Title of Invention: Fluorescence detection enhancement using photonic crystal extraction Abstract: Enhancement of fluorescence emission from fluorophores bound to a sample and present on the surface of two-dimensional photonic crystals is described. The enhancement of fluorescence is achieved by the combination of high intensity near-fields and strong coherent scattering effects, attributed to leaky photonic crystal eigenmodes (resonance modes). The photonic crystal simultaneously exhibits resonance modes which overlap both the absorption and emission wavelengths of the fluorophore. A significant enhancement in fluorescence intensity from the fluorophores on the photonic crystal surface is demonstrated. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS Pursuant to the provisions of 35 U.S.C. § 119(e), this application claims priority to U.S. Provisional Application Ser. No. 60/916,462 filed May 7, 2007, the content of which is incorporated by reference herein. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention was made, at least in part, with United States governmental support awarded by the National Science Foundation under NSF BES 0427657. The United States Government has certain rights in this invention. BACKGROUND OF THE INVENTION Photonic crystals, also commonly referred to as photonic bandgap structures, are periodic dielectric structures exhibiting a spatially periodic variation in refractive index that forbids propagation of certain frequencies of incident electromagnetic radiation. The photonic band gap of a photonic crystal refers to the range of frequencies of electromagnetic radiation for which propagation through the structure is prevented in particular directions. A photonic crystal structure may be designed to exhibit extraordinarily high reflection efficiency at particular wavelengths, at which optical standing waves develop and resonate within the photonic crystal structure. Such optical resonances are known to occur at the wavelengths adjacent to the photonic band gap, sometimes referred to as the photonic band edge. The spatial arrangement and refractive indices of these structural domains generate photonic bands gaps that inhibit propagation of electromagnetic radiation centered about a particular frequency. This anomalous resonant phenomenon (termed guided-mode resonance) arises due to the introduced periodicity which allows phase-matching of externally incident radiation into modes that can be reradiated into free-space. Due to the fact that these modes possess finite lifetimes within such structures, they are referred to as ‘leaky eigenmodes’ of the structures. More recently, guided-mode resonances have been studied in crossed gratings or two-dimensional (2D) photonic crystal (PC) slabs. The leaky nature of these modes has been exploited towards the development of light emitting diodes (LEDs) with improved extraction efficiency, biosensors (see Cunningham et al., Colorimetric Resonant Reflection as a Direct Biochemical Assay Technique, Sensors and Actuators B, 2002, 81, pgs 316-328 (2002)) and vertically emitting lasers. The ability of photonic crystals to provide high quality factor (Q) resonant light coupling, high electromagnetic energy density, and tight optical confinement can also be exploited to produce highly sensitive biochemical sensors. Here, Q is a measure of the sharpness of the peak wavelength at the resonant frequency. Photonic crystal biosensors are designed to allow a liquid test sample to penetrate the periodic lattice, and to tune the resonant optical coupling condition through modification of the surface dielectric constant of the crystal through the attachment of biomolecules or cells. Due to the high Q of the resonance, and the strong interaction of coupled electromagnetic fields with surface-bound materials, several of the highest sensitivity biosensor devices reported are derived from photonic crystals. Such devices have demonstrated the capability for detecting molecules with molecular weights less than 200 Daltons (Da) with high signal to noise margins, and for detecting individual cells. Because resonantly coupled light within a photonic crystal can be effectively spatially confined, a photonic crystal surface is capable of supporting large numbers of simultaneous biochemical assays in an array format, where neighboring regions within ˜10 μm of each other can be measured independently. See Li, P., B. Lin, J. Gerstenmaier, and B. T. Cunningham, A new method for label-free imaging of biomolecular interactions. Sensors and Actuators B, 2003. Given substantial advances in their fabrication and their unique optical properties, photonic crystal-based sensors are under development for a variety of applications. Biosensors are one application. Biosensors incorporating photonic crystal structures are described in the following references, which are hereby incorporated by reference in their entireties: U.S. Pat. Nos. 7,118,710, 7,094,595, and 6,990,259; U.S. Published applications 2007/0009968; 2002/0127565; 2003/0059855; 2007/0009380; 2003/0027327; Cunningham, B. T. J. Qiu, P. Li, J. Pepper and B. Hugh, A Plastic Calorimetric Resonant Optical Biosensor for Multi-parallel Detection of Label Free Biochemical Interactions, Sensors and Actuators B, 2002, 85, pgs 219-226. U.S. Pat. No. 6,707,561 describes a grating-based biosensing technology that is sometimes referred to in the art as Evanescent Resonance (ER) technology. This technology employs a submicron scale grating structure to amplify a fluorescence signal, following a binding event on the grating surface, where one of the bound molecules carries a fluorescent label. ER technology enhances the sensitivity of fluorophore based assays enabling binding detection at analyte concentrations significantly lower than non-amplified assays. ER technology uses grating generated optical resonance to concentrate laser light on the grating surface where binding has taken place. In practice, a laser scanner sweeps the sensor at some angle of incidence (θ), typically from above the grating, while a detector detects fluoresced light (generally at longer optical wavelength) from the sensor surface. By design, ER grating optical properties result in nearly 100% reflection, also known as resonance, at a specific angle of incidence and laser wavelength (λ). Confinement of the laser light by and within the grating structure amplifies emission from fluorophores bound within range of the evanescent field (typically 1-2 μm). Hence, at resonance, transmitted light intensity drops to near zero. The spectral width and