Probing Stark-induced nonlinear phase variation with opto-optical modulation

Abstract

We extend the recently developed technique of opto-optical modulation (OOM) to probe stateresolved ac-Stark-induced phase variations of a coherently excited ensemble of helium atoms. In a joint experimental and theoretical study, we find that the spatial redirection of the resonant emission from the OOM process is different for the low-lying 1s2p state as compared with the higher-lying Rydberg states, and that this redirection can be controlled through the spatial characteristics of the infrared (IR) probe beam. In particular, we observe that the intensity dependence of the IR-induced Stark phase on the 1s2p emission is nonlinear, and that the phase accumulation changes sign for moderate intensities. Our results suggest that OOM, combined with precise experimental shaping of the probe beam, could allow future measurements of Stark-induced phase shifts of excited states.

Population transfer to high angular momentum states in infrared-assisted XUV photoionization of helium

“…At the intermediate intensities considered in this work the NIR field induces substantial AC Stark shifts on both the Rydberg and continuum states. AC Stark shifts of Rydberg states in helium were investigated most recently in reference [23]. Following reference [23], we model the AC Stark shift as a ponderomotive shift U p (t) = |F NIR (t)| 2 /4ω 2 NIR , except for the |1s2p states.…”

“…AC Stark shifts of Rydberg states in helium were investigated most recently in reference [23]. Following reference [23], we model the AC Stark shift as a ponderomotive shift U p (t) = |F NIR (t)| 2 /4ω 2 NIR , except for the |1s2p states. The AC Stark shift of the |1s2p state is known to have a complex NIR intensity dependence [23].…”

Spatial control of extreme ultraviolet light with opto-optical phase modulation

“…The technique to control XUV light that we focus on here, however, is opto-optical modulation (OOM), where a non-resonant light pulse is instead used to control the XUV in an interaction gas [4,5]. The reversal has also been shown, where the behaviour of the XUV light is used as a probe into the interaction between the gas and the non-resonant pulse [6].…”

“…In this article we are focusing on a few closely spaced high-lying p-states in Helium, from 1s8p at 24.38 eV [12]. Previous studies show that in general, high-lying p-states behave approximately ponderomotively, that is their phase shift is approximately linear with the intensity of the control pulse [6]. Henceforth we will confine the discussion to the behaviour of these high-lying p-states in Helium and approximate their phase shifts to be equal to the ponderomotive phase shift, given by the energy shift ∆E(r a , t) ∝ I(r a , t) [13].…”

“…Experiments were undertaken using the setup described in [4][5][6]. The control pulse is an IR pulse that is non-concentric and slightly non-colinear with the XUV light, but overlaps spatially with the XUV light in the mutual focus.…”