Patent Application
20250155623
The present invention relates to nanostructured birefringent optical elements, and the use of such optical elements in differential interference contrast microscopes.
A Wollaston prism is an optical device or element that can be used to manipulate polarized light. Propagation through a Wollaston prism transforms an incident light beam into an output comprising two linearly polarized output light beams which have wavefronts at an angle to one another so that the two output beams are spatially separated with diverging propagation directions. A Wollaston prism is formed from two individual right-angled prisms or wedges of a birefringent material which are cemented together with their hypotenuse faces abutting, and with the optical axes of the two wedges orthogonal to one another. The refractive index differences provided by the orthogonal orientations of the optical axes causes the divergence of ordinary and extraordinary components of the propagating light beam, giving the two separate output beams. The angle of divergence depends upon the wedge angle. Uniaxial birefringent crystal materials such as quartz or calcite may be used for the wedges.
A related device is a Nomarski prism. This also comprises two prisms or wedges of birefringent material with orthogonal optical axes. However, in a Wollaston prism both optical axes are parallel to the input and output faces of the prism, while in a Nomarski prism the input wedge has its optical axis oriented at an angle to the input face. The effect of this is that the angled wavefronts of the two output beams cause the spatially separated output beams to converge to a focal point outside the prism, rather than to diverge.
The cutting and polishing of the wedges from bulk crystals, and subsequent cementing to create the prism can be complex and difficult, leading to a costly finished product.
An important application of Wollaston prisms and Nomarski prisms is in differential interference contrast (DIC) microscopes. This is a type of microscope that produces a monochromatic shadow-cast image of the optical path (refractive index) gradient in a transparent specimen, which enables observations such as of structure and motion in unstained living cells and isolated organelles. A more sophisticated version is the orientation-independent DIC microscope. In both microscope types, the imaging is achieved by passing a pair of spatially separated and orthogonally polarized light beams through the specimen being observed, the specimen introducing the required optical path difference between the two beams. Wollaston and/or Nomarski prisms are used to provide the two beams. A first prism is placed before the specimen, with an associated condensing lens to make the beams parallel for propagation through the specimen. An objective lens collects the beams after interaction with the specimen and converges them to a second prism where they are recombined into a single beam for observation.
Besides the cost of Wollaston and Nomarski prisms (which can be exacerbated by the requirement for a small wedge angle), a further difficulty is that prisms must be tailored to the specification of the microscope model or design for which they are intended, and cannot be exchanged between different microscopes. Also, a prism must be unmounted and cut into a particular shape to fit into a microscope. Microscope manufacturers do not provide suitable prisms with their microscopes, however. A typical delivery time of six months or more for newly-ordered prisms can therefore cause significant delays for new projects.
Accordingly, alternatives to Wollaston and Nomarski prisms are of interest.
Aspects and embodiments are set out in the appended claims.
According to a first aspect of certain embodiments described herein, there is provided a birefringent optical element for transforming an incident beam of light into two spatially separated output beams of light with orthogonal linear polarizations, comprising: a transparent substrate with an input face for receiving the incident beam, an output face, and a uniform thickness between the input face and the output face, the substrate having a non-uniform birefringence in a plane parallel to the input face, the birefringence provided by a plurality of randomly positioned nanostructures within the substrate and configured to cause the incident beam to shear within the substrate into two output beams with orthogonal linear polarisations and wavefronts at an angle to one another so that the output beams leave the output face with a spatial separation along a shear direction parallel to the output face; wherein each nanostructure has an oblate spheroidal shape with an elliptical cross-section in a plane parallel to the input face, the orientation of the elliptical cross-section giving a slow axis orientation of birefringence and a size of the oblate spheroidal shape giving a retardance value of birefringence, the orientation and the size varying between the nanostructures to provide the non-uniform birefringence of the substrate.
