Two-photon stereolithographic writing system

Abstract

A stereolithographic writing apparatus (1) for carrying out two-photon polymerization within a photopolymerizable substrate medium (100), the apparatus (1) comprising: a source of electromagnetic radiation (10) for emitting a beam of photons (B) suitable for effecting two-photon polymerization in the substrate medium (100); means (M1, M2, M3, WP1, AOM1, WP2, AOM2, M4, M5, WP3, AOD1, AOD2, M6, AL1, S, M7, AL2, M8, M9, OBJ1) for defining an optical propagation path of the photon beam (B) as it passes from the electromagnetic radiation source (10) towards the substrate medium (100); beam control means (AOM1, AOM2, AOD1, AOD2) within the optical propagation path for steering and focusing the photon beam (B) into a selected target region of the substrate medium (100) in which the two-photon polymerization can occur; and means (30) for moving the substrate medium (100) relative to the photon beam so as to sequentially expose different target regions of the substrate medium (100) to the photon beam (B) to effect two-photon polymerization sequentially at those different target regions of the substrate medium (100); wherein the beam control means comprises: at least one acousto-optic deflector (AOD1, AOD2) for spatially controlling the direction of the photon beam (B) as it passes along the optical propagation path, and at least one acousto-optic modulator (AOM1, AOM2) for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam (B) as it passes along the optical propagation path.

Description

[0001] TWO-PHOTON STEREOLITHOGRAPHIC WRITING SYSTEM

[0002] TECHNICAL FIELD

[0003] This invention relates to a two-photon stereolithographic writing system. More particularly, though not exclusively, it relates to a two-photon stereolithographic writing apparatus, which may be operated with single or plural optical writing beams, for use in creating objects or structures, especially two-or three-dimensional objects or structures, through the principle of two-photon polymerization within a substrate medium, especially a volumetric substrate medium. Such objects or structures may typically be very small or highly accurate or detailed microstructures, which find uses in a wide variety of research, development, industrial and commercial fields, e.g. in biology, medicine and physics, and in other technical applications where highly specific functional surfaces are required, e.g. as security features. The invention also relates to a method of forming such objects or structures using the apparatus, and to such objects or structures produced thereby.

[0004] BACKGROUND OF THE INVENTION AND PRIOR ART

[0005] As a general concept, two-photon polymerization (hereinafter “2PP”) is well-known. It generally involves forming a volumetric (i.e. three-dimensional) object or structure in a polymerizable substrate medium (usually a liquid) by projecting a beam delivering two (or more) photons substantially simultaneously into the medium to define a selected tiny region therein (called a “voxel”) at which two-photon absorption occurs, which causes highly selective polymerization of the medium at that point. By progressively scanning the coincident dual-photon beams over the appropriate three-dimensional space within the substrate medium, the corresponding three-dimensional object or structure can be built up from the polymerized solid material, point-upon-point and / or layer-upon-layer. This basic technique is often termed “stereolithography”. The process of creation of such objects or structures by what is in essence a “writing” technique is therefore often termed “stereolithography writing” (hereinafter abbreviated to “SLW”). Hitherto in the art many different types of objects or structures have been able to be produced by this technique, and typically they may be three-dimensionally complex or particularly small in size and scale.

[0006] However, if ones wishes to reduce the size of the polymerized object or structure down to micron or sub-micron sizes or even smaller, or if especially complex three-dimensional structures are to be produced, and then apply that to mass-production on a viable commercial scale, then a current problem in the art is that most known SLW systems do not enable the performance of the necessary optical scanning fast enough or accurately enough. These limitations on writing speeds and resolutions place significant limitations on the extent to which SLW techniques can be designed and exploited to good technical and economic effect in many technical and commercial fields.

[0007] In recent years this limitation has been partially improved by the rapid development of suitable femtosecond laser light sources and suitable recording media, which have allowed the creation of some complex three-dimensional microstructures. Furthermore, some known UV single-photon SLW systems achieve high writing speeds with a relatively large size of the written voxel, but these are themselves limited in their efficacy and usefulness by the diffraction limit of the focusing or imaging optical system. Some two-photon SLW systems are also known which are able to write sub-diffraction structures with details down to e.g. tens of nanometers. However, even with such high-end SLW devices - which typically use galvanic mirrors to control the photon beam - the writing speed is still significantly slower than is often needed or is ideal, especially in the context of ever more complex and accuracydependent structures whose creation by SLW techniques is becoming more and more desirable.

[0008] It is therefore a primary object of the present invention to address and ameliorate the above shortcomings and limitations of the known art of SLW systems and apparatuses, and to provide a new and improved form of SLW system and apparatus which is based on a new optical control system that allows for the attainment of significantly faster SLW writing speeds and / or writing resolutions.

[0009] SUMMARY OF THE INVENTION

[0010] Accordingly, in a first aspect the present invention provides a stereolithographic writing apparatus, especially a stereolithographic writing apparatus for carrying out two-photon polymerization within a photo-polymerizable substrate medium, the apparatus comprising: a source of electromagnetic radiation for emitting a beam of photons suitable for effecting two-photon polymerization in the substrate medium; means for defining an optical propagation path of the photon beam as it passes from the electromagnetic radiation source towards the substrate medium; beam control means within the optical propagation path for steering and focusing the photon beam into a selected target region of the substrate medium in which the two-photon polymerization can occur; and means for moving the substrate medium relative to the photon beam so as to sequentially expose different target regions of the substrate medium to the photon beam to effect two-photon polymerization sequentially at those different target regions of the substrate medium; wherein the beam control means comprises: at least one acousto-optic deflector for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and at least one acousto-optic modulator for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path.

[0011] In a second aspect the present invention provides a method of creating an object or structure through the principle of two-photon polymerization within a photo-polymerizable substrate medium, the method comprising:

[0012] (i) providing a stereolithographic writing apparatus according to the first aspect of the invention or any embodiment thereof; and

[0013] (ii) operating the apparatus so as to emit a photon beam from the electromagnetic radiation source and propagate the beam along an optical propagation path towards the substrate medium, and controlling the photon beam by steering and focusing it into a selected target region of the substrate medium such that two-photon polymerization occurs therein, and moving the substrate medium relative to the photon beam so as to sequentially expose different target regions of the substrate medium to the photon beam to effect two-photon polymerization sequentially at those different target regions of the substrate medium, wherein the controlling of the photon beam comprises spatially controlling the direction of the photon beam, as it passes along the optical propagation path, by operation of the at least one acousto-optic deflector, and modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam, as it passes along the optical propagation path, by operation of the at least one acousto-optic modulator.

[0014] In many embodiments of the above method of the second aspect, the method may be a method of creating a two- or (especially) three-dimensional object or structure, through the principle of two-photon polymerization within a volumetric photo-polymerizable substrate medium.

[0015] In a third aspect the present invention provides an object or structure, especially a two- or (especially) three-dimensional object or structure, formed through the principle of two-photon polymerization within a photo-polymerizable substrate medium by use of the stereolithographic writing apparatus according to the first aspect of the invention or any embodiment thereof, or by the method of the second aspect of the invention or any embodiment thereof.

[0016] In a fourth aspect the present invention provides a control system for controlling a photon beam emitted by a source of electromagnetic radiation in a stereolithographic writing apparatus for carrying out two-photon polymerization within a photo-polymerizable substrate medium, the control system being provided within an optical propagation path of the photon beam as it passes from the electromagnetic radiation source towards the substrate medium, and the control system being constructed and operable for steering and focusing the photon beam into a selected target region of a substrate medium in which the two-photon polymerization can occur, wherein the control system comprises beam control means which comprise: at least one acousto-optic deflector for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and at least one acousto-optic modulator for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path.

[0017] In embodiments of the above control system of the fourth aspect, the apparatus may be a stereolithographic writing apparatus with features, including (especially) the beam control means, as defined in the first aspect of the invention or any embodiment thereof defined or described herein.

[0018] Thus, the SLW system of the present invention in its various aspects may be used to create, using 2PP principles, objects or structures which are typically definable as being either substantially two-dimensional only - e.g. surfaces or faces (or a surface or facial layer) of a particular material or a material itself in the form of a substantially thin planar film, web or layer, in either case with fine surface details or textures that lie substantially within the thickness of such surfaces / faces (or surface / facial layers) or films / webs / layers themselves) - or alternatively, and indeed often more usually and more usefully, are truly three- dimensional in nature. The SLW writing system of the invention may be used to especially good effect for the production by 2PP of often complex three-dimensional structures, e.g. microstructures, complex functional surfaces with complex three-dimensional surface features, and suchlike.

[0019] Indeed, because of the present invention’s new, faster and more accurate way of controlling the photon beam in steering and focusing it onto / into any given selected target region of the substrate medium, it is possible to write structures (i.e. cause them to be formed voxel-by- voxel through 2PP, especially within successive or sequential writing planes) with significantly greater speeds and accuracy than has hitherto been possible with known SLW systems, which lead not only to the ability to form more complex structures quicker but also with greater accuracy, especially in terms of forming and joining up adjacent voxels with higher accuracy as well as speed.

