Ultrafast Heterodyne Optical Sampling applied to Picosecond Ultrasonics and Nanoscale Heat Transfer

Pump-probe techniques are widely used to measure events on time scales much shorter than the resolution of electronic detectors, and are applied in such diverse fields as ultrafast spectroscopy, photo-acoustics, TeraHertz imaging, etc. In ultrafast photoacoustics measurements for instance, a pump beam launches in the sample acoustic waves, which are detected by a second, temporally shifted probe beam. Typical detection methods rely on very small changes in the reflection coefficient of the sample surface, requiring an averaging of the signal to improve the signal to noise ratio. Traditional pump-probe methods use a mechanical delay line to shift the two pulses in the time domain, where each measurement point corresponds to a single mechanical position of the delay line. Although very efficient for small measurement ranges, extending this method in the hundreds of picoseconds or nanosecond lead to a very long acquisition time, and unpractical length for the delay line. We present a new, compact detection system, using a compact dual-oscillator ultrafast laser system, specifically designed for pump-probe measurements over time scales as long 20 ns, with a sub-picosecond resolution. This system does not use any mechanical delay line, and allows for extremely fast acquisition time