According to a second aspect of certain embodiments described herein, there is provided an optical assembly for transforming an incident beam of light into two spatially separated output beams of light with orthogonal linear polarizations, comprising: a first birefringent optical element according to the first aspect; a second birefringent optical element according to the first aspect; and an optical rotator sandwiched between the first birefringent optical element and the second birefringent optical element.
According to a third aspect of certain embodiments described herein, there is provided a differential interference contrast microscope comprising one or both of: a birefringent optical element according to the first aspect, and arranged to shear an illuminating beam of light in the microscope into two spatially separated beams of light with orthogonal linear polarizations for illuminating a specimen placed in the microscope; and a birefringent optical element according to the first aspect, and arranged to receive two spatially separated beams of light with orthogonal linear polarizations from the specimen by transmission or reflection, and converge the two spatially separated beams of light into a single output beam of light for observation or detection.
According to a fourth aspect of certain embodiments described herein, there is provided an orientation-independent differential interference contrast microscope comprising one or both of: an optical assembly according to the second aspect, and arranged to shear an illuminating beam of light in the microscope into two spatially separated beams of light with orthogonal linear polarizations for illuminating a specimen placed in the microscope; and an optical assembly according to the second aspect, and arranged to receive two spatially separated beams of light with orthogonal linear polarizations from the specimen by transmission or reflection, and converge the two spatially separated beams of light into a single output beam of light for observation or detection.
These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, devices and apparatus may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.
For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:
Aspects and features of certain examples and embodiments are discussed/described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed/described in detail in the interests of brevity. It will thus be appreciated that aspects and features of devices and apparatus discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.
A differential interference contrast (DIC) microscope is a beam-shearing interference system in which a reference or illuminating light beam is sheared by a small amount (generally by somewhat less than the diameter of an Airy disk) before being transmitted through a specimen under observation. By “sheared” it is meant that the beam is divided into two spatially separated beams, which in this case also have orthogonal linear polarization states owing to the conventional use of Wollaston or Nomarski prisms to produce the shear. By illuminating the specimen with the sheared beams, the microscope produces a monochromatic shadow-cast image that displays the gradient of optical paths. Those regions of the specimen where the optical paths increase along a reference direction appear brighter (or darker), while regions where the path differences decrease appear in reverse contrast. As the gradient of the optical path grows steeper, image contrast is significantly increased. Another important feature of the DIC technique is that it produces effective optical sectioning of the specimen. This is particularly marked when high numerical aperture objective lenses are used together with high numerical aperture condenser illumination.
The insert to
The DIC microscopy enabled by DIC microscopes such as the examples of
As noted in the background section, a problem affecting DIC microscopes (of both the standard and the orientation-independent types) is the suitability and availability of appropriate Wollaston and Nomarski prisms. The prisms must be designed to be compatible with a specific microscope model, of which a number are commercially provided by different manufacturers. Hence, prisms designed for a microscope made by Olympus cannot be used with microscopes made by other manufacturers, such as Zeiss, Leica and Nikon. For compatibility, a prism must have the appropriate optical performance (particularly relating to the shear angle and the bias), as well as being unmounted and cut to the required shape for the microscope design. Microscope manufacturers may not provide appropriate prisms with their microscopes, however, so the user must obtain prisms separately, which is costly and can take several months. Factors contributing to a high cost for Wollaston and Nomarski prisms are the requirement for cutting and polishing the birefringent crystal material into the required wedge shape with the appropriate wedge angle (which is typically required to be small for DIC microscopy), and subsequent stage of cementing the prisms together in an optically suitable manner.
Accordingly, it is proposed to provide an alternative to Wollaston and Nomarski prisms, for use in DIC microscopes and for other applications, that offers the same optical functionality and performance in a simpler and less costly format, and also enables straightforward tailoring of the optical characteristics to meet the requirements of specific designs of microscope and other optical devices.
