Electron beam device and method for generating a pulsed electron beam and their uses

Abstract

An electron beam device 100 for generating a pulsed electron beam 1 including a sequence of electron pulses comprises an irradiation source device 10 for generating a sequence of emitter excitation pulses 2, in particular laser pulses; an electron source device 20 having a photoemission electron source 21 for irradiation-induced emission of source electron pulses 3 in response to irradiation with the emitter excitation pulses 2; a number-state disperser 30 for spatially separating the source electron pulses 3 into sub-pulses 4, 4A, each sub-pulse 4, 4A including an integer number n of electrons, where n = 1, 2, 3, ...; and a number-state selection device 40 for selecting sub-pulses 4, 4A including at least one set of predetermined electron number states as the generated pulsed electron beam 1. Preferably, the photoemission electron source 21 is configured to generate countable low-charge electron pulses, e.g., including 1, 2, 3, or 4 electrons per pulse. Methods for generating the pulsed electron beam 1 and for using the electron beam device are also described.

Description

[Technical Field]

[0001] The present invention relates to an electron beam device and / or method for generating a pulsed electron beam comprising a sequence of electron pulses. Applications of the invention can be found, for example, in the fields of electron microscopy or materials processing by electron lithography. [Background technology]

[0002] This disclosure refers to the following prior art, which provides technical background and related art for the present invention: [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] DNKlyshko,Phys.-Usp.39,573(1996) [Non-patent document 2] MIKolobov,Rev.Mod.Phys.71,1539(1999) [Non-patent document 3] G. Kotliar and D. Vollhardt, Physics Today 57,53(2004) [Non-patent document 4] E.Morosan,D.Natelson,AHNevidomskyy,andQ.Si,Adv.Mater.24,4896(2012) [Non-Patent Document 5] TDLadd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and JLO'Brien, Nature 464, 45 (2010) [Non-patent document 6] O.Kfir,V.Di Giulio,FJGde Abajo,and C.Ropers,Sci.Adv.7,eabf6380(2021) [Non-Patent Document 7] S.Asban and FJGarc▲i▼a de Abajo,npj Quantum Inf 7,42(2021) [Non-licensed document 8] V.Di Giulio,M.Kociak,and FJGde Abajo,Optica6,1524(2019),arXiv:1905.06887

Non-licensed literature 9

Non-licensed literature 10

Non-licensed Document 11

Non-licensed Document 12

Non-licensed Document 13

Non-licensed Document 14

Non-licensed Document 15

Non-licensed Document 16

Non-licensed Document 17

Non-licensed Document 18

Non-licensed Document 19

Non-licensed Document 20

Non-licensed Document 21

Non-licensed Document 22

Non-licensed Document 23

Non-licensed Document 24

Non-licensed Document 25

Non-licensed Document 26

Non-licensed Document 27

Non-licensed Document 28

Non-licensed Document 29

Non-licensed Document 30

Non-licensed Document 31

Non-licensed Document 32

Non-licensed Document 33

Non-licensed Document 34

Non-licensed Document 35

Non-licensed Document 36

Non-licensed Document 37

Non-licensed Document 38

Non-licensed Document 39

Non-licensed Document 40

Non-licensed Document 41

Non-licensed Document 42

Non-licensed Document 43

Non-licensed Document 44

Non-licensed Document 45

Non-licensed Document 46

Non-licensed Document 47

Non-licensed Document 48

Non-licensed Document 49

Non-licensed literature 50

Non-licensed Document 51

Non-licensed Document 52

Non-licensed Document 53

Non-licensed Document 54

Non-licensed Document 55

Non-licensed Document 56

Non-licensed Document 57

Non-licensed Document 58

Non-licensed Document 59

Non-licensed Document 60

Non-licensed Document 61

Non-licensed Document 62

[0004] Controlling the quantum statistics of fermionic and bosonic modes is at the heart of non-classical light sources [Non-Patent Documents 1, 2] and strongly correlated functional materials [Non-Patent Documents 3, 4], and enables (noise-corrected) quantum computers [Non-Patent Document 5]. Developing these concepts in the context of free-electron quantum optics promises coherent manipulation of quantum states [Non-Patent Documents 6, 7] and nanoscale sensing and imaging [Non-Patent Document 8], while generating new opportunities from structural biology and materials science to electron beam lithography. In particular, recent theoretical and experimental studies have explored quantum-enhanced electron microscopy, relying on interference [Non-Patent Document 9] or multi-pass [Non-Patent Documents 10, 11] schemes, quantum light emission [Non-Patent Document 12], or tailored interactions with optical modes [Non-Patent Documents 6, 8, 13-15].

[0005] While correlated many-electron states are ubiquitous in experimental condensed-matter physics [16, 17], their analogues in free particle beams have yet to be observed. Proposed mechanisms include indirect coupling in long-lived optical modes [13, 18], common-path interference

[19] , or correlated photoemission [20, 21] and ionization [22, 23] processes. As a characteristic and necessary condition for entanglement, particle correlations in free electron beams have been studied in transverse [19, 24] and longitudinal

[25] phase spaces, taking into account contributions from exchange-mediated

[24] or Coulomb

[26] interactions. In particular, classical Coulomb repulsion leads to stochastic transverse and longitudinal emittance enhancement of electron beams, described by the Boersch [27, 28] and Löffler

[29] effects, respectively, limiting the brightness of state-of-the-art electron sources

[30] . In highly charged electron pulses, mean-field-induced space-charge effects dominate the achievable pulse duration, energy spread, and focusability

[31] , posing major experimental challenges in ultrafast electron microscopy and diffraction, particle accelerators, and free-electron lasers

[32] .

[0006] Coincidence spectroscopy is well established in atomic and molecular science [33, 34] and has revealed complex collision mechanisms and correlation effects in solids [20, 35], COLTRIMS, reaction microscopy, and other correlation detection techniques.

[0007] Miniaturization of electron sources to generate coherent electron pulses with nanoscale pointed emitters, enabling ultrafast dark-field [Non-Patent Document 36] or phase-contrast imaging [Non-Patent Document 37], nanoscale diffraction probes [Non-Patent Documents 38, 39], and photon-induced near-field electron microscopy [Non-Patent Documents 40-43], requires a detailed analysis of stochastic Coulomb and mean-field effects on low-charge electron fluxes [Non-Patent Document 31].

[0008] However, disentangling both contributions requires single-particle resolved event-based detection, which has been applied to fast STEM [Non-Patent Documents 44, 45] and EELS [Non-Patent Document 42] and has been introduced very recently in electron microscopy to detect electron correlated X-ray emission [Non-Patent Document 46], as well as cathodoluminescence in quantum materials [Non-Patent Document 47] and integrated photonic resonators [Non-Patent Document 15]. Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide an improved electron beam device and / or method for generating a pulsed electron beam, which can avoid the limitations of the prior art for generating electron beams. In particular, the pulsed electron beam should be generated with tailored pulse statistics, thus enabling a wide range of new applications, such as, in particular, electron microscopy applications with reduced noise and / or improved imaging quality, and / or improved applications in electron lithography and / or quantum computing, and / or new applications in electron manipulation. [Means for solving the problem]

[0010] These objects are solved by an electron beam device and / or a method for generating a pulsed electron beam, each comprising the features of the independent claims. Advantageous embodiments and applications of the invention are defined in the dependent claims.

