In several years — maybe in one or two decades, but maybe sooner or never — one of today’s problems will be solved in an original way: our computers, nanoantennas and other kinds of equipment will operate on the base of photons, rather than electrons. Even now, we are practically prepared to accomplish this switch. If it happens, the spheres studied by an international group of Russian, French and Spanish scientists will be able to become one of the elementary components of new photonic devices. The results of the study were published in the latest issue of Scientific Reports.
The potentialities of conventional electronic computers are going to be exhausted. During the last four decades, Moore’s law (according to which computer’s processor speed doubles every 18 months) was fulfilled due to increasing of the operating frequency of a single processor. Now, the same result is reached by means of the parallel computing — we have dual-core processors as well as quad-core ones. It means that single-core processors are not able to cope with the computation speed demanded; moreover, it is not possible to increase this speed anymore, because modern computers’ processor operating frequency is close to the theoretical limit. The process of multiplying the number of cores is also not endless: by all accounts, it will soon come to the end. That’s why plenty of research teams all around the world are working on the creation of super-fast optical systems, which would be able to replace the electronic computers.
On one hand, such systems should be as small as possible. On the other hand, optical radiation has its own scale — the wavelength (in the visible range of the spectrum it is about 0.5 micrometers). This scale is too big to be implemented in modern electronic devices with ultra-dense arrangement of elements. In order to compete with such electronic devices, optical systems should work on scales much shorter than the wavelengths. These problems fall within the domain of modern discipline, called subwavelength optics. The aim of subwavelength optics is to manipulate with electromagnetic radiation on scales shorter than its wavelength — in other words, to do the things, which were considered as conceptually impossible in the traditional optics of lenses and mirrors.
Until recently, subwavelength optics laid great hopes on the effects related to interactions of light with the, so-called, plasmons — collective oscillations of free electron gas in metals. In the case of metal particles with sizes about 10 nm, the frequencies of the oscillations of the free electron gas fall within the range of the optical band. If such a particle is irradiated with an electromagnetic wave, whose frequency is equal to one of the particle’s plasmon oscillation frequencies, a resonance occurs. At the resonance, the particle acts as a funnel, which “grabs” the electromagnetic wave’s energy from the external environment and converts it into the energy of the electronic gas oscillations. This process can be accompanied with a wide range of very interesting effects that, in principle, could be employed in various applications.
Unfortunately, the best part of the expectations related to the plasmonics has not been justified. The fact is that even very good electric conductors (for example, copper or platinum) exhibit large electric resistance when the frequency of the electric current becomes of the same order of magnitude as that of the visible light. Therefore, as a rule, the plasmon oscillations are strongly damped, and the damping kills the useful effects that could be utilized.
That’s why, recently, scientists turned their attention to dielectric materials with high refractive index. There are no free electrons in these materials, because all of them are connected with their atoms; and the impact of light does not induce conduction current. At the same time, electromagnetic wave affects electrons inside the atom and shifts them from the equilibrium positions. As a result, atoms acquire induced electric moment; this process is called “polarization.” The higher the degree of polarization, the higher the refractive index of the material. It turned out that, when a sphere made of a material with high refraction index interacts with light, the result of this interaction to a large extent resembles the above-described plasmon resonance in metals with one very important exception: a wide range of dielectric materials — as distinct from metals — have weak damping at the optical frequencies. We often use this property of dielectrics in our everyday life — for example, weak damping at the optical frequencies is the key for the transparency of glass.
