THz Electromagnetic Radiation Research Articles

Dynamically Controllable Terahertz Electromagnetic Interference Shielding by Small Polaron Responses in Dirac Semimetal PdTe2 Thin Films

AbstractTerahertz (THz) electromagnetic interference (EMI) shielding materials is crucial for ensuring THz electromagnetic protection and information confidentiality technology. Here, it is demonstrated that high electrical conductivity and strong absorption of THz electromagnetic radiation by type‐II Dirac semimetal PdTe2 film make it a promising material for EMI shielding. Compared to MXene film, a commonly used metallic 2D material, the PdTe2 film demonstrates a remarkable 40.36% increase in average EMI shielding efficiency per unit thickness within a broadband THz frequency range. Furthermore, it is demonstrated that a photoinduced long life‐time THz transparency in Dirac semimetal PdTe2 films is attributed to the formation of small polarons due to the strong electron‐phonon coupling. A 15 nm‐thick PdTe2 film exhibits a photoinduced change of EMI SE of 1.1 dB, a value exceeding three times that measured on MXene film with a similar pump fluence. This work provides insights into the fundamental photocarrier properties in type‐II Dirac semimetals that are essential for designing advanced THz optoelectronic devices.

Plasma instability and amplified mode switching effect in THz field effect transistors with a grating gate

We developed a theory of collective plasma oscillations in a dc current-biased field effect transistor with an interdigitated dual grating gate and demonstrated a new mechanism of electron plasma instability in this structure. The instability in the plasmonic crystal formed in the transistor channel develops due to conversion of the kinetic energy carried by the drifting plasmons into electromagnetic energy. The conversion happens at the opposite sides of the gate fingers due to the asymmetry produced by the current flow, and occurs through the gate finger fringing capacitances. The key feature of the proposed instability mechanism is the behavior of the plasma frequency peak and its width as functions of the dc current bias. At a certain critical value of the current, the plasma resonant peak with a small instability increment experiencing redshift with increasing current changes to the blue-shifting peak with a large instability increment. This amplified mode switching effect has been recently observed in graphene-interdigitated structures. The obtained theoretical results are in very good qualitative agreement with these experiments and can be used in future designs of the compact sources of THz electromagnetic radiation.

Experimental study of the effect of corneal hydration and its biomechanical properties on the results of photorefractive keratectomy

To study the effect of hydration of the cornea determined by non-contact THz scanning and its biomechanical parameters on the results of photorefractive keratectomy (PRK) in an experiment. PRK was performed using the Nidek EC-5000 QUEST excimer laser on 8 rabbit eyes. Corneal hydration was evaluated by determining the reflection coefficient (RC) in the THz electromagnetic radiation range before PRK, after 3-5 days, and after 1, 2, 3, and 4 months. Clinical examination included autorefractometry, assessment of corneal thickness and other anatomical and optical parameters of the anterior eye segment (Galilei G6, Ziemer Ophthalmic Systems AG 6.0.2, Switzerland), measurement of corneal hysteresis (CH) and corneal resistance factor (CRF) using the Ocular Response Analyzer (ORA; Reichert, USA), as well as tear production (Schirmer test). The initial water content in the cornea has a significant effect on the thickness of the removed layer, i.e. on the PRK effect, with correlation coefficient of Rs= -0.976 (p<0.01). The correlation between CH and the ablation depth is less pronounced (Rs=0.643), and CRF had no correlation with it (Rs= -0.089). Biomechanical indicators of the cornea depend on its hydration: changes in CH and CRF after excimer laser ablation qualitatively coincide with changes in RC, the correlation coefficient between RC and the initial value of CH is R= -0.619 (moderate negative correlation). THz scanning is an effective non-contact technology for monitoring corneal hydration level. The mismatch of the hypoeffect of keratorefractive excimer laser intervention planned for patients with presbyopia with the actual outcome can be caused by individual decrease in the initial water content in the cornea.

Dynamical Mass Generation in Graphene by Bicircular Laser Fields

Electron-hole creation in single-layer intrinsic graphene interacting with bicircular laser fields is investigated. Due to the linear dispersion relation near the degeneracy points, the electron and hole dynamics are treated according to the Dirac theory, i.e., it is assumed that the charge carriers behave as ultrarelativistic massless fermions. Of special interest is their dynamical mass generation induced by a THz electromagnetic radiation. The harmonic response from graphene and electron currents induced by the bicircular laser field are also analyzed.

