SearcharxivSearch

arXiv subjects

Paul Hockett

Publications and source records attributed to Paul Hockett.

18 recordsLinked to original sources

Highly efficient DUV generation at 100 kHz via Yb pumped four-wave mixing in stretched hollow-core fibers

We report the generation of the fifth harmonic of Yb at 206 nm with pulse energies exceeding 16 $μ$J and durations of approximately 100 fs at a repetition rate of 100 kHz. The deep ultraviolet pulses are produced using four-wave difference frequency mixing in a He-filled stretched hollow-core fiber, driven by a pump at 343 nm and seeded at 1030 nm. Guided by simulations, we carefully optimize the process, resulting in a conversion efficiency of $\sim$30% from the 343 nm pump beam.

physics.optics

Efficient ($\sim$10$\%$) Generation of Vacuum Ultraviolet Femtosecond Pulses via Four-Wave Mixing in Hollow-Core Fibers

We report the generation of the 5th harmonic of Ti:sapphire, at 160 nm, with more than 4~$μ$J of pulse energy, and a pulse length of 37 fs with a 1 kHz repetition rate. The VUV pulses are produced using Four-Wave Difference Frequency Mixing (FWDFM) in a helium-filled stretched Hollow-Core Fiber (HCF), driven by a pump at 267 nm and seeded at 800 nm. Guided by simulations using Luna.jl, we are able to optimize the process carefully. The result is a conversion efficiency of $\sim$10$\%$ from the 267 nm pump beam, rivaling the efficient optical mixing schemes in nonlinear crystals.

physics.optics

Ultrafast Molecular Frame Quantum Tomography

We develop and experimentally demonstrate a methodology for a full molecular frame quantum tomography (MFQT) of dynamical polyatomic systems. We exemplify this approach through the complete characterization of an electronically non-adiabatic wavepacket in ammonia (NH$_3$). The method exploits both energy and time-domain spectroscopic data, and yields the lab frame density matrix (LFDM) for the system, the elements of which are populations and coherences. The LFDM fully characterizes electronic and nuclear dynamics in the molecular frame, yielding the time- and orientation-angle dependent expectation values of any relevant operator. For example, the time-dependent molecular frame electronic probability density may be constructed, yielding information on electronic dynamics in the molecular frame. In NH$_3$, we observe that electronic coherences are induced by nuclear dynamics which non-adiabatically drive electronic motions (charge migration) in the molecular frame. Here, the nuclear dynamics are rotational and it is non-adiabatic Coriolis coupling which drives the coherences. Interestingly, the nuclear-driven electronic coherence is preserved over longer time scales. In general, MFQT can help quantify entanglement between electronic and nuclear degrees of freedom, and provide new routes to the study of ultrafast molecular dynamics, charge migration, quantum information processing, and optimal control schemes.

physics.chem-ph

Topical Review: Extracting Molecular Frame Photoionization Dynamics from Experimental Data

Methods for experimental reconstruction of molecular frame (MF) photoionization dynamics, and related properties - specifically MF photoelectron angular distributions (PADs) and continuum density matrices - are outlined and discussed. General concepts are introduced for the non-expert reader, and experimental and theoretical techniques are further outlined in some depth. Particular focus is placed on a detailed example of numerical reconstruction techniques for matrix-element retrieval from time-domain experimental measurements making use of rotational-wavepackets (i.e. aligned frame measurements) - the ``bootstrapping to the MF" methodology - and a matrix-inversion technique for direct MF-PAD recovery. Ongoing resources for interested researchers are also introduced, including sample data, reconstruction codes (the \textit{Photoelectron Metrology Toolkit}, written in python, and associated \textit{Quantum Metrology with Photoelectrons} platform/ecosystem), and literature via online repositories; it is hoped these resources will be of ongoing use to the community.

physics.chem-ph

Molecular Frame Photoelectron Angular Distributions in Polyatomic Molecules from Lab Frame Coherent Rotational Wavepacket Evolution

