DE102016110301B3 - Method for generating an electron beam and / or electron pulse and an electron source and their use - Google Patents

Abstract

The invention relates to a method for generating an electron beam and / or an electron pulse, in which a tapered metallic structure (14) with a nanotip (15) for emitting electrons is used, the metallic structure (14) being a cathode (11). is applied, a voltage is applied to the metallic structure (14), and with a first light beam (19), the metallic structure (14) is irradiated. In order to enable a higher temporal resolution, the method is characterized in that the metallic structure (14) is irradiated with a further light beam (25), due to the irradiation of the metallic structure (14) with the first light beam (19) and the other Light beam (25) of the electron beam and / or the electron pulse is generated.

Description

The invention relates to a method for producing at least one electron beam and / or one electron pulse, wherein a tapered metallic structure having a nanotip for discharging electrons is used, the metallic structure being used as a cathode, a voltage is applied to the metallic structure , and with a first light beam the metallic structure is irradiated. Furthermore, the invention relates to an electron source for generating at least one electron beam and / or electron pulse having a cathode having a tapered metallic structure and a nanotip for discharging electrons, with a first laser system for irradiating the metallic structure with a first light beam.

Such a method or a corresponding electron source is known from DE 10 2015 108 893 B3 known.

Moreover, it is known to use an electron source and / or an electron microscope to study atomic spatial resolution structures. High-resolution investigations can be carried out by means of a laser which, due to broad spectra and / or short wavelengths, allows a pulse duration in the range of femtoseconds (abbreviation: fs) and below.

In order to simultaneously enable a high spatial and a high temporal resolution, the spatial resolution of an electron source, in particular an electron microscope, with the temporal resolution and / or pulse duration of lasers can be connected. In such so-called ultrafast electron microscopes, short electron pulses can be generated by means of photoemission due to an ultrashort laser pulse. Such a photoemission is known, for example, from P. Dombi et al., Ultrafast strong-field photoemission from plasmonic nanoparticles, Nano Letters 13 (2013), pp. 674-678. In this case, the temporal resolution of the laser pulse, at least in a first approximation, pass to the electron pulse. With the ultrashort electron pulse, a sample can be irradiated, wherein preferably the sample was previously excited by means of at least a second ultrashort energy pulse and / or electron pulse. By varying the delay between the arrival of the two pulses on the sample, the temporal evolution of the sample can be studied.

In particular, in individual known electron sources, such as from DE 11 2009 003 724 T5 , and / or electron microscopes, especially ultrafast electron microscopes, there are some disadvantages. For example, if the electrons are produced by a diffraction-limited light spot, which may be a few microns in size with moderate focus, there is a risk that a sample will be moved into the light spot. This can lead to unwanted heating of the sample, potentially triggering unwanted extra electrons, and / or potentially shading an electron-emitting tip of the electron source. Thus, there is the risk that no controlled electron pulse and / or no more electrons are triggered.

For spatially high-resolution projection images, the smallest possible distance between the electron source and a sample is necessary. In particular, it is desirable that the distance is only a few hundred nanometers. Even for measurements with high temporal resolution, the distance between the electron source and the sample should be as small as possible, which can be found, for example, in the US 2011/0220792 A1 described ultrafast electron microscope is not given. For this purpose, it is known to use imaging optics or to use a laser-based electron point source for imaging a sample in a point projection microscope. As a result, the distance between the electron source and the sample can be reduced to about 10 microns. By means of suitable measures, the temporal resolution and / or the electron pulse duration can be increased to about 100 femtoseconds. However, a reduction of the distance may be limited due to the risk of irradiation of the sample by a light source, in particular a laser for triggering the electrons.

In particular, with a strong focusing of the electron-triggering light beam and / or laser pulse to reduce the light spot at the location of the triggering of the electrons, a lesser distance between the electron source and the sample can be realized, but in this case increases the opening angle of the light cone. This cone of light, in particular with an opening angle of a few 10 °, must be kept free and is therefore not suitable for the placement of z. B. electron lenses available. This creates the risk that the design and / or construction of corresponding lens systems is severely limited.

Furthermore, it is disadvantageous that the size of the focused light and / or light spot, in particular in the range of a few micrometers, and the electron source, in particular in the range of a few nanometers, usually do not coincide. This has the disadvantage that a large part of the laser energy can not be used for the emission of electrons.

The temporal resolution of previously known electron sources and / or electron microscopes, in particular ultrafast electron microscopes, is limited to about 100 femtoseconds. Thus, investigations of temporally faster processes with such an electron source and / or such an electron microscope are not possible. In particular, the time resolution is limited by the propagation length of the electrons from their generation to the interaction with the sample. This limitation of the temporal resolution can result from a widening of the electron pulse over time, in particular due to chromatic aberration and / or electrostatic repulsion effects.

It is the object underlying the invention to develop a method and / or an electron source for generating an electron beam and / or an electron pulse of the type mentioned above such that a higher temporal resolution and / or a smaller distance between the electron source and a sample possible is. In particular, a temporal compression of the electron pulse at the sample location is to be achieved and / or improved. Preferably, an alternative to previously known methods and / or electron sources should be provided.

The object underlying the invention is achieved by means of a method of the type mentioned, wherein the metallic structure is irradiated with a further light beam, wherein due to the irradiation of the metallic structure with the first light beam and the further light beam, the electron beam and / or the electron pulse generated becomes. Furthermore, the object underlying the invention is achieved by means of an electron source of the type mentioned, in which a further laser system for irradiating the metallic structure with a further light beam is present, due to the formation of the first laser system and the other laser system for irradiating the metallic Structure of the electron beam and / or the electron pulse can be generated.

It is advantageous that the first light beam and the further light beam are individually and / or independently selectable and / or adjustable. In particular, the first light beam and the further light beam can be selected and / or adjusted in such a way that the desired electron beam and / or electron pulse is generated only with a suitable and / or predetermined combination of the irradiation of the metallic structure with the first light beam and the further light beam. Preferably, the first light beam and the further light beam are formed such that the generated electron beam and / or electron pulse has an improved and / or higher temporal resolution.