[0011] According to a first general aspect of the present invention, the above object is solved by an electron beam apparatus configured to generate a pulsed electron beam comprising a sequence of electron pulses, the electron beam apparatus comprising: an irradiation source device arranged to generate a sequence of emitter excitation pulses, in particular laser pulses; an electron source device having a photoemission electron source arranged for irradiation-induced emission of source electron pulses in response to irradiation with the emitter excitation pulses; a number state disperser device arranged to spatially separate the source electron pulses into sub-pulses, each sub-pulse comprising an integer number of electrons (n), where n=1, 2, 3, ...; and a number state selection device arranged to select sub-pulses comprising at least one set of predetermined electron number states for the generated pulsed electron beam.

[0012] According to a second general aspect of the present invention, the above object is solved by a method for generating a pulsed electron beam comprising a sequence of electron pulses, the method comprising the steps of: generating a sequence of emitter excitation pulses, in particular laser pulses, by an irradiation source device; irradiating a photoemission electron source of the electron source device with the emitter excitation pulses so that source electron pulses are generated by irradiation-induced emission; spatially separating the source electron pulses into sub-pulses by a number-state dispersing device, each sub-pulse comprising an integer number of electrons (n), where n = 1, 2, 3, ...; and selecting sub-pulses comprising at least one set of predetermined electron number states by a number-state selecting device for the generated pulsed electron beam. Preferably, the method of the second general aspect of the present invention or an embodiment thereof is carried out using an electron beam device according to the first general aspect or an embodiment thereof. All preferred embodiments disclosed herein with respect to the device are also deemed to be corresponding preferred embodiments of the method, and vice versa.

[0013] According to a third general aspect of the present invention, the above object is solved by a method of using an electron beam device according to the first general aspect or embodiments thereof as a beam source in at least one of an electron microscopy device, an electron lithography device, an electron pair (source) device, an electron herald device, an electron counting device (for measuring precise electron counts), an information processing device, a communication device, and a quantum computing device. An electron microscopy device, an electron lithography device, and / or a quantum computing device comprising an electron beam device according to the first general aspect or embodiments thereof and / or configured to perform the method of the second general aspect of the present invention or embodiments thereof is considered an independent subject of the present invention.

[0014] The term "pulse" as used in this disclosure generally refers to a waveform that provides a predetermined, discrete time structure of source electrons. Pulses may be provided as a periodic pulse sequence. The step of selecting sub-pulses including at least one set of predetermined electron number states includes selecting a single set of sub-pulses having a single electron number state, e.g., n=2, or selecting multiple sets of sub-pulses, each having a different electron number state, e.g., n>1 or n=1. Thus, when selecting multiple sets of sub-pulses, different electron number states may be combined.

[0015] The number state dispersion device spatially separates pulses of integer electron numbers n=0, 1, 2, 3, ... and projects the electron beam onto a number state selection device, which may comprise, for example, apertures, slits, and / or beam blocks to block / reject a subset of the quantized number states n=0, 1, 2, 3, ...

[0016] We have observed Coulomb-correlated electron pair states and triplet states generated by femtosecond-pulse photoemission from nanoscale, point-tip emitters in an ultrafast transmission electron microscope. Event-based electron spectroscopy allows unambiguous identification of specific number states due to their characteristic interparticle few-electron-volt kinetic energy separation. State-selected beam caustics exhibit increased virtual source size for higher electron populations induced by stochastic Coulomb scattering, and energy state separation affects the averaged spectrum. We propose a scheme in which simple beam filtering, e.g., spectral, spatial, and / or spin filtering, allows for state selection and tuning of pulse statistics. Furthermore, by employing an electrostatic emitter configuration, the electron kinetic energy separation and the relative occurrence of desired number states may be altered to optimize specific applications. This enables sophisticated control schemes, e.g., for generating sub-Poissonian electron beams or implementing heralded single-electron sources, eliminating the limitations of electron microscope aberration correction and shot noise in electron beam lithography.

[0017] According to a preferred embodiment of the present invention, the photoemission electron source is configured to generate countable low charge electron pulses, preferably containing 1, 2, 3 or 4 electrons per pulse.

[0018] According to a preferred embodiment of the present invention, the photoemission electron source comprises a beam-limiting aperture configured to reduce high-charge electron pulses to countable low-charge electron pulses, which may be located immediately downstream of the electron source device and upstream of optional accelerator and collector optics.

[0019] In accordance with a preferred embodiment of the present invention, the photoemission electron source is configured to generate multi-electron states having distinguishable properties in addition to the pulsed charge.

[0020] According to a preferred embodiment of the present invention, the number state dispersing device comprises an energy dispersing device, such as a spectrometer device. The spectrometer device may be configured to spectrally resolve the source electron pulse and apply an energy filter to the source electron pulse. The spatial filter device may be arranged in an energy selection plane of the spectrometer device. A second spectrometer device may be provided to merge the selected electron subset into a common beam.

[0021] According to a preferred embodiment of the present invention, the energy dispersing device may comprise a beam monochromator, in particular an Omega, Alpha, Wien filter, double Wien, electron mirror or other type of monochromator, and / or a spectrometer device configured to separate the spectral components of the source electron pulse, in particular a spectrometer device including a magnetic prism, a spectrometer device including electrostatic multipole electron optics, or another type of spectrometer.

[0022] According to a preferred embodiment of the present invention, the number state dispersion device comprises a spatial dispersion device, in particular a rotationally symmetric electron lens and / or a cylindrical electron lens, which may also be called an (electron) momentum dispersion device, a position dispersion device, an electron orbital momentum dispersion device or an electron spin dispersion device.

[0023] According to a preferred embodiment of the present invention, the number state selection device includes a spatial modulator of electron beam intensity, in particular a spatial modulator configured to pass selected sub-pulses having a predetermined electron number state and block the remaining sub-pulses.

[0024] According to a preferred embodiment of the present invention, the number state selection device comprises at least one of a mechanical slit, a grating, a linear beam block, a hole, a ring, or a disk, and the selection device is preferably made as a conductive material or electrode adapted to absorb, preferably completely absorb, the electron number states to be rejected.

[0025] According to a preferred embodiment of the present invention, the electron beam device further comprises at least one of a detector device configured to measure the number of electrons in at least one of the electron number states of the pulsed electron beam, and a beam-forming electron-optics system configured to illuminate a sample, such as a sample under investigation, and / or a workpiece, such as a semiconductor workpiece.

[0026] According to a preferred embodiment of the present invention, the photoemission electron source comprises a pointed photoemission electron source, preferably configured for linear photoemission, in particular at least one of a Schottky emitter, a cold field emitter, and a thermionic emitter.

[0027] The photoemission electron source may comprise a tungsten tip with a (100) crystal facet covered with a thin film of zirconium oxide, or a composite tip made from a material with an electron work function equal to or lower than that of lanthanum hexaboride, LaB6, or cerium hexaboride, CeB6, or a metal tip with a crystal tip facet with a reduced electron work function compared to the rest of the tip material, or a pure metal tip, in particular a pure metal tip made of W, Mo, Re, Ir, Ta, Hc, Pt, or Ni, a transition metal carbide tip, in particular a transition metal carbide tip made of HfC, ZrC, NbC, TaC, TiC, or VC, or a carbon cone emitter or a single carbon nanotube tip.

