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)
Metamaterials are artificial resonant composite structures formed by common materials; however, the sizes of the resonators and the distances among them are much smaller than the targeted wavelength of the radiation. A proper choice of materials and of their arrangement can induce an unusual electromagnetic behavior. In this way, it is possible to conceive e.g. a medium with a negative refractive index (i.e. with simultaneously negative dielectric permittivity and magnetic permeability) allowing one to overcome the diffraction limit in the optical imaging.
Although crystals of TiO2 (rutile) do not exhibit magnetic properties, it is possible to use rutile in a suitable geometrical configuration to create a magnetic response. We proposed and studied a metamaterial which can exhibit a negative effective permeability in the terahertz spectral range. Its preparation is based on spray-drying of a suspension of TiO2 nanoparticles which then self-assemble into microspheres. This metamaterial shows a magnetic resonance near 1 THz. We also developed a new method of measuring unambiguously such a magnetic response [H. Němec et al., Appl. Phys. Lett. 100, 061117 (2012)]
Left: Scanning electron microscope images of the fabricated TiO2 microspheres before the sorting procedure. Sorting procedure allows one to obtain a narrower distribution of microsphere diameters d. Right: effective magnetic response (real and imaginary part of the effective permeability) of samples with a 10% volume filling fraction of TiO2 microspheres and their sizes d = 45 ± 4 μm and 39 ± 3 μm. Symbols: experiment, lines: results of electromagnetic simulations.
Dye-sensitized nanostructured semiconductors represent a new generation of prospective materials for solar cell fabrication. Their operation relies on a cascade of complex physical processes. The efficiency of solar cells crucially depends on the speed of long-range charge carrier transport and the character of this transport constitutes the key knowledge for its improvements. Here we are interested in processes occurring on the sub-nanosecond time scale which include namely the electron injection into the semiconductor and the initial phase of the electron transport towards the anode (see Fig.). We use time-resolved terahertz spectroscopy as a contact-free probe of ultrafast carrier transport complemented with numerical simulations.
We investigated carrier injection and subsequent transport in dye-sensitized nanostructured ZnO and TiO2. The generally accepted picture of the photoconductivity of these systems was that mobile electrons appear in the semiconductor conduction band in concert with their injection from the dye. Our results show that charge injection and formation of mobile charges are not necessarily connected, and that charge transport in the sensitized solar cell material can differ from that in bulk or nanocrystalline nonsensitized semiconductors. For ZnO an electron-cation complex is formed within 5 ps which causes fast charge recombination. Moreover, the electron mobility is significantly decreased even after the dissociation of the complex (100 ps) due to strong electrostatic interaction between injected electrons and dye cations. In contrast, sensitized TiO2 nanocrystals does not suffer from this problem due to their high permittivity efficiently screening the charges. We believe that the described processes are responsible for the different power conversion efficiencies of TiO2 and ZnO-based Grätzel cells. [H. Němec et al., Phys. Rev. Lett. 104, 197401 (2010)]. (more...)
Left panel. Scheme of a Grätzel photovoltaic cell. Incident radiation first excites dye molecules. Subsequently, the electron (e−) is injected into a semiconductor nanoparticle and it is transported to the anode. The oxidized dye cation (D+) is reduced by redox electrolyte. Right panel. In TiO2, an electron is injected to the semiconductor nanoparticle in less then 1 ps after photoexcitation. After injection, the electron is free to diffuse through the nanoparticle network to the electrode. In contrast, injection into ZnO occurs via an intermediate electron-cation complex in which the electron and cation are strongly bound to each other. This state is formed within 5 ps and it breaks within 100 ps. After that, the electron is released, but it remains weakly attracted by the cation which makes its transport to the electrode much slower.
Theoretically predicted ferroelectric phase induced by tensile stress was confirmed in SrTiO3 epitaxial films and SrTiO3/DyScO3 heterostructures on DyScO3 substrates, prepared in top world laboratories. It occurs near 270 K and it is drive by a soft phonon mode in the THz range which couples to a low-frequency overdamped excitation (at ~ 10 cm–1) in the whole measured temperature range of 20-300 K. The soft mode is strongly tunable by the electric field which results in a tunable permittivity of such heterostructures up to the THz range, attractive for applications. We proposed a general model of the soft-mode behavior in strained SrTiO3 films applicable in a wide range of temperatures and applied fields.
For details see:
C. Kadlec et al., J. Phys.: Condens. Matter 21, 115902 (2009),
C. Kadlec et al., Phys. Rev. B 80, 174116 (2009),
D. Nuzhnyy et al., Appl. Phys. Lett. 95, 232902 (2010),
V. Skoromets et al., J. Appl. Phys. 107, 124116 (2010).
Temperature dependence of polar mode frequencies and high-frequency permittivity in SrTiO3 thin films on DyScO3 substrate as evaluated from the IR reflection and THz transmission spectra and from microwave resonance measurements. The film thicknesses are listed in the figure.
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)].
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)]. (more...)
(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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