[0020] In particular, embodiments of the SLW apparatus and method aspects of the present invention may be designed and operable so as to constitute a plural-beam - especially a two- beam or a four-beam - SLW system in order to write / record by effecting 2PP in or at plural regions / sites, e.g. in or at plural regions / sites within a given voxel or in adjacent yet discrete voxels, within the substrate medium substantially simultaneously.

[0021] Thus, in some such two-beam systems according to embodiments of the invention, the beam control means - or especially the at least one acousto-optic deflector thereof - may be constructed and operable so as to effect simultaneous yet separate spatial control, i.e. the simultaneous yet separate control of the deflection of, first and second independent photon sub-beams which propagate substantially parallel to each other along at least a portion of the optical propagation path leading to the substrate medium.

[0022] In some such two-beam embodiment systems, the beam control means may comprise at least first and second ones of at least two acousto-optic deflectors, the first said acousto- optic deflector being constructed and operable to split a primary photon beam into two discrete sub-beams in a x-axis direction, and the second said acousto-optic deflector being constructed and operable to deflect the split sub-beams together in a y-axis direction and into a substantially parallel arrangement relative to each other and thereby into a desired final output direction for onward propagation to the substrate medium.

[0023] Correspondingly, in some such four-beam systems according to embodiments of the invention, the beam control means - or especially the at least one acousto-optic deflector thereof - may be constructed and operable so as to effect simultaneous yet separate spatial control, i.e. the simultaneous yet separate control of the deflection of, first, second, third and fourth independent photon sub-beams which propagate substantially parallel to each other along at least a portion of the optical propagation path leading to the substrate medium.

[0024] And further correspondingly, in some such four-beam embodiment systems, the beam control means may comprise at least first and second ones of at least two acousto-optic deflectors, the first said acousto-optic deflector being constructed and operable to split a primary photon beam into two discrete sub-beams in a x-axis direction, and the second said acousto-optic deflector being constructed and operable to further split each of the two subbeams into two sub-sub-beams (i.e. to form four sub-sub-beams in total) in a y-axis direction and into a substantially parallel arrangement relative to each other and thereby into a desired final output direction for onward propagation to the substrate medium.

[0025] In either of the above-defined plural-beam embodiment species of SLW apparatus (i.e. two- beam or four-beam) of the invention, each of the respective splittings and / or configurations of the respective beams or sub-beams may be effected by any of the following (i), (ii) or (iii):

[0026] (i) the nature of a synthesised control signal applied to one or more of the at least one acousto-optic deflectors;

[0027] (ii) a dynamic change of a synthesised control signal applied to one or more of the at least one acousto-optic deflectors;

[0028] (iii) a combination of (i) and (ii).

[0029] Moreover, in some such embodiments as in the preceding paragraph, such a dynamic change in the synthesised control signal applied to one or more of the acousto-optic deflector(s) may be used not only for controlling a cross-section of one or more of the deflected beams, but it may also be usefully used for controlling the size of the or each respective recorded / written voxel (or at least one or more of them) together with controlling the position of the or each respective recorded / written voxel (or at least one or more of them).

[0030] In many embodiments of the invention, the beam control means may comprise: a plurality of acousto-optic deflector devices for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and a plurality of acousto-optic modulator devices for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path

[0031] In many embodiments of the invention, the beam control means, including the one or more acousto-optic deflectors and the one or more acousto-optic modulators, may form part of the means for defining the optical propagation path, whereby the at least one acousto-optic deflector and the at least one acousto-optic modulator are present within the optical propagation path and the photon beam interacts with both the at least one acousto-optic deflector and the at least one acousto-optic modulator as the beam passes along the optical propagation path. In some embodiments of the invention, the control of the photon beam by the one or more acousto-optic modulators in terms of modulating the intensity and / or frequency of the photon beam may involve controlling the photon beam in terms of modulating any one or more of the following parameters of the photon beam:

[0032] (i) the amplitude of the electromagnetic radiation forming the beam;

[0033] (ii) the cross-sectional shape of the beam;

[0034] (iii) the timing of the exposure dose of the radiation deliverable by the beam;

[0035] (iv) the frequency (or wavelength) of the electromagnetic radiation forming the beam.

[0036] In the context of the present invention, the or each acousto-optic modulator may be any suitable acousto-optic modulator device that acts as or is operable to control the transmitted intensity, power, cross-section or frequency of a laser beam using an electrical drive / control signal - which is to say, it may be a device that acts as or is operable to modulate one or more properties, especially one or more of transmitted intensity, power, cross-section or frequency, of the electromagnetic radiation via an acousto-optic effect. As a technical concept, acousto-optic effects are well-known perse in the art, and are based on the periodic modulation of the refractive index of a medium due to the presence of sound waves in that medium. The sound waves produce what is effectively a refractive index grating in the medium, and the electromagnetic waves interact with this grating, especially by diffraction. Thus, as used in embodiments of this invention, an acousto-optic modulator may be a device that uses an acousto-optic effect to diffract and shift the frequency of electromagnetic radiation using sound waves. Such an acousto-optic modulator may also serve to modulate the intensity of the propagating photon beams of the electromagnetic radiation.

[0037] In general terms an acousto-optic modulator may typically comprise a piezoelectric transducer that creates sound waves in various types of materials, e.g. glass, quartz or various crystalline materials, and various different types of acousto-optic modular devices are known per se in the art for various practical applications that exploit such acousto-optic effects.

[0038] Suitable practical examples of acousto-optic modulator devices that may be used in embodiments of the invention are widely available commercially, e.g. any of various series thereof available from CSRayzer or AAA Opto (as well as possibly other manufacturers / suppliers).

[0039] In many practical embodiments of the invention, at least one acousto-optic modulator, especially at least one of a plurality of acousto-optic modulators, forming part of the beam control means may be of a “fast” type - which is to say, it may be an acousto-optic modulator in which the speed of the sound wave in the medium is at least, or is greater than, about 2000 or 3000 or even 4000 m / s (for example, in the case of a crystal of TeO2, to name just one example of such a medium, the speed of the sound wave in the crystal is in the region of -4200 m / s). This is in contrast to a “slow” type of acousto-optic modulator (e.g. which operates in a “shear” mode), in which the speed of the sound wave in the medium is at most, or less than, about 1000 m / s (for example, in the same case of a crystal of TeO2, the speed of the sound wave in the crystal is in the region of -650 m / s).

[0040] In such embodiments which comprise a plurality of acousto-optic modulators, at least one of which is a “fast” type, it may be that at least one other of those acousto-optic modulators, apart from that at least one “fast” one, is of a “slow” (or “regular” / “standard”) type.

[0041] In the context of the present invention, the or each acousto-optic deflector may be any suitable acousto-optic deflector device that acts as or is operable to spatially control the direction of the photon beam, or its direction of propagation, as it passes through or impinges on the medium within the device - which is to say, it may be a device that acts as or is operable to spatially modulate, or to modulate the direction of propagation of, the electromagnetic radiation via an acousto-optic effect. Acousto-optic deflectors are devices which can be used to deflect an electromagnetic radiation (e.g. laser) beam in a given direction by a variable angle by use of an acousto-optic effect, in which an electrical drive signal of (typically) constant power but a variable frequency is applied to the acoustic transducer. Thus, in the operation of an acousto-optic deflector the power driving the acoustic transducer is kept on, especially at a constant level (though possibly at a variable level instead), while the acoustic frequency is varied to deflect the radiation beam to different angular positions. The acousto-optic deflector makes use of the acoustic frequencydependent diffraction angle, where a change in the diffraction angle is a function of the change in the acoustic frequency.

[0042] The amplitude of the acoustic grating in the or each acousto-optic deflector determines the modulation of the power of the deflected electromagnetic radiation beam. Thus, a combination of the amplitude of the acoustic grating formed in the or each acousto-optic deflector and the frequency of the drive signal applied thereto modulates both the electromagnetic radiation beam in terms of its deflected angle and the power of the deflected radiation beam together. Thus in basic physical terms an acousto-optic deflector is similar to a acousto-optic modulator, but it is connected and operated in a different manner in order to modulate an interacting electromagnetic radiation beam in a different way.

[0043] Like an acousto-optic modulator, in general terms an acousto-optic deflector may typically comprise a piezoelectric transducer that creates sound waves in various types of materials, e.g. glass, quartz or various crystalline materials, and various different types of acousto-optic deflector devices are known per se in the art for various practical applications that exploit such acousto-optic effects when operated as acousto-optic deflectors using a frequencyvariable drive signal.

