THz spectroscopy
Terahertz time-domain spectroscopy
The terahertz pulses are generated via optical rectification in a nonlinear crystal using femtosecond near-infrared pulses. For detection, the terahertz pulses overlap with near-infrared pulses in another nonlinear crystal. Due to the electro-optic effect, the polarization of the near-infrared pulses rotates according to the strength of the terahertz pulses. By scanning the delay of the near-infrared pulses, both the magnitude and phase of electric field of the terahertz pulses can be recorded. By compariing the complex transmission coefficient with and without passing through a sample, the complex value of the material parameters such as refractive index or dielectric constant in terahertz ranges can be measured.
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Figure: A typical terahertz waveform and its Fourier transform. The FID is due to water absorption.
Pump-probe spectroscopy
A strong (optical or terahertz) pump pulse is used to excite the sample into a non-equilibrium state. Then, a weak (optical or teraherz) probe pulse is used to monitor the pump-induced change in material parameters. By scanning the delay of the pump and probe pulses, the relaxation dynamics of the sample can be studied.
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Figure: A transient reflectivity trace from photo-excited bismuth.
Single-shot spectroscopy
Conventionally, recording a terahertz waveform or a pump-probe trace requires repetitively moving a translation stage to tune the delay. The data acquisition time is long and we cannot study irreversible processes like phase transitions or photochemistry. To address these issues, we have developed the single-shot detection technique. We can obtain a pump-probe trace or a terahertz spectrum with one laser shot.
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Figure: The convensional pump-probe spectroscopy and our single-shot pump-probe spectroscopy.
Experimental setup