According to a further embodiment, the electron beam and / or the electron pulse is generated on the basis of a spatial and / or temporal superimposition of the first light beam and of the further light beam. Preferably, the metallic structure is irradiated temporally after the first light beam with the further light beam. Thus, the irradiation with the further light beam can be done after completion of the irradiation with the first light beam. In particular, the first light beam and the further light beam can spatially overlap at least partially or completely. According to an alternative embodiment, the first light beam and the further light beam may overlap in time at least partially or completely. The irradiation of the metallic structure with the further light beam can take place within less than 100 fs after the end of the irradiation with the first light beam. The irradiation with the first light beam and the further light beam can begin and / or occur at a predetermined time and / or spatial distance. Alternatively, the irradiation of the metallic structure with the first light beam and with the further light beam can take place simultaneously and / or at least partially overlapping in time. Here, the first light beam and the further light beam may be focused on the same location and / or the same area of the metallic structure. Alternatively, the first light beam and the further light beam may be focused on deviating points and / or regions of the metallic structure.

Preferably, upon irradiation of the metallic structure, no electron beam and / or electron pulse is caused exclusively with the first light beam or exclusively with the further light beam. Only on account of a predetermined combination of the irradiation of the metallic structure with the first light beam and the further light beam is the electron beam and / or the electron pulse generated. In particular, irradiation of the metallic structure exclusively with the first beam or exclusively with the further light beam causes a negligible electron emission, in particular for further use. Preferably, further use is understood to mean the use of an electron beam and / or electron pulse in an electron microscope for examining a sample.

The first light beam can be generated by a first light source and / or the further light beam by a further light source. In particular, a single and / or common light source is used to generate the first light beam and / or the further light beam. Preferably, the first light beam, in particular as a first light pulse, is generated by a first laser system. The further light beam, in particular as a further light pulse, can be generated by another laser system. The first laser system and the further laser system can have a single, common light source or each have its own light source. Preferably, the first light beam is selected from a first wavelength range and the further light beam is selected from a further wavelength range that is different from the first wavelength range. Thus, the first light beam and the further light beam may have different wavelengths. In particular, the first laser system and the further laser system each emit a light beam and / or light pulse in different wavelength ranges. Thus, light pulses of different colors can be used as the first light beam and as a further light beam. In particular, the first light beam and / or the further light beam is designed as an electromagnetic wave.

Preferably, the first light beam has a, in particular average, wavelength in the visible range. In particular, the wavelength of the first light beam is in the range from 250 nm to 750 nm. The further light beam may have one, in particular average, wavelength in the infrared and / or near-infrared range. In particular, the wavelength of the further light beam is in the range from 750 nm to 1000 nm. The wavelength of the further light beam is preferably in the range from 750 nm to 1400 nm or in the range from 750 nm to 3000 nm.

According to a development, electrons in the metallic structure, in particular in the nanotip, are converted by means of the first light beam into an intermediate state, in particular above the Fermi energy, and / or into an image charge state. Thus, electrons can be decoupled from the Fermi level with the first light beam and stored above the Fermi level and / or the Fermi energy. In particular, the intermediate state and / or the image charge state is an atom-like surface state. Preferably, the image charge state represents an, in particular long-lived, intermediate state for the electrons above the Fermi energy and / or with regard to the electron beam and / or electron pulse to be generated. In particular, the electrons remain in the intermediate state for a time of up to 100 fs / or the image charge state. Thus, a long lived intermediate state may persist in this regard for a time of up to 100 fs. Thus, by means of the first light beam, an optical excitation of the metallic structure, in particular the nanotip, take place. The electrons emitted due to the optical excitation in the Bildleiungszustand produce a polarization of the charge distribution in the interior of the metallic structure, in particular the nano-tip, which leads to an attractive interaction with the exiting electron. In particular, the electrons in the image charge state are in a predetermined and / or defined energy state, preferably above the Fermi level.

Preferably, the electrons are triggered by the further light beam from the image charge state for generating the electron beam and / or the electron pulse. Thus, the electrons can be released from the intermediate state by means of the further light beam and released as electron beam and / or electron pulse. Preferably, the irradiation of the metallic structure with the further light beam takes place temporally after the irradiation of the metallic structure with the first light beam. In this case, the time interval between the first light beam and the further light beam can be predetermined. Preferably, the time interval is less than 100 fs. In particular, the irradiation with the further light beam takes place within 100 fs after the termination of the irradiation with the first light beam. Preferably, the electrons remain in the charged state after completion of irradiation with the first light beam for a period of up to 100 fs. Thus, the metallic structure can be irradiated with the first light beam and a temporally delayed further light beam. In particular, the metallic structure is irradiated with a sequence of a first light pulse and a further light pulse. In this case, the first light beam and the second light beam can be emitted successively in time to the metallic structure or at least partially overlap spatially and / or temporally when irradiating the metallic structure. A field strength of the first light beam and / or of the further light beam can be adjusted such that the single first light beam or further light beam leads to a negligible electron emission, while the irradiation of the metallic structure with both light beams in the desired emission of photoelectrons for generating the electron beam and / or electron pulse results. In particular, a field strength in the range of a few V / nm, in particular in the range of 1 V / nm to 20 V / nm, is set for the first light beam and / or the further light beam. Preferably, a relative field amplitude and a total intensity, in particular of the first light beam and / or the further light beam must be chosen exactly so that electrons are decoupled with the first light beam from the Fermi level and stored in the image charge state. In particular, the electron beam and / or electron pulse is caused due to multi-photon absorption or multi-photon excitation. Preferably, the multi-photon absorption by the further light beam is caused with effect on the electrons in the image charge state. A trigger An electron from the image charge state may require absorption of up to 10 photons. In particular, absorption of less than 15 photons to trigger an electron from the image charge state is necessary.

