Periodic Reporting for period 4 - NanoBeam (Quantum Coherent Control: Self–Interference of Electron Beams with Nanostructures)
Reporting period: 2022-09-01 to 2024-01-31
What is the problem/issue being addressed?
Within the context of the ERC grant Nanobeam, we have proposed a method for improving the time resolution of ultrafast electron microscopes by devising and realizing a spectral interferometry technique. Normally, photoemission electron sources are used to produce electron pulses that have sub-picosecond durations, and interact with photo-induced excitations in the sample. The electron beam interacting with photo-induced coherent excitations, experiences stimulated emission and absorption processes, in contrast with conventional electron energy-loss spectroscopy and cathodoluminescence (CL) spectroscopy that are based on spontaneous emission processes. The combined conventional pump-probe photon-electron spectroscopy technique is referred to as photon-induced Near-field electron microscopy (PINEM). In contrast with PINEM that is based on high-intensity laser fields, we propose a technique that is based on a few-photon source, hence enhancing the sensitivity, and allowing us to combine it with spectral interferometry and shaped electron beams for coherent control of the materials excitations and polaritonic states, as well as energy and charge-transfer dynamics. Our method should allow us to infer the femtosecond dynamics and to measure the time-frequency mutual correlations of the cathodoluminescence responses of the sample with respect to the EDPHS, ultimately at the single particle and molecule levels.
Why is it important for society?
Understanding and controlling coherent electron dynamics enable various technological advances, encompassing design and characterization of functional materials that utilize efficient energy and charge transfer mechanisms. We merge the unprecedented spatial resolutions of the electron microscopes with a novel spectroscopy technique that benefits from quantum processes at the few-photon level. Being able to coherently control processes at the molecular and single-particle level with a few-photon process can pave the way towards implementing cost-effective methods that harness the sensitivity of a quantum spectroscopy approach.
What are the overall objectives?
We intend to explore the interaction of shaped electron beams and shaped light with samples, as a goal to explore the quantum dynamics in solid-state and molecular system with unprecedented spatial and temporal resolutions. Particularly our method will utilize the quantum sensitivity of few-photon processes. Our objectives are thus two-fold:
Objective 1- We fabricate and design electron-driven photon sources (EDPHs) as tools to tailor the shape of the generated photons and explore their interactions with sample inside and electron microscope. In other words, the photon sources inside the microscope, in contrast with the PINEM, generates photons that are perfectly synchronous with the electron beam. Moreover, advanced nanostructuring techniques will allow us to shape the spatial profile of the generated light pulses on demand, using methods like holography.
Objective 2- We explore the methodologies to shape the electron beams by virtue of their interactions with designed masks, laser light, and a combination of both, and investigate their interactions with samples, to control the charge density distributions in materials.
Objective 3- We explore and lay strategies to use the combination of both objectives mentioned above to coherently control the electronic responses of the materials, as a tool to better decompose material responses, and / or conduct the propagation dynamics, and energy-transfer dynamics in molecular systems, quantum materials, and hybrids.
Within the context of the ERC grant Nanobeam, we have proposed a method for improving the time resolution of ultrafast electron microscopes by devising and realizing a spectral interferometry technique. Normally, photoemission electron sources are used to produce electron pulses that have sub-picosecond durations, and interact with photo-induced excitations in the sample. The electron beam interacting with photo-induced coherent excitations, experiences stimulated emission and absorption processes, in contrast with conventional electron energy-loss spectroscopy and cathodoluminescence (CL) spectroscopy that are based on spontaneous emission processes. The combined conventional pump-probe photon-electron spectroscopy technique is referred to as photon-induced Near-field electron microscopy (PINEM). In contrast with PINEM that is based on high-intensity laser fields, we propose a technique that is based on a few-photon source, hence enhancing the sensitivity, and allowing us to combine it with spectral interferometry and shaped electron beams for coherent control of the materials excitations and polaritonic states, as well as energy and charge-transfer dynamics. Our method should allow us to infer the femtosecond dynamics and to measure the time-frequency mutual correlations of the cathodoluminescence responses of the sample with respect to the EDPHS, ultimately at the single particle and molecule levels.
