Development and Application of Advanced Ultrafast Spectroscopy

<p><strong>Ultrafast spectroscopy is a critical tool for probing the photophysics of advanced materials. An optimal system requires high temporal resolution, broadband detection, high signal-to-noise ratio, and rapid data acquisition, all of which depend on the choice of light source. While titanium-sapphire (Ti-sapphire) lasers deliver short and high-energy pulses, their inherent nature restricts simultaneous operation at high power and high repetition rates. In contrast, Ytterbium-based (Yb-based) lasers offer a compact, stable, and low-maintenance alternative. Their significantly higher repetition rates potentially improve signal-to-noise ratio and accelerate data acquisition, addressing the major drawbacks in Ti-sapphire-based spectroscopic systems. However, their longer pulse duration necessitates external light source for temporal compression to achieve optimal time resolution to probe ultrafast processes. To overcome this limitation, this thesis develops several spectroscopic systems integrating Yb-based lasers with advanced external light sources to provide high-performance measurements. We anticipate these setups will establish a new standard for ultrafast spectroscopy.</strong></p><p>First, transient absorption spectroscopy (TAS) systems were developed using Yb-based lasers, with performance further enhanced by integrating multiple-plate compression (MPC) light sources. This setup achieves a temporal resolution of 3.2 fs with an octave-spanning detection range. The high-intensity and broadband spectrum of MPC enables flexible wavelength selection via commercial spectral filters, eliminating the need for complex optical parametric amplification. The high brightness of MPC facilitates measurements of materials with strong absorption, reflection loss, or scattering. Additionally, the red-shifted central wavelength of Yb-based lasers compared to Ti-sapphire lasers is particularly advantageous for studying near-infrared (NIR) photoresponse. Based on these advantages, we established a platform for the research and development of NIR optoelectronic materials. This is demonstrated by direct device ultrafast TAS measurements on a state-of-the-art NIR organic photodetector under conditions with and without an external bias to simulate operational scenarios and elucidate the photophysical mechanisms underlying its exceptional performance.</p><p>Second, Yb-based lasers paired with fiber amplifiers and MPC light sources were employed to improve the performance of transient grating photoluminescence spectroscopy (TGPLS). These advanced light sources deliver ultrashort pulses and elevated repetition rates, enabling broadband time-resolved photoluminescence (TRPL) measurements with excellent temporal resolution, high signal-to-noise ratio, and rapid data acquisition. To validate these capabilities, the TGPLS system was applied to study an organic light-emitting material, Pt(II) complex 4H. This investigation successfully captured vibrational coherence during the fast intersystem crossing and elucidated excited-state deactivation pathways via phosphorescence.</p><p>A novel merged spectroscopic system was developed by integrating TAS and TGPLS within a single experimental setup based on a Yb-based fiber amplifier. This unified configuration enables broadband detection in both techniques with similar temporal resolution of <200 fs and identical excitation conditions, minimizing discrepancies between measurements. These features allow for direct comparison of spectral and kinetic features obtained through global fitting. With these advantages, we study the ultrafast solvation processes in a laser dye, where distinct spectral evolution over time were captured by the two spectroscopic methods, yet their global fitting results exhibited identical features. Overall, this thesis presents significant advancements in ultrafast spectroscopy by leveraging Yb-based laser systems and next-generation light sources. These innovations enhance detection sensitivity, reduce data acquisition time, and expand detection bandwidth, establishing a robust foundation for future ultrafast spectroscopic research and the exploration of emerging materials.</p>