A Field-Enhancing Device

Abstract

A field-enhancing device includes at least one metal layer or a metal grating consisting of metal stripes or a dielectric grating. Usually the device is constructed on some substrate. The adhesive layer is advantageous when the next layer is metallic but is not needed with dielectric layers. The next layers to be constructed form a mirror structure that can also be omitted for simple field-enhancing device constructs. The mirror structure can be either a metal mirror structure or a distributed Bragg reflector structure (DBR). The next layer is the thin metal layer. This layer can be covered with a 1-D metal grating consisting of metal stripes or with a dielectric grating having similar geometry. The structure can also be fabricated without metals when dielectric grating is used as the field-enhancing part. Finally, a protective layer can be added on top of the structure.

Claims

1. A field-enhancing device to enhance optical processes in samples lying on or in the proximity of a surface of the device, the device comprising: a substrate, a field-enhancing structure arranged on the substrate and comprising dielectric grating, said dielectric grating consisting of dielectric stripes.

2. The device of claim 1, wherein the device comprises additionally an adhesion layer and/or mirror structure, where the mirror structure is a metal mirror structure or a distributed Bragg reflector mirror structure.

3. The device of claim 1, wherein the field-enhancing structure comprises a metal grating, wherein the metal grating of the device comprises elongated metal stripes and elongated empty spacing or grooves between the stripes.

4. The device according to claim 1, wherein the field-enhancing structure comprises a full metal layer and a dielectric grating.

5. The device of claim 4, wherein the dielectric grating of the device comprises elongated dielectric stripes and elongated empty spacing or grooves between the stripes.

6. The device of claim 2, wherein the total number of alternating dielectric layers in the DBR mirror structure is in a range of 2-50.

7. The device of claim 1, wherein the thickness of the underlying substrate is in a range of 50 μm-5 mm.

8. The device of claim 2, wherein the thickness of the adhesion layer is in a range of about 0.5-50 nm.

9. The device of claim 2, wherein the thicknesses of the metal mirror structure are in a range of 10 nm-500 nm for the metal layer and in a range of 50 nm-10 μm for the dielectric layer.

10. The device of claim 6, wherein the thicknesses of the alternating dielectric layers of the DBR mirror structure are in a range of 10 nm-500 nm for the dielectric layer and in a range of 10 nm-500 nm for the dielectric layer.

11. The device of claim 1, wherein the field-enhancing structure comprises a full metal layer and the thickness of the full metal layer is in a range of 1 nm-100 nm, preferably at least 40 nm.

12. The device of claim 1, wherein the field-enhancing structure comprises a metal grating, wherein the thickness of the metal layer for the metal grating is in a range of 5-500 nm.

13. The device of claim 3, wherein the width of the elongated metal stripes in the metal grating is in a range of 10-1000 nm.

14. The device of claim 3, wherein the empty spacing or grooves between the two adjacent elongated metal stripes in the metal grating is in a range of 10-1000 nm.

15. The device of claim 3, wherein a periodicity of the adjacent elongated metal stripes in the metal grating comprises the sum of the width of one elongated metal stripe and the width of the empty spacing or grooves of two adjacent elongated metal stripes, and wherein the periodicity is selected to resonate with either the molecular vibrational frequency of a substance in the sample or the frequency of the exciting laser light or both of them.

16. The device of claim 15, wherein the periodicity in the metal grating is in a range of 10-1000 nm.

17. The device of claim 4, wherein the thickness of the dielectric layer for the dielectric grating is in a range of 5-500 nm.

18. The device of claim 4, wherein the width of the elongated dielectric stripes in the dielectric grating is in a range of 10-1000 nm.

19. The device of claim 4, wherein the empty spacing or grooves between the two adjacent elongated dielectric stripes in the dielectric grating is in a range of 10-1000 nm.

20. The device of claim 4, wherein a periodicity the empty spacing or grooves between the two adjacent elongated dielectric stripes in the dielectric grating comprises the sum of the width of one elongated dielectric stripe and the width of the empty spacing of two adjacent elongated dielectric stripes, and wherein the periodicity is selected to resonate with either the molecular vibrational frequency of a substance in the sample or the frequency of the exciting laser light or both of them.