Why is it important for society?
Understanding and controlling coherent electron dynamics enable various technological advances, encompassing design and characterization of functional materials that utilize efficient energy and charge transfer mechanisms. We merge the unprecedented spatial resolutions of the electron microscopes with a novel spectroscopy technique that benefits from quantum processes at the few-photon level. Being able to coherently control processes at the molecular and single-particle level with a few-photon process can pave the way towards implementing cost-effective methods that harness the sensitivity of a quantum spectroscopy approach.
What are the overall objectives?
We intend to explore the interaction of shaped electron beams and shaped light with samples, as a goal to explore the quantum dynamics in solid-state and molecular system with unprecedented spatial and temporal resolutions. Particularly our method will utilize the quantum sensitivity of few-photon processes. Our objectives are thus two-fold:
Objective 1- We fabricate and design electron-driven photon sources (EDPHs) as tools to tailor the shape of the generated photons and explore their interactions with sample inside and electron microscope. In other words, the photon sources inside the microscope, in contrast with the PINEM, generates photons that are perfectly synchronous with the electron beam. Moreover, advanced nanostructuring techniques will allow us to shape the spatial profile of the generated light pulses on demand, using methods like holography.
Objective 2- We explore the methodologies to shape the electron beams by virtue of their interactions with designed masks, laser light, and a combination of both, and investigate their interactions with samples, to control the charge density distributions in materials.
Objective 3- We explore and lay strategies to use the combination of both objectives mentioned above to coherently control the electronic responses of the materials, as a tool to better decompose material responses, and / or conduct the propagation dynamics, and energy-transfer dynamics in molecular systems, quantum materials, and hybrids.
Work performed toward satisfying Objective 1:
We have designed, fabricated, and characterized EDPHSs that in interaction with electron beams, generate photons with tailored properties. Particularly using concepts from photon sieves, metamaterials, and holography, the directionality, duration, carrier wavelength, polarization, angular momentum, and the intensity of the generated light pulses have been controlled, as demonstrated in a number of publications by us (Fig. 1)(Nanophotonics 9 (15), 4381-4406 (2020); Nano Letters 20 (8), 5975-5981 (2020); Nature communications 10 (1), 1-8 (2019)).
We have examined as well various mechanisms of radiation from the electron beams, covering the transition radiation, plasmon-induced radiation, as well as exciton-enhanced CL radiation (Communications Materials 2 (1), 1-11 (2021); arXiv:2101.01465; arXiv:2103.14442 (accepted in ACS Applied Nanomaterials); arXiv:2101.11516). Excitons particularly can lead to more photon yield compared to plasmonic lattices, though the far-field coherence is not as pronounced as for plasmonic materials. Hence as a second step, we aim at hybrid materials structures enabling higher photon yields and spatio-temporal coherent features.
Work performed toward satisfying Objective 2:
Interaction of electron beams with light and nanostructures lead to both elastic and inelastic processes. The so-called Kapitza-Dirac effect (KDE) is a prominent example of a two-photon process that lead to electron diffraction by a standing-wave pattern of light. In contrast with the KDE, PINEM is predominantly an inelastic process, that happens due to the interaction of electron beams with near-field distributions, hence momentum criterion for single-photon processes can be easily satisfied. Both processes though, are generally modelled using an Eikonal approximation known as Volkov states or nonrecoil approximation, that constitutes the electron-beam phase modulations caused by the electromagnetic interactions. We have recently developed a numerical Maxwell-Schrödinger toolbox that allows for exploring the dynamics of electron and light interactions beyond adiabatic approximations.
