Pump-Probe Spectroscopy

Pump-probe schemes are used to measure ultrafast phenomena using short laser pulses. As the pump pulse impinges on the sample, various physical phenomena can be induced – for instance, an electronic excitation. After an adjustable time delay often controlled with an optical delay line, a probe pulse hits the sample: its transmission and/or reflection is consequently measured. By monitoring the probe signal as a function of the time delay, it is possible to obtain information on the decay dynamics of the generated excitation.

A key aspect of pump-probe measurements is that the time resolution is governed by the duration of the laser pulses and not the bandwidth of the photodetectors or the signal-recording electronics. Numerous techniques investigating ultrafast phenomena are based on the pump-probe scheme, such as THz and Raman spectroscopy.

The repetition rates of pulsed lasers are typically between a few kHz and hundreds of MHz. As the signal-to-noise ratio (SNR) of individual pulses is often very small, sensitive detection electronics and heavy averaging are required. Two of the most common approaches are:

• Use fast photodetectors in combination with boxcar averaging. This makes it possible to record the signal over a short duty cycle, and therefore excludes the vast amounts of data recording time where only noise is present. This method offers the highest SNR but is also the most demanding for the electronics.

• Adjust the bandwidth of the photodetector such that the resulting signal covers the entire period and is close to a sinusoidal. This technique can rely on a lock-in amplifier for signal detection, which is easy to set up and provides sufficient SNR as all non-synchronous noise components are efficiently rejected.

• With Zurich Instruments, you can pursue the two discussed measurement strategies. In fact, both can run simultaneously on the UHFLI and can thus be directly compared. For a low repetition rate and experiments on a lower budget, the HF2LI or the MFLI are attractive alternatives for the second approach.
• The UHF-BOX is the only synchronous boxcar averager. It rejects all noise sources not synchronized with your lasers.

• The UHF-BOX acquires data without dead times, thus minimizing the total measurement time.
• With the Periodic Waveform Analyzer, a highly averaged view of your basic signal allows you to easily define the boxcar windows.
• If your setup enables you to “blank” every second pump-pulse, our background subtraction feature allows you to take measurements independently of background noise and DC shifts of the signal.