We demonstrate coherent control of multiphoton and above-threshold photoemission from a single solidstate nanoemitter driven by a fundamental and a weak second harmonic laser pulse.Depending on the relative phase of the two pulses, electron emission is modulated with a contrast of the oscillating current signal of up to 94%.Electron spectra reveal that all observed photon orders are affected simultaneously and similarly.We confirm that photoemission takes place within 10 fs.Accompanying simulations indicate that the current modulation with its large contrast results from two interfering quantum pathways leading to electron emission.
We demonstrate coherent control of multiphoton and above-threshold photoemission from a single solidstate nanoemitter driven by a fundamental and a weak second harmonic laser pulse.Depending on the relative phase of the two pulses, electron emission is modulated with a contrast of the oscillating current signal of up to 94%.Electron spectra reveal that all observed photon orders are affected simultaneously and similarly.We confirm that photoemission takes place within 10 fs.Accompanying simulations indicate that the current modulation with its large contrast results from two interfering quantum pathways leading to electron emission.
Coherent control steers a quantum system from an initial state to a target state by controlling quantum interference phenomena via an external field, which is central to vast applications ranging from quantum information processing to attosecond physics. Here, we analyze the quantum pathways interference in two-color coherent control of photoemission using exact analytical solutions of the time-dependent Schr\"odinger equation. The theory includes all possible quantum pathways and their interference terms. Constructive (or destructive) interferences among the pathways leads to the maximum (or minimum) emission with varying phase delay of the two-color lasers. It is found that increasing the intensity ratio of the second harmonic $(2\ensuremath{\omega})$ to fundamental $(\ensuremath{\omega})$ lasers results in less contribution from the $\ensuremath{\omega}$ pathway (absorption of $\ensuremath{\omega}$ photons only) and more contribution from multicolor pathway (simultaneous absorption of both $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ photons) and $2\ensuremath{\omega}$ pathway (absorption of $2\ensuremath{\omega}$ photons only), and therefore stronger pathways interference and increased visibility larger than 95%. Increasing bias voltages shifts the dominant emission to processes with lower-order photon absorption, which sequentially decreases the interference between the $\ensuremath{\omega}$ and the $2\ensuremath{\omega}$ pathways, and between single-color and multicolor pathways, leading to two peaks in the visibility.
Ultrafast high harmonic beams provide new opportunities for coherently controlling excitation and ionization processes in atoms, molecules, and materials on attosecond time scales by employing multiphoton two-pathway electron-wave-packet quantum interferences. Here we use spectrally tailored and frequency tuned vacuum and extreme ultraviolet harmonic combs, together with two phase-locked infrared laser fields, to show how the total single and double photoionization yields of argon can be coherently modulated by controlling the relative phases of both optical and electronic-wave-packet quantum interferences. This Letter is the first to apply quantum control techniques to double photoionization, which is a fundamental process where a single, high-energy photon ionizes two electrons simultaneously from an atom.
In coherent control schemes, pathways connecting an initial and a final state can be independently controlled by manipulating the complex amplitudes of their transition matrix elements. For paths characterized by the absorption of multiple photons, these quantities depend on the magnitude and phase between the intermediate steps, and are expected to be strongly affected by the presence of resonances. We investigate the coherent control of the photoemission process in neon using a phase-controlled two-color extreme ultraviolet pulse with frequency in proximity of an excited energy state. Using helium as a reference, we show that the presence of such a resonance in neon modifies the amplitude and phase of the asymmetric emission of photoelectrons. Theoretical simulations based on perturbation theory are in fair agreement with the experimental observations.
Abstract Ultrashort, coherent light pulses in the extreme ultraviolet (XUV) wavelength region are excellent tools for a wide range of experiments. These XUV pulses are in particular used to characterize electron dynamics. Additionally, the high photon energy enables quantum control of short-lived transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Coherent XUV pulses would be even more useful if the tools to control and manipulate them existed. Presently, however, no modulators exist for these frequencies, which significantly reduces the number of possible ultrafast optics experiments and applications in the XUV regime. This Ph.D. tutorial addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light we have developed a method for opto-optical modulation (OOM), which modulates the phase of XUV light emitted from a gas of atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation and emit light after the excitation pulse passed. By shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse, the phase of the emitted light, and therefore its wavefront, is controlled. With OOM the direction of XUV light emitted from noble gases has been controlled, and the temporal dynamics of the emission studied. By varying the delay between the excitation and the control pulse, the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, a proof-of-principle experiment with two control pulses was performed, resulting in a shaped XUV pulse with controlled duration. The technique of OOM presented in this article is still at a very early stage and in order to obtain complete control of the amplitude and phase of the XUV pulses further development is necessary. Such development would present new opportunities for ultrafast XUV quantum control.
We have investigated multiphoton ionization of Ba 6snl Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the 6sns and 6snd 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of Ba+ Rydberg states and Ba2+ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto Ba+ Rydberg states.
Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field. They are ubiquitous in physics with applications in different areas such as photonics, nano-electronics, electron microscopy, and quantum information. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interaction within the rotating wave approximation\cite. Although intense nanometer-wavelength light-sources have been available for more than a decade, Rabi dynamics at such short wavelengths have not been observed directly. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser can drive Rabi oscillations between the ground state and an excited state in helium atoms. The measured photoemission signal revealed an Autler-Townes doublet as well as an avoided crossing, phenomena that are both trademarks of quantum optics. Using theoretical analyses that go beyond the strong-field approximation, we found that the ultrafast build-up of the doublet structure follows from a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond and few-femtosecond extreme-ultraviolet pulses, our results offer opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.