21. The device of claim 20, wherein the periodicity in the dielectric grating is in a range of 10-1000 nm.

22. The device of claim 1, wherein the device comprises a protective layer, and wherein the thickness of the protective layer is in a range of 1 nm-500 nm.

23. The device of claim 1, wherein the substrate of the device comprises for example coverslip glass, normal glass, calcium fluoride (CaF2), silicon.

24. The device of claim 2, wherein the adhesion layer is deposited using materials, such as chromium, titanium and TiO.sub.2.

25. The device of claim 2, wherein the metal mirror of the device comprises an underlying metal layer, that can be any light reflecting metal material, such as gold, silver, aluminium, or copper.

26. The device of claim 2, wherein the metal mirror layer is separated from the field-enhancing structure by a dielectric layer comprising any dielectric material, such as Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2.

27. The device of claim 2, wherein the dielectric layers of the DBR mirror structure are any dielectric materials having dissimilar dielectric constants ε.sub.1 and ε.sub.2, such as Al.sub.2O.sub.3, TiO.sub.2, or SiO.sub.2.

28. The device of claim 1, wherein the field-enhancing structure comprises a full metal layer, wherein the full metal layer and/or the metal grating comprises any plasmonic materials, such as gold, silver, copper, platinum, palladium, aluminium, or any other material which enhances the optical processes.

29. The device of claim 4, wherein the dielectric grating comprises any dielectric materials, such as Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2.

30. The device of claim 22, wherein the protective layer comprises any dielectric materials, such as Al.sub.2O.sub.3, TiO.sub.2, SiO.sub.2.

31. The device of claim 1, wherein the field-enhancing device is configured to enhance the optical processes of Raman scattering (RS), linear and nonlinear surface enhanced Raman scattering (SERS), coherent anti-Stokes Raman scattering (CARS) and surface enhanced coherent anti-Stokes Raman scattering (SECARS).

32. The device of claim 1, wherein the device is configured to enhance the optical processes of fluorescence, second harmonic generation (SHG), sum frequency generation (SFG), and two photon excited fluorescence (TPEF).

33. The device of claim 1, wherein the field-enhancing structure comprises nanograting structures with elongated grooves and comprises predefined continuous shape and patterns for enhancing four wave mixing (FWM) signal intensity without two photon excited luminescence (TPEL) background in SECARS imaging.

34. The device of claim 1, wherein the field-enhancing structure comprises an adhesion layer comprising TiO.sub.2 and a dielectric grating comprising TiO.sub.2.

35. The device of claim 34, wherein a depth of the grating is 20-60 nm, preferably 20 nm.

36. The device of claim 34, wherein a periodicity of the grating is 250-700 nm, preferably 300 nm.

37. The device of claim 34, wherein the thickness of the adhesion layer is 20-150 nm, preferably 69 nm.

38. The device of claim 1, wherein the field-enhancing structure comprises an adhesion layer comprising Ti, a full metal layer comprising Ag, a metal grating comprising Ag, and a protective layer comprising Al.sub.2O.sub.3.

39. The device of claim 38, wherein the wherein a depth of the grating is 20-60 nm, preferably 25 nm.

40. The device of claim 38, wherein a periodicity of the grating is 250-350 nm, preferably 300 nm.

41. The device of claim 38, wherein the thickness of the adhesion layer is 2-6 nm, preferably 5 nm.

42. The device of claim 38, wherein the thickness of the full metal layer is 50-100 nm, preferably 80 nm.

43. The device of claim 38, wherein the thickness of the protective layer is 2-10 nm, preferably 5 nm.

44. The device of claim 1, wherein the field-enhancing structure comprises an adhesion layer comprising Ti, a full metal layer comprising Au, and a metal grating comprising Au.

45. The device of claim 44, wherein a depth of the grating is 20-60 nm, preferably 25 nm.

46. The device of claim 44, wherein a periodicity of the grating is 500-650 nm, preferably 580 nm.

47. The device of claim 44, wherein the thickness of the adhesion layer is 2-6 nm, preferably 5 nm.

48. The device of claim 44, wherein the thickness of the full metal layer is 50-100 nm, preferably 80 nm.

49. A method for manufacturing a field-enhancing device of claim 1, wherein the method comprises steps of: providing a field-enhancing structure comprising a dielectric grating, said dielectric grating consisting of dielectric stripes, on a substrate layer using electron beam lithography (EBL) or nanoimprint lithography (NIL) techniques and lift-off or wet or dry etching process.

