| Two of the first systematic studies of high harmonic generation are explored with low-binding energy atoms, cesium and rubidium, under the influence of a strong mid-infrared field, 3–4 μm. High harmonic generation (HHG) is the result of the nonlinear response of a collection of atoms to an intense laser field. HHG consists of odd harmonics of the driving field with spacing of twice the laser frequency between consecutive odd orders. The first experiment involves studying the spectral and temporal characteristics of HHG. Since the long-wavelength driving field produces harmonic radiation with visible/UV spectral content, the emitted harmonic radiation is experimentally accessible with standard temporal and spectral metrology. We present pulse width measurements of harmonics 5 through 9 with their corresponding measured bandwidths. From these measurements, we find orders 5 through 9 have a large residual chirp. We assess the measured chirp values against two possible mechanisms, the intensity dependent phase and dispersion due to free electrons. We find the intensity dependent phase underestimates the measured chirp by 3 orders of magnitude, while the dispersion due to free-electrons has good agreement.; Since the D1 and D2 transitions in alkali metals are easily driven by visible light, harmonic generation from excited atoms is studied for the first time. With an incoherent ensemble of excited and ground state Rubidium atoms, we have observed harmonic yields enhanced by up to 4 orders of magnitude when compared to ground state alone. We find the enhancement is both intensity and pressure dependent with the greatest enhancement occurring for the low intensity and low pressure. We have measured increased harmonic bandwidths from the excited atomic medium as large as a factor of 3 when compared to the ground state. These widths are nominally 20 times greater than the mid-infrared bandwidth. The enhanced harmonic signal is believed to come from the 1% population in the 5D3/2 state, which is populated through a transition that is nearly degenerate with the D2 line. |
