WO2008045123A2

WO2008045123A2 - Nanofabricated optical interference array

Info

Publication number: WO2008045123A2
Application number: PCT/US2007/001581
Authority: WO
Inventors: Henri Lezec; James Jay Mcmahon; Pulickel M. Ajayan; Omkaram Nalamasu
Original assignee: Rensselaer Polytechnic Institute; California Institute Of Technology
Priority date: 2006-01-23
Filing date: 2007-01-22
Publication date: 2008-04-17
Also published as: WO2008045123A3; WO2008045123A9

Abstract

A display or sensor device includes a nano-stepped or nano-dimpled solid surface. The device produces a color readout in response to an optical interference effect between light reflected from a bottom and a top of a step or a dimple of the surface. The device is adapted to detect a change in the reflected light in response to a presence of a fluid and/or a change in a property of the fluid. A fluid source is adapted to deliver or remove a transmissive fluid to or from the step or dimple, which causes a change in the color readout or reflectance spectrum.

Description

NANOFABRICATED OPTICAL INTERFERENCE ARRAY

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application 60/761 ,513, filed on January 23, 2006, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention is generally directed to optical interference arrays and more specifically to microfluidic, nanofabricated optical interference arrays.

[0003] A transparent thin-film coating on a reflective substrate displays a modulated reflection spectrum with alternating high- and low-intensity features. This effect results from optical interference between light reflected at the two interfaces defined by the thin film. When such a sample is illuminated with white light, a distinct color is perceived. This color is a sensitive function of the thickness d and the index of refraction n of the thin film. See R. C. Bailey et al , "Sensing via optical interference," Mater. Today 2005, April, 46-52.

[0004] Porous silicon and porous aluminum have been investigated as materials for chemical or biological sensing. The reflection spectrum of thin films of such a material generates interference fringes as a function of wavelength. Detectable shifts in these interference fringes occur when the "average" index of refraction of the porous matrix is changed upon permeation with a chemical liquid or upon selective binding of biological molecules. This porous matrix can be treated as being laterally homogeneous, while having optically smooth interfaces (e.g., porous Si/air and porous Si/bulk Si) in the transverse direction. See, e.g., V. S. Lin et al, "A Porous Silicon-Based Optical Interferometric Biosensor," Science 1997, Vol. 278, 840; M. Anderson et al, "Sensitivity of the optical properties of porous silicon layers to the refractive index of liquid in the pores," Phys. Status Solidi (A) 2003, Vol. 197, 528; S. Pan & L. Rothberg, "Interferometric Sensing of Biomolecular Binding Using Nanoporous Aluminum Oxide Templates," Nano Lett. 2003, Vol. 3, 81 1. [0005] However, the porosity of the matrix is not easily controlled or varied across the lateral portions of the surface, thus making it difficult to use disordered matrices for multi- pixel displays or for sensor assays that detect multiple analytes in parallel at different parts of the surface.

BRIEF SUMMARY OF THE INVENTION

[0006] An embodiment of the present invention provides a display device that includes a nano-stepped or nano-dimpled solid surface. The surface produces a color readout in response to an optical interference effect between light reflected from a bottom and a top of a step or a dimple of the surface. A fluid source is adapted to deliver or remove a transmissive fluid to or from the step or dimple, wherein the color readout is altered when the fluid is delivered or removed.

[0007] Another embodiment of the present invention provides a sensor device that includes a nano-stepped or nano-dimpled solid surface having an ordered arrangement of steps or dimples, such that surface exhibits an optical interference effect between light reflected from a bottom and a top of a step or a dimple of the surface. The device is adapted to detect a change in the reflected light in response to at least one of a presence of a fluid on the step or dimple or a change in a chemical property of the fluid on the step or dimple.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure IA is an optical microscope image (in reflection, at normal incidence, using a 10^χ objective lens) of a row of 23 dimple fields each of fixed periodicity (P = 800 nm), size (1 1 x 1 1 dimple array) and cavity diameter (f= 600 nm). Dimple depth d is increased from array to array in 32-nm increments, from d = 96 nm at the leftmost array position (Field #1) to d = 800 nm at the rightmost array position (Field #23).