[0028] The irradiation source device may include a laser source and / or may be configured to generate emitter excitation emission having a wavelength selected depending on the application conditions, in particular the emitter tip material, for example a wavelength in at least one of the ranges of 1 nm to 200 nm, 200 nm to 1500 nm, and 1500 nm to 16 μm.

[0029] According to a preferred embodiment of the method of the present invention, adjusting the irradiation source device and / or the photoemission electron source to generate multi-electron states that are distinguishable by characteristics other than pulse charge (= number of electrons); adjusting the number state disperser according to distinguishable characteristics of the source electron pulses, in particular energy, angle / momentum, and spatial focus; adjusting the number state selection device to reject or block specific number states from the beam, of particular interest to reject or block all n>2 or to reject or block all subpulses with n≠2; At least one of the steps of adjusting the illumination source device, the photoemission electron source, the number-state dispersing device and / or the number-state selecting device based on the output of the additional number-state sensitive detector may be provided.

[0030] Further details and advantages of the invention are explained below with reference to the accompanying drawings, which are shown schematically below. [Brief explanation of the drawings]

[0031] [Figure 1] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 2] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 3] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 4] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 5] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 6] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 7] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 8] 1 is an illustration of features of a preferred embodiment of the present invention; [Figure 9] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 10] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11A] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11B]10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11C] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11D] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11E] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11F] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11G] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 11H] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 12] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13A] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13B] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13C] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13D] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13E] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. [Figure 13F] 10 illustrates further features and experimental testing of a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] Figures 1 to 8 show in schematic form the general principles of the present invention (particularly Figure 1) and features of preferred embodiments for practical implementation of the present invention (particularly Figures 2 to 8). Further details of practical implementations based on the inventors' experimental findings will be described with reference to Figures 9 to 13.

[0033] Embodiments of the present invention are described with particular reference to the preparation and / or analysis of pulsed electron beams having well-defined number or charge states. The electron beam apparatus may be implemented based on a transmission electron microscope, as outlined below. Details and / or operating parameters of the transmission electron microscope, such as adjusting the accelerating voltage, and / or details and / or operating parameters of a laser source for exciting the electron source of the transmission electron microscope, such as controlling the power of the laser source, will not be described insofar as conventional electron microscopy techniques are known per se.

[0034] The implementation of the present invention is not limited to electron microscope applications, but can also be used in other applications as mentioned above. Furthermore, the present invention is not limited to the application conditions provided for the actual tests shown in Figures 10 to 14. Depending on the application, the embodiments of the present invention may be modified, for example, with respect to the design of the number-state distribution device and / or the number-state selection device.

[0035] 1 shows a schematic diagram of an electron beam device 100 for generating a pulsed electron beam 1 comprising a sequence of electron pulses. The electron beam device 100 comprises a radiation source device 10, in particular a pulsed radiation source such as a pulsed laser, which is arranged to generate a sequence of emitter excitation pulses 2, in particular a pulsed laser beam of laser pulses. The electron beam device 100 further comprises an electron source device 20 having a photoemission electron source 21. Exemplary details of the photoemission electron source 21 are shown in particular in FIGS. 3 to 9.

[0036] The photoemission electron source 21 is configured to receive an emitter excitation pulse 2 and to emit a source electron pulse 3 in response to irradiation of the emitter excitation pulse 2 directed onto the photoemission electron source 21. The source electron pulse 3 provides a pulsed electron beam having a small number of (countable) electrons per pulse. In particular, each source electron pulse 3 includes a number n of electrons, e.g., 1, 2, 3, or more. A pulse charge of each source electron pulse 3 is determined corresponding to the number of electrons per pulse. In addition to the pulse charge / electron number, the multi-electron states (number states) of the source electron pulse 3 are identifiable by at least one physical property, such as the electron energy, angular / momentum, position, spin, and / or spatial defocus, determined by the angular / transverse momentum of the source electron pulse, resulting from, for example, a number state disperser.

[0037] The source electron pulses 3 are directed along the optical axis z (electron beam axis z) to the number-state disperser 30 of the electron beam device 100. The number-state disperser 30 is configured to spatially separate the sequence of source electron pulses 3 into separated subsequences of source electron pulses 4, each subsequence containing source electron pulses 4 with a common integer number of electrons (n), i.e., n=1, 2, 3, .... The number-state disperser 30 provides dispersion along at least one distinguishable physical property. Thus, the number states are dispersed into one or more portions. Depending on the physical properties of the number states, each source electron pulse 3 is directed to a different one of the pulse paths of the sub-pulses 4 (see Figures 3-9).

[0038] Furthermore, electron beam device 100 comprises a number state selection device 40 arranged to receive spatially separated sub-pulses 4 and to select one of the sub-pulses 4 comprising at least one set of predetermined electron number states as the generated pulsed electron beam 1. Based on at least one distinguishable physical characteristic, pulses having a particular number state are transmitted by number state selection device 40 as pulsed electron beam 1, while pulses having number states other than the particular number state are rejected, particularly blocked, or optionally split by number state selection device 40, as exemplarily shown in particular Figures 3-9.

[0039] The pulsed electron beam 1 is directed towards an application site 50 which contains a sample and / or an application site 50 which contains, for example, a sample to be investigated with the pulsed electron beam 1 or a workpiece to be processed with the pulsed electron beam 1. Only a certain number of states, for example n<2 or n=2, are used at the sample and / or application site 50. The sample and / or application site 50 comprises, for example, a sample stage of a microscopy, spectroscopy or lithography apparatus. For example, only a subsequence of source electron pulses 4 having two electrons per pulse is used for investigating the sample or irradiating the workpiece by electron microscopy.

[0040] Optionally, a multi-state reporting channel 60 may be provided that includes an electron pulse detector device. Using the multi-state reporting channel 60, one channel of number state n may be detected while state n-1 is used at the sample and / or application site 50, for example, to realize a heralded single electron source.

[0041] 2 shows further details of an optional application involving a particular number-state herald, such as that shown in FIG. 1, including a number-state selection device 40, a sample and / or application site 50, and a number-state reporting channel 60. The number-state selection device 40 transmits a pulsed electron beam 1 in state m=n−1 to the sample and / or application site 50, and splits unselected source electron pulses 4A in state n to the number-state reporting channel 60. The unselected subsequence of source electron pulses 4A includes, for example, a single electron pulse detected by a detector device, which is included in the number-state reporting channel 60.

[0042] The number state reporting channel 60 may be coupled to the sample and / or application site 50 via an information channel 61. For example, an accurately countable number of electrons may be injected into the number state reporting channel 60 and the sample, or into the number state reporting channel 60 and the application site 50. This provides knowledge of the exact number of electrons in an electronic state that are delivered to the sample and / or application site 50, making it suitable, for example, for microscopy and lithography applications with well-defined electron doses.

