DE102014200742B4 - Process for anti-reflective coating of an optical element, optical element and terahertz system - Google Patents

Abstract

A method for anti-reflective coating of a surface (1) of an optical element (2), in which the surface (1) is provided with a reflection-reducing surface structure, the surface structure being generated by means of at least one laser beam (5) directed onto the surface (1) and guided over the surface (1), characterized in that the laser beam (5) is pulsed with a pulse duration of less than 20 ps and the optical element (2) is a lens suitable for beam guidance of terahertz radiation.

Description

The invention relates to a method for anti-reflective coating of a surface of an optical element, in which the surface is provided with a reflection-reducing, reflection-reducing surface structure, and an optical element with a surface which is anti-reflective by such a method.

It is known from the prior art to antireflect surfaces of optical elements by providing these surfaces with microscopic structures which result in a refractive index gradient in a surface layer. Compared to conventional anti-reflective coatings through anti-reflective coatings, surface structures of this type have the advantage that they significantly reduce the degree of reflection of the respective surface over a comparatively large wavelength range. Such an anti-reflective coating is described, for example, in the publication DE 10 2005 048 365 A1 described. Due to the etching process proposed there for the production of the surface structure, however, this is only suitable for an anti-reflective coating for electromagnetic waves in an environment in the visible range.

In the publication US 2013/0156053 A1 describes a method for anti-reflective coating of a surface of an optical element, in which the surface is provided with a reflection-reducing surface structure. The method has the features mentioned in the preamble. In particular, this publication describes a THz emitter with a main body (“main body”) which serves as an optically parametric resonator for THz radiation and for this purpose is formed from an optically non-linear crystal, for example from lithium niobate.

In the publication US 2011/0051250 A1 describes an optical element on and below the surface of which a hole structure is incorporated by means of a laser. During the production of the hole structure, the material of the optical element is heated by the laser radiation to such an extent that it thermally decomposes and gaseous oxygen is generated, which fills the holes that are created.

The publication US 2012/0026591 A1 describes the production of a surface structure of an optical element by means of an etching process.

The publication US 2008/0073438 A1 describes a method for structuring silicon wafers with femto laser pulses, for example for producing an inscription on the silicon wafer.

The publication US 2006/0217601 A1 describes a system with a THz lens made of silicon.

In the publication of Clapham, PB, Hutley, MC, entitled Reduction of Lens Reflexion by the Moth Eye Principle, published in Nature, Vol. 244, 1973, pp. 281-282 , describes the production of surface structures that reduce the reflections of visible light on glass materials.

In the publication of Bruckner, C., et al., Entitled “Broadband antireflective surface-relief structure for THz optics”, published in Optics Express, Vol. 15, 2007, no. 3, pp 779-789 THz lenses with anti-reflective surface structures are described, whereby the anti-reflective structures are mechanically introduced into a plastic surface using a diamond tool.

In the publication of Bruckner, C., et al., With the title “Broadband antireflective structures applied to high resistive float zone silicon in the THz spectral range”, published in Optics Express, Vol. 17, 2009, no. 5, pp. 3063-3077 , describes a manufacturing process for an anti-reflective structure on flat silicon substrates by reactive ion depth etching (DRIE).

The present invention is based on the object of proposing a method that is as easy to implement as possible, with which an optical element can be antireflective as effectively as possible, largely regardless of its nature and also for longer wavelengths, as well as a device with which such antireflection can be carried out. The invention is also based on the object of proposing a correspondingly effectively anti-reflective optical element and in particular a terahertz system with a correspondingly low-loss optics for guiding terahertz radiation.

This object is achieved according to the invention by a method with the characterizing features of the main claim in conjunction with the features of the preamble of the main claim, by an optical element according to claim 10 and a terahertz system with the features of claim 11.

The method according to the invention for anti-reflecting a surface of an optical element provides that the reflection-reducing surface structure is generated by means of at least one laser beam which is directed onto the surface and guided over the surface. The laser beam can be focused on or above or below the surface of the optical element in order to create a To cause material removal. The surface structure is typically generated by ablation, which is caused by the laser beam. However, other effects would also be conceivable, for example local melting due to the action of heat caused by the laser beam. It is irrelevant whether the laser beam is guided over the surface by moving the laser beam or by moving the optical element or by moving both the laser beam and the optical element. According to the invention, the laser beam is pulsed with a pulse duration of less than 20 ps. The optical element is a lens suitable for guiding terahertz radiation.

