Field of invention

The present invention relates to light filtering, in particular to switchable colour filtering.

State of the art

Switchable colour filters are disclosed for instance in US patent 7,110,054 B2 or in European patent application 1 933 193 Al.

General description of the invention

The present invention provides an efficient and original alternative to existing systems and method for light filtering.

It relates to a light filter comprising a plate defined by a length along a direction x, a width along a direction y and a height along a direction z; said plate including at least one chamber containing Liquid Crystals (LC) molecules and which is oriented substantially along a direction parallel to the xy, xz or yz-plane; said filter furthermore being adapted to let a flow be present or temporally and spatially modified within said chamber, wherein said LC molecules are oriented along a first direction in the absence of said flow and along a second direction when said flow is present, resulting thus in the emergence or modification of a birefringence.

The invention also relates to a method for using said filter.

Liquid crystals such as nematic liquid crystals (NLCs) and others such as also cholesteric, smectic, flexoelectric et al liquid crystals may advantageously be confined in Polydimethylsiloxane (PDMS) microscale chambers, where the surface area in the xy-plane is set larger than the ones in the xz-plane (figure la). In this configuration, the LC molecules align perpendicularly to a PDMS surface on the xy plane as illustrated in figurelb.

Figure 1 illustrates a static LCs alignment: a) microscale chamber settings, b) representation of the homeotropic alignment and non-homogeneities (defects) in the cross-section channel (yz-plane). The LC in this particular embodiment is nematic for the purpose of illustration, as alternative material types can be applied.

In the absence of localized flow, the transmitted light through the microchamber is low when the chamber is placed between crossed polarisers (see figure 2a). When flow is applied, the NLC director reorients, resulting thus in the emergence of a birefringence. When a light undergoes birefringence, the ordinary and an extraordinary waves experience a phase delay between them, which in turn induces changes in the polarization. When the incoming linear polarization undergoes a π/2 phase shift, it becomes circular and half of the incoming intensity is transmitted through the analyser (figure 2c, green circle). When the phase shift is π, the incoming linear polarization still linear but has rotated by an angle of 90° and then the incoming light is fully transmitted (figure 2c, red line).

Figure 2 illustrates:

(a) Reorientation of NLCs director under flow, with the microchamber between crossed polarizers and illuminated for the top.

(b) Phase shift δ induced by the birefringence An(u) with ο the wavelength of the light source, h the height of the chamber and u the average flow velocity.

(c) Representation of the output polarization when the phase shift is null, π/2, π and in- between (ellipsoid).

(d) Transmittance of a single wavelength light for varying flows.

In the present invention, the birefringence is controlled via flow : In applying higher or lower flow rates inside the microscale chamber(s), the birefringence may be tuned, controlling thus the transmission (see figure 2d). Flow can be induced via peristaltic means, for instance by modulating the channel volume. This is possible when the chamber is made from elastomeric materials, for instance PDMS. Channel volume modulation can be induced for example by integrating a piezoelectric transducer in the proximity of the chamber (see figure 3). Figure 3 illustrates a schematic representation of the flow actuation method. The PDMS chamber is placed on top of an electrically actuated piezoelectric transducer, which in turn is placed between two contacts (ITO, or similar). When actuated, the transducer oscillates, causing in turn the oscillation of the chamber volume. This induces flow inside the PDMS chamber in accordance with the principle of the present invention.

Tuneable colour filters can be achieved within the aforementioned platform. Different wavelengths undergo different phase shifts between their ordinary and extraordinary waves as they propagate through a birefringent medium. When a wavelength is absorbed by the analyser, another one is fully transmitted. As the birefringence depends on the applied flow, in applying higher or lower flow, the birefringence is tuned and thus the transmitted or absorbed wavelengths are shifted (figure 4). Preliminary results are shown in figure 4 (right), where the transmission between a nematic liquid crystal filled chamber placed between crossed polarisers is shown of varying flow rates. The colour information was captured with a colour CCD camera. Alternative embodiments of the present invention involve the doping of the liquid crystal with chromophores capable of polarization dependent dichroic behaviour. Distance dependent response can also achieved to confine or modify the birefringence response (figure 9).

Figure 4 illustrates:

Left: Different wavelengths undergo different phase shifts. In modifying the phase shift via a change of birefringence/flow, the wavelength that is fully transmitted or fully absorbed can be selected.

Right: Preliminary experimental results from a chamber filled with E7 NLC (Merck, Japan) at varying flow rates. The chamber width is 100 μιη.