Recently, we have predicted and explored by means of our numerical experimentations, conditions for quantum-path interferences between both single and two photon process, that are achieved by including two inclined light beams of certain intensity and wavelengths (New Journal of Physics 21 (9), 093016 (2019)). Our methods paves the way towards novel kinds of boson-sampling devices that exploit matter waves, though yet to be explored.
In addition, we have simulated the recoil that the electron receives when interacting with laser-induced photonic modes of nanostructures (Fig. 2) (Physical Review Letters 125, 080401 (2020)). Particularly we showed that when the electron wave packet strongly interact with near-field photons, a prominent KDE effect is observed that allows for exploring dynamics by observing the diffraction patterns rather than detecting the spectrum.
Most recently, we exploited the interaction of spin-polarized two-electron wave packets with laser-induced plasmon oscillations, with the aim to understand the role of exchange correlations in transferring the phase information among electron-orbital constituents.
We have designed, fabricated, and characterized EDPHSs that in interaction with electron beams, generate photons with tailored properties. Particularly using concepts from photon sieves, metamaterials, and holography, the directionality, duration, carrier wavelength, polarization, angular momentum, and the intensity of the generated light pulses have been controlled, as demonstrated in a number of publications by us (Fig. 1)(Nanophotonics 9 (15), 4381-4406 (2020); Nano Letters 20 (8), 5975-5981 (2020); Nature communications 10 (1), 1-8 (2019)).
We have examined as well various mechanisms of radiation from the electron beams, covering the transition radiation, plasmon-induced radiation, as well as exciton-enhanced CL radiation (Communications Materials 2 (1), 1-11 (2021); arXiv:2101.01465; arXiv:2103.14442 (accepted in ACS Applied Nanomaterials); arXiv:2101.11516). Excitons particularly can lead to more photon yield compared to plasmonic lattices, though the far-field coherence is not as pronounced as for plasmonic materials. Hence as a second step, we aim at hybrid materials structures enabling higher photon yields and spatio-temporal coherent features.
Work performed toward satisfying Objective 2:
Interaction of electron beams with light and nanostructures lead to both elastic and inelastic processes. The so-called Kapitza-Dirac effect (KDE) is a prominent example of a two-photon process that lead to electron diffraction by a standing-wave pattern of light. In contrast with the KDE, PINEM is predominantly an inelastic process, that happens due to the interaction of electron beams with near-field distributions, hence momentum criterion for single-photon processes can be easily satisfied. Both processes though, are generally modelled using an Eikonal approximation known as Volkov states or nonrecoil approximation, that constitutes the electron-beam phase modulations caused by the electromagnetic interactions. We have recently developed a numerical Maxwell-Schrödinger toolbox that allows for exploring the dynamics of electron and light interactions beyond adiabatic approximations.
Recently, we have predicted and explored by means of our numerical experimentations, conditions for quantum-path interferences between both single and two photon process, that are achieved by including two inclined light beams of certain intensity and wavelengths (New Journal of Physics 21 (9), 093016 (2019)). Our methods paves the way towards novel kinds of boson-sampling devices that exploit matter waves, though yet to be explored.
In addition, we have simulated the recoil that the electron receives when interacting with laser-induced photonic modes of nanostructures (Fig. 2) (Physical Review Letters 125, 080401 (2020)). Particularly we showed that when the electron wave packet strongly interact with near-field photons, a prominent KDE effect is observed that allows for exploring dynamics by observing the diffraction patterns rather than detecting the spectrum.
Most recently, we exploited the interaction of spin-polarized two-electron wave packets with laser-induced plasmon oscillations, with the aim to understand the role of exchange correlations in transferring the phase information among electron-orbital constituents.
Our main goal is to exploit EDPHS in a scanning electron microscope in near future, as photon sources that exhibit an unprecedented degree of controllability in electron-photon mutual correlations. For this purpose, we have designed and fabricated a peculiar nanopositioner that can tune the lateral and longitudinal distances between the sample and the EDPHS with nanometer accuracy.