Peak Electric Field Strength Research Articles

Generation of Circularly Polarized Strong-Field Terahertz Waves

Abstract Terahertz (THz) circular dichroism (TCD) spectroscopy has been widely used to study the chiral characteristics of biological macromolecules and various other materials. With the rapid development of strong-field THz generation and field-modulated techniques, the advancement of tunable strong-field TCD spectroscopy technology is of great significance. In this work, we built an integrated strong-field TCD spectroscopy system. We employed the tilted-pulse-front technique to generate linearly polarized strong-field THz radiation and a reflective metasurface for linear-to-circular conversion of the strong-field THz radiation. We obtained circularly polarized THz radiation with an ellipticity of over 0.9 in the frequency range from 0.38 THz to 0.61 THz, with a LTC conversion efficiency of over 90%. The peak electric field strength of the obtained strong-field circularly polarized THz radiation exceeded 100 kV/cm. This system is expected to be used to study the chiral characteristics of materials under strong-field conditions, as well as changes in the chiral characteristics under different field conditions.

Influence of beam polarization on underwater femtosecond laser machining of silicon wafer

This paper investigates the effect of polarization directions on underwater femtosecond laser machining of silicon wafer. Firstly, laser ablation in liquids produces laser-induced periodic surface structures (LIPSS) on both edges of the groove. The direction of LIPSS has direct correlation with the polarization of the laser beam. Secondly, the electric field intensity distribution within the groove is analyzed by numerical simulation employing the finite-difference-time-domain (FDTD) method. The simulation and experimental results reveal that the distribution of the inter electric field intensity within the grooves varies with polarization direction. As the angle between the polarization and scanning directions decreases, the peak electric field strength and groove depth gradually increase. Finally, changing the direction of polarization of the beam affects the symmetry of the groove profile. The groove profile produced by ablation is symmetrical when the beam polarization direction is parallel to the scanning direction. By modulating and optimizing the direction of polarization, grooves with large depth and symmetrical shape can be obtained.

Single-shot local measurement of terahertz correlated second harmonic generation in laser air plasma filaments.

We present a single-shot detection method of terahertz correlated second harmonic generation in plasma-based sources by directly mixing an optical probe into femtosecond laser-induced plasma filaments in air. The single-shot second harmonic trace is obtained by measuring a second harmonic generation on a conventional CCD with a spatiotemporally distorted probe beam. The system shows a spectrometer resolution of 22 fs/pixel on the CCD and a true resolution on the order of the probe pulse duration. With considerable THz peak electric field strength, this formalism can open the door to single-shot THz detection without bandwidth limitations.

Optimum Optical Designs for Diffraction-Limited Terahertz Spectroscopy and Imaging Systems Using Off-Axis Parabolic Mirrors

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy.

Generation of 13.9-mJ Terahertz Radiation from Lithium Niobate Materials.

Extremely strong-field terahertz (THz) radiation in free space has compelling applications in nonequilibrium condensed matter state regulation, all-optical THz electron acceleration and manipulation, THz biological effects, etc. However, these practical applications are constrained by the absence of high-intensity, high-efficiency, high-beam-quality, and stable solid-state THz light sources. Here, the generation of single-cycle 13.9-mJ extreme THz pulses from cryogenically cooled lithium niobate crystals and a 1.2% energy conversion efficiency from 800nm to THz are demonstrated experimentally using the tilted pulse-front technique driven by a home-built 30-fs, 1.2-Joule Ti:sapphire laser amplifier. The focused peak electric field strength is estimated to be 7.5MVcm-1 . A record of 1.1-mJ THz single-pulse energy at a 450 mJ pump at room temperature is produced and observed that the self-phase modulation of the optical pump can induce THz saturation behavior from the crystals in the substantially nonlinear pump regime. This study lays the foundation for the generation of sub-Joule THz radiation from lithium niobate crystals and will inspire more innovations in extreme THz science and applications.

Intense narrowband terahertz pulses produced by obliquely colliding laser pulses in helium gas

A practical configuration for generating narrowband terahertz (THz) pulses based on plasma dipole oscillations (PDOs) is studied using two-dimensional particle-in-cell simulations. In this scheme, two slightly detuned laser pulses collide obliquely in a helium gas. Plasma strips are generated along the paths of the laser pulses by field ionization. The PDO created in the overlap region of the two laser pulses emits a THz pulse with a peak electric field strength of a few gigavolt per meter. An energy conversion efficiency of 0.542 × 10 − 3 is achieved for laser pulse intensities 4.82 × 10 16 W / c m 2, a spot radii of 5 μ m, and a collision angle of 10.8 °. A force balance model is extended for the obliquely colliding configuration of the pulses. As the complications, such as generating plasmas separately or aligning the beams with preformed plasma, are eliminated from our new configuration, this makes a future experimental study of PDO more straightforward.