To achieve this, an optical element comprising a substrate in which birefringence is provided by nanostructures formed within the material of the substrate is proposed. Procedures for “writing” the nanostructures can be readily adjusted to create nanostructures that give a profile of varying birefringence across the substrate which mimics the beam-shearing function of Wollaston and Nomarski prisms and allows the shearing angle and bias to be set as required for any application. The optical element simulates the optical action of the prisms by similarly transforming (splitting, dividing or shearing) a single input beam into two output beams. The varying birefringence achieved by appropriate shaping of the nanostructures corresponds to the varying birefringence produced by the sloped boundary between the wedges in a prism. The substrate is a single piece of transparent optical material of uniform thickness, so the manufacturing precision required for cutting, polishing and cementing the wedges of Wollaston and Nomarski prisms is avoided. Also, the requirement for optical crystals which are naturally and inherently birefringent is negated, allowing costly crystalline materials such as quartz and calcite to be replaced with inexpensive silica-based glasses.
The nanostructures and techniques for forming the nanostructures will be described in more detail later. First, we look at the principles of operation of Wollaston and Nomarski prisms to understand the optical properties required to be duplicated by the proposed birefringent optical element, particularly with reference to use in a DIC microscope.
As explained above,
In a DIC microscope, the shear amount or shear distance d is then set by passing the diverging beams through a condenser lens (see
In order to substitute a particular conventional Wollaston or Nomarski prism with the proposed birefringent optical element, it is necessary to determine the shear angle of a prism in order to replicate this property in the optical element. The derivative of retardance with respect to the coordinate along the shear direction has been measured for a number of DIC microscope prisms in order to yield the shear angle in accordance with the geometry in
The birefringent optical elements proposed herein to replace Wollaston and Nomarski prisms are a simple planar substrate in which the birefringence is non-uniform and varies across the plane of the substrate, where in use the optical element is oriented with this plane orthogonal to the propagation direction of an incident optical plane wave. In terms of the DIC microscopes already described, the plane of the substrate therefore lies in the x-y plane, with the substrate having a thickness along the propagation direction z. The required shearing effect can be produced with a profile of varying birefringence. The profile is made up of the value of birefringence along the z-direction where different x-y locations exhibit different birefringence values (the variation is only present along one of these directions, giving the shear direction, with no variation along the other direction). Hence, for an incident beam with a finite beam cross-sectional area, different parts of the beam across its transverse profile (in the x-y plane) experience different birefringences. The birefringence at any location comprises two components: the retardance amount and the orientation (azimuth) of the slow optical axis direction (as is usual for birefringence). Various birefringence profiles can provide the required beam shearing, with the shear amount (angular separation of the two output beams) and the bias (optical path difference between the two output beams) which are achieved being selectable by tailoring the retardance and slow axis orientation across the profile. Two examples will be discussed in detail.
A first example has a birefringence profile made up of a variable retardance and a non-varying (fixed) slow axis direction. A second example has the converse arrangement, with a non-varying (fixed) retardance and a variable slow axis direction. More complex configurations in which both the retardance and the slow axis direction vary are also envisaged. However, the process for forming the nanostructures (described further below) can be simplified if only one component of the birefringence is variable, so this may be preferred. Also, in the examples, the birefringence profile is one-dimensional, such that in the plane of the substrate it varies along one direction (the x direction as described, corresponding to the shearing direction) and is constant along the other direction (the y direction as described). This corresponds with a beam shearing prism, owing to the geometry of the wedge boundary. However, it is also possible to vary the profile along both the x and y directions to achieve beam shearing.
Assume that the substrate of the optical element has a birefringence profile defined within a field in the x-y plane with dimensions X mm by Y mm. A square field is practical given that the incident beam will typically have a circular cross-section. Hence, the birefringence profile exists in the area 0 mm<x<X mm, 0 mm<y<Y mm where X and Y are equal for a square field. A retardance derivative of a is required along the x direction (so the shear direction is along the x direction), to give an absolute retardance value Δ at any x position. Along the x direction, the variable retardance is chosen to have a zero value at the centre of the field (x=X/2), increasing to a maximum positive value at one edge of the field (x=X) and a maximum negative value at the opposite edge of the field (x=0), the maximum values being equal. In the half of the field with a positive retardance, the slow axis orientation q is fixed and chosen to be parallel to the x direction. In the other half of the field, with a negative retardance, the slow axis orientation is also fixed but orthogonal to the slow axis orientation in the positive half, so is parallel to the y direction. The orthogonal slow axis orientations may be reversed, however, or set at an angle to the x and y directions.
