WO2001011408A1

WO2001011408A1 - Device for focussing light onto an object

Info

Publication number: WO2001011408A1
Application number: PCT/EP2000/007672
Authority: WO
Inventors: Gerhard Leuchs; Susanne Quabis; Ralf Dorn; Oliver GLÖCKL; Manfred Eberler; Marc D. Levenson
Original assignee: Gerhard Leuchs; Susanne Quabis; Ralf Dorn; Gloeckl Oliver; Manfred Eberler; Levenson Marc D
Priority date: 1999-08-09
Filing date: 2000-08-08
Publication date: 2001-02-15
Also published as: EP1163548A1; ATA136499A

Abstract

The invention relates to a device for focussing light onto an object. Said device comprises at least one light source that is preferably formed by a laser. The inventive device also comprises at least one optical focussing element and a device for fixing the relative position of the focussing element and the object. The light which is sent through the focussing element is present in the form of a radially polarised ring mode.

Description

Device for focusing light on an object

The invention relates to a device for focusing light onto an object, with at least one light source preferably formed by a laser, with at least one optical focusing element and a device for determining the relative position of the focusing element and the object.

In a large number of optical applications, in particular in the field of microscopy, lithography and optical or magneto-optical data storage, it is desirable to be able to concentrate light on the smallest possible area. (The term "optical" refers to all types of electromagnetic radiation including visible light, infrared and ultraviolet radiation.)

In optical and magneto-optical data storage methods, the recording density is essentially limited by the size of the focal spot focused on the medium. As is well known, applies to the

Diameter of this focal spot approximately the relationship diameter = g * λ / NA, where λ denotes the wavelength of the light used and NA stands for the numerical aperture of the focusing optics, g has the value 1, 22 for homogeneous illumination of the entrance pupil of the focusing optics. To the

There are several ways to reduce the focal spot diameter:

The wavelength λ can be reduced by using new laser light sources.

Conventional CD read / write devices are operated at a wavelength of 780nm, with DVDs a wavelength of 635nm is used. The cost of laser diodes, the light with an even shorter wavelength, preferably in the blue

Spectral range, emit, prevent the use in devices for the

Mass market.

Another starting point is aimed at increasing the numerical aperture NA. This can be done in several ways:

If the object is illuminated through a small aperture, the extent of which is smaller than the illumination wavelength, the angular spectrum of the emerging light also contains waves with an imaginary wave vector. These are in the far field non-propagating (evanescent) parts negligible, since their amplitude decreases exponentially with increasing distance from the aperture. One finds approximately the field distribution of a dipole there. At a distance from the diaphragm that is smaller than the illumination wavelength, the proportion of evanescent waves in the total field cannot be neglected and leads to the size of the diaphragm essentially determining the region in which the light is concentrated. The applicability of this method for optical data storage has been demonstrated by E. Betzig, M. Isaacson and A. Lewis (Appl. Phys. Lett. 61, 142-144, 1992), among others. In another variant (see patent WO 98/18122), the light is guided to the tip of two crossed, tapering prisms which have an exit slit at the end, the width of which is smaller than half the wavelength.

Since the numerical aperture is defined as NA = n * sin (q) (q = half opening angle of the focusing lens), you can also increase the refractive index n of the medium into which you are focusing. This has been used for a long time in so-called immersion microscopes by adding high-index immersion oil between the objective and the examined object. This method cannot be used for optical and magneto-optical data memories in which the read / write head moves relative to the storage medium.