[0043] Further details of an electron beam device 100, e.g., according to Figure 1 or Figure 2, are exemplarily shown in Figures 3 to 9. In the embodiment of Figures 3 to 9, the irradiation source device 10 comprises a laser generating a pulsed laser beam, e.g., a femtosecond laser generating laser pulses having a duration of 160 fs and a repetition period of e.g., about 2 μs. The pulsed laser beam is focused onto a photoelectron source 21 of an electron source device 20 as a sequence of emitter excitation pulses 2 with focusing optics, such as a lens 11.

[0044] The photoemission electron source 21 of FIGS. 3-7 and 9 includes an emitter tip (see especially FIG. 9). In response to irradiation with a focused pulsed laser beam accompanied by an emitter excitation pulse 2, a corresponding pulsed electron beam of source electron pulses 3 is generated at the emitter tip. To direct the source electron pulses 3 along the optical axis z toward the number-state dispersion device 30, the electron source device 20 of each illustrated embodiment may optionally further include a suppressor anode 22, an extractor anode 23 (see also FIG. 9), a beam-limiting aperture 20A, and accelerator and collector optics 24 (see FIG. 3). Preferably, the beam-limiting aperture 20A is disposed between the extractor anode 23 and the accelerator and collector optics 24.

[0045] According to the embodiment of FIGS. 3 and 4, the number-state dispersion device 30 is an energy dispersion device including an energy-dispersive magnetic prism 31, which may be configured as described, for example, in [Non-Patent Documents 58, 61, 62]. Preferably, a single energy-dispersive magnetic prism 31 may be provided. The energy-dispersive magnetic prism 31 applies different polarizations with respect to the optical axis z to source electron pulses with different charge / energy in the deflection field of the energy-dispersive magnetic prism 31. In the illustrated example, sub-pulse 4 with number state n=2 is deflected by a larger output angle compared to sub-pulse 4A with number state n=1. The deflection due to the spatial separation of the sub-pulses is provided to a number-state selection device 40.

[0046] The number-state selection device 40 includes a mechanical beam block 41 that acts as a spatial modulator of the electron beam intensity of the spatially separated subpulses. The mechanical beam block 41 in FIG. 3 is configured to pass only the selected subpulse 4 of number state n=2 to the sample and / or application site 50, while blocking the subpulse 4A of number state n=1. For this purpose, the mechanical beam block 41 may include a beam-blocking material that is disposed in the path of the blocked subpulse 4A and transmits the selected subpulse 4. Alternatively, the mechanical beam block 41 may be described as a slit, e.g., a circular slit that passes the selected subpulse 4. As a further alternative, to select only the subpulse 4A of number state n=1 and block the subpulse 4 of number state n>1, particularly n=2, the mechanical beam block 41 may include a circular beam-blocking material with a central hole, as shown in FIG. 4.

[0047] 5 shows further details of the electron beam device 100, e.g., according to FIG. 1 or 2, in which the number-state dispersion device 30 comprises an in-column omega monochromator filter 32 including a number-state selection device 40 provided by a slit 42. The in-column omega monochromator filter 32 may be configured, e.g., as described in [Non-Patent Documents 58, 61]. The monochromator filter 32 deflects the pulsed electron beam of the source electron pulse 3 into the shape of the Greek letter Ω and comprises four magnetic prisms 32A-32D acting as energy filters.

[0048] Under the influence of the deflection fields of the first pair of magnetic prisms 32A and 32B, and depending on the number state of the received source electron pulse 3, the source electron pulse 3 is dispersed as sub-pulses 4, 4A on different beam paths through the monochromator filter 32 toward the slit 42. The slit 42 can block the outer n>1, e.g., n=2, sub-pulses 4 and pass the central n=1, sub-pulse 4A. Alternatively, the slit 42 can be replaced by a beam stop (not shown) at the slit position that transmits only the outer n>1, e.g., n=2, sub-pulses and blocks the central n=1, sub-pulse.

[0049] Due to the effect of the deflection field of the second pair of magnetic prisms 32C and 32D, the selected sub-pulses passed by the slit 42 (or beam stop) are re-deflected in the initial direction of the source electron pulse 3 and directed as the resulting electron pulse 1 towards the sample and / or application site 50. To adjust the electron kinetic energy to the requirements of the application / sample 50, an accelerator 33 is arranged after the monochromator 32, but can optionally also be arranged before 32.

[0050] According to the embodiment of Fig. 6, an electron beam device 100, for example according to Fig. 1 or 2, comprises a number-state dispersion device 30 having an in-column alpha monochromator 34 including a slit 43. The slit 43 provides a mechanical beam block for the number-state selection device 40. A pulsed electron beam of source electron pulses 3 generated at the emitter tip 21 is directed to the alpha monochromator 34 via collector optics and an accelerator 25.

[0051] The alpha monochromator 34 may be configured as described in, for example, [Non-Patent Documents 58, 61, 64]. The monochromator 34 includes imaging optics and three magnetic prisms 34A-34C that deflect the pulsed electron beam of the source electron pulse 3 into the shape of the Greek letter α and act as energy filters. Similar to the monochromator filter 32 in FIG. 5, the source electron pulse 3 is dispersed as subpulses 4 and 4A on different beam paths through the monochromator 34 toward the energy-selection slit 43. The slit 43 can block the outer subpulses 4 in the n>1 (e.g., n=2) state and pass the central subpulse 4A in the n=1 state. Alternatively, a central beam stop (not shown) at the slit position can be used to transmit only the outer subpulses in the n>1 state and block the central subpulse in the n=1 state. The selected subpulses transmitted by the slit 43 (or around the central beam stop) are imaged toward the sample and / or application site 50.

[0052] In a further alternative embodiment, the electron beam device 100 of FIG. 7 includes a Wien monochromator 35 as the number-state dispersion device 30. The Wien monochromator 35, which may be configured as described in, for example, [61, 64], is arranged in combination with an accelerator 36 immediately downstream of the extractor anode 23. As an alternative to the Wien monochromator 35 shown, a double Wien monochromator may be used. The Wien monochromator 35 functions as an energy-selective imaging system, directing pulses with different electron number states onto beam paths at different angles relative to the optical axis z of the electron source device 20. Thus, the sub-pulses 4 and 4A are spatially separated, and sub-pulses can be selected according to their number states. The schematic color scale diagram in FIG. 7 encodes the kinetic electron energy dispersed by the Wien filter.

[0053] The number state selection device 40 includes a slit 44 located at a distance downstream of the Wien-type monochromator 35. The slit 44 acts as a spatial modulator of the electron beam intensity of the spatially separated subpulses. Depending on the position of the slit 44 relative to the optical axis z, the slit 43 can either block the n>1 state or transmit only the n=2 state.

[0054] 8 shows the formation of the size and position of a virtual source at the apex of a pointed photoemission electron source 21 of an electron beam device 100. In response to a focused pulsed laser beam irradiation from an irradiation source device 10 via a lens 11, an emitter excitation pulse 2 generates a pulsed electron beam of source electron pulses 3, with the virtual source size and virtual source shift at the emitter tip depending on the number state of the pulse. In particular, the virtual source size further increases (see double arrow) as the number state of the excited electron pulse increases, and the virtual source further shifts along the optical axis z (see arrow) as the number state of the excited electron pulse increases.