The application of a matrix-based reconstruction protocol for obtaining Molecular Frame (MF) photoelectron angular distributions (MFPADs) from laboratory frame (LF) measurements (LFPADs) is explored. Similarly to other recent works on the topic of MF reconstruction, this protocol makes use of time-resolved LF measurements, in which a rotational wavepacket is prepared and probed via photoionization, followed by a numerical reconstruction routine; however, in contrast to other methodologies, the protocol developed herein does not require determination of photoionization matrix elements, and consequently takes a relatively simple numerical form (matrix transform making use of the Moore-Penrose inverse). Significantly, the simplicity allows application of the method to the successful reconstruction of MFPADs for polyatomic molecules. The scheme is demonstrated numerically for two realistic cases, $N_2$ and $C_2H_4$. The new technique is expected to be generally applicable for a range of MF reconstruction problems involving photoionization of polyatomic molecules.

physics.chem-ph

Multivariate Discrimination in Quantum Target Detection

We describe a simple multivariate technique of likelihood ratios for improved discrimination of signal and background in multi-dimensional quantum target detection. The technique combines two independent variables, time difference and summed energy, of a photon pair from the spontaneous parametric down-conversion source into an optimal discriminant. The discriminant performance was studied in experimental data and in Monte-Carlo modelling with clear improvement shown compared to previous techniques. As novel detectors become available, we expect this type of multivariate analysis to become increasingly important in multi-dimensional quantum optics.

quant-ph

Multidimensional quantum-enhanced target detection via spectro-temporal correlation measurements

In this work we investigate quantum-enhanced target detection in the presence of large background noise using multidimensional quantum correlations between photon pairs generated through spontaneous parametric down-conversion. Until now similar experiments have only utilized one of the photon pairs' many degrees of freedom such as temporal correlations and photon number correlations. Here, we utilized both temporal and spectral correlations of the photon pairs and achieved over an order of magnitude reduction to the background noise and in turn significant reduction to data acquisition time when compared to utilizing only temporal modes. We believe this work represents an important step in realizing a practical, real-time quantum-enhanced target detection system. The demonstrated technique will also be of importance in many other quantum sensing applications and quantum communications.

quant-ph

Quantum Beat Photoelectron Imaging Spectroscopy of Xe in the VUV

Time-resolved pump-probe measurements of Xe, pumped at 133~nm and probed at 266~nm, are presented. The pump pulse prepared a long-lived hyperfine wavepacket, in the Xe $5p^5(^2P^{\circ}_{1/2})6s~^2[1/2]^{\circ}_1$ manifold ($E=$77185 cm$^{-1}=$9.57 eV). The wavepacket was monitored via single-photon ionization, and photoelectron images measured. The images provide angle- and time-resolved data which, when obtained over a large time-window (900~ps), constitute a precision quantum beat spectroscopy measurement of the hyperfine state splittings. Additionally, analysis of the full photoelectron image stack provides a quantum beat imaging modality, in which the Fourier components of the photoelectron images correlated with specific beat components can be obtained. This may also permit the extraction of isotope-resolved photoelectron images in the frequency domain, in cases where nuclear spins (hence beat components) can be uniquely assigned to specific isotopes (as herein), and also provides phase information. The information content of both raw, and inverted, image stacks is investigated, suggesting the utility of the Fourier analysis methodology in cases where images cannot be inverted.

physics.atom-ph

Angle-resolved RABBIT: theory and numerics

Angle-resolved (AR) RABBIT measurements offer a high information content measurement scheme, due to the presence of multiple, interfering, ionization channels combined with a phase-sensitive observable in the form of angle and time-resolved photoelectron interferograms. In order to explore the characteristics and potentials of AR-RABBIT, a perturbative 2-photon model is developed; based on this model, example AR-RABBIT results are computed for model and real systems, for a range of RABBIT schemes. These results indicate some of the phenomena to be expected in AR-RABBIT measurements, and suggest various applications of the technique in photoionization metrology.

quant-ph

Response to 'Comment on "Time delays in molecular photoionization"': Extended Discussion & Technical Notes

