WO2010091754A1 - Photoconductive measurement tip, measurement setup, and use of the photoconductive measurement tip and/or the measurement setup - Google Patents

Abstract

The invention relates to a photoconductive measurement tip for field detection of the electrical component of an electromagnetic field, having an arrangement of at least one carrier and a first electrode and a second electrode insulated from each other and disposed in lateral extension on the carrier, connected to each other by means of a photoconductive material in the area of a photoconductive gap at a distal end, wherein, at least in a front area of the arrangement comprising the front end of the electrodes, a lateral bounding structure of the first and second electrode is not adapted to the wavelength in design relative to a wavelength of the electrical component of the electromagnetic field to be detected, and tapers toward the distal end, wherein a dimension of the taper is continuous and the arrangement has a lateral dimension of less than 20 µm at the distal end in the area of the photoconductive gap. The invention further relates to a near-field measurement setup. The invention further relates to a use of the near-field measurement tip and/or the near-field measurement setup.

Description

 Photoconductive measuring tip, measuring setup and use of the photoconductive measuring tip and / or the measuring setup

The invention relates to a photoconductive measuring tip for, in particular for direct temporal and spatial high-resolution field detection of the electrical component of an electromagnetic field, in particular for near-field detection and / or far-field detection with an arrangement of at least one carrier and a lateral Erstre- ckung on the Carrier disposed, mutually insulated first electrode and second electrode, which are connected to each other via a photoconductive material in the region of a photoconductive gap at a distal end. Furthermore, the invention relates to a measurement setup. The invention also relates to a use of the measuring tip and / or the measuring structure.

THz radiation is the subject of diverse research and application. A generation and / or detection of high-power THz radiation in the material analysis is possible, for example, with arrangements of the type described in DE 10 2006 059 573 B3; but usually with photoconductive measuring tips, as in WO 2004/023566 A1. A A comparatively unusual arrangement of three measuring tips of WO 2006/072762 A1 serves for polarization-dependent detection of THz radiation.

Particularly for imaging techniques, the application of THz radiation has received increasing attention as a promising method for detecting spatially resolved information of different material properties in the far-infrared range. However, diffraction effects limit the spatial resolution of imaging processes, especially in the THz range, namely, mainly due to the optical components of the structure such as lenses or parabolic mirrors. Usually, a local resolution limit is above a few hundred micrometers. In order to achieve field detection of the electrical component of an electromagnetic field below, for example, 500 μm spatial resolution, complex near-field optical microscopes (SNOM) have been used. Thus, SNOM approaches to a local resolution below the wavelength of the electrical component to be detected could be achieved. Both static and dynamic apertures were used for this purpose. However, the sensitivity and bandwidth of the aperture-based approaches is severely limited because the amplitude of the transmitted fields decreases with the cube of the aperture diameter.

For example, in an alternative SNOM method, THz radiation may be incident on a metallic wire tip in the sub-wavelength diameter region which is placed near the sample surface. Scattered light from the metal tip can provide information about the sample in the sub-wavelength local resolution range and can be collected in the far field (s-SNOM). In this configuration, a spatial resolution in the range of 150 nm at THz frequencies has been reported. However, the spatial resolution of a THz-s-SNOM structure critically depends on the radius of the stylus tip and the distance between the tip and the sample surface. Unfortunately, the scattered near-field THz signal is also relatively small and underlain by a relatively large background contribution of scattered THz radiation resulting from reflected THz radiation of the sample surface. In addition, the time resolution is significantly reduced.

A very efficient way to improve the signal-to-noise ratio in this type of s-SNOM configuration is to scan the near-field electric field directly or at least in close proximity to the probe tip via an electro-optically active crystal plate. Although the bad time resolution is not reduced, but is the full THz excitation bandwidth available for electro-optical sampling. In this case, the spatial resolution is diffraction limited to the size of the optical probe beam diameter of usually 10 μm. Such results have been provided, inter alia, by NA Seo et. al. in Opt. Express 15, 1 1781-11789 (2007).

A promising SNOM alternative to near field scanning is based on miniaturized photoconductive probes (PC-SNOM). Photoconductive probes are based on low temperature gained galium arsenide (LT-GaAs) as switches with insulated electrodes. Such switches have been used to form a contact lens with a dielectric substrate to detect electrical signals on coplanar striplines. In direct contact mode and with a time resolution of 2.3 ps, a voltage sensitivity of 4 μV / (Hz) ^{1/2 could be} achieved. In order to reduce the invasiveness of such photoconductive switches with measuring tip, it is advantageous to avoid large-volume dielectric substrates and instead to use ultra-thin photoconductive (photoconductive PC) films (thickness about 1 μm) on LT-GaAs as self-standing substrates reported, for example, by RK Lai et al. in Applied. Phyics. Letters. 69, 1843-1845 (1996). Although the use of isolated PC switches offers high field sensitivity, the spatial and temporal resolution is limited due to the high RC time constant and the spatially quite large field coupling of the photoconductive switch (PC switch) in a non-contacted mode. By using a dipole antenna with a narrow integrated PC gap, RC constants can be reduced - for example as described in US 2003/0184328 A1 - but the spatial field coupling is still defined by the λ / 2 length of the dipole arms. Thus, spatial and temporal resolutions in the range of 100 microns and a few picoseconds were reported, which, however, were hardly reproducible.

It has been shown that the approach of a direct temporal and spatial high-resolution field detection by means of near-field photoconductive measuring probes has good potential due to the - with wavelength-resonant design as a dipole - high field sensitivity for the large-scale applications mentioned above. However, it is desirable to achieve improved spatial resolution and broadband.

At this point, the invention begins, whose object is to provide a photoconductive measuring tip and a test setup and an associated use, by means of - A -

a high spatial resolution and broadband detection with good time resolution can be achieved, yet a high field sensitivity should be given.

The object is achieved by the invention by means of a photoconductive measuring tip of the type mentioned above, wherein according to the invention it is provided that at least in a front region of the arrangement comprising the front end of the electrodes a lateral limiting structure of the first and second electrodes with respect to a wavelength of the electrical component of the electro-magnetic field to be detected is designed to be non-wavelength-adapted and decreases towards the distal end, a measure of the reduction being continuous and the arrangement having a lateral dimension at the distal end in the region of the photoconductive gap below 20 μm.

According to the concept of the invention, it is provided that the lateral confinement structure of the first and second electrodes is non-wavelength-adjusted to a wavelength of the electrical component of the electromagnetic field to be detected. In other words, the limiting structure is free of a wavelength-resonant structure. For this purpose, the invention provides in principle, this to the distal end towards constantly downsize interpreted, with the extent of reduction, d. H. the decrease itself, runs steadily. In particular, the distal end in the region of the photoconductive gap should have a lateral dimension below 20 μm.

