Emission Of Terahertz Radiation Research Articles

Transit‐Time Instability Mechanism in Lateral Graphene n+‐i‐n‐i‐n+ Structure with Transverse Terahertz Pasmonic Resonant Cavity

The terahertz (THz) plasmonic instability in the lateral graphene structure (LGS) with the n+‐i‐n‐i‐n+ junction in its graphene channel (GC) is analyzed. The i‐regions in the LGS n+‐i‐n‐i‐n+ GC plays the role of the transit spaces of the electrons emitted from the n+‐regions (the emitters) and collected by the gated n‐region (collector) and the side contact to the latter. The interband Zener–Klein tunneling (ZKT) in the i‐regions also contributes to the currents received by the n‐region. Simultaneously, the n‐region constitutes the transverse resonant plasmonic cavity, in which the plasmons with the wave vector directed perpendicular to the electron propagation direction are excited. The plasmonic instability in the LGSs is associated with the transit‐time negative dynamic conductance of the i‐regions supported by the plasmonic resonances in the n‐region. The self‐excitation of plasmonic oscillations associated with the instability in the LGS with sub‐micrometer i‐regions and supplied with a proper antenna can lead to the emission of THz radiation. The LGS can form periodic arrays, which can serve as an active distributed antenna.

Variable laser profiles and polarization states in the wave-mixing scheme for the generation of THz radiation in collisional-magnetized clustered-plasma

The four-wave mixing (FWM) mechanism for the generation of Terahertz (THz) radiation in magnetized-collisional clustered-plasma is studied. Clusters can trap the incident laser fields, leading to efficient energy transfer to plasma electrons and the creation of a strong nonlinear current. The size and density of clusters can be tuned to control the emission of THz radiation. The smaller clusters can bring about higher intensity THz radiation, while larger clusters can contribute to the broader THz frequency range. The THz radiation angular distribution patterns in the forward direction have been assessed, and the impact of plasma interaction length, cluster radius, and plasma electrons collision frequency on the generated THz wave patterns has been investigated. The FWM analysis highlights the importance of plasma clusters as well as magnetic fields in the efficient generation of THz emission. The model also takes into consideration how an induced electron current density initiated by DC external magnetic field can boost radiation power without changing the directivity diagram.

Terahertz Generation through Coherent Excitation of Slow Surface Waves in an Array of Carbon Nanotubes

In this paper, we present a scheme for generating terahertz (THz) radiation using an array of parallel double-walled carbon nanotubes (DWCNTs) subjected to a direct current (DC). The longitudinal surface plasmon polaritons (SPPs) in the DWCNTs are coherently excited by two near-infrared laser beams with slightly different frequencies. Through numerical methods, we investigate the spectral characteristics of the SPPs in the presence of a DC current in the nanotubes. We identify high-quality plasmonic modes with a slowdown factor exceeding 300 in the terahertz frequency region. The amplification of these slow SPP modes is facilitated by the DC current in the DWCNTs, fulfilling a synchronism condition. This condition ensures that the phase velocity of the SPPs is closely matched to the drift velocity of the charge carriers, allowing for an efficient energy exchange between the current and the surface electromagnetic wave. The high-frequency currents on the nanotube walls in the DWCNT array enable the emission of THz radiation into the far field, owing to an antenna effect.

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.

Highly conformable terahertz metasurface absorbers via two-photon polymerization on polymeric ultra-thin films

Abstract The continuously increasing interest in flexible and integrated photonics requires new strategies for device manufacturing on arbitrary complex surfaces and with smallest possible size, respectively. Terahertz (THz) technology can particularly benefit from this achievement to make compact systems for emission, detection and on-demand manipulation of THz radiation. Here, we present a novel fabrication method to realize conformable terahertz metasurfaces. The flexible and versatile character of polymeric nanomembranes is combined with direct laser writing via two-photon polymerization to develop free-standing ultra-thin quasi-perfect plasmonic absorbers with an unprecedentedly high level of conformability. Moreover, revealing new flexible dielectric materials presenting low absorption and permittivity in the THz range, this work paves the way for the realization of ultra-thin, conformable hybrid or all-dielectric devices to enhance and enlarge the application of THz technologies, and flexible photonics in general.

Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations

Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.

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.

Coherent emission from surface Josephson plasmons in striped cuprates

The interplay between charge order and superconductivity remains one of the central themes of research in quantum materials. In the case of cuprates, the coupling between striped charge fluctuations and local electromagnetic fields is especially important, as it affects transport properties, coherence, and dimensionality of superconducting correlations. Here, we study the emission of coherent terahertz radiation in single-layer cuprates of the La2-xBaxCuO4 family, for which this effect is expected to be forbidden by symmetry. We find that emission vanishes for compounds in which the stripes are quasi-static but is activated when c-axis inversion symmetry is broken by incommensurate or fluctuating charge stripes, such as in La1.905Ba0.095CuO4 and in La1.845Ba0.155CuO4. In this case,terahertz radiation is emitted by surface Josephson plasmons, which are generally dark modes, but couple to free space electromagnetic radiation because of the stripe modulation.