[0055] The number-state dispersion device 30 of the embodiment of Figure 8 includes a pair of electron lenses 37, 38 arranged with a common optical axis z perpendicular to the surface of the emitter tip photoemission electron source 21. The electron lenses 37, 38 are arranged axially spaced apart from each other and include a number-state selection device 40. The number-state selection device 40 includes a circular opening 45 made of a beam-blocking material with a hole centered on the optical axis z. Alternatively, the number-state selection device 40 includes a circular beam blocker (not shown) centered on the optical axis z.

[0056] The emitter tip of the photoemission electron source 21 provides a virtual source size that depends on the number states of the excited source electron pulse 3. This characteristic produces an angular / spatial dispersion of the source electron pulse 3 visible in the beam caustic after the first electron lens 37, as shown schematically in FIG. 8 (see also FIG. 13). As the virtual source size increases, i.e., as the number of number states increases, the diameter of the beam caustic after the first electron lens 37 also increases. Therefore, depending on the type of aperture or blocker and the position of the number state selection device 40, sub-pulses 4 of a specific number state, e.g., n=1, can pass through the number state selection device 40, while sub-pulses 4A of other number states, e.g., n=2 and n=3, are blocked by the number state selection device 40. The sub-pulses 4 transmitted by the number state selection device 40 (the generated pulsed electron beam) are imaged onto the sample and / or application site 50 by the second electron lens 38.

[0057] With respect to each of Figures 5-8, it should be noted that the average kinetic energy of the electron pulse can be adjusted using an electrostatic electron accelerator (see, for example, 33 in Figure 5 or 36 in Figure 7), which can be placed at any position in the beam path between the emitter unit and the application / sample, preferably before the number-state dispersion or after the number-state selection device.

[0058] In the following, further practical embodiments and experimental tests of the present invention will be described with reference to FIGS.

[0059] Our practical experiments demonstrate strong Coulomb correlations in two- and three-electron states generated in a laser-driven Schottky field emitter, as shown below. Using event-based electron spectroscopy and imaging, the kinetic energy distributions of electron ensembles emitted by a single laser pulse are recorded and events are classified by the number of free electrons. For example, characteristic double- and triple-lobed spectra are found for events containing two and three electrons, respectively. This invention allows us to quantitatively characterize interparticle correlations in both energy and transverse momentum, observing that stochastic few-body interactions dominate mean-field (space charge) effects. The two-particle energy correlation function reveals a prominent peak at an energy difference of approximately 1.7 eV, indicating an effective junction emission region for electron pair states much smaller than the physical and virtual source sizes. This finding sheds light on fundamental correlations in many-electron emission and enables statistical control of electron beams for on-demand correlated few-particle imaging and spectroscopy.

[0060] 9 illustrates the creation of Coulomb-correlated few-electron states in an electron beam device 100 according to an embodiment of the present invention. The electron beam device 100 is based on a transmission electron microscope, such as the Ultrafast Transmission Electron Microscope (UTEM) described in

[48] , with a microscope column 101 having an optical axis z.

[0061] The microscope electron source at the top of the microscope column 101 includes an electron source device 20 with a pointed photoemission electron source 21, shown in an enlarged view in the upper right section of FIG. 9. The pointed photoemission electron source 21 comprises a Schottky field emitter, e.g., a tungsten (W) / ZrOx nanotip with a W(100) facet 21A covered with a ZrOx layer. The photoemission electron source 21 has a radius of curvature of, e.g., r=490 nm, and is formed by, e.g., U ext = 2 kV extraction voltage and U bias The photoemission electron source 21 is operated at a bias voltage of -0.3 kV. The photoemission electron source 21 may be cooled during operation. Cooling may be performed to just below the continuous Schottky emission threshold.

[0062] A photoemission electron source 21 is arranged for focused illumination by an emitter excitation pulse 2 and for generating pulsed photoemission, in particular linear photoemission. The emitter excitation pulse 2 (e.g., 160 fs pulse duration, 515 nm central wavelength) is generated using a femtosecond laser source 10 at a repetition rate of, for example, 600 kHz and focused by a lens 11 onto the pointed photoemission electron source 21.

[0063] Additionally, the electron source device 20 includes a laser-assisted Schottky field emitter (nano-tip) with a repetition period T rep A suppressor anode 22 and an extractor anode 23 are provided for directing and accelerating onto the optical axis z an ultrashort source electron pulse 3 emitted at 1. The minority electron state is prepared by pulsed photoemission, i.e. the source electron pulse 3 is generated by a pulsed charge of minority electrons, which then passes through the extractor anode 23 and optionally another beam-limiting aperture 23A.

[0064] Each source electron pulse 3 represents an n-electron event, as shown in the top right section of Figure 9. Each emitter excitation pulse 2 results in the generation of, for example, n = 0 (no electron pulse), n = 1 (one electron per pulse), n = 2 (two electrons per pulse), n = 3 (three electrons per pulse), or n greater than 3. However, experimental testing by the inventors has shown that pulses with n > 4 are less likely. The source electron pulse 3 may have a low pulse charge, i.e., the source electron pulse 3 contains an average of less than one electron per pulse in the sample plane.

[0065] At the lower end of the microscope column 101, a number-state dispersing device 30 is located. The number-state dispersing device 30 includes an imaging energy filter (shown in FIGS. 3 / 4) that deflects the source electron pulse 3 at a deflection angle relative to the optical axis z. The deflection angle depends on the number of states of the source electron pulse 3, and as a result, source electron pulses 3 with different number of states are spatially separated as sub-pulses 4 of the sequence of electron pulses. This spatial separation is shown schematically by the diagram of a total source electron pulse 3 containing any number of states, e.g., further separated sub-pulses 4 of number states n=1, n=2, and n=3.

[0066] Subsequent selection of sub-pulses 4 having one of the number states is provided using a number state selection device 40 which transmits the selected sub-pulse as the acquired pulsed electron beam 1. The number state selection device 40 may be provided, for example, as described in one of Figures 1 to 8 above.

[0067] Downstream of the number-state selection device 40, a time-resolved event-based electron detector camera 105, such as a Timepix3 ASIC (EMCheeTah T3, Amsterdam Scientific Instruments BV), may be positioned for event-based electron spectroscopy, enabling number-state-selective beam analysis, particularly for the tests described herein. The time resolution of the electron detector camera 105 allows for distinguishing between successive incident electron pulses, providing a clear measure of the number of transmitted electrons per laser pulse, n. In practical applications of the present invention, the detector camera 105 is preferably replaced by a sample, such as a sample stage, and / or application site 50, or the detector camera 105 may be combined with the sample and / or application site 50.

[0068] A microscope is shown in FIG. 9 for illustrative purposes only, with a typical configuration including objective lenses 102 and 103 and a sample stage 104 in the sample plane. Electrons pass through the sample plane of the microscope. In some embodiments, objective lenses 102 and 103 and sample stage 104 are not used to practice the techniques of the present invention and may be omitted. However, in the embodiment of FIG. 8, objective lenses 102 and 103 may provide a pair of electron lenses 37 and 38, sample stage 104 may be used to provide a number state selection device 40, and units 30 and 40 at the downstream end of column 101 may be omitted.