[0009] Figures IB, 1C and ID are scanning electron microscope (SEM) images (45° tilt) of detailed areas of fields with ./=96, 416, and 800 nm, respectively (Fields ## 1 , 12, and 23, respectively), in the fields of Figure IA. [0010] Figures 2A and 2B contain optical microscope images (in reflection, at normal incidence, using a 2Ox objective lens) of selected fields in Figure IA before and after the addition of methanol, respectively. The insets of Figures 2A and 2B are schematic cross sectional views of the device surface facing air and methanol, respectively.

[0011] Figure 3 A is a plot of reflectance spectra, R(Y), versus wavelength for arrays with dimple depths ct=544, 608, 672, 736, and 800 nm, respectively (Fields U 15, 17, 19, 21 and 23, respectively), with sample facing air (w=l).

[0012] Figure 3B is a plot of measured reflectance data (depicted as data points) for array with d = 800 nm (field 23), facing fluids of different dielectric constant n. Wavelength positions of minima and maxima in R, λ_min and λ_max, are plotted as a function of n. Theoretical predictions based on the interference model of Equations 3a and 3b are shown as solid lines. Adjustable parameter) represents the interference orders explicit in Equations 3a and 3b which lead to best fit to experimental data for λ_min and λ_max. The inset is a plot of measured spectrum and corresponding interference model prediction (Equation 2) for the case n = 1.64,

[0013] Figure 4A is a schematic to view of a microfluidic sensor and display with color readout according to an embodiment of the invention.

[0014] Figures 4B-D are optical microscope images of the progression of an air-water interface progressing from right to left in the microfluidic channel of Figure 4A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A fluid-based interference approach is provided for producing bright color fields, without the need for two parallel interfaces such as those of a thin solid film. The two reflective interfaces of the solid film are conceptually replaced by a single solid surface having one or more dimples or steps on the surface. Colors of the entire visible spectrum can be generated by choosing appropriate depths of the steps or dimples, as well as the lateral distances between adjacent steps or dimples. The topological permeability of such an open surface readily allows infusion of fluids, having different refractive indices, for color switching or detection. [0016] Figures IA-D show an array of bi-level cavities ("dimples") on a substrate surface according to an embodiment of the present invention. A polished Si wafer was used as the substrate. The Si wafer was milled using a focused ion (Ga^"", 30 keV, 300 pA) beam (FIB) to create a periodic, two-dimensional array of flat-bottomed cylindrical dimples of varying depths. For notational convenience, the bottom surface of the step or dimple is designated as surface "B," and the top surface adjacent to the step or dimple (for instance, the raised area surrounding the cavity opening of a dimple) is designated as surface "A." Array geometry of the present invention is configured according to a few simple design rules. Dimple diameter φ is chosen to ensure that the cylindrical cavity forms a waveguide below cutoff, i.e., allows propagation of light down and back up the cylindrical cavity. This can be achieved by choosing φ > λγ/2, where λ_v corresponds to the maximum wavelength of interest for device operation. For wavelengths in the visible and near-infrared range, λ_v = 1 ,000 nm. The spacing between steps or dimples can, but need not, be periodic. For instance, the spacing between steps or dimples may be substantially randomized. Alternatively, an array period P is chosen to ensure that the unit-cell area corresponding to the bottom surface B, A_B = πφ²/4, is approximately equal to the unit-cell area corresponding to the top surface A, ^.^=P²-πφ²/4. For visible to near-infrared wavelengths, the areas A_A and A_B are preferably greater than approximately 190,000 nm², for instance approximately 283,000 to 357,000 nm². Maximum interference contrast can be obtained under plane wave illumination at normal incidence when the reflected intensity from surfaces A and B are substantially equal. Because the material in the present embodiment (i.e., silicon) is identical for surfaces A and B, the reflected intensity from these equal-area surfaces should also be substantially equal. However, if different materials are used for surfaces A and B, then both A_A and AB can be chosen to ensure substantially equal reflected intensity from these surfaces. Dimple diameter φ is chosen to be wavelength or subscale, i.e., to be comparable to the central wavelength of the range of operation of interest, i.e., φ ~ λς. This ensures significant diffraction at the exit of each cavity leading to strong lateral overlap and interference between light reflected from surfaces A and B.