50. The method of claim 49, further comprising fabricating additionally an adhesion layer and/or mirror structure on the field-enhancing device, wherein the mirror structure is a metal mirror structure or a distributed Bragg reflector (DBR) mirror structure.

51. The method of claim 49, wherein the method comprises steps of fabricating the field-enhancing device on a substrate so that the adhesion layer is first deposited by a metal evaporator, followed by fabricating an intermediate layer, and after fabricating at least one adhesion layer and intermediate layer the electron beam lithography or nanoimprint lithography and lift-off processes are applied.

52. The method of claim 49, wherein a periodicity P is the periodicity of the two adjacent elongated grooves and the periodicity P is selected in relation to a wavelength so that λ.sub.SP (i,j) the formula: ${\lambda }_{\mathrm{SP}\left(i,j\right)}=\sqrt{\frac{{\mathrm{.Math.}}_d.Math.{\mathrm{.Math.}}_m}{{\mathrm{.Math.}}_d+{\mathrm{.Math.}}_m}}.Math.\frac{P}{\sqrt{i^2+j^2}},$ is fulfilled where the integers (i, j) represent the Bragg resonance orders, and ε.sub.d and ε.sub.m are the dielectric functions of the metal and measurement medium, respectively.

53. The device of claim 1, wherein the field-enhancing structure additionally comprises a full metal layer and/or a metal grating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

[0049] FIG. 1 illustrates exemplary constituting parts of the device according to an advantageous embodiment of the invention,

[0050] FIG. 2 illustrates the structure of nanogratings according to an advantageous embodiment of the invention

[0051] FIG. 3 illustrates three different examples of devices according to an advantageous embodiment of the invention,

[0052] FIG. 4 illustrates exemplary reflectance measurements,

[0053] FIG. 5 illustrates exemplary reflectance spectra of the exemplary device according to an advantageous embodiment of the invention,

[0054] FIG. 6 illustrates example of calculated transverse magnetic (TM) and transverse electric (TE) reflectance spectra of a device according to an advantageous embodiment of the invention,

[0055] FIG. 7 shows one exemplary structure of a device according to an embodiment of the invention where the SEBI substrate is optimized for green fluorescent protein (GFP),

[0056] FIG. 8 illustrates a reflectance spectrum that may be obtained with a device according to the embodiment of FIG. 7,

[0057] FIG. 9 gives one more exemplary structure of a device according to an embodiment of the invention,

[0058] FIG. 10 illustrates yet one exemplary structure of a device according to an embodiment of the invention,

[0059] FIG. 11 shows a reflectance spectrum that may be obtained with a device according to the embodiment of FIG. 9, and

[0060] FIG. 12 shows a reflectance spectrum that may be obtained with a device according to the embodiment of FIG. 10.

DETAILED DESCRIPTION

[0061] The different embodiments of field-enhancing devices according to the invention is next described by referring to FIGS. 1-6.

[0062] According to an embodiment of the invention the field-enhancing device (100) can be constructed in several ways, but it always contains at least one metal layer (005) or a metal or dielectric grating (006, 007) consisting of metallic or dielectric stripes. Usually the device is constructed on some foreign substrate (001). The adhesive layer (002) is advantageous when the next layer is metallic, but may not be needed with dielectric layers. The next layers to be constructed form a mirror structure, that can also be omitted for simple device constructs. The mirror structure can be either a metal mirror structure (003) or a distributed Bragg reflector structure (DBR) (004). The next layer is the thin metal layer (005), that can also be omitted. This layer can be covered with a 1-D metal grating (006) consisting of metal stripes or with a dielectric grating (007) having similar geometry. Finally, a protective layer (008) can be added on top of the structure.

[0063] The object of the invention is a device that enhances electric field at the surface and in the proximity of the device. This enhancement is advantageous in certain microscopic and spectroscopic measurements that utilize laser light to excite optical processes in samples lying on the surface of the device.

