Picosecond laser ultrasonics (PLU) measurement is the study of materials using high frequency acoustic pulses generated and detected by ultrashort optical pulses typically <1 ps in duration. This technique, which is primarily used in material characterization, can detect properties of opaque thin films that are not observable by conventional optical inspection. The modulation imposed by the pump is weakly transferred to the probe beam resulting in a very low level of modulation on a large DC background. The measurement of this modulation is normally performed with a single point detector in combination with a lock-in amplifier. These single point measurements are extremely time consuming, this leads to impracticalities in the measurement of sufficient data to create thickness maps of even small areas. However, using sufficiently sensitive custom CMOS cameras allows for measurements to be made in parallel, greatly speeding up the measurement procedure. A description of the measurement technique used is given below.
A schematic of the optical set-up is shown to the left.The femtosecond laser used (not shown), is a Spectra Physics Tsunami (780 nm wavelength, 100 fs pulse width, 80 MHz repetition rate). The output of this laser is divided into a pump arm and a probe arm with a beam splitter. The optical power of the pump arm is approximately 1 nJ per pulse, the power of the probe arm is around 1000 times less than this. The pump beam is modulated with an optical chopper, the frequency of which is derived from the same clock that generates the camera control logic. The energy of the pump pulse is absorbed by the sample causing localised heating and expansion, which generates a very high frequency ultrasonic pulse to propagate. By varying the optical delay in the probe arm, time-resolved measurements can be made of the acoustic wave as it propagates into the sample.
The figure to the right shows the origin of the Brillouin oscillations. The surface of the sample and the propagating longitudinal wave form an interferometer, where the phase between the interfering beams varies with the position of the sound wave. Changing the delay between the probe and pump beams enables the interference pattern (Brillouin oscillations) to be recovered.
A pulse echo method of measurements is also performed. This method takes advantage of repeating reflections of the ultrasonic pulse between the film surface and the film-substrate interface. The time between the repeating echoes is measured, and if the longitudinal velocity of the sample film is known, the thickness of the sample film can be calculated.