[0044] Suitable practical examples of acousto-optic deflector devices that may be used in embodiments of the invention are widely available commercially, e.g. any of various series thereof available from AAA Opto (as well as possibly various other manufacturers / suppliers).

[0045] In some practical embodiments of the invention, at least one acousto-optic deflector, especially at least one of a plurality of acousto-optic deflectors, forming part of the beam control means may be of a “slow” type - which is to say, it may be an acousto-optic deflector in which the speed of the sound wave in the medium is at most, or less than, about 1000 m / s (for example, in the case of a crystal of TeO2, to name just one example of such a medium, the speed of the sound wave in the crystal is in the region of -650 m / s). This is in contrast to a “fast” type of acousto-optic deflector, in which the speed of the sound wave in the medium is at least, or is greater than, about 2000 or 3000 or even 4000 m / s (for example, in the same case of a crystal of TeO2, the speed of the sound wave in the crystal is in the region of -4200 m / s). The use of such “slow” type acousto-optic deflector(s) may be acceptable (or even desirable) from the point of view of the electronics (in terms of its nature, complexity, operation and / or cost) that is used to control it / them.

[0046] However, in some alternative practical embodiments of the invention, if desired or appropriate it may be possible for at least one acousto-optic deflector, or at least one of a plurality of acousto-optic deflectors, forming part of the beam control means to be of the “fast” type instead.

[0047] In embodiments which comprise two or more such acousto-optic deflector devices, those two, or any such two, acousto-optic deflector devices may be oriented or arranged or tilted - in terms of respective reference planes of those acousto-optic deflector devices - at a nonzero or non- 180 degrees angle relative to each other, especially oriented or arranged at 90 degrees, i.e. perpendicularly, relative to each other. In other words, in such two, or any two, acousto-optic deflector devices they may be mutually oriented or arranged or tilted with their respective reference planes at a non-zero or non- 180 degrees angle relative to each other, especially oriented or arranged at 90 degrees, i.e. perpendicularly, relative to each other. Each respective such reference plane is defined by a respective plane coincident with (or containing) or parallel to the respective mounting base of the respective acousto-optic deflector device. By this perpendicular relative positioning or orientation, such plural acousto-optic deflector devices may be able to function to deflect the photon beam independently in each of two orthogonal axis directions.

[0048] In embodiments of the invention, the means defining the optical propagation path, especially that or those portion(s) thereof apart from the at least one acousto-optic deflector and the at least one acousto-optic modulator, may comprise any number and any type of optical components of either optically transmissive or optically reflective properties, so that together they form or define a general path of propagation or travel of the photon beam that is emitted by the radiation source and which passes towards the substrate sample.

[0049] Such optical components of either optically transmissive or optically reflective properties may include one or more mirrors for reflecting the photon beam into a desired altered direction of travel, and / or one or more retardation plates for modifying the polarization (or other appropriate parameter) of the photon beam transmitted therethrough. Suitable practical examples of such optically transmissive or optically reflective optical components are widely commercially available and well-known in in their forms and functions in the optics art.

[0050] It will thus be apparent from the foregoing definitions and general descriptions that a key difference between SLW systems according to embodiments of the present invention and any known single-photon-polymerization SLW writing system lies primarily in the achieved resolutions of the writing by our new systems. Furthermore, as compared with other known two-photon-polymerization SLW writing systems, the new systems of embodiments of this invention may achieve significantly higher writing speeds - this is achieved by (a) using faster optical devices (namely the acousto-optic modulator(s) and deflector(s) instead of galvanic micro-mirrors as are most commonly used in the prior art) that may need to have compensation for their imperfections (e.g. non-homogeneity, non-linearity), and (b) enabling plural-beam writing modes that may allow for an order-of-magnitude faster writing speed in certain specific cases. For example, the writing speed of a typical example of a known 2PP- SLW writer is specified as 100-625 mm / sec, whereas in prototype embodiments of our new system of this invention we may achieve speeds of -600-3000 mm / sec (which is approx, five times as fast). Thus, in our new systems we have significantly increased the writing speed primarily by using a new optical beam control system based on the new combiantio of acousto-optic deflectors and acousto-optic modulators, especially pluralities of each thereof and often of certain types, and in unique combinations.

[0051] In practical embodiments of the present invention, the photo-polymerizable substrate medium may comprise any suitable component(s) or composition(s) that offer or are capable of two-photon photo-polymerization using the stereolithographic writing apparatus of embodiments of the invention. Such two-photon-polymerizable substrate media or compositions may for example comprise any such substances or compositions that are already known for their suitability in the art of two-photon polymerization (2PP) for the production of two- or (especially) three-dimensional objects or structures by a 2PP reactive process.

[0052] As used herein the term “photo-polymerization” is intended to be construed broadly, and encompasses not only true polymerization reactions of monomers initiated by electromagnetic radiation (e.g. UV or other wavelengths of light) but also photo-induced cross-linking or curing of already partially polymeric or oligomeric precursors of final polymerised or cross-linked or cured polymeric materials.

[0053] The recording material - i.e. the substrate medium in which 2PP is to occur - must absorb and polymerize (or cross-link or cure, as the term “polymerize” and its linguistic equivalents is intended to encompass as it is used in this specification) at the appropriate wavelength given by the electromagnetic radiation source (e.g. a laser). In practical embodiments of the present invention, the recording material (i.e. the substrate medium in which 2PP is to occur) may typically be mounted in or on, or contained in, a suitable carrier, e.g. a pad, plate, wafer, mat, receptacle or other carrying structure, which may be of any suitable material and size, shape and configuration, e.g. depending on the shape and configuration of the end structure to be formed. By way of example, suitable forms of such carriers may include e.g. silicon polished wafers and glass plates, as well as various other shaped carrier structures.

[0054] Typically, in practical embodiments of the invention the recording material (i.e. the substrate medium in which 2PP is to occur) may comprise a monomer or polymer precursor base and an initiator (i.e. a photo-initiator) of the two-photon polymerization reaction.

[0055] Various photo-initiators may be used in practical embodiments of the invention, and there are many examples of suitable photo-initiators in the art. One such suitable example of a photo-initiator is 4,4'-Bis(diethylamino)benzophenone (also abbreviated to DEAB) (which is available from various commercial manufacturers / suppliers), which is an organic compound belonging to the class of aromatic ketones. It has the empirical molecular formula C21 H28N2O and a molecular weight of 324.5 g / mol. Like other photo-initiators, DEAB has the ability to initiate (i.e. trigger) chemical reactions using light. When exposed to ultraviolet (UV) light, the electrons in the DEAB molecule are excited, causing the molecule to break down and reactive radicals to form. These radicals can then react with other molecules and trigger various reactions, e.g. the polymerization of photopolymer precursor materials or the formation of polymer backbones.

[0056] DEAB is useful as a photo-initiator in various applications including not only two-photon lithography but also photoreactive printing, and the production of copolymers and other materials. It is also known for use as part of some photocatalytic systems that serve to oxidize organic compounds and other chemical reactions. It should be noted that DEAB is considered a highly toxic substance and can be hazardous to human health if handled improperly. Therefore, it is important to take safety precautions when working with this substance and follow the appropriate safety procedures.

[0057] In some cases the photo-initiator may for example be selected according to the requirements of the finished 2PP-written structure (e.g. its biocompatibility, among other possible end properties of the final structure that may be desirable).

[0058] Various other known photo-initiator substances or compounds may be used instead of DEAB, and the art contains many such examples of other such photo-initiators which may be suitable for use in practical embodiments of the invention.

[0059] The monomer or polymer precursor base component may in principle be any of a wide range of photo- (i.e. 2PP-) -polymerizable monomers or polymer precursors, of which there are many suitable known examples in the art. One such range of suitable polymer precursor materials are the hybrid organic-inorganic photopolymers that belong to the class of Ormocers (Organically Modified Ceramics) available under the trade mark Ormocomp® from Micro Resist Technology GmbH. These reagents are photosensitive polyimides and polyimide precursors, and are suitable for various microlithography and microstructureproduction systems.

[0060] Ormocomp® polymers are photosensitive and react, with suitable initiation, with ultraviolet light to form hard and solid layers. These polymers are designed to have low viscosity at low temperatures, allowing for easy and precise application to the carrier on which the 2PP reaction is to take place. Upon initiation and exposure to ultraviolet light, these polymers polymerize by cross-linking to form high strength solid materials, enabling use in a wide variety of microlithography and microsystems applications. Ormocomp® polymers may thus be especially useful as precursors for the production of final polymeric structures for use as e.g. microfluidic devices, micro-optical elements, microsystems for biomedical applications, microelectronic devices and many other kinds of microsystems. These polymers may also be suitable for the production of thin films and high-resolution microstructure features.

[0061] In practical embodiments of the invention, various other polymer precursors or bases or monomers may be used instead of Ormocomp® polymer precursors, of which there are many known examples in the art. Polymeric materials can sometimes change their properties after exposure to light. Thus, particular polymer precursors or bases or monomers may be chosen for use depending on the specific type of photopolymer being formed.