wavelength of the resonance phenomena describes the important externally measurable parameters of a device. Resonance width refers to the full width at half maximum, in wavelength measure, of a resonance feature plotted as reflectance (or transmittance) versus wavelength. Resonance width also refers to the width, in degrees, of a resonance feature plotted on a curve representing reflectance or transmittance as a function of θ. In practice, one can make adjustments to the incident angle to “tune” the resonance towards maximum laser fluorophore coupling. In one embodiment of this invention, a biosensor is constructed as a photonic crystal structure which has a periodic surface grating in which a so-called evanescent resonance is created. Conceptually, resonance phenomena can occur in planar dielectric layer gratings where almost 100% switching of optical energy between reflected and transmitted waves occurs when the grooves of the grating have sufficient depth and the radiation incident on the corrugated structure is at a particular angle. This phenomenon is exploited in the sensing area of the platform where that sensing area includes grating structures (e.g., grooves, or holes or posts) of sufficient depth and light is caused to be incident on the sensing area of the platform at an angle such that evanescent resonance occurs in that sensing region. This creates in the sensing region an enhanced evanescent field which is used to excite samples under investigation. It should be noted that the 100% switching referred to above occurs with parallel beam and linearly polarized coherent light and the effect of an enhanced evanescent field can also be achieved with non-polarized light of a non-parallel focused laser beam. Excitation photons incident on the sample (chip, for example) under resonance conditions couple into a thin corrugated surface (such as a metal oxide layer) at the site of incidence. As a result of the transducer geometry, the energy is locally confined into the thin corrugated layer of high refractive index material. Consequently, strong electromagnetic fields are generated at the surface of the chip. The effect has been attributed as evanescent resonance and leads to increased fluorescence intensity of chromophores (fluorescent material) close to the surface of the sensor. The effective field strength can be increased up to 100-fold by the confinement of the available excitation energy, depending on the optical properties of the optical detection system used. The inventive sensors and method of this disclosure are useful in conjunction with a variety of different types of fluorophores. Such fluorophores have excitation and emission spectra which are typically well characterized and available from the manufacturer, or can be determined experimentally. Quantum Dots (QDs) are fluorescent, nanometer-sized inorganic semiconductor crystals that have rapidly emerged as an important class of nanomaterials which promise to revolutionize a wide range of nanotechnology-enabled fields. QDs derive their unique optical properties (broad absorption spectrum, narrow, size-tunable emission spectrum, high photostablity, quantum efficiency and strong nonlinear response) from quantum confinement effects. These attributes, coupled with the ability to functionalize QDs, has made them important candidates for light sources, solar cells, optical switches and fluorescent probes in sensitive biological assays. The ability to more efficiently excite and extract the light emitted by QDs would thus be of vital importance in realizing high brightness light sources, enhanced nonlinear effects and lowering the detection limits in biological assays. Fluorescent dyes represent a broad class of organic and inorganic fluorescent molecules that are capable of emitting light. Generally, electrons within the fluorescent molecule are excited from a ground state to an excited state through the absorption of a photon from an external source of illumination. The electron in the excited state may return to the ground state through a variety of mechanisms. One such mechanism is through the release of heat in the form of a phonon. Another such mechanism is through the release of light in the form of a photon. Absorption of energy by the fluorophore occurs at a particular range of incident photon energies (or equivalently wavelengths) that are unique for each type of molecule. Due to conservation of energy, the emitted photon energy must be less than or equal to the energy of the incident photon, and therefore the emitted wavelength must be larger than the incident photon wavelength. Therefore, a fluorescent molecule has two distinct spectra associated with it: the range of wavelengths for which it is capable of absorbing photons, and the range of wavelengths for which it is capable of emitting photons. The difference between the absorption and emission wavelength is known as the Stokes shift. SUMMARY Photonic crystal sensors are disclosed for use in testing samples in which a fluorophore, e.g., inorganic crystalline semiconductor (“quantum dot”) or fluorescent dye is present in the sample. The sample and fluorescent dye are in close proximity, or more typically bound, to the photonic crystal surface, e.g., by depositing the sample with fluorophore on the sensor surface in a dry or aqueous environment. In one aspect of this disclosure, the photonic crystal sensor is constructed and arranged with a surface in the form of a periodic surface grating structure which simultaneously exhibits multiple resonance modes for light at a given incident angle. The resonance modes overlap both the excitation and emission spectra of the fluorophore. In particular, when light is incident upon the photonic crystal at an appropriate incident angle θ, the photonic crystal sensor simultaneously exhibits multiple resonance modes (referred to herein as leaking eigenmodes or leaky modes) which have spectra that overlap both the absorption (excitation) and emission spectra of the fluorophore present in the sample. In this document, the term “spectrum” in the context of a resonant mode refers to the band of wavelengths of incident light in which a guided mode resonance is created in the photonic crystal as the angle of incidence θ varies. A photonic crystal constructed and arranged so as to possess such a doubly resonant scheme (i.e., exhibiting resonance modes overlapping both the excitation and emission spectra of the fluorophore simultaneously at a given incident angle θ) yields strongly enhanced fluorescent emission and the ability to extract such emission in