ES2363955A1 - Method for the analysis of the refractive index of a dielectric medium adjacent to a plasmonic medium, and corresponding device - Google Patents

Abstract

The method comprises the steps of directing light from a light source (1) towards a plasmonic medium to excite a superficial plasmon resonance, and analysing light from said plasmonic medium (4). A laser diode is used as light source (1), the wavelength of the light emitted by the laser diode being periodically varied. The invention moreover relates to a corresponding device.

Description

Method for refractive index analysis of a dielectric medium adjacent to a plasmonic medium, and corresponding device.

Technical Field of the Invention

The invention is included in the field of detection of changes in media refractive indexes dielectrics, based on the phenomenon of plasmon resonance superficial (for example, SPR or LSPR).

Background of the invention

The detection of index changes of refraction in dielectric media adjacent to a surface metal by detecting Plasmon Resonance Superficial (SPR): "Surface Plasmon Resonance "), including variants such as plasmon resonance localized surface (LSPR).

A surface plasmon wave is a wave magnetic transverse electromagnetic that propagates in the intercalates of a metal and a dielectric when the metal behaves similar to a free electron gas. Plasma wave It is characterized by a propagation vector (wave vector) which defines the necessary conditions to be excited. If he metallic medium and dielectric are semi-infinite, the plasmon propagation vector k_ {SP} is given by the following expression:

one

where \ lambda is the length of wave and n_ {m} and n_ {d} are, respectively, the indices of refraction of metal and dielectric (y ε m) and \ varepsilon_ {d} are its dielectric constants, with n = \ sqrt {\ varepsilon}).

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For the resonance phenomenon to occur of plasmon it is necessary that

- the real part of the dielectric constant of the metal is negative, Re [\ varepsilon_ {m}] <0,

- that Re [\ varepsilon_ {d}] <- Re [\ varepsilon_ {m}],

- and that the wave produced is transverse magnetic (TM).

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These conditions are met for various metals, among which the most used are gold and silver. The electromagnetic field of a surface plasma wave is characterized by having the maximum intensity in the interface of the metal and dielectric and an exponential decay in both means, as schematically illustrated in Figure 1 (this Figure illustrates the exponential decay of the wave in the intercara of metal 1001 and dielectric medium 1002).

As a consequence, the excitation of the wave of surface plasma will depend heavily on the constant dielectric (or refractive index) of the dielectric medium.

There are different ways to excite these waves superficial, for example, with electrons or with light. But nevertheless, the excitation of this surface plasmon wave cannot be carried out directly with light on the metal. This is because the wave vector of light is given by the following expression:

2

where? the angle of incidence of light and λ wavelength. So that produce excitation it is necessary that both wave vectors be same. If we compare the plasmon and light wave vectors it is true that, for any angle of incidence of the light:

3

To be able to excite the plasmon with light superficial different techniques are used, including highlight:

a) Prism coupling (illustrated in shape schematic in Figure 2): a prism 1003 with an index is used of refraction n_ {p} and dialectic constant \ varepsilon_ {p} greater than those of the dielectric medium 1002 in which they will be produced the optical changes (\ varepsilon_ {p}> \ varepsilon_ {d}), and a thin sheet or metal layer 1001 with a determined thickness (which depends on the wavelength of the light and the metal used) interposed between prism 1003 and dielectric medium 1002. In the Figure 2, k_ {x0} is the component of the light wave vector in the air parallel to the reflection surface (and \ varepsilon_ {0} is the dielectric constant of air), k_ {xp} is the component of vector of light waves in the prism parallel to the surface of reflection (y?) is the dielectric constant of prism), and k_ {SP} is the plasmon propagation vector.

The excitation is carried out by means of the total internal reflection of the light in the interface between the prism and the metal, and the plasmon is generated in the interface of the metal and the dielectric medium in which the measurement is to be carried out. In this configuration, the thickness of the metallic layer is an essential parameter to be able to observe the plasmon resonance. The optimum thickness can be calculated by various methods, for example, through the formalism described in the publication M. Shubert, Polarization-dependent optical parameters of arbitrarily anisotropic homogeneous layered media , Physical Review B, vol. 53, p. 4265 (1996).

b) Designing a periodic structure, such as a grid, in the metallic layer. In this way a phenomenon occurs of diffraction of the light that affects the periodic structure, and which leads to an increase in the vector of light waves:

4

where \ Lambda is the period of the periodic structure and N is the order of diffraction of light. With this method the thickness of the metal layer is not a very parameter important, however, if the period will be important and structures depth periodic.

c) Through guided light in a waveguide or in an optical fiber The excitation is done across the field evanescent of the light confined in the core of the guide or of the optical fiber.

