Generation Of Terahertz Radiation Research Articles

Generation of Intense Few-Cycle Terahertz Radiation in Organic Crystals Pumped by 1.24-μm Multigigawatt Chirped Laser Pulses

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

Generation of terahertz radiation by a Hermite–Gaussian laser beam inside magnetoplasma with a density ramp

In the present scheme of work, the Hermite–Gaussian (HG) laser beam dynamics has been investigated under the influence of an upward density ramp inside magnetized plasma, where both relativistic and ponderomotive nonlinearities are operative. One can achieve self-focusing of laser beam due to the change in the medium's dielectric function, which comes into operation due to the expulsion of plasma electrons from the high intensity to the low-intensity region by ponderomotive force and their motion at relativistic speeds. The dynamics of the laser beam and terahertz generation have been investigated by using the moment theory approach. It has been observed from the present analysis that the dynamics of the laser beam and the production of terahertz radiations strongly depends upon the HG laser beam and plasma parameters. In addition to this, the effect of density ramp and magnetic field has also been investigated on the efficiency of terahertz generation. It has been observed that higher-order modes of the HG laser beam play a dominant role in the production of terahertz radiations.

Terahertz radiation generation driven by beating of chirped laser pulses in single-walled carbon nanotubes by applying tapered magnetic field

Terahertz radiation generation driven by beating of chirped laser pulses in single-walled carbon nanotubes by applying tapered magnetic field

Nonlinear Photonic Waveguides: A Versatile Platform for Terahertz Radiation Generation (a Review)

AbstractContinued development in the areas of communication, security, medicine, and safety is calling for terahertz‐wave technologies to evolve beyond laboratory research and into the realm of real‐world applications. Due to the compatibility of waveguides with on‐chip fabrication techniques and their ability to realize a small footprint, such structures provide a unique opportunity to achieve terahertz radiation generation in an on‐chip arrangement, ideal for practical applications requiring terahertz electric fields. This review considers waveguiding arrangements that produce terahertz radiation using the nonlinear frequency‐conversion techniques of optical rectification and difference frequency generation. The investigated terahertz radiation waveguiding sources are categorized into three different classes: those that confine and guide both the excitation electric fields and the generated terahertz radiation, those that confine and guide the excitation electric fields but not the generated terahertz radiation, and those that confine and guide the generated terahertz radiation but not the excitation electric fields. Various types of waveguides are surveyed from each category, including dielectric waveguides, metallic waveguides, photonic crystal waveguides, poled waveguides, and multiple waveguides embedded within one another. This review provides a detailed overview of state‐of‐the‐art terahertz radiation waveguide sources, with the intent of aiding in the continuing development of such sources.

Terahertz Radiation Generation by Hermite-cosh Gaussian and Hollow Gaussian Laser Beams in Magnetized Inhomogeneous Plasmas

Terahertz Radiation Generation by Hermite-cosh Gaussian and Hollow Gaussian Laser Beams in Magnetized Inhomogeneous Plasmas

Generation of High-Power Terahertz Waves in a Collisional and Magnetized Plasma by Two-Color Femtosecond Laser Beams

Terahertz (THz) radiation generation based on a nonlinear induced electron current density in the interaction of two-color femtosecond laser beams with collisional and magnetized plasma is investigated. The nonlinear electron current density that is responsible for THz radiation includes a share from nonlinear ponderomotive force and a part from relativistic term. In this work, relativistic effects along with the effects of variation of interacting plasma density, external magnetic field, and electron collision frequency on THz wave generation have been considered. The reduction of electron collision frequency along with rising in the magnitude of the external magnetic field generates more power without any deformation of radiation patterns. The angular distribution showing the radiation pattern of THz radiation in the forward direction is obtained and the effect of laser pulse second harmonic and variation of electric field on the directivity of generated waves is examined. The change in two-color laser beam intensities both in relativistic and nonrelativistic regimes leads to the generation of THz wave electric fields with different orders of magnitude. The numerical analysis indicated that the utilization of laser pulse second harmonic in a two-color scheme enhances the THz electric field leading to more intense radiation compared to single-color mechanism. The model also shows the effect of external magnetic field and electron collision frequency on the radiation efficiency of THz generation for linear and circular polarizations of two-color laser pulses.

Picosecond pulses with 40 W peak power from a passively mode‐locked tapered quantum well laser

In this letter, the authors present a monolithic edge emitting diode laser intended as a potential key component for the generation of terahertz radiation in a compact time-domain spectroscopy system. The 6 mm long multi-section InGaAsP double quantum well (DQW) laser, featured with a 3.6 mm long tapered (TP) gain section, operates at a wavelength of ∼832 nm and generates pulses at a fundamental repetition rate of ∼6.3 GHz in passive mode-locking operation. Pulses with 2.6 ps width and 40 W peak power are measured. The generated pulses are not bandwidth-limited and should be compressible for further applications.

Emission and sensing of high-frequency terahertz electric fields using a GaSe crystal.