[0017] The above conditions were satisfied for the visible and near- infrared range (λ_v = 1 ,000 nm, and λς = 650 nm). Dimple diameter φ was chosen as 600 nm (thus, A_B ~ 283,000 nm²). Periodicity P was chosen as 800 nm (thus, A_A ~ 357,000 nm²). Under these conditions, the reflected intensity from Si surfaces A and B is substantially equal because A_A and A_B are substantially equal. Different values for parameters φ and P can be used for different dimple or step geometries (e.g., square dimples, multi-terraced steps) or for different wavelengths of interest (e.g., UV). Additionally, different manufacturing processes other than FIB can be used to create the nano-stepped or nano-dimpled surface. For instance, e- beam lithography, photolithography, nanoimprint lithography, or CD stamping may be used. Different substrates besides Si wafers, for instance, glass or other reflective surfaces can also be used.

[0018] Figure IA shows a color readout of a dimpled surface according to an embodiment of the present invention. The surface contains a row of 23 dimple fields (labeled 1 to 23). Within each field is an 1 1 x 1 1 square array of flat-bottomed, cylindrical dimples of equal depth d. From field to field, the depth d increases linearly in constant increments of Ad > 32 nm. Figure IB is an SEM image of the left-most field, Field #1 , which has d = 96 nm. Figure 1 C is an SEM image of a middle field, Field #12, which has d = 448 nm. Figure ID is an SEM image of the right-most field, Field #23, which has d = 800 nm. Each l l x l l array is visible in Figure IA in reflection under white light illumination at normal incidence under optical microscopy, at a magnification of 10. Each 1 1 x 1 1 array appears as a single colored pixel. However, the pixels can be larger or smaller depending on the number of dimples or steps used or the magnification used. Preferably, parallel processing by nano-imprint lithography or CD stamping may provide more efficient fabrication of larger step/dimple arrays across larger surfaces for direct visualization without the need for microscopy. For a review of nano-imprint lithography techniques, see C. M. Sotomayor Torres et al. , "Nanoimprint lithography: an alternative nanofabrication approach," Mat. Sci. Eng,. C 23 (2003) 23-31 ; S. Zankovych et al, "Nanoimprint lithography: challenges and prospects," Nanotechnology, Vol. 12 (2001) 91 -95.

[0019] Figures 2A-B show a dimpled array according to an embodiment of the present invention before and after methanol is delivered to the surface. Figure 2A is an optical microscope image of a selected area of the device row of Figure IA, showing dimple Field #5 (d = 224 nm) through Field #21 ( d= 736 nm), viewed at a magnification of 20 (reflection mode, under white light, normal incidence). Note that the white dots apparent in some of the color fields correspond to missing dimples resulting from fabrication error. This indicates that a single dimple or step may be used as a single pixel. In Figure 2A, the surface is facing air. In Figure 2B, a thick (milimeter-range) liquid film of methanol (»=1.3) is applied to the device. Each field is observed to undergo a dramatic color change between the color readouts of Figures 2A-B. Without wishing to be bound by any particular theory, the insets of Figures 2A-B illustrate a proposed explanation for the interference process leading to the observation of color and color change. Colors are produced when light reflected from the bottom of the dimples (surface A) interferes with light reflected for the area surrounding the dimple exit surfaces (surface B). Angular diffraction of light when exiting the cavity as well as scattering from the top surface between the cavities leads to mixing and interference in the far field, including in-the reconstructed microscope image. The variation of the net reflected beam as a function of wavelength (known as Fabry-Perot fringes) is what leads to the perception of a colored field. This mechanism is further supported by the observation that the colors perceived at the array locations begin to disappear at higher magnification (such as 4Ox). In that case, the microscope can separate light reflected from the bottom of the dimples from that reflected for the top surface; interference does then not take place in the image plane, leading to loss of color contrast. Adding a fluid, such as a liquid, above the sample surface readily fills the cavities and establishes a new difference in optical path length δ between light reflected from surface A and surface B, from δ = 2d to δ = 2nd.

