Pulsed Terahertz Radiation Research Articles

Temperature Dependence of the Sensitivity of PVDF Pyroelectric Sensors to THz Radiation: Towards Cryogenic Applications.

The application of terahertz (THz) science in industrial technology and scientific research requires efficient THz detectors. Such detectors should be able to operate under various external conditions and conform to existing geometric constraints in the required application. Pyroelectric THz detectors are among the best candidates. This is due to their versatility, outstanding performance, ease of fabrication, and robustness. In this paper, we propose a compact pyroelectric detector based on a bioriented poled polyvinylidene difluoride film coated with sputtered metal electrodes for in situ absorption measurement at cryogenic temperature. The detector design was optimized for the registration system of the electron paramagnetic resonance (EPR) endstation of the Novosibirsk Free Electron Laser facility. Measurements of the detector response to pulsed THz radiation at different temperatures and electrode materials showed that the response varies with both the temperature and the type of electrode material used. The maximum signal level corresponds to the temperature range of 10-40 K, in which the pyroelectric coefficient of the PVDF film also has a maximum value. Among the three coatings studied, namely indium tin oxide (ITO), Au, and Cu/Ni, the latter has the highest increase in sensitivity at low temperature. The possibility of using the detectors for in situ absorption measurement was exemplified using two typical molecular spin systems, which exhibited a transparency of 20-30% at 76.9 cm-1 and 5 K. Such measurements, carried out directly in the cryostat with the main recording system and sample fully configured, allow precise control of the THz radiation parameters at the EPR endstation.

Pulsed THz radiation under ultrafast optical discharge of vacuum photodiode

In this paper, we first present an experimental demonstration of terahertz radiation pulse generation with energy up to 5 pJ under the electron emission during ultrafast optical discharge of a vacuum photodiode. We use a femtosecond optical excitation of metallic copper photocathode for the generation of ultrashort electron bunch and up to 45 kV/cm external electric field for the photo-emitted electron acceleration. Measurements of terahertz pulses energy as a function of emitted charge density, incidence angle of optical radiation and applied electric field have been provided. Spectral and polarization characteristics of generated terahertz pulses have also been studied. The proposed semi-analytical model and simulations in COMSOL Multiphysics prove the experimental data and allow for the optimization of experimental conditions aimed at flexible control of radiation parameters.Graphical

High repetition rate coherent terahertz synchrotron radiation via self-amplified energy modulation

Coherent terahertz (THz) radiation sources based on a storage ring hold a significant future for generating high average power terahertz radiation, as the electron beam possesses a high revolution frequency. We propose a novel scheme to generate a high repetition rate coherent THz radiation leveraging the energy modulation self-amplification technology. In this scheme, utilizing the interaction of external intensity-modulated laser pulses with the electron beam can lead to a customized manipulation of the electron beam, such as a periodic longitudinal density modulation with a submillimeter scale for coherent THz radiation. Positively, it can propel the repetition rate when the peak power requirement of an external laser system is decreased. Thus, we employ a self-modulation method to enhance the initial weak energy modulation induced by the laser directly. In the meanwhile, the use of self-modulation can bypass the limitation that the bandwidth of a long modulator should be smaller than that of the external wideband laser source, which is necessary to shape the intensity envelope flexibly. The one-dimensional analytical description of electron longitudinal dynamic during the self-modulation process is given, which confirms the effective enhancement of the modulation at THz band. Three-dimensional time-dependent simulation results based on the parameters of Hefei light source-II show that the coherent THz radiation pulses with a repetition rate of 10 kHz can be generated. Published by the American Physical Society 2024

Interaction of Terahertz Radiation Pulse with a Plasma Layer in a Magnetic Field

Interaction of Terahertz Radiation Pulse with a Plasma Layer in a Magnetic Field

Terahertz Emission via Optical Rectification in a Metal-Free Perovskite Crystal.