Preferably, the further light beam causes strong field photoemission. In particular, an electric field is generated in the region of the metallic structure by means of the further light beam, resulting in a field increase due to the tapered shape of the metallic structure and / or the nanotip. Due to the tapered metallic structure, in particular the nanotip, the irradiated field strength of the further light beam and the geometric constriction caused by the field lines can be additionally amplified and / or increased. In this way, a lower power of the light source for the further light beam, in particular a lower laser power, can be used. Thus, without exceeding a damage threshold of the material, strong field effects, in particular as in an above-threshold ionization, can be transferred to solids. Thus, by means of the irradiation of the further light beam, an above-threshold ionization or at least one comparable process can be caused. In particular, a strong field interaction with the periodic, on the metallic structure, in particular the nano tip, greatly inflated electric field of the further light beam to the electrons a comb-like and / or step-like energy and / or phase. The phase of the electronic wave function can be controlled by the time interval between the first light beam and the further light beam and by the spectrum and / or the spectral phase of the first light beam and / or the further light beam. In terms of time, in particular a time domain, this can result in a sequence of electron pulses after a certain propagation distance. In particular, a single electron pulse has a duration in the range of less than 10 fs and / or in the attosecond region. A correspondingly generated electron beam and / or electron pulse can interact with a sample.

According to a further embodiment, a light pulse having a time duration and / or full half-life of less than 100 fs, less than 50 fs or less than 20 fs is used as first light beam and / or as further light beam. Particularly preferably, the time duration and / or full half-life of the first light pulse and / or the further light pulse is in the range of 15 fs, 10 fs or less. Due to a sequence of a plurality of first light beams and a plurality of further light beams, in particular first light pulses and further light pulses, a series of a plurality of individual electron beams and / or electron pulses can be generated. Preferably, a single electron beam and / or electron pulse has a time duration and / or full half-life of less than 100 fs, in particular less than 50 fs. Particularly preferably, the time duration and / or full half-life of a single electron beam and / or electron pulse is less than 10 fs. In particular, the time duration and / or full half-life of a single electron beam and / or a single electron pulse is in the range of 5 fs, 500 attoseconds or 400 attoseconds. The time duration and / or full half-life of the electron beam and / or electron pulse can be influenced by the time duration and / or full half-life of the further light beam or light pulse.

According to a development, the nanotip is irradiated with the first light beam and / or the further light beam. Alternatively, a coupling element of the metallic structure spaced from the nano-tip can be irradiated with the first light beam and / or the further light beam. Preferably, the first light beam and / or the further light beam is focused on the metallic structure, the coupling element and / or the nanotip. The method can be carried out or carried out by means of direct illumination of the nanotip and / or by means of excitation via the coupling element.

Preferably, the first light beam and / or the further light beam is focused on a focus diameter that at least substantially corresponds to the dimensions and / or the surface of the metallic structure, a coupling element and / or the nanotip. Here, the focusing can be done by means of a lens, a lens system and / or a reflection optics. In particular, the first laser system and / or the further laser system is designed to focus the first light beam and / or the further light beam. Due to the focusing, energy losses during the conversion of the first light beam and / or the further light beam, in particular into a plasmon, can be reduced. This allows the use of a light source, in particular a laser, with a lower power. Preferably, the focusing and / or adjustment of the first light beam, the further light beam, the focus diameter and / or the light source with respect to a central axis of the metallic structure. In particular, the irradiation of the first light beam and / or of the further light beam onto the metallic structure takes place at an angle to the surface of the metallic structure, the coupling element and / or the nanotip. The irradiation of the first light beam and / or of the further light beam preferably takes place with an angle of incidence to the surface which corresponds to half the opening angle of the metallic structure and / or the nanotip.

For generating an electron beam and / or an electron pulse, a voltage of a voltage source is applied to the cathode. In particular, a voltage of the voltage source is applied to the cathode and to an anode for generating an electric field between the cathode and the anode. As a result, the induced electron emission can be facilitated and / or supported. After the release and / or detachment of the electrons from the nanotip, the released electrons are accelerated in the direction of the anode. The electrons thus accelerated and accelerated away from the nanotip by the applied voltage can be used for investigations of a sample. By means of the applied voltage, sufficient electrons are supplied to the cathode, the metallic structure and / or the nanotip for further triggering and / or detaching processes for generating further electron beams and / or electron pulses.

According to a development of the electron source according to the invention for generating an electron beam and / or electron pulse, in particular with a method according to the invention, the metallic structure and / or the nanotip is cone-shaped and / or cone-shaped. In particular, the metallic structure and / or the nano-tip is conical and / or conical. The metallic structure may be formed of gold, silver, tungsten or aluminum. For example, a polycrystalline gold wire, in particular having a fine grain structure, can be used as the starting material. The metallic structure and / or the nano-tip may have an opening angle in the range of 10 ° to 40 °, in particular in the range of 20 ° to 30 °. For example, the opening angle of the metallic structure and / or the nano-tip is 22 °. Due to the cone-like and / or conical shape of the metallic structure, at least one generated plasmon may be caused to converge and / or focus in the nano-tip. Thus, the metallic structure, in particular the nanotip, can be designed to focus at least one plasmon. In this case, a plasmon generated on the basis of the first light beam and / or of the further light beam and / or several generated plasmons can run into and be focused into a nanometer-sized tip and / or a volume in the nanometer range, namely the aforementioned nanotip. In particular, a plasmon or several plasmons are contracted spatially when focusing in the direction of the nanotip. Due to the focusing of the at least one plasmon in the region of the nanotip, an electric field in the region of the nanotip can be amplified and / or increased, whereby electrons can be transferred to the image charge state and / or triggered or detached from the nanotip. Due to a compound of the metallic structure as a cathode with a voltage source, electrons can be replenished. In particular, an anode is provided, which may also be connected to the voltage source.

