Generation of terahertz electromagnetic pulses from quantum-wellstructures

Terahertz generation from semiconductor quantum-well structures pumped by a femtosecond optical pulse is studied. We propose a three-level model for the electrons and holes in the quantum wells. We then solve the coupled optical Bloch equations directly using a Runge-Kutta method and calculate the terahertz radiation field. We study optical rectification and quantum beats caused by charge oscillations in 1) a coupled quantum well in which quantum beats occur between two electron states of the coupled system and 2) a single-quantum-well structure in which quantum beats occur between light-hole and heavy-hole excitons. Our theoretical results agree very well with the experimentally measured terahertz data     

Generation of short pulses of coherent electromagnetic radiation ina free-electron laser amplifier

Theoretical and experimental studies of the evolution of a frequency-chirped pulse under the influence of both phase and gain dispersion effects induced by the free-electron laser interaction are presented. For the experimental parameters used (electron beam voltage V=150 kV, wiggler periodicity l_w=3.5 cm, gain ~10 dB, input pulse width Δt~200 ns, frequency w ₀/2π=10 GHz, and frequency chirp α/2π~5 MHz/ns), pulses of a few nanoseconds were generated after an interaction length of 2.30 m, in good agreement with theoretical expectations     

Generation of surface electromagnetic waves in semiconductors under the action of femtosecond laser pulses

It is demonstrated that intense photoexcitation of the surface of a semiconductor with femtosecond laser pulses can induce fundamental changes in its optical response and ensure conditions for the generation of surface electromagnetic waves of various types. The connection between electronic processes initiated in the surface layer by photoexcitation and the formation of periodic surface microstructures observed in experiments on irradiation of Si targets is considered. The importance of photoemission in the processes taking place under ultrashort excitation is confirmed.     

Propagation of ultra-wide-band electromagnetic pulses throughdispersive media

We develop an efficient method for the analysis of ultra-wide-band (UWB) electromagnetic pulses (e.g., double-exponential pulse) propagating through a waveguide or cold plasma (i.e., the ionosphere). First we show that the inverse Fourier-transform representations for the electric and magnetic fields satisfy second order, nonhomogeneous, ordinary, differential equations. These differential equations are solved analytically, thereby yielding closed-form expressions involving incomplete Lipschitz-Hankel integrals (ILHIs). The ILHIs are computed using efficient convergent and asymptotic series expansions. We demonstrate the usefulness of the ILHI expressions by comparing them with the fast Fourier-transform technique (FFT). Because of the long tails associated with UWB pulses, a large number of sample points are required in the FFT, to avoid aliasing errors. In contrast, the ILHI expressions provide accurate and efficient numerical results, regardless of the number of points computed. An asymptotic series representation for the ILHIs is also employed, to obtain a relatively simple, late-time approximation for the transient fields. This approximate late-time expression is shown to accurately model the waveform over a large portion of its time history     

Propagation of electromagnetic pulses in terrestrial waveguides

The propagation of electromagnetic waves over the earth's surface is considered under transient conditions. The source is taken to be a vertical electric dipole whose current moment is suddenly established. The build-up of the radiated field is calculated under various assumed corditions. It is shown, even in the absence of an ionospherically reflected wave, that the influence of earth curvature has a pronounced effect on the distortion of the original pulse shape. For great distances (i.e., d>2000 km), it is found to be more convenient to regard the field as a sum of modes. Particular attention is given to the transient characteristics of the dominant mode as a function of the source waveform.     

A compact radiator of high-power electromagnetic pulses

The design and parameters of a compact radiator of high-power electromagnetic nanosecond pulses designed for simulation of ultra-wideband exposures in electromagnetic compatibility problems are described. It is shown that the effective pulse power of the radiator is 3 GW in the forward traveling wave and 15 GW in the wave reflected from the reflector. The effective density of the continuous energy spectrum is 17 mJ/MHz at 0.3 GHz.     

