The invention relates to the field of photonic device manufacturing. More particularly, the present invention relates to the field of trimming of one or more photonic components, photonic devices or photonic circuits.
Optical filter structures integrated on a chip are already being used in optical communication networks (e.g. wavelength division multiplexing). The requirements for these devices are often very stringent as very narrow wavelength bands need to be filtered out of a broad spectrum. Silicon-on-insulator (SOI) is a widely used material system for integrated optics as it allows mass production of on-chip devices. The waveguides typically are defined with deep-UV lithography. At present the wavelength used is 193 nm or smaller and very detailed fabrication is possible. However, the characteristics of optical filters can still be unpredictable in a less than perfectly fabricated device. It is therefore still impossible to guarantee that the designed specifications will be met. For devices like optical filters with a very narrow bandwith it may be necessary to adjust them after fabrication so that they operate according to the desired specifications. This process is called trimming. The most straightforward method to do this is to incorporate a resistive heater on the devices, as described by Dong et al. in Optics Express 18 (2010) 20298, so they can be tuned thermally. This is however a power consuming technique as it requires a constant current supply. Another method that has been researched in SOI is compacting the oxide layer around the waveguide as described by Schrauwen et al. US 2011/0013874. The effective index of the waveguide mode is then altered due to strain. This method is expensive, slow and difficult and cannot be used in mass production. Yet another method uses UV-sensitive PMMA as a top cladding on slotted ring resonators, as described by Zhou et al in Photonic technology Letters 21 (2009) 1175. The trimming here is done with UV illumination. The refractive index variation in the cladding is only very small and the trimming range is limited, even with slot waveguides.
It is an object of embodiments of the present invention to provide good methods and systems for trimming photonic devices and photonic devices thus obtained. It is an advantage of embodiments according to the present invention that a relative large range for trimming photonic devices can be obtained, such as for example inducing a wavelength shift of 30 to 35 nm for the photonic component, allowing not only compensating for manufacturing errors but also allowing the fabrication of standard components to which a large change can be applied using trimming to bring them into particular specs. In other words, it is an advantage of embodiments of the present invention that trimming can be used for generating custom-made photonic components based on standard components.
It is an advantage of at least some embodiments according to the present invention that the trimming can be evaluated using properties expressing the functionality of photonic devices, thus allowing correction for all imperfections having an effect on the functionality of the photonic devices.
It is an advantage of embodiments according to the present invention that a permanent solution for trimming components is provided, so that after an initial trimming process no power is further required to maintain the trimmed state.
It is an advantage of embodiments according to the present invention that, for trimming a plurality of chips on a device, at least part of the steps of the method for trimming can be done in batch, thus resulting in methods being more efficient than methods where individual trimming of components is required. It is an advantage of embodiments according to the present invention that an efficient method for trimming can be obtained, e.g. less time consuming than e-beam.
The above objective is accomplished by a method and device according to the present invention.
The present invention relates to a method for trimming at least one photonic device, the method comprising obtaining one or more photonic devices comprising at least one component supporting propagation of electromagnetic radiation and a covering layer comprising a polymerisable liquid crystal, determining, for a selected photonic device selected from the one or more photonic devices, a selected liquid crystal orienting condition to be applied to the covering layer comprising the polymerisable liquid crystal resulting in a preferred value for an electromagnetic property of the selected photonic device, and while applying the liquid crystal orienting condition, polymerizing the polymerisable liquid crystal covering layer of said selected photonic device, thus obtaining a polymerized liquid crystal on said selected photonic device.
The present invention also relates to a trimmed photonic device comprising at least one component for supporting propagation of radiation and a polymerized liquid crystal covering layer on top of the component, the polymerized liquid crystal covering layer being polymerized in a state adapting the effective refractive index of the at least one component.