The at least one plasmon may be a surface plasmon. In particular, energy is transferred from an electromagnetic wave of the first light beam and / or the further light beam into at least one surface plasmon polariton bonded to the metallic structure. In particular, by means of a light pulse, a plurality of plasmons, surface plasmons and / or surface plasmon polar ions can be excited on the metallic structure, in particular a coupling element, and / or focused in the area of the nanotip.

Preferably, the metallic structure, the nano-tip and / or at least a portion of the metallic structure from the coupling element to the nano-tip is free of impurities and / or free of grain boundaries. In particular, the region of the coupling element and / or the region including the coupling element up to and including the nano-tip is free of material contamination and / or free of grain boundaries. As a result, unwanted interference effects in the focusing of the plasmons and / or field strengths in the direction of the nano tip can be avoided. In particular, unwanted detachment of electrons in an area outside the nanotip, in particular in the region of the coupling element and / or in a region between the coupling element and the nanotip, can be avoided.

In particular, the nanosphere has a volume in the nanometer range, in particular of about 10 nm ³ . The nano tip may alternatively be referred to as apex. The nano tip may have a tip radius of less than 20 nm, in particular less than 15 nm or less than 12 nm. In particular, the tip radius of the nano tip is in a range of 5 nm to 12 nm.

According to a further embodiment, the first laser system and / or the further laser system has a femtosecond laser as a light source for the first light beam and / or the further light beam. Thus, a sequence of the first light beam and the further light beam, in particular a first laser pulse and a further laser pulse, can be generated by means of a suitable femtosecond laser. The first laser system and the further laser system can have a common or identical light source, in particular a laser light source. In particular, the first laser system has a first optical path and the further laser system has a further optical path. Preferably, the first optical path and the further optical path are at least partially formed separated from each other. By means of the first optical path and the further optical path, the path length of the first light beam or of the further light beam can be controlled and / or adjusted. In particular, the first light beam and the further light beam at a common light source by means of the first optical path and the further optical path can be separated from each other. In addition, the first light beam and the further light beam can be set and / or manipulated by means of the two paths with regard to their optical properties, in particular independently of each other. Preferably, the first optical path and the further optical path are formed such that the first light beam and the further light beam are spatially superimposed on the metallic structure, the nanotip and / or the coupling element. The light source for the first light beam and / or the further light beam, the first laser system and / or the further laser system can be designed such that phase-stabilized, few-cycle laser pulses having at least two different central wavelengths and / or pulse durations, in particular in the range of 10 femtoseconds , are producible.

The metallic structure may have, spaced from the nanotip, at least one coupling element for exciting at least one plasmone by means of the first light beam and / or the further light beam. A first coupling element may be present for the first light beam and a further coupling element for the further light beam. Here, the training and / or position of the first coupling element may differ from the training and / or position of the further coupling element. Due to the coupling element spaced from the nano-tip, direct illumination of the electron source, in particular the nano-tip, can be avoided. Thus, a laterally, in particular arbitrarily far, extended sample, preferably as desired, can be brought close to the electron source, in particular the nanotip. Lens systems, in particular electrostatic lens systems, can be constructed more compactly around the electron source, in particular the nanotip. As a result, particularly short pulse durations of the electron pulses can be realized on a sample. A heating of the electron source, in particular the nanotip, is considerably reduced. In particular, heating of the electron source and / or the nanotip results merely from the absorption of a plasmone and / or several plasmons. Thus, at most, only a small input of energy or only a slight warming result.

Furthermore, the use of a coupling element spaced from the nano-tip may allow a stronger focusing of the electromagnetic wave of the first light beam and / or of the further light beam. In this way, a lower power at a light source for the first light beam and / or the further light beam can be selected as in a direct illumination of the electron source, in particular the nanotip. In particular, a mismatch between the size of the illumination of the coupling element and the size of the emission point for the electrons is significantly reduced or eliminated due to the distance from the nanoclip coupling element and the focusing of the at least one plasmone.

In particular, the coupling element is designed as a grating coupler. By means of the coupling element can be excited on the surface of the metallic structure due to an incoming electromagnetic wave of the first light beam and / or the further light beam at least one plasmon. The grating coupler can have up to 15 grids or more. Preferably, the number of grid lines is not limited and can be arbitrarily selected. In particular, the grating coupler has 7 grid lines. The grid lines may have a width in the range of 100 nm to 1,000 nm and / or a depth in the range of 200 nm to 600 nm. In particular, the grating lines are 800 nm wide and 400 nm deep. The grating coupler preferably has a lattice constant in the range from 1200 nm to 3000 nm, in particular in the range from 1500 nm to 2500 nm. More preferably, the grating coupler has a lattice constant of 1600 nm or 2000 nm. The lattice constant can be the wavelength of the electromagnetic radiation radiated onto the coupling element Wave of the first light beam and / or the other light beam correspond.

Preferably, the distance of the coupling element from the nanotip is greater than a focus diameter of the first light beam and / or the further light beam for irradiating the coupling element. The distance between the coupling element and the nanotip may be greater than 10 microns or greater than 20 microns. Preferably, the distance between the coupling element and the nano tip is in the range of 10 .mu.m to 200 .mu.m. Particularly preferably, the distance between the coupling element and the nano tip is 50 microns. In particular, the distance between the coupling element and the nano tip is selected such that a simultaneous illumination and / or irradiation of the nano tip is avoided when the coupling element is illuminated and / or irradiated. In particular, the distance of the coupling element from the nanotip is selected such that this distance is less than an average propagation length of the plasmons generated by means of the electromagnetic wave of the first light beam and / or of the further light beam. For example, at a wavelength of 1,600 nm arriving at the coupling element electromagnetic wave is the average propagation length of the plasmons generated thereby about 100 microns. Thus, for example, at a distance of the coupling element of the nanotip of about 50 microns can be ensured that the plasmon reach the nanotip.