Generation of high-average-power ultrabroad-band infrared pulses

This paper summarizes the results of analytical and numerical studies on a novel technique that is capable of providing high average power ultra broadband radiation that extends from approximately 2 to 16 μm. Such a spectrum has several potential applications, including telecommunications and remote sensing. Additional attractive features of the new source are its anticipated compact size, light weight, ruggedness, and affordable cost. The technique is based on the interaction of a beat wave with a nonlinear medium. The beat wave is formed from the mixing of two CO₂ laser beams with closely spaced wavelengths, such as 9.5 and 9.6 μm. The discrete ultrabroad-band spectrum is generated in a nonlinear optical medium by the self-phase modulation process, a third-order nonlinearity. The long-wavelength portion of the spectrum, i.e., from 5 to 16 μm is produced directly from the interaction of the beat wave with a GaAs crystal. The short-wavelength portion of the spectrum is produced from the interaction of a frequency-doubled beat wave with a GaAs crystal following the chirping of the pulse by a different GaAs crystal and its subsequent optimal compression by a thin sapphire slab     

Generation of the Gaussian-like HE11 mode from gyrotron TEOn mode mixtures at 70 GHz

This work reports calculations and measurements on mode converters for the transformation of TEOn gyrotron mode mixtures (primarily TEO2) into the linearly polarized HE11 hybrid mode at 70 GHz. This mode is ideal for quasi-optical launching systems for ECRH of plasmas. Mode transducers with axisymmetric radius perturbations convert the gyrotron TEOn mode composition into the TEO1 mode. Proper matching of the phase differences between the various modes and of the perturbation amplitudes of the several converter sections is required. The TEO1 wave is used for long-distance transmission through smooth-walled overmoded waveguides. A mode converter with constant diameter and periodically perturbed curvature transfers the unpolarized TEO1 mode into the polarized TE11 wave. The experimentally determined ΣTEOn-to-TEO1 conversion efficiency is (98.5±1)% (99% predicted) while the TEO1-to-TE11 transformer has a (94±2)% efficiency (93% theoretically); ohmic losses are included. The “Gaussian-like” HE11 mode with axisymmetric power distribution and almost no cross polarization is produced in a circumferentially corrugated TE11-to-HE11 mode transducer with a measured conversion efficiency of (98.3±1.5)% (98.5% predicted). The overall efficiency of the complete mode converter system in the desired mode was determined to be (91±2.5)%. High-power operation (200kW, 100ms) has been successfully demonstrated.     

Distortion of electromagnetic pulses at totally reflecting layers

The plane-wave reflection of electromagnetic pulses from homogeneous and inhomogeneous layered media is discussed. Special attention is given to the case of total reflection which is characterized in the frequency domain by phase distortion without amplitude distortion. For this case it is shown that the waveform in the time domain undergoes a rather remarkable change.     

Generation and amplification of ultrashort optical pulses

During the last two years since the 1966 Iternational Quantum Electronics Conference further progress toward obtaining high-power light pulses has taken place. An effective method of ultrashort light pulse generation (mode locking) has been developed, further development of the method of coherent amplification of light pulses has been recorded, and new prospective ideas have been put forward. The present paper briefly reports on the data obtained by us in these directions.     

Generation of balanced subnanosecond pulses using step-recoverydiodes

A method for generating balanced subnanosecond pulses is described. By using pulsed bias techniques to excite a step-recovery diode via a conventional ferrite balun transformer, a balanced step waveform can be taken directly from the diode itself, eliminating the requirement for a broadband output balun transformer. The resulting pulses are suitable for applications in broadband antenna excitation, time-domain reflectometry and high-speed sampling bridges     