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These ways to excite surface plasmon through the incidence of light they are conventionally used (such time, especially the system based on coupling with prism) in measurement / detection systems for changes in the rates of refraction of dielectric media.

These measurement and detection systems are based in the fact that the condition of excitation of the resonance of plasmon depends on the refractive index, n d of the medium dielectric. This implies that if the refractive index changes Plasmon excitation condition will change. This change in resonance condition can be detected in different ways, by example, analyzing the light reflected by the metallic layer as a function of the angle of incidence of the light, maintaining the wavelength fixed, and in a prism coupling configuration.

Figure 3A illustrates a known configuration for the detection of changes in the refractive index of a medium dielectric, comprising a monochromatic light source 1004 with magnetic cross polarization (also known as "TM polarization" or "p polarization", that is, with the electric field within the plane of incidence of light), a 1005 light intensity detector connected to media 1006 electronic data processing configured to analyze the signals at the output of the light intensity detector 1005. In addition, the configuration comprises a coupling prism 1003, a thin metallic layer 1001 (typically, of gold) located on a surface of prism 1003 and, on the other side of the metal layer, that is, in contact with the surface of the metal layer that does not is in contact with the prism, the dielectric medium 1002 (for example, a fluid). Light 1007 is reflected when the layer strikes metallic and the reflected light strikes the light detector 1005, which detects its intensity, which is recorded by the means of electronic data processing 1006.

Figure 3B schematically reflects how the prism 1003 and the metallic layer 1001 can be rotated with with respect to the light source 1004, so that the angle varies the of incidence of light 1007 (this can be done displacing the light source and / or the set consisting of prism 1003 and metallic layer 1001).

As it follows from the explained more above, the condition of plasmon excitation with light depends on several factors, including the wavelength of light, the angle of incidence the and refractive index n_ {d}. Yes with the configuration illustrated, it starts from an angle of incidence small? and this angle is increased, it reaches a moment in which a total reflection of light occurs in the intercalates between prism 1003 and sheet or metal layer 1001. A from that angle, if the incidence angle continues to increase the, there is a sharp decrease in intensity reflected, until reaching a minimum, which coincides with the excitation of the surface plasma wave at the other side of the metal. Since the condition of excitation of plasmon resonance depends on both the angle of incidence? and the index of refraction (n_ {d}) of the dielectric medium, if the other variables they remain constant (for example, the dielectric constant \ varepsilon_ {m} and other characteristics of the metal layer, the wavelength of light, etc.), a change in the rate of refraction (n_ {d}) of the dielectric medium will correspond to a change at the angle of incidence? for which a minimum in the intensity of the reflected light.

Figure 4 reflects two curves that relate the intensity R_ {pp} of the reflected light with polarization TM (measured with detector 1005 of the configuration described above) in incidence angle function? for two indices of different refraction (n_ {d1}, n_ {d2} with n_ {d1} <n_ {d2}). As can be seen, the increase in refractive index from n_ {d1} to n_ {d2} is reflected in a certain displacement to the right in the diagram of the curve R_ {pp} (\ theta) due to the increase in the angle of incidence for which the plasmon excitation. In this way, making sweeps of the, the change in the angle for which it is detected can be detected produces plasmon excitation and relate this change to the Variations in the refractive index of the dielectric medium 1002.

That is, displacement quantification of the angle for which the resonance occurs provides a measurement of the refractive index change.

On the other hand, the sensitivity with which can detect these resonance angle changes depends on what narrow that resonance curve. The narrower, the greater it will be the sensitivity, and that will depend, in this case, on the metal used, the thickness of the layer and the wavelength of the light. A configuration that is usually used is a 50 nm layer of gold and light with a wavelength of 632 nm.