[0069] FIG. 10A shows the number of n electron states per pulse detected by the electron detector camera 105 of FIG. 9 depending on the laser power of the emitter excitation pulse 2. For the measurement, the detector camera 105 may be operated as follows: The detector camera 105 may generate a stream of data packages containing the positions of the electron-activated detector pixels, their time of arrival (ToA) digitized in 1.56 ns time bins, and the energy associated with the incident electron events (time over threshold, ToT). For example, at a beam voltage of 200 kV, for each individual electron, a variable size (N pixel,avg ≒8 pixels), shape and energy (ToT avgThe ToT-corrected raw data stream activates clusters of pixels with a ToT of approximately 280 a.u. Localization of single electron events in the data stream is achieved using, for example, a partition of the event clustering code from the Nanoscopy Department M4I at Maastricht University, based on the Python3 hierarchical density-based spatial clustering (HDBSCAN) algorithm

[59] . The algorithm reconstructs the timing and position of individual electrons incident on the detector from clusters of activated pixels (hits). Individual electrons are then differentiated by their ToA and attributed to the same cluster a total pixel energy ToT ranging between three and nine adjacent pixels activated within a 100 ns time window and between 200 a.u. and 400 a.u. (see

[60] ). Photoelectrons are clustered according to the femtosecond laser pulse that generated them. The time resolution of the detector (e.g., 1.56 ns) is much faster than the time pulse separation given by the laser repetition rate (≈1 μs), but much slower than the time separation of correlated electrons in the detector (≈1 ps). This results in a Δt n Electrons arriving at the detector within 50 ns are assigned to several classes of electron states n=1, 2, 3, ... determined by the number of electrons in one laser pulse.

[0070] As shown by the power scaling of the one-, two-, and three-electron states in Figure 10A, the ratio of single-electron pulses to the total number of luminescence events scales linearly with optically pumped laser power, consistent with the process used for near-threshold laser-assisted Schottky photoemission [48, 49]. Figure 10B shows, in an exemplary manner, how one-photon laser-assisted near-threshold Schottky emission generates a one-electron state.

[0071] Similarly, the fraction of two-electron and three-electron events increases as a power of n. Considering the relative distribution of n-electron events at a given laser power, a few sub-Poisson statistics are specified. Specifically, the probability of detecting n electrons in a pulse is defined as P n Then, the Poisson process is r n At =1

number

[0072] A survey of the kinetic energies of number-selected electronic states is shown in Figures 10C-10F, which show how the event-averaged spectrum (Figure 10C) can be separated into number-state-resolved contributions (n ​​= 1, 2, 3, Figures 10D-10F). The two-electron and three-electron spectra show distinct shapes with n peaks, indicating the discrete energy separation of the electrons involved.

[0073] The spectral distribution of one-electron events (Fig. 10D) consists of a single peak that dominates the entire spectrum (averaged over all events) but is centered at an accelerating voltage of E = 200 keV. In stark contrast, the spectra of two- and three-electron events show pronounced double- and triple-lobe structures, respectively, with mean energy at E.

[0074] Beyond averaging over similar events, as shown in Figures 10C-10F, our measurement scheme further allows us to relate spectral features to two- and three-particle correlations within individual electron pulses, as shown in Figure 11. Figure 11 illustrates the generation of electron pair states in a pointed photoemission electron source 21 (see Figure 9). Figure 11A shows an energy histogram of matched electron pairs revealing a strong correlation in relative kinetic energies, as seen in the spectral correlation function (inset, integrated along the diagonal line). Figure 11B shows the normalized one-sided pair correlation function (n=2) for various laser powers. Figure 11C shows the power scaling of the peak position of the n=2 correlation function relative to the spectral width (FWHM) of the n=1 state (spectra for various laser powers are shown in Figure 11D). Figure 11E shows the normalized n=2 spectrum for various laser powers, and Figure 11F shows the pair correlation function for photoemission due to two delayed laser pulses. At temporal overlap, a strong correlation gap is observed, which disappears with a pulse delay of about 200 fs (see cross section in the inset). Figure 11G shows the correlation gap at the extraction voltage U ext Figure 11H shows the horizontal (r tra ) and vertical (r lon ) dimensions are shown (inset: illustration of the correlation amount versus the virtual source size).

[0075] For the generation of two laser pulses described with reference to Figures 11F and 11H, a Michelson interferometer may be used that splits the incident laser pulse into two separate pulses. One of the interference arms has a variable optical path length implemented by a retroreflector attached to a delay stage with bidirectional repeatability of (value). The optical path difference can be adjusted to a delay time difference between the two pulses of, for example, up to 10 ps.

[0076] More specifically, FIG. 11A shows the electron energy E associated with two electrons A and B assigned to the same electron pulse. A and E BThe Coulomb repulsion is due to the energy difference E A -E B and partially reduces the observed gap in the overall spectral density. A +E B This strong correlation demonstrates that the observed splitting of the n=2 spectrum into a double-lobed structure is the result of two-electron interactions arising from Coulomb repulsion.

[0077] Similar to conventional (non-laser-triggered) Schottky sources, only a fraction of the electrons generated at the emitter surface are transmitted to the microscope column 101 (see Figure 9). Therefore, the mean field (space charge) and stochastic interactions with random neighboring electrons not incident on the beam can be considered and distinguished from the correlations observed in the electron pair states. Laser-power-dependent measurements allow for the evaluation of these different contributions. The corresponding spectral distributions for n = 1 and n = 2 (Figures 11D and 11E) show a broadening with increasing laser power (see circle C1 in Figure 11C), i.e., with the mean photocurrent. This corresponds well to previous non-event-selective measurements [28, 31, 50] and is typically attributed to stochastic Coulomb interactions and mean-field effects.

[0078] In contrast, the set of two-electron correlation functions shown in Figure 11B is remarkably independent of laser power, exhibiting a pronounced gap approximately 1 eV wide, a peak at approximately 1.8 eV, and an extended tail toward higher-energy separations of over 4 eV. Increasing the photocurrent only moderately varies the depth of the gap and the shape of the high-energy tail. Notably, the position of the main correlation peak (circled C2 in Figure 11C) approaches a fixed value of 1.7 eV toward vanishing laser power and, hence, average current. This demonstrates that the observed correlations are dominated by two-electron correlations, with only minor variations due to multiple Coulomb interactions with electrons blocked by the aperture.

[0079] To investigate the time range over which such strong Coulomb correlations prevent the observation of independent single electrons, measurements were performed at constant integrated laser power using a pair of laser pulses with variable delay (see Figure 11F). Two distinct ranges were identified. Temporally overlapping laser pulses reproduce the n = 2 correlation function described above. In contrast, a temporal separation of more than 200 fs corresponds to a significantly reduced energy difference, indicating two individual, uncorrelated electron emission events.

[0080] Although the correlations only have a slight dependence on the emission current, we find that the extraction field applied to the tip has a more pronounced effect. Decreasing the extraction voltage substantially changes the observed gap and the slope of the high-energy tail (see the semi-logarithmic plot in Figure 11G). Physically, changes in the extraction voltage affect the Schottky barrier height and the beam acceptance angle. The spectral shape of these correlation functions can be modeled in a simple way by an ensemble of electron doublets prepared with Gaussian-distributed interparticle distances, and it can be assumed that the initial interparticle Coulomb energy is magnified by acceleration in the electrostatic field, resulting in a larger kinetic energy difference. Separate standard deviations r are given for the longitudinal (perpendicular to the surface) and transverse (parallel to the surface) distributions, respectively. lon and r tra Using this, this simple model nicely describes the main features of the measured correlation function (see solid line in Figure 11G).

