A SURFACE REFRACTIVE INDEX SCANNING SYSTEM AND METHOD

Abstract

The invention relates to a surface refractive index scanning system for characterization of a sample. The system comprises a grating device for holding or receiving the sample, the device comprising at least a first grating region having a first grating width along a transverse direction, and a second grating region having a second grating width in the transverse direction. The first grating region and the second grating region are adjacent in the transverse direction, wherein the first grating region has a grating period Λ.sub.1 in a longitudinal direction, and the second grating region has a grating period Λ.sub.2 in the longitudinal direction, where the longitudinal direction is orthogonal to the transverse direction. A grating period spacing ΔΛ=Λ.sub.1-Λ.sub.2 is finite. Further, the first and second grating periods are chosen to provide optical resonances for light respectively in a first wavelength band and a second wavelength band, light is being emitted, transmitted, or reflected in an out-of-plane direction, wherein the first wavelength band and the second wavelength band are at least partially non-overlapping in wavelength. The system further comprises a light source for illuminating at least a part of the grating device with light at an illumination wavelength band. Additionally, the system comprises an imaging system for imaging the emitted, transmitted or reflected light from the grating device. The imaging system comprises an optical element, such as a cylindrical lens or a bended mirror, configured for focusing light in a transverse direction and for being invariant in an orthogonal transverse direction, the optical element being oriented such that the longitudinal direction of the grating device is oriented to coincide with the invariant direction of the optical element, and an imaging spectrometer comprising an entrance slit having a longitudinal direction oriented to coincide with the invariant direction of the optical element. The imaging spectrometer further comprises a 2-dimensional image sensor. The invention further relates to a method.

Claims

1. A surface refractive index scanning system for characterization of a sample comprising: a grating device for receiving the sample, the device comprising at least a first grating region having first grating width along a transverse direction., and a second grating region having a second grating width in the transverse direction, the first grating region and the second grating region being adjacent in the transverse direction, wherein the first grating region has a grating period Λ.sub.1 in a longitudinal direction, the longitudinal direction being orthogonal to the transverse direction, and the second grating region has a grating period Λ.sub.2 in the longitudinal direction, a grating period spacing ΔΛ=Λ.sub.1-Λ.sub.2 being finite, the first and second grating periods are chosen to provide optical resonances for light respectively in a first wavelength band and a second wavelength band, light is being emitted, transmitted, or reflected in an out-of-plane direction, wherein the first wavelength band and the second wavelength band are at least partially non-overlapping in wavelength, a light source for illuminating at least a part of the grating device with light at an illumination wavelength band, an imaging system for imaging the emitted, transmitted or reflected light from the grating device, the imaging system comprising an optical element, configured for focusing light in a first transverse direction and for being invariant in an orthogonal transverse direction, the optical element being oriented such that the longitudinal direction of the grating device is oriented to coincide with an invariant direction of the optical element, and an imaging spectrometer comprising an entrance slit having a longitudinal direction oriented to coincide with the invariant direction of the optical element, the imaging spectrometer further comprising a 2-dimensional image sensor.

2. The surface refractive index scanning system according to claim 1, wherein the optical element is a cylindrical lens.

3. The surface refractive index scanning system according to claim 1, wherein the optical element is a bended mirror.

4. The surface refractive index scanning system according to claim 1, wherein the grating device comprises multiple structured regions, including the first grating region and the second region, such as a number of structured regions in the range 2-100, or 10-80, or even 20-50, wherein each of the grating regions are arranged to provide optical resonances for light respectively in corresponding wavelength bands, light is being emitted, transmitted, or reflected in an out-of-plane direction, the corresponding wavelength bands being at least partially non-overlapping.

5. The surface refractive index scanning system according to claim 4, wherein the first structured region and/or the second structured region are configured as a second order distributed Bragg reflector, DBR, for providing the optical resonance.

6. The surface refractive index scanning system according to claim 4, wherein the grating device comprises a light-emitting material having an emission spectrum, wherein first structured region and the second structured region are configured such that the first wavelength range and the second wavelength range at least partially fall within the emission spectrum of the light-emitting material.

7. The surface refractive index scanning system according to claim 6, wherein grating period spacing ΔΛ between two adjacent grating regions is in the range 0.05 nm-10 nm, such as in the range 0.1 nm-5 nm, or even in the range 0.5 nm-2 nm.