[0020] Taking an effective medium approach, Y_A and V_B are the area-averaged complex amplitude reflection coefficient from surfaces A and B respectively. The net far-field intensity resulting from lateral mixing of light reflected from A and B is given by:

where λ „ and Λare the vacuum wavelength and intensity of the incident light, respectively. Since surfaces A and B are composed of the same material, rA and rs have the same phase. Equation (1) can therefore be written as:

/_Λ (λ) (2)

Minima and maxima occur for IR at respective vacuum wavelength positions of And

A._,_ — (3a)

2; - i

and

Λ 7 nax = — ^2nd / ("³1M^b)

J where/ denotes the order of the interference.

[0021] Figures 3A-B show plots of experimental and theoretical reflectance spectra according to an embodiment of the present invention. Figure 3 A shows experimental reflectance spectra,

for arrays with dimple depths d = 544, 608, 672, 736, and 800 nm, respectively (Fields ## 15, 17, 19, 21 and 23, respectively), with sample facing air (« = 1). With regard to depth d, it is notable that Figure 3 A shows steeper slopes for larger values of d, indicating higher sensitivities to changes of wavelength. A broad, periodic modulation is evident across the spectral range. Varying the index of refraction n of the dielectric space facing the dimple array leads to a notable shift in the reflectance spectrum, This effect is demonstrated in Figure 3B for the array with d = 800 nm (Field #23). In this figure, experimental values of λ_mm and λ_max (in the range 400 - 900 nm) are plotted as a function of n, where n is varied by successively immersing the array in air, methanol and two different index-matching fluids (Cargille series A and AA), respectively. Predicted positions for λ_mm and ^"k_max based on Equations 3a and 3b are also shown in Figure 3B, using a self- consistent set of values for/ which give the best match to the discrete experimental values. Interference orders/ = 3, 4, and 5 are inferred to be effective over the considered spectral range for producing minima and maxima in the reflected signal. A maximum for a given interference order/ occurs when the round-trip distance down and up the dimple, 2nd, is equal to an integer number/ of dielectric-adjusted wavelengths λo/n. This brings the wave reflected from the bottom of the dimple in phase with the wave reflected from the surface between the dimples, leading to a condition of constructive interference and maximum net reflected intensity. A minimum for the same order/ entails fitting an extra half-wavelength λo/2n within that round-trip distance, leading to destructive interference and a minimum in the net reflected signal. [0022] The inset of Figure 3B displays the corresponding experimental reflectance spectrum R(K) for the case n = 1.64. The theoretical spectral dependence of R based on the analytic expression of Equation 2 is also plotted, using reflection amplitudes \r_A\ and \r_B\ as fitting parameters. A good match between the two curves is obtained, demonstrating the basic validity of the simple interference model proposed above.

