefore, metamaterials have found their application in biosensorics, thermotherapy, solar batteries, information storage, etc. In particular, new photonic elements have been developed – devices for controlling light on microscales (for example, for future applications in fast computing).
The problem with optical metamaterials is that their optical properties cannot be rearranged. Many needs require controllable metamaterials whose properties can be changed using electric or magnetic fields after manufacture. And for use in fast calculations, the properties of metamaterials must also change extremely quickly under external influences.
One of such superfast changes was recently discovered by scientists from the Lomonosov Moscow State University together with colleagues from the USA and Germany. Ultrafast light switching is realized in gallium arsenide metamaterials. We are talking about a technique for ultrafast pump-probe measurements (see Attoseconds: 3. How to cut an atom), which uses two laser pulses with a duration of several tens of femtoseconds. The process resembles in its logic the work of an electronic transistor, in which applying a voltage to the base changes the current through the transistor. The first pulse changes the optical properties of the medium: it can quickly heat it up and thereby change the reflection coefficient of light from the medium. The second impulse interacts with the changed environment in a different way than with the “cold” one: it is reflected weaker or stronger. Then, in a few picoseconds, the material returns to its original state.
The idea itself is far from new. The technique has already been applied to many different materials. However, in most cases, the properties of the material do not change strongly enough, by a few percent, and to implement a logical switching (switching a logical “zero” to “one”), you need to change one of the coefficients – reflection, refraction or absorption – at least twice.
To increase the switching effect, it would be natural to use semiconductors in which light (laser pulse) is able to generate free current carriers (electrons), which were bound to the lattice before excitation. In this case, the semiconductor is transformed into a metal for a short time, which is a qualitative difference for optics: electrons free in a metal and bound in a dielectric react differently to an alternating electric field of a light wave. A piece of metal looks like a mirror, and a piece of plastic is rather matte.
Historically, researchers have worked most with silicon, developing the idea of integrating silicon electronics and photonics. However, silicon is a so-called indirect-gap semiconductor, in which, in order to generate free electrons by radiation in the near-infrared range, it is necessary to change the momentum of these electrons in order to fulfill the law of conservation of momentum. Therefore, optical generation of carriers in silicon is not so efficient. Gallium arsenide is a direct-gap semiconductor, optical generation of carriers in it is much more efficient, but still insufficient for practical applications.
For ultrafast light control, this work used the concept of metasurfaces made from a direct gap semiconductor. The size of the gallium arsenide nanodisks (Fig. 1) is selected in such a way that the light of the required wavelength is effectively retained in it, forming a kind of standing wave. In fig. 2 shows such a resonant increase in the electromagnetic field inside the nanodisk (numerical calculation).
Fig. 2. Amplification of the electromagnetic field of a light wave in a gallium arsenide nanodisc (numerical calculation). Image from the discussed article in Nature Communications
For this, in addition to the correct dimensions, a high contrast of the refractive index of the disk with the surrounding materials is required. With a small difference between the refractive indices of the substance and the environment, the reflection will be weak, the optical resonator will not work. This is easy to understand if a piece of glass is immersed in water. In this case, the light practically “does not notice” the boundary between water and glass: we do not see glass, since the refractive indices of water and glass are close, and the reflection coefficient in this case is small. For a good optical contrast between gallium arsenide and the environment, additional discs are used – “pads” made of dielectric AlGaO and SiOx, therefore, the structure is multi-layered. In this metamaterial, the light reflectance has a resonant character, that is, it increases near the resonance wavelength (schematically shown by the blue line in Fig. 3).
The position of the resonant wavelength is very sensitive to the refractive index of the nanodisk material. If we change it with a laser pulse, then the resonance curve is shifted by an amount comparable to the width of this resonance (red curve). The effect of high-quality switching of the reflection coefficient is now clearly visible. For the selected light wavelength of 1000 nm (dashed line in Fig. 3), the reflectance changed from almost unity to almost zero. In a real experiment, it was possible to dynamically reduce the reflection coefficient by more than two times in 1 picosecond; the system returned to its original state in 6 picoseconds. And, which is most important for practical applications, due to the resonant amplification of the optical field inside the semiconductor, switching was achieved with a pulse energy density of only 380 μJ / cm2… In a pump-sensing technique, this is a very small value to produce such strong effects.
The result of the work poses new exciting problems and opens up the possibilities of practical applications, such as the study of ultrafast changes in other characteristics of the optical field – for example, the direction of radiation, the phase of its front, the influence of the environment (for biosensorics) and, of course, fast optical calculations in a photonic computer. Electronic processors have reached sufficiently high frequencies, that is, operating speeds, using 14-nanometer technology. However, with such dimensions of the minimum element, the “bottleneck” became the conductors connecting them, since they must be very thin, while their resistance to electric current is high, which, in turn, increases the transit time of this current in proportion to the resistance. In fact, the fundamental limit of the frequency of the electronic processor has already been reached.
Conceptually new ideas are proposed to overcome the speed limitations of processors, such as quantum computing (see Quantum computer) or the photonic computer. Photonic computers try to speed up computations and data transfer by using light instead of electrons in all functional parts of the device – processor, data transfer bus, memory.
True, it turned out that it was not enough to replace individual functions of a computer from electronic to photonic, since the very conversion of an electronic signal into a photonic signal and vice versa is strongly limited in speed. To make a qualitative leap, we must replace all the functional parts of the computer at once – data transmission, memory, processor. And if the problem of transmitting data via optical fiber has long been successfully solved, then the problem of optical computing requires the search for new fundamental physical effects that can form the basis of an optical transistor. One of these effects is the discovered effect of ultrafast optical switching in the gallium arsenide metasurface.
A source: Maxim R. Shcherbakov, Sheng Liu, Varvara V. Zubyuk, Aleksandr Vaskin, Polina P. Vabishchevich, Gordon Keeler, Thomas Pertsch, Tatyana V. Dolgova, Isabelle Staude, Igal Brener & Andrey A. Fedyanin. Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces // Nature Communications… 2017. V. 8. Article number: 17. DOI: 10.1038 / s41467-017-00019-3.