The proposed generation of the surface structure, which is produced by material removal that varies depending on the location, that is to say differently from location to location, has several advantages. On the one hand, properties of the surface structure, in particular parameters relevant for the reflection-reducing effect, can be freely selected within relatively wide limits. This applies, for example, to a structure depth or an order of magnitude of lateral period lengths or extensions of other typical lateral structural features of the surface structure and thus for parameters that are very decisive for the wavelengths at which the surface structure created in this way develops its anti-reflective effect. A reflectivity of the surface can thus be effectively reduced with the proposed method, in particular for radiation of longer wavelengths, for example for terahertz radiation (THz radiation). In addition, the method is not only comparatively easy to implement, but also largely independent of a shape of the surface to be anti-reflective and largely independent of the material from which the optical element or a surface layer of the optical element is made. The proposed use of a laser beam makes it possible in particular to implement structures with very steep flanks. As a result, the surface structure can be designed with a relatively large aspect ratio, that is to say with a relatively large structure depth for a given typical lateral scale of the surface structure, which is advantageous for the greatest possible reduction in the degree of reflection.

The optical element according to the invention, which has a surface that is type anti-reflective with the method according to the invention, is a lens for guiding terahertz radiation. In the present document, terahertz radiation refers to electromagnetic radiation with a frequency of between 0.1 THz and 10 THz or a vacuum wavelength of between 3 mm and 30 μm.

Thus, a terahertz system (THz system), which includes a terahertz transmitter for generating terahertz radiation and / or a terahertz receiver for receiving terahertz radiation, can be made particularly efficient by the proposed measures if it has at least one anti-reflective optical element for guiding the terahertz radiation with a method described here. This optical element can be a component of the terahertz transmitter and / or a component of the terahertz receiver or also an optical element additionally provided in a beam path of the terahertz system.

The optical element will typically be formed from a material that has a relatively high refractive index and the lowest possible absorption in a wavelength range or frequency range for which the optical element is intended. In particular for applications in the terahertz range (THz range), e.g. silicon or sapphire are suitable as a material for forming the optical element. However, the material can also be a plastic or a plastic mixed product, in particular, for example, HDPE, Zeonex, TPX or Teflon.

The surface structure, which is an anti-reflective structure that is comparatively broadband compared to conventional anti-reflective coatings, forms a gradient index layer with an increasing refractive index, which prevents an abrupt change in the refractive index. This minimizes reflections, which in turn increases transmission.

The anti-reflective optical element with the proposed method is suitable for guiding terahertz radiation. For this application, the reflection-reducing surface structure is preferably designed in such a way that it reduces a reflectivity of the surface for terahertz radiation and accordingly increases the transmission, that is to say improves the quality of a TH signal passed through the optical element. Because of the relatively long wavelengths, it is expedient if the surface structure is designed with a structure depth of at least 80 μm, preferably even with a structure depth of 100 μm or more. The structure depth can be defined as the difference in height between the highest elevations and deepest depressions of the surface structure. As a rule, however, the structure depth will not be more than 300 µm or 400 µm or 500 µm, because this is not necessary for an effective reduction in the degree of reflection for terahertz radiation and because the creation of the surface structure naturally becomes more difficult with increasing structure depth.

A particularly advantageous embodiment provides that the surface structure is designed as a so-called moth's eye structure, which comprises a large number of microscopic columns. Such structures are particularly effective in creating a surface layer within which an effective refractive index results over a relatively large depth, the amount of which is between the refractive index of the material forming the optical element and a refractive index of a surrounding medium - typically air. With regard to a refractive index gradient that is not too large, it is particularly advantageous if the columns are each at least slightly conical. The term “column” in the present document is therefore not intended to be restricted to exactly cylindrical shapes. Rather, it is favorable for a refractive index profile that is as constant as possible if the thickness of the columns decreases towards the top. In the case of a moth's eye structure of the type described, the structure depth corresponds to a height of the microscopic columns.

