Photokinetic Sampling

Photo-kinetic Sampling of Ultrafast Waveforms

Introduction

The photo-kintic effect in high Tc superconductors has been used to develop photomixers based on the optical modulation of the supercarrier density and it's effects on the kinetic inductance (KI).

A corrolary to this effect is the potential for sampling of ultrafast waveforms by using the ultrafast photo-modulation of the KI.

We have conducted experiments to investigate this process suign specimens designed to generate ultrafast waveforms on photoexcitaion which are coupled to the sampling structure:

The sampling structure is composed of a coplanar waveguide microwave resonator to ensure that the ultrafast signals generated are easily identified.

The specimen is exceited with two ultrashort laser pulses, one applied to the left microbridge (the source) and the other to the reight (the sampling bridge). The source bridge is biased by a constant transport current whilst the sampling bridge is unbiased.
The two laser beams are modulated at different orthogonal frequencies (for square wave modulation) using a spinning slotted disc. A lockin amplifier is used to record the amplitude of the modulated votages generated by the sampling bridge in response to photoexcitation and the incident voltge pulse from the source bridge. The lockin is referencd to the difference freqeuncy between the two laser beams to isolate the signal generated by sampling alone.

With the device cooled to 10K in a He flo cryostat the time of arrival between the laser pulses is varied to record the time evulution of the electrical signal:

The figure shows the reconstructed signal after scanning the delay line through ~30mm equivalent to ~100ps of time delay. The inital signal at time =10ps (when the signal arives at the sampling bridge) rises very quickly, ~1ps whilst then later signal is generally slower reflecting the low bandwidth of the microwave filter structure that surround ths sampling and source bridges.

Reversal of the polarity of the bais voltage (direction of bais current) across the source bridge reverses the signal and results in an exact mirror image of the signal for forward bias confirming the signal's origin in the modulatio of the source bridge current.

Future developments include the fabrication of meander line structures to increase Lk abnd hence the strcnght of the mixing signal, and design, build and testing of teahertz photo-transciever sets using bow tie antennas along with photokinetic devices.