However, it is possible to focus the light in a highly refractive medium, so that the focus is at the interface between this and an optically thinner medium behind it. The advantage of a smaller focal spot size due to the high refractive power of the medium is lost again after the light enters the optically thinner medium. As with the method described above, which uses a sub-λ aperture as the light source, the light remains reduced to a smaller focal spot within a distance smaller than the light wavelength. Care must therefore be taken to ensure that the medium to be illuminated is within this range. If the geometry of the highly refractive medium is chosen in the form of a hemisphere and the flat side faces the medium to be illuminated, this so-called solid immersion lens (SIL) can be introduced into the beam path of a focusing lens without causing further aberrations (see also US -Patent 5,125,750). Finally, by taking advantage of nonlinear effects, the focal spot size that is effectively effective on a storage medium to be read out can be reduced. If, for example, the reflectivity of the medium is dependent on its temperature (phase transition between solid and liquid phase), the part of the medium that has already been read out and is therefore highly heated is in the molten (ie weakly reflecting) state. In the area of the focal spot, only the part that has not yet been read is able to reflect the light strongly ("1" bit) or weakly ("0" bit) (see Gijs Bouwhuis and JHM Spruit, Appl. Opt. Vol. 29 (1990) pp. 3766-3768;

Jpn. J. Appl. Phys. Vol. 32 (1993) pp. 5210-5213

The object of the present invention is to provide an apparatus by means of which light can be focused on objects described in more detail below, the focal spot size being reduced in comparison to conventional illumination systems with diffraction limits.

This is achieved according to the invention in that the light transmitted by the focusing element is in the form of a radially polarized ring mode. The basic idea is not primarily to shorten the wavelength of the focused light or to increase the numerical aperture of the illuminating optical system with the help of immersion or near-field techniques, but by modifying the field distribution to be focused with regard to its polarization and intensity distribution. A radially polarized ring mode is used which, after focusing, has a focal spot size that is half as large compared to a linearly polarized beam with a homogeneous intensity distribution. Depending on the illuminated object, there are various embodiments which are described in detail below. For applications in lithography, the object consists of a carrier material, preferably made of silicon, to which a light-sensitive layer is applied. For applications in the field of microscopy, a sample to be examined is illuminated and scanned point by point. In a further embodiment as a data storage system, the illuminated object consists of an optical or magneto-optical storage medium. For the purpose of repairing photomasks, material is evaporated or deposited at the location of the focal spot. In general, the present invention can be used wherever light is to be focused on the smallest possible area.

The invention is explained in more detail below with reference to the drawings.

Fig. 1: Intensity distribution of a radially polarized ring mode as a function of

Distance p from the axis of symmetry. Fig. 2: By superimposing two orthogonally polarized TEM ₀₁ - and TEM ₁₀ -

A radially polarized ring mode can be generated in modes. The arrows indicate that

Direction of polarization.

Fig. 3: A collimated radially polarized ring mode is created with the help of a lens

Focal length f focused. Fig. 4: Before focusing, part of the radially polarized ring mode is faded out by means of a ring diaphragm inserted into the beam path.

Fig. 5: Dependence of the focal spot area on the NA of the focusing element for homogeneous, linearly polarized field distribution and radially polarized ring mode, each with and without a ring diaphragm. Fig. 6: Schematic structure of an optical data storage system in which one

Light source is used, which provides a radially polarized ring mode.

Fig. 7: Schematic structure of a lithography system using a light source that provides a radially polarized ring mode.

Fig. 8: Schematic structure of a system suitable for repairing lithographic masks, in which a light source is used that uses a radially polarized one

Ring fashion provides.

Fig. 9: Schematic structure of a confocal microscope using a light source that provides a radially polarized ring mode.

Fig. 10 Schematic structure for generating a radially polarized ring mode. Fig. 11 Part b) shows the schematic structure of a

Polarization converter, which is made of four half-wave plates. The main axes are indicated by lines. Parts a) and c) show the polarization state of the light before (a) and after

(b) the passage through the polarization converter.

Fig. 12 Schematic structure of an optical system for illuminating a

Object with the help of an immersion lens, which further reduces the focal area of a radially polarized ring mode pre-focused by an optical focusing element.

Fig. 13 Schematic structure of an optical system for the evanscent

Illumination of an object with the help of a highly reflective coated immersion lens that acts as a resonator. Fig. 14 Schematic structure of a resonator, in which light at the base of a

Partially reflected prism and partially coupled to an object and associated lighting device.

Fig. 15 Schematic structure of a laser system in which the beam path runs in such a way that the light is partially reflected on the base surface of a prism and partially coupled out onto an object.

