John F. Whitaker

Picosecond switching time measurement of a resonant tunneling diode

Picosecond bistable operation has been experimentally observed for the first time in a double‐barrier resonant tunneling diode. A rise time of 2 ps was measured using the electro‐optic sampling technique; this is the fastest switching event yet observed for an electronic device. This time domain measurement adds necessary information to the understanding of the transport mechanisms in the resonant tunneling diode and is consistent with switching time limitations computed for the device. It also demonstrates that appropriately designed double‐barrier quantum well diodes have a response time comparable to that of the fastest all‐optical logic elements, and that they may be very useful in high‐speed logic applications.

Propagation model for ultrafast signals on superconducting dispersive striplines

The algorithm suitable for the computer-aided design of transmission lines is used to model the propagation of picosecond and subpicosecond electrical signals on superconducting planar transmission lines. Included in the computation of a complex propagation factor are geometry-dependent modal dispersion and the frequency-dependent attenuation and phase velocity which arise as a result of the presence of a superconductor in the structure. The results of calculations are presented along with a comparison to experimental data. The effects of modal dispersion and the complex surface conductivity of the superconductor are demonstrated, with the conclusion that it is necessary to incorporate both phenomena for accurate modeling of transient propagation in strip transmission lines. >

High‐frequency characterization of thin‐film Y‐Ba‐Cu oxide superconducting transmission lines

We report the first measurements of picosecond pulse propagation on transmission lines patterned from YBa2Cu3O7−x films. Distortion‐free propagation of high current density transients is demonstrated. The high‐frequency properties were analyzed by careful study of the relative phase delays of the electrical transients as the temperature of the sample was varied from 1.8 K to Tc. Simulations using Mattis–Bardeen complex conductivities showed good agreement with the measured results. High‐frequency critical current densities in excess of 105 A/cm2 were measured.

Pulse Dispersion and Shaping in Microstrip Lines

A method for determining the modal dispersion of gigahertz-bandwidth pulses on microstrip transmission lines is described. We have investigated the evolution of temporal waveforms propagating on microstrips, with very good agreement noted between experimental pulse shapes and numerical simulations. The resulting pulse distortion contributed to a pulse-shaping application where 100-ps rise times were stretched to the nanosecond durations necessary to control the shape of high-energy optical pulses used in fusion research. The tunability of the shape of the rising edge was investigated through variation of the stripline geometry and the substrate material. Additional effects due to high-frequency attenuation in several dipolar liquid dielectrics were substantiated experimentally, and the relevance of the results has been discussed.

Propagation characteristics of picosecond electrical transients on coplanar striplines

Using a cryogenic electro‐optic sampling technique, we have studied the transient propagation characteristics of superconducting and normal indium lines in the picosecond regime. Transient dispersion effects, including increased rise time and increased pulse width, the introduction of ringing on the waveform, and a novel ‘‘pulse sharpening’’ were observed. A model that takes into consideration the effects of modal dispersion and superconducting complex conductivity was established, and an algorithm was developed that accurately describes all of the experimental findings.

Ultrafast response of superconducting transmission lines

The authors report investigations of picosecond transient propagation on normal and superconducting transmission lines and results of a variety of lines that include YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) coplanar lines, a superconducting coaxial cable, and a dielectric-matched gold-line structure. A previously developed algorithm for analyzing transient propagation was used to identify the dominant mechanisms for signal distortion in most of these cases, and the essential properties of all tested to date are summarized for a direct comparison. >