For an application in the field of terahertz radiation, the distance between directly adjacent columns of the moth's eye structure must in any case not be less than 6 μm. Typically, a sufficiently fine moth-eye structure can also be realized with a distance between directly adjacent columns of the moth-eye structure of not less than 20 μm or 10 μm. However, it is advantageous if this distance is at most 90 μm and preferably not more than 50 μm.

gilt, wobei n_s die Brechzahl des für das optische Element verwendeten Materials oder Substrats ist und α den maximalen Einfallswinkel der THz-Strahlung beschreibt. Die Effektivität der als Antireflexstruktur dienenden Oberflächenstruktur hängt von der nachfolgend mit d bezeichneten Strukturtiefe ab, für die am besten $$d\ge \frac{\lambda }{4n_{eff}}$$

gilt, wobei n_eff die effektive gemittelte Brechzahl in der durch die Oberflächenstruktur gebildeten Oberflächenschicht beschreibt. Diese hängt von einem sogenannten Füllfaktor ab, also davon, wie groß ein vom strukturierten Material des optischen Elements ausgefüllter Anteil eines Volumens dieser Schicht ist. Das wiederum hängt natürlich von der Strukturform der Oberflächenstruktur ab, also im Fall der Mottenaugenstruktur z.B. von einem mittleren Durchmesser der Säulen und der Periode Λ. Dabei gilt in grober Näherung n_eff = Füllfaktor × n_s mit 0 < Füllfaktor < 1.In the case of regularly arranged columns, this distance defines a period Λ of the surface structure. How the surface structure should be designed for the best possible anti-reflective coating results from simple physical relationships. If λ is the smallest wavelength in the spectrum of the THz radiation to be carried, then particularly good results are achieved if for period A of the surface structure $$\Lambda \le \frac{\lambda }{n_s+sin\left(\alpha \right)}$$

applies, where n _{s is} the refractive index of the material or substrate used for the optical element and α describes the maximum angle of incidence of the THz radiation. The effectiveness of the surface structure serving as an anti-reflective structure depends on the structure depth, designated below with d, for which the best $$d\ge \frac{\lambda }{\mathrm{4th}{n}_{eff}}$$

applies, where n _{eff describes} the effective average refractive index in the surface layer formed by the surface structure. This depends on a so-called fill factor, that is, on how large a portion of a volume of this layer is filled by the structured material of the optical element. This in turn depends of course on the structural shape of the surface structure, i.e. in the case of the moth's eye structure, for example, on an average diameter of the columns and the period Λ. As a rough approximation, n _eff = fill factor × n _s with 0 <fill factor <1.

The microscopic pillars of the moth's eye structure can be formed in that the at least one laser beam is guided over the surface in such a way that the pillars remain between areas in which material is removed by the laser beam. For this purpose, the at least one laser beam can be guided over the surface along a first set of parallel paths and along a second set of parallel paths that form a non-vanishing angle - preferably a right angle - with the paths of the first set, and typically many at a time Times. For example, each of the named paths can be run over 100 times with the laser beam in order to complete the surface structure. The laser beam can also travel along the paths in a statistical or quasi-statistical sequence, staggered over time. This allows individual areas of the processed optical element to cool down from time to time, which helps to avoid undesired melting. A path actually traversed by the laser beam does not have to completely traverse the named paths in each case before a next path is followed. Rather, this path can be guided as desired within wide limits and can have a large number of changes in direction. The tracks themselves can also be curved and, in other designs, also include different angles, without necessarily running parallel to the other tracks in each case.

According to the invention, the laser beam guided over the surface to generate the surface structure is pulsed with a pulse duration of less than 20 ps or even less than 10 ps. By using short or ultra-short laser pulses, it can be ensured that the laser beam has as little thermal influence as possible on areas that surround a point hit by the laser beam and in which no material is to be removed. In this way, a locally narrowly limited energy input into the material of the surface of the optical element and thus a surface structure with steep flanks and / or laterally small dimensions can be implemented particularly well.

A wavelength of the laser used can, for example, be in a range between 500 nm and 1500 nm. It is preferably an ultrashort pulse laser with a repetition rate of, for example, between 10 kHz and 100 kHz. A pulse energy of the individual laser pulses can be between 10 µJ and 50 µJ, for example. A focus of the laser beam, which can be localized either directly on the surface or just above or just below it, should have a diameter suitably selected depending on the desired surface structure, for example a diameter of between 5 μm and 30 μm. In order to ensure that the material is removed sufficiently quickly, but in a controlled, localized manner, the laser beam can be guided over the surface, for example, at a speed of between 100 mm / s and 300 mm / s.

