Terahertz (THz) radiation lies in the frequency range between the infrared and microwaves, i.e. between 100 GHz and 10 THz. These waves are able to excite soft polar phonons in solids, vibrations of larger chains in biomolecules, to induce plasma oscillations of free charge carriers with concentrations of ~1014–1018 cm-3 and to interact with carriers localized in nanoparticles. Due to the lack of bright sources and sensitive detectors the THz spectroscopic applications about 20 years ago concerned mainly the astronomy and analytical science. Recent technological innovations in photonics and nanotechnology have lead to a dramatic increase in the interest of the scientific and industrial community in the THz research and applications.
We use an optoelectronic approach to the generation and detection of broadband THz pulses which makes use of ultrashort optical pulses and of their frequency conversion into the THz range. This technique is called the time domain THz spectroscopy and it is able to measure complex dielectric and conductivity spectra of various kinds of samples in a spectral range of 5 to 80 cm-1. In addition, the use of laser pulses for the THz generation makes it possible to perform so called pump–probe experiments where the sample is first excited by an optical (UV, VIS, IR) pulse and, subsequently, it is probed by a delayed THz pulse. This technique allows us to access the far-infrared fingerprints of the ultrafast dynamics on sub-picosecond to nanosecond time scales.
(More information on the website of the THz group.)
Nanostructured and organic semiconductors represent a new generation of prospective materials for solar cell fabrication. The efficiency of solar cells crucially depends on the speed of long-range charge carrier transport and the transport nature constitutes the key knowledge for its improvements. Here we deal with processes undergoing on a nanosecond time scale and for their study we use time-resolved terahertz spectroscopy and numerical Monte-Carlo simulations. The simultaneous use of experimental and theoretical techniques allowed us to elucidate the transport mechanisms in a number of complex materials.
We described the connection of the terahertz spectra with the inter- and intra-nanoparticle transport processes in thin films made of ZnO and TiO2 nanoparticles [H. Němec et al., Phys. Rev. B 79, 115309 (2009)]. Subsequently, these findings allowed us to determine the role of small and large grains in the carrier transport in microcrystalline silicon [L. Fekete et al., Phys. Rev. B 79, 115306 (2009)]. We also studied a blend of polymer and electron acceptor with the conclusion that the motion of holes along polymer chains is significantly reduced by potential barriers which may be connected to the torsional disorder of the chains [H. Němec et al., Phys. Rev. B 79, 245326 (2009)].
Fig. 1: Scheme of the principle of the charge carrier transport in a blend of polymer LBPP1 and a fulleren acceptor. We show at the top the model of potential barriers used for the calculations of the charge transport between individual segments of the polymer; at the bottom we plot a dramatic decrease of the conductivity due to the charge localization between potential barriers observed at sub-picosecond time scale.
Metamaterials are artificially created composite periodic structures with a unit cell much smaller than the targeted wavelength of the radiation. These materials may exhibit electromagnetic properties not found in nature. It appear that by using a suitable combination of composite constituents it is possible to conceive for example an “invisibility cloak” or plates with a negative refractive index allowing one to overcome the diffraction limit in the optical imaging. However, these properties can be used in a narrow spectral range only restricted by the width of very sharp magnetic resonances (in permeability). For this reason we proposed and experimentally realized a metamaterial with a tunable range of negative effective permeability in the terahertz spectral range (0.2 – 0.36 THz) [H. Němec et al., Phys. Rev. B 79, 241108 (2009)].
This structure consists of an array of nonmagnetic rods made of an incipient ferroelectric SrTiO3 which shows a high tunable permittivity. The magnetic response and its tuning are achieved by a temperature control of the permittivity of SrTiO3, which defines the resonant confinement of the electromagnetic field within the rods (so called Mie resonances). The spectral positions of resonances depend on the geometrical parameters of the rods and on their tunable permittivity. The electromagnetic coupling between the adjacent rods is negligible. With a suitable aspect ratio of the rods, a broadband magnetic response can be obtained [R. Yahiaoui et al., Opt. Lett. 34, 3451 (2009)]. (více...)
Fig. 2: (a) Scanning electron microscope image of the metamaterial. (b) Transmittance spectra of the metamaterial for various temperatures. The dip corresponds to the lowest Mie resonance which is associated with strong effective magnetic behavior. (c) Effective magnetic permeability (Re μ and Im μ) of the metamaterial.
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