An alternative way to detect changes in the refractive index can consist of maintaining the angle of constant the incidence and measure changes in the reflectivity (in the case of Figure 4, if you choose to keep the angle of incidence? = 72 degrees, an increase in the rate of refraction of n_ {d1} to n_ {d2} would be detected as an increase of reflectivity, etc.). As in the previous case, the sensor sensitivity depends on how narrow the peak of resonance.

If instead of varying the angle of incidence se the wavelength of light is varied, it happens exactly the same, the appearance of a resonance peak that travels to the vary the refractive index of the dielectric medium adjacent to the gold cape This also applies to the case of excitation by a periodic structure or by a waveguide.

There is a large number of detection systems of changes in refractive indices based on the resonance of the superficial plasmon; Examples of such systems are described in:

US-A-5912456

US-A-5485277

US-A-2 0 03103208

Logically, a direct application of this type of sensors is the refractometer (to measure index changes of refraction). However, another important application of this type of Sensors today is the biosensor or chemical sensor. The penetration distance of the evanescent field of the plasma wave surface within the dielectric medium is around 100 nm. Therefore, a biomolecular interaction that takes place in the metal layer surface will vary locally the index of surface refraction. This variation, in turn, will produce a change in the plasmon propagation vector and, as consequence, in the condition of resonance. This change can be detected with the methods described above.

The use as a biosensor can be based in the previous immobilization of receptor biomolecules 2001 on the surface of the metal layer 1001, as illustrated schematically in Figure 5. These receptor biomolecules they can selectively bind to the 2002 analyte molecules that are they want to detect and that they may be present in a liquid with the that the metal layer is in contact. By joining the molecules analyte 2002 with the receptor molecules 2001, will be returned to produce a local change in the refractive index over the metal surface that will vary, in turn, the condition of plasmon resonance.

There is also at least one variant of the Surface Plasmon Resonance (SPR) systems that does not require the coupling means (prism coupling, with periodic structure in the metal layer, or by guided light in a waveguide or in a optical fiber). This is what is known as the Localized Surface Plasmon Resonance (LSPR): "Localized Surface Plasmon Resonance"). Basically, the term SPR (Surface Plasmon Resonance) is usually used when the plasmonic medium is a flat metal structure (for example, as described above, and that includes some type of coupling means) and the term LSPR when the plasmonic medium comprises metal structures of a nanometric size, for example, a surface of metal nanoparticles. The optical properties of metal nanoparticles (especially Au and Ag) are marked by their localized surface plasmon resonance (LSPR). Unlike the thin layer superficial plasmon (SPR), whose nature is propagating, the collective movement of conduction electrons induced by the LSPR is confined in the nanoparticles, and the phenomenon can be understood, in a simplified way, as that of a resonant induced dipole. When LSPR is excited, the absorption and dispersion of photons by the nanoparticles greatly increases and very intense electromagnetic fields are generated around the nanostructure. The excitation of the LSPR does not require coupling means as in the SPR, and can be achieved directly through a beam of light at a certain wavelength. Said excitation wavelength is highly dependent on the shape, size and composition of the nanostructure, as well as the refractive index of the surrounding environment. This last dependence constitutes the principle of biosensor detection of metallic nanostructures. Thus, when there is a local change in refractive index on the surface of the nanoparticle, such as that induced by a biomolecular interaction, there is a change in the wavelength of the resonance that can be detected spectroscopically. As the LSPR is confined in the nanoparticles, the multiplexing capacity is enormous and its limit is in a single nanoparticle. In addition, the sensitivity of nanoparticles to local changes in refractive index is greater than that of the SPR in certain spectral areas of the
resonance.

Borja Sepúlveda , et al ., " LSPR-based biosensors ", Nano Today (2009) 4, pages 244-251 summarizes some aspects of LSPR-based biosensors .

Figure 6A schematically reflects a detail of a Localized Surface Plasmon Resonance sensor in a biosensor application. On a 3001 dielectric substrate 3002 metal nanoparticles have been fixed, for example, by chemical bonds, or said metal nanoparticles have been formed 3002 by, for example, nanolithography. This represents a metallic nanostructure, in which LSPR can be induced.