Preferably, the metallic structure is disposed within a vacuum chamber. As a result, damage and / or destruction of the electron source, in particular due to ionization of the otherwise existing gases, can be avoided. A light source, in particular a laser and / or a laser light source, for irradiating the metallic structure, the nanotip and / or the coupling element can be arranged inside or outside the vacuum chamber. Preferably, the light source is arranged outside the vacuum chamber. The electromagnetic wave or the first light beam and / or the further light beam can be supplied to the metallic structure, the nanotip and / or the coupling element by a window element, which is associated in particular with the vacuum chamber. The first light beam and the further light beam can be guided through a single, common window element in the vacuum chamber. Alternatively, a first window element for the first light beam and a further window element different from the first window element for the further light beam may be present. The window element can be arranged in a wall of the vacuum chamber. Furthermore, a lens, a lens system and / or a reflection optics may be provided. The lens, the lens system and / or the reflection optics can be arranged inside or outside the vacuum chamber. The lens, the lens system and / or the reflection optics is positioned between the light source and the coupling element. In particular, a first optical path and a further optical path each have an associated lens, a lens system and / or a reflection optical system. In particular, the lens, the lens system and / or the reflection optics serve for focusing the first light beam and / or the further light beam onto the surface of the metallic structure, the nanotip and / or the coupling element. Preferably, the focusing of the first light beam and / or the further light beam is such that the light and / or the associated electromagnetic wave substantially completely hits the metallic structure, the nanotip and / or the coupling element.

According to another embodiment, a detector system for detecting electrons is provided. By means of the detector system, the electrons imaging a sample can be detected. In particular, it is possible by means of the detector system to determine a spatial position of an electron when hitting the detector system. The detector system preferably has a detection device for determining a kinetic energy of the electrons. Thus, by means of the detection device, the kinetic energy of the electrons can be detected when hitting the detector system. In particular, the detector system allows both an investigation and / or determination of the location as well as a kinetic energy and / or velocity of the detected electrons. The detection device may comprise a delay line detector. In particular, the detection device is designed as a delay line detector. The detection device and / or the delay line detector can be arranged in combination with at least one microchannel plate. The use of one or more microchannel plates, in particular with channels running obliquely to the plane of the plate, is known from conventional detector systems. In conventional detector systems, such microchannel plates are used in combination with a phosphor screen. Here, the phosphor screen is used to represent the impinging electrons or the resulting electron spots. Instead of the phosphor screen, a delay line detector may be present.

The lifetime of the electron source, in particular the metallic structure and / or the nanotip, can be optimized by improving the vacuum. For example, instead of a turbo pump for generating the vacuum, an ion getter pump can be used, whereby the vacuum is improved. Furthermore, it is advantageous if the mechanical stability of the structure of the electron source, in particular of an electron microscope, is increased. For example, the mechanical stability can be increased by means of a more compact design. The mechanical stability in the region of the energy source and / or the sample can be increased by focusing the first light beam and / or the further light beam outside the vacuum chamber. In particular, a lens, a lens system and / or a reflection optics may be arranged outside the vacuum chamber. This can minimize the number of objects within the vacuum chamber. For example, the number of motorized traversing tables in the vacuum chamber can be reduced. This can also lead to a reduction of costs. Another advantage is a vibration-damped suspension of the electron source, in particular the metallic structure and / or nanotip. Additionally or alternatively, the sample may be suspended vibration damped. Due to the vibration-damped suspension vibrations can be minimized. Thereby, the spatial and / or temporal resolution in the use of the electron source in an electron microscope for examining a sample can be improved. After all For example, the electron source, the metallic structure and / or the nanotip may be surrounded by a shield, in particular of magnetically highly permeable metal. This is preferably a passive shield. For example, the vacuum chamber may be formed of a corresponding material. Due to such a shield, an influence of the electrons in their propagation by external, in particular electromagnetic, fields can be reduced or prevented. For example, a so-called Mu metal can be used.

An electron can be detached and / or triggered at the nanotip due to a multi-photon excitation. In particular, an undesired detachment and / or release of electrons due to, for example, one-photon excitation in the region of the metallic structure and outside the nanotip is avoided. The light pulse, the first light beam and / or the further light beam may have an energy of less than 50 picojoules.

The metallic structure may be made of a polycrystalline material and / or a wire, in particular a gold wire. Preferably, the material of the metallic structure is heated in a predetermined time to dissolve grain boundaries. In particular, the material of the metallic structure is heated to a temperature of at least 500 ° Celsius or 800 ° Celsius. The material of the metallic structure may be maintained in a heated state for a predetermined period of time and / or cooled to ambient temperature in a predetermined period of time. The predetermined time can be at least 6 hours or exactly 8 hours.

Thus, the material of the metallic structure and / or the nano-tip can first be heated to a temperature of 800 ° Celsius within, for example, 8 hours. Subsequently, the heated material can be maintained at a temperature of 800 ° C for a further 8 hours. Subsequently, the material of the metallic structure and / or the nano-tip can be cooled to ambient temperature over a period of 8 hours. By this method, the material is relaxed. In particular, larger grain structures are formed. Preferably, the metallic structure and / or the nanotip is approximately einkristallin ausbildbar. In particular, the material of the metallic structure and / or the nano-tip is heated to dissolve grain boundaries in the area of the tapering metallic structure and / or the nano-tip, in particular in a region between the coupling element and the nano-tip.

