A Sagnac interferometer splits an incident beam of light into two components which travel in opposite directions of the same path. Consequently, each beam travels an equivalent distance. However, by rotating the entire apparatus at a sufficient speed, a noticeable change in the beams’ interference pattern is observed. This pattern results from one beam travelling against rotation and the other travelling with rotation, resulting in an increase or decrease in apparent path length, respectively. This is known as the Sagnac Effect. By using a traditional mirror-and-laser interferometer setup and a large turntable, we demonstrate the Sagnac Effect by showing that a given angular velocity results in a phase shift which matches what is predicted.

We describe theoretically and experimentally a laser-based method to control the rotations of asymmetric top molecules in three-dimensional space. Our method relies on keeping one axis of a molecule essentially fixed in space along the polarization vector of a nanosecond laser pulse (termed the long pulse) and forcing the molecule to rotate about the aligned axis by an orthogonally polarized, femtosecond laser pulse (termed the short pulse). Experimentally, we use femtosecond timed Coulomb explosion to image the three-dimensional (3D) alignment of the 3,5-difluoroiodobenzene molecule as a function of time after the short pulse. Strong 3D alignment is observed a few picoseconds after the short pulse and is repeated periodically, reflecting directly the revolution of the molecular plane about the aligned axis. Our numerical results, based on nonperturbative solution of the time-dependent Schrodinger equation, are in good agreement with the experimental findings and serve to unravel the underlying physical mechanism of the observations. The experiments and theory explore the influence of the laser parameters on the rotational control, in particular the role played by the intensity of the long and the short laser pulses. To illustrate the generality of our method, we illustrate its applicability to a molecule (3,4-dibromothiophene), with significantly different inertia and polarizability tensors. Finally, our theory shows that the strong 3D alignment obtained by the combined laser pulse method can be converted in to field-free alignment by rapid truncation of the long laser pulse.

We illustrate, experimentally and theoretically, a laser-based method to control the rotations of polyatomic molecules in 3D space. A linearly polarized nanosecond pulse strongly aligns the most polarizable axis of an asymmetric top molecule along its polarization axis while an orthogonally polarized, femtosecond pulse sets the molecules into controlled rotation about the aligned axis. As a result, strong three-dimensional (3D) alignment occurs shortly after the femtosecond pulse and is repeated periodically, reflecting coherent revolution about the molecular axis. Our method opens new directions for research in orientationally confined complex molecules.

We combine newly developed experimental and theoretical methods to explore and control the rotations of asymmetric top molecules. In the low-temperature limit, the revival spectra of laser-driven rotational wave packets exhibit a fascinating quantum-mechanical structure, which is qualitatively different for asymmetric tops compared to symmetric or linear tops. With increasing fluence, the structure gradually simplifies, illustrating a controllable transformation of the rotational dynamics. Our theoretical and experimental predictions are in qualitative agreement.

The alignment dynamics of polyatomic molecules subject to two temporally overlapping nonresonant laser pulses is studied theoretically and experimentally. We examine the potential advantage of combining a long (compared to the molecular rotational periods) with a short pulse, where both laser fields are linearly polarized with common polarization direction. Experimentally, iodobenzene molecules are irradiated by a 2 ps pulse at 800 nm synchronized to the peak of a 9 ns pulse at 1064 nm. The alignment dynamics following the short laser pulse is measured using time-resolved Coulomb explosion with a delayed 25 fs-long intense laser pulse. The alignment attained with the two-pulse combination is significantly enhanced compared to that attainable with either a short or a long pulse alone under nonionizing conditions. Numerically, we solve the time dependent Schrodinger equation nonperturbatively for asymmetric top, symmetric top, and linear molecules subject to similar combinations of long and short excitation pulses. The alignment dynamics, and in particular the alignment enhancement, are explained and their generality is tested.

We review the theory of intense laser alignment of molecules and present a survey of the many recent developments in this rapidly evolving field. Starting with a qualitative discussion that emphasizes the physical mechanism responsible for laser alignment, we proceed with a detailed exposition of the underlying theory, focusing on aspects that have not been presented in the past. Application of the theory in several pedagogical illustrations is then followed by a review of the recent experimental and theoretical advances in the field. We conclude with a discussion of new directions, future opportunities, and areas where we expect intense laser alignment to play a role in the future. Throughout we emphasize the recent evolution of the method of nonadiabatic alignment from isolated diatomic molecules to complex media.

Nonadiabatic alignment of an asymmetric top molecule induced by a short, moderately intense laser pulse is studied theoretically and experimentally. Numerically, we solve nonperturbatively the time-dependent Schr\"odinger equation for a general asymmetric top molecule subject to a moderately intense laser field, and analyze the dependence of the alignment dynamics on the field strength and on the rotational temperature. Experimentally, we use time-resolved photofragment imaging to measure the time-dependent angular distributions of the spatial orientation of the molecules. Our studies, using iodobenzene as a test molecule, focus on the short-time alignment dynamics, during and after the pulse.

The rotational revival structure of asymmetric top molecules, following irradiation by an intense picosecond laser pulse, is explored theoretically and experimentally. Numerically we solve nonperturbatively for the rotational dynamics of a general asymmetric top subject to a linearly polarized intense pulse, and analyze the dependence of the dynamical alignment on the field and system parameters. Experimentally we use time-resolved photofragment imaging to measure the alignment of two molecules with different asymmetry, iodobenzene, and iodopentafluorobenzene. Our numerical results explain the experimental observations and generalize them to other molecules. The rotational revival structure of asymmetric tops differs qualitatively from the intensively studied linear top case. Potentially it provides valuable structural information about molecules.