THz time-domain spectroscopy is used to characterize material properties by measuring the complex frequency response within the frequency range from 0.1 THz to tens of THz. In this regime it is possible to observe various fundamental resonances, such as electronic and phononic excitations in solid-state materials.
To measure the complex frequency response of a material, a THz transient with tailored spectrum is generated by an ultrashort pump pulse in a non-linear process. The interaction with the sample modifies the transient, which is then either reflected or transmitted. The resulting waveform is detected by an ultrashort probe pulse using a non-linear technique based on electro-optic sampling or photoconductive antennas, for example, so that its instantaneous electric field is revealed. The time delay between the probe pulse and the THz field is varied to enable the full reconstruction of the waveform in amplitude and phase of the electromagnetic field. Similarly to other ultrafast optical techniques, e.g. pump-probe spectroscopy, the temporal resolution is given by the duration of the probe laser pulse and not by the bandwidth of the photodetectors or the measurement electronics. This means that THz time-domain spectroscopy can uncover changes on the THz wave packet with sub-cycle temporal resolution.
The signal change induced by the THz field on the probe pulses is minute. To recover this signal, a high signal-noise ratio (SNR), sensitive electronics and averaging are required. It is performed on the basis of the modulation frequency, which is typically between few kHz and hundreds of MHz. Two typical approaches are:
- Exploit the short duty-cycle of the probe pulses using fast photodetectors with boxcar averaging. This method confines the measurement to the part of the period where the signal is present, excluding the part with noise only. It gives access to the highest SNR but is very demanding for the detection electronics.
- Another strategy is to limit the bandwidth of the photodetector so that the signal is distributed over the entire period and is thus close to a sinusoidal. It is then possible to use a lock-in amplifier for signal detection, which guarantees sufficient SNR given that all non-synchronous noise components are rejected efficiently.
The Benefits of Choosing Zurich Instruments
- With Zurich Instruments you can pursue both measurement strategies. In fact, both can run simultaneously on the UHFLI and can be directly compared. For low repetition rates and experiments on a lower budget, the HF2LI or MFLI are attractive alternatives for the second approach.
- The UHF-BOX is a uniquely synchronous boxcar averager, which means that it rejects all noise sources not synchronized with your laser pulses.
- The UHF-BOX acquires data without dead times, thus minimizing the required measurement time.
- With the Periodic Waveform Analyzer, you obtain a highly averaged view of your basic signal. This helps to optimally define the boxcar windows.
- If you can "blank" every second pump pulse in your setup, our background-subtraction feature allows you to take measurements independently of background noise and DC shifts of the signal.