We present measurements on terahertz generation by optical rectification, in which both the thickness of the generation crystal and the wavelength of the generating optical pulse are varied. The difference between the group velocity at optical frequencies and the phase velocity at terahertz frequencies affects the time trace and spectrum of the generated terahertz pulse. For the thickest crystal, we find that the phase mismatch gives rise to the generation of two pulses, separated by 4.5 ps. The physical origin of the observed features is clarified with a simple model that includes phase matching and absorption.
The generation of half-cycle pulses, using laser illumination with femtosecond pulses of a biased GaAs wafer, induces strong electronic noise. We found that electronic noise can be significantly reduced by mounting an RC circuit directly on the semiconductor wafer.
In order to improve both the reliability and stability of the conventional colliding pulse mode-locked (CPM) laser the authors have added an electronic feedback to the system. By linking the integrated output of the CPM laser to the output of the Ar+ laser the average energy in the CPM cavity is kept constant. In this way, the stability of the CPM laser can be maintained for several days on an optical table of moderate quality. In addition, the alignment is substantially easier and the pulse height distribution of the autocorrelation function is improved by a factor of two due to the feedback loop.
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