8. The surface refractive index scanning system according to claim 6, wherein the light-emitting material is or comprises a dye-doped material.

9. The surface refractive index scanning system according to claim 8, wherein the dye-doped material comprises a polymer.

10. The surface refractive index scanning system according to claim 1 where the grating device is a passive structure for reflecting or transmitting light in the wavelength ranges.

11. The surface refractive index scanning system according to claim 10, wherein the grating period spacing ΔΛ between two adjacent grating regions is in the range 0.5 nm-500 nm, such as about 5 nm-200 nm or even in the range 10 nm-100 nm.

12. The surface refractive index scanning system according to claim 10, wherein the system further comprises an optical gain material and a first cavity end reflector, and wherein the grating device is arranged as a second cavity end reflector, the gain material being disposed between the first cavity end reflector and the second cavity end reflector in an external cavity laser configuration.

13. The surface refractive index scanning system according to claim 1, wherein the system is or comprises a microscope system.

14. A method of acquiring a surface refractive index image of a sample, the method comprising: providing a grating device for receiving the sample, the device comprising at least a first grating region having a first grating width along a transverse direction, and a second grating region having a second grating width in the transverse direction, the first grating region and the second grating region being adjacent in the transverse direction, wherein the first grating region has a grating period Λ.sub.1 in a longitudinal direction, the longitudinal direction being orthogonal to the transverse direction, and the second region has a grating period Λ.sub.2 in the longitudinal direction grating period spacing ΔΛ=Λ.sub.1-Λ.sub.2 being finite, wherein the first and second grating periods are chosen to provide optical resonances for light respectively in a first wavelength band and a second wavelength band, light is being emitted, transmitted, or reflected in an out-of-plane direction, wherein the first wavelength band and the second wavelength band are at least partially non-overlapping in wavelength, positioning the sample to be characterized onto the grating device so as to come into contact with the first and/or the second grating region, illuminating at least a part of the grating device with light at an illumination wavelength band, imaging the emitted, transmitted or reflected light from the grating device with an imaging system into a 2-dimensional raw image, the imaging system comprising an optical element, configured for focusing light in a first transverse direction and for being invariant in an orthogonal transverse direction, the optical element being oriented such that the longitudinal direction of the grating device is oriented to coincide with an invariant direction of the optical element, and an imaging spectrometer comprising an entrance slit having a longitudinal direction oriented to coincide with the invariant direction of the optical element, the imaging spectrometer further comprising a 2-dimensional image sensor, oriented along the longitudinal direction of the entrance slit, processing the 2-dimensional raw image to obtain a 2-dimensional map of refractive index change of the sample the grating regions, by utilizing that each row of pixels in the raw image perpendicular to the longitudinal direction of the entrance slit contains spectrally resolved resonance peaks for each grating region, each resonance peak corresponding to a position along the transverse direction of the grating device, and positions in the raw image along the longitudinal direction translates directly to positions along the longitudinal direction of the grating device.

15. The method according to claim 14 wherein the step of processing the image comprises tracking changes in spectral positions of the resonance peaks to provide a time-resolved image of refractive index changes in the sample.

16. The method according to claim 14, wherein processing the 2-dimensional raw image comprises: for each row of pixels perpendicular to the longitudinal direction of the entrance slit, detect peak positions in pixels for resonance peaks in the row associated with each grating region, convert each peak position in pixels to a peak wavelength value of resonance, calculate a wavelength shift for each of the peak wavelength values of resonance with regards to reference peak values corresponding to the grating device without the sample present, and calculate refractive index values from the wavelength shifts.

17. The method according to claim 16, wherein finding the peak position in pixel for each of the grating region comprises: finding a peak pixel as a pixel with a highest intensity reading within a range of pixels associated with that grating region, calculate the peak position in pixels from a range of pixels around the peak pixel using a numerical method.

18. The method according to claim 17 wherein the numerical method is a center of mass calculation or a Lorentzian fit.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0034] The surface refractive index scanning system according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0035] FIG. 1 illustrates a grating device as comprised by the inventive system.

[0036] FIG. 2 illustrates a simplified setup according to the invention.

[0037] FIG. 3 illustrates an embodiment of the surface refractive index scanning system according to the invention.

[0038] FIG. 4 illustrates an embodiment of the surface refractive index scanning system as arranged in a passive photonic crystal configuration according to the invention.

