Method of making a frequency doubling structure in an optically non-linear medium

Abstract: In an optically non-linear (NLO) polymeric material a frequency doubling structure composed of alternatingly poled NLO polymer is made. Said structure is made by forming a layer of NLO polymer, effecting poling in it with the use of an electric field, partially removing the layer of NLO polymer by etching, such that in the polymer a grating structure is formed composed of etched grooves and projecting polymer parts, the width of the grooves matching that of the polymer parts, dividing the polymer layer containing the grating structure into at least two sections, and placing the two sections one on top of the other, such that grooves of the one section are backfilled by projecting polymer parts of the other section. To prevent the enclosure of air a filler may be added, preferably in the form of a solution of the NLO polymer.

Description: The invention relates to a method of making a frequency doubling structure in an optically non-linear polymeric material, in which process alternating poling is effected in the optically non-linear material. Nowadays, because of developments in the field of solid state lasers, it is possible in many optical techniques to employ electromagnetic radiation of which the wavelength falls at the near-by infrared end of the electromagnetic spectrum or even within that region thereof in which there is question of visible light (red). However, for many optical applications it is desired to be able to use light of a wavelength which falls within the middle region of the visible light range or at the far removed (blue) end thereof. Examples of applications for which this is particularly desired include optical data storage, optical communication technics, scanning, and medical applications of optics. To provide a light source emitting a single wavelength in the desired region, it is known to pass electromagnetic radiation emitted by an existing light source, e.g. a laser having a wavelength in the range of, say, approximately 700 to approximately 1300 nm, through a frequency doubler, which will give a light source emitting a wavelength of half that length, i.e. in the range of approximately 350 to 650 nm. In such a method it is known to employ optically non-linear materials as a frequency doubling structure. Optically non-linear materials, also called non-linear optical (NLO) materials, are known. In such materials non-linear polarisation occurs under the influence of an external field of force (such as an electric field). Non-linear electric polarisation may give rise to several optically non-linear phenomena, including the generation of second harmonics, i.e., frequency doubling. Polymeric NLO materials also are known. Obtaining the desired NLO effect in these macroscopically requires that first the groups present in such a material, mostly hyperpolarisable sidegroups, be aligned (poled). Such poling is usually effected by exposing the polymeric material to electric (dc) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation. For example, a method as described in the opening paragraph is known from US Patent Specification 4 865 406. The frequency doubler disclosed in said specification comprises a substrate-supported thin layer of an alternatingly poled NLO polymer. The frequency doubling structure is made by providing the layer of NLO polymer on either side with layers of cladding and subsequently with aluminium electrodes. The Al electrodes are given a grating structure with the aid of a photoresist. Next, with heating, the layer of NLO polymer is exposed to an electric field. According to the description, this gives poled sections alternating with unpoled sections. However, guidelines on how to produce the disclosed alternating alignment, which is essential to practicability, are not provided. The envisaged alternating structure is needed to prevent light subjected to frequency doubling from being wholly or partially extinguished prior to leaving the frequency doubler. Such extinction is connected with the so-called "coherence length". This is the distance between two spaced apart points A and B, with the frequency doubled component of the light of the original wavelength travelling through the frequency doubler generated at point B being in counterphase to a frequency doubled component of the original light already generated at point A. To prevent such exctinction the periodicity of the alternatingly poled polymeric NLO material should be equal to twice the coherence length. This is known per se , int. al. from US Patent Specification 4 971 416. The present invention has for its object to provide a method by means of which an alternatingly poled frequency doubling structure can be incorporated into an NLO polymer in a comparatively simple manner. Further, it is envisaged to provide a frequency doubling structure having the highest possible modulation depth of the poling-induced second-order NLO coefficient χ (2) , which gives maximum doubling yield, and not having any change of the index of refraction occurring in the optical path, which also contributes to an improved yield. For an explanation of the term χ (2) reference is made to David J. Williams, Angew. Chem. Int. Ed. Engl, 23 (1984), 690-703. To this end the invention resides in that the following steps are incorporated into a method of the above-mentioned known type: forming a layer of NLO polymer exposing the layer of NLO polymer to an electric field in order to effect poling of the NLO polymer partial removal of the layer of NLO polymer by etching, such that in the NLO polymer a grating structure is formed composed of etched grooves and projecting NLO polymer parts, the width of the grooves matching that of the projecting NLO polymer parts separating the NLO polymer layer containing the grating structure into at least two sections positioning one section on top of the other, such that grooves of the one section are backfilled by projecting NLO polymer parts of the other section. Optionally, the NLO polymer may be poled after etching but prior to separation. Alternatively, the separated parts may each be poled individually prior to being fitted together. However, these embodiments are not very functional. As a rule, a layer of viscous filler will be added before, during, or after the two sections are placed one on top of the other in order to prevent the enclosure of air. However, the optical properties of the filler should not differ greatly from those of the NLO polymer. Preferably, the selected filler will have an index of refraction which is identical or virtually identical with