[0081] The physical source size and back-projected virtual source size of a Schottky field emitter should be in the range of more than 20 nm

[51] . However, because the correlation function nearly vanishes at zero energy difference, there are few additional events involving two uncorrelated (or weakly correlated) electrons. This is noteworthy because the fraction of electron doublets with Coulomb correlation reaches 85% of that expected from the Poisson number distribution and the fraction of one electron. The missing 15% of two-electron events (antibunching) can be attributed to local Coulomb blockade [52, 53], Pauli blocking

[24] , or lateral interparticle deflection and spatial filtering.

[0082] In other words, the statistical occurrence frequency of two-electron events is fairly close to that expected from uncorrelated emission events from an extended nanoscale source.

[0083] To explain the simultaneous enhancement of electron-pair emission, we focus on several mechanisms previously proposed in atomic and molecular contexts, similar to two-electron photoemission. Enhanced "discontinuous" double ionization has been observed in atoms exposed to strong laser fields, first in helium

[54] and subsequently in many other elements. Various mechanisms have been used to explain this observation, including cotunneling, shake-up processes, and field-driven electron rescattering, but the latter mechanism appears to be responsible in most cases [33, 34]. At the rather modest local intensities in our experiments, the ponderomotive potential is significantly below 1 meV, so recollisions can be ruled out as a dominant factor. In the linear regime, one-photon, two-electron emission is common in the Auger effect, where the core hole resulting from photoemission is filled during the simultaneous emission of a second electron. Coulomb interactions are the dominant factor in this process

[55] .

[0084] Figure 12 shows the characterization of spatial beam properties of minority electron states. Figure 12A is a schematic diagram of spatial filtering due to the effect of Coulomb interactions. For integer-increasing pulse charges, the virtual source increases in size (see double arrow) and shifts along the electron beam axis z (see vertical arrow). Figure 12B shows the caustics of an n-selected electron beam recorded by varying the final focusing lens 38 of the electron beam device 100. The insets in Figure 12B show images of the beam profile for n=2 in underfocus (left), focus (center), and overfocus (right). Figure 12C is an image of the beam profile at underfocus, with the correlation angle φ between electron pairs relative to the beam center. The long angular leg of the underfocus state allows for accurate measurement of angular correlation, and Figure 12D shows that a strong anisotropic angular correlation is observed for n=2, compared to the isotropic distribution (shown in Figure 12B by the black circles around the data points) used to depict random events.

[0085] More specifically, Figure 12 shows that the minority-electron states observed here exhibit distinctive spatial characteristics, along with spectral distributions and correlations, as discussed below. Specifically, Figure 12B shows n-dependent beam caustics, which exhibit discrete differences in both the minimum spot size and focal position, resulting in separated subpulses. Variations in laser power result in changes in the caustics (higher powers increase the spot size somewhat), but these are much smaller than the differences between event classes. Under given conditions, the focusability can be limited by the spherical aberration of the objective lens and the virtual source size, resulting in typical spot profiles for positive and negative defocus (inset in Figure 12B). Clearly, caustics for n ≥ 1 are the result of a larger effective source, and the beam waist is shifted toward positive defocus.

[0086] Both observations can be understood from the mutual transverse deflection shown in Figure 12 A. Specifically, the transverse deflection is expected to spread the minority electron orbits laterally, such that the virtual source size increases and moves forward

[31] , as previously predicted by simulations [52, 57].

[0087] A more detailed analysis of the spatial characteristics of the minority-electron state can be obtained by analyzing the correlation of transverse momentum. To this end, we measured the position correlation for sufficiently large negative defocus (Figure 12C). The spatial correlation is quantified via the angle φ between the two electrons and the beam center. Figure 12D shows the angular correlation density for a two-electron state compared to the random correlation obtained from the corresponding single-electron state at the same spot size (15 nm). For the electron pair state, a strong anisotropic correlation is obtained, peaking at an angle of around 180°, corresponding to electron events localized on either side of the defocused beam and therefore with nearly opposite transverse momentum.

[0088] These observations demonstrate that averaging over few states has a significant impact on beam characteristics, including uncorrectable stochastic aberrations. Therefore, control of the number statistics of photoemission beams can directly benefit microscopy applications using such sources. More generally, stochastic Coulomb interactions are a fundamental challenge in electron microscopy, limiting the brightness of electron sources by altering the beam's transverse (Löffler) and longitudinal (Börsch) momentum distributions. The moderate antibunching observed here and in previous work

[25] means that the overall photocurrent exhibits slight sub-Poissonian noise characteristics, a highly sought-after property in focused-system scenarios (e.g., achieved by Coulomb blockade). In the context of electron microscopy, this feature can be directly applied to shot noise reduction in imaging, spectroscopy, and lithography. However, perhaps even greater potential arises from the strong Coulomb correlations identified for the electron doublet (n = 2) state. The fact that both electrons in this state are energetically well separated from each other and from the central energy allows for energy selection of each few state. This facilitates a powerful approach to control the statistics of one- and two-electron events by energy selection.

[0089] Figure 13 illustrates the statistical control of one-electron and two-electron states using spatial and spectral filtering. Figure 13A shows the spatial filtering scheme using a circular aperture, and Figure 13B shows the transmittance of spatial filtering. Figure 13C (two-electron state suppression) demonstrates that an energy slit significantly reduces the transmission of the n=2 electronic state generated by the laser pulse relative to the n=1 electronic state, with the spectra of the n=1 and n=2 electronic states shown. The electron energies in the dark shaded region are truncated by the energy slit. Figure 13D (two-electron state enhancement) demonstrates that an energy beam stop significantly reduces the transmission of the n=1 electronic state relative to the n=2 electronic state, with the spectra of the n=1 and n=2 electronic states shown. The electron energies in the dark shaded region are truncated by the energy beam stop.

[0090] Figure 13E shows the transmission T for various energy slit widths with up to 8-fold enhancement of the n=1 electronic state (for the setup shown in Figure 13C). n and the transmission ratio T1 / T2, and FIG. 13F shows the transmission T1 / T2 for various energy stop widths with over 20-fold enhancement of the n=2 electronic state (for the setup shown in FIG. 13D). n and a plot of the transmission ratio T2 / T1.

[0091] More specifically, pre-specimen energy filters, commonly used in state-of-the-art electron microscopes

[58] , can be used to selectively favor specific number states. Specifically, such an energy slit with a pinhole diameter d truncates the transmitted energy spectrum, and its width can be adjusted to strongly favor the transmission probability of n = 1 over n = 2 states (see Figure 13C). Specifically, in experimentally measured one- and two-electron spectra (Figure 13E), the transmission probability of n = 1 exceeds the transmission probability of n = 2 by a factor of 8 at small slit widths, significantly enhancing the sub-Poissonian nature of the electron number distribution and facilitating electron currents with reduced shot noise. Conversely, a central beam stop at an energy level can suppress a significant proportion of one-electron states, enhancing the transmission of pair states relative to the n = 1 state by up to 20 times (see Figures 13D and 13F). This approach enables new forms of correlated electron microscopy and spectroscopy for a variety of novel two-point or two-times measurement schemes in correlated materials and free-electron quantum optics.