A GaSe crystal cut along the (001) crystallographic plane is investigated for the emission and detection of high-frequency (i.e. up to ∼20 THz) electric fields. To date, a comprehensive analysis on high-frequency difference frequency generation and electro-optic sensing in GaSe has not been performed and should consider aspects such as electric field polarization orientation, symmetries inherent to the crystal structure, and the various possible generation and detection phase-matching arrangements. Herein, terahertz radiation generation is investigated for various excitation electric field polarizations as the GaSe crystal is rotated in the (001) plane. Subsequently, the crystal is rotated out-of-plane to investigate the difference frequency generation and electro-optic sampling phase-matching conditions for various arrangements. The measured terahertz radiation spectra show peak generation at the frequencies of 10, 16, and 18 THz (dependent on the GaSe crystal orientation), in agreement with the frequencies exhibiting perfect phase-matching. GaSe has the potential to emerge as the primary crystal for the emission and detection of high-frequency electric fields, such that this comprehensive analysis is necessary for the widespread adoption and practical implementation of GaSe as a high-frequency source crystal.

Nd3+-Doped La2CaB8O16 Crystals for Orthogonally Polarized Dual-Wavelength Near-Infrared Lasers

The Nd3+-based laser crystal is one of the key gain media for all-solid-state laser technology. Herein, by investigating the relationship between structure and optical properties, we found that a La-containing borate, La2CaB8O16, with a long rare-earth ion distance more than 4.27 Å, allows a high doping concentration and exhibits a weak fluorescence quenching effect. For the first time, Nd3+-doped La2CaB8O16 crystals with different concentrations were grown by the top seeded solution growth method with large size and high optical quality. Its spectral characters were measured, suggesting a strong absorption band at 800 nm and a fluorescence band at 1055 nm. Furthermore, a diode-pumped orthogonally polarized dual-wavelength laser operation at 1052 and 1055 nm was realized, with a slope efficiency of 16.3% and output power of 859 mW. These results suggest that the Nd3+-doped La2CaB8O16 crystal is a promising laser gain medium for 1 μm lasers and possible generation of terahertz radiation.

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 high repetition rate THz radiation at the mill-watt-level via optical rectification in an enhancement cavity

We propose an experimental method for the generation of coherent terahertz radiation in the spec-tral region between 0.2 THz and 2 THz, with a high repetition rate of nearly 100 MHz, and with an average power at the milliwatt level. An Ytterbium-doped mode locking laser is amplified to 60 W, and pulses are stacked into an optical cavity up to 750 W. There, they interact with a Gallium Phosphide crystal producing THz radiation via optical rectification. With the cavity enhanced configuration, we show that more than one order of magnitude can be gained with respect to simply focalize the 60 W beam into the GaP crystal.

Excitation of terahertz radiation in p-n-heterostructures based on a-Si:H/c-Si

Studies on the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H/c-Si upon their photoexcitation by a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of observed THz radiation allow to explain its nature by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits THz electromagnetic waves. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in the structures. The intensity of THz radiation observed in the studied p-n-heterostructures based on a-Si:H/c-Si is comparable to that generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H/c-Si p-n-heterostructures can be used as THz emitters for need of THz spectroscopy. Keywords: femtosecond laser photoexcitation, heterostructures, fast photocurrent, terahertz electromagnetic radiation.

Возбуждение терагерцевого излучения в p-n-гетероструктурах на основе a-Si:H/c-Si

The results of studying the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H//c-Si upon their photoexcitation by radiation from a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of the observed THz radiation can be explained by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits a THz electromagnetic wave. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in structures. In terms of intensity, the THz radiation observed from the studied p–n heterostructures based on a-Si:H//c-Si is comparable to the THz radiation generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H//c-Si p-n-heterostructures can be used as THz emitters for solving tasks of THz spectroscopy.

Research Progress on Intense and Broadband Terahertz Radiation Generation with Multi‑Color Laser Field

Research Progress on Intense and Broadband Terahertz Radiation Generation with Multi‑Color Laser Field

Femtosecond laser-induced sub-wavelength plasma inside dielectrics. III. Terahertz radiation emission

Electromagnetic radiation within the terahertz (THz) frequency range is of great interest for applications in remote sensing and time-domain spectroscopy. The laser-induced plasmas are promising mediums for generating THz radiation. It has been recently reported that focusing femtosecond Bessel pulses inside dielectrics induces a high aspect ratio over-critical plasmas. Here, we show that the intense resonantly driven electrostatic fields at the so-called critical surface lead to THz radiation emission. Through three-dimensional particle-in-cell simulation and analytical derivation, we have investigated the emission of THz radiation. We show that the THz radiation is associated with a hot population of electrons trapped in ambipolar electric fields of the double layers.

Advances in terahertz technology for cancer detection applications.