We report on the emission of high-intensity pulsed terahertz radiation from the metal-free halide perovskite single crystal methyl-DABCO ammonium iodide (MDNI) under femtosecond illumination. The power and angular dependence of the THz output implicate optical rectification of the 800 nm pump as the mechanism of THz generation. Further characterization finds that, for certain crystal orientations, the angular dependence of THz emission is modulated by phonon resonances attributable to the motion of the methyl-DABCO moiety. At maximum, the THz emission spectrum of MDNI is free from significant phonon resonances, resulting in THz pulses with a temporal width of <900 fs and a peak-to-peak electric field strength of approximately 0.8 kV cm-1-2 orders of magnitude higher than any other reported halide perovskite emitters. Our results point toward metal-free perovskites as a promising new class of THz emitters that brings to bear many of the advantages enjoyed by other halide perovskite materials. In particular, the broad tunability of optoelectronic properties and ease of fabrication of perovskite materials opens up the possibility of further optimizing the THz emission properties within this material class.

Semiconductor Characterization by Terahertz Excitation Spectroscopy.

Surfaces of semiconducting materials excited by femtosecond laser pulses emit electromagnetic waves in the terahertz (THz) frequency range, which by definition is the 0.1-10 THz region. The nature of terahertz radiation pulses is, in the majority of cases, explained by the appearance of ultrafast photocurrents. THz pulse duration is comparable with the photocarrier momentum relaxation time, thus such hot-carrier effects as the velocity overshoot, ballistic carrier motion, and optical carrier alignment must be taken into consideration when explaining experimental observations of terahertz emission. Novel commercially available tools such as optical parametric amplifiers that are capable of generating femtosecond optical pulses within a wide spectral range allow performing new unique experiments. By exciting semiconductor surfaces with various photon energies, it is possible to look into the ultrafast processes taking place at different electron energy levels of the investigated materials. The experimental technique known as the THz excitation spectroscopy (TES) can be used as a contactless method to study the band structure and investigate the ultrafast processes of various technologically important materials. A recent decade of investigations with the THz excitation spectroscopy method is reviewed in this article. TES experiments performed on the common bulk A3B5 compounds such as the wide-gap GaAs, and narrow-gap InAs and InSb, as well as Ge, Te, GaSe and other bulk semiconductors are reviewed. Finally, the results obtained by this non-contact technique on low-dimensional materials such as ultrathin mono-elemental Bi films, InAs, InGaAs, and GaAs nanowires are also presented.

Active THz beam shaping using a one-dimensional array of photoconductive emitters

The spatial profile of a beam of pulsed terahertz (THz) radiation is controlled electrically using a multi-pixel photoconductive emitter, which consists of an array of interdigitated electrodes fabricated on semi-insulating GaAs. Activating individual pixels allows the transverse position of the THz beam's focus to be varied off-axis, as verified by spatial beam profiles. Enabling multiple pixels simultaneously permits non-Gaussian beam shapes to be created. The diffraction-limited performance of the system is established by comparison with the Abbé and Sparrow criteria, and a condition for effective beam steering using this design is derived. The spatial resolution of the approach is linked to the frequency of the THz radiation and the f-number of the collection optic.

Widely tunable electron bunch trains for the generation of high-power narrowband 1–10 THz radiation

Terahertz radiation enables the coherent excitation of many fundamental modes, such as molecular rotation, lattice vibration and spin precession. To excite the transient state of matter far out of equilibrium, high-power and tunable narrowband terahertz radiation sources have been in great demand for years. However, the terahertz source available at present cannot meet these tunability and power demands, leaving a large scientific gap that has yet to be fully explored. Here we convert the energy modulation induced by the nonlinear longitudinal space charge force to a density modulation and experimentally demonstrate the generation of electron bunch trains with modulation frequencies that are adjustable between 1 and 10 THz. The electron bunch trains can be directly used to produce tunable high-power narrowband terahertz radiation that fully covers the long-standing ‘terahertz gap’, which will open up many more possibilities in terahertz science. A tunable terahertz radiation pulse is demonstrated based on a linear accelerator. The emission frequency of this terahertz radiation is tunable from 1 to 10 THz by changing the bunching frequency of a 34 MeV electron beam. The pulse energy is at the submillijoule level.

