Filip Kadlec

Ultrafast opto-terahertz photonic crystal modulator

We present an agile optically controlled switch or modulator of terahertz (THz) radiation. The element is based on a one-dimensional photonic crystal with a GaAs wafer inserted in the middle as a defect layer. The THz electric field is enhanced in the photonic structure at the surfaces of the GaAs wafer. Excitation of the front GaAs surface by ultrashort 810 nm laser pulses then leads to an efficient modulation of the THz beam even at low photocarrier concentrations (approximately 10(16) cm(-3)). The response time of the element to pulsed photoexcitation is about 130 ps.

Phase-sensitive time-domain terahertz reflection spectroscopy

An approach to time-domain terahertz reflection spectroscopy is proposed and demonstrated. It allows one to obtain very accurately the relative phase of a reflected THz wave form, and consequently the complex dielectric function can be precisely extracted. The relevant setup was demonstrated to allow measurements of a variety of samples: we present results for doped silicon and for ferroelectric SrBi2Ta2O9 (bulk ceramics as well as thin film on sapphire substrates).

Optical rectification at metal surfaces

We report on experimental observation of freely propagating pulses of terahertz (THz) radiation produced by optical rectification of femtosecond pulses at metal surfaces. This opens a qualitatively new way of investigation of nonlinear phenomena at metal surfaces and can be also exploited in the future for development of new THz emitters.

Gouy shift correction for highly accurate refractive index retrieval in time-domain terahertz spectroscopy.

Terahertz spectroscopic measurements are usually performed in focused beam geometry while the standard routine for the retrieval of the sample refractive index assumes plane-wave approximation. In this paper we propose a model for the transmission function which accounts for spatially limited Gaussian terahertz beams. We demonstrate experimentally its validity and applicability for an accurate extraction of the refractive index from experimental data.

A metal-dielectric antenna for terahertz near-field imaging

We present an antenna-based approach to near-field imaging and spectroscopy, which can be used for both continuous-wave and pulsed broadband electromagnetic radiations from microwave to terahertz frequencies. Our near-field antenna consists of a rectangular-shaped block of low-loss dielectric material sharpened to a pyramidal tip which is partially metallized and terminated by a micron-sized plane facet. At this facet the entire energy of the incident wave is concentrated as a very high but strongly localized electric field, which can be used as a sensitive near-field microprobe for electromagnetic radiation. Currently, experiments in reflection geometry with pulsed terahertz radiation and continuous-wave radiation near 80GHz reveal a frequency-independent spatial resolution of about 20μm corresponding to λ∕200 at 80GHz, which is only limited by the size of the facet terminating the tip.

Study of terahertz radiation generated by optical rectification on thin gold films

Emission of terahertz (THz) radiation as a result of optical rectification of intense femtosecond laser pulses on thin gold films has been studied by time-domain THz spectroscopy. The THz amplitude was measured as a function of film thickness and incidence angle. The experiments reveal that the emitted THz field is suppressed for a thickness below 100 nm, which gives evidence of the nonlocal character of the response. The variation of incidence angle allows us to estimate the components of susceptibility tensor chi2ijk. For thicker films and near grazing incidence, the emitted THz field attains a peak value of 4 kV/cm.

Dielectric tunability of SrTiO3 thin films in the terahertz range

We have developed an interdigited electrode structure for applying a static electric field to a ferroelectric thin film, which enables nearly full transmission of a perpendicularly polarized terahertz wave. This approach has been used to determine the electric field dependence of the complex permittivity of a SrTiO3 thin film on a sapphire substrate up to about 2THz employing time-domain terahertz spectroscopy. We have demonstrated up to 10% variation of the film permittivity at 300GHz at room temperature induced by an applied electric field of 100kV∕cm. No dielectric dispersion is observed between 1MHz and about 500GHz. The field-induced changes are attributed to soft mode hardening.

Broadband dielectric response and grain-size effect in K0.5Na0.5NbO3 ceramics

Dielectric spectra of two K0.5Na0.5NbO3 ceramics with different grain sizes (10 and 0.5 μm) were measured from 102 to 1014 Hz in a broad temperature range. The sequence of first-order phase transitions (cubic-tetragonal-orthorhombic-rhombohedral) was detected by differential scanning calorimetry, dielectric spectroscopy, and time-domain terahertz spectroscopy. The grain size affects all the phase transitions, which are more smeared in the small-grain sample. In the large-grain ceramics, two well-separated near-Debye relaxations are seen in the tetragonal phase, which suddenly merge on cooling across the tetragonal-orthorhombic transition, and on further cooling the lower-frequency relaxation strongly broadens. On reducing the grain size, the higher-frequency relaxation shifts from ∼1 to ∼20 GHz and the lower-frequency one strongly broadens. Without quantitative understanding, these effects could be assigned to domain-wall dynamics and its temperature and grain-size dependences. Similar to pure KNbO3, an o...

Propagation of terahertz pulses in photoexcited media: Analytical theory for layered systems

Optical pump-terahertz probe spectroscopy has become a widely used experimental tool for the investigation of the ultrafast far-infrared response of polar systems. In this paper the authors present an analytical method of calculating the propagation of ultrashort terahertz pulses in photoexcited media. The transient terahertz wave form transmitted through the sample is equal to a product of the incident terahertz field (at a mixed frequency), transient susceptibility, and a so called transfer function which depends on the properties of the sample in equilibrium. The form of the transfer function is derived for general layered systems and for specific cases including one-dimensional photonic crystals, thin films, and bulk samples. Simplified expressions directly applicable to the analysis of the experimental results related to the most common sample geometries are shown and discussed.

Highly tunable SrTiO3∕DyScO3 heterostructures for applications in the terahertz range

The terahertz dielectric properties of several heterostructures based on epitaxial SrTiO3 and DyScO3 thin films were characterized without and under applied static or low-frequency electric field. We demonstrate up to 65% variation of the permittivity of SrTiO3 films and up to 35 % modulation of the power transmission of terahertz waves at 500 GHz and 112 V (75 kV/cm) bias.