  Information technology demands high-speed optoelectronic devices, but going
beyond the one terahertz (THz) barrier is challenging due to the difficulties
associated with generating, detecting, and processing high-frequency signals.
Here, we show that femtosecond-laser-driven phonons can be utilized to
coherently manipulate the excitonic properties of semiconductors at THz
frequencies. The precise control of the pump and subsequent time-delayed
broadband probe pulses enables the simultaneous generation and detection
processes of both periodic lattice vibrations and their couplings with
electronic states. Combining ultralow frequency Raman spectroscopy with
first-principles calculations, we identify the unique phonon mode-selective and
probe-energy dependent features of electron-phonon interactions in layered
PdSe2. Two distinctive types of coherent phonon excitations could couple
preferentially to different types of electronic excitations: the intralayer
(4.3 THz) mode to carriers and the interlayer (0.35 THz) mode to excitons. This
work provides new insights to understand the excited-state phonon interactions
of 2D materials, and to achieve future applications of optoelectronic devices
operating at THz frequencies.
