Frustrated tunnel ionization in the few-cycle regime

Frustrated tunnel ionization (FTI) is one of the dominant channels in strong field ionization that results in the excitation of atoms. Recent studies have shown that there is a significant number of FTI events for multi-cycle pulses with the theory predicting that the excitation efficiency increases with pulse duration decreasing into the few-cycle regime. Our work concentrates on experimentally investigating the effect of few-cycle pulses on the FTI excitation process. We use pulses with duration 6 fs centred at 800 nm at intensities up to 0.8 PW crossed with an atomic Ar beam. We find that with few-cycle pulses there is more FTI per tunneling event and that for the same pulse energy more FTI is generated.R. D. Glover, D. Chetty, B. A. deHarak, A. J. Palmer, M. A. Dakka, J. L. Holdsworth, I. V. Litvinyuk, A. N. Luiten, P. S. Light, R. T. San

Relativistic nondipole effects in strong-field atomic ionization at moderate intensities

We present a detailed experimental and theoretical study on the relativistic nondipole effects in strong-field atomic ionization by near-infrared linearly polarized few-cycle laser pulses in the intensity range of 10¹⁴–10¹⁵  W/cm². We record high-resolution photoelectron momentum distributions of argon using a reaction microscope and compare our measurements with a truly ab initio fully relativistic 3D model based on the time-dependent Dirac equation. We observe counterintuitive peak shifts of the transverse electron momentum distribution in the direction opposite to that of laser propagation as a function of laser intensity and demonstrate an excellent agreement between the experimental results and theoretical predictions.Nida Haram, Igor Ivanov, Han Xu, Kyung Taec Kim, A. Atia-tul-Noor, U. Satya Sainadh, R.D. Glover, D. Chetty, I.V. Litvinyuk, and R.T. San

Carrier-Envelope Phase-Dependent Strong-Field Excitation

We present a joint experimental-theoretical study on the effect of the carrier-envelope phase (CEP) of a few-cycle pulse on the atomic excitation process. We focus on the excitation rates of argon at intensities in the transition between the multiphoton and tunneling regimes. Through numerical simulations, we show that the resulting bound-state population is highly sensitive to both the intensity and the CEP. The experimental data clearly agree with the theoretical prediction, and the results encourage the use of precisely tailored laser fields to coherently control the strong-field excitation process. We find a markedly different behavior for the CEP-dependent bound-state population at low and high intensities with a clear boundary, which we attribute to the transition from the multiphoton to the tunneling regime.D. Chetty, R. D. Glover, X. M. Tong, B. A. deHarak, H. Xu, N. Haram, K. Bartschat, A. J. Palmer, A. N. Luiten, P. S. Light, I. V. Litvinyuk, and R. T. San

Relativistic Nondipole Effects in Strong-Field Atomic Ionization at Moderate Intensities

© 2019 American Physical Society.We present a detailed experimental and theoretical study on the relativistic nondipole effects in strong-field atomic ionization by near-infrared linearly polarized few-cycle laser pulses in the intensity range of 1014-1015 W/cm2. We record high-resolution photoelectron momentum distributions of argon using a reaction microscope and compare our measurements with a truly ab initio fully relativistic 3D model based on the time-dependent Dirac equation. We observe counterintuitive peak shifts of the transverse electron momentum distribution in the direction opposite to that of laser propagation as a function of laser intensity and demonstrate an excellent agreement between the experimental results and theoretical predictions11sciescopu

Extreme ultraviolet interferometer using high-order harmonic generation from successive sources

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs. © 2012 American Physical Society

Measuring laser carrier-envelope-phase effects in the noble gases with an atomic hydrogen calibration standard

We present accurate measurements of carrier-envelope-phase effects on ionization of the noble gases with few-cycle laser pulses. The experimental apparatus is calibrated by using atomic hydrogen data to remove any systematic offsets and thereby obtain accurate CEP data on other generally used noble gases such as Ar, Kr, and Xe. Experimental results for H are well supported by exact time-dependent Schrödinger equation theoretical simulations; however, significant differences are observed in the case of the noble gases. © 2017 American Physical Society1