Currently, there is an increasing demand for the diagnostic techniques that provide functional and morphological information with early cancer detection capability. Novel modern medical imaging systems driven by the recent advancements in technology such as terahertz (THz) and infrared radiation-based imaging technologies which are complementary to conventional modalities are being developed, investigated, and validated. The THz cancer imaging techniques offer novel opportunities for label free, non-ionizing, non-invasive and early cancer detection. The observed image contrast in THz cancer imaging studies has been mostly attributed to higher refractive index, absorption coefficient and dielectric properties in cancer tissue than that in the normal tissue due the local increase of the water molecule content in tissue and increased blood supply to the cancer affected tissue. Additional image contrast parameters and cancer biomarkers that have been reported to contribute to THz image contrast include cell structural changes, molecular density, interactions between agents (e.g., contrast agents and embedding agents) and biological tissue as well as tissue substances like proteins, fiber and fat etc. In this paper, we have presented a systematic and comprehensive review of the advancements in the technological development of THz technology for cancer imaging applications. Initially, the fundamentals principles and techniques for THz radiation generation and detection, imaging and spectroscopy are introduced. Further, the application of THz imaging for detection of various cancers tissues are presented, with more focus on the in vivo imaging of skin cancer. The data processing techniques for THz data are briefly discussed. Also, we identify the advantages and existing challenges in THz based cancer detection and report the performance improvement techniques. The recent advancements towards THz systems which are optimized and miniaturized are also reported. Finally, the integration of THz systems with artificial intelligent (AI), internet of things (IoT), cloud computing, big data analytics, robotics etc. for more sophisticated systems is proposed. This will facilitate the large-scale clinical applications of THz for smart and connected next generation healthcare systems and provide a roadmap for future research.

Scaling Law of THz Yield from Two-Color Femtosecond Filament for Fixed Pump Power

In 3D + time numerical simulations, we study the wavelength scaling law for the energy of terahertz (THz) radiation emitted from a two-color femtosecond filament, which forms during cofocusing into air the fundamental and second harmonics of the laser pulse. In our simulations, the central wavelength of the fundamental harmonic varied from 0.8 to 8 μm and the numerical aperture varied from 0.006 to 0.03. While the harmonics and supercontinuum development are not extreme, so the harmonics spectra are clearly separated, the energy of the generated THz radiation is proportional to the oscillation energy of the electrons, which grows as the squared pump wavelength, and the total number of free electrons in the filament, which decreases quasi-exponentially with the pump wavelength. As a result, the scaling law for the THz energy on the pump wavelength is nonmonotonic with the maximum at 1.6–4 μm depending on the focusing conditions.

Interaction of obliquely incident lasers with anharmonic CNTs acting as dipole antenna to generate resonant THz radiation

The present paper explores the terahertz (THz) generation by the interaction of obliquely incident laser beams with the array of vertically aligned anharmonic carbon nanotubes (CNTs) acting as dipole antennas. In the scheme, anharmonicity arises due to the nonlinear variation of restoration force on the various electrons of CNTs and it plays a key role in the enhancement of THz generation. The anharmonic CNTs help in broadening the resonance peak, which paves the way for the enhancement of the normalized THz amplitude. The laser beams incident obliquely on the close-packed array of vertically aligned anharmonic CNTs grown over the glass substrate and set oscillations in the CNTs so that each CNT act as the oscillatory dipole to generate THz radiation. The THz electric field shows enhancement at surface plasmon resonance frequency , where is the characteristic parameter of CNTs, is the relative permittivity of lattice and is the plasma frequency. This scheme is quite suitable to generate THz radiations in the milliwatt range of optimized values of the laser and CNTs parameters. We also explore the impact of polarization, S-parameter, critical angle, and length-matching effects of CNT antennas on the THz generation.

Generation of Terahertz Radiation under the Femtosecond Laser Excitation of a Multilayer Structure Based on a-Si:H/a-SiC:H/c-Si

Coherent terahertz radiation has been generated in p–n heterostructures based on a-Si:H/a-SiC:H/c-Si excited by 800-nm femtosecond laser pulses at room temperature. Terahertz radiation is generated when a reverse bias voltage is applied to heterostructures. The properties of the generated terahertz radiation strongly depend on the bias voltage, which reflects the dynamics of nonequilibrium charge carriers produced by femtosecond laser pump in the heterostructure.

Generation of terahertz radiation under the effect of a femtosecond pulse on plasma in a magnetic field.

The generation of terahertz (THz) radiation under the effect of a femtosecond laser pulse on a transparent semibounded plasma in a constant magnetic field is studied. It is shown how the generation patterns of THz radiation depend on the laser pulse duration and the ratio between the cyclotron and plasma frequencies of electrons. In each of the modes considered, the total energy per unit area, spectral composition, and pulse shape of the generated THz radiation are found. It has been established that optimal generation conditions are realized at laser pulse duration comparable to the reciprocal frequency of plasma oscillations and at electron cyclotron frequency lower than plasma frequency. When cyclotron frequency exceeds plasma frequency, the possibility of increasing the generation frequency up to frequencies comparable to the cyclotron frequency is revealed.