The invention has recognized that, with regard to broadband, it is primarily the problem of learning that the approach via dipole antennas leads to deliberately giving up the broadband in order to enable high field sensitivity.

The invention is based on the consideration that a photoconductive measuring tip-when the dipole shape is dispensed, or when a wavelength-adapted lateral limiting structure of the arrangement is used-is capable of particularly broadband for high-temporal field detection of the electrical component of an electromagnetic field To be available. The invention has recognized that a high sensitivity of the photoconductive measuring tip can be achieved in spite of abandoning resonant structures, that with a particularly small lateral dimension, ie transverse dimension, of the photoconductive measuring tip at the distal end not only a spatial resolution well below the wavelength to be detected the electrical component of the electromagnetic field is achieved, but over the at the distal end with a small transverse dimension caused electromagnetic "peak effect" a significant Field elevation is achieved. Surprisingly, it was found that this more than compensates for the loss of measurement sensitivity by abandoning the resonant structure. As a result, not only an improved measuring sensitivity is achieved with significantly improved spatial resolution, but in the beginning already a high bandwidth and thus frequency-wide use of the photoconductive near field or far field measuring tip.

In particular, an improved PC-SNOM near-field measuring principle can be realized using the photoconductive measuring tip with a freely positionable photoconductive near-field measuring tip which is capable of high-resolution field detection with high sensitivity directly in time and space due to the peak effect at the distal end of the measuring tip to enable. Due to the substantial avoidance of wing-like electrode boundary structures or other resonant or wavelength-matched antenna boundary structures can be a high broadband with high sensitivity z. B. achieve with spatial submillimeter resolution and / or temporal subpicosecond resolution.

The invention also leads to a near field and / or far field measurement setup, in particular

PC-SNOM measurement setup. This is preferably used for high-resolution field detection of the electrical component of an electromagnetic field. Preferably, the measuring tip is attached to a body, in particular vitreous body holder.

Furthermore, the invention leads to the use of the photoconductive measuring tip for direct temporally and spatially high-resolution field detection of the electrical component of an electromagnetic field in imaging methods. In particular, a use in the field of medical technology or material testing has proven to be advantageous.

Advantageous developments of the invention can be taken from the subclaims, from which further advantages and additions to the concept of the invention result.

Preferably, at least in the front region of the arrangement, the profile of the lateral limiting structure is determined by an electrode spacing between the first and second electrodes and / or an electrode dimension of the first or second electrode. Under development of the concept of the invention, a course of the electrode spacing and / or the electrode dimension is non-wavelength-adjusted, ie free from a wavelength-resonant structure. An electrode spacing and / or an electrode dimension have proven to be relatively critical to the realization of the inventive concept, and preferred electrode spacing and / or electrode dimension designs have been found to be particularly effective.

Particularly advantageously, an electrode spacing between the first and second electrodes and / or an electrode dimension of the first and / or second electrode can decrease towards the distal end. In particular, an electrode spacing and / or an electrode dimension in the region of the photoconductive gap can advantageously be less than 20 μm, preferably less than 5 μm.

In a further development of the concept of the invention, it has proven particularly advantageous for a decrease in the electrode spacing and / or a decrease in an electrode dimension to run continuously towards the distal end. As a result, it is achieved in a particularly advantageous manner that a measure of the reduction of the delimiting structure of the first and second electrodes is continuous. Advantageously, for example-in the mathematical sense-a first derivative of a profile of the lateral limiting structure, in particular an electrode spacing and / or an electrode dimension, may be continuous. For example, a decrease in the electrode spacing and / or a decrease in the electrode dimension may be linear. It may also be advantageous for a decrease in the electrode spacing and / or the electrode dimension to be curved, in particular conical or convex. As a result, an application-adapted response behavior of the photoconductive measuring tip can advantageously be achieved with regard to its sensitivity, taking into account the concept of the invention.

Put simply, this type of refinement provides that the first and second electrodes converge towards one another in the region of the photoconductive gap, and advantageously the electrodes themselves are pointed. A measure of the reduction of the boundary structure is continuous. By eliminating resonant antenna elements, spatial resolution, bandwidth, sensitivity and non-invasiveness are significantly improved. As a result, applications are developed in a variety of forms that could not be achieved with previous, known from the prior art resonant, relatively low spatial resolution and high invasiveness. This includes in particular the quality control and development of integrated ultra-high-frequency circuits or high-resolution THz imaging in the field of medical technology, bioanalytics, safety technology, eg As for spatially resolved spectroscopy for the detection of hazardous substances or the like. In addition, the present concept can be used in basic research, especially in the field of novel photonic materials and structures.

The measuring tip has proven to be particularly advantageous for detecting the electrical component of an electromagnetic field in a wavelength range between 10 .mu.m and 10 mm. In addition, taking into account the concept of the invention, it is also possible to tap into wavelength ranges between 20 μm and 1 mm without problems. According to the inventive design of the photoconductive measuring tip, the limiting structure is designed to be non-wavelength-adapted in the wavelength ranges mentioned.

In principle, different constructive developments of the near field and / or far field measuring tip can be realized. A measuring tip is advantageous in which the lateral boundary structure at the distal end, in particular in the region of the photoconductive gap, is comparatively small. This has considerable advantages for achieving the electromagnetic "peak effect" since, according to the concept of the invention, it contributes significantly to increasing the sensitivity of the photoconductive measuring tip according to the invention of 20 .mu.m, in particular 10 .mu.m, preferably below 3 .mu.m Application-specific also allows a lower limit of the lateral limiting structure to be realized above 10 nm, preferably above 100 nm.

To improve the broadband, the distal ends of the electrodes can advantageously be rounded. Preferably, diameters between 1 .mu.m and 10 .mu.m, in particular in the range below 3 microns are provided. On the one hand, such dimensions ensure the realization of the electromagnetic "peak effect" and, on the other hand, take account of the broadband design of the near-field measuring tip according to the concept of the invention.

Preferably, an opening angle of a clearance space of the restriction structure is provided between the electrodes, which is in a range between 20 ° and 70 °. Advantageously, an opening angle of a distance space to the limiting structure of an electrode is provided which is between 5 ° and 25 °. The further design of the opening angle advantageously ensures that the lateral boundary structure of the electrodes is realized as far as possible from a dipole structure. Under development of the concept of the invention, it has been recognized that a dipole structure largely combines wavelength-resonant properties and, in its simplest form, comprises parallel, coplanar or wing-like electrode structures. As a result of the further design of the opening angle of the distance space or dimension space, a lateral delimiting structure of the first and / or second electrode can be achieved, which is comparatively dissimilar to a dipole structure. In the context of particularly preferred developments, an opening angle of a distance space, preferably between 30 ° and 60 °, in particular between 40 ° and 50 ° can be realized. An opening angle of a dimensioning space can be realized in a particularly advantageous manner between 10 ° and 15 °.

