THz Time-Domain Spectroscopy

Related products: UHF-BOX, UHFLI, HF2LI, MFLI

THz time-domain spectroscopy is used to characterize material properties by measuring the complex frequency response within the frequency range of 0.1 THz to tens of THz. In this regime various fundamental resonances can be found, such as electronic and phononic excitations in solid state materials.

To measure the complex frequency response, a THz transient with tailored spectrum is generated by an ultrashort pump pulse in a non-linear process. On meeting the sample the transient is modified as it is reflected or transmitted. The resulting waveform is detected by an ultrashort probe pulse using a non-linear technique, such as electro-optic sampling or photoconductive antennas, which reveals its instantaneous electric field. The time delay between the probe pulse and the THz field is varied to allow full reconstruction of the waveform in amplitude and phase of the electromagnetic field. Similar 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 measurement electronics. This means THz time-domain spectroscopy can reveal changes on the THz wave packet with sub-cycle temporal resolution.

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Measurement Strategies

The signal change induced by the THz field on the probe pulses is minute. To recover the signal,a high signal-noise ratio, sensitive electronics and averaging is 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 withboxcar averaging. This method confines the measurement to the part of the period when the signal is present and omits the rest with noise only. It promises highest SNR but is the very demanding for the detection electronics as well.
  • Another strategy is to limit the bandwidth of the photodetector so the signal is distributed over the entire period and is therefore close to a sinusoidal. This allows using the lock-in amplifier for signal detection, which is easy to set up and provides sufficient SNR because all non-synchronous noise components are rejected efficiently.

Your benefits measuring with Zurich Instruments

  • With Zurich Instruments equipment you can pursue both measurement strategies. In fact, on the UHFLI both strategies can run simultaneously and can be directly compared. For low repetition rate and experiments on a lower budget, the HF2LI or MFLI are attractive alternatives for the second approach.
  • The UHF-BOX is uniquely asynchronous boxcar averager and consequently rejects all noise sources not in synchronization with your laser pulses.
  • The UHF-BOX Boxcar acquires data without dead-time and therefore minimizes the required measurement time.
  • With the Periodic Waveform Analyzer, you get a highly averaged view of your basic signal. This helps you to define the boxcar windows in an optimal way.
  • If your setup allows the blanking of everysecond pump-pulse, our background-subtraction feature allows you to take measurements independent of the background noise and DC shifts of the signal.

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