An ancient work of Professor Michael Tribelsky (Mikhail Tribel’skii), Faculty of Physics, M.V. Lomonosov Moscow State University, and Moscow State University of Information Technologies, Radioengineering and Electronics MIREA, gave the initial impulse to the research described above. The scientist says: “If we use the language of quantum physics while speaking about the plasmon excitation, we can say that a quantum of light, photon is converted into a quantum of plasmon oscillations. In the middle of 80th, I got the following idea: since all processes in quantum mechanics are reversible, the inverted process of the plasmon-to-photon conversion should exist too. Then, I arrived at the conclusion that a new type of light scattering exists. This was the case indeed. Moreover, it occurred that this new type of light scattering has very little in common with the described in all textbooks Rayleigh scattering.” As a result, paper “Resonant scattering of light by small particles,” Tribel’skii M.I., Sov. Phys. JETP 59, 534 (1984): http://www.jetp.ac.ru/cgi-bin/dn/e_059_03_0534.pdf came out. However, in 1984, this work could not attract the attention of scientists, because nanotechnologies did not exist yet. The first citation of this paper occurred in 2004 — exactly 20 years after its publication. Nowadays, this type of scattering, named “anomalous,” is widely acknowledged. Unfortunately, even in the case of the anomalous scattering, once again, we face the fatal role of dissipation. In order to observe the anomalous scattering, it is necessary to use metals with very weak damping at optical frequencies.
The very natural question in this case is: if we take the advantage of the weak damping of dielectrics, will the sphere made of dielectric materials with high refraction index be able to demonstrate the effects which cannot be observed in the case of plasmon resonances in metals with strong damping? To answer the question, Professor Tribelsky’s laboratory (Faculty of Physics, M.V. Lomonosov Moscow State University) began a carry out joint research with French and Spanish colleagues. Scientists experimented with a dielectric sphere with a diameter about two centimeters, made of special ceramics, and “taught” it to redirect the incident electromagnetic waves in a desired manner. Moreover, the directionality of the scattering may be controlled and changed dramatically just by fine tuning of the frequency of the incident wave.
According to Tribelsky’s explanation, this sphere has rather narrow resonance lines related to its polarization oscillations. In a sense, it is quite analogous to a metal sphere, which has the resonance frequencies related to the oscillations of the free electron gas. Every line corresponds to the excitation of a particular oscillation mode, called harmonics or partial modes. Every harmonic is characterized by a fixed dependence between the scattering intensity and the scattering angle. This dependence is determined by the nature of a given harmonic. The sphere’s total scattering field is a sum of the contributions of every harmonic (partial wave). Partial waves interfere with each other. The narrow width of these lines allows to excite partial modes selectively and to control the interference. This, in turn, allows redirecting the incident radiation in a desired way. That’s it! The controlled manipulation with the radiation is achieved.
However, why do we speak about nanoscales if the sphere’s diameter is about two centimeters? That’s just the point. Professor Tribelsky says: “I can freely speak about the experimental beauty of this work, as I’m a theoretician. I just participated in the planning of the experiment, while the entire difficult experimental work was done by my French colleagues. As for the experimental beauty of this work, it is in the following: with the help of the microwave radiation — similar to the one used in a mini oven — we have managed to simulate on a centimeter scale all the processes occurred on a nanoscale with the visible light. It is widely known: if we have two objects of the same shape but of different sizes and with the same refractive index, they will scatter the electromagnetic waves in the same way, provided the ratio of objects’ linear dimensions to the wavelength is the same for both the objects. This was the idea of our experiments. However, the way from the idea to the results was very difficult. It’s sufficed to say that the researchers managed to separate the desired signal from background whose amplitude sometimes was 3000 times larger than that of the signal.”
Bearing in mind possible practical applications of the obtained results, it is important to stress that the fabrication technique of such nanospheres for manipulation of optical and near infrared radiation is rather cheap and simple. It does not require any “exotic,” expensive materials, and/or sophisticated equipment. Besides the optical computers (nowadays, still in the sphere of virtual reality), nanoscale spheres described in the paper by Tribelsky and co-authors can be used in the wide range of different fields: telecommunication systems; recording, processing and storing of information; diagnosis and treatment of different diseases including oncological, etcetera.
Citation: Michael I. Tribelsky, Jean-Michel Geffrin, Amelie Litman, Christelle Eyraud, Fernando Moreno. Small Dielectric Spheres with High Refractive Index as New Multifunctional Elements for Optical Devices. Scientific Reports, 2015; 5: 12288 DOI: 10.1038/srep12288