Figure 5 illustrates:

A triangular shaped microfluidic chamber in-between its inlet (left) and outlet (right). The chamber triangular shape leads flow modifications at each spatial location of the chamber which in turn leads to the spatial dependence of the birefringence. The latter is manifested by the different colors at each location. By modifying the actuation pressure this spatial response adapts (800 mbar/upper and 1200 mbar/lower). In comparison to the state-of-the-art (e.g. US 7,110,054 B2), the present invention shows a distinctively different actuation method and also occurs in novel confinement media capable of simultaneous alignment, flow and pixelation via cost-effective methods (e.g. soft- lithography).

This method provides several advantages with respect to existing filtering systems and methods, in particular:

1. Full compatibility with other microfluidics components; in this way the filters can be integrated in the proximity of the chambers, but can also be actuated with conventional microfluidic methods.

2. Low manufacturing costs.

3. High repetition speeds, > 100's Hz.

The invention may be used in several fields or applications such as:

1. Lab-on-a-chip spectrometer: a tunable colour filter can change its transmission peak, enabling the determination of the fluorescence or absorption band of an entity. This flow driven filter is integrated with a microfluidic chambers that carries the stationary or moving object being interrogated. In the same embodiment, the chamberdimensions may vary to enable distance dependent response (size relates to flow rate and thus birefringence response), also tunable by modifying the actuation (either amplitude or phase).

2. Integrated tunable wavelength sources: a white light source illuminates the filter; by tuning the filter, different wavelengths of illumination can be selected. Such embodiment enables the integration of cost-effective white-light lamps for lab-on-a- chip spectroscopy applications.

3. Imaging applications: by modifying the filter transmission, different colours are used to form the image of objects on a conventional sensor. The objects can be labelled specifically or un- specifically, enabling thus spectral multiplexing of optical information (e.g. hyperspectral imaging). High speed colour separation applications: rapid filter tuning can enable fast absorption experiments, enabling the mapping of associated dynamics (e.g. transient absorption). Cytometry methods: integrated high speed filters for separating bioentities by colour instead of employing a full colour CCD or three separate filters and photo-diode based detectors; apart from the overall reduction of cost, another comparative advantage of the present invention is higher level of colour multiplexing and single detector employment (see US 2010/0138774 Al) Colour multiplexing in OCT (optical coherence tomography): for retrieving natural colour object information, depth selectivity, the possibility to employ cost-effective low coherence white light sources, high speed measurements (> 30 Hz) and biocompatibility associated with PDMS enabling potential implantations (see US 7,095,503 B2). Pressure, viscosity, flow rate/velocity and temperature measurements (colour or light intensity modulations translate into the aforementioned quantities, providing thus a way of measuring their changes). Arrays of modulators forming pixelated devices, where actuation is enabled by mechanical, electromechanical or piezoelectric systems. Typical devices involve high speed Spatial Light Modulators and high resolution displays (in the spatial and temporal domain) with applications is the scientific domain (e.g. imaging, optical tweezers) and consumer products (e.g. screens etc.). Permanent switch-on state via permanent membrane deformation or high flow rates that induce a certain alignment in the micro-chambers which forms an equilibrium state in the LC alignment. Integrated optics approaches (the microfluidic chamber filled with the liquid crystal acts as a waveguide). Beam control either in shape (see 8) or its temporal or spatial propagation direction (e.g. Q-switching components in laser systems, or beam deflectors).

Detailed description of the invention

As an exemplary embodiment, a liquid crystal (LC) optical modulator based on peristaltic microflows is presented. Peristalsis, an externally controlled chamber volume modulation, induced oscillatory flows and, as a result, periodic director reorientation occurred. The work is based on Nematic Liquid Crystals (NLC), however alternative liquid crystal media are also possible to employ. The microfluidics were realised in poly(dimethylsiloxane) (PDMS) enabling peristalsis due to its elastomeric nature, aligning the LC and cost-effective patterning via cast molding. LCs have dominated societal needs in optical information processing and displays (LCD); however their long and asymmetric response times hinder their performance, placing thereby a severe limitation over emerging cost-effective technologies. Peristaltic LC modulation exhibits symmetric and sub-millisecond on-off response and together with large area lithography and piezoelectric nanotechnologies, this device can address such NLC limitations.