Depending on the type of optical element, the surface can be, for example, a spherical surface or a rotationally symmetrical aspherical surface or a freeform surface. With the proposed method, the surface structure can be implemented without any problems, largely independently of the shape of the surface. In particular, two particularly expedient variants are conceivable. For example, the laser beam can be directed perpendicular to the surface. In the case of a moth's eye structure, this means that the microscopic pillars created in this way are each perpendicular to the surface. In another variant that is relatively easy to implement, the laser beam can be directed onto the surface in each case parallel to a defined axis of the optical element, for example parallel to an axis of symmetry of the optical element. In the typical case of a moth's eye structure, the columns are then each parallel to one another and also inclined in places with respect to a surface normal to the surface, but this is not harmful to the anti-reflective effect.

A device is also proposed which is advantageously suitable for anti-reflective coating of a surface of an optical element using a method of the type described here. This device, which itself is not part of the claimed invention, comprises a holder for holding the optical element, at least one laser light source for irradiating the surface with a laser beam and a drive for moving the holder and / or the at least one laser light source and / or one of the Beam guiding element guiding the laser beam. This device also has a control unit for controlling the drive. This control unit is set up by means of appropriate programming to control the drive in such a way that the laser beam of the at least one laser light source is guided along several paths over the surface in such a way that surface areas remain between the paths that are not swept over by the laser beam.

In particular, the control unit can be set up to control the drive in such a way that the laser beam of the at least one laser light source runs along a first set of parallel paths and along a second set of parallel paths that form a non-vanishing angle - preferably a right angle - with the paths of the first set. include, is passed over the surface. The moth-eye structures described above as being particularly advantageous can thus be produced with the device. The control unit is preferably programmed in such a way that the laser beam traverses each of the paths multiple times, e.g. over 100 times each, for example at a speed of between 100 mm / s and 300 mm / s. Depending on the desired dimensioning of the moth's eye structure to be created, the directly adjacent tracks can run side by side offset at least 6 µm and / or at most 90 µm, for example.

The at least one laser light source of the device can be a pulse laser, preferably an ultrashort pulse laser for generating laser pulses with a pulse duration of less than 20 ps and / or a pulse energy of between 10 μJ and 50 μJ. The laser can be set up to generate the laser pulses, for example, with a repetition rate of, for example, between 10 kHz and 100 kHz. Its wavelength can, for example, be in a range between 500 nm and 1500 nm. A focus of the laser beam that can be generated with this laser can, for example, have a diameter of between 5 μm and 30 μm.

Preferred areas of application for the methods and devices described here are the manufacture of optical components for the THz range and their design. Every THz system requires optics for beam guidance. Up to now, mirror optics have often been used for this, as the THz radiation is absorbed by most materials. Lenses and other refractive optical elements made of silicon or other highly refractive materials, on the other hand, have so far found little use due to the high losses, in particular due to reflections at boundary layers in the THz range. The anti-reflective coating of these materials in the manner proposed here can significantly improve the optical design of THz systems, because it allows lenses and other refractive or diffractive optics to be used without too great a loss of intensity. Time-resolved measurements, in particular of THz pulses, are also made easier by avoiding reflections that would otherwise cause additional pulses in a detected signal.

• 1 eine schematische Ansicht einer Vorrichtung zum Entspiegeln eines optischen Elements,
• 2 in entsprechender Darstellung eine alternative Ausführung einer solchen Vorrichtung,
• 3 einen Querschnitt durch eine Oberfläche eines durch ein entsprechendes Verfahren mit einer derartigen Vorrichtung entspiegelten optischen Elements,
• 4 eine Aufsicht auf die so entspiegelte Oberfläche aus 3,
• 5 einen Querschnitt durch eine in entsprechender Weise entspiegelte Terahertz-Linse,
• 6 in einer der 5 entsprechenden Darstellung einen Querschnitt durch eine mit einer geringfügigen Abwandlung des Verfahrens entspiegelte Terahertz-Linse und
• 7 in schematischer Darstellung ein Terahertz-System, das mit entsprechend entspiegelten Terahertz-Linsen ausgestattet ist.

