Multi quantum pathway interference with phase-locked pulses

Coherent phase control is a very well established approach to discriminating molecular dissociation pathways, spatial and energetic properties of photoelectron emission, direction of photocurrent, orientation of molecular dipoles in polychromatic laser fields, and many other areas. In the vast majority of cases, these processes involve two alternative pathways for optical excitation and/or photo-ionization, and the outcome of the light-matter interaction is determined by the relative phase between the waves of different colors that comprise the laser waveform. Using the latest advances in the generation of composite multi-color ultrashort light pulses and the techniques of carrier-envelope phase stabilization, we propose to create and investigate fully phase controlled-i.e. identically reproducible in each laser shot-multi-color waveforms capable of promoting several quantum pathway interferences (QPIs) simultaneously. The new moment here, which, according to our understanding, is still experimentally unexplored is the opportunity for a strong interference enhancement similar to the effect of mode- locking of coherent optical waves. Pronounced intensity spikes in a mode-locked field correspond to a periodic re- phasing of the optical modes, whereas the low intensity between the spikes is a consequence of de-phasing or destructive interference among the modes. Accordingly, the result of a multiple QPI corresponding to a set of various multiphoton excitation processes should also be able to exhibit a prominent dependence on the common, frequency independent phase of the pulse. We expect, therefore, a sharp phasing-in of the interference cross-terms within a narrow set of the carrier envelope phase (CEP) values, as well as a prompt de-phasing (cancelling randomization) when the CEP value is detuned from the range that provides constructive interference. If this is true, than the selectivity (chemical, directional, etc.) of multi-photon-induced processes can be dramatically improved. Moreover, it could potentially create a highly attractive method for determining and calibrating the absolute value of the common phase of a laser field, as opposed to the techniques for tracking of the pulse-to-pulse CEP drift by methods sensitive only to the "relative", not "absolute" phase such as the f-to-nf nonlinear interferometry. The Austrian and the Taiwanese teams propose a program for joint studies based on complementary approaches aimed at (a) demonstrating the multi-pathway interference enhancement and (b) at the phase calibration in absolute terms in the weak-field regime, rather than relying on strong-field interactions that trace the time- domain field evolution and, consequently, are exclusive sources of absolute phase calibration available to date. The need for bilateral cooperation is very strongly justified: whereas the Austrian partner shall provide a significant contribution in the development of the technological knowhow, the Taiwanese partner shall contribute spectroscopic knowhow and theory support, as well the intellectual property right to the idea of the multiple QPI enhancement.

The main point of this project is to demonstrate and exploit enhancement of complex interactions between light and matter by adding together several interaction channels in a way that the overall outcome exceeds the sum of individual parts. This is a well-known idea of the so-called coherent addition which requires that electromagnetic waves of the same frequency but from different sources are brought to oscillate in phase such that they interfere constructively. We apply this principle of enhancement, which is universal in optical sciences, to a class of optical multi-photon phenomena in which a system of atoms or molecules emits coherent light at a higher frequency, such that each new photon is created by adding together several photons of lower frequency. The lower-optical-frequency photons are impinged on the target system by a high-intensity laser pulse from a laser specially built for this project. If the frequency spectrum of the driver laser field is narrow, then each nonlinearity order of the multi-photon interaction, corresponding to the number of laser photons consumed to emit each high-frequency photon, will be responsible for a discrete high-frequency spectral band. By progressively broadening the frequency bandwidth of the laser pulse and simultaneously reducing its duration, the resultant high-frequency bands can start overlapping first with their nearest neighbours and then with the bands with more distant nonlinearity orders. In our studies, we investigate multi-photon conversion of long-wavelength, infrared laser pulses into high-frequency, visible to X-ray pulses. If the spectral overlap between high-frequency bands is limited to just the adjacent band, the spectral intensity will exhibit a trivial dependence on the relative phase between two successive multi-photon orders. However, spreading the inter-band interference such that at a given frequency, not just two but more multi-photon orders contribute at once, leads to a fascinating transformation of the phase dependence from relative to global. In practical terms, this means that the maximum enhancement of the spectral brightness in the output high-frequency spectrum follows when the peak oscillation of the laser electric field has no phase delay with respect to the peak of the pulse intensity. To demonstrate the effect, we had performed a proof of principle experiment using high-intensity pulses at a very low, mid infrared optical frequency applied to a target filled with noble gas. Because of the high bandwidth to carrier-frequency ratio, the resultant multi-photon processes emit spectra that overlap even for very low multi-photon orders. After successfully proving the existence of the effect, we carried out the development of an advanced multi-colour laser system based on active as well as passive phase locking of long-wavelength pulses. The few examples of the interferences of multiple quantum paths, demonstrated during the project, indicate an intriguing potential to control the interplay of multi-photon interactions of different orders by minute changes of the electric field shape in the laser pulse. These early examples constitute a solid basis for follow-up investigations. The main technological achievement of the project is the demonstration of unprecedented - in terms of wavelength, energy, phase control, etc. -laser tools that made it possible to observe these multi-photon effects.