Investigating Quasiparticle Interactions in the Far-Infrared

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2026

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DOI:
https://doi.org/10.22029/jlupub-20986

Abstract

This dissertation investigates ultrafast charge-carrier dynamics and quasiparticle interactions in semiconductors using terahertz (THz) spectroscopy. The THz spectral range provides direct access to low-energy excitations such as intraexcitonic transitions and free-carrier dynamics, making it a powerful probe of many-body interactions in photoexcited semiconductor systems. Using optical pump–terahertz probe spectroscopy and broadband THz emission measurements, several aspects of carrier dynamics in bulk and low-dimensional semiconductors are explored. First, broadband and gapless THz radiation is generated in bulk germanium via phase-controlled quantum interference currents driven by two-color optical excitation. This approach enables efficient emission across the entire THz spectral range and provides insight into the underlying photocurrent generation mechanisms. The dissertation further investigates the formation dynamics of excitons in (Ga,In)As multi-quantum wells following nonresonant optical excitation. Time-resolved THz spectroscopy reveals that excitons emerge from an initially created electron–hole plasma on two distinct timescales: a fast component of approximately 10 ps and a slower component of about 250 ps. By introducing a differential probing method based on weak and strong THz fields, the longstanding discrepancy in reported exciton formation times is resolved. In addition, the interaction of incoherent excitons with an additional electron–hole plasma is studied using a double optical pump–THz probe scheme. The results show that elastic and inelastic scattering processes are governed primarily by the excess energy of the additional carriers rather than their density. Despite excitation energies exceeding the exciton binding energy, inelastic scattering occurs only when suitable final states are available, consistent with predictions from Fermi’s golden rule. The screening of excitonic states by additional charge carriers is investigated by monitoring the transient shift of the intraexcitonic 1s–2p transition. The measurements provide direct access to the time-dependent exciton binding energy and reveal that the screening strength scales linearly with carrier density while the screening time is determined by the carrier excess energy. Finally, transient excitonic states induced by intense optical fields are demonstrated. Detuned optical excitation produces short-lived excitonic resonances that exist only during the temporal overlap of pump and probe pulses and manifest as blue-shifted THz absorption features. Together, these results provide new insight into ultrafast exciton formation, many-body interactions, and THz generation mechanisms in semiconductor systems, highlighting the capabilities of terahertz spectroscopy as a powerful tool for probing quasiparticle dynamics in condensed matter.

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