[0023] Figures 4A-D show a microfluidic device according to an embodiment of the present invention. The device is used as a simple, environmentally-friendly fluid-based display or sensor device having a direct color readout. Preferably, the device allows direct visual observation of a color change upon the introduction or removal of a liquid to or from the steps or dimples of the surface. Alternatively or concurrently, a reflectance spectrum, such as the spectra previously discussed with respect to in Figures 3A-B, may be used to detect the change in the reflected light. The surface may comprise a plurality of steps or dimples. The steps may include ledges, cliffs, or terraces, and may be have rounded or sharp corners. The dimples may include cavities having different shapes, such as cylindrical, rectangular, or other shapes. The dimples may be positioned on the terraces of the steps. Optionally, the dimples may be positioned within other dimples to form tri- or multi- level dimples. Rather than being flat, the surfaces A and B may be rounded, curved, or disposed at non-parallel angles to one another. For instance, such off-angle surfaces may accommodate different angles of incidence or reflectance.

[0024] Figure 4A illustrates a display or sensor device,according to an embodiment of the present invention. A dimple array is disposed on the bottom of a microchannel. The channel is used to deliver or remove liquids of various indices of refraction to the surface of the arrays in a controlled manner. The depth d is preferably less than about 5 microns, such as about 2 microns, for example about 96 nm to 800 nm. A column of dimple arrays (each array having an area 10 μm x 10 μm) with geometrical parameters identical to those of Field #8 previously discussed with respect to in Figures 1-2 (P= 800nm, φ=600nm, and c?=320nm) was fabricated by FIB milling on the surface of an undoped, polished Si wafer. A transparent molded PDMS (polydimethyl siloxane) microchannel (channel height 10 μm, channel width 200 μm, channel length 5 mm), connecting two fluid sources (for example, fluid wells) on either end, was then placed over the dimple arrays. One well was filled with water, the other with air, and the sample was placed under an optical microscope (10^χ objective) for observation in reflection. Optionally, the air can be replaced with a second fluid having a different index of refraction from that of water. Optionally, the air can be replaced with a gas other than air, such as nitrogen, argon, carbon dioxide, or other gas. Any fluid, for example any liquid or gas fluid, can be used so long as the fluid is sufficiently transmissive to allow incident light to reach the underlying stepped or dimpled surface.

[0025] Figure 4B shows a portion of the microchannel containing the dimple arrays facing air. A color readout initially shows only blue arrays. An air-water interface was then positioned in the channel and repeatedly moved across the dimple arrays by pneumatic action. Figure 4C shows a portion of the arrays covered with water. That water-facing portion produces a color readout of orange arrays, while the air-facing portion remains blue. As the fluid moves across the dimples (from left to right in the channel), the color of the dimple arrays switches to orange, as seen in Figure 4D. From a visual perspective, the color switch takes place concurrently and instantaneously with the progression of the air-water interface over the arrays, implying rapid filling of the cavities with liquid. To alter the color switching effect, other types of liquids and surfaces can be used, such as nonpolar liquids or hydrophobic surfaces. The liquid is preferably transmissive to allow a portion of the incident light to pass through the liquid and reflect from the solid surface. Preferably but not necessarily, the stepped or dimpled surface is disposed within the microchannel. Alternatively, the stepped or dimpled surface is contained in a chamber different from, but fluidly connected to, the microchannel. Optionally, the microchannel is adapted to deliver or remove a plurality of different liquids to or from the steps or dimples. For instance, different arrays of dimples or steps are fluidly connected to the different microchannels, each of which is adapted deliver or remove a fluid having a different index of refraction, which thereby produces a different color readout or reflectance spectrum at each array,

