Steve Summers and Dr. Bret C. Hess, Physics and Astronomy
Electrical processes within conducting solids happen on the order of a few femtoseconds (femto = 10 ) which is much too short for conventional -15 electronic equipment to be used for analysis. However, as we enter the age of faster electronics, it has become necessary to understand these processes. In order to acquire this understanding it is necessary to develop techniques with femtosecond resolution.
The most feasible technique for acquiring such resolution is based on the use of ultra short laser pulses. Pulses as short as eight femtoseconds have been produced, allowing for a “strobe photography” technique useful in mapping out changes taking place within conducting solids.
There are numerous ways of using short laser pulses as a diagnostic tool. An extremely sensitive application is the use of pulses to generate radiation in the terahertz range. Figure I illustrates the setup used to generate and collect terahertz radiation. Pulses from the laser are passed through a beam splitter and are directed through two separate arms. The pump pulse is directed onto the sample which lies within an electric field. The electric field is set just below the level needed to excite charge carriers in the solid. The pump pulse provides the additional energy needed to set the carriers into motion.
As the pump pulse passes through the sample, the carriers accelerate, move, and then decelerate as the pulse finishes its pass. The acceleration of the charges creates radiation which is collected by a set of parabolic mirrors into a collimated beam. This beam is focused onto another conducting solid which acts as a receiving antenna for the radiation. The antenna is not contained within an electric field, but the combination of the probe pulses and the terahertz radiation produces an electric field proportional to the amount of radiation produced in the first sample. This electric field can be measured and related to properties of the sample solid.
To make these measurements it is necessary to have an antenna which is capable of detecting radiation in the terahertz range. As we conducted our experiments, we found this to be a difficult task. In order to generate an electric field which could oscillate at such high frequencies, the antenna had to be composed of two conducting strips of aluminum laid on a non-conducting plate of silicon. We made our antennae with aluminum strips 20 pm wide and separated by a distance of only 20 pm. To deal with such small sizes, we relied on help from the Electrical Engineering Department. Using their Integrated Microelectronic Laboratory, we were able to lay the antenna strips.
We found that the challenge we faced was not only laying the aluminum strips, but also avoiding ruining the silicon on which we deposited the strips. After several trials and adaptations of our original plans, we were able to develop some antennae which exhibited some of the properties we were looking for. However, when we tested the antennae in the experimental setup, we found that they were not effective in detecting terahertz radiation. We are investigating possible explanations for this failure.
At the same time that we were working on the antennae we were also working on developing a short-pulse laser. Using a design created at Washington State University, we were able to create a short pulse laser with pulses as short as 17 fs. This time gives us the resolution we need to be able to conduct research using terahertz radiation as well as several other methods.
Now that we have developed the short-pulse laser, we are working on the completion of methods utilizing these pulses. The experiment funded last year by the Office of Research and Creative Activities greatly contributed to this effort, allowing us to gain valuable experience in the formation of terahertz antennas which will soon be implemented into future experiments. These experiments will provide insights into the mechanisms by which charge carriers are excited, accelerate, and move through conductors. Such insights will be helpful in both industry and in the academic world, as we push the boundaries of our current knowledge.