[0092] In summary, this invention demonstrates unprecedented control over the statistics in pulsed charged particle beams, demonstrating a novel approach for generating femtosecond pulses containing a well-defined integer number of electron charges, Q = n e. The number of electrons in each state, n, can not only be directly tracked by event-based detection but also significantly influence the beam's spectral and angular distribution. This allows for the modification of electron beam particle statistics, thus efficiently filtering specific number states, simply by blocking a portion of the time-averaged intensity in the energy dispersive plane, transverse momentum, or focused beam position. Rejecting beam states with n > 1 efficiently generates sub-Poissonian beams, which has direct implications for reduced shot noise electron imaging and lithography. Furthermore, selecting specific number states allows for borrowing concepts from quantum optics. For example, n = 2 constitutes a highly classical state and can be used to implement high-fidelity, electron-heralded single-electron states, enabling shot-noise-free (or reduced) electron imaging and lithography with precisely countable electron numbers, eliminating previously considered fundamental limitations. Furthermore, the fundamental scattering processes accompanying the creation of many-electron states can be assumed to induce entanglement among many electrons, typically in the absence of additional entanglement-breaking reporting channels, such as electron-holes potentially remaining within the photoemission device, or coupling to an external thermal bath. Future research must address the quantum coherence of such many-electron states, promising promising new quantum technologies using free electrons, potentially enabling interaction-free measurements and ghost imaging, quantum teleportation and information processing, and ultimately entangled free-electron qubits for fermionic quantum computing.

[0093] The features of the invention disclosed in the above specification, drawings and claims may be important both individually and in combination or subcombination for the realization of the invention in its various embodiments. The invention is not limited to the preferred embodiments described above. Rather, multiple variations and derivations are possible that use the inventive concept and thus fall within the scope of protection. Furthermore, the invention also claims protection for the subject matter and features of the dependent claims apart from the referenced features and claims.

Claims

1. 1. An electron beam device (100) configured to generate a pulsed electron beam (1) comprising a sequence of electron pulses, comprising: an irradiation source device (10) arranged to generate a sequence of emitter excitation pulses (2), in particular laser pulses; an electron source device (20) having a photoemission electron source (21) arranged for irradiation-induced emission of a source electron pulse (3) in response to irradiation with the emitter excitation pulse (2); a number-state disperser (30) arranged to spatially separate the source electron pulse (3) into sub-pulses (4, 4A), each sub-pulse (4, 4A) containing an integer number of electrons (n), where n=1, 2, 3, ...; a number state selection device (40) arranged to select sub-pulses (4, 4A) including at least one set of predetermined electron number states as the generated pulsed electron beam (1); An electron beam device comprising:

2. the photoemission electron source (21) is configured to generate countable low charge electron pulses, preferably containing 1, 2, 3 or 4 electrons per pulse; 2. The electron beam device according to claim 1.

3. the photoemission electron source (21) comprises a beam limiting aperture (20A) configured to reduce high-charge electron pulses to countable low-charge electron pulses; 3. The electron beam device according to claim 1 or 2.

4. The photoemission electron source (21) is configured to generate multi-electron states having distinguishable properties in addition to a pulsed charge.

4. The electron beam device according to claim 1.

5. Electron beam device according to any one of claims 1 to 4, wherein the number state dispersing device (30) comprises an energy dispersing device.

6. The energy dispersing device is Electron beam device according to claim 5, comprising a beam monochromator, in particular an Omega, Alpha, Wien filter, double Wien or electron mirror type beam monochromator.

7. The energy dispersing device is Spectrometer apparatus, in particular a spectrometer apparatus including a magnetic prism or an electrostatic multipole electron optics system 6. The electron beam device of claim 5, comprising:

8. 8. The electron beam device according to claim 1, wherein the number state dispersion device (30) comprises a momentum dispersion device or a position dispersion device or an electron orbital momentum dispersion device, in particular at least one of a rotationally symmetric electron lens and a cylindrical electron lens.

9. Electron beam device according to any one of the preceding claims, wherein the number state dispersion device (30) comprises an electron spin dispersion device.

10. the number state selection device (40) comprises a spatial modulator of electron beam intensity, in particular a spatial modulator configured to pass selected sub-pulses (4, 4A) having a predetermined electron number state and to block the remaining sub-pulses (4, 4A); 10. The electron beam device according to claim 1.

11. the number-state selection device (40) comprises at least one of a mechanical slit, a grating, a linear beam block, a hole, a ring, or a disk; 11. The electron beam device according to claim 1.

12. A detector device configured to measure the number of electrons in at least one of the electron number states of the pulsed electron beam (1). The electron beam device according to any one of claims 1 to 11, further comprising:

13. Beam-forming electron optics configured to illuminate a sample, such as a sample under investigation, and / or a workpiece, such as a semiconductor workpiece. The electron beam device according to any one of claims 1 to 12, further comprising:

14. The photoemission electron source (21) includes a pointed photoemission electron source. Electron beam device according to any one of claims 1 to 13.

15. An electron microscope apparatus comprising the electron beam apparatus according to any one of claims 1 to 14.

16. A method for generating a pulsed electron beam (1) comprising a sequence of electron pulses, comprising: generating a sequence of emitter excitation pulses (2), in particular laser pulses, by means of an irradiation source device (10); irradiating a photoemission electron source (21) of an electron source device (20) with the emitter excitation pulse (2) so that a source electron pulse (3) is generated by irradiation-induced emission; spatially separating the source electron pulse (3) into sub-pulses (4, 4A) by a number state disperser (30), each sub-pulse (4, 4A) containing an integer number of electrons (n), where n=1, 2, 3, ...; selecting sub-pulses (4, 4A) including at least one set of predetermined electron number states as the generated pulsed electron beam (1) by a number state selection device (40); A method comprising:

17. adjusting the irradiation source device (10) and / or the photoemission electron source (21) to generate multi-electron states that are distinguishable by characteristics other than pulse charge; 17. The method of claim 16, comprising:

18. adjusting said number-state dispersion device (30) according to distinguishable characteristics of said source electron pulses (3), in particular energy, angle / momentum, spatial focus; 18. The method of claim 16 or 17, comprising:

19. adjusting said number state selection device to reject or block certain number states from said beam, of particular interest to reject or block all n>2 or all sub-pulses (4, 4A) with n≠2; The method according to any one of claims 16 to 18, comprising:

20. adjusting the illumination source device (10), the photoemission electron source (21), the number-state dispersion device (30) and / or the number-state selection device (40) based on the output of an additional number-state sensitive detector; The method according to any one of claims 16 to 19, comprising:

21. 16. Use of an electron beam device according to any one of claims 1 to 15 as a beam source in at least one of an electron microscopy device, an electron lithography device, an electron pair (source) device, an electron herald device, an electron counting device, an information processing device, a communication device, and a quantum computing device.

Images