In a comment on our article Time delays in molecular photoionization [1], Baykusheva & Wörner reproduce canonical scattering theory, and assert that our results are inconsistent with this well-established theory [2]. We absolutely refute this assertion and the spirit of the comment, although we do agree with Baykusheva & Wörner that the textbook theory is correct. In a short response, Response to Comment on "Time delays in molecular photoionization" [3], we have already provided a clear rebuttal of the comment, but gave no technical details. In this fuller response we extend those brief comments in the spirit of completeness and clarity, and provide three clear rebuttals to Baykusheva & Wörner based on (1) logical fallacy (category error), (2) theoretical details of the original article, (3) textural content of the original article. In particular, rebuttal (1) clearly and trivially points to the fact that there is no issue here whatsoever, with recourse to theoretical details barely required to demonstrate this, as outlined in the short version of our response. Our numerical results are correct and reproduce known physical phenomena, as discussed in the original article hence, as careful readers will recognise, the formalism used is canonical scattering theory, and cannot be anything other. In fact, there is no new fundamental physics here to dispute whatsoever, and nor was this the raison d'etre of the original article. Additionally, rebuttal (2) provides the opportunity to discuss, at length, some of these textbook aspects of photoionization theory, and we hope this discussion might be of service to new researchers entering this challenging field.

physics.chem-ph

Time-resolved multi-mass ion imaging: femtosecond UV-VUV pump-probe spectroscopy with the PImMS camera

The Pixel-Imaging Mass Spectrometry (PImMS) camera allows for 3D charged particle imaging measurements, in which the particle time-of-flight is recorded along with $(x,y)$ position. Coupling the PImMS camera to an ultrafast pump-probe velocity-map imaging spectroscopy apparatus therefore provides a route to time-resolved multi-mass ion imaging, with both high count rates and large dynamic range, thus allowing for rapid measurements of complex photofragmentation dynamics. Furthermore, the use of vacuum ultraviolet wavelengths for the probe pulse allows for an enhanced observation window for the study of excited state molecular dynamics in small polyatomic molecules having relatively high ionization potentials. Herein, preliminary time-resolved multi-mass imaging results from C$_2$F$_3$I photolysis are presented. The experiments utilized femtosecond UV and VUV (160.8~nm and 267~nm) pump and probe laser pulses in order to demonstrate and explore this new time-resolved experimental ion imaging configuration. The data indicates the depth and power of this measurement modality, with a range of photofragments readily observed, and many indications of complex underlying wavepacket dynamics on the excited state(s) prepared.

physics.chem-ph

Bootstrapping to the Molecular Frame with Time-domain Photoionization Interferometry

Photoionization of molecular species is, essentially, a multi-path interferometer with both experimentally controllable and intrinsic molecular characteristics. In this work, XUV photoionization of impulsively aligned molecular targets ($N_2$) is used to provide a time-domain route to "complete" photoionization experiments, in which the rotational wavepacket controls the geometric part of the photoionization interferometer. The data obtained is sufficient to determine the magnitudes and phases of the ionization matrix elements for all observed channels, and to reconstruct molecular frame interferograms from lab frame measurements. In principle this methodology provides a time-domain route to complete photoionization experiments, and the molecular frame, which is generally applicable to any molecule (no prerequisites), for all energies and ionization channels.

physics.chem-ph

ePSproc: Post-processing suite for ePolyScat electron-molecule scattering calculations

ePSproc provides codes for post-processing results from ePolyScat (ePS), a suite of codes for the calculation of quantum scattering problems, developed and released by Luchesse \& co-workers (Gianturco et al. 1994)(Natalense and Lucchese 1999)(R. R. Lucchese and Gianturco 2016). ePS is a powerful computational engine for solving scattering problems, but its inherent complexity, combined with additional post-processing requirements, ranging from simple visualizations to more complex processing involving further calculations based on ePS outputs, present a significant barrier to use for most researchers. ePSproc aims to lower this barrier by providing a range of functions for reading, processing and plotting outputs from ePS. Since ePS calculations are currently finding multiple applications in AMO physics (see below), ePSproc is expected to have significant reuse potential in the community, both as a basic tool-set for researchers beginning to use ePS, and as a more advanced post-processing suite for those already using ePS. ePSproc is currently written for Matlab/Octave, and distributed via Github: https://github.com/phockett/ePSproc.

physics.comp-ph

Augmented Reality with Hololens: Experiential Architectures Embedded in the Real World

