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Melbourne, Australia

Ast S.,Leibniz University of Hanover | Mehmet M.,Leibniz University of Hanover | Mehmet M.,Space Time Research | Schnabel R.,Leibniz University of Hanover
Optics Express | Year: 2013

We report the generation of squeezed vacuum states of light at 1550 nm with a broadband quantum noise reduction of up to 4.8 dB ranging from 5MHz to 1.2 GHz sideband frequency. We used a custom-designed 2.6mm long biconvex periodically-poled potassium titanyl phosphate (PPKTP) crystal. It featured reflectively coated end surfaces, 2.26 GHz of linewidth and generated the squeezing via optical parametric amplification. Two homodyne detectors with different quantum efficiencies and bandwidths were used to characterize the non-classical noise suppression. We measured squeezing values of up to 4.8 dB from 5 to 100MHz and up to 3 dB from 100MHz to 1.2 GHz. The squeezed vacuum measurements were limited by detection loss. We propose an improved detection scheme to measure up to 10 dB squeezing over 1 GHz. Our results of GHz bandwidth squeezed light generation provide new prospects for high-speed quantum key distribution. © 2013 Optical Society of America. Source

Lein M.,Space Time Research
Journal of Modern Optics | Year: 2011

Motivated by recent experimental progress in precision investigation of strong-field ionisation by angular streaking 1, we employ streaking by a linearly polarised half-cycle pulse as a theoretical tool to analyse the strong-field ionisation of atoms. For moderate field strengths, the electron momentum distribution is consistent with the interpretation that ionisation occurs most probably at the peak of the field followed by classical electron motion under the combined force of the laser field and the atomic potential. At high field strengths, the momentum distributions together with the classical model indicate that ionisation occurs preferentially before the peak of the field. Surprisingly, the momentum at the peak of the distribution scales almost linearly with the intensity of the half-cycle pulse. © 2011 Taylor & Francis. Source

Zhao J.,Space Time Research | Zhao J.,National University of Defense Technology | Lein M.,Space Time Research
Physical Review Letters | Year: 2013

From the numerical solution of the time-dependent Schrödinger equation, we obtain the times of ionization and return of the laser-driven electron in high-order harmonic generation by probing the dynamics with a second harmonic field polarized orthogonal to the fundamental field and observing the harmonic emission in dependence on the two-color delay. Our retrieval method using complex-time evolution gives ionization and return times in excellent agreement with the quantum-orbit model, while a retrieval based on real-time classical dynamics can introduce substantial errors. Because of the imaginary parts, the harmonic signal polarized along the probe field is nonzero for any two-color delay. The tunneling time can be retrieved under an assumption for the return time. © 2013 American Physical Society. Source

Zhao J.,Space Time Research | Zhao J.,National University of Defense Technology | Lein M.,Space Time Research
New Journal of Physics | Year: 2012

In this paper autoionizing states in the one-dimensional helium atom are investigated by numerical solution of the time-dependent twoelectron Schrodinger equation. The atom is irradiated by an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared few-cycle laser pulse. The XUV pulse populates a superposition of doubly excited states, leading to Fano resonances in the photoelectron spectrum. It is demonstrated that the Fano line profile is strongly modified by the presence of the laser field. Laser-induced coupling between the different doubly excited states causes the population of autoionizing states that cannot be reached by absorbing a single XUV photon from the ground state. The resulting additional peaks in the photoelectron spectrum are modulated as a function of time delay. Furthermore, the photoelectron spectrum exhibits a fringe pattern that is determined by the time delay but is independent of the details of the laser pulse. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Source

Tudorovskaya M.,Space Time Research | Tudorovskaya M.,University of Kassel | Lein M.,Space Time Research
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance. From the numerical solution of the time-dependent Schrödinger equation, we calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase. The analysis reveals the separate contributions of the short and long trajectories as well as the resonance. A range of harmonics is strongly enhanced by the presence of the resonance irrespective of the pulse length. The signature of the resonance remains significant after coherent summation over intensities as a simple method to simulate macroscopic effects. The time-frequency analysis supports the recently proposed four-step mechanism of the enhanced harmonic generation process. © 2011 American Physical Society. Source

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