Fig. 16 Schematic structure of a resonator in which light is guided in a glass fiber and partially coupled out to an object at one point of the fiber and the associated lighting device.

Fig. 17 Schematic structure of a resonator, which is designed in the form of a two-dimensional waveguide structure with associated lighting device.

In this patent specification, a radially polarized ring mode is understood to mean an electromagnetic field distribution with the following characteristics:

• The electric and magnetic fields show radial symmetry with respect to an axis determined by the direction of propagation.

• The field strength on the axis of symmetry assumes a value that does not differ significantly from zero and has one or more intensity maxima at fixed distances R ₀ , R _{1 t} ... from the axis of symmetry (see Fig. 1).

• The electric field is locally linearly polarized in each case in a direction lying radially to the axis of symmetry.

• In any plane perpendicular to the axis of symmetry, the phase has the same value for all points that are at the same distance from the axis of symmetry. Such a field distribution can be generated, for example, by superimposing two orthogonally polarized TEM _0m and TEM _m0 modes (m odd). A particularly suitable mode results from the superposition of a TEM ₁₀ mode polarized in the x direction and a TEM ₀₁ mode polarized in the y direction (see FIG. 2).

Ring mode = polarized in the x direction polarized in the y direction

TEM ₁₀ mode + TEM ₀₁ mode

r

E ( ^χ , y) ex xe - (χ ² + y ² ) / w ² r 2. ~ 2 _w 2

+ e- • ye - (x ² + y ² ) /

^χ l _e - ( ^χ2 + y ² ) / ² ₌ J _e -p ² w ² ^E ring fashion ( ^x > y) ^∞ vyj

An elegant setup to create a radially polarized ring mode is described below (see also Fig. 10):

A light beam provided by a coherent, polarized (preferably consisting of a laser) light source 35 is first widened with the aid of a telescope consisting of the lenses 36 and 37. If the light source is disturbed by back-reflected light (as is typically the case with a diode laser), an optical isolator 38 can be introduced into the beam path after the light source. Higher transverse modes are selected out by a mode filter (pinhole) 39 attached between the lenses 36 and 37, so that the light after the telescope is in the form of a linearly polarized TEM ₀₀ mode.

The central element of the structure is the polarization converter 40, which is attached behind the telescope. It consists of at least three half-wave plates which are cut in the form of circular segments and which are joined to one another in such a way that a circular plate is formed again. In order to minimize disruptive effects caused by wedge angles between the individual segments, the polarization converter can be held in a transparent container with the Index matching oil is filled, whose refractive index is equal to the refractive index of the material from which the half-wave plates are made. The main axes of the half-wave plates are oriented such that the direction of polarization of the incident, linearly polarized beam within each segment is rotated in a direction that points radially away from the center of the incident light beam.

Fig. 11b shows a polarization converter consisting of four half-wave plates 40i, 40ii, 40iii and 40iv. In Fig. 11a and Fig. 11c the polarization of the beam is shown in front of 11a behind 11c the polarization converter. The field distribution thus created after the polarization converter consists of a mode mixture and contains, among other things, the desired TEM ₀ r and TEM ₁₀ mode in orthogonal polarization. In order to select out these two desired modes and to suppress all others, a Fabry-Perot resonator 41 is used as a spatial mode filter. The telescope and the radii of curvature of the mirrors that make up the Fabry-Perot resonator are matched to one another in such a way that there is a maximum overlap between the modes to be selected and the associated eigenmodes of the resonator. In order to actively stabilize the resonator, the mirror spacing can be varied periodically with the aid of a piezoelectric adjusting element. The light transmitted by the resonator is then also periodically modulated in intensity. Part of this light can be used to constantly adjust the mirror distance by means of a lock-in control so that the TEM ₁₀ and TEM ₀₁ modes are in resonance.