that of the poled NLO polymer. The use of a glue or some other adhesive is not excluded. An advantage of the present method is that a structure of alternatingly poled NLO polymer is formed, with the alignment being wholly homogeneous. It is pertinent to note here that the object in US 4 865 406 is presumably to apply an alternating electric poling field to obtain alternating alignment. However, the use of an alternating poling field is attended with several drawbacks, the principal one being that electrodes of different polarities have to meet too closely, which makes for a major chance of lateral burnout. Because of the electrodes' edge effects (the deviating concentration of lines of force which always occurs at the electrode edge) the field profile of the poling field in the polymeric layers will deviate significantly from the desired homogeneous vertical profile. Such drawbacks can be avoided using the method according to the present invention, since the use of electrodes extending along the entire polymeric layer makes it possible to have these edge effects occur outside of the polymer to be poled. It should also be noted that the electric voltage generated by the poling field generally is d.c. voltage, but that it has also proved possible, under certain conditions described in the literature, to make use of a.c. voltage, cf. Paul R. Ashley and Thomas E. Tumolillo, Opt. Soc. Am. Technical Digest Series , Vol. 8, p. 87 (1991). Conceivable embodiments of the method according to the invention will be exemplified hereinbelow. The step in which the layer of NLO polymer is formed may consist of applying a polymer solution to a suitable substrate, e.g. by means of spincoating, followed by evaporating the solvent. Suitable substrates are, int. al., silicon wafers or plastics laminates, such as those based on epoxy resin which may be reinforced or not. Suitable substrates are known to the skilled man. The substrate is not essential to the method according to the present invention. The layer of NLO polymer can also be formed by moulding, injection moulding, or other processing techniques known in themselves. The layer of NLO polymer so formed is partially removed by etching, generally after having been poled, such that a grating structure composed of grooves etched into the polymer is formed. For the etching any suitable etching technique may be employed, e.g., those known in the manufacture of integrated circuits (ICs). For example, use might be made of wet-chemical etching techniques, e.g. with organic solvents or strong bases being employed. It is preferred, however, to make use of dry-chemical etching techniques, such as sputter etching, reactive ion etching, or reactive plasma etching. Such techniques are known to the skilled man and require no further elucidation here. Alternatively, there may be etching using a laser, mechanical etching, such as grinding, cutting, drilling, or etching through bombardment with sanding particles, such as alumina, silica, and more particularly, pumice. The preferred etching technique is dependent on the NLO polymer employed. The skilled man is expected to be able to select an appropriate etchant for the NLO polymer in question without undue experimentation. Preferably, use is made of O₂ plasma, and a very well defined grating structure can also be obtained using the so-called laser ablation technique. To obtain the desired toothed structure when using non-mechanical etching techniques, a mask is applied to cover those parts which should remain free from attack by the etchant. These masks, the chief prerequisite of which is that they be resistant to the action of the etchant, are known, int. al., from IC technology. Such a mask may be preformed and made up of, e.g., metal or synthetic material; alternatively, it can be made by applying a photosensitive material (photoresist) and subsequently exposing and developing it in accordance with the desired pattern. Instead of by etching it is also possible to produce a toothed structure without removing material, e.g. by means of embossing. In such a method the formation of grooves is attended either with heightening at the side of the grooves or with increased density of the material tamped down in the grooves. The invention also relates to methods in which the etching step may be omitted because the layer of NLO polymer is already provided with the desired toothed structure as it is formed. Such may be achieved using shaping and processing techniques known in themselves for polymers which are known to the skilled man and hence require no further elucidation here. As an example may be mentioned (injection) moulding in a mould. In this method a thermosetting composition which is very readily mouldable in the uncured state and, in general, easy to release from the mould in the cured state may advantageously be employed as NLO polymer. The thermosetting composition is then cured in the mould with simultaneous exposure to the poling field. The grating structure is of vital importance to the action of a frequency doubler manufactured according to the present invention, since it determines the structure of the alternatingly poled NLO polymer as it is ultimately obtained. As has been indicated above, the periodicity of the alternating structure should be equal to twice the coherence length. As appears from, int. al., US Patent Specification 4 971 416 mentioned hereinbefore, the following can be said to hold for the spatial periodicity Λ: Λ = 2 l c = 2π/Δk wherein l c equals the coherence length and Δk stands for the difference in wave vectors between the frequency doubled optical wavefront and the original (frequency generating) optical wavefront. On the basis of the wavelength of the light subjected to frequency doubling the skilled man is able to select the correct grating constant. Hence the NLO polymer layer comprises grooves (preformed or applied later) having a width equal to half of the coherence length, thus leaving polymeric sections which could be called "teeth", which teeth also have a width equal to half of the coherence length. The layer of NLO polymer can now be divided into sections, e.g. by cutting, sawing, or otherwise cleaving in two, preferably such that two halves are formed. These sections, preferably halves, can be simply fitted together, such that the grooves of the one section will match the teeth of the other section. The resulting structure is then alternatingly poled and can be used in a frequency doubling device. It should be noted that a method of making a frequency doubling structure in which sections of a poled NLO polymer are separated and placed one on top of the