Unipolar Pulse Research Articles

Broadband X waves with orbital angular momentum

We describe the minimal diffraction-free and dispersion-free, superluminal wave packets (X-waves and generic superluminal localized waves) that can carry a given amount of units $m$ of orbital angular momentum (OAM) per photon. Even if their frequency spectrum is wide enough to synthesize a unipolar pulse, OAM imposes a temporal pulse shape with as many zeroes as units of OAM, and therefore $|m|/2$ temporal oscillations. All the frequencies in the broadband spectrum are displayed, like in a rainbow, from the bluer ones in the vicinity of the vortex to the redder ones in the wave periphery. At the same time, the whole wave paclet experiences a blue shift proportional to $|m|$. As a result of the radial red shift and the OAM blue shift, the colour of the brilliant ring around the central vortex is independent of OAM, and solely determined by the broadband source spectrum. Given the very peculiar properties of X-waves with OAM, their generation and use, instead of standard Laguerre-Gauss modes, could improve the performance of OAM-based communication and quantum cryptographic systems, as well as the efficiency of the generation of high harmonics and attosecond pulses with OAM.

Nonharmonic Spatial Population Difference Structures Created by Unipolar Rectangular Pulses in a Resonant Medium

In the case of coherent interaction with a medium of extremely short light pulses (ESPs) having a carrier frequency and harmonic shape (when the pulse durations are shorter than the population relaxation times T1 and polarization relaxation time T2 of the medium), electromagnetically induced gratings (EMIGs) of the population difference, which have a pronounced harmonic dependence on the coordinates, may appear in it. These structures can occur when pulses do not overlap or overlap in the medium. Recently, the possibility of obtaining unipolar electromagnetic pulses in the optical and adjacent ranges of non-harmonic shape, for example, rectangular and triangular, with a duration less or comparable to the duration of the extremely-short pulse in this range, has attracted interest. In this work, using the numerical solution of the system of Maxwell-Bloch equations, we study EMIG formation by rectangular attosecond pulses in a two-level resonant medium. The possibility of inducing an EMIG of a non-harmonic shape in the form of light-induced channels microresonators, (microcavities) with a size of the order of the wavelength of the resonant transition of the medium, whose parameters can be controlled, for example, by the amplitude of the incident pulses, is shown. It has been suggested that it is possible to create an EMIG of a predetermined non-harmonic shape only in the general case of using unipolar pulses. Keywords: attosecond pulses, unipolar pulses, rectangular pulses, electromagnetically induced gratings, polarization waves, light-induced microresonators.

Generation of unipolar pulses of terahertz radiation with a large electric area

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on "three-step scheme" --- ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion). Keywords: unipolar pulses, ultrafast optics, electric pulse area, terahertz radiation, three-step model.

Generation of isolated attosecond pulses with large electric area in a dense resonant medium

Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a few-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its "atomic scale" is carried out. Keywords: attosecond pulses, unipolar pulses, subcycle pulses, electrical area of pulses.

Негармонические пространственные структуры разности населенностей, создаваемые униполярными прямоугольными импульсами в резонансной среде

In the case of coherent interaction with a medium of extremely short light pulses from a carrier of a harmonic form (when the pulse durations are shorter than the population relaxation times T_1 and polarization T_2 of the medium), electromagnetically induced gratings (EMIGs) of the population difference, which have a pronounced harmonic dependence on the coordinates, may appear in it. These structures can occur when pulses do not overlap at the same time or overlap in the medium. Recently, the possibility of obtaining unipolar electromagnetic pulses of the optical and adjacent ranges of non-harmonic shape, for example, rectangular and triangular, with a duration less than or comparable to the duration of the PCR in this range, has attracted interest. In this work, using the numerical solution of the system of Maxwell-Bloch equations, we study EMIG induced by rectangular attosecond pulses in a two-level resonant medium. The possibility of inducing an EMIG of a non-harmonic form in the form of light-induced channels, a kind of microresonators, with a size of the order of the wavelength of the resonant transition of the medium, whose parameters can be controlled, for example, by the amplitude of the incident pulses, is shown. It has been suggested that it is possible to create an EMIG of a predetermined non-harmonic form only in the general case of using unipolar pulses.