[0062] Some of the more common photopolymer base substances whose precursors or monomers may be polymerised by suitably initiated 2PP in accordance with embodiments of the present invention include the following:

[0063] - Acrylate bases: These photopolymer materials are made from acrylate monomers that are polymerized using light. Acrylates are very popular for photopolymer layers because they are highly sensitive to UV light and have a wide range of end-applications.

[0064] - Epoxy bases: Epoxy photopolymers are made from epoxy monomers that polymerize with light. Epoxy photopolymers are often used to create high-precision structures such as microelectronic components.

[0065] - Cyclopentenone bases: These photopolymer materials are made from cyclopentenone and polymerize with light. Cyclopentenone photopolymers are often used to make optical elements such as lenses and optical fibers.

[0066] - Silicone bases: Silicone photopolymers are made from silicone monomers and polymerize with light. Silicone photopolymers are often used to create soft materials and elastomers such as sealants and silicone patches.

[0067] - Styrene bases: These photopolymer materials are made from styrene monomers and polymerize with light. Styrene photopolymers are often used to create structures with high hardness and resistance, such as plastic components.

[0068] For use in embodiments of the present invention, the relevant / applicable polymer precursor or base or monomer and the photo-initiator may be mixed ready for use in any suitable relative proportions in order to give the final prepared substrate medium in which the 2PP reaction is to be carried out. Such suitable relative proportions will be readily known or calculable to / by the skilled person, especially on the basis of the known or suppled reagent data and the skilled person’s ordinary skill and knowledge in the 2PP art.

[0069] By way of example, however, in one practical example embodiment of the present invention using DEAB and Ormocomp® reagents, a mixing weight ratio of DEAB to Ormocomp® of 1% may typically be employed. The DEAB photo-initiator may suitably be pre-dissolved in a suitable solvent, e.g. in methylisobutylketone, e.g. in a weight ratio of 1 :2 initiator to solvent.

[0070] As a rule, in practical implementations of embodiments of the present invention, any polymeric materials producible by 2PP that will have suitable properties, e.g. flatness, adhesion to the recording medium carrier material, resistance to the laser beam, insolubility in any developer used, etc, may be designed to be produced by 2PP methods within the scope of embodiments of the present invention. If desired or necessary, by making suitable adjustments in various parameters of any given embodiment system, e.g. tailoring of the focusing technology or the choice of the most suitable recording material (i.e. the substrate medium in which 2PP is to occur), it may in theory be possible to produce virtually any two- or three-dimensional structures using embodiments of the apparatus and method aspects of the present invention.

[0071] In some practical implementations of embodiments of the invention, in order to be able to write structures successfully into such substrate media / materials, it may be important or desirable for an accurate and stable setting of the exposure dose for the polymerization of the material to be effected. In our systems, the size of the exposure dose may be controlled by very fast switching of the femtosecomd (fs) laser beam by at least one of the one or more acousto-optic modulator(s). Angular deflection of the recording beam may be ensured by a pair of the acoustic-optical deflectors, of which a plurality thereof may desirably be employed in many practical embodiment systems of the invention. In certain embodiments there may also be present at least one further acousto-optic deflector in the optical beam control system whose function is to compensate for the temporal and spatial dispersion of the recording laser beam so that minimal dispersion occurs after the passage of the fs laser beam through the entire optical beam control system. After being transformed by the optical beam control system, the laser beam may then be focused by a microscope lens into the photo- polymerizable monomer of the substrate medium.

[0072] In putting practical embodiments of the present invention into effect, the electromagnetic radiation generated by the source, e.g. a laser, and used to effect the 2PP may be electromagnetic radiation within any desired region or sub-region of the electromagnetic spectrum. In some practical embodiments light from within the visible region (e.g. with wavelengths within the overall approximate range of from about 380 nm up to about 750 nm, especially in a relatively narrow band of wavelengths (e.g. within a ~10 to ~40 or ~50 nm range) therewithin) may be used. Alternatively, in other embodiments, electromagnetic radiation in the ultra-violet (UV) (e.g. -100 - 380 nm) or infra-red (IR) regions (e.g. -750 nm - 500 pm) of the spectrum may be used instead. Practically speaking, the most appropriate wavelength of radiation appropriate to use in any given practical example system may depend on the identity and chemistry of the polymerizable monomer(s) in the substrate medium that is to be subjected to the 2PP SLW writing operation. Whatever the appropriate electromagnetic radiation wavelength that is to be used in any given practical example system, an appropriate source thereof, e.g. a laser, may be employed, of which there are many readily commercially available examples in the art.

[0073] Within the scope of this specification it is envisaged that the various aspects, embodiments, examples, features and alternatives, and in particular the individual constructional, configurational or operational features thereof, set out in the preceding paragraphs, in the claims and / or in the following description and accompanying drawings, may be taken independently or in any combination of any number of same. For example, individual features described in connection with one particular embodiment, or described singly or in combination with another feature in any one or more embodiments, are applicable on their own or in combination with one or more other features to all embodiments and may be found and used in combination with any other feature in any given embodiment, unless expressly stated otherwise or such features are incompatible.

[0074] BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Embodiments of the present invention and various features thereof will now be described in detail, all by way of non-limiting examples only, with reference to the accompanying drawings, in which:

[0076] FIGURES 1(a) and 1(b) are explanatory drawings illustrating the basic principles of one-photon and two-photon excitation, the latter of which is exploited in the putting into practice of embodiments of the present invention;

[0077] FIGURE 2 is a schematic view of the general arrangement of the principal components of a first, general, embodiment of the SLW apparatus according to the invention;

[0078] FIGURE 3 is a perspective view of a physical prototype example of the two-beam SLW apparatus illustrated schematically in FIG. 2, showing its optomechanical construction; FIGURES 4(a), (b) and (c) are, respectively, front, left-side and top-plan views of the apparatus of FIG. 3;

[0079] FIGURE 5 is a schematic view of the general arrangement of the principal components of a second embodiment of the SLW apparatus according to the invention, which is designed and operable as a two-beam SLW system;

[0080] FIGURE 6 is a schematic view of the general arrangement of the principal components of a third embodiment of the SLW apparatus according to the invention, which is designed and operable as a four-beam SLW system;

[0081] FIGURES 7(a), 7(b) and 7(c) are explanatory drawings illustrating some of the basic principles involved in the synthesis and use of an electrical control signal for controlling the operation of an AOD device as used in the SLW apparatuses according to the above embodiments of the invention;

[0082] FIGURE 8 is an explanatory drawing illustrating the basic principle involved in the use of a dynamically changing electrical control signal in the controlling of the operation of an AOD device as used in the SLW apparatuses according to the above embodiments of the invention.

[0083] DETAILED DESCRIPTION OF EMBODIMENTS AND FEATURES OF THE INVENTION

[0084] Referring firstly to FIGS. 1(a) and 1 (b), these diagrams explain the basic principles of one- photon versus two-photon excitation, it being the latter which is exploited in carrying out 2PP using new SLW apparatuses according to embodiments of the present invention. FIG. 1(a) shows a comparison of the absorption energy of a single photon by UV light (left-hand drawing) as compared with the absorption energy of two photons by near-infrared light (righthand drawing). FIG. 1 (b) shows schematically a comparison of the excitation volume of one- photon excitation (i) and two-photon excitation (ii). In 2PP regions outside the laser focus are less likely to exceed the polymerization threshold of the polymerizable substrate (e.g. a photoresist). This phenomenon allows the fabrication of complex three-dimensional structures, since the proximity effect in 2PP is significantly less than that in 1 PP (singlephoton polymerization).

[0085] In embodiments of the 2PP SLW systems of the present invention, the key difference between these and any known 1 PP (single-photon polymerization) systems lies in the attainable SLW writing resolutions of the new systems. Two-photon excitation needs two photons at the same time at the same location, but the probability of this is in reality somewhat limited to the small spatial volume close to the beam’s focus, which small spatial volume is typically termed a “voxel” in the context of 2PP technology. Therefore, the voxel height and / or width may typically be in the range of around 1 up to several (e.g. up to around 5 or 10 or 20) microns, in many practical embodiments of the system of the present invention.

[0086] Moving on to FIG. 2, this shows schematically the general set-up of the main components of a first embodiment of a SLW apparatus 1 according to the present invention. The SLW apparatus 1 comprises a multi-stage sequential arrangement of various optical components M1 , M2, M3, WP1 , AOM1 , WP2, AOM2, M4, M5, WP3, AOD1 , AOD2, M6, AL1 , S, M7, AL2, M8, M9 and OBJ1 - whose identities and functions will be described in further detail below - which together define an optical propagation path of a beam B of electromagnetic radiation, i.e. photons, emitted from a laser source 10 through to a substrate sample medium 100 in which the 2PP writing process is to take place. The majority of the optical propagation path components just downstream of the laser source 10 and as far as just upstream of a final magnifying lens AL2 are mounted on a table or platform 20 of granite (or other suitable ceramic or suitably dense, stability-providing material).

