Picosecond Ultrasonics

Picosecond ultrasonics has been established as a technique for non-destructively measuring sound velocity and attenuation at frequencies between 2 and 500 GHz in metals, semiconductors, dielectrics, and superlattices from 50 K to room temperature. It has been increasingly applied to analyze nanostructures. 

In picosecond ultrasonic measurements, for example of a thin film sample, the pump light pulse of typically 1 nJ of energy and 1 picosecond duration is absorbed at the film surface. This sets up a thermal stress and sends out a strain pulse into the film. The strain pulse will travel to the far side of the film and be partially reflected there. When the strain pulse returns to the free surface of the film, it will induce a change in the optical constant of the film. Thus the returing strain pulse can be deteced by measuring the change in the optical properties of the film. 

The picosecond ultrasonic measurements are usually performed using optical pump probe technique. A detailed diagram of the pump-probe system for the picosecond ultrasonics measurements is shown below. A Ti:sapphire laser is employed as the optical source. The laser beam is split into a pump and probe beam by polarizing cube beamsplitter PCB. The λ/2 waveplate before the PCB1 is to rotate the polarization of the laser beam and therefore to control the ratio between the amplitudes of the two beams. A second λ/2 waveplate-PCB pair is used to adjust the power of the pump beam. The pump beam is chopped by as acousto-optic modulator AOM at 1 MHz and focused on the sample by L3. The probe beam is delayed relatively to the pump beam by the delay stage, focused into a single mode polarization preserving fiber by lens L1 and onto the sample by lens L2. The mandrel wrap filter MF is incorporated to attenuate any fiber cladding modes. After reflecting off of the sample, the light is collected by L4 and detected by the signal detector. The polarizer 90° POL is used to remove any pump light scattered into the probe beam. A small portion of the light reflected by the sample is diverted through a low frequency chopper and detected by the normalization detector. 

Figure 1. Schematic of an picosecond ultrasonic experiment

Figure 2.  Measured change in the reflectivity of the probe beam as a function of delay time. The echos indicate the arrival of the strain pulse at the free surface of the film.