If such a radially polarized ring mode is focused with the aid of a lens or a lens system 1 (see FIG. 3), the focal spot also shows complete radial symmetry. In contrast, this symmetry is broken in the focal spot of a linearly polarized field distribution, since one direction is distinguished by the linear polarization. With a homogeneous intensity distribution and strong focus, the focal spot has the shape of a twisted ellipse (dog bones).

The term focal spot here means the energy density distribution of the electric field in the focal plane. With a focused, radially polarized Ring mode, the area of this focal spot (area occupied by the energy density distribution at half the maximum value) is only slightly smaller with very strong focusing (i.e. high numerical aperture of the focusing element) than in the case of a linearly polarized field distribution with an approximately constant intensity profile. However, the focal area can be further reduced if (as shown in Fig. 4) an annular diaphragm 2 is brought into the beam path. This suppresses the low-frequency components of the angular spectrum of the focused field distribution.

Fig. 5 shows the dependence of the focal spot area on the numerical aperture NA of the focusing element 1, in each case for linearly polarized, homogeneous field distribution and radially polarized ring mode. In the case of the curves marked with a triangle and cross, the fields each meet directly with the focusing element, with the other two curves an aperture is inserted which only allows an annular portion between 90% and 100% of the radius of the entrance aperture of the focusing element to pass. If the focus is strong (NA> 0.85) and a ring diaphragm is used, the radially polarized ring mode can be used to achieve focal spot areas that are significantly smaller than the focal spots that can be generated when illuminated with linearly polarized light. A solid immersion lens, which consists of a material with a high refractive index, can additionally be attached between the actual focusing element and the object. Since the wavelength in the material is reduced by a factor of n, the area of the focal spot is reduced by a factor of n ² . The hemispherically shaped immersion lens must be attached so that the curved surface faces the focusing element and the flat surface coincides with the focal plane of the system without an immersion lens. However, the illuminated object must then be at a distance from the flat surface of the immersion lens that is smaller than the illumination wavelength.

Fig. 6 shows the schematic structure of an optical data storage system in which a light source 3 is used which provides a radially polarized ring mode. The ring mode is then filtered by means of a ring diaphragm 2 and focused on a data carrier 4 with the aid of an optical focusing element 1. On Servo system 5 makes it possible to move one or more components of the focusing element and thereby to focus the focal spot exactly to a certain depth in the data carrier material or on the surface of the data carrier. The light source emits with high intensity for writing information. This burns holes in the surface of the data storage layer or changes other physical properties of the layer. In order to be able to write zero or one bits, the intensity of the light source is modulated either directly or by an external modulator 6. To read the information, the light source is set to a weaker intensity, which is not sufficient to change the information stored on the data storage layer. The light reflected from the data memory is directed via a beam splitter 7 to a detector 8 which is connected to a decoding unit. The data carrier is moved relative to the focal spot with the aid of adjusting elements and, in the simplest case, is designed in the form of a rotating disk. The amount of information that can be stored on the data carrier is determined by the size of the focal spot. Due to the radially polarized ring mode and when using a ring diaphragm 2, this spot size can be halved and the storage density can thereby be doubled.

Fig. 7 shows the schematic structure of a lithography system for point-by-point writing of structures, in which a light source 3 is used which provides a radially polarized ring mode. The ring mode is filtered by means of a ring diaphragm 2 and focused by a focusing element 1 onto a wafer 11 provided with a light-sensitive layer 10. The focusing element 1 can be displaced by means of a servo system 5 such that the focal spot lies in the plane of the light-sensitive layer 10. To write structures, the wafer is moved parallel to the focal plane by an actuating element 12 and exposed at certain points at the corresponding points. The light source can be varied either directly or with the aid of a modulator 6.