Advantageously, the length dimension can be interpreted under development of the inventive concept for photoconductive probe. Preferably, the front portion has a length dimension of less than 2 mm to the distal end. As a result, in particular the broadband design of the photoconductive measuring tip can be supported. Preferably, the length dimension of the front region to the distal end is designed below 1 mm, in particular below 500 μm. Depending on the application, it is also possible to realize length dimensions of the front region below 300 μm, in particular below 100 μm. In addition, with development of the inventive concept, a rear region of the arrangement of the photoconductive near-field measuring tip which is closer to the proximal end of the near field measuring tip can also be advantageously designed. In a particularly preferred manner, a boundary structure of the rear area in the geometric sense is designed similarly to a delimiting structure of the front area. So z. B. in the front and rear areas of the electrodes converge. As a result, geometric transitions, which can cause wavelength-resonant structures, advantageously avoided and thus support the broadband design of Nahfeldmessspitze. If necessary, however, limiting structures can also be realized in principle in the rear area, which are formed unlike a boundary structure of the front area. So can, for. B. in the front region, the electrodes converge and run parallel in the rear area. Preferably, the rear region has a length dimension of not more than 10 mm, in particular below 2 mm, preferably below 1 mm. The proximal end preferably has a transverse dimension below 10 mm, preferably below 3 mm, in particular below 2 mm, preferably below 1 mm. By limiting the length dimension of the rear region and / or the transverse dimension of the proximal end of the near field measuring tip, a broadband design of the photoconductive near field measuring tip is advantageously further improved while developing the concept of the invention.

The near-field measuring tip can basically be realized with different materials familiar to the person skilled in the art. A photoconductive material for the photoconductive gap in the form of an LT-GaAs-based material has proved to be particularly advantageous. LT-GaAs is characterized by a particularly short relaxation time of excited electrons and can thus provide a particularly broadband frequency sensitivity. In particular, a chromium or gold-based material has proved to be advantageous for the electrode material.

With regard to the measurement setup, it has been shown that this can be realized both as a far field or near field measurement setup, preferably in the form of a photoconductive SNOM measurement setup. The photoconductive Nahfeldmessspitze can particularly advantageous to a body, for. As glass body, and be implemented with the body in the measurement setup. For reasons of accessibility, the photoconductive measuring tip can advantageously be inclined to the front plane of the radiation. Of course, other body can be used for mounting the photoconductive probe tip or other ways to implement the photoconductive probe used in the test setup. The body serves in particular as a carrier of electrical services, which are provided for electrical connection of the electrodes. In addition, a glass body has proved to be advantageous in order to make the photoconductive measuring tip available for a freely propagating excitation radiation and / or for the radiation to be detected. It has been found that the measuring structure with development of the concept of the invention particularly advantageously has optical components such that they are provided for a freely propagating excitation radiation and / or radiation to be detected. The measuring structure and / or the measuring tip can be used in particular in the context of imaging processes, preferably in the field of medical technology or in material testing.

Embodiments of the invention will now be described below with reference to the drawing. This is not necessarily to scale the embodiments, but the drawing where appropriate for explanation, executed in a schematic and / or slightly distorted form. With regard to additions to the teachings directly recognizable from the drawing reference is made to the relevant prior art. It should be noted that various modifications and changes may be made in the form and detail of an embodiment without departing from the general idea of the invention. The disclosed in the description, in the drawing and in the claims features of the invention may be essential both individually and in any combination for the development of the invention. In addition, all combinations of at least two of the features disclosed in the description, the drawing and / or the claims fall within the scope of the invention. The general idea of the invention is not limited to the exact form or detail of the preferred embodiment shown and described below or limited to an article which would be limited in comparison to the subject matter claimed in the claims. For the given design ranges, values within the stated limits should also be disclosed as limit values and be arbitrarily usable and claimable.

In detail, the drawing shows in

1 shows a schematic representation of a first embodiment of a photoconductive measuring tip according to the concept of the invention in a comparatively simple embodiment;

FIG. 2 shows a schematic illustration of a second embodiment of a photoconductive measuring tip in a further development of the first embodiment; FIG.

3 shows a schematic representation of a third embodiment of a photoconductive measuring tip in the form of a variant, in view (a) as a plan view and in view (b) as a side view; 4 shows a schematic representation of a fourth embodiment of a photoconductive measuring tip according to the concept of the invention as a modification of the first embodiment;

5 shows a schematic illustration of a fifth embodiment of a photoconductive measuring tip according to the concept of the invention as a modification of the third embodiment, in view (a) as a top view and in view (b) as a side view;

Fig. 6 in view (a) is a detailed schematic representation of a photoconductive

Measuring tip according to the second embodiment with dimension and material designation, in view (b) an enlarged view of the front portion of the arrangement of the photoconductive probe tip and in view (c) an SEM image of the previously shown photoconductive probe tip in a freestanding realization on a Sapphire substrate formed form;

7 is a schematic representation of a measurement setup for field detection, in FIG

View (a) a far field setup with optical components for excitation, transmission of excitation radiation and detection, the latter comprising the photoconductive far field measuring tip according to the concept of the invention, according to the second embodiment, and in view (b) a spatial detail of the optical Near-field detection component comprising the near-field photoconductive probe according to the second embodiment;

Fig. 8 exemplary far field and near field measurement results, which were achieved with the measurement setups shown in Fig. 6 and Fig. 7 and the photoconductive measuring tip shown there - on the left side of FIG. 8 corresponding

Waveforms in the time domain as detected in the far field and in the near field - the two traces show an incident THz wave with polarization in both the X and Y directions, with the arrows evenly spaced at 4.5 ps (picoseconds) are justified. The right illustrations show the Fourier-transformed signal of the time domain for the polarization in the X direction and Y direction, respectively; 9 in view (a) is a schematic representation of a single element of a frequency-selective surface, which was used for experimental characterization of the previously explained photoconductive probe tip with dimensions in microns and in view (b) a spatially high-resolution pictorial representation of the individual element of the frequency-selective

Surface using a THz near field image. The image consists of 50 x 50 pixels with 3 x 3 μm ² pixel size and the arrow represents the polarization direction of the transmitted electric field. In views (c) and (d), the THz field amplitude is shown over a 10 μm wide metallic structure of view (b) along the X direction and Y direction - the shaded one

Area indicates the areas of the underlying metallic structure.