[0087] Actually, and as will be apparent from the description which follows below, the overall optical system of the SLW apparatus 1 comprises two distinct paths which the photon beam B, or a portion of the beam B, follows in its overall travel through the apparatus 1 : firstly there is the SLW recording path, which effects the 2PP writing in the substrate medium 100, and secondly there is a detection path, involving the use of a camera CMOS (with associated optics), for observing and monitoring the 2PP writing process as it proceeds within the substrate medium 100.

[0088] The substrate sample medium 100 is mounted on a mechanically moveable (e.g. motorized) two-axis stage / positioning table 30 e.g. of a known commercially available type, for supporting the sample medium, e.g. in a suitable container and via a vacuum fixture, and effecting movement, relative to the photon beam B, of the substrate sample 100 in which the 2PP writing process is to take place. The mechanically moveable stage 30 may be of any suitable known type that is able to effect high-speed translative two-axis movement, especially in at least two orthogonal axis directions, of the substrate sample medium 100, in order to sequentially expose different target regions or sites within the medium 100 to the photon beam B to effect two-photon polymerization sequentially at those different target regions or sites within the medium 100. The component 50 represents a calibration photodiode which may be used in a system calibration procedure, if that should be desirable or necessary in practice (as the skilled person will readily recognise and be well-versed in how to do). The stage / positioning table 30’s movement is controlled by a voltage generated by a gate array unit. The fast CMOS camera is used to monitor, set up and calibrate the apparatus. Program control of a field programmable gate array (FPGA) and the setting of the writing parameters may all be controlled by a PC running appropriate software, as is the general case with known SLW writing systems.

[0089] As the laser source 10 there may be used a femtosecond laser operating typically for example at a wavelength of 515 nm, with a laser power of over 2.3 W, a repetition frequency of 80 MHz and a pulse length of 110 fs. One suitable commercially available femtosecond laser of this type and specification is that of the FLINT series available from Light Conversion. The photon beam B emanating from the laser 10 is initially reflected by a mirror periscope formed by mirrors M1 , M2 from the lower face of the granite table 20 to its upper face. As seen more clearly in FIG. 3, that upper face of the granite table 20 has mounted stably thereon (e.g. by screws (not shown)) a mounting plate 22 of Duralumin™ (the name of a well-known range of especially strong, hard, yet lightweight alloys of aluminium and copper, usually with minor amounts of manganese and magnesium), which mounting plate 22 is provided with an array of drilled threaded holes 24 therein for the mounting thereon of the various components forming the majority part of the photon beam B’s optical propagation path.

[0090] The key deflecting and modulating components of the beam control mean or system are located on this Duralumin™ mounting plate 22. To guarantee the correct position and direction of the laser beam B as it proceeds along the initial portion of the optical propagation path, and to facilitate its directional adjustment if that should be necessary, a mirror M3 is placed immediately downstream of the mirror periscope M1 , M2, which mirror M3 has a low group velocity dispersion (GVD) and is mounted in a kinematic positioning holder (not shown). Then the laser beam B propagates through a first zero-order half-wave retardation plate WP1 for precise polarization control and impinges on a first fast-type acousto-optic modulator AOM1 , e.g. from CSRayzer. This first, fast, acousto-optic modulator AOM1 serves as a fast optical shutter for effecting a precise timing of the exposure dose of the photon beam B that the substrate sample medium 100 ultimately receives, and also for controlling the fs-wave amplitude of the laser beam B, and further also for acting as a fine compensator of the temporal dispersion fs of the laser beam B. Part of the fs energy of the laser beam B diffracted by the fast acousto-optic modulator AOM1 to the first order is used for the SLW writing operation.

[0091] Subsequently, the polarization twist angle of the propagating laser beam B is rotated by a second zero-order half-wave retardation plate WP2 in the appropriate direction, and the propagating laser beam B then falls on a second acousto-optic modulator AOM2, which of a slow regular / standard type, e.g. from AAA Opto, which serves as a main fs dispersion compensator of the laser beam B. The laser beam B is then reflected by a pair of low GVD mirrors M4, M5 onto a third zero-order half-wave retardation plate WP3. After passing through this WP3 retardation plate, the laser beam B - now with the correct polarization - falls on the first of a pair of “slow”-type acousto-optic deflectors AOD1 and AOD2, e.g. from AAA Opto, which are oriented with their reference planes at 90 degrees (i.e. perpendicularly) relative to each other. These acousto-optic deflectors AOD1 , AOD2 serve to angularly deflect the laser beam B into the required precise position for it to effect the desired 2PP writing process in or at the intended target region or site in the substrate medium 100.

[0092] The deflected laser beam B is then reflected by 90 degrees by a further low-GVD mirror M6, and then passes through a first achromatic lens AL1 . Then the beam B passes through a circular aperture S in order to filter out unwanted parasitic diffraction orders generated by the AOD1 / AOD2 combination. The transmitted beam B is then re-reflected by a further low-GVD mirror M7 housed in a Thorlabs cage system kinematic holder, which is fixed to the granite table 20 of the apparatus via a reduction to a vertical linear slide table (not shown). Then the beam B passes through a second achromatic lens AL2.

[0093] The first and second achromatic lenses AL1 , AL2 together form part of an optical lens system which prepares the photon beam B for the final portion of its propagation path towards the substrate sample medium 100 and to the detection system which allows the 2PP writing process to be observed and monitored as it proceeds within the substrate medium 100. In that lens system the deflected photon beam B is magnified by the two achromatic lenses AL1 , AL2 so that the diameter of the beam B fills the back aperture of a microscope objective OBJ1 (e.g. LIPLAN SAPO 100X Oil, from Olympus) so as to achieve a diffraction-limited focal size of the beam B. At the same time, the distance, mutual position and focal length of the two achromatic lenses AL1 and AL2 are chosen so that the first and second acousto-optic deflectors AOD1 , AOD2 lie in a conjugate plane with the back focal plane of the microscope objective OBJ1 in order to minimize the axial movement of the beam B in a given writing plane (within the substrate sample medium 100) in which the 2PP writing is to take place at any point in the overall 2PP writing procedure. This configuration allows one to achieve very fast writing by deflecting the laser beam B using the two acousto-optic deflectors AOD1 , AOD2 into the substrate sample medium 100 over a deviation range of for example at least 30 micrometers (for a 100X microscope objective OBJ1). For microscope objectives with lower magnifications, the increase in deviation range may be proportional to the magnification ratio of the microscope objective used and the microscope objective with 100X magnification. Now, the laser beam B is reflected by a further pair of low-GVD mirrors M8, M9, each of which is provided in a kinematic mount, whereby the beam B is reflected by 90 degrees each time, and the beam B is then incident on the entrance pupil of the microscope objective 0BJ1. The beam B is then focused to the working distance of the microscope objective OBJ1 on the substrate sample medium 100. Both mirrors M8, M9 are attached via a Duralumin™ adapter to a vertical linear positioning table (e.g. from Zolix) (not shown, for clarity) which is attached to the granite table 20 of the apparatus 1. The substrate sample medium 100, for example a silicon or glass body with a photo-monomer layer applied thereon, is fixed by a vacuum spacer (not shown) mounted on a high-speed two-axis scanning table (e.g. ANT95-L, from Aerotech) which constitutes the moveable stage 30 shown in FIGS. 2, 3 and 4.

[0094] The Zolix vertical positioning table is used for vertical adjustment of the possible writing planes (within the substrate sample medium 100) for substrate samples of different thicknesses and for increasing the distance between the microscope objective OBJ1 and the vacuum spacer (via which the sample 100 is fixed to the scanning table / stage 30) for the purpose of convenient substrate placement.

[0095] The SLW apparatus 1 is capable of writing onto or into substrate media with thicknesses ranging from a few micrometers (e.g. from around 1 or 5 or 10 or 20 pm) up to a few centimeters (e.g. up to around 1 or 2 or 3 or 4 or 5 or 8 or 10 cm). In order to minimize thermal fluctuations of the apparatus’s frame structure on which the various optical components defining the optical propagation path etc are mounted, the granite table or platform 20 may be mounted on an air-damped optical table 70, as shown in FIGS. 3 and 4. The entire collection of operational components of the apparatus 1 may also very desirably be enclosed within a light-proof box (not shown) for preventing premature or unwanted optical illumination of the photo-monomer in the substrate sample medium 100 during the overall SLW procedure, and for meeting the health & safety requirements for protection of personnel from laser radiation, and also for minimizing temperature fluctuations experienced by the various optical components of the apparatus 1.