The present invention further relates to a system for obtaining a trimmed photonic device, the system comprising at least one component for supporting propagation of radiation and a polymerisable liquid crystal cladding layer, and a liquid crystal orienting condition application means being positioned for inducing a liquid crystal orienting condition in the polymerisable liquid crystal cladding layer.
The present invention also relates to the use of a system as described above for trimming a photonic device.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.
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The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.
Any reference signs in the claims shall not be construed as limiting the scope.
In the different drawings, the same reference signs refer to the same or analogous elements.
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.
It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.
Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.
Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.
In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
Where in embodiments of the present invention reference is made to trimming, reference is made to the fact that properties of a photonic device or component are adapted towards preferred values, e.g. towards a set of specs, to reach a desired functionality of the photonic device. Such properties may be electromagnetic properties of the device, such as for example optical or microwave properties. Trimming thereby also may be referred to as fixed or static tuning or adapting, as after the trimming process, the properties are fixed or static. The specs thereby can initially be not reached due to manufacturing errors or variability on manufacturing. Alternatively, the specs may also be defined after mass processing whereby trimming to a customer determined specification is performed on particular devices or components.
Where in embodiments of the present invention reference is made to a covering layer comprising a polymerisable liquid crystal, reference may be made to a covering layer comprising a mixture of liquid crystal materials containing a significant amount of polymerisable mesogens.
In one aspect the present invention relates to a photonic device comprising a covering layer comprising a polymerized liquid crystal. The trimmed photonic device 100, an example according to an embodiment of the present invention being indicated in
In one aspect, the present invention relates to a method for trimming one or more photonic components. Photonic components that typically can benefit from a method and system for trimming according to embodiments of the present invention may be photonic components wherein radiation propagation is influenced by a covering layer. The latter may for example be a cladding material or an electromagnetic radiation guiding layer, such as for example in slotted waveguides or microwave applications. Such photonic components may for example be waveguide based photonic components. Such photonic components may for example be optical filters, optical resonators, optical couplers, etc. Such photonic components may for example be photonic components wherein the refractive index of one or more components is deterministic for operation of the component. A method according to embodiments of the present invention comprises obtaining one or more photonic devices comprising at least one component supporting propagation of electromagnetic radiation and a covering layer comprising a polymerisable liquid crystal. The covering layer may in particular examples be a cladding layer. It furthermore comprises determining, for a selected photonic device selected from the one or more photonic devices, a selected liquid crystal orienting condition to be applied to the polymerisable liquid crystal in the covering layer resulting in a preferred value for an electromagnetic property of the selected photonic device. It also comprises, while applying the liquid crystal orienting condition, polymerizing the polymerisable liquid crystal in the covering layer of said selected photonic device, thus obtaining a polymerized liquid crystal in said selected photonic device.
Further features and advantages of embodiments of the present invention will be illustrated below with reference to an exemplary method for trimming and with reference to
A first step 210 of the exemplary method comprises obtaining a photonic device comprising a covering layer comprising a polymerisable liquid crystal. The covering layer may be a cladding layer and the cladding layer may be part of, or on top of a waveguide. The polymerisable liquid crystal layer may be any type of polymerisable liquid crystal layer as described above. In the exemplary method trimming of silicon-on-insulator waveguide based photonic components is performed, but as already indicated, it will be clear that the method is not restricted by the particular type of photonic device used, or by the particular materials used. The effective index of the electromagnetic radiation guiding portion of the device typically is determined by the interaction of the electromagnetic radiation with the materials in which it propagates. As the propagating modes typically also have evanescent tails in cladding materials, these layers will contribute to the electromagnetic radiation behavior in the device. In embodiments of the present invention wherein for example an optical radiation property is influenced, this effect is used for adjusting the effective refractive index. In the present example, as the mode in the SOI waveguides has evanescent tails extending into the cladding layers, the cladding layers contribute to the effective index. In the present example, different cladding layers are present, one of these being a polymerisable liquid crystal cladding layer. The refractive index of the liquid crystal is determined by the interaction of the electric field components of the light with the relative dielectric constants of the LC. While the devices are designed for TE-polarized light, the mode has nonzero y- and z-components due to the small dimensions. The transverse x-component of the electric field is the strongest component in the Si, but near the sidewalls of the waveguide the longitudinal z-component is very strong. The y component is generally very small and we will not take it into account here. In the cells the liquid crystal director has an orientation parallel to the propagation direction of the light in the absence of an electric field, i.e. without applying an electric field, as shown in