Electric-field control of nonlinear THz spintronic emitters

Energy-efficient spintronic technology holds tremendous potential for the design of next-generation processors to operate at terahertz frequencies. Femtosecond photoexcitation of spintronic materials generates sub-picosecond spin currents and emission of terahertz radiation with broad bandwidth. However, terahertz spintronic emitters lack an active material platform for electric-field control. Here, we demonstrate a nonlinear electric-field control of terahertz spin current-based emitters using a single crystal piezoelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) that endows artificial magnetoelectric coupling onto a spintronic terahertz emitter and provides 270% modulation of the terahertz field at remnant magnetization. The nonlinear electric-field control of the spins occurs due to the strain-induced change in magnetic energy of the ferromagnet thin-film. Results also reveal a robust and repeatable switching of the phase of the terahertz spin current. Electric-field control of terahertz spintronic emitters with multiferroics and strain engineering offers opportunities for the on-chip realization of tunable energy-efficient spintronic-photonic integrated platforms.

Terahertz Radiation as a Tool for Exploring Material Properties

Researchers from Japan highlight, in a recent review, the range of material properties that can be and have been studied using the phenomenon of femtosecond laser-induced terahertz radiation emission.

Spintronic terahertz emitters: Status and prospects from a materials perspective

Spintronic terahertz (THz) emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) thin films, have demonstrated remarkable potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. Since the discovery that novel FM/NM heterostructures can be utilized as sources of THz radiation, researchers have endeavored to find the optimum combination of materials to produce idealized spintronic emitters capable of generating pulses of THz radiation over a large spectral bandwidth. In the last decade, researchers have investigated the influence of a wide range of material properties, including the choice of materials and thicknesses of the layers, the quality of the FM/NM interface, and the stack geometry upon the emission of THz radiation. It has been found that particular combinations of these properties have greatly improved the amplitude and bandwidth of the emitted THz pulse. Significantly, studying the material properties of spintronic THz emitters has increased the understanding of the spin-to-charge current conversion processes involved in the generation of THz radiation. Ultimately, this has facilitated the development of spintronic heterostructures that can emit THz radiation without the application of an external magnetic field. In this review, we present a comprehensive overview of the experimental and theoretical findings that have led to the development of spintronic THz emitters, which hold promise for use in a wide range of THz applications. We summarize the current understanding of the mechanisms that contribute to the emission of THz radiation from the spintronic heterostructures and explore how the material properties contribute to the emission process.

Enhanced directive terahertz radiation emission from a horn antenna-coupled W/Fe/Pt spintronic film stack

We demonstrate enhanced directive terahertz (THz) radiation emission from a horn-antenna coupled spintronic THz radiation emitter without the use of additional lenses. The waveguide-fed horn antenna is fabricated using 3D lithography and is coupled directly to the THz-emitting film serving to direct the highly divergent THz radiation emitted by the spintronic THz radiation emitter. The antenna-coupled emitter provides a 19.6 dB increase in the measured signal power at 1.5 THz compared to a bare emitter. Finite-difference time-domain simulations are conducted to gain insight into the behavior of the antenna at different frequencies, providing results that match those observed experimentally. Our device is well suited for platforms where footprint size is a constraint and can be modified to act as a directive bandpass filter.

Observation of the Three-Dimensional Polarization Vector in Films of Organic Molecular Ferroelectrics Using Terahertz Radiation Emission

For the application of a ferroelectric thin film to optical and electric devices, it is important to align its ferroelectric polarization parallel or perpendicular to the film surface. In this context, an effective method to characterize the polarization directions in the whole area of devices is needed. Here, we report an alternative method to determine a polarization vector in three dimensions using an emission of terahertz waves. The target material is an organic hydrogen-bonded molecular ferroelectric 2-methylbenzimidazole (MBI). By the irradiation of a femtosecond laser pulse on a bulk single-crystal thin film of MBI, we observe the emission of the terahertz waves originating from the polarization modulation of infrared-active phonons via the impulsive stimulated Raman-scattering mechanism. By measuring the terahertz electric field parallel to the ferroelectric polarization with rotating the single-crystal film around two independent axes, we succeed in determining the polarization vector in three dimensions, the direction of which is tilted by 45\ifmmode^\circ\else\textdegree\fi{} normal to the film surface and substrate. This method can be a powerful tool to characterize directions of ferroelectric polarizations in any kinds of thin-film samples.

Terahertz emission by multiple resonances under external periodic electrostatic field.

In the presence of electron neutral collisions of a frequency (ν) of the order of ν≥0.5ω_{p}(ω_{p} is the plasma frequency), the collisional effects play an adverse role in the mechanism of terahertz (THz) radiation. The present work describes an approach for the efficient emission of THz radiation with the application of an external periodic electric field in the density rippled collisional plasma wherein an additional transverse component of the current is generated that adds to the THz radiation mechanism. The THz field obtained by coupling of the lasers' field with the external field is termed as an external field induced THz (EFIT). Here, the periodic wave number of the external field enables tuning of the THz radiation and helps achieve multiple resonance conditions for the excitation of large amplitude nonlinear plasma currents.