[0096] As already mentioned above, the SLW apparatus 1 comprises not only the main SLW recording path that the laser beam B follows in its travel through the apparatus 1 , but there is also a detection path which a reflected portion of the beam B follows for the purpose of observing and monitoring the 2PP writing process as it proceeds within the substrate sample medium 100. For the purpose of such detection and monitoring, a CMOS (or alternatively CCD) camera (e.g. a model acA2000-50gc, from Basler) is used to display the position and shape of the focus of a reflected portion of the laser beam B emitted back from the substrate sample medium 100. That reflected beam portion is collected via the objective OBJ1 and transmitted back to the mirror M9. A part of this reflected light is not reflected by that mirror M9 but is transmitted through the mirror M9, through a third achromatic lens AL3, and into the camera CMOS. Vertical positioning of the microscope objective OBJ1 may, if desired or if necessary, be performed by virtue of the microscope objective OBJ1 being mounted on its own nano-positioning table (e.g. from MC Labs) (not shown). Alternatively or additionally, and as already mentioned, the apparatus 1 contains a vertical linear table (e.g. from Zolix) primarily for permitting vertical adjustment of the possible writing planes for substrate samples of different thicknesses and for increasing the distance between the microscope objective OBJ1 and the vacuum spacer (via which the sample 100 is fixed to the scanning table / stage 30) for the purpose of convenient substrate placement, but that vertical linear table may also be used for positioning the microscope objective OBJ1 itself with a micrometer-scale degree of accuracy.

[0097] Thus, as will be apparent from the above description, the optical propagation path of the apparatus 1 is composed of two paths: the recording one and the detection one. The recording path comprises the writing laser beam source 10, the periscope composed of mirrors M1 , M2, the reflecting mirror M3, the zero-order half-wave retardation plate WP1 , the first acousto-optic modulator AOM1 , the second zero-order half-wave retardation plate WP2, the second acousto-optic modulator AOM2, the mirrors M4, M5, the third zero-order halfwave retardation plate WP3, the two mutually perpendicular first and second acousto-optic deflectors AOD1 , AOD2, the reflecting mirror M6, the first achromatic lens AL1 , the aperture S1 , the reflecting mirror M7, the second achromatic lens AL2, the reflecting mirrors M8, M9, and finally the microscope objective OBJ1. On the other hand, the detection path comprises the microscope objective OBJ1 , the third achromatic lens AL3, and the camera CMOS. The mirror M9 has no optical function in the detection path. Indeed, an advantage here is that only a small part of the transmitted light from the substrate sample medium 100 and passing through the mirror M9 is enough for detection by the camera CMOS.

[0098] The overall control system of the apparatus 1 , by which the various optical components may be controlled to effect the desired overall SLW procedure, may comprise for example a fast FPGA card (e.g. Zynq-5000, from Xilinx) which runs software that allows for changes to be made to the various recording parameters, such as the exposure dose, number of simultaneous beam focuses, the position of the laser focus, and the positioning of the two- axis scanning table / stage 30. The software may also be used to process the relevant input datasets.

[0099] One aspect of the above description of the functions and operation of the main optical components of the apparatus 1 which may require some further explanation, for clarity, is the issue of the femtosecond laser beam’s dispersion compensation. Such dispersion compensation may be important for a successfully operable optical system arrangement. This is explained in the following paragraph:

[0100] The transmission or reflection of the laser beam B by the various optical components forming the optical propagation path results in a temporal dispersion effect, which manifests itself in an extension of the laser pulse length. This physical property of fs lasers may need to be taken into account and appropriately compensated for in order to achieve high power density in / at the focal point of the beam. Another factor that causes dispersion, both angular and temporal, is the nature of the acousto-optic deflectors AOD1 , AOD2 themselves. By a suitable combination of geometrical couplings and modulation frequencies, the angular and temporal dispersion of the fs pulse may be reduced. In our prototype set-up as described above, we do this as follows: We calculate the influence of the individual optical components and pre-compensate the laser pulse length by this value so that the first and second acousto- optic modulators AOM1 , AOM2 are turned at an angle of 45 degrees and the wave modulation direction in the crystal is opposite to the modulation vector of the pair of deflecting acousto-optic crystals in the acousto-optic deflectors AOD1 , AOD2. The distances between AOM1 and AOM2 and between AOD1 and AOD2 are just such that a pre-chirp of the fs laser pulse occurs on AOM1 and AOM2, which compensates for the chirp caused by AOD1 and AOD2 (and possibly other of the optical components) during the passage of the laser beam into the downstream focussing portion AL2, OBJ1 , etc of the apparatus.

[0101] One of the key operational features of many embodiments of the present invention, and indeed one of the key characteristics of the above-described embodiment apparatus 1 , is the ability of the apparatus 1 to effect unusually effective and high-speed SLW writing in the substrate sample medium 100. The first acousto-optic modulator AOM1 is of a fast type. Thus, the sound wave propagates through its crystal at a speed of -4200 m / s, i.e. unlike the slow and regular / standard-type second acousto-optic modulator AOM2 where the sound wave travels at -650 m / s. The combined use of such first (fast) and second (slow / regular) acousto-optic modulators AOM1 , AOM2 therefore enables both dispersion compensation and fast switching on-and-off of the laser beam B. Thus, and to clarify, the fast switching on- and-off of the laser beam B is effected by the first (fast) acousto-optic modulator AOM1 , and its control signal (i.e. its amplitude and / or frequency) is changed during the switching operation. On the other hand, the control signal of the second (slow / regular) acousto-optic modulator AOM2 does not change during the switching operation, since its working speed is not critical.

[0102] In addition to the above key feature that underpins of the above-described apparatus 1 vested in its high writing speeds in the substrate sample medium 100, a further new and useful feature of embodiments of the present invention is the ability for the system to be designed and operated with plural writing beams, for example so as to constitute a two-beam or a four-beam SLW system. FIGS. 5 and 6, respectively, show the basic set-ups of each of these species of multi-beam SLW apparatuses T, 1” according to second and third embodiments of the invention. With such plural-beam embodiments it is possible to write / record by effecting 2PP in or at different yet plural regions / sites within a given voxel of the substrate medium substantially simultaneously, or even in different yet adjacent discrete voxels simultaneously.

[0103] In such a multi-beam SLW system according to such embodiments within the scope of the invention the advantageous employment of the new form of beam control means permits the simultaneous yet separate spatial control, i.e. the simultaneous yet separate control of the deflection of, first and second independent photon beams which propagate substantially parallel to each other along at least a portion of the optical propagation path leading to the substrate medium. In this manner the practical SLW performance and utility of the system may be yet further increased as compared with a single-beam embodiment version of the system.

[0104] FIG. 5 shows schematically the main components of the second embodiment of SLW apparatus according to the invention, which is a two-beam SLW system, and employs most of the same components as in the more general first embodiment system of FIG. 2, as indicated by the general use of the same reference numerals therefor. In the case of such two-beam SLW recording, the main photon beam B is split by the first acousto-optic deflector AOD1 into two separate sub-beams B1 , B2, each such sub-beam B1 , B2 then continuing to propagate in the defined x-axis direction of the optical propagation path. In the drawing of FIG. 5 it will be noted that the sub-beams B1 , B2 are illustrated as being angled relative to one another but this is for clarity and schematic purposes only: in reality the two sub-beams B1 , B2 will be oriented substantially parallel (or almost parallel, but for practical purposes this means substantially parallel) to one another so that both sub-beams B1 , B2 propagate substantially parallel to each other in the defined x-axis direction of the optical propagation path leading to the substrate medium. The sub-beams B1 , B2 are then each further deflected by the second acousto-optic deflector AOD2 as required in the defined y-axis direction of the optical propagation path, as illustrated by the changing of the sub-beams B1 , B2’s directions B1 in, B2in — > B1out, B2out in the left-hand magnified portion of FIG. 5. The two outgoing sub-beams B1out, B2out are thus substantially parallel yet separate once they have passed through the first and second acousto-optic deflectors AOD1 , AOD2, so they continue to propagate in parallel along the optical propagation path through the remaining optical components of the system and towards the substrate sample 100 in the usual way as for the single-beam embodiment system. In the microscope objective OBJ1 both sub-beams B1out, B2out are then finally focused in the usual way as for the single-beam embodiment system - each onto / into a different yet closely adjacent site or region within a given voxel (or in neighbouring voxels) of the substrate sample medium 100, as illustrated in the right-hand magnified portion of FIG. 5.