The system outlined in Fig. 8 is used to repair masks, which are mainly used in lithography. The mask 13 to be repaired, which usually consists of a glass plate to which a chromium structure is applied, is made using a microscope objective 14, tube lens 15 and eyepiece 16 Existing microscope examined for defects. The light source 17, the lenses 18 and 19 and the beam splitter 20 form the lighting system. In addition to the incident light arrangement outlined here, illumination with transmitted light arrangement or dark field illumination is also possible. A radially polarized ring mode, which is made available by the light source 3, is coupled into the beam path of the microscope via a second beam splitter 21. After filtering with the aid of a ring diaphragm 2, the microscope objective 14 focuses it on the mask 13. When the mask is inspected, the light source 3 operates at low intensity. The mask is shifted by means of an xyz shift element 22 until a defective spot comes to lie under the focal spot of the ring mode. The shutter 23 is then closed and the intensity of the light source 3 is increased to a suitable value. After opening the shutter, the ring mode evaporates excess material (e.g. chrome) at the location of the focal spot. Alternatively, a missing structure can also be deposited on the mask. For this purpose, the displacement unit and mask are located in a container 24 which is filled with a suitable process gas containing chromium. In order to protect the optical elements from deposits of the process gas, the mask is illuminated by a window 25. Due to the high intensity in the focal spot, the process gas is decomposed and chromium separates (chemical vapor deposition). The focal spot area of the focused ring mode, which is greatly reduced compared to conventional methods, allows a more precise and targeted repair.

Confocal microscopy is a procedure in which an object to be examined is illuminated and scanned point-by-point and which allows three-dimensional measurement for transparent objects (e.g. cells). The lateral spatial resolution is essentially limited by the smallest achievable cross-sectional area of the illumination beam focused in the medium. When illuminated with a focused, radially polarized ring mode, this cross-sectional area can be reduced.

Fig. 9 shows schematically the structure of a confocal miroscope, in which a light source 3 is used to illuminate the object, which provides a radially polarized ring mode. After filtering through a ring diaphragm 2, the ring mode is focused by means of a microscope objective 26. Object 27 rests an xyz displacement element 28 and can thus be positioned so that the focus comes to lie at any desired point within the object. The scattered light which arises in the area of the focus is directed onto a detector 31 by means of the microscope objective 26, the beam splitter 34, the tube lens 29 and the lens 30.

A special feature of the confocal microscope is the pinhole 32, which is attached between the tube lens 29 and the lens 30 and prevents light from an area outside the focus from reaching the detector. This is indicated in Fig. 9 by the rays drawn in dashed lines. For a measurement, one volume element of the object after the other is moved into focus by the control unit 33 and the backscattered intensity measured by the detector 31 is evaluated. In addition to the illumination in incident light shown in Fig. 9, illumination in a transmitted light arrangement is also possible.

Fig. 12 shows the schematic structure of an optical system with the aid of which a radially polarized ring mode provided by the light source 3 can be focused on an object 42. The object (as described in more detail elsewhere) can be an optical data storage medium, a photomask, a wafer coated with photosensitive material or a sample to be examined with a microscope.

After filtering with the aid of a ring diaphragm 2, the ring mode is focused using an optical focusing element 1. In addition, a solid immersion lens (so-called solid immersion lens) 43 is attached symmetrically to the optical axis 44 in such a way that its flat underside faces the object and, moreover, lies in the plane in which the focal spot of the lens is located even in the absence of the immersion lens Focusing element 1 is focused light.

In this case, all the rays emanating from the focusing element 1 hit the curved surface of the immersion lens 43 perpendicularly, so that no further deformation of the wavefront takes place. However, the wavelength of light within the immersion lens is reduced by a factor of n, so that the area of the focal spot changes reduced by a factor of n ² on the flat side of the immersion lens, n denotes the refractive index of the material from which the immersion lens is made. In order to take advantage of the reduced focal spot area, however, the illuminated object must be at a distance from the flat side of the immersion lens that is smaller than the illumination wavelength.

