Frequency of the radiation of 1 terahertz (1 THz) refers to an electromagnetic wave with 1012 cycles per second, which corresponds to a vacuum wavelength of 0.3 mm. The frequency range around 1 THz (roughly 0.1–3 THz) was frequently referred to as the “terahertz gap” dividing the electromagnetic spectrum to the world of electronics at lower frequencies and to the world of optics at higher frequencies (see Fig. 1). Indeed, the THz spectral range was difficult to access using both conventional optical methods and classical electronic transistor-based devices due to the absence of powerful sources. A breakthrough came with the development of ultrafast lasers delivering femtosecond optical pulses (i.e. typically in the visible or near infrared range, see Fig. 1). The so-called optoelectronic approach to the generation and detection of broadband THz pulses makes use of a coherent frequency conversion of optical pulses into the THz range. This technique, born at the beginning of 1990's, is called the time-domain THz spectroscopy. The ultrashort laser pulses are converted in an emitter (which may be e.g. a non-linear crystal or a photoconductive antenna) into picosecond broadband pulses of terahertz radiation. The coherent gated detection process provides access to the time profile of the THz electric field (so-called THz waveform); consequently, it is a phase-sensitive detection. Recent technological innovations in photonics and nanotechnology have led to a dramatic increase in the interest of the scientific and industrial community in the THz research and applications.
In our research we use and develop time-domain THz spectroscopy applied to various materials. The technique is able to measure complex dielectric, magnetic and/or conductivity spectra of samples in a spectral range of typically 0.1–3 THz. The pulsed nature of the technique makes it possible to carry out optical pump – THz-probe experiments, where the sample is first excited by an optical (UV, VIS, IR) pulse and, subsequently, probed by a delayed THz pulse. These measurements allow us to obtain far-infrared fingerprints of the ultrafast dynamics on sub-picosecond to nanosecond time scales.
It is possible to investigate a broad range of physical systems and phenomena. The THz waves excite soft (i.e. low-frequency) polar phonons in solids, vibrations of larger chains in biomolecules, induce plasma oscillations of free charge carriers with concentrations of ~1014 – 1018 cm−3 and interact with carriers localized in nanoparticles.
Our investigations focus on four major subjects:
- We study structural phase transitions in ferroelectric and related materials and use their nonlinear properties close to phase transitions to electric-field tunable applications.
- We investigate time-resolved photoconductivity and charge carrier transport in ultrafast semiconductors and semiconductor nanostructures, which have a huge impact in opto-electronic applications.
- We devote a considerable effort to the understanding of magneto-electric coupling in materials showing both electric and magnetic ordering (so called multi-ferroic materials); these materials are promising for the development of electrically controlled magnetic memories.
- We are interested in photonic and artificial resonant structures (metamaterials) for the THz spectral range.
More information can be obtained here.