a highly efficient manner, resulting in a high sensitive sensors suitable for a very broad range of applications, as will be explained below. A sample testing system for testing a sample having a fluorophore bound to the sample is also described. The testing system includes a detection instrument comprising a light source and a detector; and a photonic crystal sensor having a periodic grating structure. The sample including the fluorophore is placed on the periodic grating structure. The light source of the detection system is oriented relative to the photonic crystal sensor such that the light source illuminates the photonic crystal sensor at a incident angle θ in which the photonic crystal simultaneously exhibits a plurality of resonant modes, the resonant modes including an excitation mode having a first resonant spectrum and an extraction mode having a second resonant spectrum. The periodic grating structure is constructed and arranged such that the resonant spectrum of the photonic crystal in the excitation mode at least partially overlaps the excitation spectrum of the fluorophore and the resonant spectrum in the extraction mode at least partially overlaps the emission spectrum of the fluorophore. The detector operates to detect radiation from the fluorophore in the emission spectrum. A method of testing a sample with a fluorophore present in the sample with the photonic crystal sensors of this disclosure are also described. The method includes the step of placing the sample onto the surface of a photonic crystal sensor; illuminating the photonic crystal biosensor with light at an angle of incidence θ, the biosensor responsively and simultaneously exhibiting (1) an excitation resonance mode having a spectrum which at least partially overlaps the excitation spectrum of the fluorophore; and (2) an extraction resonance mode having a spectrum which at least partially overlaps the emission spectrum of the fluorophore, the illumination and the resulting excitation and extraction resonance modes causing the fluorophore to emit light; and collecting the emitted light from the fluorophore and directing the emitted light onto a detector. In yet another aspect, a photonic crystal sensor is disclosed which includes a periodic surface grating structure which exhibits a resonance mode at a given incident angle which overlaps the emission spectrum of a fluorophore which is present with a sample deposited on the sensor, and does not have a resonance mode which overlaps the excitation spectrum of the fluorophore. The photonic crystal sensor produces an enhanced extraction effect without also producing an enhanced excitation effect. Sample testing systems suitable for the doubly resonant photonic crystal sensors are also useful with this embodiment. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is an illustration of a photonic crystal featuring the enhanced excitation and extraction features of this disclosure and comparing the directionally enhanced extraction of emitted radiation from fluorophores from the photonic crystal with a dielectric slab which does not feature the resonance modes of this disclosure. FIG. 2a is an illustration of a two-dimensional photonic crystal device having a periodic surface grating structure constructed as a two dimensional array of holes in accordance with one exemplary embodiment of the invention. FIG. 2b shows Scanning Electron Micrograph (SEM) images of a cleaved photonic crystal device of FIG. 2a. In FIG. 2a, the lines joining the features Γ, X and M are axes of high symmetry in the photonic crystal surface. Λ=300 nm is the period of the surface grating structure (with a square unit cell in X and Y directions) and t=125 nm, the thickness of a high index of refraction material layer which is deposited onto the grating layer. Θ is the angle the incident beam of light makes with the vertical. FIGS. 3a and 3b are graphs of the calculated and experimental dispersion spectra, respectively, of the fabricated photonic-crystal sensor of FIGS. 2a and 2b for white, S-polarized incident illumination incident along the Γ-M direction. The sensor exhibits resonant mode at λ=488 nm when the incident beam makes an angle θ=11.2° with the surface normal. The excitation of the resonant mode at this wavelength, where the quantum dot fluorophore is strongly absorbing, provides the required near-field enhancement for evanescent resonance (enhanced excitation). The shading scale shows the efficiency of transmission. Higher Q factors for a resonance mode are indicated by thinner lines whereas broader Q factors for a resonance mode are indicated by relatively thicker lines. FIGS. 4a and 4b are calculated near electric-field intensities (E2) at resonance for the photonic crystal structure of FIG. 2a, with FIG. 4a showing the intensity for the lower surface of the surface grating (bottom of the hole) and FIG. 4b showing the intensity for the upper surface of the surface grating. The intensities are for the leaky mode (resonance) with λ=488 nm when the incident beam makes an angle θ=11.2° with the surface normal. The intensity is normalized to the unit amplitude incident wave. FIGS. 5a, 5b and 5c are graphs of the dispersion spectrum showing the resonance modes of the photonic crystal sensor showing the possibility of enhanced extraction for all polarizations and directions of incident white light. The graphs were experimentally determined from the photonic crystal of FIG. 2a. FIG. 5a is the graph for P-polarized and incident along the Γ-M direction, FIG. 5b is the graph for P-polarized and incident along the Γ-X direction, and FIG. 5c is the graph for S-polarized, incident along Γ-X direction. FIGS. 6a and 6b are fluorescence (pseudocolor) scan images of the photonic crystal of FIG. 2a with quantum dots dispensed on the surface. FIG. 6a is a scan image taken when the photonic crystal is resonant with respect to the incident beam (θ=11.2°), showing an enhancement factor of over 108 times. FIG. 6b is a scan image taken when the photonic crystal is not resonant with the incident beam (θ=0°), showing an enhancement factor of over 13 times. The circular regions represent the area over which intensity information was averaged. In both the images, the circle to the left shows the control region where no photonic crystal is present. FIG. 7 is a near-field scanning optical microscopy (NSOM) image of the near-fields on the surface of a fabricated photonic crystal device. FIG. 8 is an illustration of an angle-resolved fluorescence measurement from the photonic crystal surface for the Γ-X direction. FIG. 9 is