Generation of THz radiation by wakefield under a wiggler magnetic field: An analytical study

In this article, the generation of terahertz (THz) radiation owing to wakefields produced by Gaussian-like laser pulse propagating throughout magnetized plasma has been investigated analytically under a wiggler magnetic field. Non-linear equations and analytical formulations governing the laser pulse THz radiation generation at a mildly relativistic regime have been derived. Besides, the total electric and magnetic wakefields inside and behind the laser pulse by employing the relativistic equations of motion, Maxwell’s equations, and perturbation technique in the attendance of a planar magnetostatic wiggler have been obtained. It is shown that the maximum transverse electric wakefield amplitude has an increasing trend with enchanting the wiggler frequency for inside and behind the laser pulse and the peak of the maximum transverse electric wakefield amplitude behind of laser pulse is higher compared to the inside of laser pulse for different values of the wiggler magnetic field. Furthermore, it is observed that electric wakefield amplitude has a fluctuating behavior inside and behind the laser pulse due to the existence of the planar magnetostatic wiggler. Also, it is shown that by using a wiggler magnetic field and components of perturbed velocities, we can generate THz waves. Moreover, it is found that we can achieve the needed values of the wiggler magnetic field for THz radiation generation for different values of the laser strength parameters. Indeed, we can control the generation of THz radiation throughout magnetized plasma by adjusting the wiggler magnetic field and laser pulse intensity.

Generation of Unipolar Pulses of Terahertz Radiation with a Large Electric Area

Generation of Unipolar Pulses of Terahertz Radiation with a Large Electric Area

Generation and control of localized terahertz fields in photoemitted electron plasmas.

Dense micron-sized electron plasmas, such as those generated upon irradiation of nanostructured metallic surfaces by intense femtosecond laser pulses, constitute a rich playground to study light-matter interactions, many-body phenomena, and out-of-equilibrium charge dynamics. Besides their fundamental interest, laser-induced plasmas hold great potential for the generation of localized terahertz radiation pulses. However, the underlying mechanisms ruling the formation and evolution of such plasmas are not yet well understood. Here, we develop a comprehensive microscopic theory to predictably describe the spatiotemporal dynamics of laser-pulse-induced plasmas. Through detailed analysis of electron emission, metal screening, and plasma cloud interactions, we investigate the spatial, temporal, and spectral characteristics of the so-generated terahertz fields, which can be extensively controlled through the metal morphology and the illumination conditions. We further describe the interaction with femtosecond electron beams to explain recent ultrafast electron microscopy experiments, whereby the position and temporal dependence of the observed electron acceleration permits assessing the associated terahertz field. Besides its potential application to the design of low-frequency light sources, our work contributes fundamental insight into the generation and dynamics of micron-scale electron plasmas and their interaction with ultrafast electron pulses.

Terahertz Electromodulation Spectroscopy for Characterizing Electronic Transport in Organic Semiconductor Thin Films

Terahertz (THz) spectroscopy is a well-established tool for measuring the high-frequency conductance of inorganic semiconductors. Its application to organic semiconductors, however, is challenging, because of the low carrier mobilities in organic materials, which rarely exceed 10cm2/Vs. Furthermore, low charge carrier densities in organic field-effect devices lead to sheet conductivities that are often far-below the detection limits of conventional THz techniques. In this contribution, we present the application of THz electromodulation spectroscopy for characterizing charge transport in organic semiconductors. Pulses of THz radiation are transmitted through organic field-effect devices and are time-resolved by electro-optic sampling. A differential transmission signal is obtained by modulating the gate voltage of the devices. This controls charge injection into the semiconductors, where the charge carriers reduce the THz transmission by their Drude response. Advantageous is that a nearly noise-free differential transmission can be obtained. Furthermore, electromodulation allows to sense specifically either injected electrons or holes. Because the method exclusively probes transport of mobile carriers, it provides access to fundamental transport properties, which are difficult to access with conventional characterization methods, such as conductance measurements of organic field-effect transistors. The outstanding property that a relative differential signal is measured allows to obtain charge carrier mobilities with high reliability. Mobilities as small as 1cm2/Vs can be probed, which makes THz electromodulation spectroscopy an attractive tool for studying charge transport in most technologically relevant organic semiconductors.

Electrooptical Effect in Silicon Induced by a Terahertz Radiation Pulse

Electrooptical Effect in Silicon Induced by a Terahertz Radiation Pulse

Terahertz radiation generation in doped semiconductor interacting with femtosecond pulse in constant magnetic field

The generation of terahertz (THz) radiation in a doped semiconductor with a sufficiently large bandgap interacting with a femtosecond pulse of IR range has been investigated. Generation occurs due to the simultaneous effect on the conduction electrons of a constant magnetic field and time-dependent ponderomotive force. The spectral composition, total energy and shape of the THz radiation pulse are found.