Early hands-on experiences with the Microsoft Hololens augmented/mixed reality device are reported and discussed, with a general aim of exploring basic 3D visualization. A range of usage cases are tested, including data visualization and immersive data spaces, in-situ visualization of 3D models and full scale architectural form visualization. Ultimately, the Hololens is found to provide a remarkable tool for moving from traditional visualization of 3D objects on a 2D screen, to fully experiential 3D visualizations embedded in the real world.

cs.GR

Coherent Control of Photoelectron Wavepacket Angular Interferograms

Coherent control over photoelectron wavepackets, via the use of polarization-shaped laser pulses, can be understood as a time and polarization-multiplexed process. In this work, we investigate this multiplexing via computation of the observable photoelectron angular interferograms resulting from multi-photon atomic ionization with polarization-shaped laser pulses. We consider the polarization sensitivity of both the instantaneous and cumulative continuum wavefunction; the nature of the coherent control over the resultant photoelectron interferogram is thus explored in detail. Based on this understanding, the use of coherent control with polarization-shaped pulses as a methodology for a highly multiplexed coherent quantum metrology is also investigated, and defined in terms of the information content of the observable.

physics.atom-ph

General phenomenology of ionization from aligned molecular ensembles

Single and multi-photon ionization of aligned molecular ensembles is examined, with a particular focus on the link between the molecular axis distribution and observable in various angle-integrated and angle-resolved measurements. To maintain generality the problem is treated geometrically, with the aligned ensemble cast in terms of axis distribution moments, and the response of observables in terms of couplings to these moments. Within this formalism the angular momentum coupling is treated analytically, allowing for general characteristics - independent of the details of the ionization dynamics of a specific molecule - to be determined. Limiting cases are explored in order to provide a phenomenology which should be readily applicable to a range of experimental measurements, and illustrate how observables can be sensitive to fine details of the alignment, i.e. higher-order moments of the axis distribution, which are often neglected in experimental studies. We hope that this detailed and comprehensive treatment will bridge the gap between existing theoretical and experimental works, and provide both quantitative physical insights and a useful general phenomenology for researchers working with aligned molecular ensembles.

physics.chem-ph

Complete photoionization experiments via ultra-fast coherent control with polarization-multiplexing

Photoelectron angular distributions (PADs) obtained from ionization of potassium atoms using moderately intense femtosecond IR fields ($\sim$10$^{12}$Wcm$^{-2}$) of various polarization states are shown to provide a route to "complete" photoionization experiments. Ionization occurs by a net 3-photon absorption process, driven via the $4s\rightarrow4p$ resonance at the 1-photon level. A theoretical treatment incorporating the intra-pulse electronic dynamics allows for a full set of ionization matrix elements to be extracted from 2D imaging data. 3D PADs generated from the extracted matrix elements are also compared to experimental, tomographically reconstructed, 3D photoelectron distributions, providing a sensitive test of their validity. Finally, application of the determined matrix elements to ionization via more complex, polarization-shaped, pulses is demonstrated, illustrating the utility of this methodology towards detailed understanding of complex ionization control schemes and suggesting the utility of such "multiplexed" intra-pulse processes as powerful tools for measurement.

physics.atom-ph

Probing Ultrafast Dynamics with Time-resolved Multi-dimensional Coincidence Imaging: Butadiene

Time-resolved coincidence imaging of photoelectrons and photoions represents the most complete experimental measurement of ultrafast excited state dynamics, a multi-dimensional measurement for a multi-dimensional problem. Here we present the experimental data from recent coincidence imaging experiments, undertaken with the aim of gaining insight into the complex ultrafast excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm light. We discuss photoion and photoelectron mappings of increasing dimensionality, and focus particularly on the time-resolved photoelectron angular distributions (TRPADs), expected to be a sensitive probe of the electronic evolution of the excited state and to provide significant information beyond the time-resolved photoelectron spectrum (TRPES). Complex temporal behaviour is observed in the TRPADs, revealing their sensitivity to the dynamics while also emphasising the difficulty of interpretation of these complex observables. From the experimental data some details of the wavepacket dynamics are discerned relatively directly, and we make some tentative comparisons with existing ab initio calculations in order to gain deeper insight into the experimental measurements; finally, we sketch out some considerations for taking this comparison further in order to bridge the gap between experiment and theory.

physics.chem-ph