Sub-THz Imaging Using Non-Resonant HEMT Detectors

Plasma waves in gated 2-D systems can be used to efficiently detect THz electromagnetic radiation. Solid-state plasma wave-based sensors can be used as detectors in THz imaging systems. An experimental study of the sub-THz response of II-gate strained-Si Schottky-gated MODFETs (Modulation-doped Field-Effect Transistor) was performed. The response of the strained-Si MODFET has been characterized at two frequencies: 150 and 300 GHz: The DC drain-to-source voltage transducing the THz radiation (photovoltaic mode) of 250-nm gate length transistors exhibited a non-resonant response that agrees with theoretical models and physics-based simulations of the electrical response of the transistor. When imposing a weak source-to-drain current of 5 μA, a substantial increase of the photoresponse was found. This increase is translated into an enhancement of the responsivity by one order of magnitude as compared to the photovoltaic mode, while the NEP (Noise Equivalent Power) is reduced in the subthreshold region. Strained-Si MODFETs demonstrated an excellent performance as detectors in THz imaging.

Sub‐THz Response of Strained‐Silicon MODFETs

Plasma waves in gated 2‐D systems can be used to detect THz electromagnetic radiation. This work reports on a two‐dimensional hydrodynamic‐model (HDM) applied to investigate the sub‐THz photovoltaic response of Schottky‐gated strained‐Si MODFETs. TCAD simulation results are validated through comparison with measurements on the transistors. Simulation and experimental results show an excellent agreement in terms of the efficiency of the transconductance. The measurement of the photovoltaic response of s‐Si MOSFETs with 100‐nm gate length is carried out at two sub‐THz frequencies: 0.15 and 0.3 THz. The THz photovoltaic response of the transistor is implemented in TCAD, as in measurements, grounding the source, biasing the gate, and floating the drain contact while a sub‐THz sinusoidal signal is superimposed to the bias gate voltage. In agreement with measurements, a non‐resonant sub‐THz photovoltaic response is found for the simulated devices. As THz detection by any plasma wave FET depends on many parameters (device dimensions, material system, the excitation frequency, etc.), its optimization is a complex problem that needs to be addressed using physics‐based tools, like the HDM. Since the HDM is able to describe in detail the photovoltaic response, its use may be generalized to design plasma wave s‐Si MODFET detectors.

Terahertz radiation induces non-thermal structural changes associated with Fröhlich condensation in a protein crystal.

Whether long-range quantum coherent states could exist in biological systems, and beyond low-temperature regimes where quantum physics is known to be applicable, has been the subject to debate for decades. It was proposed by Fröhlich that vibrational modes within protein molecules can order and condense into a lowest-frequency vibrational mode in a process similar to Bose-Einstein condensation, and thus that macroscopic coherence could potentially be observed in biological systems. Despite the prediction of these so-called Fröhlich condensates almost five decades ago, experimental evidence thereof has been lacking. Here, we present the first experimental observation of Fröhlich condensation in a protein structure. To that end, and to overcome the challenges associated with probing low-frequency molecular vibrations in proteins (which has hampered understanding of their role in proteins' function), we combined terahertz techniques with a highly sensitive X-ray crystallographic method to visualize low-frequency vibrational modes in the protein structure of hen-egg white lysozyme. We found that 0.4 THz electromagnetic radiation induces non-thermal changes in electron density. In particular, we observed a local increase of electron density in a long α-helix motif consistent with a subtle longitudinal compression of the helix. These observed electron density changes occur at a low absorption rate indicating that thermalization of terahertz photons happens on a micro- to milli-second time scale, which is much slower than the expected nanosecond time scale due to damping of delocalized low frequency vibrations. Our analyses show that the micro- to milli-second lifetime of the vibration can only be explained by Fröhlich condensation, a phenomenon predicted almost half a century ago, yet never experimentally confirmed.

