Jan L. Chaloupka

High-order harmonic generation with an annular laser beam.

High-order harmonics have been generated by the use of an annular laser beam. The nonlinearity of harmonic production and the shorter wavelengths involved cause the harmonics to emerge strongly peaked on the laser axis. Thus the harmonics emerge from the focus inside the missing portion of the laser beam. This permits the laser to be blocked by an aperture that passes the harmonics.

Dynamics of Strong-Field Double Ionization in Two-Color Counterrotating Fields.

The double ionization of helium in bichromatic, circularly polarized intense laser fields is analyzed with a classical ensemble approach. It is found that counterrotating fields produce significant nonsequential double-ion yield and drive novel ionization dynamics. It is shown that distinct pathways to ionization can be modified by altering the relative intensities of the two colors, allowing for unique control of strong-field processes. Electrons are observed to return to the ion at different angles from the angle of ionization, opening new possibilities for probing electronic and molecular structure on the ultrafast time scale.

Single-beam, ponderomotive-optical trap for free electrons and neutral atoms

A ponderomotive-optical trap for energetic free electrons has been generated with a single, high-peak-power laser beam. The focal region consists of an intensity minimum at the center of the focus, with increasing intensities in all directions. The focus can be generated with a two-zone binary phase plate, or with a novel, coaxially segmented wave plate. This scheme can also be used to trap neutral atoms.

Measuring the topological charge of ultrabroadband, optical-vortex beams with a triangular aperture

A simple technique for determining the topological charge of supercontinuum optical vortices is presented. The spatial dispersion inherent to generating broadband vortices with a single forked grating is compensated with a double-pass arrangement from a single spatial light modulator. The vortex charge is determined by inspecting the diffraction pattern through a triangular aperture. It is shown that the topological charge is constant, and can be consistently measured, across a wide range of colors.

Experimental realization of the devil’s vortex Fresnel lens with a programmable spatial light modulator

We present a unique method for experimentally generating multiple vortices by way of a devils vortex lens combined with a Fresnel lens using a spatial light modulator. These lenses have the multifocal properties of fractal zone plates combined with the orbital angular momentum of a spiral phase plate and can be tailored to fit within a small space on an optical bench. Results are presented alongside numerical simulations, demonstrating the robust nature of both the experimental method and the predictive power of the Huygens-Fresnel wavelet theory.

Characterization of a tunable, single-beam ponderomotive-optical trap

The generation and numerical and optical characterization of a single-beam, ponderomotive-optical trap are discussed. A novel segmented wave-plate technique is described, and experimental results of the trap formation are presented. Several methods to tune the traps are discussed, including techniques to realize a bright trap for dark-seeking electrons. The effects of non-Gaussian incident beams and beams with phase aberrations on trap formation are also described.

Strong-field double ionization of rare gases

We have studied the double ionization of helium and other rare gases using an electron-ion coincidence technique. With this scheme, the electron energy spectrum correlated to the creation of a doubly charged ion may be compiled. In all cases, the observed double ionization electron distributions are similar and enhanced at high energies, while the single ionization spectra exhibit distinct differences.

Color schlieren imaging with a two-path, double knife edge system

The traditional arrangement for visualizing optical phenomena with the schlieren technique is modified to include a Mach-Zehnder geometry. This allows for the implementation of two independent knife edges along two different beam paths, resulting in an enhanced combined image that is uniquely adjustable. Post-processed combined images are also generated by spatially separating the paths from each arm and then colorizing and combining the images into a single composite. In this way, bidirectional, color schlieren images have been produced using both white-light and monochromatic sources.

Ionization Dynamics in Intense Two-Color Circularly Polarized Laser Fields

Ionization in intense two-color circularly polarized laser pulses is explored numerically and experimentally. Double ionization is enhanced with counterrotating fields, and diverse dynamics are uncovered that are impossible with linear polarization.

Color Schlieren Imaging with Monochromatic Sources

Color, direction-indicating schlieren images are generated with a two-path system using monochromatic light. Two simultaneous views of physical processes are captured that can be combined into a single, synthesized color composite.