Accordingly, if we select the x direction as horizontal and the y direction as vertical, the birefringence profile (along the x direction) can be mathematically described as:
Consider a specific example. The birefringence field is set to be 10 mm by 10 mm, so can be described by 0 mm<x<10 mm, 0 mm<y<10 mm. The retardance derivative α is selected to be 15 nm/mm, to correspond with the actual Olympus U-DIC40HR prism mentioned above. Hence, each half of the field in the x-direction has a width of 5 mm, giving a maximum retardance at the edges of the field of ±75 nm. The birefringence profile is therefore given by:
In order to assess the performance of the experimental optical element of the first example it was used in a microscope configured for DIC microscopy to obtain images of a specimen, which were compared with images of the same specimen taken using the same microscope equipped with a conventional shearing prism. The microscope was a research microscope, Olympus BX61, equipped with a 40× objective lens, a high resolution DIC slider U-DICTHR, a bandpass filter 576/10 nm, and a monochromatic CCD camera, Lumenera Infinity 3-1M to record the images. In the illumination path (so, the prism position between the light source and the specimen-see
The specimen comprised short segments of glass rod, used as spacers in liquid crystal cells, embedded in Fisher Permount Mounting Medium (from Fisher Scientific Company LLC). This was selected as being optically similar to transparent filaments in a cell structure, a typical subject of observation via DIC microscopy. The refractive indices of the glass rods and the Permount at a wavelength of 546 nm were measured using a Jamin-Lebedeff microscope (from Zeiss), as being 1.554 and 1.524 respectively. A drop of the suspension was placed between a microscope slide and a coverslip, and the resulting specimen was placed on the rotatable stage of the microscope, as in
In this example, the birefringence profile of the birefringent field of the optical element has a non-varying (fixed) retardance and a variable slow axis orientation. Again, assume that the substrate of the optical element has a birefringence profile defined within a field in the x-y plane with dimensions X mm by Y mm, so that the profile is present in the area 0 mm<x<X mm, 0 mm<y<Y mm where X and Y are equal for a square field. In order to give a shear direction along with x direction, the slow axis orientation or azimuth φ is varied along the x direction. The variation is uniform in that the value of the slow axis azimuth has a constant gradient across the field; hence the value of the slow axis azimuth is proportional to the value of x. The retardance Δ is fixed at a constant value along the x direction, so there is the same amount of retardance at all x positions. In this example, a retardance value of half the wavelength λ of the intended incident light beam is selected; other values might be used, however.
Accordingly, if we select the x direction as horizontal and the y direction as vertical, the birefringence profile (along the x direction) can be mathematically described as:
(where β is the gradient magnitude of rotation of the slow axis).
This design gives an optical element in which the evolution of the propagating wavefront though the substrate follows the geometric phase principle.
Consider a specific example. The birefringence field is set to be 10 mm by 10 mm, so can be described by 0 mm<x<10 mm, 0 mm<y<10 mm. The slow axis gradient magnitude of rotation β is chosen to be 5.5°/mm. The wavelength λ is selected as 560 nm, so that the retardance Δ is half of this value, 280 nm.