A second step 220 of the exemplary method comprises applying a liquid crystal orienting condition, such as for example an electric field or temperature condition or magnetic field condition or a combination thereof, to the covering layer of polymerisable liquid crystal in the device. Applying such a liquid crystal orienting condition, e.g. electric field, may be applying a liquid crystal orienting condition in a direction so that the liquid crystal is responsive thereto, such as for example when applying an electric field typically in a direction perpendicular to the substrate or the polymerisable liquid crystal layer. Applying such a liquid crystal orienting condition may comprise applying subsequently different liquid crystal orienting conditions. For example, in some embodiments, this may comprise applying subsequently electric fields with different strengths, although embodiments of the present invention are not limited thereto. Applying an electric field perpendicular to the substrate reorients the director vertically, as illustrated in
A third step 230 of the exemplary method comprises determining a selected liquid crystal orienting condition, e.g. an electric field, for which a preferred value for the properties of the optical device resulting in a desired functionality of the optical device is obtained. In some embodiments, the applying step 220 also can considered being part of step 230. The latter may be performed by scanning a range of conditions, e.g. electric fields strengths, and by simultaneously measuring the electromagnetic parameter or parameters of the photonic device to be evaluated so that an optimum value can be selected from the obtained values for the parameter(s), or altering the electric field strength until an appropriate value is obtained, etc. The latter typically requires in situ measurement of the parameter. The electromagnetic parameter or parameters advantageously may be representative for part or all of the functionality of the optical device wherein the photonic component is used, the present step thus allowing to provide a feedback for the trimming based on the functionality of the optical device.
In a fourth step 240, when the preferred parameter value for the photonic component or the device using the component is determined, the polymerisable liquid crystal is fixated using polymerization e.g. by illumination with UV light. The latter has as an effect that the orientation of the director in the liquid crystal is fixed, thus fixing the refractive index and thus fixing the parameter value of the component or the device using the component. During this step the polymerisable liquid crystal becomes a polymerized liquid crystal. Illumination during application of the liquid crystal orienting condition over the polymerisable liquid crystal is illustrated in
In a fifth step 250, application of the liquid crystal orienting condition, e.g. applying an electric field, is ended. As the polymerization of the liquid crystal has resulted in a static adjustment of the photonic device by freezing the state of the liquid crystal, as indicated in
In some embodiments of the present invention, determining a selected liquid crystal orienting condition and polymerization may be performed on an individual photonic device or photonic integrated circuit. Typically an electric field strength then is applied to the cladding layer of the individual photonic device and the cladding layer is polymerized. An advantage of such an approach is that it is typically far less critical how focused the illumination of the polymerisable cladding layer is, as typically no other radiation sensitive layers are present.
In some embodiments of the present invention, determining a selected liquid crystal orienting condition and polymerization may be performed at least partly on a plurality of photonic devices or photonic integrated circuits. In one embodiment, the application of the condition can be local and specified for each photonic device or photonic integrated circuit separately but in a simultaneous way, i.e. using local condition application means, such as for example a patterned conductive layer allowing to induce different electric field strengths for different photonic devices. If for some or each selected photonic device the appropriate condition, polymerization can be performed simultaneously for these photonic device. If for some photonic devices in the group, the condition cannot be obtained simultaneously, such devices can be shielded from polymerization during polymerization of the other devices.
In cases wherein a plurality of photonic devices is to be trimmed, embodiments of the present invention also may be adapted for applying a liquid crystal orienting condition to the full group of photonic devices, although the condition is only optimum for one of these devices, and locally polymerizing that device, e.g. by focused irradiation and optionally masking.