The larger the numerical aperture of the focusing element 1 is selected and the fewer rays near the axis are let through by the ring diaphragm 2, the greater the angle α which the flattest rays make to the optical axis. If the condition sin (α)> n ^{"1 is} fulfilled, all rays are totally reflected on the flat side of the immersion lens. At the location of the focal spot, a purely evanescent field then arises below the immersion lens, which field decreases exponentially with increasing distance from the immersion lens To illuminate the object with a reproducible, constant intensity, it must therefore be brought into a precisely controllable distance from the immersion lens, which generally requires an expensive positioning device 45. If the immersion lens is provided with a highly reflective coating 46 on its curved surface (see Fig 13), so that the intensity on the object is less dependent on the distance between the object and the immersion lens The coated curved surface of the hemisphere, together with the flat underside, on which total reflection takes place, forms an optical resonator in which a strong electromagnetic Field builds up when the light coupled into the curved surface meets the resonance condition. If there is no object in the vicinity of the underside of the immersion lens, the field strength or the number of photons stored in the resonator is only limited by the limited reflectivity of the mirrors and adjustment errors. The amplitude of the evanescent waves then also becomes maximum. If you approach an object to the flat underside of the immersion lens, part of the light reflected there is coupled out to the object via the evanescent field (frustrated total reflection). The closer the object comes to the interface, the more the amplitude of the evanescent field increases at the location of the object and the more light is coupled out onto the object. At the same time, the loss channel thus created also reduces the number of photons stored in the resonator and thus the amplitude of the evanescent waves, so that the intensity on the object drops again. The resonator thus acts like a control loop, which ensures that the intensity on the object is significantly less sensitive to the distance between the object and the immersion lens than in the case of the uncoated immersion lens.

Fig. 14 shows the schematic structure of a resonator, which consists of mirrors 51 and 52 and a prism 53. The geometry of the resonator and the refractive index n of the material from which the prism 53 is made are selected so that the light guided in the resonator experiences total reflection at the interface between the base of the prism and the environment (typically air). At the point of total reflection, evanescent (non-propagating) waves then arise, the amplitude of which decreases exponentially with increasing distance from the base surface of the prism. An object 54 which is approximated to the interface with the aid of a positioning element 55 must therefore be at a distance of the order of the wavelength or below the interface if it is to be illuminated with detectable intensity.

The illuminated area on the object is essentially determined by the beam diameter of the mode propagating in the resonator at the point of total reflection. In order to illuminate the object on the smallest possible area, the radius of curvature and the position of the mirrors is selected so that the beam waist of this resonator mode is at the point of total reflection. The presence of the medium 53 with the refractive index n additionally reduces the focal spot area (cross-sectional area of the beam waist) by the factor n ² (immersion effect).

The pump light provided by a light source 56, which preferably comprises a laser, is coupled into the resonator with the aid of a focusing device 57. For an effective coupling, the focusing element has to be adjusted in such a way that the overlap between the mode of the pump light and the excited natural mode of the resonator becomes as large as possible. If the resonator fulfills the resonance condition, either by detuning the wavelength of the pump light or by varying the resonator length by means of an adjusting element 57, which is attached to one of the both mirrors is attached can happen, a strong field is built up in the resonator. If there is no object in the vicinity of the prism base, the light circulating in the resonator experiences total total reflection there and the number of photons stored in the resonator (or the field strength) is only limited by the limited reflectivity of the mirrors and adjustment errors etc. The amplitude of the evanescent waves then also becomes maximum. If one approaches an object 54 to the medium embodied here as a prism, part of the light reflected at the prism base is coupled out onto the object (frustrated total reflection). The closer the object comes to the interface, the more the amplitude of the evanescent waves increases at the location of the object and the more light is coupled out onto the object. At the same time, however, the loss channel created in this way also reduces the number of photons stored in the resonator and thus the amplitude of the evanescent waves, so that the intensity on the object drops again. The resonator thus acts like a control loop, which ensures that the intensity on the object depends much less sensitively on the distance between the object and the medium (prism) than in the case of illumination of the interface (prism base) with total reflection without an additional resonator.

As shown schematically in Fig. 15, the light source can also be integrated into the resonator. The combination of laser-active medium 59 and optical pump source 60 then forms a laser system together with the mirrors 51 and 52 and the interface of the medium 53 illuminated under total reflection, which function as a resonator.

