Takeshi Suzuki - THz spectroscopy

Terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is a technique used to measure the properties of materials in the terahertz frequency range (typically from 0.1 to 10 THz, or 100 GHz to 10 THz). It combines aspects of both time-domain and frequency-domain spectroscopy, using ultrafast pulses of light to probe materials.

Key Concepts:

Generation of THz pulses: A short laser pulse is directed onto a photoconductive antenna or other generation source, which produces a broadband THz pulse. The pulse typically lasts femtoseconds (fs), which is much shorter than the period of the THz radiation itself.
Transmission/Reflection through a sample: This THz pulse is then passed through or reflected off a material sample. The material will interact with the THz pulse, which can cause changes in the amplitude or phase of the pulse depending on the properties of the material, like its conductivity, permittivity, and molecular structure.
Detection of the transmitted pulse: After the pulse has interacted with the material, it is detected by a second photoconductive antenna or other detector. The timing of this pulse’s arrival is recorded, providing a time-domain signal.
Analysis: By analyzing how the shape, phase, and amplitude of the pulse changes as it travels through or is reflected off the material, you can extract a wealth of information about the material's optical and electronic properties, such as its dielectric response, conductivity, and phonon modes (vibrations at the atomic scale).

This technique is particularly useful in studying materials like semiconductors, polymers, biological tissues, and other materials that interact with THz radiation. THz-TDS is valued for its high temporal resolution and its ability to probe properties of materials in the low-frequency, yet high-energy, THz range. It also doesn’t require contact with the material, and is non-destructive, which is great for examining sensitive samples.

We have particularly used this technique in a pump and probe geometry to investigate the electron-hole system in semiconductors generated by the photo-excitation. The experimental scheme is the pump-probe geometry, where optical pump is used to excite electron hole pairs and terahertz probe is transmitted through sample to detect the carrier dynamics (実験配置). As a playground, we select a bulk silicon, where a lot of phases are existent shown as the phase diagram as a function of temperature and electron hole pair density (シリコンの相図). We have particularly study ①formation dynamics of the exciton and electron-hole droplets, ②cooling dynamics of electron and holes after the photoexcitation, and ③exciton Mott transition that is the insulator metal transition accompanied by the increase of electron hole pairs to screen the Coulomb attractive interaction.