FIGS. 1 to 5 show, in the following, five particularly preferred embodiments of photoconductive measuring tips in a schematic representation which are suitable for near-field and far-field detection. FIGS. 6 to 9 furthermore show a concrete realization based on the second embodiment with exemplary THz measurement results in the far field and near field of a photoconductive measuring tip. All of the measuring tips shown in the following are particularly suitable for direct field and spatially high-resolution field detection of the electrical component of an electromagnetic field both in the far field and in the near field. It has been found that an implementation and use of a photoconductive measuring tip according to the concept of the invention and in particular according to the embodiments described herein is suitable, on an industrial scale, for. For example, for imaging methods, without problems advantageously possible - such methods with imaging function using in particular THz radiation have proven to be particularly useful in the field of medical technology or material testing. In principle, the photoconductive measuring tips shown below are suitable not only for the detection of electromagnetic radiation in the THz range, but in principle for the detection of electromagnetic radiation in the infrared or far infrared wavelength range between 10 .mu.m and 10 mm, in particular with comparatively high sensitivity in the wavelength range between 20 μm and 1 mm. The illustrated near-field photoconductive probes according to the concept of the invention are provided, at least in these wavelength ranges, with a boundary structure for the electrodes which is non-wavelength-matched, ie a lateral confinement structure substantially free of a wavelength-resonant structure. 1 shows a photoconductive near-field measuring tip 100 according to a first embodiment with an arrangement 140, at least one support 110 and lateral extension on the support 110, insulated from each other first electrode 120 and second electrode 130. In the present case, the support 110 is complete formed in the form of LT-GaAs. Freestanding self-supporting LT-GaAs has proven to be advantageous in the present case, but does not generally have to be designed in such a way. For example, the support may also be an AIA with deposited LT GaAs. Since the carrier 110 in the present case is formed entirely from LT-GaAs, the electrodes 120, 130 are thus at their distal end 150 - there in particular in the region of a photoconductive gap 235 exemplified in Fig. 6 - via photoconductive material in the form of LT GaAs interconnected. In other words, the electrodes 120, 130 are electrically insulated from each other via the photoconductive gap at the distal end 150, whereby photooptically excited charge carriers can be generated in the photoconductive material, which then cause measurable electric fields via the electrodes. In the present case, LT-GaAs has proved to be particularly suitable as a photoconductive material, since the charge carrier lifetime in LT-GaAs is comparatively short, and thus LT-GaAs enables frequency-wide field detection.

The electrodes 120, 130 have a limiting structure 170, which is formed by the course of the inner, outer, upper, lower boundary of the first and second electrodes 120, 130. In the present case, the course of the boundary is such that the lateral delimiting structure 170 decreases overall toward the distal end 150, specifically in such a way that a measure of the reduction runs steadily. In mathematical terms, the progression of the boundary change is continuous and results in a reduced dimension of the delimiting structure 170 on the distal end 150 and moreover a first derivative of a measure of the profile of the boundary of the lateral delimiting structure 170 towards the distal end is continuous. According to the concept of the invention, this is ensured by the fact that a measure of the reduction, that is, a reduction course decreasing the delimiting structure 170 itself, is continuous. In the present specific case of the photoconductive near-field measuring tip 100, in particular the electrode spacing 180 between the first electrode 120 and the second electrode as well as an electrode dimension 181, 182 decreases toward the distal end 50. Thus, if one were to plot an electrode spacing 180 or an electrode dimension 181, 182 as a function of path X from proximal end 190 of near field photoconductive probe 100 to distal end 150, the decrease in electron gap 180 and decrease in electrode dimension 181, 182 will be continuous also the first derivative of the same. This is according to In the first embodiment of the near-field photoconductive probe 100, the decrease in the electrode spacing 180 and the decrease in the electrode dimension 182 are chosen to be linear.

In contrast to comparatively narrowband and resonant or wavelength-adapted designed dipole electrode structures - whose boundary structure regularly parallel boundary curves - provides, is in the present design of the boundary structure of the electrodes to a non-wavelength-matched structure with respect to a relatively wide range of wavelengths electrical component of the electromagnetic field to be detected. According to the concept of the invention, the limiting structure of the electrodes selected in the first embodiment is adapted for a wavelength range of between 10 μm and 10 mm, which is to be detected in a broadband manner, in particular in the wavelength range between 20 μm and 1 mm. With this background, it is an embodiment that can detect not only THz radiation but also a wide far-infrared wavelength range.

The loss of sensitivity according to the concept of the invention by expressly dispensing with a wavelength-resonant design is over-compensated according to the concept of the invention in that a lateral dimension of the arrangement 140 below the 20 μm is present at the distal end 150 in the region of the photoconductive gap Fig. 6 z. B. a photoconductive gap below 5μm at 1.8 microns. Depending on requirements, a lateral dimension can also be selected clearly below 10 .mu.m, preferably below 3 .mu.m, as is explained, for example, in the specific design example of FIGS. 6 to 9. As a result, according to the concept of the invention, due to the electromagnetic peak effect, a field elevation in the region of the distal end 150 is achieved, which not only provides superior sensitivity to resonant electrode arrangements, in particular dipole arrangements, but also significantly increases spatial resolution.

The first embodiment described here ensures the specifications according to the concept of the invention, in particular in a front region 160 comprising the distal end 150 of the electrodes 120, 130. This ensures that, in particular in a front region 160 of the photoconductive measuring tip 100 relevant to the coupling of an electromagnetic field, wavelength-matched or resonant generating structures are avoided - so ensures the broadband and high sensitivity of the photoconductive probe 100 for direct temporal and spatial high-resolution field detection in both the near field and far field.

FIG. 2 shows a second embodiment of a photoconductive measuring tip 200 which, like the photoconductive measuring tip 100, has an arrangement 240 of a carrier 210 with electrodes 220, 230 which are in the region via a photoconductive material - in the present case LT-GaAs of the carrier 210 a photoconductive gap at a distal end 250 are interconnected. Again, in accordance with the concept of the invention, the lateral confinement structure of the first and second electrodes 220, 230 is designed to be non-wavelength-matched and downslope toward the distal end 250 with a progressive reduction in size, with a lateral dimension below 20 at the distal end 250 of the photoconductive gap μm is located. In a further development of the first embodiment of the photoconductive near field measuring tip 100, in the second embodiment of the photoconductive near field measuring tip 200, a field shield 261 is additionally applied to the electrodes 220, 230. This has advantages if the detection is to be carried out in strongly field-scattering areas. In addition, carrier generation can be effectively limited to the area of the photoconductive gap 235 (Figure 6). In the present case, the shielding 261 is formed in the form of a metal shield that continuously reduces in size, similar to the electrodes 220, 230, and covers it almost up to the distal end 250. In order to enable a freely propagating charge carrier excitation in the region of the photoconductive gap, the arrangement 240 at the distal end 250 is not covered by the shield 261.