THz and above THz electron or hole oscillations in DNA dimers and trimers

A non conventional source or receiver of THz and above THz electromagnetic radiation is proposed. Specifically, electron or hole oscillations in DNA dimers (two interacting DNA base‐pairs or monomers) are predicted, with frequency in the range 0.25–100 THz (period 10–4000 fs) i.e. potentially absorbing or emitting electromagnetic radiation mainly in the mid‐ and far‐infrared with wavelengths ≈ 3–1200 μm. The efficiency of charge transfer between the two monomers which make up the dimer is described with the maximum transfer percentage p and the pure maximum transfer rate . For dimers made of identical monomers , but for dimers made of different monomers . The investigation is extended to DNA trimers (three interacting DNA base‐pairs or monomers). For trimers made of identical monomers the carrier oscillates periodically with 0.5–33 THz ( 30–2000 fs); for 0 times crosswise purines , for 1 or 2 times crosswise purines . For trimers made of different monomers the carrier movement may be non periodic. Generally, increasing the number of monomers above three, the system becomes more complex and periodicity is lost; even for the simplest tetramer the carrier movement is not periodic.

High power THz sources and applications at ENEA-Frascati

ENEA has a long term expertise in the development of powerful short-pulse mm-wave and THz radiation sources driven by free electrons. Various electron-wave interaction schemes were successfully tested in the past, including Cerenkov and Smith-Purcell radiators as well as undulator devices. Two THz-FEL sources are currently available, covering altogether the spectral range from 90 GHz to 0.7 THz. Recently a novel Electro-Magnetic pulser, capable of providing both nanosecond THz electromagnetic (EM) radiation pulses and electrostatic (ES) pulses with identical time duration in one device has been designed and is currently under development. The pulser will allow, for the first time, direct comparison of EM and ES pulses on biological systems with peak electric fields that are significantly higher than any reported to date.

Terahertz Radiation from Magnetic Excitations in Diluted Magnetic Semiconductors

We probed, in the time domain, the THz electromagnetic radiation originating from spins in CdMnTe diluted magnetic semiconductor quantum wells containing high-mobility electron gas. Taking advantage of the efficient Raman generation process, the spin precession was induced by low power near-infrared pulses. We provide a full theoretical first-principles description of spin-wave generation, spin precession, and of emission of THz radiation. Our results open new perspectives for improved control of the direct coupling between spin and an electromagnetic field, e.g., by using semiconductor technology to insert the THz sources in cavities or pillars.

THz response of nonequilibrium electrons of highly doped graphene on a polar substrate

High-frequency response of a system with drifting electrons in a highly doped graphene and surface polar optical phonons of a polar substrate is considered. For this interacting system, we obtained a dielectric function, frequencies and decrement/increment of cooperative plasmon-optical phonon oscillations. We found, that the response is significantly depends on the degree of nonequilibrium of the electrons. In particular, interaction between drifting plasmons and surface polar optical phonons leads to an instability of the electron subsystem due to Vavilov-Cherenkov effect. We suggest that the hybrid system - graphene on a polar substrate - is capable to be used in amplifiers or generators of THz electromagnetic radiation.

Correlated terahertz acoustic and electromagnetic emission in dynamically screened InGaN/GaN quantum wells

We investigate acoustic and electromagnetic emission from optically excited strained piezoelectric In0.2Ga0.8N/GaN multiple quantum wells (MQWs), using optical pump-probe spectroscopy, time-resolved Brillouin scattering, and THz emission spectroscopy. A direct comparison of detected acoustic signals and THz electromagnetic radiation signals demonstrates that transient strain generation in InGaN/GaN MQWs is correlated with electromagnetic THz generation, and both types of emission find their origin in ultrafast dynamical screening of the built-in piezoelectric field in the MQWs. The measured spectral intensity of the detected Brillouin signal corresponds to a maximum strain amplitude of generated acoustic pulses of 2%. This value coincides with the static lattice-mismatch-induced strain in In0.2Ga0.8N/GaN, demonstrating the total release of static strain in MQWs via impulsive THz acoustic emission. This confirms the ultrafast dynamical screening mechanism in MQWs as a highly efficient method for impulsive strain generation