Lateral electromagnetic waves and pulses on open microstrip

The propagation of lateral electromagnetic waves and pulses on microstrip is investigated. Interference patterns generated by the superposition of the lateral and direct waves along the air-substrate surface are shown. The field generated by the pulse excitation of a horizontal dipole on the air-substrate boundary is shown to consist of a lateral-wave pulse and a slower direct-wave pulse. Their differences in shape and decay rate are clarified. It is shown that the shape of a Gaussian pulse propagating along an open microstrip transmission line is closely related to the shape of the lateral electric-field pulse generated by a Gaussian current pulse in a dipole on the air-substrate boundary     

New sensors for measuring very short electromagnetic pulses

In order to detect very short electromagnetic pulses under 100 ps in width, a wide-band receiving antenna with a bandwidth from 0 to 14 GHz is needed. The usual short dipoles or monopoles do not provide both the required fidelity (bandwidth) and sensitivity. A novel type of electromagnetic pulse (EMP) sensor has been designed, analyzed, and tested. It is simple, inexpensive, and can detect the electromagnetic pulse with both high fidelity and high sensitivity. An approximate analysis is given for better understanding and optimum design of the proposed EMP antenna     

Propagation of electromagnetic pulses in an infinite Drude medium

We analyze the propagation of an harmonic pulse launched at t=0 with some boundary condition at z=0 in an infinite Drude medium. This initial-boundary value problem is studied analytically in the low- and high-frequency limits by use of the Laplace transformation and subsequent application of asymptotic methods for the evaluation of the requisite integrals. We discuss the qualitative features of these approximations     

45fs光脉冲的产生和测量

本文报道了由CPM飞秒激光器产生的45fs光脉冲,最优化激光器设计提供了稳定可靠的激光器运转,还报告了激光器的各种参数。     

紫外微微秒高功率激光脉冲的产生

采用被动锁模染料激光器作为振荡器,其输出为2～10微微秒的锁模脉冲系列,单脉冲能 量为5～10微焦耳。用XeCl准分子激光器泵浦的染料激光放大器放大,同时利用染料激光器可调谐特性,将其准确地调到616毫微米处,倍频后刚好落在XeCl准分子激光器的最大增益带宽中心,再经XeCl激光器放大后,可获得波长为308毫微米,带宽0.2毫微米,脉宽小于10微微秒,能量为8～10毫焦耳的紫外激光单一脉冲输出。     

Generation and emission of high-power ultrabroadband picosecond pulses

The generation and emission of high-power ultrabroadband electromagnetic pulses are studied. The possibility of high-voltage (about 100 kV) bipolar voltage pulses with a duration of about 200 ps is demonstrated. The electromagnetic pulses with a FWHM of less than 100 ps, an effective potential of up to 400 kV, and a repetition rate of 100 Hz are generated using the 16-element antenna array.     

用LBO晶体产生紫外超短脉冲

将激光波长自由转换并加以利用是物理工作者追求的目标。为此人们正在不断开发波长转换用的各种非线性光学晶体。 三硼酸锂晶体(简称LBO)是首先由福建物构所开发成功的新型非线性光学晶体。除了有适中的有效倍频系数、损伤阈值高等特点外,最突出的特点是紫外透光区域可短至160nm,适     

Generation and radiation of high-power ultrawideband nanosecond pulses

The results of investigation of the generation and radiation of high-power ultrawideband electromagnetic pulses are presented. The possibility of obtaining wave beams of radiation with orthogonal polarizations of the electric field is shown. The radiating system is a 16-element antenna array that can be separated into two subarrays operating in orthogonal polarizations and excited by pulses separated in time. Two design versions of orthogonal subarrays are studied.     

Generation of ultrashort electrical pulses in semiconductor waveguides

We report a novel device capable of generating ultrashort electrical pulses on a coplanar waveguide (CPW) by means of optical rectification. The device consists of a completely passive GaAs-based optical waveguide, which is velocity matched to a CPW line. Optical pulses are injected into the device and electrical pulses are collected at the output. Experimental results obtained in the laboratory show the potential of this device for high speed optical-to-electrical conversion.