Performance hindrance in liquid crystals is due to their long response times, associated with their mechanical control. Albeit the emergence of novel media (ferroelectric ref. (3), flexoelectrics ref. (4), and nematogens doped with dyes, nanoparticles ref. (5)), pure nematogens remain attractive (simplicity, optical quality and cost) and a rapid actuation method is needed for further improvements. To address this, an optofluidic strategy was and direct flow of liquid crystals was implemented to induce modulation of polarized light incident on the microfluidic cell. The origin of this effect is the reorientation of the nematogen director, due to the coupling between translational and rotational molecular motions ref. (6, 7).

Figure 6 illustrates a Schematic of the microfluidics coupled to the mechanical oscillator (upper, a), the effect of the needle actuation (lower, a).

Peristaltic microfluidics was used to directly induce NLC flow. Peristalsis resulted in rapid oscillatory flow and as a result, the director periodically oriented parallel to the flow and induced optical modulation with sub-millisecond response, and symmetric rise- and fall- profiles.

Microfluidic chips were realized in PDMS, immobilized on glass coverslips and placed between crossed-polarizers, oriented at ±45° with respect to the flow direction. The optical transmission was measured either with a laser and a photodiode, or with a white light source and CCD camera. Peristalsis was induced by an oscillating metallic needle inside the inlet, coupled to a loudspeaker.

The optimal microfluidic geometry was identified by varying the cross-sectional dimensions and performing a steady-state analysis. For chambers with widths greater than their heights, the E7 aligned homoetropically. When the channel height became comparable to its width, the orthogonal chamber surfaces the alignment is perturbed and becomes planar. The homoetropic area variation as a function of the channel width exhibits a clear threshold (fig. 7, which illustrates the homoetropic area dependence on the chamber width). Below threshold, the alignment is planar and above threshold, the homoetropic area increases. It has to be noted that for chamber thicknesses greater than 8 μιη, the PDMS surfaces needed to be treated with an ethanol solution of polypropylene sulphide to obtain uniform alignment.

To induce flow, a metallic needle is fixed at one end at the loudspeaker and the other end is inserted in the PDMS chamber inlet. When the needle is actuated, a mechanical deformation of the microchamber is induced (fig. 7). When the needle moves downwards, the

microchamber volume decreases; it increases when the needle moves upwards. As a result, an alternating backward and forward flow occurs respectively. This behaviour was

experimentally confirmed by dispersing silver particles in the microchamber PDMS. These microchambers were subsequently filled with a liquid containing the same particles. Particle tracking revealed simultaneously the PDMS deformation and the flow inside the chambers. Typical results are shown in Figure 8 (Figure 8 illustrates Particle velocity under flow) for a frequency of 200 Hz.

The peristalsis and thus microfluidic flow are periodic, translating into a periodically modulated optical retardation. A typical transmission measurement is shown in Figure 9 at 1000 Hz. Under no peristalsis, the transmission is zero. The rise and fall times were sub- millisecond (250 and 290 μ&&ο respectively) and symmetric, which is due to the direct actuation of the chamber walls, which in turn can induce rapid flow switching inside the microfluidic s. It must be mentioned that by connecting a mechanically actuated syringe to induce flow in the chambers, it is not possible to achieve flow at high frequencies due to losses and dispersion associated with the viscoelasticity of the intermediate connecting parts.

Figure 9 illustrates the Optical transmission at a peristaltic flow rate of 1 kHz. The optical transmission was localized within approximately 200 μιη from the inlet, indicating that the flow rate depends on the distance from the actuation. Numerical analysis confirmed this by showing that that closer to the perturbation source, the volume deformation is higher and decreases further away. This translates into phase retardation that is distance dependent, forming thus an individual pixel. The integration of piezoelectric oscillators within composite media can enable the realization of rapid, cheap and miniature NLC based modulators. In addition, cast- molding is compatible with multi-layer lithography and can thus permit 3D, large-scale integration of such NLC devices with microfluidic circuits for analytical and imaging methods.

Of course, all the examples and embodiments are given by way of non-limiting examples and many modifications are possible with equivalents steps and means within the scope of the present invention.

The NLC behaviour suggests that other media may be applicable. Their alignment may not be homeotropic, but planar and we may need to do a small treatment or choose the right chamber dimensions.

The alignment may be externally imposed, for example through light (optics), electric fields, and flow which can drive the liquid crystal; these 'external fields' can be combined. For example we can use flow to control the optical non-linearity (light matter interactions depend on the orientation), or electric fields to control the viscosity (viscosity depends on the LC orientation).

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