Time-Resolved THz Spectroscopy

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Time–resolved THz spectroscopy (TRTS) is an optical pump/THz probe technique which is used to study systems in which a visible excitation initiates a change in far-infrared absorption properties on a sub-picosecond timescale. 

The base of our TRTS setup is a regeneratively amplified Ti:sapphire laser which outputs 100-fs, 800-nm pulses at a repetition rate of 1 kHz.  The output is split into two beams: one for the pump beam and one for THz generation and detection.  THz radiation is generated via optical rectification in a ZnTe(110) crystal and detected via free-space electo-optic sampling in an additional ZnTe(110) crystal.  The THz beam is focused onto the sample at the same spatial and temporal position as the pump beam.  A schematic of our setup is presented in Figure 1.  

TRTS

Figure 1. The experimental setup for TRTS.

Two different types of experiments are possible with TRTS.  In one type, the change in the peak THz time-domain transmission is monitored as a function of the time delay between the pump and probe beams.  Because the pump beam is chopped, the measured signal is the change in THz transmission observed upon photoexcitation (see figure 2). These “pump scans” provide information about the timescale associated with the onset of photoconductivity and the lifetime of that conductivity.

Pump Scans

Figure 2. A THz-TDS probe scan (a) and a TRTS pump scan for GaAs (b).  The blue circle in (a) is centered on the time-domain peak of the THz pulse. In (b), the change in that peak absorption is shown as a function of the delay between the optical pump pulse and THz probe pulse.

With the other technique, the full THz waveform is collected at a fixed pump delay time.  From the THz waveform, the photo-excited absorption coefficient, refractive index, and thus conductivity can be calculated.

Figure 3. If the THz pulse precedes the optical pump pulse, the THz pulse will be delayed and attenuated due to absorption by the sample. If the optical pulse photoexcites the sample first, the THz pulse will be further attenuated due to absorption by mobile electrons. Comparison of the pulses obtained in (a) and (b) gives the photoexcited optical constants of the materia when it is photoexcitedl.
 

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