I. N. Duling

Generation of subpicosecond electrical pulses on coplanar transmission lines

Electrical pulses shorter than 0.6 ps were generated by photoconductively shorting a charged coplanar transmission line with 80 fs laser pulses. After propagating 8 mm on the line the electrical pulses broadened to only 2.6 ps.

Capacitance free generation and detection of subpicosecond electrical pulses on coplanar transmission lines

By reanalyzing an earlier experiment to generate subpicosecond pulses using photoconductive switches (M.B. Ketchen, et al., Appl. Phys. Lett., vol.48, pp.751-753, 1986), it is shown that to first order, the sliding-contact generation site has no capacitance. This conclusion is further supported by a double sliding-contact experiment where, to first order, neither the generation nor the detection site has any capacitance. This result removes the parasitic capacitance of the electrical circuit as one of the major difficulties to short electrical pulse generation using photoconductive switches. >

Subpicosecond optoelectronic study of resistive and superconductive transmission lines

We have studed the propagation of subpicosecond electrical pulses on coplanar resistive and superconductive Nb transmission lines. Pulses with 0.9 ps full width at half maximum were generated and detected by shorting fast photoconductive switches with 80 fs laser pulses. Dramatic improvements in propagation characteristics were achieved when the Nb was superconductive. We observed the strong dispersion and attenuation predicted to occur for frequency components near the superconducting enery gap frequency.

Far infrared spectroscopy with subpicosecond electrical pulses on transmission lines

Optically generated and detected electrical pulses on transmission lines in the subpicosecond range have frequencies extending up to 1 THz, thereby covering the far infrared region of the spectrum from 0 to 30 cm−1. We have studied the propagation of these short pulses through a section of the transmission line covered with erbium iron garnet which shows distinct absorption lines in the far infrared at low temperatures (2–30 K). The absorption and dispersion of the garnet modify the shape of the pulse, and the absorption spectrum is obtained by Fourier transforming the propagated pulse shape.

Subpicosecond optoelectronic study of superconducting transmission lines

We have studied the propagation of subpicosecond electrical pulses on coplanar superconducting Nb transmission lines. Pulses with 0.6 ps full width at half maximum were generated by photoconductively shorting a \sim 10 \mu m region between two charged 1 to 5 μm lines separated by a 2 to 10 μm gap. The propagating pulses were sampled by the delayed shorting of a fast phototconductive switch between a sampling probe and one of the transmission lines at variable distances away from the generation point. Silicon-on-sapphire wafers served as the transmission line substrate, with the 0.5 μm thick Si layer heavily damaged by an oxygen implant to provide the subpicosecond carrier life time for the excitation and probe switches. Measurements and analyses of pulses propagated up to 8 mm distance at temperatures from 2 K to 10 K showed a threshold for strong attenuation and dispersion at a frequency reflecting the onset of pair breaking in the superconducting transmission lines. The results at least qualitatively confirm the superconducting microstrip transmission line calculations of Kautz based on Mattis and Bardeens formulae for the complex conductivity of superconductors.

Photoconductive Generation of Subpicosecond Electrical Pulses and Their Measurement Applications

The generation of short electrical pulses via optical methods has for some time been performed by driving Auston switches (photoconductive gaps) with short laser pulses.[l] The shape of the electrical pulse depends on the laser pulseshape, the material properties of the semiconductor, the nature of the charge source, and the characteristics of the associated electrical transmission line. The same techniques can also measure the generated electrical pulses by sampling methods. An alternate measurement approach has been to use the electro-optic effect in a crystal.[2] In this case, the field of the electrical pulse is sampled through the rotation of the polarization of the optical sampling pulse. Because it has demonstrated 460 fsec time resolution,[3] the electro-optic method is presently considered to be the fastest sampling technique. However, recent work utilizing photoconductive switches has generated and measured subpicosecond electrical pulses.[4] This large reduction in the generated pulsewidth demonstrates the ultrafast capability of the Auston switches and challenges the ultrafast time resolution of the electro-optic methods. We will now discuss subpicosecond pulse generation using photoconductive gaps,[4] together with the initial interpretation of these experimental results. In addition, we will present some recent measurements, which will allow us to conclude with a relatively complete description of the pulse generation process.

Spectroscopy with Ultrashort Electrical Pulses

Recently optoelectronic techniques have been used to generate and detect subpicosecond electrical pulses on coplanar transmission lines.[1,2] The frequency bandwidth of these short electrical pulses ranges up to 1 THz and covers an important part of the far infrared energy spectrum from 0 to 30 cm-1, in which can be found the gap frequencies of superconductors, magnetic excitations, and the far infrared modes of lattices and molecules. With the proper generation geometry, these pulses propagate as a single mode excitation of the transmission line, and the pulse reshaping is determined by the frequency dependent dielectric response of the transmission line materials. These features, plus the fact that the earlier observations showed that the subpicosecond pulses broadened to only 2.6 psec after propagating 8 mm on the transmission line, allow for the following spectroscopic applications of these guided wave electrical pulses. In this paper we will discuss the study of far infrared absorption in superconductors and some magnetic modes in rare-earth garnets.