If an optical element with the birefringence profile is illuminated by a linearly polarised beam with a vertical polarisation, the azimuth χ of the polarisation plane of the output beam is given by χ=2βx (so, twice the slow axis azimuth as noted above). However, the rotation of the linear polarisation by the angle χ can be described as a phase shift between two orthogonal circular (left and right) eigenwaves ψ, so that ψ=2χ=4βx. Then, the derivative of the phase shift between the left and right circular eigenwaves is given by dψ/dx=4β. Accordingly, the optical path difference or retardance derivative ε can be described by ε=(λ/360° dψ/dx=λ(β/90°. (As an aside, it is mentioned that the optical element of the second example, which can be described as a geometric phase optical device, operates similarly to a Fresnel triprism, which is a further example of a prism with beam shearing capability, but uses a different principle of transformation of the beam polarisation).
Recall that the retardance derivative corresponds to the shear angle. In the present example, λ=560 nm and β=5.5°/mm, so the shear angle introduced between two orthogonal circularly polarised beams is 34 μrad (2 millidegree). This is similar to the shear angle of the Olympus DIC prism U-DIC20HR, which as already noted is 30 μrad. Hence, it is feasible to compare experimental optical element with the Olympus prism to assess its performance.
Accordingly, a similar experiment was performed as with the experimental optical element of the first example, by using the experimental optical element of the second example in a microscope configured for DIC microscopy to obtain images of a specimen, and comparing these with images of the same specimen taken using the same microscope equipped with a conventional shearing prism. The microscope was again an Olympus BX61 microscope, this time equipped with a 20× objective lens, a high resolution DIC slider U-DICTHR, a bandpass filter 576/10 nm, and a monochromatic CCD camera, Lumenera Infinity 3-1M to record the images. In the illumination path (so, the prism position between the light source and the specimen-see
Accordingly, DIC microscopes arranged as shown in the example of
Additionally, birefringent optical elements as described herein may be used in orientation-independent (OI-) DIC microscopes [1, 2]. These have a largely similar structure as the non-Ol versions of DIC microscopes, but an important difference for enabling the orientation independence is that the or each beam shearing prism is replaced with a beam shearing assembly that comprises a pair of beam shearing prisms (Wollaston or Nomarski) with their optical axes arranged orthogonally, and a 90° optical rotator sandwiched between them. The optical rotator may be a liquid crystal device, for example. These prisms may be replaced with birefringent optical elements as described herein (according to the first or second detailed examples, for instance), to provide a beam shearing assembly comprising a pair of birefringent optical elements with a 90° optical rotator sandwiched between them.
Hence, birefringent optical elements as proposed herein can be successfully used in both orientation-independent and non-orientation independent DIC microscopes. They are expected to be less expensive than conventional beam-shearing prisms, and owing to the fabrication methodology with is described further below, can be readily customised on an individual basis for each particular model or configuration of microscope (including commercially available DIC microscopes for which it is currently costly to obtain suitable prisms). Such customised optical elements can be more precise, can be configured to compensate for manufacturing or alignment errors in a microscope, and can provide better image contrast. Furthermore, the optical elements exhibit a negative birefringence with a maximum retardance that may be as low as about 200 nm. This is in contrast with conventional quartz beam-shearing prisms that have a positive birefringence and a much larger retardance of about 5000 nm. The negative birefringence and the smaller retardance amount will enable an increase in the field of view of DIC microscopes.
The birefringence of the optical elements described herein is provided by a nanostructure created within the optical element. The nanostructure comprises a collection of nanopores in a periodic or random distribution in an optically transparent material, typically silica glass. The nanopores are nanometre-scale structural modifications or changes in the bulk material that alter its refractive index and provide a negative birefringence. Although not yet well-understood, the nanopores are considered to be voids within the bulk material. The nanopores have a shape and orientation which confers the birefringent properties to the bulk material. They are created or “written” by the action of ultrashort (femtosecond) pulses of focused laser light directed into the substrate. The optical characteristics of the pulses determine the shape and orientation of the nanopores. Hence, the birefringent properties can be tailored by modifying the pulse characteristics, enabling a birefringent profile that emulates the beam-shearing capabilities of a Wollaston or Nomarski prism to be formed within the substrate by appropriate control of the laser pulses as they are applied to the substrate.