Comparison of the effects of unipolar half-cycle and resonant multi-cycle electromagnetic pulses on quantum systems

It is shown that not only multicycle resonant pulses with zero electric area, but also nonresonant unipolar pulses can be used to efficiently excite quantum transitions in a medium. Arguments are presented in favor of the advantages of unipolar pulses for excitation of quantum systems in comparison with resonant radiation. A simple relationship has been found showing that in order to compare the effects of unipolar half-cycle and resonant multicycle pulses on atomic systems. It shows that it is necessary to compare the electric area of ??a unipolar pulse and the area of ??the envelope of a bipolar pulse, rather than their energies. Keywords: unipolar pulses, electric pulse area, pulse area, extremely short pulses, attosecond pulses.

32 Element Piezoelectric Micromachined Ultrasound Transducer (PMUT) Phased Array for Neuromodulation

Interest in utilizing ultrasound (US) transducers for non-invasive neuromodulation treatment, including for low intensity transcranial focused ultrasound stimulation (tFUS), has grown rapidly. The most widely demonstrated US transducers for tFUS are either bulk piezoelectric transducers or capacitive micromachine transducers (CMUT) which require high voltage excitation to operate. In order to advance the development of the US transducers towards small, portable devices for safe tFUS at large scale, a low voltage array of US transducers with beam focusing and steering capability is of interest. This work presents the design methodology, fabrication, and characterization of 32-element phased array piezoelectric micromachined ultrasound transducers (PMUT) using 1.5 μm thick Pb(Zr0.52 Ti0.48)O3 films doped with 2 mol% Nb. The electrode/piezoelectric/electrode stack was deposited on a silicon on insulator (SOI) wafer with a 2 μm silicon device layer that serves as the passive elastic layer for bending-mode vibration. The fabricated 32-element PMUT has a central frequency at 1.4 MHz. Ultrasound beam focusing and steering (through beamforming) was demonstrated where the array was driven with 14.6 V square unipolar pulses. The PMUT generated a maximum peak-to-peak focused acoustic pressure output of 0.44 MPa at a focal distance of 20 mm with a 9.2 mm and 1 mm axial and lateral resolution, respectively. The maximum pressure is equivalent to a spatial-peak pulse-average intensity of 1.29 W/cm2, which is suitable for tFUS application.

Excitation and ionization of a particle in a one-dimensional potential well of zero radius by an extremely short light pulse

The Migdal sudden perturbation approximation is used to solve the problem of excitation and ionization particles in a one-dimensional potential of zero radius with an extremely short pulse. There is only one energy level in such a one-dimensional the delta-shaped potential well. It is shown that for pulse durations shorter than the characteristic period of oscillations of the wave function of the particle in the bound state, the population of the level (and the probability of ionization) is determined by the ratio of the electric area of the pulse to the characteristic "scale" of the area inversely proportional to the size of localization of the particle in a bound state. Keywords: unipolar pulses, electrical pulse area, atomic scale of electric area.

Генерация униполярных импульсов терагерцового излучения с большой электрической площадью

A physical situation is proposed and theoretically analyzed, in which, in our opinion, it is possible to generate unipolar terahertz pulses with a large electric area. In this case, the gas in the tube is excited by a femtosecond IR laser pulse. In this case, the tube with gas is placed in a constant external electric field. The generation of a unipolar pulse is based on ”three-step scheme" - ionization of gas atoms by a femtosecond pulse, subsequent acceleration of a free electron in a dc external field and subsequent annihilation of an electron upon collision with a tube wall or another atom (ion).

Coherent control and creation of population gratings for a pair of attosecond pulses in a resonant medium based on one-dimensional rectangular quantum wells

Attosecond pulses can be used to create and control coherence in resonant media, since their duration is shorter than the population relaxation times T1 and medium polarization T2. Previously, the possibility of creating and ultrafast control of electromagnetically induced gratings (EMIG) of atomic populations in a resonant medium was shown using a sequence of extremely short light pulses, when the pulses coherently interact with the medium and do not simultaneously overlap in the medium. These studies were carried out in various approximations, when a finite number of energy levels of the medium is taken into account, or when the pulse amplitude is small. In this paper, based on a direct numerical solution of the time dependent Schredinger equation without the indicated approximations, we study the possibility of ultrafast coherent control of populations and the creation of an EMIG by a pair of attosecond pulses in a multilevel resonant medium with a low density of particles. The medium is modeled using a one-dimensional rectangular potential well with infinitely high walls. The studies performed show the possibility of ultrafast coherent control of the properties of resonant media based on quantum wells using attosecond pulses. Keywords: electromagnetically induced gratings, coherent interaction, extremely short pulses, unipolar pulses, attosecond pulses, medium coherence.