other is in itself known from EP 355 915. However, at issue in this document is a frequency doubling structure of another type, i.e. one in which there is question of an interface between two layers polarised in opposite directions, with an abrupt inversion of the polarisation taking place at the interface. From an infrared laser beam passing parallel to the interface a light beam having double the original frequency is formed. In general, the structure of alternatingly poled NLO polymer formed using the method according to the invention will be enveloped in a cladding to realise a waveguide in the structure. The index of refraction of such a cladding is lower than that of the enclosed polymer. Depending on the NLO polymers used, it is also possible to realise a waveguide by creating in the NLO polymer a channel of a higher index of refraction, e.g. by the method set forth in EP 358 476. Poling the NLO polymers in the method according to the invention is by means of the application in a known manner of an electric field. To this end the material to be poled, which is commonly deposited on a substrate, is provided with electrodes connected to a rectified voltage feed and, e.g., placed on a controlled temperature table. Voltages of some tens to several hundreds of volts per µm of polymer layer thickness are common. The period of exposure to the electric field is generally in the range of a few seconds to several minutes, but may also be from some tens of minutes to one hour - notably when use is made of a thermosetting NLO composition. The period of time required is further dependent on the temperature at which poling takes place. As has been stated above, this temperature is dependent on the NLO polymers used, but as a rule it will be in the range of about 50° to about 350°C, more particularly in the range of 80° to 200°C. In general, the poling field is maintained as the poled material is cooled down to ambient temperature. Representative poling temperatures and the periods of time required in such cases are known, int. al., from patent publications. Thus, it is known from EP 378 185 to expose an NLO copolymer described therein at a temperature of 85°C to an electric field strength of 8x10⁵ V/cm for a period of 20 minutes. An NLO polymer described in US 4 865 406 is exposed for 10 minutes to an electric field strength of 70 V/µm at a temperature of 90°C. EP 396 172 describes the alignment of an NLO polymer by means of corona discharge, the temperature being 127°C. In EP 445 864, in which there is question of an NLO thermoset, a thermosetting composition is cured and poled over a period of 15 to 45 minutes at a temperature of 145°C. In EP 359 648 there is question of poling for a few seconds under the influence of an electric field of 50 V/µm at a temperature of 100°C. In a frequency doubler manufactured by the method according to the invention use may be made of known NLO polymers. Examples of such NLO polymers are those described in EP 350 112, EP 350 113, EP 358 476, EP 445 864, EP 378 185, EP 359 648, as well as those described in the US patent specifications mentioned hereinbefore. The invention is not restricted to any particular type of NLO polymer. In addition, the frequency doubling device manufactured according to the invention may comprise all structural characteristics conceivable, c.q. required in the case of such an apparatus. For example, the device comprises a component for coupling light into the frequency doubler. For research purposes it is typical to employ a coupling prism to this end. The well-known prism coupling technique is described in, int. al., Applied Physics Letters , 55 (1989), 616-618, and it is also used in the aforementioned US patent specifications. In actual practice a coupling prism is not very functional and for the coupling of frequency generating light generally use will be made of an optical fibre or a laser lens. The device further comprises a waveguide, i.e. a layer or a channel through which the coupled and the frequency doubled light can be propagated. Commonly such a waveguide will be made up of a core layer of polymer which is to some degree transparent to at least a portion of the coupled light, enveloped by cladding layers of a different polymer having a lower index of refraction for the envisaged light than the core layer does. According to the invention, the core layer comprises a frequency doubling structure as described hereinbefore. Preferably, the core layer is composed in its entirety of such an alternatingly poled structure of NLO polymer. For background data on frequency doubling devices reference is made to EP 361 602, EP 355 915, EP 254 921, and GB 2 187 566, as well as to an article in Electronics Letters , 26 (1990), 2105-2107.

Claims: A method of making a frequency doubling structure in an optically non-linear polymeric material, in which process alternating poling is effected in the optically non-linear material, characterised in that the process comprises the following steps: forming a layer of optically non-linear polymer exposing the layer of optically non-linear polymer to an electric field in order to effect poling of the optically non-linear polymer partial removal of the layer of optically non-linear polymer by etching, such that in the optically non-linear polymer a grating structure is formed composed of etched grooves and projecting optically non-linear polymer parts, the width of the grooves matching that of the projecting optically non-linear polymer parts dividing the optically non-linear polymer layer containing the grating structure into at least two sections placing two sections one on top of the other, such that grooves of the one section are backfilled by projecting optically non-linear polymer parts of the other section. A method according to claim 1, characterised in that between the two sections placed one on top of the other a filler is provided. A method according to claim 2, characterised in that the filler used is a solution of the optically non-linear polymer. A method according to any one of the preceding claims, characterised in that the layer of optically non-linear polymer is formed by applying a polymeric film to a substrate. A method according to any one of the preceding claims, characterised in that the width of the grooves is equal to half of the coherence length of the second harmonics to be generated by the frequency doubling structure.

Inventors: HORSTHUIS WINFRIED HENRI GERAR, MOHLMANN GUSTAAF RONALD, HORSTHUIS, WINFRIED HENRI GERARD, MOHLMANN, GUSTAAF RONALD
Applicants: AKZO NOBEL NV, AKZO NOBEL N.V.
Application year: 1992
Main classification: G02F1
IPC codes: G02F1