[0105] FIG. 6 shows schematically the main components of the third embodiment of SLW apparatus according to the invention, which is a four-beam SLW system. In the case of such a four- beam system, the main photon beam is again split initially by the first acousto-optic deflector AOD1 into two separate sub-beams B1 , B2, each sub-beam B1 , B2 then continuing to propagate in the defined x-axis direction of the optical propagation path. As with the drawing of FIG. 5, in the drawing of FIG. 6 it will again be noted that the sub-beams B1 , B2 are illustrated as being angled relative to one another but again this is for clarity and schematic purposes only: again, in reality the two sub-beams B1 , B2 will be oriented substantially parallel (or almost parallel, but for practical purposes this means substantially parallel) to one another so that both sub-beams B1 , B2 propagate substantially parallel to each other in the defined x-axis direction of the optical propagation path leading to the substrate medium. However, now each of these sub-beams B1 , B2 is further split by the second acousto-optic deflector AOD2 into two sub-sub-beams (to form four sub-sub-beams B1out, B2out, B3out, B4out in total) in the defined y-axis direction of the optical propagation path, as illustrated by the transformation of the sub-beams as B1 in, B2in — > B1out, B2out, B3out, B4out in the lefthand magnified portion of FIG. 6. All four outgoing sub-beams B1out, B2out, B3out, B4out are thus substantially parallel yet separate once they have passed through the first and second acousto-optic deflectors AOD1 , AOD2, so they continue to propagate in parallel along the optical propagation path through the remaining optical components of the system and towards the substrate sample 100 in the usual way as for the single-beam embodiment system. As with the two-beam embodiment of FIG. 5, in the microscope objective OBJ1 all four sub-sub-beams B1out, B2out, B3out, B4out are then finally focused in the usual way as for the single-beam embodiment system - each onto / into a different yet closely adjacent site or region within a given voxel (or in neighbouring voxels) of the substrate sample medium 100, as illustrated in the right-hand magnified portion of FIG. 6.

[0106] The multi-beam writing that is attainable in the second, two-beam, and third, four-beam, embodiment SLW systems of the invention may be effected by any of three different approaches in the design and control of each of the first and second acousto-optic deflectors AOD1 , AOD2, namely:

[0107] (i) the nature of a synthesised control signal applied to the AOD;

[0108] (ii) a dynamic change of the control signal; or

[0109] (iii) a combination of (i) and (ii).

[0110] As regards (i) the synthesis of a control signal:

[0111] An acousto-optic deflector (AOD) device comprises as its key component a crystal, which is substantially transparent to the laser photon beam B. However, if a control signal is connected to the AOD device an acoustic wave is generated inside the AOD crystal. This wave forms an optical grating inside the crystal and as a result of passing through it the photon beam B is diffracted. Such diffraction is to + (plus) and - (minus) diffraction orders. The required diffracted beam for use is selected by shading the non-needed diffraction orders.

[0112] FIG. 7(a) illustrates a case of no electric control signal being applied to the crystal of an AOD. In such as case, when no electric control is applied, the laser photon beam is transmitted straight through the crystal, i.e. in the direction of the 0thdiffraction order.

[0113] However, FIG. 7(b) illustrates a case of a control signal f1 being applied to the crystal of the AOD. As illustrated here, an electric control signal with a frequency f1 is applied to the AOD device input. An acoustic wave travels through the crystal and regular denser and sparser grating-like areas are created within the crystal. Now, the laser photon beam travels through the crystal as if it were a volume grating with areas of a slightly different refractive index. So, the photon beam is split into several directions according to the standard diffraction equation. Next to the AOD there is a slit (aperture). Only one split beam, e.g. the +1 -diffraction order as illustrated here in FIG. 7(b), passes through an opening of the slit. Other diffraction orders, e.g. +2, +3, ... etc and 0, -1 , -2, -3, ... etc are blocked by the slit.

[0114] The diffraction angle a1 is proportional to the grating density, i.e. GD = 1 / A1 , and this is proportional to the control signal frequency f 1 , i.e. a1 ~ 1 / A 1 ~ f1. It may be noted here that, in our prototype system for example, the signal control frequency may be approx. 100 MHz (time period 10 ns). The traveling speed of the acoustic wave is 650 m / s. Thus, the spatial period of the grating is approx. 6500 nm. By way of example, for e.g. the green A the diffraction angle - arc sin A - is 532 nm, and therefore arc sin (532 / 6500) = 4.7 degrees.

[0115] An increase in the control signal frequency implies an increase of the grating density (GD), and this therefore results in a larger diffraction angle. It may be noted here that, in our prototype system for example, the range of control signal frequencies is relatively low, e.g. only a few percent of the frequency value. For example, ±1 % of the 100 MHz nominal value gives a frequency range of 99 - 101 MHz.

[0116] FIG. 7(c) illustrates a case of a control signal mixed from basic signals f1 and f2 being applied to the crystal of the AOD. As illustrated here, when the control signal is synthesized as a mixture (i.e. bundle) of two (or more) frequencies, then the laser photon beam splits simultaneously into two (or more) directions (but still in the +1 -diffraction order). By proper selection of the f1 and f2 basic signals, two separate beams can be generated. It is thus possible to control the direction of each single beam separately yet at the same time. In this way it is possible to multiply and control numbers of simultaneous SLW writing beams - and this is what we exploit in the design and operation of the two- and four-beam embodiment SLW systems within the scope of this invention.

[0117] As regards (ii) the dynamic change of the control signal:

[0118] FIG. 8 illustrates a case of dynamic change of a control signal applied to the crystal of the AOD. As illustrated here, if control frequency is changed quicker than the timing of the propagation of the acoustic wave across the AOD crystal, it is possible to obtain different grating-like areas in different parts of the AOD crystal. As a result, the laser photon beam is diffracted into different directions from different parts of the AOD crystal.

[0119] As illustrated in FIG. 8: At time t1 there is no deflection, so the laser photon beam passes directly through the AOD crystal. However, at time t2 a part of the laser photon beam is deflected (i.e. in the left half of the AOD crystal in the drawing), and part of the laser beam passes through the AOD crystal without deflection (i.e. in the right half of AOD crystal in the drawing), i.e. there are 2 beams at the crystal’s output. One can assume in this example that the laser beam illuminates the whole width of the AOD crystal. Next, there are two laser photon beams deflected from the AOD crystal in different directions at time t3 as a result of different grating-like areas in left and right portions of the AOD crystal. And lastly at time t4, the laser photon beam deflection is in the right part of the AOD crystal only, and in addition a part of the laser beam passes directly through the AOD crystal without deflection in the left part of the AOD crystal.

[0120] Thus, in this manner such a dynamic change in the synthesised control signal applied to one or more of the acousto-optic deflector(s) can be used not only for controlling the crosssection of one or more of the deflected beams, but it can also be used to control the size of the recorded / written voxel(s) together with controlling the position of the recorded / written voxel(s). For example: if changes of the control signal are with a higher frequency then the frequency used for deflecting a respective laser beam, e.g. minimally two times or more over the time during the writing of a single voxel, then a diffraction grating is generated in the respective acousto-optic modulator, which may be specific to each individual voxel being written. The laser signal is dispersed on this grating into a broader beam. Because the parameters of such a grating can be varied over time for each voxel being written, the size of each voxel being written can thus be varied over time in this manner. This may be advantageous because it thus makes it possible to vary the area writing rate over time according to the parameters of the currently written details of the structure being written.

[0121] As regards (iii) a combination of dynamic change and synthesis of the control signal:

[0122] Theoretically, it is possible to mix the synthesis of a control signal with its dynamic change of control signal, as will be readily apparent to the skilled person from the foregoing description of how to perform each aspect separately.

[0123] Throughout the description and claims of this specification, the words “comprise” and “contain” and linguistic variations of those words, for example “comprising” and “comprises”, mean “including but not limited to”, and are not intended to (and do not) exclude other moieties, additives, components, elements, integers or steps.

[0124] Throughout the description and claims of this specification, the singular encompasses the plural unless expressly stated otherwise or the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless expressly stated otherwise or the context requires otherwise.

[0125] Throughout the description and claims of this specification, features, components, elements, integers, characteristics, properties, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith or expressly stated otherwise.

[0126] Furthermore, it is expressly envisaged in this disclosure of the present invention that the various aspects, embodiments, examples, features and alternatives, and in particular the individual constructional, configurational or operational features thereof, set out in the preceding paragraphs, in the claims and / or in the following description and accompanying drawings, may be taken independently or in any combination of any number of same. For example, individual features described in connection with one particular embodiment, or described singly or in combination with another feature in any one or more embodiments, are applicable on their own or in combination with one or more other features to all embodiments and may be found and used in combination with any other feature in any given embodiment, unless expressly stated otherwise or such features are incompatible.

Claims

CLAIMS1. A stereolithographic writing apparatus for carrying out two-photon polymerization within a photo-polymerizable substrate medium, the apparatus comprising: a source of electromagnetic radiation for emitting a beam of photons suitable for effecting two-photon polymerization in the substrate medium; means for defining an optical propagation path of the photon beam as it passes from the electromagnetic radiation source towards the substrate medium; beam control means within the optical propagation path for steering and focusing the photon beam into a selected target region of the substrate medium in which the two-photon polymerization can occur; and means for moving the substrate medium relative to the photon beam so as to sequentially expose different target regions of the substrate medium to the photon beam to effect two-photon polymerization sequentially at those different target regions of the substrate medium; wherein the beam control means comprises: at least one acousto-optic deflector for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and at least one acousto-optic modulator for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path.