The medium at whose interface the total reflection takes place can also be designed as an optical optical fiber 61, as shown in FIG. 16. In contrast to the embodiments described above, the light propagating in the medium is then not only totally reflected at one point, but is guided in the medium with periodic total reflection. The resonator can either be formed by a highly reflective coating on the fiber ends or by mirrors (62 and 63) attached to the beginning and end of the fiber. In order to couple light onto an object 54, the fiber cladding 65, which has a refractive index which is slightly lower than that of the fiber core, must be removed to such an extent that the object 54 is removed from the interface to a distance which is less than the light wavelength between the fiber core and the fiber cladding, on which the evanescent waves arise, can be approximated.

With the aid of a light source 66 and a focusing system 67, light is also coupled into the resonator from the outside. The fiber itself can also be doped with a laser-active material and optically pumped, so that a fiber laser is created.

In the arrangement shown in FIG. 17, the resonator or the medium in which the total reflection takes place is in the form of a two-dimensional waveguide structure 75. This consists of a hemisphere or spherical cap, the outer shell of which has a higher refractive index n 'than the inner region, which is made of a material with the refractive index n. Such a structure can be produced, for example, from a glass sphere, in which the sodium ions are partially exchanged for silver ions in a suitable melt on the surface of the sphere and which is then sawn into two hemispheres or spherical sections. Light that is coupled into the shell with the refractive index n ^' on the flat side of the hemisphere propagates therein under total reflection to pole 76 and further to a point opposite the coupling-in point. The flat side is provided with a highly reflective coating 77 so that the outer shell becomes a resonator. With the aid of the lighting device 78, light is coupled in at all points of the shell on the flat side of the hemisphere or spherical cap. The further this light propagates to the pole, the steeper the angle between the direction of propagation and the optical axis 79. At the pole, all rays meet at an angle of 90 ° to the optical axis. The hemisphere therefore acts like a lens with a numerical aperture of NA = n ^' sin (90 °) = n ^' . As mentioned above, the minimum focal spot area that can be achieved on a focusing element decreases with increasing numerical aperture. The area of the focal spot forming at the pole of the hemisphere is therefore smaller than with conventional focusing elements (e.g. Microscope objectives) can be achieved, since these do not reach the maximum aperture angle of 90 ° in practice.

So that only purely evanescent waves occur at the pole, which can be coupled out to an object 54 located near the pole, the light coming from the lighting device may only be coupled into the area of the shell structure with the refractive index n ^' and not into the inner area with the refractive index n , The easiest way to do this is through an aperture 80, which is arranged between the hemisphere and the light source and absorbs all light that would strike the interior of the hemisphere. Alternatively, the flat side of the hemisphere can be provided with an absorbent layer on the inside. A particularly small focal spot area can be achieved by coupling in a radially polarized ring mode. For efficient coupling into the shell structure, there is a suitable diffractive element 81 between the lighting device, which provides a radially polarized ring mode, and the resonator, which ensures that the majority of the light emitted by the light source is directed onto the shell structure. The outer shell of the hemisphere can also consist of a laser-active material in this embodiment. The resulting laser resonator must then also be optically pumped from the outside.

Claims

Patent claims:

1. Device for focusing light onto an object, with at least one light source preferably formed by a laser, with at least one optical focusing element and a device for determining the relative

Position of the focusing element and object, characterized in that the light transmitted through the focusing element is in the form of a radially polarized ring mode.

2. Device according to claim 1, characterized by a beam-shaping element for generating a radially polarized ring mode from at least one incident linearly polarized wave with a preferably Gaussian intensity distribution.

3. Device according to claim 2, characterized in that said beam-shaping element contains a polarization converter, which consists of at least three half-wave plates, which are mounted substantially perpendicular to the direction of propagation of the light beam striking the polarization converter in a segment arrangement, so that the half-wave plates occupies the major part of the beam cross-section and the semiaxes are each aligned so that the direction of polarization of the incident light within each plate is rotated in a direction that points radially away from the center of the incident light beam.

4. Device according to claim 3, characterized in that said polarization converter is stored in a transparent in the area of the incident and outgoing light beam, which is completely or partially filled with a medium whose refractive index is greater than that of air and substantially equal to the refractive index of the material from which the half-wave plates are made.

