Abstract
Helium is the simplest multi-electron atom and its electronic structure can be calculated very precisely by QED many-body calculations. Comparisons between experimental results and theoretical predictions therefore provide the best testing ground for atomic calculations and enable our better understanding of QED effects in bound systems. In this thesis, precision spectroscopy of the transitions in the helium singlet states including 21S0-21P1 transition at 2058 nm and 21P1-31D2 transition at 668 nm has been performed. Electromagnetically induced transparency (EIT) with a ladder-type system of the 21S0-21P1-31D2 transitions has also been studied. At first, a 2.06 μm tunable single-frequency volume Bragg grating (VBG)-based short-cavity Tm:Ho:YLF laser is constructed. The laser has hundreds of mW output power and a mode-hop free tuning range of 10 GHz. The frequency stabilization of this laser to a CO2 absorption line with a stability of 3×10-9 is achieved using frequency modulation and fluorescence detection. Precision spectroscopy measurements of the 21S0-21P1 transition at 2058 nm are performed using two Tm:Ho:YLF lasers and an Er:fiber-based optical frequency comb (OFC). This represents the first Doppler-free measurement on this transition. The absolute frequency of this transition in 4He is measured to be 145622892886(183) kHz with a relative uncertainty of 1.3×10-9. This result can be combined with other precisely measured transitions to derive the ionization energy of the 21P1 state with an uncertainty of approximately 200 kHz. The 3He-4He isotope shift of this transition is also determined to be 4248.7(5.3) MHz, 10 times more precise than previous measurement. Furthermore, the line shapes of the measured spectra are also studied. The frequency metrology of the 21P1-31D2 transition at 668 nm is performed using an OFC-stabilized external cavity diode laser (ECDL). The absolute frequency of this transition in 4He is measured to be 448791399113(268) kHz with a relative uncertainty of 6×10-10. This result combined with other precisely-known transitions enables us to derive the ionization energy of the 21P1 and the 21S0 states and the separation between the 31D2 and 33D1 states in 4He. By comparison with the theories, a serious discrepancy with the most precise atomic calculation is found on the ionization energy of the 21P1 state. This will stimulate more theoretical investigations on the singlet states of helium. In addition, the ladder-type EIT signal of the 21S0-21P1-31D2 transition is observed. The spectral width is reached to sub-natural linewidth. The EIT signals in different experimental conditions are also studied.