In one aspect the present invention also relates to a system for obtaining a trimmed photonic device. The system typically comprises at least one component for supporting propagation of electromagnetic radiation and a covering layer comprising a polymerisable liquid crystal. The system typically also comprises a liquid crystal orienting condition application means being position for inducing a liquid crystal orienting condition in the polymerisable liquid crystal. The liquid crystal orienting condition application means may for example be an electric field generator comprising a conductive layer on top of the polymerisable liquid crystal, e.g. in the form of a conductive layer on a substrate like a glass substrate. The conductive layer may be spaced from the optical component using spacers, and the polymerisable liquid crystal may be provided in between the optical component and the conductive layer. An additional contacting layer for providing contact between the conductive layer and the polymerisable liquid crystal also may be provided. The system alternatively may comprise a non contact electric field providing means. The electric field application means is selected transparent for UV radiation, if the latter is used for polymerization. The system also may comprise a polymerization assisting means for polymerization of the polymerisable liquid crystal. Such a system may for example be a UV irradiation system. Further features of the system may correspond with features providing the functionality of the method embodiments as described above.
By way of illustration, embodiments of the present invention not being limited thereto, results for a number of experiments on ring resonators are discussed below, illustrating features and advantages of some embodiments. In the experiment below, the polymerizable liquid crystal (PLC) used is a a combination of three types of reactive mesogens (13.2% RM23, 22.1% RM82 and 53% RM257, all from Merck), a non-reactive liquid crystal (8.8% 5CB), an initiator (0.3% irgacure from Ciba) and an inhibitor (2.6% t-butylhydroquinone). The initiator enables polymerization by UV illumination. The inhibitor avoids chemical reactions with the environment. A small amount of non-reactive liquid crystal was added to obtain nematic phase at room temperature.
The optical properties of the PLC were determined with spectrometry. It was found that the ordinary refractive no index of the material increases from 1.55 to 1.65 for wavelengths from 400 nm to 700 nm. The extraodinary refractive index ne changes from 1.75 to 1.95 in this region. The birefringence was found between 0.24 and 0.28 for wavelengths between 400 nm and 700 nm. When a voltage was applied over the PLC, the molecules reorient themselves along the electric field, causing a decrease in birefringence. A low-frequency AC voltage was applied to prevent drift of ions in the LC. The material in each cell was polymerized under a different voltage by UV illumination. Polymerization caused a small decrease in Δn (<5%), but the orientation of the mesogens is preserved for the most part. When the voltage was removed after polymerization, the birefringence did not change anymore. The molecules were frozen into their reoriented state. The calculated values of Δn at λ=750 nm for five samples are given in
As indicated above, the experimental results obtained in the present example are based on a silicon-on-insulator chip whereby ring resonators are used, the ring resonators being the subject of the trimming. The silicon-on-insulator chip consists of a Si substrate, a 2 μm thick SiO2 layer and a 220 nm thick monocrystalline Si layer in which the waveguides and the ring resonators are defined. The SiO2 layer acts as an optical insulation layer in order to prevent leakage losses from the waveguides to the substrate. The waveguide dimensions can be very due to the high confinement factor of the material system. The waveguide width in the present example is 450 nm and the height is 220 nm. Bend radii of only a few μm are possible. In our experiments, the rings have a 6 μm radius. With UV-curable glue we attach a glass plate on top of the chip. Silica spacers with a radius of 3.4 μm control the spacing. The device is then heated on a hotplate together with the PLC. The PLC in its isotropic state is deposited near the gap between the chip and the glass. Capillary forces then cause the gap to fill with PLC. Finally, the device is cooled gradually to avoid the formation of domains. At room temperature the PLC is in its nematic state. Prior to assembly the glass plate was spin-coated with an alignment layer. In the experiments discussed here polyimide (PI) was used to form the alignment layer. After spin-coating and baking, the alignment layer was rubbed with a cloth. When LC comes into contact with the rubbed layer, the director will orient itself along the rubbing direction. In this way we can control the initial orientation of the director. The structure used for trimming, is illustrated in