Amplification of the electric area of a pulse during reflections from a moving mirror

An analysis was made of the transformation of the electric area of an electromagnetic pulse upon reflection from a moving mirror. Conditions are found under which reflection leads to an increase (in absolute value) of the electric area. For a dynamic resonator, one of the mirrors of which periodically oscillates, an expression is obtained for the threshold for generating pulses with an ever-increasing area. Nonlinear factors that limit the exponential growth of the pulse electric area are discussed. Keyword: pulse electric area, unipolar pulse, dynamic resonator.

Сравнение воздействия униполярных полуцикловых и резонансных многоцикловых электромагнитных импульсов на квантовые системы

It is shown that not only multicycle resonant pulses with zero electric area, but also nonresonant unipolar pulses can be used to efficiently excite quantum transitions in a medium. Arguments are presented in favor of the advantages of unipolar pulses for excitation of quantum systems in comparison with resonant radiation. A simple relationship has been found showing that in order to compare the effects of unipolar half-cycle and resonant multicycle pulses on atomic systems. It shows that it is necessary to compare the electric area of ​​a unipolar pulse and the area of ​​the envelope of a bipolar pulse, rather than their energies.

Discharge Characteristics and Mechanism of Bubble Flow in FC-72 under Unipolar Repetitive Pulse Voltage

This article investigates the discharge characteristic of the bubble flow in FC-72 under unipolar repetitive pulsed voltage, which supports the insulation design of two-phase cooling in an HV power electronic device. The effect of thermal bubble flow on the insulation of FC-72 refrigerant was evaluated. According to the partial discharge (PD) characteristics, it is found that there are two stages in the discharge characteristics of the bubble flow as voltage rises. Combining the PD inception voltages of FC-72 vapor under unipolar pulse voltage and dc voltage, it reveals the discharge thresholds of the vapor inside the bubble under the transient electric field and the steady electric filed, respectively, which proves the mechanism of the staged PD evolution. Based on the motion calculation of the bubble, the PD probability of the bubble flow within a voltage period is analyzed. It simulates the phase-resolved PD pattern, which reveals the PD mechanism of the bubble flow. The research provides a reference for the insulation design for the two-phase cooling technology in power electronics.

On the nonlinear optics of extremely short pulses

A brief scientific-methodological review of theoretical works and approaches created in the nonlinear optics of extremely short pulses from the 1970s to the present is presented. Important features are emphasized that distinguish the nonlinear optics of extremely short pulses from the nonlinear optics of quasi-monochromatic signals. Keywords: Extremely short pulse, Unipolar Pulse, Soliton.

Peculiarities of pulsed plasma jet initiation

The generation of a cold plasma jet of atmospheric pressure in helium by unipolar positive pulses excitation at a voltage of 2-6 kV and the impact on a dielectric target made of aluminum oxide has been studied. It is shown that the main parameter influencing the regular nature of the streamer breakdown is the pulse duration, and the achievable parameters of cold plasma impact on the target identical to those for sinusoidal excitation. Keywords: plasma jet of atmospheric pressure, pulse excitation.

ОЦІНЮВАННЯ ВИМІРЮВАЛЬНИХ СИГНАЛІВ НА ОСНОВІ ТРИКУТНОЇ ЧАС-ІМПУЛЬСНОЇ МОДУЛЯЦІЇ ДЛЯ КОНТРОЛЮ ТЕХНІЧНОГО СТАНУ ЗАСОБІВ ЗВ’ЯЗКУ ТА ПЕРЕДАЧІ ДАНИХ