2. A stereolithographic writing apparatus according to claim 1 , which is constructed and operable so as to be a plural-beam SLW system in order to write by effecting 2PP in or at plural regions or sites within a given voxel or in adjacent yet discrete voxels within the substrate medium substantially simultaneously.

3. A stereolithographic writing apparatus according to claim 2, which is constructed and operable so as to be a two-beam or a four-beam SLW system.

4. A stereolithographic writing apparatus according to claim 3, which is constructed and operable so as to be a two-beam system, and wherein the beam control means, or the at least one acousto-optic deflector thereof, is constructed and operable so as to effect simultaneous yet separate spatial control of the deflection of first and second independent photon sub-beams which propagate substantially parallel to each other along at least a portion of the optical propagation path leading to the substrate medium.

5. A stereolithographic writing apparatus according to claim 4, wherein the beam control means comprises at least first and second ones of at least two acousto-optic deflectors, the first said acousto-optic deflector being constructed and operable to split a primary photon beam into two discrete sub-beams in a x-axis direction, and the second said acousto-optic deflector being constructed and operable to deflect the split sub-beams together in a y-axis direction and into a substantially parallel arrangement relative to each other and thereby into a desired final output direction for onward propagation to the substrate medium.

6. A stereolithographic writing apparatus according to claim 3, which is constructed and operable so as to be a four-beam system, and wherein the beam control means, or the at least one acousto-optic deflector thereof, is constructed and operable so as to effect simultaneous yet separate spatial control of the deflection of first, second, third and fourth independent photon sub-beams which propagate substantially parallel to each other along at least a portion of the optical propagation path leading to the substrate medium.

7. A stereolithographic writing apparatus according to claim 6, wherein the beam control means comprises at least first and second ones of at least two acousto-optic deflectors, the first said acousto-optic deflector being constructed and operable to split a primary photon beam into two discrete sub-beams in a x-axis direction, and the second said acousto-optic deflector being constructed and operable to further split each of the two sub-beams into two sub-sub-beams (i.e. to form four sub-sub-beams in total) in a y-axis direction and into a substantially parallel arrangement relative to each other and thereby into a desired final output direction for onward propagation to the substrate medium.

8. A stereolithographic writing apparatus according to any one of claims 2 to 7, wherein each of the respective splittings and / or configurations of the respective beams or sub-beams is effectable by any of the following (i), (ii) or (iii):(i) the nature of a synthesised control signal applied to one or more of the at least one acousto-optic deflectors;(ii) a dynamic change of a synthesised control signal applied to one or more of the at least one acousto-optic deflectors;(iii) a combination of (i) and (ii).

9. A stereolithographic writing apparatus according to claim 8, wherein a dynamic change in the synthesised control signal applied to one or more of the acousto-optic deflector(s) is such as to effect not only a control of a cross-section of one or more of the deflected beams, but it is also such as to effect a control of a size of the or each respectivevoxel to be recorded / written (or at least one or more of them) together with a control of the position of the or each respective voxel to be recorded / written (or at least one or more of them).

10. A stereolithographic writing apparatus according to any preceding claim, wherein the beam control means comprises: a plurality of acousto-optic deflector devices for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and a plurality of acousto-optic modulator devices for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path11. A stereolithographic writing apparatus according to any preceding claim, wherein the beam control means, including the one or more acousto-optic deflectors and the one or more acousto-optic modulators, forms part of the means for defining the optical propagation path, whereby the at least one acousto-optic deflector and the at least one acousto-optic modulator are present within the optical propagation path and the photon beam interacts with both the at least one acousto-optic deflector and the at least one acousto-optic modulator as the beam passes along the optical propagation path.

12. A stereolithographic writing apparatus according to any preceding claim, wherein the control of the photon beam by the one or more acousto-optic modulators in terms of modulating the intensity and / or frequency of the photon beam involves controlling the photon beam in terms of modulating any one or more of the following parameters of the photon beam by an acousto-optic effect:(i) the amplitude of the electromagnetic radiation forming the beam;(ii) the cross-sectional shape of the beam;(iii) the timing of the exposure dose of the radiation deliverable by the beam;(iv) the frequency (or wavelength) of the electromagnetic radiation forming the beam.

13. A stereolithographic writing apparatus according to any preceding claim, wherein at least one acousto-optic modulator, or at least one of a plurality of acousto-optic modulators, forming part of the beam control means is of a “fast” type.

14. A stereolithographic writing apparatus according to claim 13, wherein there are a plurality of acousto-optic modulators, and at least one other thereof, apart from that at least one “fast” one, is of a “slow” (or “regular” / “standard”) type.

15. A stereolithographic writing apparatus according to any preceding claim, wherein at least one acousto-optic deflector, or at least one of a plurality of acousto-optic deflectors, forming part of the beam control means is of a “slow” type.

16. A stereolithographic writing apparatus according to any preceding claim, wherein there are a plurality of acousto-optic deflectors, and at least two thereof are oriented or arranged or tilted with their respective reference planes at 90 degrees, i.e. perpendicularly, relative to each other, whereby such plural acousto-optic deflectors are able to function to deflect the photon beam independently in each of two orthogonal axis directions.

17. A stereolithographic writing apparatus according to any preceding claim, wherein the means defining the optical propagation path, or that or those portion(s) thereof apart from the at least one acousto-optic deflector and the at least one acousto-optic modulator, comprises any number and any type of optical components of either optically transmissive or optically reflective properties, so that together they form or define a general path of propagation or travel of the photon beam that is emitted by the radiation source and which passes towards the substrate sample, and wherein the said optical components of either optically transmissive or optically reflective properties include one or more mirrors for reflecting the photon beam into a desired altered direction of travel, and / or one or more retardation plates for modifying the polarization of the photon beam transmitted therethrough.

18. A method of creating an object or structure through the principle of two-photon polymerization within a photo-polymerizable substrate medium, the method comprising:(i) providing a stereolithographic writing apparatus according to any one of claims 1 to 17, and(ii) operating the apparatus so as to emit a photon beam from the electromagnetic radiation source and propagate the beam along an optical propagation path towards the substrate medium, and controlling the photon beam by steering and focusing it into a selected target region of the substrate medium such that two-photon polymerization occurs therein, and moving the substrate medium relative to the photon beam so as to sequentially expose different target regions of the substrate medium to the photon beam to effect two-photon polymerization sequentially at those different target regions of the substrate medium, wherein the controlling of the photon beam comprises spatially controlling the direction of the photon beam, as it passes along the optical propagation path, by operation of the at least one acousto-optic deflector, and modulating the intensity and / or frequencyand / or a cross-sectional parameter of the photon beam, as it passes along the optical propagation path, by operation of the at least one acousto-optic modulator.

19. A method according to claim 18, wherein the operation of one or more of the acousto- optic deflector(s) and / or acousto-optic modulator(s) includes effecting a dynamic change of a synthesised control signal applied to one or more thereof, wherein the said dynamic change in the control signal is such as to effect not only a control of a cross-section of one or more of the deflected beams, but it is also such as to effect a control of a size of the or each respective voxel to be recorded / written (or at least one or more of them) together with a control of the position of the or each respective voxel to be recorded / written (or at least one or more of them).

20. A method according to claim 18 or claim 19, which is a method of creating a three- dimensional object or structure through the principle of two-photon polymerization within a volumetric photo-polymerizable substrate medium.

21. An object or structure formed through the principle of two-photon polymerization within a photo-polymerizable substrate medium by use of the stereolithographic writing apparatus according to any one of claims 1 to 17 or by the method of any one of claims 18 to 20.

22. An object or structure according to claim 21 , which is a three-dimensional object or structure.

23. A control system for controlling a photon beam emitted by a source of electromagnetic radiation in a stereolithographic writing apparatus for carrying out two-photon polymerization within a photo-polymerizable substrate medium, the control system being provided within an optical propagation path of the photon beam as it passes from the electromagnetic radiation source towards the substrate medium, and the control system being constructed and operable for steering and focusing the photon beam into a selected target region of a substrate medium in which the two-photon polymerization can occur, wherein the control system comprises beam control means which comprise: at least one acousto-optic deflector for spatially controlling the direction of the photon beam as it passes along the optical propagation path, and at least one acousto-optic modulator for modulating the intensity and / or frequency and / or a cross-sectional parameter of the photon beam as it passes along the optical propagation path.

24. A control system according to claim 23, wherein the apparatus is a stereolithographic writing apparatus with features, including the beam control means, as defined in any one of claims 1 to 17.

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