In the following experiments are discussed illustrating features of the trimming process. For optimizing the parameter of the photonic device, light from a tunable laser is coupled into the waveguide on the chip using grating couplers and the output is measured with a power meter for evaluation. The applied electric field used for controlling the polymerisable liquid crystal is a square wave of 1 kHz. Below a certain threshold value, the electric field is too weak to overcome the elastic forces between the LC molecules. Above threshold the director of the liquid crystal reorients allowing adjusting the resonance wavelength being the parameter to which the photonic device is trimmed For increasing voltage, it was found that the resonance wavelength of the photonic device gradually shifts towards lower wavelengths. When the molecules of the liquid crystal were reoriented to their maximum angle, the shift saturates. The results of two experiments are shown. In
For some embodiments, the shift may be negligible. The voltage or any other parameter at which polymerization takes place may then be set such that Γpoly−, i.e. the value of the electromagnetic property before polymerization, corresponds to the preferred value of the electromagnetic property (Γdes). Neglecting of the shift that takes place due to polymerization is illustrated in
In configurations for which the shift is not negligible or intolerable, different techniques can be applied to make sure that ⊖poly+, i.e. the value of the electromagnetic property after polymerization, is equal to Γdes, i.e. the preferred value of the electromagnetic property.
In a first technique the shift in electromagnetic property is taken into account by using calibrated data correlating the liquid crystal orienting condition to be applied to the polymerisable liquid crystal cladding layer on the one hand and the electromagnetic property of the selected photonic device obtained after polymerizing the liquid crystal cladding layer with the selected liquid crystal orienting condition on the other hand, thus taking into account a shift in electromagnetic property of the selected photonic device due to the polymerizing. In this first technique, the curves Γpoly−(V) and Γpoly+(V) are determined for predicting the shift due to polymerization. In order to obtain the preferred property after polymerization Γdes, the polymerization voltage can be set to the appropriate voltage taking into account this shift. Although this method is simple to implement, it requires a lot of measurements to determine the curve Γpoly+(V), because each point in the curve requires the polymerization of a cell (or part of a cell). The technique is illustrated in
Another technique makes use of a stepwise polymerization whereby the selected liquid crystal orienting condition to be applied to the polymerisable liquid crystal cladding layer resulting in a preferred value for an electromagnetic property of the selected photonic device are determined repeatedly and intermediately partly polymerizing the polymerisable liquid crystal cladding layer is applied. In this technique the shift due to polymerization thus is taken into account using a multi-step polymerization. In the previously described methods the polymerization occurs in one step, i.e. the required illumination energy for full curing is applied in one step by regulating the UV intensity and illumination time. According to the current technique, the polymerization occurs in different steps. For each step, the applied illumination energy is smaller than the energy for full curing. This means that after one step the material is not fully polymerized and it is still possible to alter its properties by applying different voltages. After each step, the voltage is adapted in order to set the value of Γ to Γdes. As an example, a 3-step polymerization is shown in
Another technique to take into account the shift due to polymerization uses a combination of the first and the second technique. Such combination allows to obtain the preferred property after curing more accurately, without the need to determine the curve Γpoly+. In this method the polymerization occurs in steps, but instead of starting from a voltage corresponding to Γdes a reasonable guess is used for the voltage such that the decrease (or increase) of Γ is anticipated after polymerization.