Morphological Changes of Melanoma Cells Induced by Pulsed Terahertz Radiation

Morphological Changes of Melanoma Cells Induced by Pulsed Terahertz Radiation

Direct bandgap dependence of bismuth films on their thickness

Thin bismuth films of various thicknesses between 5 and 32 nm grown by molecular beam epitaxy on Si (111) substrates were investigated. The samples were characterized by the x-ray diffraction method, which allowed us to identify two types of Bi crystallographic structures—α and β bismuth. Terahertz radiation pulses emitted from the samples after their illumination by femtosecond optical pulses with different wavelengths were characterized. The main THz emission features were similar for both types of Bi layers. Due to 2D confinement, the electron energy band structure depends on the thickness. With the terahertz excitation spectroscopy method, direct bandgaps were determined to be in the range from 0.25 to 0.5 eV—much greater than the indirect bandgaps of the layers. A simple model was used to describe the nature of THz emission from these films, which is the cause of uncompensated lateral photocurrents occurring because of the diffusive electron scattering at the Bi/Si interface.

Optical conductivity of an electron gas driven by a pulsed terahertz radiation field

Optical conductivity of an electron gas driven by a pulsed terahertz radiation field

Inertia of the oscillatory mechanisms of giant nonlinearities of optical materials in the terahertz spectral range

Subject of study. The inertia of the oscillatory mechanisms of the nonlinearities of isotropic dielectric media in the field of the terahertz-frequency electromagnetic waves was investigated for resonant and nonresonant interactions between radiation and matter. The purpose of this work was to construct a dynamic model of the nonlinear polarization responses of optical media with oscillatory nature in the field of terahertz pulses and to estimate the time constants characterizing the inertia of such responses during resonant and nonresonant interactions between radiation and media molecular vibrations. Method. The model of anharmonic vibrations of the atoms of each molecule as an oscillator in the general case for an isotropic medium, with both quadratic and cubic nonlinearities, was reduced for a macroscopic optical characteristic—its polarization—to a model as a system of parametrically coupled equations with only cubic nonlinearities. The system parameters were determined from well-known characteristics of a medium, such as its thermal expansion coefficient, stretching molecular vibration frequency, and refractive index. Main results. Expressions were obtained for the inertial time constants of the cubic susceptibilities of optical media with nonlinear vibrations in two- and one-photon resonant interactions with quasi-monochromatic terahertz pulses as well as in nonresonant interactions with broadband terahertz pulsed radiation through the thermal, spectral, and optical characteristics of materials known in the literature. Numerical estimates were obtained for the inertial time constants of the nonlinear susceptibilities of media with particularly high vibrational nonlinearities in the refractive indices, namely, α-pinene and water, as well as silicon dioxide. For these materials, the inertial time constants of the resonant oscillatory mechanisms of the nonlinearities for radiation in the terahertz spectral range were shown to be of the order of hundreds of femtoseconds; for the nonresonant interactions, the time constants decreased to ten femtoseconds or less. Practical significance. The obtained inertial time constant estimations of the polarization responses of materials indicate that their giant nonlinearities in the far infrared spectral range could be used to develop ultrafast photonic devices for pulsed terahertz radiation parameter control.

Self-Focusing of Pulsed Terahertz Radiation with a Spectrum in the Region of Anomalous Group-Velocity Dispersion of a Dielectric Medium

Self-Focusing of Pulsed Terahertz Radiation with a Spectrum in the Region of Anomalous Group-Velocity Dispersion of a Dielectric Medium

Highly efficient generation of narrowband terahertz radiation driven by a two-spectral-line laser in PPLN.

We demonstrate record ∼0.9% efficiencies for optical conversion to narrowband (<1% relative bandwidth) terahertz (THz) radiation by strongly cascaded difference frequency generation. These results are achieved using a novel, to the best of our knowledge, laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (≥250 ps). THz radiation generation in 5% MgO-doped periodically poled lithium niobate (PPLN) crystals of varying poling period was explored at cryogenic and room temperature operation as well as with different crystal lengths. This work addresses an increasing demand for high-field THz radiation pulses which has, up to now, been largely limited by low optical-to-THz radiation conversion efficiencies.