Terahertz radiation emission from three-color laser-induced air plasma

Based on the photocurrent model, terahertz (THz) radiation generation in three-color (fundamental ($$\omega $$), second-harmonic ($$2\omega $$), and third-harmonic ($$3\omega $$)) laser-induced air plasma is studied using particle in cell simulation. Our simulation results show that the amplitude of THz radiation in the three-color laser scheme is several times larger than the amplitude of THz radiation in the two-color laser scheme (fundamental ($$\omega $$), second harmonic ($$2\omega $$)). In addition, the effect of the phase difference between the first and second harmonics $$\left( \varphi _{2} \right) $$ and the phase difference between the fundamental and third harmonics $$\left( \varphi _{3} \right) $$ on THz generation is investigated in order to obtain the optimum conditions. It is indicated that the effect of $$\varphi _{3}$$ on THz radiation strongly depends on $$\varphi _{2}$$ in the three-color scheme and it is found that the THz radiation is maximum in the case with $$\varphi _{2}=\pi / 6$$ and $$\varphi _{3}= \pi / 6$$. The influence of laser wavelength and pulse duration on THz generation is also studied. Our results reveal that the THz radiation increases proportionally to the square of the laser wavelength. Moreover, it is shown that THz radiation increases by increasing pulse duration.

Selective terahertz emission due to electrically excited 2D plasmons in AlGaN/GaN heterostructure

Terahertz radiation emission from an electrically excited AlGaN/GaN heterostructure with a surface metal grating was studied under conditions of two-dimensional (2D) electron heating by the lateral electric field. Intensive peaks related to nonequilibrium 2D plasmons were revealed in the terahertz emission spectra with up to 4 times selective amplification of the radiation emission in the vicinity of 2D plasmon resonance. This selective emission was shown to be frequency-controllable by the grating period. Exact spectral positions of the 2D plasmon resonances were preliminarily experimentally detected with the help of equilibrium transmission spectra measured at various temperatures. The resonance positions are in a satisfactory agreement with the results of theoretical simulation of the transmission spectra performed using a rigorous solution of Maxwell’s equations. The effective temperature of hot 2D electrons was determined by means of I–V characteristics and their analysis using the power balance equation. It was shown that for a given electric field, the effective temperature of nonequilibrium 2D plasmons is close to the hot 2D electron temperature. The work may have applications in GaN-based electrically pumped emitters of terahertz radiation.

Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave.

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene devices.

A Polarization-Resolved Study of Nanopatterned Photoconductive Antenna for Enhanced Terahertz Emission

Terahertz (THz) frequencies, despite having the potential for several important applications, have been relatively underexplored in the past owing to the unavailability of proper sources and detectors. The scenario has been changing over the past few decades due to the advent of convenient THz sources and detectors. THz photoconductive antennas (PCA), due to their attractive features, such as cost effectiveness and room temperature operation, are playing a key role in current and future research prospect in the field of THz spectroscopy, both as sources and detectors. Complex PCA designs have been proposed and studied to boost the THz emission efficiencies. Elucidating the underlying physics in such devices requires a thorough investigation of a few physical parameters. This requires the integration of several experimental techniques under identical conditions. In this paper, we show such a study, including a parametric variation of pump polarization, conducted on a PCA with a nanopatterned active region, which boosts the emitted THz radiation. Through the set of measurements, we unravel the subtle interplay of the various physical processes responsible for the emission of THz radiation from the device.

Interaction of surface plasmon–phonon polaritons with terahertz radiation in heavily doped GaAs epilayers

We report on experimental studies of the surface plasmon-phonon polariton excitations in heavily doped GaAs epitaxial layers. Reflection and emission of radiation in the frequency range of 2–19 THz were investigated for samples with surface-relief grating, as well as for samples with planar surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances attributed to excitations of different surface plasmon-phonon polariton modes. The observed resonances lie beyond the limits of the Reststrahlen band. Terahertz radiation emission from the samples was studied in nonequilibrium conditions under the pulsed electric field excitation. Two contributions to the spectral density of the terahertz radiation have been revealed, the first being due to bulk plasmon–phonon polaritons (PPhPs), while the second originating from the surface PPhPs. A field dependence of the effective temperature of the bulk PPhPs has been established. Polarization dependence of the terahertz radiation related to surface PPhPs has been experimentally examined for the first time.

Strong emission of THz radiation from GaAs microstructures on Si

Remarkably strong emission of terahertz radiation from illuminated GaAs microstructures on a Si substrate is reported. The peak–to–peak amplitude of terahertz radiation from the sample is 9 times larger than that of THz radiation from a semi-insulating GaAs wafer. The spectral width of the sample is larger than that of a semi-insulating GaAs wafer; in particular, the spectral amplitude increases at higher frequencies. The presented GaAs microstructures on a Si substrate can be suitable for practical and efficient THz sources required in various THz applications.