on 19.10.2020 by Romain Stomp
Standard scanning probe microscopy (SPM) offers high spatial resolution at the expense of poor temporal discrimination. The detection bandwidth in an atomic force microscope (AFM) rarely goes beyond 5 MHz, and the preamplifier bandwidth in an ultra-low-noise scanning tunneling microscope (STM) goes down further to a few kHz due to higher requirements on the amplification gain factor. In the time domain, with direct modulation techniques or fast data capture this bandwidth limitation translates into a temporal resolution no better than a few milliseconds or microseconds, in the best-case scenarios, and corresponds to the fastest detectable change.
Applying pump-probe methods from other fields combines the best of the two techniques to investigate transient phenomena over short timescale or to capture the temporal evolution of nanoscale objects at surfaces. The SPM probe acts as a spatial filter where only the fraction of the light directly under the tip - or the short electric pulse carried by the tip - contributes to the overall sensed force, whereas the temporal resolution is determined solely from the precise time lapse between pump and probe pulses.
Time-resolved SPM methods take different forms depending on the excitation source – electrical or optical – and the detection scheme, which relies on a current preamplifier or on a photodetector. The detection strategy consists in adding a slow modulation frequency that averages the probe response over the time scale detectable by the probe bandwidth. This can be achieved with a mechanical chopper or with an AC modulation frequency carried by the probe pulse. With every pump pulse, a delay value is set between the pump and the next probe pulse, which is then averaged over many repetition periods and demodulated by a lock-in amplifier (LIA) such as the Zurich Instruments MFLI, as in standard optical pump-probe spectroscopy. In time-resolved SPM, the measured amplitude corresponds to the SPM sensor's response for the chosen pump-probe delay, which is also spatially resolved. By sweeping this delay, it is then possible to reconstruct the decay curve of the transient phenomenon over the complete settling process (see figure).
While optical methods offer the highest possible temporal resolution, up to fs, electrical methods can be used in broader contexts – from electron-spin resonance (ESR)-STM to pump-probe Kelvin probe force microscopy – and over a wide range of time scales, from ms down to sub-ns. As with all pump-probe techniques, the temporal resolution does not depend on the slow force or current detector bandwidth but solely on the duration of the stroke pulses and the control of the delay between pump and probe.
The advantage of using electrical pump-probe methods is that all signals can be produced directly from an arbitrary waveform generator such as the Zurich Instruments HDAWG or UHFAWG, with the latter being able to host a lock-in amplifier for detection as well. Having an instrument that controls a wide range of parameters – pulse width, repetition rate, carrier frequency, delay and sweeping speed of delays – from an arbitrary wave generator (AWG) with built-in or in combination with a lock-in detection facilitates the implementation of the full SPM setup.
on 19.10.2020 by Romain Stomp