An example of a multi-polymerization method and device will be described hereafter. A 20 μm thick liquid crystal cell with a polymerizable liquid crystal mixture was placed between crossed polarizers. The same composition of the liquid crystal mixture was used as described in Example 1 below. The transmission spectrum of the polarizers and liquid crystal cell was measured at the start and after each illumination step with the combination of a Xenon lamp (with UV filter) and a USB spectrometer. Each photopolymerization step was performed with an intensity of approximately 9 mW/cm2 for 0.2 s. The UV light was generated by a UV illumination system (Omnicure S1000) consisting of a mercury lamp coupled to an optical fiber light guide with a collimation lens. Most of the intensity of the UV light was situated around 365 nm. The photopolymerization was performed with a 1.5 Vrms AC signal (1 kHz) applied to the liquid crystal cell. The different transmission spectra for different steps in the polymerization can be found in
In the table, the wavelength of a minimum in the transmission spectrum is plotted after each illumination step. It is clear from the table that there is an overall shift to shorter wavelengths, although individual photopolymerization steps may also exhibit a shift to longer wavelengths. There is a clear threshold for polymerization since the first three illumination steps do not lead to any shift in the wavelength. Only after step 4 a distinct shift in the wavelength is observed. After step 9 the reactive liquid crystal mixture appears to be fully cured. This experiment demonstrates some of the possibilities of the stepwise polymerization of the liquid crystal.
In the following example, using an almost identical setup, it is shown that after a number of polymerization steps, a change of voltage still leads to a shift in the wavelength.
The method and device according to the present invention is not being limited to trimming of SOI ring resonators, but may be used in any application considered suitable by the person skilled in the art. Other applications, without being limited thereto, can be LC tunable filters, tunable lenses, tuning the transmission of microwaves through thin metal slits, as will be described hereunder. A first example is the use of a method and device according to the present invention in a LC tunable filter. Some optical devices are fabricated while being optimized for a certain parameter, such as the operating wavelength. These devices may share the same design, but contain one or more components which are used for optimizing the device for the preferred parameter. An example in which a retardation plate with certain retardation is necessary is in liquid crystal tunable filters.
A second example is the use in tunable lenses. Liquid crystals can be used to steer optical beams by inducing a blazed grating in a liquid crystal cell as shown in the
Another example is the use of the device and/or method according to the present invention in filtering a desired frequency component in microwave or terahertz devices. In “J. R. Sambles, A. P. Hibbins, R. J. Kelly, J. R. Suckling and F. Yang, Microwaves: thin metal slits and liquid crystals., Integrated Optical Devices, Nanostructures, and Displays, Vol. 5618, pp. 1-14, 2004” it is shown that liquid crystals can be used to tune the transmission of microwaves through thin metal slits. It is shown that controlling the liquid crystal orientation by applying a voltage, allows switching on and off of the signal at 59.20 GHz. Also in the terahertz range, liquid crystals can be used to tune the transmission. In “S. A. Jewell, E. Hendry, T. H. Isaac and J. R. Sambles, Tuneable Fabry-Perot etalon for terahertz radiation, New Journal of Physics, Vol. 10, pp. 033012, 2008” the transmission of the signal at 0.6 THz can be changed by applying a voltage over the liquid crystal cell. Such systems can also be implemented with polymerizable liquid crystals, according to embodiments of the present invention. The anisotropy of non-reactive and reactive liquid crystals is similar in the terahertz and microwave region of the electromagnetic spectrum. The voltage is chosen in such a way that the transmission of a certain wavelength is as desired after which the orientation of the liquid crystal is fixed by photopolymerization.
It is an advantage of at least some embodiments according to the present invention that the component, after polymerization, is less influenced by temperature. In order to obtain this advantage, polymers may be chosen for the polymerisable liquid crystal, that have an opposite refractive index change as function of temperature with respect to one or more of the remaining components in the photonic device, e.g. with respect to silicon in case a silicon photonic device is trimmed With design of the device design, the thermal behavior of the liquid crystal can be selected such that temperature influence can be very small or even cancelled out entirely.
Number | Date | Country | Kind |
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1110948.5 | Jun 2011 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2012/062649 | 6/28/2012 | WO | 00 | 12/23/2013 |