According to a development, the tapered, in particular conical and / or conical, shape of the metallic structure and / or the nano-tip is produced by means of etching in an acid. In this case, the etching can be carried out electrochemically. The acid used can be a hydrochloric acid, in particular a 37% solution. In particular, to form the tapered metallic structure and / or the nanotip, a wire, preferably a gold wire, immersed in an opening of a platinum ring and / or at a predetermined depth is immersed in the acid. Here, the wire 20 microns to 100 microns, preferably 50 microns, are deeply immersed in the acid. The platinum ring can serve as a counter electrode for the wire. To perform the electrochemical etching process, a voltage can be applied to the wire and the platinum ring. In particular, the tapered metallic structure and / or the nano-tip is produced at a free end of a wire. Due to the immersion of the free end of the wire, the acid creeps up some distance on the wire due to capillary forces, with the acid breaking away from the metallic structure and / or the nano-tip after sufficient removal of material. Due to the material removal, the tapered, conical and / or conical shape of the metallic structure and / or the nano-tip can be produced.

Preferably, the coupling element is applied spaced from the nano-tip on the surface of the tapered metallic structure. In this case, the coupling element, in particular in the form of a grating coupler, by means of ion beam lithography and / or FIB (English: Focussed Ion Beam milling) can be applied.

Of particular advantage is a use of a method according to the invention and / or an electron source according to the invention in an electron microscope, in particular in an ultrafast electron microscope, in a point projection microscope, in a scanning tunneling microscope, a transmission electron microscope, in a raster photocurrent microscope and / or for time-resolved imaging of Currents and / or electromagnetic fields in materials.

The invention will be explained in more detail with reference to the figures. Show it:

1 a schematic side view of an electron source according to the invention, and

2 a schematic flow diagram for a method according to the invention.

1 shows a schematic side view of an electron source according to the invention 10 , The electron source 10 has a cathode 11 and an anode 12 on. The cathode 11 and the anode 12 are with a voltage source 13 connected. The cathode 11 and the anode 12 are spaced apart, due to the applied voltage source 13 between the cathode 11 and the anode 12 a not shown here in detail electric field can be generated.

The cathode 11 has a tapered metallic structure 14 on. The tapered metallic structure 14 has a conical shape with a nano tip 15 , The nano tip 15 has a nanometer-sized volume and is the anode 12 facing. In this embodiment, the nano-tip has 15 an opening angle of 30 °. Furthermore, the nano tip has 15 a tip radius of 12 nm. Between the cathode 11 and the anode 12 can be arranged a sample not shown here.

Spaced from the nano tip 15 has the tapered metallic structure 14 a coupling element 16 on. In this embodiment, the coupling element 16 about 50 microns from the nanotip 15 spaced. The coupling element 16 is formed as a grating coupler and on a surface 21 the tapered metallic structure 14 applied. In this embodiment, the coupling element shown schematically 16 four grid lines 17 on. For better clarity, not all grid lines are 17 provided with a reference numeral. Alternatively, you can also use more gridlines 17 , in particular seven grid lines 17 , be provided. The coupling element 16 serves to excite surface plasmons on the surface 21 the tapered metallic structure 14 due to one on the coupling element 16 irradiated first light beam 19 and / or further light beam 25 ,

The electron source 10 has a first laser system 18 and another laser system 26 , In this embodiment, the first laser system 18 a first light source 27 and the other laser system 26 another light source 28 , In an alternative embodiment, the first laser system 18 and the other laser system 26 have a common light source. The first laser system 18 has a first optical path 29 for the first ray of light 19 and the other laser system 26 another optical path 30 for the further light beam 25 on. The optical paths 29 . 30 are separated from each other and are completely independent of each other in this embodiment.

The light sources 18 . 27 as well as the optical paths 29 . 30 are arranged or formed such that in this embodiment, the first light beam 19 and the further light beam 25 on the coupling element 16 is aligned. An axis of light rays 19 . 25 in this embodiment is perpendicular with respect to a central axis 20 the metallic structure 14 aligned. Here is the middle axis 20 schematically indicated as a dashed line. Furthermore, the light rays 19 . 25 in this embodiment obliquely to the surface 21 the metallic structure 14 or of the coupling element 16 aligned.

In an alternative embodiment, the first light beam 19 and / or the further light beam 25 instead of the coupling element 16 directly on the nano tip 15 be self-directed. While in this embodiment, the first light beam 19 and the further light beam 25 to two different, here by way of example remote from each other areas of the coupling element 16 , are directed, the two beams of light can 19 . 25 in an alternative embodiment, on the same location and / or same area of the metallic structure 14 be directed.

For focusing the first light beam 19 is a lens 22 between the light source 18 and the coupling element 16 arranged. The Lens 22 is in a beam path or in the first optical path 29 for the first ray of light 19 positioned. Furthermore, another lens 31 for focusing the further light beam 25 between the light source 28 and the coupling element 16 arranged. The Lens 31 is in a beam path or in the first optical path 30 for the further light beam 25 positioned. Instead of a single lens 22 or 31 a lens system or a reflection optics may be present.

In this embodiment, the metallic structure 14 inside a vacuum chamber 23 arranged. Also the anode 12 is inside the vacuum chamber 23 positioned. In a wall of the vacuum chamber 23 is a first window element 24 arranged. In another wall of the vacuum chamber 23 is another window element in this embodiment 32 arranged. The window elements 23 and 32 are here on two opposite sides of the vacuum chamber 23 , In an alternative embodiment, both window elements 24 . 32 next to each other and / or on the same side or the same wall of the vacuum chamber 23 be. In a further alternative, only a single window element may be present, with the optical paths 28 . 29 are arranged so that the first light beam 19 and the further light beam 25 through the same window element in the vacuum chamber 23 can reach.

Outside the vacuum chamber 23 are the light sources 18 . 28 , wherein the first light beam 19 through the window element 24 and the further light beam 25 through the further window element 32 into the interior of the vacuum chamber 23 and to the coupling element 16 can get. The lenses 22 . 31 are outside the vacuum chamber 23 arranged. Alternatively, the lenses can 22 . 31 inside the vacuum chamber 23 be. In another alternative, the light source 18 and or 28 inside the vacuum chamber 23 be positioned, in which case the window element 24 respectively. 32 is dispensable.