A method is proposed for triangular modulation of data on the temporal parameters of the carrier, followed by narrow-band filtering of measuring signals for monitoring the technical condition of communication and transmission facilities, which, compared with the binary modulation method, provides a decrease in the harmonic coefficient of the envelope at approximately the same, fairly small error in setting the amplitude modulation coefficient of the amplitude-modulated signal. The analysis of the obtained relations for the amplitude modulation coefficient of measuring signals with a triangular modulation law showed the following. Unipolar and bipolar carrier pulse sequences lead to the same values of the amplitude modulation coefficient of the generated amplitude-modulated signal, but with a bipolar carrier pulse sequence, the amplitude of the output signal doubles with the same amplitude of carrier pulses (without pulses). With phase modulation, the value of the amplitude modulation coefficient is much less than with width modulation. At the same time, two-way width modulation provides twice the value of the amplitude modulation coefficient than the modulation of the front or cutoff of the carrier pulses. This statement, however, is valid only for small values of the modulation coefficients proportional to the modulation coefficients of the temporal parameters of the carrier pulses with the corresponding numerical coefficients depending on the type of modulation. If the amplitude modulation coefficients are not small, then they are determined by more complex dependencies, but also more accurate, allowing to reduce the methodological error. In this case, for the required values of the amplitude modulation coefficient, the time modulation parameters can be calculated in advance or using the built-in microprocessor and stored in the memory of the measuring device (calibrator).

A Broadband Time-Domain Detection System for Electromagnetic Compatibility Conducted Susceptibility Test

In this paper, a novel conducted susceptibility (CS) test system is proposed to analyze broadband interference for equipment under test (EUT). This system can quickly locate the susceptibility frequency bands and effectively find the susceptibility thresholds. The system can generate a unipolar or bipolar Gaussian pulse signal with a pulse width of less than 720 ps to perform these functions. In addition, the pulse amplitude can be adjusted from ±50 V to ±1.5 kV under a 50 Ω resistance load. The repetition frequency is adjustable within the range of 1 Hz to 1 kHz. The pulse spacing of the bipolar Gaussian pulse signal can be controlled from 0.25 ns to 12 ns in the step of 0.25 ns. In detail, a CS test on an electric vehicle was performed with the proposed system and an existing CS system. By using the proposed system, three potential problems of electric vehicles were predicted, susceptible frequency bands were located, and susceptibility thresholds were determined. The proposed method has high test efficiency and high reliability.

Electrophysical and thermal parameters of the atmospheric pressure plasma jet in helium excited by sinusoidal and pulsed voltages

Comparative studies of cold plasma jet of generation at atmospheric pressure in helium excited by sinusoidal and unipolar positive pulses voltage at 2-6 kV for medical applications were carried out. It has been shown that the achieved parameters are identical, but the absence of the effect of frequency self-organization of streamer breakdown leading to irregularity in streamer propagation under pulse excitation makes it preferable in biophysical experiments. A limiting factor for plasma jet exposure is the achievable target temperature, which can be reduced by limiting the current duration. Keywords: cold plasma jet, excitation by sinusoidal and pulsed voltage, temperature.

Импульсная проводимость в Ag-Pd-резисторах, индуцированная импульсами лазера

Pulsed EMF in resistive thick-film Ag - Pd elements is experimentally studied when their surface is irradiated with a laser beam. The dependences of the pulsed EMF on the coordinate of the action of the laser beam on the film surface are obtained. It is shown that applying a constant bias voltage to a thick-film resistive element can significantly increase the amplitude of unipolar EMF pulses induced by laser pulses. It was found that the amplitude and frequency of the EMF signals depend, respectively, on the power and frequency of laser pulses. It is shown that by the value of the pulsed EMF it is possible to control the parameters of the high-power laser radiation incident on the surface in real time. The design of a receiver for recording the parameters of laser radiation based on an Ag - Pd sensitive element has been developed. The sensitivity of the device for measuring the power and pulse repetition rate is estimated.

Pulse Conductivity in Ag-Pd Resistors Induced by Laser Pulses

Pulsed EMF in resistive thick-film Ag-Pd elements is experimentally studied when their surface is irradiated with a laser beam. The dependences of the pulsed EMF on the coordinate of the action of the laser beam on the film surface are obtained. It is shown that applying a constant bias voltage to a thick-film resistive element can significantly increase the amplitude of unipolar EMF pulses induced by laser pulses. It was found that the amplitude and frequency of the EMF signals depend, respectively, on the power and frequency of laser pulses. It is shown that by the value of the pulsed EMF it is possible to control the parameters of the high-power laser radiation incident on the surface in real time. The design of a receiver for recording the parameters of laser radiation based on an Ag-Pd sensitive element has been developed. The sensitivity of the device for measuring the power and pulse repetition rate is estimated. Keywords: measurement of laser parameters, thick-film resistor, thermo EMF.