Half Cycle Pulses

Typical half-cycle pulse waveform produced in our lab

To probe the impulse response of Rydberg atoms and magnetic materials, we use unipolar half-cycle electromagnetic pulses (of which a typical example is shown to the right) to pump the system and then observe its response. These pulses have pulse lengths on the order of 1 ps (or equivalently bandwidths of approximately 1 THz) and are predominantly unipolar (in that the positive peak field is 2 to 3 times stronger than the negative maximum field).

Half-cycle pulses are generated by the current surge caused when a GaAs wafer biased with a high electric field across it is pumped by a pump pulse from a femtosecond oscillator. The pump pulse excites carriers into the conduction band which are accelerated by the biasing electric field. This current surge radiates an electric field in the far field proportional to the time derivation of the current. The unipolar pulse shape is hence caused by the asymmetry in the duration of the excitation of the charge carriers by the pump pulse as compared to the decay in photo-current set by the carrier lifetime which is on the order of 100's of picoseconds for semi-insulating GaAs.

Half-cycle pulse generation and detection setup

Due to the extremely broadband nature of these pulses (they have a spectral width greater than the center frequency), diffraction effects during their propagation in free space can have large effects on their pulse shape and amplitude. We have optimized our emitters by comparing various biasing gap size. Larger gap sizes result in a lower peak field likely due to reduced optical intensity when averaged over the gap size but increased asymmetry as the length scale upon which diffraction begins is set by the gap size.