The device presented here as well as exemplary embodiments of the invention are described below with reference to FIG 1 until 7th described. It shows

• 1 a schematic view of a device for anti-reflective coating of an optical element,
• 2 a corresponding representation of an alternative embodiment of such a device,
• 3 a cross section through a surface of an anti-reflective optical element using a corresponding method with such a device,
• 4th a plan view of the anti-reflective surface 3 ,
• 5 a cross section through a correspondingly anti-reflective terahertz lens,
• 6th in one of the 5 corresponding illustration shows a cross section through a terahertz lens with an anti-reflective coating with a slight modification of the method and FIG
• 7th a schematic representation of a terahertz system that is equipped with appropriately anti-reflective terahertz lenses.

1 shows a device which can be used for anti-reflective coating of a surface 1 an optical element also shown there 2 suitable. This device comprises a holder 3 to hold the optical element 2 , as well as a laser light source 4th for irradiating the surface 1 with a laser beam 5 . The device also has a drive 6th to move the bracket 3 on. Finally, the device also comprises a control unit 7th to control the drive 6th . This control unit 7th is set up by appropriate programming, the drive 6th so control that the laser beam 5 along several tracks so across the surface 1 is guided so that surface areas remain between the paths traveled over, which are not exposed to the laser beam 5 be painted over. The bracket 3 through the drive 6th both around an axis of symmetry of the optical element 2 rotated as well as horizontally shifted in one or two directions, which is in 1 illustrated by arrows. The drive 6th can also be designed so that the drive 6th the bracket 3 and thus the optical element 2 tipped in a way that is in 1 is illustrated by a curved double arrow. Then the laser beam can 5 eg so about the surface 1 be driven so that it is always perpendicular to the surface 1 shine.

At the laser light source 4th It is an ultrashort pulse laser with a wavelength of 1030 nm, which generates laser pulses with a pulse duration of 8 ps and a pulse energy of 20 µJ to 24 µJ with a repetition rate of 40 kHz. One on or just above or just below the surface 1 lying focus of the laser beam 5 has a size of about 20 µm. Of course, other designs of the device are possible in which the laser light source 4th has other parameters. The drive 6th can for example be controlled in such a way that the laser beam 5 repeatedly traverses the said tracks, for example 300 times, with the laser beam 5 the surface 1 for example, can sweep at a speed of 200 mm / s. Of course, these parameters can also be selected differently. It is also conceivable that instead of the individual laser light source 4th multiple laser light sources are provided so that the surface 1 is crossed by several laser beams at the same time, which then, for example, different parallel paths on the surface at the same time 1 can leave.

In 2 is a modification of the device from 1 shown. Recurring features are again provided here and in the following figures with the same reference numerals and no longer have to be specifically described. From the device 1 differs in 2 device shown only by having another drive 6 ' for moving the laser light source 4th is provided. Instead, the drive can 6 ' also be designed so that he instead of the whole laser light source 4th just one the laser beam 5 leading beam guiding element moves to the laser beam 5 across the surface 1 respectively.

Through the drive 6 ' can use the laser light source 4th horizontally in an in 2 with the direction designated x and possibly also vertically in a direction designated there with z. When the drive 6 ' what is also possible is designed so that it is the laser light source 4th can also move in a horizontal direction perpendicular to the x direction, the drive 6th the bracket 3 can also be omitted. The drive 6 ' can also be designed so that it is the laser light source 4th tipped in a way that is in 2 is illustrated by a curved double arrow. This also allows the laser beam 5 eg so about the surface 1 be driven so that it is always perpendicular to the surface 1 shine. Like the drive 6th so will the drive too 6 ' so by the appropriately programmed control unit 7th controlled that the laser beam 5 along several tracks so across the surface 1 is guided so that surface areas remain between the paths traveled over, which are not exposed to the laser beam 5 be painted over.

In the case of the optical element 2 In the present exemplary embodiment, it is a THz lens, that is to say a lens suitable for guiding terahertz radiation, which is anti-reflective with the described device by providing the surface with a reflection-reducing surface structure which makes the surface reflectivity 1 especially for terahertz radiation. The surface structure is created by ablation by means of the laser beam 5 generated to do this on the surface 1 directed and by the drive 6th and or 6 ' across the surface 1 to be led.