This illustrated in Fig. 2 preferred second embodiment of a photoconductive Nahfeldmessspitze 200 is described in more detail in Fig. 6 in detail in the context of a structural design of a probe tip 200. There, it is also shown that the disadvantages of a low spatial resolution and low bandwidth previously known from resonant dipole structures of photoconductive measuring tips are deliberately avoided by measuring tips of the present type. Both the measuring tip 100 and the measuring tip 200 avoid the use of large-area photoconductive switches with a single contact line and resonant antenna elements. Rather, two adjacent pointed tapered planar electrodes 120, 130 and 220, 230 are provided in both embodiments according to the concept of the invention, which are applied in the context of the arrangement 140, 240 on a thin self-supporting photoconductive substrate - in the present case LT-GaAs - and their Electrode distance as well as their electrode dimension is to Tip down, with a measure of the reduction itself being steady. In other words, there are also kinks or the like. At least avoided in the front region of the arrangement. As a result, a comparatively high field elevation in the region of the photoconductive gap at the distal end 150, 250 is achieved, which makes it possible to limit the field detection to the tip region bounded by the photoconductive gap. The generation of the field enhancement in the present case is largely frequency-independent and the RC constant is comparatively small due to the extremely small switch area, so that a significantly higher bandwidth than with the previous solutions can be achieved. Thus, a particularly high degree of spatial resolution, bandwidth, sensitivity and non-invasiveness is achieved.

FIG. 3 shows a third embodiment of a photoconductive measuring tip 300, whose arrangement 340 is formed with carrier 310 and electrodes 320, 330 similar to the measuring tip 100 of the first embodiment, so that with respect to the construction of the arrangement 340 to the corresponding description of FIG 1 is referred to. In the present case, the arrangement 340 of the photoconductive measuring tip 300 is connected to a body which forms a further arrangement 341. The arrangement 341 has a further carrier 311 as well as a first electrical line 321 arranged thereon and a second electrical line 331. For this purpose, the further carrier 311 of the further arrangement 341 is connected to the carrier 310 of the arrangement 340, in the present case formed of transparent material in the form of a quartz glass. Likewise, the first electrical lead 321 is connected to the first electrode 320 via an electrical contact 322, and the second electrical lead 331 is electrically connected to the second electrode 330 via a second electrical contact 332. In other words, as can be seen from view (b) of FIG. 3, the arrangement 340 of the photoconductive near-field measuring tip 300 is applied to the further arrangement 341 such that a front region 360 of the arrangement 340 encompassing the distal end 350 of the electrodes 320, 330 beyond the further arrangement 341 protrudes. In this way, a freely propagating excitation beam can generate charge carriers in the photoconductive gap at the distal end 350 of the electrodes. In addition, the support 311 of the further arrangement can in principle be formed from any sufficiently mechanically stable material. Nevertheless, as described above, a transparent support 311, in the present case made of quartz glass, has proven to be advantageous, since it may be desirable in applications to view areas below the photoconductive near field probe 300, for example via a CCD camera or similar optical apparatus or opti - see adjustment. In addition, a non-wavelength-adapted design of the lateral boundary structure of the first electrical line 331 and second electrical line 321 is also ensured in the body of the further arrangement 341-analogous to the design of the delimiting structure of the first and second electrodes 120, 130, as exemplified by all Embodiments have been described with reference to the first embodiment of the photoconductive Nahfeldmessspitze 100 in Fig. 1. In the present case, analogous to the limiting structure 370 of the arrangement 340 in the delimiting structure 370 'of the arrangement 341, it is provided that the line spacing 380 between the first and second electrical lines 321, 331 and the line dimensions 371, 372 of the first and second electrical lines 321, 331 decreases distal end 350 and decreases the decrease of the line spacing and the decrease of the line dimension 380, 371, 372 to the distal end 350 towards steadily.

4 shows a fourth embodiment of a photoconductive measuring tip 400, in turn comprising an arrangement 440 of a carrier 410, on which a first electrode 420 and a second electrode 430 are attached, and their lateral limiting structure - only in the front end comprising the distal end 450 Area 460 - a lateral boundary structure 470 of the first and second electrodes 420, 430, which is designed non-wavelength-adapted and substantially the Begrenzungsstruck- tures 170, 270, 370 of the embodiments of FIGS. 1 to 3 corresponds. As a result, it is also ensured in the fourth embodiment of the measuring tip 400 that in a significant area for the detection area - namely in the front area 460 - a broadband and highly sensitive detection of electromagnetic radiation in the wavelength range between 10 .mu.m and 10 mm, at least in the wavelength range between 20 μm and 1 mm is possible. In the context of the present embodiment of the measuring tip 400, it has been recognized that a substantially coplanar design of the electrodes 420, 430 is possible in a rear region 461 of the measuring tip 400. For this purpose, the electrodes 420, 430 are coplanar in the rear region 461 and substantially pointed towards one another in the front region 460, ie they are angled toward one another at the transition between the rear region 461 and the front region 460. Of course, in a modification of the rear region in other embodiments, alternative electrode profiles can be realized.

FIG. 5 shows a development of the measuring tip shown in FIG. 4 in the form of the photoconductive measuring tip 500. Analogous to the third embodiment of a photoconductive probe

Measuring tip 300, as described in FIG. 3, is presently the same In principle, the arrangement 540 of the measuring tip 500 described in FIG. 4 is attached to a body of a further arrangement 541. For this purpose-similarly as described in FIG. 3-the carrier 510 of the arrangement 540 is applied to the carrier 511 of the further arrangement 541 such that it projects beyond a front region 560. This ensures that the arrangement 540 of the measuring tip 500 in the region of the photoconductive gap can be excited by a freely propagating optical field. The first and second electrodes 520, 530 are electrically connected via respective contacts 522, 532 to a first and second electrical line 521, 531, the latter being mounted on the carrier 511 of the further device 541. The delimiting structure 570 'of the first and second electric lines 521, 531 is configured non-wavelength-matched, as described in connection with the third embodiment of the measuring tip in FIG. 3, ie, the limiting structure 570' decreases towards the distal end, wherein one dimension the reduction is continuous. In particular, line spacings 580 and line dimensions 571, 572 decrease towards the distal end 550, the decrease being continuous.

Fig. 6 shows in view a the photoconductive Nahfeldmessspitze 200 of the second embodiment as shown in Fig. 2 - for the same parts or parts of the same function have been used in the same reference numerals. In addition, in view (a), a dimension for a concrete implementation of the near-field photoconductive probe 200 is shown. It can be seen that the assembly 240 has a substantially triangular shape and is tapered from the proximal end 290 to the distal end 250. In particular, a non-wavelength-limited confinement structure 270 of the first and second electrodes 220, 230 decreases towards the distal end 250, a measure of the reduction being continuous. In the present case, the arrangement 240 has a lateral dimension of 1161 μm at the proximal end 290-ie at the base of the triangle. The substantially isosceles lengths of the electrodes 220, 230 correspond substantially to the side length of the assembly 240, which in the present case is 1152 μm. The shield 260 in this case has a side length of 437 microns. It can also be seen from view (a) of FIG. 6 that an opening angle 291 of a spacing space of the delimiting structure 270 between the electrodes 220, 230 is present at 40 °. In principle, such an opening angle 291 can also be between 20 ° and 70 °. According to the concept of the invention, it has been recognized that such an opening angle range is as far as possible from a wavelength-resonant dipole structure - in the present case, an opening angle of 40 ° has proven to be particularly optimal. Likewise, in the present case, an opening angle 292 of a dimensional space of the boundary structure 270 of each electrode 220 or 230 at 10 °. The opening angle 292 of the spacing space of the delimiting structure in each case one electrode 220, 230 results here from the further opening angle of 60 °, which denotes the spacing space of the delimiting structure 270 at the outer edges of the electrodes 220, 230 and in the present case at 60 °.