Effects of volume and surface conductivity on the surface charge and electric field characteristics of the tri‐post insulator in SF 6 ‐filled ± 500 kV DC‐GIL

Herein, the surface charge characteristics of a tri-post insulator in a ± 500 kV direct current gas-insulated transmission lines (DC-GIL) filled with SF6 are thoroughly studied. The influences of volume and surface conductivity of the insulator on the surface charge and consequent electric field distribution are discussed. An improved method is introduced to obtain the distribution of surface charge with considerable accuracy and efficiency for the calculation. It is found that with increasing the volume conductivity, the dominative conduction mechanism changes from surface conduction to bulk conduction, while the inverse variation appears with increasing the surface conductivity. During the variation of electric conductivity, obvious effect of surface charge accumulation is found on the electric field distribution involving the variation of peak electric field strength and its location. Besides, high charge density usually appears along with low peak field strength during this variation. It can be concluded that the surface charge accumulation should be paid special attention when analysing the insulation characteristics of tri-post insulators. The variation of volume and surface electric conductivity should be thoroughly considered.

A wideband high-power microwave radiation source based on gyromagnetic nonlinear transmission line and Vlasov antenna.

The gyromagnetic nonlinear transmission line (GNLTL) is a special kind of coaxial transmission line partially loaded with the ferrite material. A GNLTL system can modulate the input high-power pulses into wideband high-power microwaves without relying on the electron beam and confining magnetic field. The unique working mechanism gives the GNLTL system the potential to be a small portable wideband high-power microwave radiation source. In this study, a wideband high-power microwave radiation source based on a GNLTL system is designed and constructed. In order to effectively radiate the wideband microwaves into the air, a high-power wideband Vlasov antenna and a special absorption high-pass filter are developed. The designs of key subsystems and high-power radiation experiments have been introduced and discussed in detail. In the test experiments, a radiated pulse with a peak electric field strength of 23 kV/m was measured at 20m away from the transmitting antenna and the effective potential of radiation is 460 kV/m. The pulse width of the radiation pulse is about 4ns, the center frequency is about 2.25GHz, and the highest repetition rate can reach 25Hz.

Waveguide structure based electron acceleration using terahertz pulses.

We have developed a waveguide structure for electron acceleration using a few µJ energy THz pulse. The metallic device focuses the incoming linearly polarized nearly single-cycle THz pulse, hence increasing the peak electric field strength. We experimentally verified the gain and the temporal profile of the electric field in the structure using electro-optic sampling technique. The acceleration of the electron bunch from rest up to 8 keV was predicted using single-cycle THz pulses with µJ-energy level.

Fivefold Helically Corrugated Waveguide for High-Power W-Band Gyro-Devices and Pulse Compression

The design, simulation, manufacture, and measurement of a W-band fivefold (5F) helically corrugated waveguide (HCW) is reported. The 5F HCW is based on the coupling of the traveling TE₃₁ and near cutoff TE₂₁ modes to create an operating eigenwave. The fabricated test structure has circular waveguide ports and features elliptical polarizer sections and broadband TE₁₁ to TE₃₁ mode converters on either side of the 5F HCW. The optimized mode converter design, based on a fourfold (4F) HCW, has a predicted power conversion efficiency greater than 90% from 89 to 102.5 GHz, and 96% peak efficiency at 94 GHz. The optimization of the 5F HCW geometry produced an eigenwave suitable for gyro-devices, but the optimization could equally well have been directed to applications such as pulse compression and microwave undulators. Analysis of simulated electric field profiles showed that the propagating power in the 5F HCW was increased by a factor of 6 over that in the 3F HCW at equivalent peak electric field strength. This is due to the larger diameter of the waveguide. Test structures were manufactured through a combination of precision machining of a sacrificial mandrel, copper growth by electroforming followed by removal of the aluminum mandrel by chemically etching. Measurements of the 5F HCW structure's dispersion showed excellent agreement with the prediction over the design range of 90-98 GHz.