A cross section of the surface structure produced in this way is shown in 3 shown. This is a so-called moth's eye structure, which has a large number of microscopic pillars 8th includes. A plan view of this surface structure is in 4th to see. There you can see that the pillars 8th form a periodic grid or a grid structure with a period Λ, which can be, for example, 30 µm. The surface structure has the same height as the columns 8th corresponding structure depth d of between 100 µm and 260 µm. This results in a broadband anti-reflection coating, in particular for electromagnetic radiation with frequencies from 0.14 THz to 1.5 THz.

The microscopic pillars 8th are formed by the ultra-short pulsed laser beam 5 so about the surface 1 that is led to the pillars 8th stop between areas where by the laser beam 5 Material is removed. This is done using the laser beam 5 repeatedly - for example 300 times - along the already mentioned tracks over the surface, namely along a first set of parallel tracks and along a second set of parallel tracks that form a right angle with the tracks of the first set. The paths of the first set run in the in 4th with x and the trajectories of the second family in the direction indicated in 4th is denoted by y. The columns 8th thus remain exactly between the tracks of the two groups, which each run at a distance of, for example, about 30 μm next to one another in the present exemplary embodiment.

In 5 is a schematic cross section through the anti-reflective optical element 2 shown. The optical element 2 can for example be made of silicon, which has a refractive index of about 3.4 in the THz range, or of sapphire or a plastic with a particularly high refractive index in the THz range. Under certain circumstances, the optical element 2 be formed directly from a wafer piece made of silicon and, for example, be incorporated into its surface. The surface 1 can in particular be a spherical surface or a rotationally symmetrical aspherical surface or a free-form surface. The microscopic pillars 8th in this embodiment are each perpendicular to the surface 1 . That is achieved by using the laser beam 5 when anti-reflective by a corresponding movement of the optical element 2 or the laser light source 4th each perpendicular to the surface 1 is judged. Of course, other lenses or optical surfaces can also be anti-reflective in a similar way.

the 6th shows a slightly different embodiment of the optical element in a corresponding representation 2 that differs from the embodiment 5 differs only in that the microscopic pillars 8th all parallel to an axis of symmetry 9 of the optical element 2 stand. This is because the laser beam 5 in this case each parallel to the axis of symmetry 9 of the optical element 2 on the surface 1 is directed when the optical element 2 - also in this case a THz lens - is anti-reflective in the manner described. This is the surface 1 divided into different concentric circular stripes, each with a focus position of the laser beam optimized for the respective stripe 5 can be edited so that the surface structure of a terrace shape is what is shown in 5 is indicated. Because of the shape of the surface 1 In this case, the surface structure is created in most areas with an oblique incidence of the laser light, whereby the anti-reflective effect is nevertheless retained.

In 7th a THz system is shown that uses a terahertz transmitter 10 for generating terahertz radiation and a terahertz receiver 11 for receiving the terahertz radiation. Both the terahertz transmitter 10 as well as the terahertz receiver 11 each have an optical element for coupling out and coupling in the terahertz radiation 2 on, the one provided in the manner described with a moth's eye structure surface 1 Has. With the optical elements 2 These are THz lenses that can be made of silicon and are arranged on the detector or emitter to couple the radiation in and out.

With the THz system one can switch between the terahertz transmitter 10 and the terahertz receiver 11 arranged sample 12th to be examined. In addition, another lens 13th be provided for beam guidance of the terahertz radiation, which can of course be anti-reflective in the same way.

The measures proposed here result in a system and processing method that can be used flexibly for anti-reflective coating of optical elements 2 , not only, but especially for the THz range. For this purpose, structures are preferably inserted into the surface with an ultra-short pulse laser (typical pulse duration less than approx. 10 ps) 1 of the material to be coated. The laser is on, above or below the surface 1 of the optical element 2 focuses and leads to material removal there. If the laser is now over the surface with defined parameters 1 guided, any structural geometries can be realized within wide limits. The structure depth d is essentially determined by the laser and process parameters and can be increased by repeating it several times. Special structures, similar to those of moth's eyes, can thus be generated, which have an anti-reflective effect for long-wave radiation. The minimal thermal influence zone on the surrounding material that is not to be ablated when using ultrashort laser pulses for material ablation allows - compared to longer laser pulses, for example in the nanosecond range - the formation of comparatively fine structures with the required geometry parameters. The local adaptation of the position of the laser focus to the surface 1 of the optical element 2 During the machining process, it also enables the creation of anti-reflective structures on spherical, aspherical and free-form elements, even if these have a very strong curvature of the surface 1 exhibit.