[0026] The display device of the present invention can be used to implement display devices ranging from ultraminiature-scale (for microfluidic applications such as high-contrast monitoring of liquid flow in lab-on-a-chip applications) to full-scale television display screens. Recently, a color-display device based on electrowetting of dyed liquids in microfabricated pixels was demonstrated. See R. A. Hayes and B. J. Feenstra, "Video-speed electronic paper based on electrowetting," Nature, Vol. 425, 383 (2003); V. H. Kwong, M. A. Mossman and L. A. Whitehead, "Control of reflectance of liquid droplets by means of electrowetting," Appl. Opt. Vol. 43, 808 (2004), The present invention eliminates the need for colored dyes in such devices, allowing pixilated colors to be generated on each array of steps or dimples. Because colors result only from local surface topography, vast surfaces incorporating pixels of different base colors (such as RGB) could be printed in one single embossing step, on rigid or flexible substrates, using mechanical approaches ranging from nano-imprint lithography for ultra-miniaturized applications to CD stamping techniques for large-scale production. To minimize trivial diffraction effects, the relative positions of the pixels and/or the steps or dimples can be randomized with respect to one another. Optionally, rather than having depth d be the same for all steps or dimples within a given array, the depth d can be varied within the array. It is believed that varying d within each array achieves sharper spectral features and sharper response to changes in fluid index.

[0027] The sensor device of the present invention can be used for material detection and identification. The device is adapted to detect a change in the reflected light in response to at least one of a presence of a liquid on the step or dimple, or a change in a property of the liquid on the step or dimple. For instance, the device can automatically monitor for a change in the color readout or reflectance spectrum, for instance by using a photodetector and/or optical microscope interfaced to a camera. Depending on the magnitude of the change, the device can calculate the liquid's index of refraction and, in turn, identify the liquid. Because the index of refraction of a material depends on how light interacts with the electrons inside the material, the sensors can detect such changes in a material's optical, electronic, magnetic, and chemical properties. For instance, a first and a second microchannel is adapted to deliver a first and a second chemical, respectively, to the surface; and the device is adapted to detect a change in the liquid caused by a chemical reaction between the first and second chemicals. Preferably, the surface comprises an ordered arrangement of steps and/or dimples on the surface, such that the parameters φ, P, and d can be precisely controlled at any lateral position along the surface. The multiple arrays of steps and/or dimples can each assay for different types of chemicals, which can be delivered through different microchannels.

[0028] The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. A display device, comprising: a nano-stepped or nano-dimpled solid surface, wherein the surface produces a color readout in response to an optical interference effect between light reflected from a bottom and a top of a step or a dimple of the surface; and a fluid source adapted to deliver or remove a transmissive fluid to or from the step or dimple, wherein the color readout is altered when the fluid is delivered or removed.

2. The display device of claim 1 , wherein: the bottom of the step or dimple comprises a bottom surface; the top of the step or dimple comprises a top surface; and the area of the bottom surface is substantially equal to the area of the top surface.

3. The display device of claim 1 , wherein: the bottom of the step or dimple comprises a bottom surface; the top of the step or dimple comprises a top surface; the bottom surface is sufficiently large to substantially allow propagation of light to and from the bottom surface; and the bottom surface is sufficiently small to substantially cause diffraction of light exiting the bottom of the step or dimple near the top surface.

4. The display device of any of claims 2-3, wherein the bottom surface has an area greater than about 190,000 nm².

5. The display device of any of claims 2-4, wherein a lateral distance between adjacent steps or dimples is about 800 nm.

6. The display device of claim 1 , wherein a depth measured from the bottom to the top of the step or dimple is less than about 5 microns.

7. The display device of claim 6, wherein a depth measured from the bottom to the top of the step or dimple is less than about 2 microns.

8. The display device of claim 7, wherein a depth measured from the bottom to the top of the step or dimple is about 96 nm to 800 nm.

9. The display device of any of claims 1 to 8, wherein: the surface comprises an at least one array of steps or dimples; and the at least one array comprises an at least one pixel of the color readout.

10. The display device of claim 9, wherein the depth is the same among the steps or dimples within the at least one array.

1 1. The display device of claim 9, wherein the depth varies among the steps or dimples within the at least one array.

12. The display device of claim 1 , wherein: the fluid source comprises a microchannel; the fluid comprises a fluid interface; and the color readout is altered when the fluid interface is moved across the step or dimple.

