James Edward Calvert

Precise and Accurate Measurements of Strong-Field Photoionization and a Transferable Laser Intensity Calibration Standard.

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here, we present measurements of the ionization yield for argon, krypton, and xenon with percent-level accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much needed benchmark for testing models of ionization in noble-gas atoms, such as the widely employed single-active electron approximation.

Transverse electron momentum distribution in tunneling and over the barrier ionization by laser pulses with varying ellipticity

We study transverse electron momentum distribution in strong field atomic ionization driven by laser pulses with varying ellipticity. We show, both experimentally and theoretically, that the transverse electron momentum distribution in the tunneling and over the barrier ionization regimes evolves in a qualitatively different way when the ellipticity parameter describing polarization state of the driving laser pulse increases.

Optical control of collision dynamics in a metastable neon magneto-optical trap

We present results for controlled optical collisions of cold, metastable Ne atoms in a magneto-optical trap. The modification of the ionizing collision rate is demonstrated using a control laser tuned close to the (3s)3P2 to (3p)3D3 cooling transition. The measured ionization spectrum does not contain resonances due to the formation of photoassociated molecules connected to the Ω = 5 excited state potential as predicted by Doery et al (1998 Phys. Rev. A 57 3603–20). Instead, we observe a broad unresolvable ionization spectrum that is well described by established theory (Gallagher and Pritchard 1989 Phys. Rev. Lett. 63 957). In order to induce collisions between pairs of trapped metastables, a control laser is tuned to the red of the transition where we measure a factor of 4 increase in the rate for ionizing collisions. We also investigate the effect of optical shielding by tuning the laser to the blue of the transition and observe a factor of 5 suppression in the ionization rate.

The interaction of excited atoms and few-cycle laser pulses

This work describes the first observations of the ionisation of neon in a metastable atomic state utilising a strong-field, few-cycle light pulse. We compare the observations to theoretical predictions based on the Ammosov-Delone-Krainov (ADK) theory and a solution to the time-dependent Schrödinger equation (TDSE). The TDSE provides better agreement with the experimental data than the ADK theory. We optically pump the target atomic species and measure the ionisation rate as the a function of different steady-state populations in the fine structure of the target state which shows significant ionisation rate dependence on populations of spin-polarised states. The physical mechanism for this effect is unknown.

The interaction of ultrashort laser pulses and exotic atoms

In this work we present results for the ionization of metastable neon with ultrashort laser pulses. In addition to improving on the accuracy of previous measurements of ion yield, we show comparisons to advanced TDSE based theory. This work presents preliminary experimental evidence of the effects of preparing the target atoms into particular angular momentum projection (mj) states on the total ionization yield.

Photoionization yield of atomic hydrogen using intense few-cycle pulses

We present the measured photoionization yield of atomic hydrogen as a function of laser intensity for few-cycle pulses. Fits with exact ab-initio simulations produce better agreement than analytical theories and enable accurate intensity calibration.

Optical collisions in a metastable neon MOT

We present results for controlled optical collisions of cold neon atoms in the (3s)³P₂ metastable state trapped in a magneto-optical trap (MOT).