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Posted: May 04, 2016

Silicon nanoparticles pave the way towards nanoscale light emitters

(Nanowerk News) Scientists from Moscow Institute of Physics and Technology (MIPT), ITMO University (St Petersburg), and their colleagues from the Australian National University have experimentally demonstrated that silicon nanoparticles are able to significantly increase the intensity of what is known as the Raman effect. These findings will help to develop nanoscale light emitters and nanoscale amplifiers for fibre optic telecommunications lines. The results of the study have been published in Nanoscale ("Resonant Raman scattering from silicon nanoparticles enhanced by magnetic response").
Usually, when light interacts with matter, it does not change colour, i.e., the wavelength of the light remains the same. There are exceptions however, and one of them is the so-called Raman effect. In this case, incident light interacts with a molecule in such a way that energy of the molecule increases by a value corresponding to the vibrational motion of the molecule. The molecule then re-emits a photon which has smaller energy and consequently a longer wavelength, meaning that the light becomes “redder”. This process may also occur in bulk crystals.
Diagram of the Raman effect
Figure 1. Diagram of the Raman effect. The incident photon excites the vibrational level of the molecule (marked in red), which causes the molecule to emit the photon at a different wavelength. Image courtesy of the authors of the study.
The discovery of the Raman effect indicated emergence of a whole new field of applied science – Raman spectroscopy. This method allows to detect even single molecules of chemical substances. In addition, the Raman effect is widely used today in fibre optic networks for signal amplification.
Until now, waveguides and spherical microcavities, whose size is larger than the emission wavelength, have been mainly employed for the Raman effect enhancement. However,the pursuit for miniaturization of telecommunication devices requires development of smaller optical components – they consume less energy and are easier to “pack” on an electronic or optical chip.
The group of scientists, including Denis Baranov from MIPT (a postgraduate student of the Problems of Physics and Power Engineering Department) looked for ways of miniaturizing Raman amplifiers.
A schematic view of resonant Raman scattering by a nanoparticle
Figure 2. A schematic view of resonant Raman scattering by a nanoparticle. The incident radiation excites the resonance of the particle – magnetic dipole mode shown by the blue arrow. The electric field of the magnetic mode interacts with the crystal lattice vibrations in the resonant silicon nanoparticle, which causes a change in the wavelength of the scattered light. Image courtesy of the authors of the study.
The researchers used silicon nanospheres which support optical resonances - the so-called Mie resonances. They exist in any spherical particles and the resonant wavelengths of these resonances depend on the particle size. One of the resonances which occurs for the largest wavelength is the magnetic dipole resonance – its wavelength is generally comparable to the diameter of the particle. In silicon, however, due to its large refractive index, magnetic dipole resonance is observed in the optical range (at wavelengths longer 300 nanometres) for nanoparticles with a diameter of approximately 100 nanometres.
This fact allows to use tiny silicon nanoparticles as a miniature element to enhance various optical phenomena, including spontaneous light emission, enhanced light absorption, and high harmonic generation.
Dark-field image of an array of silicon particles of different diameters
Figure 3. Dark-field image of an array of silicon particles of different diameters used in the experiment. Inset: an image of an individual particle as seen under an electron microscope. Image courtesy of the authors of the study.
In the experiment, the scientists studied the behaviour of silicon nanoparticles of different sizes. In order to determine the size of the particles, they placed them under a microscope and illuminated them with white light. Particles of different diameters demonstrate Mie resonances at different wavelengths resulting in different glowing colours in the dark-field image.
The scientists then tested how the intensity of the Raman emission depends on the diameter of a silicon particle. The intensity of the Raman emission was at a maximum at the resonant diameter of the particle, which was entirely consistent with the theory the authors had developed. The intensity of Raman emission of resonant particles was more than 100 times greater than that of non-resonant particles with other diameters.
experimentally measured (points) and theoretically predicted (lines) Raman emission enhancement spectrum for particles of different diameters
Figure 4. The experimentally measured (points) and theoretically predicted (lines) Raman emission enhancement spectrum for particles of different diameters. The maximum point corresponds to the excitation of magnetic dipole resonance of a silicon nanoparticle. Inset: the electric field distribution inside a resonant particle. Image courtesy of the authors of the study.
“The Raman effect is incredibly useful in practice, and will help not only in detecting microscopic amounts of chemical compounds, but also in transmitting information over long distances. Because of the pursuit for smaller electronic and optical devices, it is becoming increasingly important for us to look for nanostructures that are able to enhance this effect. Our observations have revealed a potential candidate – silicon nanoparticles,” said Denis Baranov, a post-graduate student of MIPT and one of the authors of the paper, when commenting on the results.
Silicon nanoparticles could serve as a basis for the development of miniature optical amplifiers for fibre optic networks. In the future, these particles could provide a platform for building a compact nanolaser using the stimulated Raman scattering, which offers prospects for very interesting applications in medicine and biomicroscopy. In particular, detecting signals of the Raman emission from particles in the human body will allow specialists to track the movement of drug molecules.
Source: Moscow Institute of Physics and Technology
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