Mechanical Damage Detection in Polymer Tiles by THz Radiation

Today the ultrasonic inspection technique is probably the most popular method for nondestructive evaluation and structural health monitoring. However, ultrasonic waves are not very effective in detecting internal defects in some materials such as ceramic foam tiles used in the thermal protection system (TPS) of the space shuttle, thick polymer composites, and polymer tiles used in various applications. Ultrasonic energy is attenuated very fast in these materials. On the other hand the electromagnetic radiation in THz (1000 GHz) frequency range can penetrate deep inside these materials. Its wavelength is small enough to detect internal defects. To understand the limits of structural damage detection capability of THz electromagnetic radiation or T-ray, mechanical damage in polymer tiles is introduced by drilling holes. Then T-ray is passed through the damaged and defect-free tiles. The received signal strength is found to be affected differently by the internal defect as the frequency changes. Experimental observations are justified from the model predictions. The model takes into account the interaction between the T-ray of finite width and the tile containing the internal defect.

Strained-Si modulation doped field effect transistors as detectors of terahertz and sub-terahertz radiation

Strained-Si modulation doped field effect transistors have been studied as detectors of 0.2 THz and 1.6 THz electromagnetic radiation at room temperature. The difference in the gate voltage dependences for 0.2 THz and 1.6 THz radiation and spatial pattern of the transistor response to focused 1.6 THz radiation confirms that the mechanism of detection is linked to the excitations of the two-dimensional electrons in the device channel.

基于耦合场量子受激拉曼散射的太赫兹波辐射

基于耦合场量子受激拉曼散射的太赫兹波辐射

Plasma wave detection of terahertz radiation by silicon field effects transistors: Responsivity and noise equivalent power

Si metal oxide semiconductor field effect transistors (MOSFETs) with the gate lengths of 120–300nm have been studied as room temperature plasma wave detectors of 0.7THz electromagnetic radiation. In agreement with the plasma wave detection theory, the response was found to depend on the gate length and the gate bias. The obtained values of responsivity (⩽200V∕W) and noise equivalent power (⩾10−10W∕Hz0.5) demonstrate the potential of Si MOSFETs as sensitive detectors of terahertz radiation.

Nanometer transistors for emission and detection of THz radiation

Experimental results concerning detection and emission of THz electromagnetic radiation in nanometer Field Effect Transistors are reviewed. The experiments were performed on GaAs/GaAlAs and GaInAs/AlInAs High Electron Mobility Transistors and Si Metal Oxide Semiconductor Field Effect Transistors at room and liquid helium temperatures. The results are interpreted within a model of the electron plasma instability in the transistor channel.

Terahertz Generation and Detection by Plasma Waves in Nanometer Gate High Electron Mobility Transistors

The high electron mobility transistors can act as a resonator cavity for the plasma waves that can reach THz frequencies for a nanometer size devices. As was predicted by Dyakonov and Shur in 1993, the steady state of the current flow in a gated 2D electron gas can become unstable leading to the emission of an electromagnetic radiation at the plasma wave frequencies. The theory predicted also that the plasma waves can be used for resonant detection of THz electromagnetic radiation. In the present paper we review our recent experiments on THz emission and detection performed on high electron mobility transistors based on different semiconductor structures: InGaAs/GaAlAs, GaAs/GaAlAs, and Si.

Electrical and optical properties of polyacetylene film in THz frequency range

Electrical and optical properties such as power absorption, index of refraction, and complex conductivity of polyacetylene film are determined by transmission measurement using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation and without invoking the Kramers–Kronig relationships. Four types of polyacetylene samples, undoped pristine, FeCl4− doped pristine, undoped and stretched twice, and FeCl4− doped and stretched twice, are compared experimentally. The undoped pristine polyacetylene is found to have a resonance of 2.45 THz. The stretched polyacetylene have an angle-dependent curve along the direction of the polarized THz beam.

Optical and electrical properties of preferentially anisotropic single-walled carbon-nanotube films in terahertz region

The absorption and dispersion of aligned single-walled carbon-nanotube films were measured from 0.2 to 2.0 THz using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation. The real conductivity increased rapidly with increasing frequency up to 0.45 THz and decreased at a high-frequency range. The Maxwell–Garnett model, where the nanotubes were embedded in a dielectric host, fit the results of this study with the Drude–Lorentz model for nanotube network. We have observed the transverse phonon mode of 2.4 THz propagating along the c direction. This suggested that the carbon nanotube network is composed of metallic and semiconducting nanotubes embedded in an air dielectric host.