Compact combined antenna for high‐power ultrawideband radiation sources with subnanosecond pulse duration

AbstractThe paper presents the design of a combined antenna, optimized in numerical calculations to radiate high‐power bipolar pulses with a duration of 0.5 ns. According to the results of the calculations, a compact antenna with the dimensions of 60 × 65 × 55 mm3 (width × height × length) was made. The influence of a high‐voltage insulator position relative to the antenna input on the matching band of the antenna with the coaxial feeder is shown. In the absence of an isolator, the matching band is in the range of 0.98–9.33 GHz. The antenna efficiency by the peak electric field strength and the energy efficiency are equal to 0.75–0.8 and 0.9, respectively.

Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy.

Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics.

Increasing the Production Yield of White Oyster Mushrooms With Pulsed Electric Fields

Pulsed electric field (PEF) technology can be utilized to increase the growth rate and total yield and alter the nutritional composition of agricultural crops. We investigated the efficacy of PEF treatment on the Pleurotus Ostreatus white oyster mushroom species. Mushroom sawdust substrates in buckets were stimulated with a combination of two rod-to-plate electrode geometries with peak electric field strengths of 1.46 and 6.67 kV/cm through unipolar pulses with approximately 20 kV amplitude. Total electrical energy of 62.3 kJ (0.017 kWh) over 30 days of excitation was delivered to each mushroom bucket. The total yield of the stimulated group showed a 34% increase compared with the control group. The experiment was repeated, and the stimulated buckets showed a 45.5% increase in yield relative to the control. A proximate analysis of the fruiting bodies was examined postexcitation for the moisture, fat, protein, and ash content. The stimulated group shows a slight increase in mean crude fat and protein content compared with the control group. Furthermore, the grounding mesh electrode interface with the fruiting substrate was found to contribute to about 10% higher ash content compared with the control group.

High-throughput terahertz spectral line imaging using an echelon mirror.

This work demonstrates terahertz (THz) line imaging that acquires broadband spectral information by combining echelon-based single-shot THz spectroscopy with high-sensitivity phase-offset electrooptic detection. An approximately 40 dB signal-to-noise ratio is obtained for a THz spectrum from a single line of the camera, with a detection bandwidth up to 2 THz at the peak electric-field strength of 1.2 kV/cm. The spatial resolution of the image is confirmed to be diffraction limited for each spectral component of the THz wave. We use the system to image sugar tablets by quickly scanning the sample, which illustrates the capacity of the proposed spectral line imaging system for high-throughput applications.

Performance comparison of lithium-niobate-based extremely high-field single-cycle terahertz sources [Invited

Tilted-pulse-front-pumping (TPFP) lithium-niobate terahertz (THz) pulse sources are widely used in pump-probe and control experiments since they can generate broadband THz pulses with tens of microjoules of energy. However, the conventional TPFP setup suffers from limitations, hindering the generation of THz pulses with peak electric field strength over 1 MV/cm. Recently, a few setups were suggested to mitigate or even eliminate these limitations. In this paper, we shortly review the setups that are suitable for the generation of single-cycle THz pulses with up to a few tens of megavolts/centimeter focused electric field strength. The THz pulses available with the new layouts pave the way for previously unattainable applications that require extremely high electric field strength and pulse energy in the multi-millijoule range.

Optimal Design of Large Signal Performance of AlN/GaN Hetero-Structural IMPATT and MITATT Diodes

Hetero-structure of AlxGa1-xN/GaN exhibits important applications in high frequency and large power devices. In this paper, AlN/GaN is adopted to optimal design the large power impact avalanche transit time (IMPATT) and mixed tunneling avalanche transit time (MITATT) diodes operating at the atmospheric low loss window frequency of 0.85 THz. The static state and large signal characteristics of the devices are numerically simulated. The values of peak electric field strength, break-down voltage, avalanche voltage, the maximum generation rates of avalanche and tunneling, admittance-frequency relation, output power, conversion efficiency, quality factor of the proposed hetero-structural IMPATT and MITATT diodes are calculated, respectively. Via comparing the obtained results of (n)AlN/(p)GaN and (n)GaN/(p)AlN IMPATT diodes to those of the MITATT counterparts, there exists little performance difference between IMPATT and MITATT devices while implies significant difference between the (n)AlN/(p)GaN and (n)GaN/(p)AlN diodes.