5. Device according to claim 3, consisting of a filter element arranged behind the polarization converter, preferably in the form of a Optical resonator, characterized in that the transmission of said filter element is substantially different from zero only for the TEM _0m and TEM _m0 modes (m odd) of an electromagnetic field distribution striking the filter element.

6. Device according to claim 1 with at least one in front of or behind one of the focusing element arranged aperture, characterized in that the aperture is designed as an annular aperture and only an annular part of the light striking the aperture can pass.

7. Device according to one of claims 1 to 6, characterized in that the object illuminated by the focused, radially polarized, annular field distribution is an optical or magneto-optical data storage medium.

8. Device according to one of claims 1 to 6, characterized in that said device forms the illumination device for an object viewed with the aid of a microscope.

9. Device according to one of claims 1 to 6, characterized in that the illuminated object made of a coated with light-sensitive material

Substrate exists.

10. Device according to one of claims 1 to 6, characterized in that the illuminated object consists of a material which evaporates at the location of the focal spot under the influence of the lighting or on the chemical

Vapor separation material is deposited.

11. Device according to one of claims 1 to 9, characterized by, a solid immersion lens through which the present in the form of a radially polarized ring mode reaches the object.

12. Device for coupling light onto an object, in which the light is totally reflected in an optically denser medium at an interface and that An object at a distance below the wavelength from the interface is arranged according to one of claims 1 to 11, characterized in that a resonator is provided, in the beam path of which the optically denser medium is arranged, the light guided in the resonator being total at the interface is reflected and partially coupled out onto the object, the light coupled into the resonator or guided in the resonator being in the form of a radially polarized ring mode.

13. The device according to claim 12, characterized in that a lighting device is provided, the light in the form of a ring mode

Provides that is coupled into the resonator.

14. Device according to claim 13, characterized in that an aperture is provided which allows only an annular part of the light incident on the aperture to pass to the resonator.

15. Device according to claim 13 or 14, characterized in that at least one suitable diffractive element is attached in the beam path in front of the resonator, so that a substantial part of the light power contained in the ring mode is directed onto the waveguide structure of the resonator and coupled.

16. Device according to one of claims 12 to 15, characterized in that said resonator is part of a laser resonator and a laser-active medium is attached in the beam path of the resonator.

17. Device according to one of claims 12 to 16, characterized in that the resonator is formed by a fully or partially provided with a reflective coating solid lens.

18. Device according to one of claims 12 to 17, characterized in that the optically denser medium is made essentially in the form of a hemisphere or spherical cap, and that the light guided in the resonator into the medium occurs at its curved surface and the flat side of the hemisphere forms the interface facing the object, at which the light is totally reflected and partially coupled out onto the object.

19. The device according to claim 18, characterized in that the resonator manufactured in the form of a hemisphere or spherical cap is provided in whole or in part on its curved surface with a highly reflective coating.

20. Device according to one of claims 12 to 17, characterized in that the optically denser medium consists of an optical fiber, wherein the fiber cladding attached to the fiber core has a thickness at the point of coupling, which is smaller than the light wavelength.

21. The device according to claim 20, characterized in that the optical fiber is made entirely or partially of a laser-active material, and that a

Device for optically pumping the laser-active material is provided.

22. Device according to one of claims 12 to 17, characterized in that the resonator consists of a two-dimensional waveguide, which is made essentially in the form of a hemisphere or spherical cap, and that

Refractive index in the outer area has a greater value than in the inner area and the light guided in the resonator on the flat side, which is wholly or partially provided with a highly reflective layer, is coupled into the resonator and partially coupled out to an object in the vicinity of the pole.

23. The device according to claim 22, characterized in that the resonator is provided in whole or in part on the curved surface with a highly reflective coating.

24. Device according to claim 22 or 23, characterized in that the outer layer of the hemisphere or spherical cap, which forms the two-dimensional waveguide, is made of a laser-active material, and that a device for optically pumping said laser-active material is provided.