a graph of the normalized fluorescence spectrum of a quantum dot fluorescence as modified by the photonic crystal sensor. FIG. 10 illustrates angle-resolved fluorescence measurements from the photonic crystal surface for the light polarized in the Γ-M direction. DETAILED DESCRIPTION Photonic crystal sensors are disclosed for use in testing samples in which a fluorophore, e.g., inorganic crystalline semiconductor (“quantum dot”) or fluorescent dye is present in the sample. The sample and fluorescent dye are in close proximity, or more typically bound, to the to the photonic crystal surface, e.g., by depositing the sample with fluorophore on the sensor surface in a dry or aqueous environment. An example of the photonic crystal is shown in FIG. 2a and will be described in detail subsequently. In one aspect of this disclosure, the photonic crystal sensor is constructed and arranged with a surface in the form of a periodic surface grating structure (such as a two dimensional array of holes shown in FIG. 2a), which simultaneously exhibits multiple resonance modes for light at a given incident angle. The resonance modes are shown graphically in FIGS. 4 and 6 (transmission efficiency shown plotted as a function of wavelength for incident light at different angles to vertical), described subsequently, and are indicated in the areas of the graphs where the transmission efficiency drops to zero. The resonance modes overlap both the excitation and emission spectra of the fluorophore present in the sample. In particular, when light is incident upon the photonic crystal at an appropriate incident angle θ, the photonic crystal sensor simultaneously exhibits multiple resonance modes (referred to herein as leaking eigenmodes or leaky modes) which have spectra that overlap both the absorption (excitation) and emission spectra of the fluorophore present in the sample. A photonic crystal constructed and arranged so as to possess such a doubly resonant scheme (i.e., exhibiting resonance modes overlapping both the excitation and emission spectra of the fluorophore) to enhance fluorescent emission from the fluorophore has not, to the current knowledge of the inventors, been previously demonstrated. The presence of the photonic crystal resonance peak occurring at the fluorescence excitation wavelength gives rise to the formation of high intensity electromagnetic near-fields which serves to efficiently excite the fluorophore present in the sample. This phenomenon is referred to herein interchangeably as “evanescent resonance” or “enhanced fluorescence.” The additional feature of a resonance mode in the photonic crystal which overlaps the emission spectrum of the fluorophore serves as an effective mechanism to extract this enhanced emission. In particular, a photonic crystal with resonance modes overlapping both the emission and excitation spectra of the fluorophore increases the number of fluorescence emitted photons which can be gathered by a detector in an associated measuring instrument for use with the photonic crystal sensor. Photonic crystal resonance occurring at the emission wavelength of the fluorophore can be used to efficiently couple emitted photons at the photonic crystal surface to be selectively directed into free space at a particular exit angle, instead of uniformly directed in all directions. This phenomenon of selective direction of emitted photons is believed to be due to fluorescence coupling to the overlapping leaky modes producing Bragg scattering out of the structure, thereby greatly reducing the amount of light trapped in the photonic crystal sensor in the extraction mode. If the dispersion of these overlapping emission leaky modes is close to the Γ-point band edge (i.e., the magnitude of the in-plane wave vector for incident polarized light is close to zero), a significant amount of the emitted light can be extracted from the photonic crystal sensor within small angles of the vertical. This discovery allows for positioning of a detector (or associated optical elements such as fiber optic probe which is coupled to a detector) at the correct position relative to the photonic crystal surface, and allows for capturing more photons that would otherwise occur, e.g., as compared to a fluorophore emitting from a non-photonic crystal surface. A photonic crystal sensor as just described is shown in FIG. 1 as item 10, with a dielectric slab 20 shown next to it which does not possess a photonic crystal property for purposes of comparison. The photonic crystal sensor 10 consists of a periodic surface grating structure formed on its surface 11, which in this example takes the form of a two-dimensional array of holes 12. (Other periodic structures for the photonic crystal 10 are possible, as will be explained below). A sample containing a fluorophore 14 is applied to the surface 11. The photonic crystal is illuminated with light (in this case from above) at an angle θ relative to the vertical direction. The angle θ is shown in FIG. 2a. The properties of the periodic grating (holes) 14 on the surface 11 are such that the photonic crystal simultaneously exhibits resonance modes which overlap both the excitation and emission spectra of the fluorophore 14. The overlap of the resonance mode with the excitation spectrum produces an enhanced excitation of the fluorophore 14. The overlap of the resonance mode with the emission spectrum of the fluorophore 14 produces a directionally enhanced extraction of the emitted radiation due to Bragg scattering, which is indicated by the lines 16 all pointing in the same direction. As a practical matter, the angle of the lines 16 can be determined either by simulation or by experimentation and light collection apparatus placed in alignment with the lines 16 so as to collect this radiation. The light collection apparatus (e.g., fiber optic probe) supplies the collected radiation to a detection device such as a CCD imager or photomultiplier tube so as to make measurements or collect images of the radiation, thereby obtaining information as to the sample. Consider now the dielectric slab 20 in FIG. 1 which does not possess any photonic crystal attributes for the sake of comparison. Assuming the incident light encompasses the excitation spectrum of the fluorophore 14, at least some fluorescence can be expected. However, because there is no resonant mode in the slab 20 overlapping the emission spectrum of the fluorophore, the resulting fluorescence is radiated in an almost spatially uniform manner, indicated by the lines 16 pointing in all directions. Thus, the concentration of radiation