Single Shot Time-Resolved Terahertz Spectroscopy for Optoelectronic Materials
Authors:   , Djorović Aleksa 
Published: November 2021

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>