It is to be understood that, even while maintaining the present geometric shape and proportion, regardless of the present embodiment as shown in Fig. 6 (a), photoconductive probe tips which are particularly smaller in size than those selected herein can be realized Have dimension.

In Fig. 6 (b), a fragmentary enlarged view of the distal end 250 of the assembly 240 is shown. It can thus be seen that the distance 280 between the first and second electrodes 220, 230 and the dimensions 281, 282 of the same continuously decrease linearly as far as the distal end 250. At the "tip" of the array 240, the first electrode 220 is connected to the second electrode 230 in the region of the photoconductive gap 235 via a photoconductive material, in the present case LT GaAs The spacing 280 between the electrodes 220, 230 increases symmetrically up to one Dimension of the photoconductive gap 235 of 1.8 μm In order to limit the excitation of photoconductive carriers in the photoconductive gap 235 to a sufficient extent, the carrier 210 is initially provided with an optically transparent electrically insulating region in the area of the distal end 250 Polymer layer 215. The shield 261 is mounted on the polymer layer 215 in the form of a metallic masking layer which also partially covers the electrodes 220, 230. The shield 261 leaves the region of the photoconductive gap 235 at the distal end 250 free to excite electric charge carriers, it has been found that in a simpler version currency form - as has already been described with reference to FIG 1 in the first embodiment of the photoconductive measuring tip. 100 - 261, the shield may be omitted. It has been found that a significant charge for the detection of charge carriers is also limited to the region of the photoconductive gap 235 even without shielding 261 and, if necessary, with a corresponding adjustment of the excitation fields.

In FIG. 6 (c), an SEM image of the above-explained photoconductive measuring tip 200 is shown in the present case, corresponding reference numerals being explained above

Designate elements of the arrangement 240. The photoconductive measuring tip 200 is present ing produced as epitaxially grown layer system. On a 1, 3 .mu.m thick layer of LT GaAs of the carrier 210. The pair of "pointed" lines which form the electrodes 220, 230 is made of a layer structure of a 10 nm chromium layer and a 200 nm gold layer on the LT GaAs. In addition, the arrangement is coated with a 1 μm thick insulating and optically transparent polymer film and thermally cured.The polymer film is produced using a photo-resist layer as a masking layer and an isotropic dry etching process in an O ₂ plasma. Next, as part of the fabrication, the step of 50 nm thick chromium masking, which limits the optical excitation - but not necessarily in other embodiments - to the photoconductive gap 235 at the distal end 250 of the probe tip 200. The masking is deposited and lithographically Next, those portions of the LT-GaAs carrier 210 which are not mi t are covered by the polymer film, here by using a wet chemical etching process which gives access to an aluminum arsenide layer. Thereafter, the underlying AlAs sacrificial layer is etched away completely with a 10% HF solution, resulting in a self-supporting freestanding LT-GaAs based chip-type device 240 of the present embodiment. Finally, the measuring tip 200 is applied to a body in the form of a 250 μm thick sapphire substrate, in the present case electrically connected by a bonding process to corresponding lines on the sapphire substrate. The attachment takes place in such a way that the measuring tip 200 protrudes beyond the sapphire substrate by a few 100 μm, as can be seen in the SEM image of FIG. 6 (c).

Photoconductive probe tip 200 is experimentally characterized by a far-field THz and THz near-field as shown in FIGS. 8 and 9. The characterization is carried out in the context of a far-field measurement setup and near-field measurement setup, as shown in FIG.

7 are shown. For the setup, 150 fs pulses are used at 76 MHz repetition rate and an optical wavelength of λ = 800 nm for excitation and detection of the broadband THz pulses. In order to minimize the distance between the photoconductive probe tip 200 and the chromium surface, the photoconductive sharpened probe tip is tilted at an angle of about α = 20 ° with respect to the X-Y plane of the incident THz axis.

We'll shoot for all experiments.

FIG. 7 shows in view (a) the measurement setup 700A with the described measurement tip 200 mounted on the above-described sapphire carrier 800. THz radiation from a surface emitter 900 is transmitted to the photopolymer via two parabolic mirrors 710, 720. tende gap 235 of the measuring tip 200 adjusted. The excitation of the surface emitter 900 is effected via an optical excitation pulse 910. The detection of the THz radiation 920 takes place by means of an optical detection pulse 930 of the previously described type. The THz radiation 920 can furthermore be polarized by a polarizer 730, in the present case in the form of a wire grid polarizer become.

In view (b) of FIG. 7, a near field measuring structure 700B with the photoconductive measuring tip 200 on the sapphire body as carrier 800 is shown enlarged, from which also the previously explained inclination angle α with respect to the XY plane of the incident THz wave and, if necessary . With appropriate adjustment with respect to the sample 740, also shown, can be seen.

Schematically, the excitation side of the measurement setup 700B with a dipole excitation element 990 is also shown in dashed outline.

Fig. 7 (a) shows a measurement setup 700A used for far-field detection. Fig. 7 (b) shows a measurement setup 700B used for near-field detection.

First, the far field setup 700A was used to determine the time response of the probe tip 200 for planar wave excitation. In this construction 700A, linearly polarized THz radiation in pulse form from an InAs surface emitter polymer is focused via a pair of parabolic mirrors 710, 720 onto the photoconductive probe tip 200 with a focus for THz radiation which is not substantially larger than the photoconductive one Gap 235. To illustrate the time-domain behavior of the waveforms, these are shown in the upper left portion of FIG. 8 for the incident THz radiation, each for polarization in the X and Y directions. For both polarizations, the signal is characterized by several peaks at 25 ps, which become smaller with increasing time. Using numerical field simulations, the peak multiple reflections of the THz signal in the probe tip can be assigned. The THz radiation is conducted within the overlapping regions of the shield 260 and the pointed electrodes 220, 230. This two-electrode waveguide section has a calculated average effective dielectric e _{r eff} = 2.5 at the THz frequencies present. Taking into account a doubly transmitted transmission length of 430 μm, the calculated effective dielectricity corresponds exactly to the measured time interval of Δt = 4.5 ps of the multiple peaks. The half-width of the main peak at t = 0 ps corresponds to 0.46 ps and 0.51 ps for the X and Y Polarization, which corresponds to a usable spectral range of about 0.1 to 2.5 THz. The Fourier-transformed signals in the time domain are shown in the upper right-hand part of FIG. Both frequency spectra show an amplitude drop for frequencies below 0.3 THz, which is typical of free propagating transmission configurations, which include a high-divergent point source and optical focusing on a sub-wavelength-sized detector. As expected from the electrode arrangement, the signal amplitude for incident THz fields which are polarized in the Y direction is reduced compared to X polarized beams. The sensitivity difference between X and Y polarization is 4: 1.