in the same direction as indicated at 16 on the left hand side of FIG. 1 produces an enhanced extraction of the fluorescent signal which is not present in a non-photonic crystal structure such as shown on the right hand side of FIG. 1. (While light can be directed onto the surface 11 from above, as shown in FIG. 1, it can also be incident on the photonic crystal from below and the same resonant effects are produced in the photonic crystal). The gain in sensitivity in the photonic crystal sensor 10 of FIG. 1 obtained by (1) evanescent resonance (enhanced fluorescence due to one resonance mode of the photonic crystal overlapping the excitation or absorption spectrum of the fluorophore) and (2) enhanced extraction (due to another resonance mode of the photonic crystal overlapping the emission spectrum of the fluorophore) are multiplied together to derive the overall gain in sensitivity resulting from the combination of these features. For example, if the evanescent resonance provides for a hundred-fold increase in the amount of fluorescence emission, and the enhanced extraction provides the ability of the detector to collect a five times as many fluorescent photons, then the overall sensitivity of an assay performed on a photonic crystal surface using the combined techniques will be five hundred (500) times greater than the same assay performed on an ordinary surface (e.g., microscope slide, microplate or microfluidic flow channel) which does not possess the photonic crystal properties of this invention. Thus, one benefit of a photonic crystal sensor 10 with the doubly resonant properties as described above is that it provides a sensor platform in which a very substantial increase in the sensitivity of fluorescent assays occurs than would otherwise be possible. As will be appreciated from the following detailed description, photonic crystal sensors in accordance with the invention are useful in a number of different applications. These include: 1) Gene expression microarrays incorporating the photonic crystal sensor 10. The genes may be detected at lower expression levels and or with smaller sample volumes than previously known. 2) Protein detection assays, such as for example detection of protein biomarkers in bodily fluids for disease diagnostic tests, where proteins are present in very low concentrations. Detection by the methods of this disclosure would be more sensitive than commonly use ELISA assays, but with a simpler assay protocol. 3) Fluorescent imaging of cells, viruses, tissue samples, bacteria, proteins etc., using a microscope. The photonic crystal sensor 10 of this disclosure can be incorporated onto the surface of a microscope slide. A specimen is stained with one or more fluorescent dyes or quantum dot fluorophores (which may be conjugated to an antibody or other probe of interest) and placed on the slide such that the specimen and fluorophore(s) are in contact with the surface of the photonic crystal sensor 10. The improved sensitivity can be used to observe dye molecules at lower concentrations and/or to use lower cost imaging cameras with an improved signal to nose ratio. 4) The photonic crystal sensor 10 can be produced uniformly over large surface areas using a nano-replica molding process which is suitable for mass production at low cost. The photonic crystal structure thus produced can be incorporated into the surface of various assay or testing devices of conventional formats, such as, for example, (1) incorporation onto the surface of a microscope slide, (2) incorporation within a standard format microplate, e.g., at the bottom thereof, or (3) incorporation into any other fluorescent assay format either now known or later developed. Thus, in one embodiment of the invention, a sensor 10 is descried herein which is adapted to test a sample having a fluorophore present in the sample which is deposited on the sensor surface. The fluorophore (e.g. quantum dot, fluorescent dye such as Cy5) has an excitation spectrum and an emission spectrum. The sample may be placed on the sensor 10 in a dry or an aqueous environment. The sensor includes a photonic crystal having a periodic grating structure (12). The photonic crystal exhibits a plurality of resonant modes when illuminated with light at an incident angle θ. The resonant modes include an excitation mode having a first resonant spectrum and an extraction mode having a second resonant spectrum. The periodic grating structure is constructed and arranged such that the first resonant spectrum of the photonic crystal in the excitation mode at least partially overlaps the excitation spectrum of the fluorophore and the second resonant spectrum of the photonic crystal in the extraction mode at least partially overlaps the emission spectrum of the fluorophore. In one embodiment, the extraction resonant mode of the photonic crystal at the incident angle exhibits a relatively low Q factor, i.e., one in which the Q factor is less than 100. The Q factor of the extraction mode will determine the rate at which coupled radiation will be scattered into free space, such as in the case where the fluorophore exhibits a broad emission spectrum. In other embodiments, the extraction resonant mode has a Q factor is relatively high, i.e., between 100 and 1000. With a relatively high Q factor, a detector can obtain enhanced extraction from a narrow band of wavelengths, but with amplified extraction efficiency. A sensor constructed to produce the optimum Q factor at the extraction resonant mode for a given application will depend on several factors, such as 1) whether the sensor is designed to be used in a detection instrument which performs single-point detection (with a photomultiplier tube, as an example) where one can gather a broad range of wavelengths with a broad range of exit angles, situations where a low Q factor for the extraction resonant mode might be best, and 2) whether the sensor is designed to be used in a detection instrument which performs imaging detection (e.g., with a CCD camera), in which case a relative high Q factor would provide detection of a narrow range of wavelengths and a narrow range of exit angles for that wavelength. The Q factor for the extraction resonance mode can be changed by changing the parameters of the surface grating structure and simulated in the design phase as will be described below. The sensor as described herein can be incorporated into testing platforms suited to a variety of specific applications. In one example, the sensor is incorporated into a gene expression microarray device. In another example, the sensor