To fulfill its biosensor function, they have attached receptor molecules 3003 to metal nanoparticles 3002. These 3004 analyte molecules can be attached to these receptor molecules, producing a change in the index of refraction from n_ {d1} to n_ {d2}. Figure 6B reflects the displacement of the curve of the plasmon signal before and after the refractive index change by binding of 3004 analyte molecules to 3003 receptors immobilized in the nanostructure. As intuited in Figure 6B, Plasmon signals have curves similar to those seen in Figure 4, that is, curves showing a peak for length wave (in the case of the SPR in a metallic layer to which refer to figures 1-5, for an angle or for a wavelength, depending on which parameter is used for the "scanning") in which the peak of the resonance of plasmon

While in the case of the SPR in one layer metallic the "plasmonic signal" that is usually detected is the intensity of the reflected light (in order to determine the reflectivity depending on angle or wavelength), in the case of the LSPR can be determined, for example, through the measurement of light absorption or dispersion, both in configurations of reflection as in transmission. In all these measures a peak in the spectrum as a function of the wavelength of the light, associated with the LSPR. Now in both cases the result is a curve whose displacement is indicative of changes in the index of refraction of a medium adjacent to the metal structure. For the therefore, determining the "position" of the curve correctly can Be key to a refractive index analysis.

Both in the case of the "normal" SPR and in The case of LSPR can be referred to as a "plasmonic medium" in the that a surface plasmon resonance can be excited. In this document the term "plasmonic medium" is used to refer to this type of medium; in the case of figure 3A, the medium plasmonic could include the metallic layer 1001 and the prism of coupling 1003. In the case of Figure 6A, the plasmonic medium includes 3002 metal nanoparticles (to nano holes, or a combination of nanoparticles and nano-holes).

There are currently multiple devices commercials and a large number of publications describing the different types of measurement settings and applications of this type of sensors

Surface Plasmon Resonance (SPR and LSPR) sensors generally have a high sensitivity to detect changes in refractive index as well as low concentrations of biomolecules. However, sometimes their sensitivity may be insufficient; For example, currently, known sensors have problems in detecting changes in the refractive index below 10-5 and molecules with a small molecular weight (less than 1000 atomic mass units), when used as biosensors. This makes the detection of certain substances, such as chemical toxic substances or environmental pollutants, complex and cannot be carried out directly (using the technology described above). This problem has been described in WO-A-2005/121754 , where a way to increase the sensitivity limit of the sensors is proposed, taking advantage that not only noble metals (such as gold, silver, etc.) allow the creation of waves of surface plasma, but there are also ferromagnetic materials (such as iron, cobalt or nickel) that have optical properties that allow the creation of surface plasma waves. Ferromagnetic materials are magneto-optically active materials, that is, they are capable of changing the optical properties of the light that interacts with them when they are subjected to a magnetic field that changes their magnetization state. Although the surface plasma wave in ferromagnetic materials is highly absorbed, the magneto-optical effects can be greatly increased when the plasmon is excited in these layers. WO-A-2005/121754 describes how magneto-optical effects can be exploited in the presence of surface plasma waves to improve the sensitivity of refractive index sensors based on surface plasmon resonance. That is to say, the magneto-optical effects of ferromagnetic metals and surface plasmon resonance at the interface of a metal and a dielectric are combined. For this, the device comprises, in addition to a series of conventional components, a metal layer containing a ferromagnetic material (for example, iron, cobalt or nickel) and, in addition, optionally, magnetization means configured to magnetize the layer of metal.

Now, and although the invention described in WO-A-2005/121754 serves to increase the sensitivity limit, the use of ferromagnetic material, the magnetization thereof, etc., implies a limitation in the design possibilities of this type of dispositives. Therefore, there is a need to find alternatives that allow the use of other materials and / or configurations.

