Ultrashort Electrical Pulses Research Articles

Interpretation of photocurrent correlation measurements used for ultrafast photoconductive switch characterization

Photocurrent correlation measurements used for the characterization of ultrafast photoconductive switches based on GaAs and silicon-on-sapphire are demonstrated. The correlation signal arises from the interplay of the photoexcited carriers, the dynamics of the bias field and a subsequent recharging of the switch. By using both photocurrent measurements and terahertz spectroscopy we verify the importance of space-charge effects on the carrier dynamics. Photocurrent nonlinearities and coherent effects are discussed as they appear in the correlation signals. An analysis based on a simple model allows an estimate of the carrier lifetimes associated with the photoconductive switching process. We illustrate how pulse propagation can be studied sequentially using this technique and how a minor modification of the experimental setup enables the study of screening from long-lived carriers. We emphasize in what ways the different techniques of measuring ultrashort electrical pulses are sensitive to different aspects of the pulse forming mechanisms.

Picosecond optoelectronic on-wafer characterization of coplanar waveguides on high-resistivity Si and [formula omitted] substrates

Microwave dispersion and attenuation of coplanar waveguides on high-resistivity silicon substrates with insulation layer are characterized by optoelectronic measurements directly on the wafer. Freely positionable photoconductive switches are employed for ultrashort electric pulse injection. The electric pulses on the coplanar waveguides are sampled electro-optically in time domain. Both, attenuation and dispersion of microwaves are significantly altered by the insulation layer over the whole investigated frequency range from 10 GHz to 600 GHz. We observe a significant increase in microwave absorption for bias conditions that lead to accumulation or inversion at the Si/insulator interface.

Generation of ultrashort electrical pulses with variable pulse widths

Electrical pulses from 450 fs to 3 ps are generated using coplanar strip lines and coplanar waveguides on SI GaAs substrate. We use two optical pump beams to generate two steplike electrical transients with opposite sign and then superimpose the signals resulting in rectangular shaped pulses. By varying the delay between the two pump beams the pulse width can be adjusted.

Low-distortion and low-crosstalk characteristics of picosecond pulses in a dual-plane coupled microstrip lines structure

Investigation of the propagation and crosstalk of ultrashort electrical pulses in a dual-phase coupled microstrip lines structure is reported. The current distributions and propagation constants of the dominant c- and pi -modes are calculated by using the spectral domain method, and the full-wave analysis results obtained are incorporated into a FFT (fast Fourier transform) algorithm to study pulse distortion and crosstalk in the transmission lines. Computer simulation results reveal the superior low-distortion, low-crosstalk characteristics of this transmission line structure, which can be used as interconnects in high-density, high-speed, integrated circuits (ICs). >

Guest editorial

Guest editorial

Propagation of ultrashort electrical pulses on microstriplines with submicron dielectrics

We present and analyze a new waveguide structure for the transmission of ultrashort electrical pulses over relatively long distances. By using the method proposed in the present work, microstrip waveguides with submicron dielectric thickness can be formed with III-V semiconductors on highly conducting (metallic) substrates for the first time. In order to minimize signal attenuation, three conventional superconducting materials have been considered to form the metallization. At 4.2 K it is found that pulse propagation is best when the lines are formed with Nb3Sn although NbN performs only slightly less well, with Nb giving the poorest performance. Such waveguides are shown to be extremely good media for the transmission of signals with components in the range 1011–1012 Hz.

Characterization of picosecond pulse propagation in a microstrip line divider

Many extremely broad bandwidth circuit components have to be newly developed for practical applications of ultrashort electrical pulses. Theoretical and experimental investigations of the transmission properties of picosecond pulses through a microstrip line divider are reported. Full-wave analysis results in the frequency domain are incorporated into a Fourier transform algorithm for computer simulations of pulse propagation through a 3-dB microstrip line divider with a curved-line branching circuit pattern. Excellent agreement has been found between the theoretical predictions and the results of experimental measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical generation of picosecond electrical pulses in asymmetric quantum well structures placed in a transverse magnetic field

New technique for optical generation of ultrashort electrical pulses in semiconductor heterostructures is proposed. It is shown that in asymmetric quantum well structures placed in a transverse magnetic field, optically excited carriers have an asymmetrical distribution in k-vector space, and therefore, they have finite drift velocity in the direction perpendicular to both growth direction and field direction. It results in photogenerated voltage along the quantum well plane. The temporal response and sensitivity of the proposed scheme are evaluated, and possible applications are suggested in ultrafast detectors or as a tool for generation and study of hot carriers.