is incorporated into a protein detection assay device and the sample is in the form of a protein. In still other examples, the sensor is incorporated into a testing device, such as a microscope slide, which is used to perform fluorescent image analysis of cells, viruses, bacteria, spores, and tissue samples. In still another example the sensor is incorporated into a microwell plate having a plurality of individual sample wells. Each of the sample wells includes a photonic crystal as described herein. In one particular embodiment, the periodic surface grating structure of the photonic crystal is constructed as a grating layer arranged as a two-dimensional array of holes each having a depth D, and a relatively high index of refraction material of thickness t deposited on the grating layer. Suitable high index of refraction materials include titanium oxide (TiO2), silicon nitride, hafnium oxide, zinc sulfide, tantalum oxide and zinc selenide. In preferred embodiments the array of holes has an axis of symmetry which is either perpendicular or parallel to the polarization state of the incident light. In FIG. 2a, the lines connecting the features Γ, X and M are axis of high symmetry in the photonic crystal surface. The grating layer is positioned above a substrate layer. The substrate layer may consist of a layer of glass, quartz, polymers, plastic, polyethylene terepthalate (PET) and combinations thereof. The depth D of the holes can be selected such that the photonic crystal exhibits the excitation and extraction resonance modes having spectra which are spectrally separated from each other and which substantially overlap the excitation and emission spectra, respectively, of a predetermined fluorophore, such as a particular quantum dot or group of quantum dots or other fluorophores having similar excitation and emission spectra. In another aspect of this invention, a sample testing system is described for testing a sample having a fluorophore present in the sample which is deposited on the photonic crystal sensor. The fluorophore has an excitation spectrum and emits fluorescence in an emission spectrum. The sample testing system includes a detection instrument comprising a light source and a detector, and a photonic crystal sensor comprising a periodic grating structure. The sample including the fluorophore are placed on the periodic grating structure. The particular construction of the detection system is not particularly important and can vary widely, depending on the particular application. Examples of suitable detection systems include those systems described in U.S. Pat. Nos. 7,118,710, 7,094,595, and 6,990,259 and U.S. Published applications 2007/0009968; 2002/0127565; 2003/0059855; 2007/0009380; and 2003/0027327. The light source of the detection system is oriented relative to the photonic crystal sensor such that the light source illuminates the photonic crystal sensor at a incident angle θ in which the photonic crystal simultaneously exhibits a plurality of resonant modes, the resonant modes including an excitation mode having a first resonant spectrum and an extraction mode having a second resonant spectrum. The periodic grating structure is constructed and arranged such that the first resonant spectrum of the photonic crystal in the excitation mode at least partially overlaps the excitation spectrum of the fluorophore and wherein the second resonant spectrum of the photonic crystal in the extraction mode at least partially overlaps the emission spectrum of the fluorophore. In one embodiment, the detector may take the form of an imaging detector, e.g., charge coupled device (CCD) camera. Other types of detectors are also possible, including photomultipliers. In preferred embodiments the light source may be a laser or a broad spectrum source. The light from the source may be polarized. In one embodiment, the grating structure of the photonic crystal has an axis of symmetry which is substantially parallel or perpendicular to the polarization state of the incident light. The sensor can be incorporated into a variety of different testing device formats, as explained herein, such as a gene expression microarray device, a protein detection assay device, a microscope slide, and a microwell plate or dish having a plurality of individual sample wells, in which each of the sample wells includes a photonic crystal described herein. In a further aspect, a method is disclosed of testing a sample having a fluorophore bound to the sample, the fluorophore having an excitation spectrum and an emission spectrum, comprising the steps of: (a) placing the sample onto the surface of a photonic crystal sensor, (b) illuminating the photonic crystal biosensor with light at an angle of incidence θ, the biosensor responsively and simultaneously exhibiting (1) an excitation resonance mode having a spectrum which at least partially overlaps the excitation spectrum of the fluorophore; and (2) an extraction resonance mode having a spectrum which at least partially overlaps the emission wavelength distribution of the fluorophore, the illumination and the resulting excitation and extraction resonance modes causing the fluorophore to emit light, and (c) collecting the emitted light from the fluorophore and directing the emitted light onto a detector. EXAMPLE FIG. 2a is an illustration of a photonic crystal sensor 10 in accordance with one embodiment of the invention. The sensor 10 includes a glass substrate layer 22, a grating layer 24 providing a periodic grating structure (in this case a two dimensional array of holes 12) and a high index of refraction later 26 of TiO2 which is deposited on the grating layer 24. In order to design a PC that can support multiple guided-mode resonances, a two-dimensional structure with a sufficiently large effective index and the features arranged as a square lattice of holes was chosen, as shown in FIG. 2a. The period (A) of the structure was chosen such that it supports a relatively high Q-factor resonant modes at a wavelength where the fluorophores (in this instance, quantum dots) are excited (λ=488 nm, excitation mode) and low Q-factor modes overlapping the quantum dot fluorescence emission spectrum (centered at λ=616 nm, extraction mode). In FIG. 2a, the features Γ, X and M are points of high symmetry in the photonic crystal surface. Λ=300 nm is the period of the surface grating structure (with a square unit cell in X and Y directions) and t=125 nm, the thickness of a high index of refraction material layer 26 which is deposited onto the grating layer 24. Θ is the angle the incident beam of