In the area of the anode 12 is a detector system 33 arranged. The detector system 33 is designed to detect electrons. For this purpose, the detector system 33 a detection device 34 on. The detection device 34 is for determining a kinetic energy of the detector system 33 formed meeting electrons. In addition, electrons can be detected by means of the detector system 33 and / or the detection device 34 in terms of their spatial location. In this embodiment, the detection device is designed as a delay line detector. Furthermore, the detector system 33 a microchannel plate 35 on. The microchannel plate 35 is between the nano tip 15 and the detection device 34 arranged. Here is the microchannel plate 35 the anode 12 assigned. Usually, at least two microchannel plates 35 to be available.

2 shows a schematic flow diagram for a method according to the invention for generating an electron pulse with an electron source 10 according to 1 , After starting according to step S10, a voltage is applied by means of the voltage source in accordance with step S11 13 to the cathode 11 and the anode 12 created. Due to the applied voltage, an electric field between the metallic structure 14 , in particular the nano tip 15 , and the anode 12 educated.

According to step S12, the metallic structure becomes 14 with a first ray of light 19 irradiated. Here, the first light beam 19 optionally either on the coupling element 16 or the nano-tip 15 be focused. Due to the irradiation of the metallic structure 14 with the first light beam 19 become electrons in the nano-tip 15 is transferred to an image charge state according to step S13. The time duration or the full half width of the first light beam 19 may be less than 10 femtoseconds. Due to the optical excitation of the nanotip 15 by means of the first light beam 19 For example, the electrons are emitted to the image charge state as an atom-like surface state. In this case, a polarization of the charge distribution in the interior of the nano-tip 15 produced, which leads to an attractive interaction with the exiting electron. After the completion of the irradiation with the first light beam 19 The electrons transferred to the image charge state remain in the image charge state for a certain period of time before the electrons would fall back to their original state. In particular, the electrons transferred into the image charge state remain after the completion of the irradiation with the first light beam 19 for a period of up to 100 femtoseconds in the image charge state.

Then and in time subsequent to the irradiation with the first light beam 19 becomes the metallic structure 14 in step S14 with the further light beam 25 irradiated. The further light beam 25 may alternatively on the coupling element 16 or the nano-tip 15 be focused. The further light beam 25 becomes in this embodiment in a time range of up to 100 femtoseconds after the end of the irradiation by means of the first light beam 19 on the metallic structure 14 issued. In particular, the irradiation with the further light beam 25 within the period of 100 femtoseconds after the end of irradiation with the first light beam 19 completed. The duration or the full width at half maximum of the further light beam 25 may be less than 10 femtoseconds.

Due to the further light beam 25 For example, the electrons in the image charge state are released from the image charge state in accordance with step S15. As a result, the desired electron beam and / or electron pulse is emitted in accordance with step S16. Then, the process ends in step S17.

The first ray of light 19 and the further light beam 25 can be generated as light pulses with a duration and / or full width at half maximum in the range of 10 femtoseconds. By means of the method according to the invention, electron pulses having a short duration and / or full width at half maximum, in particular significantly less than 100 femtoseconds or less than 10 femtoseconds, can be generated. In particular, a time duration and / or full half-life in the attosecond range can be realized for a single electron pulse.

The method according to steps S10 to S17 can be repeated as often as desired. As a result, a sequence of electron pulses can be generated. This is due to the method according to the invention damage and / or change the nano tip 15 preventable. Due to the applied voltage, electrons are transferred to the nanotip 15 resupplied. It can be a sequence of laser pulses for the first light beam 19 and the other light beam 25 be generated at a user selectable refresh rate. The maximum possible repetition frequency can be limited by heating the tip.

LIST OF REFERENCE NUMBERS

10

electron source

11

cathode

12

anode

13

voltage source

14

Metallic structure

15

Nano tip

16

coupling element

17

grid line

18

first laser system

19

first light beam

20

central axis

21

surface

22

lens

23

vacuum chamber

24

window element

25

another light beam

26

another laser system

27

first light source

28

another light source

29

first optical path

30

another optical path

31

another lens

32

another window element

33

detector system

34

detector

35

Microchannel plate

S10

begin

S11

Apply voltage

S12

Irradiate metallic structure with first light beam

S13

Transfer of electrons into an image charge state

S14

Irradiate metallic structure with additional light beam

S15

Triggering electrons from the image charge state

S16

Delivering an electron beam

S17

The End

Claims (15)