13. The display device of claims 12, wherein the microchannel is adapted to deliver a plurality of fluids having different indices of refraction to or from the step or dimple.

14. The display device of any of claims 12-13, wherein: the surface comprises silicon; and the microchannel comprises polydimethyl siloxane.

15. A method of operating a display device comprising a nano-stepped or nano-dimpled solid surface, the method comprising: reflecting an incident light from the surface; generating a first color readout in response to the step of reflecting, such that the surface exhibits an optical interference effect between a bottom and a top of a step or a dimple of the surface; delivering or removing a transmissive fluid to or from the step or dimple; and generating a second color readout in response to the step of delivering or removing.

16. The method of claim 15, wherein: the step of generating a first color readout comprises reflecting an incident light from the surface substantially covered by a first fluid having a first index of refraction; and the step of generating a second color readout comprises reflecting an incident light from the surface substantially covered by a second fluid having a second index of refraction.

17. The method of claim 16, wherein:

(a) the first fluid comprises a gas; and the second fluid comprises the transmissive fluid; or

(b) the first fluid comprises the transmissive fluid; and the second fluid comprises a gas.

18. The method of claim 17, wherein the transmissive fluid comprises a liquid.

19. The method of claim 15, wherein: the incident light comprises white light; the first color readout comprises a first wavelength in the visible spectrum; and the second color readout comprises a second wavelength in the visible spectrum, such that the first wavelength is different from the second wavelength.

20. A sensor device, comprising a nano-stepped or nano-dimpled solid surface having an ordered arrangement of steps or dimples, such that surface exhibits an optical interference effect between light reflected from a bottom and a top of a step or a dimple of the surface; wherein: the device is adapted to detect a change in the reflected light in response to at least one of: a presence of a fluid on the step or dimple; or a change in a property of the fluid on the step or dimple.

21. The sensor device of claim 20, wherein the change in the reflected light comprises a change in at least one of a color readout or a reflectance spectrum.

22. The sensor device of claim 21 , wherein: the bottom of the step or dimple comprises a bottom surface; the top of the step or dimple comprises a top surface; and the area of the bottom surface is substantially equal to the area of the top surface.

23. The sensor device of claim 21 , wherein: the bottom of the step or dimple comprises a bottom surface; the top of the step or dimple comprises a top surface; the bottom surface is sufficiently large to substantially allow propagation of light to and from the bottom surface; and the bottom surface is sufficiently small to substantially cause diffraction of light exiting the bottom of the step or dimple near the top surface.

24. The sensor device of any of claims 22-23, wherein: the change in the reflected light comprises the change in the color readout; and the bottom surface has an area greater than about 190,000 nm².

25. The sensor device of claim 20, wherein: the device is adapted to detect a change in the reflected light in response to the presence of the fluid on the step or dimple; and the device is further adapted to measure the index of refraction of the fluid.

26. The sensor of claim 20, wherein: the device is adapted to detect a change in the reflected light in response to the change in the property of the fluid on the step or dimple; and the device is further adapted to measure the index of refraction of the fluid.

27. A method of operating a sensor device comprising a nano-stepped or nano-dimpled solid surface having an ordered arrangement of steps or dimples, the method comprising: reflecting an incident light from the surface; generating a reflected light in response to the step of reflecting, such that the surface exhibits an optical interference effect between a bottom and a top of a step or a dimple of the surface; and monitoring for a change in the reflected light in response to at least one of: a presence of a fluid on the step or dimple; or a change in a property of the fluid on the step or dimple.

28. The method of claim 27, wherein the step of reflecting comprises illuminating the surface with the incident light in a direction normal to the surface; and the change in the reflected light comprises a change in at least one of a color readout or a reflectance spectrum.

29. The method of claim 27, further comprising delivering the fluid on the step or dimple using a microchannel.

30. The method of claim 27, wherein the step of monitoring comprises using at least one of a photodetector or a camera.