The amount of optical energy delivered to the substrate (determined by factors including the power, duration and repetition rate of the laser pulses) determines a type of the nanostructure which is formed. The pulses can provide sufficient accumulated heat and lattice thermalisation to induce permanent material modification in the substrate. One type of nanostructure, referred to as type II [3], comprises nanogratings in which nanopores are self-organised into periodic distributions that act as optical gratings, providing an anisotropic refractive index pattern and hence birefringence. There is a strong dependence of the anisotropy on the polarisation of the laser pulses, so the induced birefringence can be tailored into a required profile.
A more useful type of nanostructure for present purposes can have a reduced optical propagation loss compared to type II structures and has been designated as type X [3]. Type X nanostructures can be written using a reduced density of applied optical pulses than is required to produce the self-assembled periodicity of type II nanostructures. They show a relatively low birefringence, around four times less than the birefringence of type II nanogratings, but by appropriate beam shaping of the laser pulses, the nanostructures can be written with a relatively long length in the intended optical propagation direction through the optical element. Lengths of the order of 50 μm or more, for example up to about 100 μm, can be written. This length of nanostructure, lying along the light propagation direction, compensates for the low birefringence, since the required parameter for birefringent operation is phase retardance, defined as the product of birefringence and optical path length. The retardance parameter can be readily varied by adjustment of the lens to control the nanopore length, and adjustment of laser energy and pulse quantity to control the density of nanopores and the aspect ratio and volume of the individual nanopores.
A type X birefringent structural modification comprises randomly distributed individual nanopores or nanostructures. However, nanostructural periodicity is not required to provide the desired birefringence, which instead depends on the orientation of the individual structures within the bulk material. Hence, the absence of self-assembly is not a barrier to the production of high quality optical elements. Also, the type X anisotropy is controlled by the polarisation of the laser writing beam. Each nanopore has an anisotropy defined by its shape, which is an oblate spheroid (ellipsoid) shape, or lenticular shape. As noted, the nanopores are randomly spaced apart within the substrate material, although lying generally within a layer at a constant depth behind the optical input surface of the optical element. The oblate spheroid shape of each nanopore is oriented with the plane of its circular cross section parallel to the optical propagation direction through the optical element and perpendicular to the input surface of the optical element. The elliptical or oval cross-section which is parallel to the input surface can be oriented with its major axis at any angle, where the major axis is formed perpendicular to the polarisation of the writing light beam. The minor axis is parallel to the polarisation of the writing light beam. Since the overall shape is lenticular, the extent of the nanostructure in the optical propagation direction, namely the length along the direction through the thickness of the optical element from the input surface to the output surface, may be the same as or similar to the major axis. The overall shape of the nanostructure is determined by the intensity distribution of the laser pulses near the focal point.
For both type II structures and type X structures, it is typical to apply a multiplicity of ultrashort laser pulses to the substrate material, in order to create a consistent and uniform birefringence. For example, about 10 or 50 or 100 pulses or more might be used. The laser light of the writing beam is polarised, where a defined polarisation direction is required in order to shape the nanopores into an oblate spheroidal shape to achieve the anisotropy that produces the desired birefringence. The polarised writing light has an anisotropic electric field distribution which is enhanced at the equators of the initially spherical nanopores induced in the substrate material by the laser pulses and causes the spheres to grow into oblate shapes with a longer dimension oriented perpendicular to the polarisation direction of the laser light. Accordingly, writing nanopores of a particular birefringence into a substrate can be achieved using ultrashort laser pulses with a polarisation direction or orientation which is selected to define the slow axis orientation of the birefringence, and an amount of optical energy which is selected to define the level of retardance of the birefringence.