[0064] The function of the device is based on excitation of surface plasmon-polaritons (SPPs) or Tamm plasmons (TPs) at the interface of a metal and a dielectric. Also diffraction grating effect can enhance the field when dielectric grating is used on the surface. These excitations provide a much enhanced electric field at the surface of the device when light is focused on it compared to the situation that light is focused on, e.g., a glass surface only. The device is advantageously designed so that the incoming light and the dimensions of the device are at resonance.

[0065] FIG. 1 shows the constituting parts of the device; some of which are optional and can be omitted in certain embodiments, as is described elsewhere in this document. With these parts several different configurations can be designed leading to multitude device constructs that provide the advantageous enhancement of the electric field.

[0066] FIG. 3 shows three different examples of devices that can be constructed.

[0067] The field-enhancing device (100) consists of a substrate (001) on which the device is manufactured, an optional adhesion layer (002), an optional mirror structure (003, 004), that can be either a metal mirror structure (003) or a distributed Bragg reflector (DBR) mirror structure (004), the plasmonic structure (005, 006) comprising a full metal layer (005) or a metal grating (006) or both of them in the order of FIG. 1, an optional dielectric grating (007), and finally an optional protective layer (008).

[0068] The substrate can be of any material, most typical being coverslip glass or normal glass. The optional adhesion layer (002) is advantageous especially when the next layer is metal. It ensures that the metal layer does not roll away from the substrate and improves heat conduction from the metal. The metal on top of the adhesion layer can be either the metal layer (0031) in the mirror structure (003) or the plasmonic metal layer (005). The adhesion layer can be metal or dielectric, most common being Ti.

[0069] When the device (100) utilizes Tamm plasmons, a mirror structure comes next in the build order. There are two options, the metal mirror structure (003) or the DBR structure (004). The metal mirror structure (003) consists of a metal layer (0031) on the bottom and a dielectric layer (0032) on top of it. The thickness of the dielectric layer is chosen so that resonance with the incoming light is achieved. The DBR structure consists of alternating dielectric layers (0041) and (0042) of different materials having different refractive indexes. The number of layers can be any integer above and including two. The most common dielectric materials for the dielectric layers in the device (100) are Al2O3, TiO2 and SiO2, but any dielectric can be used.

[0070] When utilizing TPs, a thin full metal layer (005) is fabricated on top of the mirror structure (003/004). This metal and the adjacent dielectric form the interface where the TP is concentrated. The enhanced electric field on the surface can also be achieved by forming a dielectric grating (007) on top of the structure. The grating consists of elongated dielectric stripes (0071) and empty space (0072) between the stripes. The widths of the dielectric stripes (0071) and the empty space (0072) between them are designed so that the electric field distribution is as uniform as possible to provide the advantageous enhancement as uniformly over the surface as possible. With certain metals, the metal layer (005) must be protected, e.g., against oxidation, and then a protective dielectric layer (008) is made on top of the whole structure or it is applied before the dielectric grating (007) is formed.

[0071] When the device (100) utilizes SPPs, the device usually does not need mirror structures below the metal layer (005), but the adhesive layer (002) may be used on top of the substrate (001). The full metal layer together with the adhesive layer provide better heat conduction to preserve the integrity of the metal grating (006) on top of it. The optional metal grating (006) comprises elongated metal stripes and empty spacing between the stripes. FIG. 2 (top) shows the geometry of the grating from a side profile. The widths of the metal stripes (0061) and the empty space (0062) between them are chosen together with the periodicity (0063) so that the SPPs and the incoming light are at resonance. Again, with certain metals, a protective layer (008) can be used as the topmost layer. The metal materials in the device (100) can be any metals, most common being gold, silver and aluminium. FIG. 2 (bottom) shows a scanning electron microscope image of a fabricated 1-D gold grating.

[0072] Three exemplary embodiments of the invention are shown in FIG. 3: the SP version with the metal grating (top left, device 101), the TP version with a DBR mirror (top right, device 102), and the TP version with a metal mirror structure and a dielectric grating (bottom, device 103).