light 13 makes with the vertical (direction normal to the surface 11). The logic governing the choice of low Q-factor extraction modes for some embodiments will become clear shortly, and although a relatively low Q-factor resonance mode at the emission spectrum is shown (Q<100) in other applications a relatively high Q-factor (100≦Q<1000) may be desired. The depth “D” of the holes 12 was chosen to provide the required spectral separation between the excitation and extraction modes. The thickness ‘t’ of the TiO2 high index layer 26 was chosen to fine tune the spectral location of the resonant modes. The photonic crystal sensor of FIG. 1a was cost-effectively fabricated by a nano-replica molding approach in an asymmetric configuration, so as to provide the required mechanical stability and maintain a simplistic fabrication procedure. A combination of high refractive index (RI) material (layer 26) and low refractive index material (layer 24, “Nanoglass™”, Honeywell) was important in order to provide sufficient effective index and positioning of the modes, respectively. Using a low RI material for layer 24 allows the modes to be positioned closer to the device's upper surface 11, whereas a high RI material for layer 24 which would draw the modes deeper into the device 10 and subsequently reduce their interaction with the fluorophores. The top surface 11 and cross section SEM images of a fabricated model device are shown in FIG. 2b. The selection of parameters of the device (refractive index, thickness, periodicity, etc.) can be chosen to provide the desired location of resonant modes, as further described herein. While an array of holes is shown in the embodiment of FIG. 2a, other types of periodic structures are possible. Such structures generally have a configuration of periodic high and low regions (referred to herein occasionally as “grooves”), which can take a variety of forms. In one embodiment, the thickness of the high index layer 26 is in the range 30 to 1000 nm, e.g. 50 to 300 nm, preferably 50-200 nm. The period of the corrugated structure may be in the range 200 to 1000 nm, e.g. 200 to 500 nm, preferably 250-500 nm. The ratio of the groove depth to the thickness of the high index layer 26 lies in the range 0.02 to 1 e.g. 0.25 to 1, preferably 0.3 to 0.7, and the ratio of the grooves width to the period of the grooves (“duty-cycle”) lies in the range 0.2 to 0.8, e.g. 0.4 to 0.6. Increasing the thickness of the high refractive index layer 26, the refractive index of the layer 26, the refractive index of the low index periodic grating layer 24, or the period of the grating will tend to increase the wavelength of the resonant mode. The grooves may be generally rectangular in cross-section. Alternatively, the grooves may be sinusoidal or of saw tooth cross-section. The surface structure may be generally symmetrical. Preferred geometries include rectangular, sinusoidal and trapezoidal cross-sections. Alternatively, the grooves may be of saw tooth cross-section (blazed grating) or of other asymmetrical geometry. In another aspect the groove depth may vary, e.g. in periodic modulations. The support or platform may be square or rectangular and the grooves may extend linearly along the platform so as to cover the surface. Alternatively the platform may be disc shaped and the grooves may be circular or linear. The grating structure can take variety of one and two dimensional forms, including two-level, two dimensional gratings, as disclosed in published PCT application WO 2007/0179024, the contents of which are incorporated by reference herein. These include square lattices and hexagonal lattices that have symmetry in three directions along the planar surface of the structure. The corrugated, periodic grating surface may be optimized for one particular excitation wavelength and for one particular type of polarization. By appropriate means, e.g. superposition of several periodic structures which are parallel or perpendicular one with another, periodic surface relief can be obtained that are suitable for multiple wavelength use of the photonic crystal sensor (“multicolor” applications). Alternatively, individual sensing areas on one platform may be optimized for different wavelengths and/or polarization orientations. In another embodiment, the photonic crystal is constructed so as to exhibit a first extraction resonance mode in a first spatial area of the photonic crystal and a second extraction resonance mode in a second spatial area of the photonic crystal distinct from the first spatial area. In other words, the construction of the surface grating structure can vary spatially (in X and Y directions) such that different areas of the photonic crystal exhibit different extraction resonant modes. This spatial pattern of different extraction modes can be repeating. The first extraction resonance mode has a spectrum at least partially overlapping the emission spectrum of a first predetermined fluorophore (e.g., a particular quantum dot) and wherein the second extraction resonance mode has a spectrum at least partially overlapping the emission spectrum of a second predetermined fluorophore different from the first predetermined fluorophore (e.g., a second quantum dot). Additionally, the construction of the surface grating structure 12 can vary spatially (in X and Y directions) such that different areas of the photonic crystal exhibit different extraction resonant modes. As an example, the photonic crystal exhibits different excitation resonant modes at different spatial regions on the surface so that the fluorescent dye Cy5 is excited in one location, and the fluorescent dye Cy3 is excited in another nearby location. One could alternate between a group of fluorophores (red, green, blue) much like how a single pixel of a video display is comprised of multiple color emitters arranged close to each other. In another possible embodiment the photonic crystal is constructed so as to exhibit a plurality of extraction resonance modes, each of the plurality of extraction resonance modes having a resonant spectrum at least partially overlapping an emission spectrum of a different predetermined fluorophore. Enhanced Excitation When externally incident light 13 (FIG. 2a) interacts with periodically modulated structures (holes 12) in the sub-wavelength regime for a photonic crystal, only the 0th order forward and backward diffracted waves can propagate. The periodicity however, also allows for phase-matching of higher (evanescent) orders to localized leaky modes supported by the photonic crystal 10. Once excited, the leaky