[0039] FIG. 5 illustrates an embodiment of the surface refractive index scanning system as arranged in an external cavity laser configuration according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0040] FIG. 1 shows a grating device 1 as comprised by the inventive system. Here, two grating regions are illustrated, i.e. the first 2 and the second 4 region, having a grating period Λ.sub.l and Λ.sub.2, respectively. The grating period spacing between the two grating regions ΔΛ should be chosen to spectrally separate the resonance from the first grating region from the resonance from the second grating region. Preferably the grating period should also be selected to minimize the bandwidth requirement of the whole grating device 1. In an embodiment, the grating device comprises a glass substrate 6, onto which is deposited a polymer-containing layer 8. The polymer-containing layer 8 may be doped with an active material, such as a dye for use in active embodiments of the inventive system. Alternatively, the polymer-containing layer 8 may be passive for use in passive embodiments. Finally, the grating device 1 preferably comprises a high refractive index layer, e. g. a TiO.sub.2-layer or a Ta.sub.2O.sub.5-layer 10 on a top side of the device, which acts to increase an optical overlap between a resonant light field within a grating region and a fluid sample put in contact with the top side of the device.

[0041] An example of a grating device as fabricated by the inventors, was made using a 4 inch Borofloat glass wafer as substrate. The glass wafer had a thickness of 500 μm, but could in general have a thickness in the range of about 50 μm-2 mm.

[0042] An alternative embodiment could also be made in a plastic material such as COC, or in a different glass. The polymer-containing layer was formed as a dye doped thin film, or more specifically as a Pyrromethene 597 doped Ormocomp-layer, having a thickness of 400 nm. In general the polymer-containing layer could have the thickness in the range of about 200 nm-500 nm, and could also be made in other material, such as poly-methyl methacrylate (PMMA) or SU8. Finally, the example device comprised a high index layer fabricated as an ion beam evaporated TiO.sub.2-layer, with a thickness of 25 nm (possible thickness range of about 0 nm-400 nm).

[0043] In the example device, 40 grating regions were made, with grating widths of 25 μm. The grating periods Λ.sub.i were chosen in the range Λ.sub.i=351 nm . . . 391 nm , with a constant grating period spacing ΔΛ=1 nm to result in resonances in the wavelength range λ˜540 nm-600 nm.

[0044] In another example device, 11 grating regions were made, with grating widths of 90 μm. The grating periods Λ.sub.i were chosen in the range Λ.sub.i=370 nm . . . 380 nm, with a constant grating period spacing ΔΛ=1 nm to result in resonances in the wavelength range λ˜565 nm-585 nm.

[0045] FIG. 2 schematically shows the surface refractive index scanning system 100 according to the invention. The system comprises a grating device 1, as discussed above in connection to FIG. 1. The system 100 further comprises an imaging system 110, here illustrated as a lens 112 and a cylindrical lens 114. The imaging system 110 is configured to collect light from the grating device 1 and focus it onto an entrance slit 120 on an imaging spectrometer. It is noticed that the first grating region 2 and the second grating region 4 are imaged onto the same elongated line and onto the entrance slit 120 for coupling into the imaging spectrometer. Thus, the spatial distribution of light from the grating device is only maintained along the longitudinal direction of the slit. By use of the imaging spectrometer (not shown), spectra for each position in the longitudinal direction of the slit may be recorded and analysed e.g. in a computer.