[0073] In another advantageous embodiment of the invention the device comprises the substrate, adhesion layer, metal mirror structure with dielectric layer, and a dielectric grating. The device may then also include a protective layer. This embodiment of the device uses diffraction grating effect. The metal mirror may also in this structure be substituted with a DBR mirror. In this case, an additional dielectric layer may also be added between the DBR mirror and the dielectric grating.

[0074] Advantageously, this version of the device works with both TE and TM mode laser light.

[0075] Various embodiments of the device are well suitable and stable to be adjacent to various media such as water, Phosphate Buffer Solution or cellular tissue culture media.

[0076] The components and versions of the device may be combined to achieve the desired effects, for example to achieve increased resonance at one wavelength, or resonance at several different wavelengths.

[0077] The most common manufacturing methods of the field-enhancing device (100) are described below, but the device can be constructed also with different manufacturing techniques. The adhesion layer (002), the metal in the mirror (0031), the full metal (005) and the starting layer for the metal grating (006) are typically deposited by a metal evaporator or a sputter. The dielectric layers (0032, 0041, 0042, 008) and the starting layer for the dielectric grating (007) are usually deposited by plasma enhanced chemical vapour deposition (PECVD) or by atomic layer deposition (ALD). For the grating (006, 007) fabrication, the features are typically defined by electron beam lithography (EBL) or nanoimprint lithography (NIL) after which a lift-off processes or dry and wet etching processes is applied.

[0078] FIG. 4 shows the reflectance measurements of the SP nanograting structure with groove width of 200 nm and spacing of 100 nm on an area of 30×30 pmt. The optical reflectance properties of the SP nanograting structures were characterized with varying refractive indexes of 1 (air), 1.33 (water) and 1.49 (PMMA). The incident TM polarized light was illuminated along the 1-D nanograting structure and the reflected light was collected by the optical spectrometer. The measurement spectra show the surface plasmon resonance wavelengths with respect to the predefined 1-D nanograting structures. The decreased reflectance (i.e., increased absorption) at resonance (based on the dimensions of the grating and the refractive index of the environment) shows the effectiveness of the structure. This present invention relates to the use of nanograting structures as disclosed herein to resonate with the excited laser beam and molecular vibrational frequency for enhancing linear and nonlinear Raman scattering, TPEL, SHG, SFG and FWM signal intensity.

[0079] FIG. 5 illustrates the reflectance spectra of the exemplary device 102. The reflectance spectrum of the mirror structure 004 only (curve a) shows high reflectance from 350 to 1000 nm. When the whole device 102 is measured (curve b), the characteristic reflection minimum or absorption dip related to the plasmons can be clearly seen at the designed wavelength. This wavelength can be varied over a wide range by changing the dimensions in the structure.

[0080] FIG. 6 illustrates the calculated transverse magnetic (TM) and transverse electric (TE) reflectance spectra of a device 100 construct, that uses Tamm plasmons, surface plasmons and grating diffraction that are coupled to achieve high signal amplification. As seen from the figure, this device can be used in both TM and TE modes in microscopes. The absorption dip wavelength is between 450 to 500 nm.

[0081] The nonlinear coherent emissions of FWM, TPEL, SHG and SFG signal intensities are significantly enhanced by using SP nanostructured nanograting grooves according to embodiment of the present invention. The present invention can be used in biological, bioimaging, medical diagnosis, pathology and chemical applications where it is useful to detect and identify the small number of molecules in sample

[0082] The resonance frequency of the TAMM plasmon may be adjusted by the thickness of the metal and dielectric layers of the device.

[0083] FIG. 7 shows one exemplary structure of a device 104 according to an embodiment of the invention where the SEBI substrate is optimized for green fluorescent protein (GFP). The structure depicted may be used for fixed or live cells.

[0084] The periodicity 0077 defines the surface plasmon resonance wavelength of the grating structure. The periodicity 0077 may be varied from 250 to 350 nm to resonate with the green fluorescent protein (GFP) excitation wavelength, which is at 488 nm. In an advantageous embodiment, the periodicity 0077 is around 300 nm.

[0085] The depth 0707 (essentially corresponding to the depth of the grating) determines the strength of the resonance wavelength. The value of the depth 0700 may be between 20-60 nm. In an advantageous embodiment, the depth 0707 is about 20 nm.