The laser beam 2 is done with a scanner plus control software over the surface 1 guided. The surface structure is designed in such a way that the fill factor is from air to the material of the optical element 2 is constantly changing and, for example, a conical moth-eye structure is formed, as shown in 3 is indicated.

The system can be extended by a multi-dimensionality so that strongly curved surfaces can be processed. In addition, controllable focus optics can be advantageous for the described method for antireflection coating.

The bracket 3 of the optical element 2 can be converted to the laser beam through hardware and software control, for example 5 move in rotation. From knowledge of the surface dimension - for example obtained from 3D data or by optical scanning - either the surface is obtained 1 to the laser beam 4th and / or the surface 1 rotates around its own mounting axis. Furthermore, the optical element 2 may be moved translationally in the direction of all axes (x, y, z).

The optical element 2 can also be in a fixed holder for the anti-reflective coating when the laser beam 5 is moved with the system by hardware and software control over the geometry of the sample in a rotating and / or translatory manner in the direction of all axes (x, y, z).

Finally, a combination is also possible in which both the optical element 2 as well as the laser beam 5 is moved.

An important advantage is the flexible, comparatively fast and contactless production of broadband THz anti-reflective structures on spherical, aspherical or freeform surfaces of optical elements. The flexibility of the bandwidth is given by the variation of the laser parameters. The anti-reflective coating of defined frequency ranges can be implemented in this way. In particular, broadband anti-reflective coating is possible over large frequency ranges. Due to the multi-axis guidance of the laser radiation and / or the processed optical element, largely any surfaces of optical elements can be structured. There is no tool wear and / or touching contact with the workpiece, which could negatively affect the quality of the optical element.

Claims (12)

A method for anti-reflective coating of a surface (1) of an optical element (2), in which the surface (1) is provided with a reflection-reducing surface structure, the surface structure being generated by means of at least one laser beam (5) directed onto the surface (1) and guided over the surface (1), characterized in that the laser beam (5) is pulsed with a pulse duration of less than 20 ps and the optical element (2) is a lens suitable for beam guidance of terahertz radiation. Procedure according to Claims 1 , characterized in that the reflection-reducing surface structure reduces a reflectivity of the surface (1) for terahertz radiation. Method according to one of the Claims 1 or 2 , characterized in that the surface structure is designed with a structure depth (d) of at least 80 µm and / or with a structure depth (d) of at most 500 µm. Method according to one of the Claims 1 until 3 , characterized in that the surface structure is designed as a moth-eye structure comprising a multiplicity of microscopic columns (8). Procedure according to Claim 4 , characterized in that the surface structure is designed so that a distance between directly adjacent columns is at least 6 µm and / or at most 90 µm. Method according to one of the Claims 4 or 5 , characterized in that the microscopic columns (8) are formed in that the at least one laser beam (5) is guided over the surface (1) in such a way that the columns (8) remain between areas in which the laser beam (5 ) Material is removed. Procedure according to Claim 6 , characterized in that the at least one laser beam (5) for this purpose along a first set of parallel tracks and along a second set of parallel tracks which enclose a non-vanishing angle with the tracks of the first set over the surface (1). Method according to one of the Claims 1 until 7th , characterized in that the optical element (2) is formed from silicon. Method according to one of the Claims 1 until 8th , characterized in that the surface (1) is a spherical surface or a rotationally symmetrical aspherical surface or a free-form surface, the laser beam (5) in each case perpendicular or in each case parallel to a defined axis of the optical element (2) on the surface (1) is judged. Optical element (2) for guiding terahertz radiation, the one with a method according to one of the Claims 1 until 9 having an anti-reflective surface (1), the optical element (2) being a lens suitable for beam guidance of terahertz radiation. Terahertz system, comprising a terahertz transmitter (10) for generating terahertz radiation and / or a terahertz receiver (11) for receiving terahertz radiation, the terahertz system according to at least one optical element (2) Claim 10 for guiding the terahertz radiation. Terahertz system Claim 11 , wherein the at least one optical element (2) is part of the terahertz transmitter (10) and / or part of the terahertz receiver (11).

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