In order to determine the sub-wavelength resolution of the measuring tip 200, the near-field measuring structure shown in Fig. 7 (b) was used. For practical reasons, the THz pulses were generated with a photoconductive dipole element which is illuminated by a 7 mW optical excitation beam through a dispersion compensated optical fiber. Of course, other excitation sources may also be used, for example, the surface emitter 900 of the far field structure of Fig. 7 (a). The THz beam propagates in the Z direction through a sample which is positioned at a fixed distance A = 1.0 mm above the THz emitter, which is a λ / 2 wavelength distance of a freely propagating THz wave at a frequency of f = 0.15 THz equivalent. The transmitted signals are detected by the photoconductive measuring tip, which is arranged approximately 3 μm above the sample surface.

First, the transmitted THz signals are measured in the time domain at the surface of a reference sample consisting of a polysilicon silicon (Si) wafer polished on both sides of 1 mm thick and more than 3000 Ωcm in resistivity. The sample is transparent in the corresponding THz frequency range but opaque to wavelengths of optical excitation and the detection beam. An additional 10.5 μm thick layer of black wax (Apiezon) is spin-deposited on top of the silicon wafer. This layer absorbs scattered light of the detection beam, which could otherwise be reflected from the back of the measuring tip 200. The measured data in the time domain are in the lower left part of FIG. 8 for X and Y polarization of the THz beams, respectively shown. The most significant differences compared to the far-field data are the pulse spread at the main peak with a half-life of 1.2 ps for X-polarized and 0.9 ps for Y-polarized THz radiation and an additional portion of a DC signal. Due to the distance between the near field emitter and the detector, the frequency below 0.15 THz a significant low-frequency coupling. Therefore, one finds in the frequency domain, the maximum of the signal amplitude at the frequency 0, as shown in the previously transformed data in the lower right part of Fig. 8. A moderate lowering of the high-frequency spectral components can also be observed, which is due to the absence of focusing optical elements. Another particular observation in this near-field configuration is that signal reflections from the reference sample surfaces are detected in addition to the in-peak reflections, again shown by arrows in the lower left portion of FIG. As with the far field measurements, a significant reduction in the probe sensitivity for the Y polarization is observed.

The spatial resolution that can be achieved with the photoconductive probe tip of other THz frequencies is determined using a metallic test pattern with sub-wavelength dimensions and shown in FIG. The test structure is a frequency selective surface consisting of an array of asymmetrically double slotted metal ring resonators with subwavelength dimensions. Such a metal ring resonator with corresponding radius and Schlitzbemaßung is shown in Fig. 9, view (a). Such structures are of particular interest for so-called biosensitive applications in the THz range, since they show strong resonance shifts when loaded with dielectrically active samples. The frequency-selective surface is profiled by means of a 10 nm chromium and 200 nm gold structure on the same wafer as the reference sample. The entire frequency-selective surface is located under an optically opaque layer of black wax.

The image caused by the THz image is achieved by raster scanning the sample with the distal end 250 of the photoconductive probe tip 200. For this purpose, scanning steps of 3 μm step size in the X and Y direction are carried out, namely at a time interval of 0 ps in the time domain, which corresponds to the main amplitude of the THz radiation. The obtained image is shown in Fig. 9 in view (b). The spatially resolved electric field clearly shows the optically hidden ring resonator structure. The 10 μm wide metallic stripe lines and the asymmetrical slot arrangement of the frequency-selective surface are clearly resolved. No further signal contributions are detected in a reference measurement with a blocked THz beam. The spatial resolution can be determined by line measurements as shown in view (c) of FIG. 9. Therein, the THz field amplitude along the two bright lines shown in Fig. 9b is determined. Using the "10% - 90%" criterion becomes a achieved spatial resolution of up to 5 microns - corresponding to a proportion of λ / 300 - along the X direction and a spatial resolution of 7 microns - corresponding to a proportion of λ / 140 - determined along the Y direction; this for a wavelength at 1500 microns.

In summary, in the present case, an attractive alternative to the SNOM concepts explained at the outset has been made possible on the basis of the concept of the invention, with a sub-wavelength resolution in spatial imaging. Moreover, the concept of the invention allows high temporal and spatial resolution with minimal invasiveness in combination with the transmitted THz signals. A spatial resolution below 5 microns was demonstrated at a distance of the photoconductive probe tip and a metallic test structure of about 13 microns. In the context of the invention, further developments and improvements, in particular for increasing the spatial resolution of the measuring tip, by reducing the distance between the measuring tip and sample and reducing the measuring tip itself and in particular the photoconductive gap 235. The size of the photoconductive gap certainly by more Instrumentation of suitable manufacturing tools in the field of nanotechnology can be achieved. The reduction of the measuring tip 200 itself can be achieved by simple optical masking methods. For example, the rear side of the measuring tip 200 can also be masked in order to avoid the influence of optically reflected radiation from the sample surface. Such and other improvements or developments are within the scope of the concept of the invention.