[0086] In the embodiment of FIG. 7, the device comprises a substrate 001, which may in this specific exemplary embodiment glass. With a glass substrate, the device may be used in both reflection and transmission mode.

[0087] An adhesion layer (002) may in FIG. 7 comprise TiO.sub.2, while a dielectric grating (007) may also comprise TiO.sub.2. The thickness of the adhesion layer (002) may be 20-150 nm, advantageously 69 nm. A total thickness of the device may be around 89 nm. The structure may be optimized to operate in the region of green light and may thus be advantageous with GFP.

[0088] The adhesion layer (002) of TiO.sub.2 may be deposited by atomic layer deposition (ALD) method. The dielectric grating may be formed by electron beam lithography or nanoimprint lithography techniques.

[0089] FIG. 8 shows the reflectance spectrum that may be obtained with a device according to the embodiment of FIG. 7. Diffraction peaks may be observed at 484 nm and 540 nm (measured in water, refractive index 1.33).

[0090] FIG. 9 gives one more exemplary structure of a device (105) according to an embodiment of the invention where the SEBI substrate is optimized for GFP. The periodicity (0079) of the grating may be varied from 250-350 nm to resonate with the green GFP excitation wavelength. In an embodiment, the periodicity (0079) is 300 nm. The structure of FIG. 9 may be used with fixed or live cells, and may be used in reflection mode. Here, the excitation and emission may be collected from the same direction.

[0091] The depth (0709) of the grating may be between 20-60 nm and advantageously a depth of about 25 nm may be used.

[0092] A substrate 001 may be glass or silicon, advantageously silicon, while an adhesion layer (002) may be Ti with a thickness of 2-6 nm, advantageously about 5 nm. A full metal layer (005) may be Ag with a thickness of 50-100 nm, advantageously about 80 nm. A metal grating (006) may be Ag with thickness of 25 nm so as to advantageously form the depth (0709) of 25 nm. A protective layer (008) may be Al.sub.2O.sub.3 with a thickness of 2-10 nm, advantageously about 5 nm.

[0093] The titanium adhesion layer (002) and/or the silver metal may be deposited by evaporation or sputter techniques. The protective layer (008) may be deposited by atomic layer deposition. ALD may provide the benefit of providing confocal growth which may be important to avoid the bleaching or quenching effect in fluorescence imaging.

[0094] FIG. 10 shows yet one exemplary structure of a device (106) according to an embodiment of the invention, which is optimized for mCherry protein and/or for use with SECARS and is usable mainly in the infrared region. The periodicity (0710) of the grating may be varied from 500-600 nm to resonate with the red fluorescent protein excitation wavelength, which is at 561 nm. In an embodiment, the periodicity (0710) is about 580 nm. The structure of FIG. 10 may be used for fixed or live cells, and may be used in reflection mode. Also here, the excitation and emission may be collected from the same direction.

[0095] A substrate (001) may be glass or silicon, advantageously silicon. An adhesion layer (002) may be Ti with a thickness between 2-6 nm, advantageously around 5 nm. A full metal layer (005) may be Au with a thickness of 50-100 nm, advantageously about 80 nm. A metal grating (006) may be Au with thickness of 25 nm so as to advantageously form the depth (0710) of 25 nm.

[0096] The adhesion layer (002) may be deposited by evaporation or sputtering techniques. The grating of the metal layer may be formed by electron beam lithography or nanoimprint lithography techniques, while the gold metal may be deposited by evaporation or sputtering. This surface layer quality and roughness values may be important for biomedical imaging applications. The growth and/or deposition parameters may be optimized to achieve high surface quality.

[0097] FIG. 11 shows a reflectance spectrum (measured in water, refractive index 1.33) that may be obtained with a device according to the embodiment of FIG. 9. The spectrum shows a surface plasmon dip at 494 nm.

[0098] FIG. 12 shows a reflectance spectrum (measured in air, refractive index 1) that may be obtained with a device according to the embodiment of FIG. 10. The spectrum shows a surface plasmon dip at 613 nm.

[0099] The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims.

[0100] The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.