A Subnanosecond Pulsed Electric Field System for Studying Cells Electropermeabilization

This article presents an experimental arrangement which, using 3-D numerical modeling, aims to study biomedical effects using subnanosecond pulsed electric fields (PEFs). As part of a major effort into developing contactless technology, the final aim of this study is to determine conditions of the applied PEFs (number of pulses, strength, pulse repetition frequency) able to produce electropermeabilization. The arrangement uses a pulsed power generator producing voltage impulses with an amplitude of up to 20 kV on a 50- $\Omega $ matched load, with a rise time of 100 ps and a duration of 600 ps. During the preliminary study reported here, samples containing E. Coli were exposed to PEFs in a 4-mm standard electroporation cuvette, allowing the application of a peak electric field strength of up to 60 kV/cm. The studies were facilitated by detailed 3-D electromagnetic modeling of the electric field distribution generated by voltage impulses inside the system. Due to the nature of tests, the numerical analysis played an essential role in the interpretation of results. Preliminary biological results reported in this study are very encouraging, showing that trains of 5000 to 50 000 pulses applied at a pulsed repetition frequency of 200 Hz (maximum PRF) can efficiently induce E. Coli electropermeabilization.

Inter-individual variations in electric fields induced in the brain by exposure to uniform magnetic fields at 50 Hz

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines and the Institute of Electrical and Electronics Engineers (IEEE) standard establish safety limits for human exposure to electromagnetic fields. At low frequencies, only a limited number of computational body models or simplified geometrical shapes are used to relate the internal induced electric fields and the external magnetic fields. As a consequence, both standard/guidelines derive the exposure reference levels for the external magnetic field without considering the variability between individuals. Here we provide quantitative data on the variation of the maximum electric field strengths induced in the brain of 118 individuals when exposed to uniform magnetic fields at 50 Hz. We found that individual characteristics, such as age and skull volume, as well as incident magnetic field direction, have a systematic effect on the peak electric field values. Older individuals show higher induced electric field strengths, possibly due to age-related anatomical changes in brain. Peak electric field strengths are found to increase for larger skull volumes, as well as for incident magnetic fields directed along the lateral direction. Moreover, the maximum electric fields provided by the anatomical models used by ICNIRP for deriving exposure limits are considerably higher than those obtained here. On the contrary, the IEEE elliptical exposure model produces a weaker peak electric field strength. Our findings are useful for the revision and harmonization of the current exposure standard and guidelines. The present investigation reduces the dosimetric uncertainty of the induced electric field among different anatomical induction models. The obtained results can be used as a basis for the selection of appropriate reduction factors when deriving exposure reference levels for human protection to low-frequency electromagnetic exposure.

Single-shot spatial-temporal electric field measurement of intense terahertz pulses from coherent transition radiation

Coherent transition radiation (CTR) is a widely used approach to characterize the electron beam in accelerator science and technology. Characterization of the CTR with an efficient, versatile and compact setup is in considerable demand for a wide range of applications such as free electron lasers, electron storage rings and other related scientific researches. In this article, we report on spatial-temporal electric field measurements of CTR using the single-shot electro-optic spatial decoding method. A quasi-half-cycle temporal waveform of the THz pulse with peak electric field strength of $\ensuremath{\sim}42\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ has been measured. A symmetrical field profile across the transverse center and the distorted wave front of CTR THz pulses near the focal plane were demonstrated. Moreover, timing jitters of the ultrashort electron beams are also monitored via this method with tens of femtosecond accuracy. This comprehensive method provides the temporal waveform and the one-dimensional spatial imaging of THz fields simultaneously, showing it has the potential for single-shot diagnostics of ultrashort electron beams.

Intracavity third-harmonic generation in Si:B pumped by intense terahertz pulses

We observe third-harmonic generation (THG) in boron-doped silicon (Si:B) upon pumping with picosecond 1.56-THz pulses from a free-electron laser with a peak electric field strength of up to 12 kV/cm. The measurements are performed at cryogenic temperatures where the majority of holes are bound to the acceptor dopants. The dependence of the THG on the pump intensity exhibits a threshold-free power-law behavior with an exponent close to 4. The observations can be explained by THz emission by free holes accelerated in the nonparabolic valence band, under the assumption that the density of free holes increases with the pump intensity. A quantitative treatment supports that these carriers are generated by impact ionization, initiated by the population of thermally ionized carriers, as opposed to direct tunneling ionization. In addition, we also observe intracavity THG by embedding the Si:B in a one-dimensional photonic crystal cavity. The THG efficiency is increased by a factor of eight due to the field enhancement in the cavity, with the potential to reach a factor of more than 100 for pump pulses with a spectrum narrower than the linewidth of the cavity resonance.