[0046] FIG. 3 shows an embodiment of the surface refractive index scanning system 100 according to the invention. The embodiment relates to the grating device shown in FIG. 1, and the schematic FIG. 2, wherein like reference numerals refer to identical or similar parts. Therefore, only differences with regards to the two previous figures are described here. This embodiment of the system uses an active grating device, i.e. a grating device comprising a light emitting material. Pump light 131 for exciting the light emitting material is generated in a pump light source, which is here illustrated as a laser 130 for generating 532 nm light. The skilled person will realize that the choice of pumping wavelength depends on the absorption spectrum of the light emitting material used in the grating device. Thus, the use of other pump wavelengths is also foreseen by the inventors. The pump light source may, e.g., also be a flash pump source or other types of non-laser sources. The pump light 131 is coupled via a dichroic mirror 132 onto the grating device 1 to simultaneously pump the first grating region 2 and second grating region 4. To improve coupling of the pump light to the grating device 1, the pump laser light is here shown to be focused by a microscope objective 112a, which is also used to collect and collimate the emitted light 133 generated within the grating device 1. However, in other configurations, the pump light 131 and collected light 133 will have separate optical paths onto and away from the grating device 1. The collimated emitted light 133 is focused onto the entrance slit 120 of the imaging spectrometer 136 by a focusing lens 112b and the cylindrical lens 114. As a convenience, a separate image of the grating device may be formed on an optional CCD array 138 (CCD1), e.g. to inspect the focus of the system. Light for the optional CCD array 138 is split off from the emitted light beam by a beam splitter 134. To use CCD1 to check focus of the system, CCD1 should be arranged in the same distance from the beam splitter 134 as the entrance slit 120. The imaging spectrometer is configured such that the CCD at the spectrograph (CCD2 139) delivers an image where the horizontal direction corresponds to the wavelength of the light. The vertical direction of the image on CCD2 139 corresponds to the longitudinal position along the entrance slit. A function generator 140 is used to generate trigger signals for the pump laser 130 and the imaging spectrometer 136, in particular CCD2 139.

[0047] In an example device, the grating device surface emits very narrow laser lines of approx. 0.2 nm full width at half maximum. With the spectrometer used for demonstration by the inventors, the horizontal width of the CCD2 139 will correspond to 55 nm. Thus, several laser lines are well distinguishable on the CCD2 139 image.

[0048] The spectrometer CCD signal is analysed and a central emission wavelength is found for each resonance peak/laser line e.g. by using a centre of mass algorithm or fitting a function to the data. The central wavelength of all resonance peaks is monitored and wavelength shifts are calculated with a computer. Wavelength shifts correspond to refractive index changes. An image of the surface refractive index may then be generated by the computer.

[0049] According to a demonstration example the optical system is capable of operating with a frame rate of 12 Hz, limited by the read-out time of the spectrometer CCD2 139. By continuously taking images the wavelength shift of each line can be calculated and refractive index changes can be monitored in time and space. In order to demonstrate the functionality of the laser imaging system, the inventors have added a piece of sugar to a microfluidic well with water on top of a grating device, and used the system to monitor how the wavelength of all laser lines was shifting upon the presence of small sugar molecules increasing the refractive index.

[0050] FIG. 4 shows an alternative embodiment of the surface refractive index scanning system according to the invention, wherein the grating device 1 is a passive device, i.e. not comprising a light emitting material. The embodiment has many similarities with the embodiment shown in FIG. 3 and discussed above. Therefore, only differences between the two embodiments will be discussed here. The grating device 1 is configured to be a guided mode resonance filter. A broadband light source is used for illumination, e.g. a Xenon lamp or an LED. In this case, the resonant wavelengths of the grating regions 2, 4 should be configured to fall within the emission band of the light source. Light reflected from the passive photonic crystal enters into the imaging spectrometer 136 via the cylindrical lens 114 and entrance slit 120, as described above for other embodiments.

[0051] FIG. 5 shows an alternative embodiment of the surface refractive index scanning system according to the invention, wherein the grating device 1 is a passive device. The embodiment has many similarities with the embodiment shown in FIG. 3 and discussed above. Therefore, only differences between the two embodiments will be discussed here. The grating device 1 is configured to be a cavity end mirror, the other end of the cavity being formed by mirror 152. Within the cavity, an active element 150 is disposed so as to provide optical gain to light resonant in the cavity. Examples of an active element 150 may be a semiconductor optical amplifier (SOA), a gain crystal, or an organic dye in solution. The active element 150 is here illustrated to be optically pumped by the pump source 130. However, in other embodiments, the active element 150 could alternatively be electrically pumped, depending on the choice of material. In this case the resonant wavelengths of the grating regions 2, 4 should be configured to fall within the amplified spontaneous emission spectrum of the active material. A small fraction of the cavity light is coupled out via a beam splitter 134, and entered into the imaging spectrometer 136 via the cylindrical lens 114 and entrance slit 120, as described above for other embodiments.

[0052] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. For instance, the embodiments have shown the use of a cylindrical lens as the optical element for focusing the light from the sample onto the entrance slit of the imaging spectrometer. However, the use of a bended mirror, or another optical element, for this purpose is also envisioned by the inventors and fall within the scope of the invention. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.