Claims

Patent claims A photoconductive measuring tip (100, 200, 300, 400, 500) for field detection of the electrical component of an electromagnetic field, comprising an arrangement (140, 240, 340, 440, 540) of at least one carrier (110, 210, 310, 410, 510) and in a lateral extent on the support (1 10, 210, 310, 410, 510), insulated from each other first electrode (120, 220, 320, 420, 520) and second electrode (130, 230, 330, 430, 530), which via a photoconductive material in the region of a photoconductive gap (235 ) at a distal end (150, 250, 350, 450, 550) are interconnected, characterized in that at least in a front region (160, 260, 360, 460, 560) of the arrangement (140, 240, 340, 440, 540) comprising the distal end of the electrodes, a lateral limiting structure (170, 270, 370, 470, 570) of the first and second electrode, with respect to a wavelength of the electric Component of the detected electro-magnetic field is designed non-wavelength-matched, and downsized to the distal end (150, 250, 350, 450, 550), wherein a degree of reduction is continuous, and - the assembly (140, 240, 340, 440, 540) has a lateral dimension at the distal end (150, 250, 350, 450, 550) in the region of the photoconductive gap (235) has below 20 μm. 2. Measuring tip according to claim 1, characterized in that at least in the front region (160, 260, 360, 460, 560) of the arrangement (140, 240, 340, 440, 540) an electrode spacing (180, 280, 380, 480 . 580) between the first and second electrode and / or an electrode dimension (181, 281, 381, 481, 581, 182, 282, 382, 482, 582) of the first and / or second electrode has a profile of the lateral limiting structure (170, 270 , 370, 470, 570). 3. Measuring tip according to claim 1 or 2, characterized in that an electrode spacing (180, 280, 380, 480, 580) between the first and second electrodes and / or an electrode dimension (181, 281, 381, 481, 581, 182, 282, 382, 482, 582) of the first and / or second electrode to the distal end (150, 250, 350, 450, 550) decreases. 4. Measuring tip according to one of claims 1 to 3, characterized in that a decrease in the electrode spacing (180, 280, 380, 480, 580) and / or a decrease in an electrode dimension (181, 281, 381, 481, 581, 182, 282, 382, 482, 582) is continuous towards the distal end (150, 250, 350, 450, 550). 5. Measuring tip according to one of claims 1 to 4, characterized in that the wavelength range to be detected is between 10 .mu.m and 10 mm, in particular in the wavelength range between 20 .mu.m and 1 mm, and at least in this wavelength range, the limiting structure (170, 270 , 370, 470, 570) is non-wavelength-matched. 6. Measuring tip according to one of claims 1 to 5, characterized in that a decrease in the electrode spacing (180, 280, 380, 480, 580) and / or a decrease in the electrode dimension (181, 281, 381, 481, 581, 182, 282, 382, 482, 582) is linear. 7. Measuring tip according to one of claims 1 to 6, characterized in that a decrease in the electrode spacing (180, 280, 380, 480, 580) and / or a decrease in the electrode dimension (181, 281, 381, 481, 581, 182, 282, 382, 482, 582) is curved, in particular conical or convex. 8. Measuring tip according to one of claims 1 to 7, characterized in that the lateral limiting structure (170, 270, 370, 470, 570) at the distal end, in particular in the region of the photoconductive gap (235), has a lateral dimension below 10 μm, preferably below 3 μm, in particular above 10 nm, preferably above 100 nm, preferably an electrode spacing (180, 280, 380, 480, 580) and / or an electrode dimension (181, 281, 381, 481, 581, 182 , 282, 382, 482, 582) is below 20 μm in the region of the photoconductive gap. 9. Measuring tip according to one of claims 1 to 8, characterized in that the distal ends of the electrodes are rounded, preferably with diameters in the range between 1 .mu.m and 10 .mu.m, in particular in the region below 3 microns. 10. Measuring tip according to one of claims 1 to 9, characterized in that an opening angle of a distance space of the limiting structure between the electrodes between 20 ° and 70 °. 1 1. Measuring tip according to one of claims 1 to 10, characterized in that an opening angle of a dimensioning space of the limiting structure of an electrode between 5 ° and 25 °. 12. Measuring tip according to one of claims 1 to 1 1, characterized in that the front region (160, 260, 360, 460.560) has a lateral length dimension to the distal end below 2 mm, preferably below 1 mm, in particular below of 500 microns, preferably below 300 microns, in particular below 100 microns. 13. Measuring tip according to one of claims 1 to 12, characterized in that the arrangement has a rear, the proximal end (190, 290, 390, 490, 590) closer lying area, whose boundary structure is formed geometrically similar or dissimilar to the boundary structure of the front region. 14. Measuring tip according to claim 13, characterized in that the electrodes converge towards one another in the front (160, 260, 360, 460, 560) and rear regions. 15. Measuring tip according to claim 13, characterized in that the electrodes converge in the front region (160, 260, 360, 460, 560) and run parallel in the rear region. 16. Measuring tip according to one of claims 1 to 15, characterized in that the rear region has a length dimension of not more than 10 mm, in particular below 2 mm, preferably below 1 mm. 17. Measuring tip according to one of claims 1 to 6, characterized in that the proximal end (190, 290, 390, 490, 590) has a lateral transverse dimension below 10 mm, preferably below 3 mm, in particular below 2 mm, preferably below 1 mm. 18. Measuring tip according to one of claims 1 to 17, characterized in that the photoconductive material is an LT-GaAs-based material. 19. Measuring tip according to one of claims 1 to 18, characterized in that the electrode material is a Cr or Au-based material. 20. Measuring tip according to one of claims 1 to 19, which is attached to a body (31 1, 51 1), in particular on a glass body. 21. Measuring tip according to claim 20, characterized in that the body (31 1, 511) - carries electrical lines (321, 331, 521, 531) which are connected to the electrodes (120, 220, 320, 420, 520, 130, 230, 330, 430, 530) are contacted and their lateral boundary structure with respect to a wavelength of the electrical component of the electromagnetic field to be detected is designed non-wavelength-matched and - Reduced to the distal end, with a measure of the reduction is continuous, in particular with the electrical lines converge. 22. Measuring tip according to claim 20 or 21, characterized in that the arrangement (140, 240, 340, 440, 540) comprises a shield on the carrier, which leaves the photoconductive gap. 23. Measuring setup (700A, 700B) with an excitation source for an electromagnetic field and a detector for field detection of the electrical component of the electromagnetic field, characterized in that the detector is provided with a measuring tip (100, 200, 300, 400, 500), is formed according to one of claims 1 to 22. 24. Measurement setup according to claim 23 in the form of a PC-SNOM measurement setup. 25. Measurement setup according to claim 23 or 24 in the form of a far-field measurement setup (700A). 26. Measurement setup according to claim 23 or 24 in the form of a near-field measurement setup (700B). 27. Measurement setup according to one of claims 23 to 26 with the excitation source and the detector for a THz-FeId. 28. Measurement setup according to one of claims 23 to 27, characterized in that the measuring tip (100, 200, 300, 400, 500) is inclined to the plane of a sample and / or incidence front plane of an incident radiation. 29. Measuring structure according to one of claims 23 to 28, characterized in that optical components for a freely propagating excitation radiation and / or free propagating radiation to be detected are provided. 30. Use of the photoconductive measuring tip (100, 200, 300, 400, 500) according to any one of claims 1 to 22 and / or the measuring structure according to claim 23 to 29 for direct temporal and spatial high-resolution field detection of the electrical component of an electromagnetic field, in particular especially for different polarization directions. 31. Use according to claim 30, characterized in that the wavelength range to be detected is between 10 μm and 10 mm, in particular in the wavelength range between 20 μm and 1 mm, and at least in this wavelength range, the limiting structure is non-wavelength-matched. 32. Use according to claim 30 or 31 for imaging methods, in particular in the field of medical technology or materials testing.