Strong field atomic physics is the study of the interaction between an atom and an intense laser pulse such that the field strength is "not-so-small" compared to an atomic unit (50 V/\AA) and so it could not be treated as just a perturbation to the atomic system. Depending upon the ionization potential of the target, typical laser intensity required to reach this regime ranges from ~ 10 to 1000 TW/cm2 (the corresponding peak field strength : ~0.017--0.17 V/A). In the low frequency limit, the photoionization process can be interpreted as a tunneling process in which the atomic potential is "tilted" by the laser field allowing the electron to escape via quantum tunneling. The escaped electron wavepacket quivers in the strong laser field whose kinetic energy is characterized by the ponderomotive energy Up which is proportional to the laser intensity and the square of the wavelength. Electron recollision happens when the ionized electron is driven back towards its parent ion by the laser field and it leads to numerous intriguing phenomena such as high-order above-threshold ionization, non-sequential ionization and high-order harmonic generation.

While most of the early experiments were performed in the near-infrared (NIR)(0.8 or 1 mum) wavelengths, an important advance over the last decade has been the emergence of intense mid-infrared (MIR)(~ 2--4 mum) sources. The quadratic scaling of Up with wavelength benefits the study of recollision-driven phenomena and also enables the exploration of strong field interaction deep in the tunneling regime. In addition, the MIR regime is of particular interest for studying molecules due to the presence of vibrational resonances. In this dissertation, we explore several strong field phenomena in atoms and molecules with near and mid infrared fields, including:

1. A comprehensive experimental benchmarking of strong field atomic ionization theories. We performed a comparative study between experiment and theories of the total intensity-dependent ionization yield for different atom species at different laser wavelengths (0.4--4 mum) at linear and circular polarizations in order to investigate the applicability of two commonly used strong field ionization theories, Perelomov, Popov and Terent'ev (PPT) and Ammosov, Delone and Krainov (ADK) models.

2. Ionization and fragmentation of methane in vibrationally resonant MIR fields. We measured the mass spectra of fragments of methane irradiated by MIR fields which cover the resonant frequency range of the C-H bond stretching mode vibration (3.2--3.5 mum). We observed significant enhancement in ionization and dissociation rate at resonant wavelengths compared with non-resonant wavelengths.

3. Electron recollision in tunnel ionization of C60 fullerenes in MIR fields. From the "soft" recollision for low energy electrons we found an unexpected suppression of the "low-energy structure" which might be attributed to the induced dipole field; from the "hard" recollision for high energy electrons we demonstrated the applicability of the "laser-induced electron diffraction" technique for imaging macromolecule and observed hint of laser induced deformation of the molecular structure.

4. Strong field double ionization with circularly polarized NIR fields. We searched for the existence of recollision effects in double ionization of magnesium, zinc and calcium at different wavelength in order to examine the validity of the classical interpretation for the previously observed enhanced double ionization yield in magnesium irradiated by circularly polarized 0.8 microm fields.