Fullwave analysis of picosecond photoconductive switches

A fullwave analysis method for characterizing fast photoconductive switches (PCs) is proposed. Three-dimensional Maxwell curl equations, including the conductive current components, are solved using a finite-difference time-domain scheme. It is found from simulations that PCs in coupled microstrip lines with small widths and spacings are very effective in avoiding propagation of non-quasi-TEM modes and for achieving rapid responses. Futhermore, responses for PCs with pulse-forming networks constructed of coplanar waveguide shunt electrodes are simulated. Ultrashort electrical pulses with minimum widths of 0.2 and 0.4 ps can be obtained by symmetric and antisymmetric excitation methods, respectively, for linewidth and spacing readily realized.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Propagation of terahertz bandwidth electrical pulses on YBa2Cu3O7−δ transmission lines on lanthanum aluminate

We study the propagation of terahertz bandwidth electrical pulses on high critical current density c-axis oriented YBa2Cu3O7−δ (YBCO) coplanar transmission lines deposited epitaxially on the low dielectric loss substrate lanthanum aluminate (LaAlO3). The losses on the YBCO transmission line are lower than on an equivalent gold line on the same substrate at temperatures below 50 K. At higher temperatures, absorption and dispersion of the ultrashort electrical pulses are observed in reasonable agreement with Mattis–Bardeen theory.

High speed gating of the proximity focused channel plate intensifier with an additional external electrode and MTF characteristics.

We have developed a new ultra high speed framing camera. In this paper we explain the structure, working principle and gating image capabilities, i.e., the transient behavior of this camera using the proximity focused image intensifier with an additional external transparent electrode. The experiment of subnanosecond gating of this intensifier is accomplished by supplying ultra short electrical pulse to the external electrode. Experimental results can be analyzed sufficiently and accurately, by using RLC transmission line equations. And also MTF characteristics are obtained.

Investigation of high-temperature superconductors with terahertz bandwidth electrical pulses

The authors summarize recent attempts to propagate ultrashort electrical pulses containing frequency components up to 1 THz on transmission lines made from thin films of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7- delta /. Optoelectronic generation of extremely short electrical transients and jitter-free electrooptic detection of the pulse shape and amplitude as they propagate along the superconducting line make it possible to investigate the high-frequency properties of the high-T/sub c/ material with high accuracy because of the the long interaction lengths that are possible on transmission lines. The authors discuss the influence of epitaxy, surface roughness, and magnetic field on the pulse propagation. Important parameters such as the magnetic penetration depth and the superconducting energy gap are directly determined from the experimental data. Finally, the authors compare their results with weak-coupling BCS (Bardeen-Cooper-Schrieffer) theory. >

Generation of picosecond electrical pulses with the aid of injection lasers

An injection laser was used to generate, for the first time, electrical pulses with a leading edge shorter than 15 ps and an amplitude in excess of 1.1 V. This was done employing optoelectronic pulse shapers made of semiconductors and operating on the basis of their photoconductivity. The source of ultrashort optical pulses was a three-component Q-switched AlGaAs/GaAs heterojunction laser emitting pulses of 5 ps duration and 50 pJ energy. Ultrashort electrical pulses were recorded using an S9-9 Sigma sampling oscilloscope with an extremely wide band and with a rise time of the transient characteristic amounting to about 20 ps.

Electrooptical generation and detection of femtosecond electrical transients

Recent work on the generation of ultrashort electrical pulses by optical rectification of femtosecond optical pulses in electrooptic materials is summarized. This technique, which is called electrooptic Cherenkov radiation, is described in detail with particular emphasis on the effects of dispersion due to coupling to lattice vibrational resonances. Recent experimental results in lithium tantalate are described which illustrate these effects. When the duration of the exciting femto second optical pulse is comparable to the period of the lattice vibrations, a pronounced ringing is observed in the electrical waveforms. A coupled oscillator model is used to develop a theory that accurately accounts for these effects and can be used to predict the limiting speed of response and generation efficiency of different electrooptic materials. >

Generation of ultrashort electrical pulses through screening by virtual populations in biased quantum wells.

We consider theoretically a semiconductor quantum well subject to an electrostatic field normal to the layer and a strong monochromatic laser field in the transparency region below the absorption edge. We show that ultrashort optical pulses generate ultrashort voltage pulses because the virtual electron-hole population induced by the laser field partially screens the applied static field. We predict that voltages &gt;1 V could be generated in micron-thick multiple quantum well structures by use of low-power short-pulse lasers.

Optoelectronic transmission and reception of ultrashort electrical pulses

We report on the recent advances in using integrated planar antenna technology to photoconductively generate and detect picosecond radiation. Detection of a single pulse of picosecond duration has been achieved using a coplanar antenna structure fabricated on a radiation-damaged silicon-on-sapphire substrate.

Chaos in Josephson Junctions Induced by Picosecond Current Pulses

We report studies of intermittent chaos in hysteretic Josephson junctions induced by ultrashort electrical pulses. Substantial changes in the junction dc I-V characteristics, including an excessive low frequency noise, a novel re-entry state wherein the dynamics became chaotic at low dc bias, but returned to the zero-voltage state at higher biases, and a complicated behavior for very high amplitude input pulses were experimentally observed. The threshold switching curve was different from that in the static limit. The experimental findings are in good agreement with numerical simulations based on the resistively and capatively shunted junction model.