A 10 GHz repetition rate laser system for applications in THz frequency metrology and ultrafast time-domain spectroscopy

Abstract

In this thesis a high repetition rate dual laser system based on two femtosecond Ti:sapphire lasers with a pulse repetition rate of 10 GHz is developed. Stabilization of the repetition rate offset frequency between the two lasers allows the implementation of the high speed asynchronous optical sampling technique. The configuration of the system including data acquisition and its limitation is discussed in detail. In addition, the noise reduction mechanism of the asynchronous optical sampling technique is illuminated and compared to the commonly used lock-in method. The potential of the system is evaluated by performing terahertz time-domain spectroscopy in lock-in and asynchronous optical sampling configuration as well as the generation of continuous-wave terahertz radiation by photomixing of isolated frequency comb modes. The dual laser system also provides the capability to perform absolute frequency calibration of quantum cascade lasers using a Vernier-like approach. The concept requires phase-locking of the quantum cascade laser to the repetition rates of the femtosecond lasers. This allows for the measurement of the absolute frequencies of a multi-mode quantum cascade laser emitting at 2.5 THz. Taking advantage of the large spacing between the individual laser modes associated with the high repetition rate of 10 GHz, a setup for line-by-line pulse shaping allowing for the generation of arbitrary optical waveforms is implemented. As tentative result, the multiplication of the repetition rate by phase-only modulation is presented. The selective excitation of zone-folded coherent phonons in a semiconductor superlattice is studied by using a two pulse excitation scheme. This experiment demonstrates the value of high repetition rate pulse trains for coherent control and resonant excitation of acoustic modes.