Nanostructure writing can be carried out using linearly polarised light to define the birefringence slow axis orientation. In order to provide a full range of orientations, the polarisation orientation needs to be variable between 0° to 180°, which is set using a variable retardance polarising element to produce the linear polarisation state, such as an electro-optic modulator. Alternatively, elliptically polarised light can be used to write nanostructures into substrate materials, it having been found that the orientation of an elliptical polarisation acts in the same way as the orientation of linear polarisation to define the slow axis orientation of the birefringent nanostructure. Hence, elliptically polarised light can straightforwardly replace linearly polarised light. Additionally, the retardance of the birefringent nanostructure also depends on the ellipticity of elliptically polarised light, so this property can be used to control the retardance when writing the nanostructure. This arrangement can avoid the need to adjust the amount of optical power in order to achieve a particular retardance. A constant pulse power (and writing speed, and pulse repetition rate, and other factors that determine the total amount of power delivered) can be used throughout the fabrication of a complete variable birefringence profile, and only the polarisation characteristics need be varied.
Additional information regarding birefringent nanostructures and methods of writing them into a substrate can be found in WO 2019/158910 [3], WO 2020/109767 [4] and WO 2020/109768 [5].
An attractive feature of replacing Wollaston and Nomarski prisms with birefringent optical elements described herein is the avoidance of costly optically birefringent crystal materials such as quartz and calcite which are used to make such prisms. The bulk material of the substrate used to form the optical element is a transparent material (having a significant transmission for at least some wavelengths across the spectrum from ultraviolet to the near-infrared (roughly 200 nm to 2500 nm) having a high transparency at the wavelength of the writing laser pulses, and also at the wavelength of the illumination beam in the microscope for which it is intended). Usefully, the material may be an amorphous glass material. For example, it may be silica (silicon dioxide, SiO2), including fused silica. The silica or other glass material might be doped with other materials to modify its optical properties (which may be interest for particular applications). Examples of doped or multicomponent glasses may include materials such as Al2O3, B2O3, alkaline earth oxides and Na2/K2O but other elements and compounds may be used; the disclosure is not limited in this regard. Other materials for the optical element may be any material able to support the laser induced nanostructures, including materials in which nanogratings such as a type II modification or a type X modification have previously been demonstrated. These include multicomponent glasses, GeO2 glass, porous glass, aerogel glass, silicon and silicon materials, semiconductor materials, lithium niobate and other lithium oxide compounds. Other materials are not excluded, however. The nanostructures are formed so as to be embedded within the volume of the substrate material of the optical element. They can be formed in a single layer, with a thickness of the layer in a range of about 50 μm to about 500 μm providing a suitable birefringent effect, although other layer thicknesses are not excluded.
The time taken to write the birefringent nanostructure into the substrate is proportional to the amount of retardance which is produced. As an example, the average retardance of a 10 mm by 10 mm square birefringent field in an optical element according to the first example might be 37.5 nm, whereas an optical element according to the second example might have a retardance of 280 nm. Hence, the writing time for a first example optical element can be at least five times less than for a second example optical element, so in some cases might be a preferred configuration. However, where a larger birefringent field size or a larger average retardance is required, a second example optical element may be preferred. DIC microscopy with a high resolution in the final image requires a small shear distance, for which a first example optical element will typically be a better choice. On the other hand, DIC microscopy with a high contrast in the final image requires a large shear distance, and therefore a second example optical element may be a better choice here.
The birefringent optical elements described herein simulate the operation of beam-shearing prisms such as Wollaston prisms, Nomarski prisms and Fresnel triprisms. Consequently, their application is not limited to use in DIC microscopes, and they may additionally be used to replace conventional beam-shearing prisms in other optical apparatus. Examples include a digital holographic interferometer [6] and shearing interferometers [7]. Also, they may be used to produce a high precision zero order Soleil-Babinet compensator. The optical elements described are effectively zero order birefringent wedges which can be oriented in opposite directions and used in place of quartz wedges in a Soleil-Babinet compensator with any need for an additional compensation birefringent plate. This modification can provide improved precision and a much wider angle field of view.
The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.
This invention was made with United States government support under grant number GM101701 awarded by the National Institutes of Health. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2023/050230 | 2/2/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63306523 | Feb 2022 | US |