modes, defined by a complex propagation constant, possesses a finite lifetime as they are leaked out both in the forward (transmitted) and backward (specular) directions. The backward reradiated waves are in phase and constructively interfere with the 0th backward diffracted order while the forward reradiated waves are out of phase with the 0th forward diffracted order by π radians, causing destructive interference and consequently resulting in zero transmission. Thus, the external excitation of the leaky modes by means of incident light 13 (FIG. 2a) is associated with a 100% reflection phenomenon for the resonant wavelength, assuming a defect-free, lossless system. Since the excited leaky modes are radiative but localized in space during their finite lifetimes, they can be engineered to have very high energy density within regions of the photonic lewiscarla@example.com. The magnitude of this energy density is directly related to the resonant mode lifetime or Q-factor of resonance, which in turn can be controlled by adjusting the device parameters (thickness, refractive index, depth of holes, periodicity, etc.). Therefore, the intensity of emission of a fluorophore (14, FIG. 1) (which is absorptive at the resonant wavelengths) can be greatly enhanced by placing the fluorophores in proximity to regions where the resonant modes concentrate most of their energy. In the example of FIG. 2A, this region is the bottom of the holes, as shown in FIG. 4a, as will be explained below. Enhanced Extraction Concurrently with the enhanced excitation as just described, the existence of leaky modes in the photonic crystal 10 that overlap the fluorescence emission spectrum opens up additional pathways for the emitted light to escape into free-space. Besides direct emission, the fluorescence can couple to the overlapping leaky modes and Bragg scatter out of the photonic crystal sensor, thereby greatly reducing the amount of light trapped as guided-modes, in comparison to an un-patterned substrate (as explained above in FIG. 1). If the dispersion of these overlapping emission leaky modes is close to the Γ-point band-edge, i.e. K11(magnitude of in-plane wave vector)˜0, a significant amount of the emitted light will be extracted within small angles with the vertical. It can thus be appreciated that enhancement of fluorescence can be achieved by enhanced excitation and enhanced extraction acting in concert together at the same time. Results for Example 1 Studying the reflection/transmission properties of a photonic crystal is a convenient technique to map out the dispersions of the leaky modes Rigorous Coupled-Wave Analysis (RCWA) techniques were used to simulate how the device of FIG. 2a would respond in transmission to externally incident radiation. FIG. 3a shows the computed leaky mode band structure, i.e. spectral location and transmission efficiency of the resonances as a function of the angle of incidence (θ) of light 13 (FIG. 2a), along the Γ-M direction. As θ is increased from 0°, K∥ begins to increase and results in degenerate resonances to separate into their respective constituent orders, as indicated at 40 in FIG. 3a. Experimental verification of the band structure of the fabricated device was carried out by mounting the device in a linear transmission setup, illuminating it with collimated white light and plotting the resulting transmitted spectrum as a function of θ, results of which are shown in FIG. 3(b). Excellent qualitative agreement between simulation and experiment in the 460 nm 500 nm range was seen, where the RIs of the materials vary slowly. Theoretically, it was predicted that the resonance at λ=488 nm should occur at θ=11.2° (the resonance indicated the region 42 in FIG. 3a), and this is accurately observed in experiment. At shorter wavelengths, the RI of TiO2 begins to increase divergently (nTiO2=2.7 at λ=400 nm) and is considerably less at longer wavelengths (nTiO2=2.36 at λ=600 nm), leading to deviations from simulations assuming a constant RI (nTiO2=2.46 at λ=488 nm). This is clearly seen in the theoretically predicted higher order bands originating from shorter wavelength resonances and red-shifted longer wavelength bands, which experimental results do not agree with. For the excitation mode shown in FIG. 3A, the Q-factor of resonance was found to be ˜155. The relatively high Q-factor is indicated in FIG. 3a by the thin, highly defined line 41 of zero transmission efficiency in the region 42. FIGS. 4a and 4b show the simulated electric near-field intensity (E2) (normalized to the unit amplitude incident field) at the two available surfaces of the device, for the excitation of the resonant mode at λ=488 nm. FIG. 4a shows the intensity at the bottom of the holes 12 of FIG. 2a. FIG. 4b shows the intensity at the upper surface 11 of the sensor 10. The influence of the resonance phenomenon on the resulting near-fields is clearly seen as manifested in the enhanced electric field intensity. Similar enhancement can also be seen for the magnetic near-fields. It is apparent that for the lower available surface (FIG. 4a) (bottom of the hole) the excitation mode concentrates its energy within the cavity region (the term cavity is used here strictly in relation to the shape of the cross-section). At the upper surface (FIG. 4b), the energy is concentrated at the cavity periphery and beyond. Both the bottom of the well and the areas on the surface 11 adjacent to the holes are where the fluorophore will be present during use, hence the device exhibits strong intensities in the excitation resonance mode in the areas of interest. Above the surfaces shown in FIGS. 4a and 4b, the field intensity decays exponentially (as previously shown). In practice, with a finite fluorophore which is not at the surface and unavoidable losses, the exact near-field intensity available to the fluorophore will always be lower than shown in FIGS. 4a and 4b. The amount of amplification for enhanced excitation detection is related to the power transferred from the device structure to a distribution of fluorophores on the sensor surface at the excitation wavelength of the fluorophore. The power density distribution of the sensor surface at the resonant wavelength, provided that the resonant wavelength is matched to the excitation wavelength, therefore provides a means for comparing the sensitivity of different device designs. One can define the cross product E (max) X H (max) as a field power or “magnification factor”. While a more thorough analysis of the intensity distribution of
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