Method for generating an electron beam and / or an electron pulse, in which a tapered metallic structure ( 14 ) with a nano tip ( 15 ) is used to deliver electrons, the metallic structure ( 14 ) as a cathode ( 11 ) is used, a voltage to the metallic structure ( 14 ) and with a first light beam ( 19 ) the metallic structure ( 14 ) is irradiated, characterized in that the metallic structure ( 14 ) with another light beam ( 25 ) is irradiated, wherein due to the irradiation of the metallic structure ( 14 ) with the first light beam ( 19 ) and the further light beam ( 25 ) the electron beam and / or the electron pulse is generated. A method according to claim 1, characterized in that the electron beam and / or the electron pulse due to a spatial and / or temporal superposition of the first light beam ( 19 ) and the further light beam ( 25 ), preferably the metallic structure ( 14 ) after the first light beam ( 19 ), in particular within less than 100 fs after the end of the irradiation with the first light beam ( 19 ), with the further light beam ( 25 ) is irradiated, preferably when irradiating the metallic structure ( 14 ) with only the first light beam ( 19 ) or exclusively the further light beam ( 25 ) does not cause an electron beam and / or electron pulse. Method according to claim 1 or 2, characterized in that the first light beam ( 19 ) from a first wavelength range and the further light beam ( 25 ) is selected from a different wavelength range from the first wavelength range, preferably the first light beam ( 19 ), in particular as a first light pulse, from a first laser system ( 18 ) and / or the further light beam ( 25 ), in particular as a further light pulse, from a further laser system ( 26 ), in particular give the first laser system ( 18 ) and the further laser system ( 26 ) each have a light beam ( 19 . 25 ) and / or light pulse in different wavelength ranges. Method according to one of the preceding claims, characterized in that the first light beam ( 19 ) has a, in particular average, wavelength in the visible range, in particular in the range of 250 nm to 750 nm, preferably has the further light beam ( 25 ) one, in particular average, wavelength in the infrared or near-infrared region, in particular in the range of 750 nm to 1,000 nm, preferably in the range of 750 nm to 1,400 nm or up to 3,000 nm. Method according to one of the preceding claims, characterized in that electrons in the metallic structure ( 14 ), especially in the nanosphere ( 15 ), by means of the first light beam ( 19 ) are transferred to an intermediate state above the Fermi energy, preferably into an image charge state, in particular the electrons are converted by means of the further light beam (FIG. 25 ) is triggered from the intermediate state and / or the image charge state for generating the electron beam and / or the electron pulse. Method according to one of the preceding claims, characterized in that by means of further light beam ( 25 ) a strong field photoemission is caused, in particular by means of the further light beam ( 25 ) an electric field in the region of the metallic structure ( 14 ), due to the tapered shape of the metallic structure ( 14 ) and / or the nano-tip ( 15 ) results in a field increase, preferably by means of the irradiation of the further light beam ( 25 ) causes above-threshold ionization. Method according to one of the preceding claims, characterized in that as the first light beam ( 19 ) and / or as another light beam ( 15 ) a light pulse having a duration and / or full half-life of less than 100 fs, less than 50 fs or less than 20 fs is used, more preferably the time duration and / or the full half-life is in the range of 15 fs, 10 fs or less in particular, due to a sequence of first light beams ( 19 ) and other light beams ( 25 Preferably, a single electron beam and / or electron pulse has a time duration and / or full half-life of less than 100 fs, in particular less than 50 fs, particularly preferably less than 10 fs. Method according to one of the preceding claims, characterized in that the nano-tip ( 15 ) and / or one of the nano-tip ( 15 ) spaced coupling element ( 16 ) of the metallic structure ( 14 ) with the first light beam ( 19 ) and / or the further light beam ( 25 ) is irradiated, preferably the first light beam ( 19 ) and / or the further light beam ( 25 ) on the metallic structure ( 14 ), the coupling element ( 16 ) and / or the nano-tip ( 15 ) focused. Method according to one of the preceding claims, characterized in that the first light beam ( 19 ) and / or the further light beam ( 25 ) is focussed to a focus diameter substantially corresponding to the dimensions of the metallic structure ( 14 ), a coupling element ( 16 ) and / or the nano-tip ( 15 ), preferably the focusing and / or adjustment of the first light beam ( 19 ) and / or the further light beam ( 25 ) with respect to a central axis ( 20 ) of the metallic structure ( 14 ). Electron source for generating an electron beam and / or electron pulse with a cathode ( 11 ) having a tapered metallic structure ( 14 ) and a nano tip ( 15 ) for emitting electrons, with a first laser system ( 18 ) for irradiating the metallic structure ( 14 ) with a first light beam ( 19 ), characterized by a further laser system ( 26 ) for irradiating the metallic structure ( 14 ) with another light beam ( 25 ), due to the formation of the first laser system ( 18 ) and the further laser system ( 26 ) for irradiating the metallic structure ( 14 ) the electron beam and / or the electron pulse can be generated. Electron source according to claim 10, characterized in that the metallic structure ( 14 ) and / or the nano-tip ( 15 ) is cone-shaped and / or cone-shaped, in particular the metallic structure ( 14 ) is formed from gold, silver, tungsten or aluminum, preferably the metallic structure ( 14 ) and / or the nano-tip ( 15 ) an opening angle in the range of 10 ° to 40 °, in particular in the range of 20 ° to 30 °. An electron source according to claim 10 or 11, characterized in that the first laser system ( 18 ) and / or the further laser system ( 25 ) a femtosecond laser as a light source ( 27 . 28 ), preferably the metallic structure ( 14 ) spaced from the nano-tip ( 15 ) a coupling element ( 16 ) for exciting at least one plasmone by means of the first light beam ( 19 ) and / or the further light beam ( 25 ), in particular the coupling element ( 16 ) is formed as a grating coupler, preferably the distance of the coupling element ( 16 ) from the nanosphere ( 15 ) greater than a focus diameter of the first light beam ( 19 ) and / or the further light beam ( 25 ) for irradiating the coupling element ( 16 ). Electron source according to one of Claims 10 to 12, characterized in that the metallic structure ( 14 ) within a vacuum chamber ( 23 ), preferably a light source ( 27 . 28 ), in particular a laser, for irradiating the metallic structure ( 14 ), the nanosphere ( 15 ) and / or a coupling element ( 16 ) outside the vacuum chamber ( 23 ), wherein the first light beam ( 19 ) and / or the further light beam ( 25 ) of the metallic structure ( 14 ), the nanosphere ( 15 ) and / or the coupling element ( 16 ) through a window element ( 24 . 32 ) of the vacuum chamber ( 23 ) can be fed. Electron source according to one of claims 10 to 12, characterized in that a detector system ( 33 ) for detecting electrons a detection device ( 34 ) for determining a kinetic energy of the electrons, preferably the detection device ( 34 ) a delay line detector, in particular the detection device ( 34 ) and / or the delay line detector in combination with at least one microchannel plate ( 35 ) arranged. Use of a method according to one of claims 1 to 9 and / or an electron source ( 10 ) according to one of claims 10 to 14 in an electron microscope, in an ultrafast electron microscope, in a point projection microscope, in a scanning tunneling microscope, in a scanning photocurrent microscope and / or for time-resolved mapping of currents and / or electromagnetic fields in materials.

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