Other ways of improving sensitivity in this type of systems and also in others (such as, for example, in differential liquid spectroscopy) are described in GB-A-2254693 and are based on the use of acousto-optical devices used for Modulate the beam of light that goes to the medium being analyzed. In the case of surface plasmon analysis, the angle of the beam is modified, using an acousto-optical deflector. In the case of spectroscopy, an acousto-optical filter associated with a white light source is used, so that a portion of the white light could be selected and thus perform a wavelength scan, in a differential spectroscopy application from
liquids

Description of the invention

A first aspect of the invention relates to a method for the analysis of the refractive index of a medium dielectric adjacent to a plasmonic medium. A plasmonic medium is a medium in which a plasmon resonance can be excited superficial. For example, the plasmonic medium can be a layer thin or set of thin layers of metal, in practice gold or silver is usually used, although others can also be used metals This type of medium requires a coupling structure, as explained in, for example, WO-A-2005/121754. Another example of Plasmonic measurement known in the state of the art is a medium nanostructured metal, for example, metal nanoparticles, nano holes in a metallic layer, or combinations of these.

The method comprises the steps of:

- direct light from a light source to the plasmonic medium, to excite a surface plasmon resonance (SPR OR LSPR);

- and analyze light coming from said medium plasmonic

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As is conventional, light can be detect with, for example, a photodetector that produces a signal of output depending on the characteristics of the incident light on the same, for example, the intensity of light, and can be analyzed This output signal. The analysis of this light allows to take out conclusions on the index of refraction of the dielectric medium and / or of variations in said index, as is conventional.

According to the invention, as a light source a laser diode is used, and the wavelength is periodically varied of the light emitted by the laser diode.

The wavelength of the light emitted by a laser diode varies with the emitted power, which depends on the supply current of the laser diode, and the temperature of the same. This makes it very easy to implement a control of the wavelength of light, and introduce a modulation of the Wavelength of light according to an electrical signal. This it may imply advantages over the state of the art, for example, against the system of WO-A-2005/121754, since it can be difficult to generate high frequency external magnetic fields, While laser diodes do not have this disadvantage: you can vary the length of the light with very high frequencies. Too may require a lower energy expenditure compared to that It involves the generation of magnetic fields.

At first glance the use of a laser diode to vary the wavelength in surface plasmon applications It may seem inappropriate: in the case of laser diodes, the range in which the wavelength can be varied is very limited, typically on the order of about 3 nm (i.e. 1.5 nm at each direction from a central wavelength). This can seem inappropriate for a search of, for example, peaks in reflectivity, where a scan is traditionally performed at over a substantially greater wavelength range, to times several hundred nm. The use of a specific type of laser diode (the "distributed feedback laser diode") in others optical analysis systems, at least in the spectroscopy of wavelength modulation, see, for example,

- Stéphane Schilt , et al ., " Wavelength modulation spectroscopy: combined frequency and intensity laser modulation "; Applied Optlcs, Vol. 42, No. 33, 11/20/2003 ; Y

- Yuntao Wang , et al ., " Logarithmic conversion of absorption detection in wavelength modulation spectroscopy with a current-modulated diode laser "; Applied Optics, Vol. 48, No. 21, 01/20/2009 ,

but in these cases these are applications of wavelength modulation spectrometry for analysis of gases, an application where precisely a beam of light is required with extremely thin spectral width, to analyze a range Very narrow spectrum.

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Now, in the context of the SPR and LSPR, it has detected that varying to wavelength using the laser diode it can be detected, instead of the "absolute" value of the signal plasmonic, the variation thereof or the relative variation or derived from it, something that serves to increase sensitivity of the method. With plasmonic signal is understood in this context, by For example, the reflectivity or intensity of the reflected light detected in the case of SPR or, for example, the measure of absorption or dispersion of light by nanostructures in the case of LSPR.

The step of analyzing light from said plasmonic medium may comprise analyzing at least one characteristic of said light as a function of the wavelength variation of the light emitted by the laser diode. That is, it can be analyzed how the plasmonic signal varies depending on the wavelength of the laser diode light, and thus determine the variation of the plasmonic signal - or the relative variation - of the plasmonic signal, as a function of the wavelength In this way, instead of analyzing the absolute value of the plasmonic signal, its variation is analyzed, for example, its derivative, something that as is known serves to increase the sensitivity of the method. In this way, the
Laser diode is an extraordinarily simple medium that, however, allows to increase the sensitivity of the method.

The step of analyzing light coming from said plasmonic medium may comprise analyzing said light from said plasmonic medium synchronously with the variation of the Wavelength of the laser diode light. This detection or synchronized analysis of the plasmonic signal with the signal that excites Plasmon resonance allows to determine the variation, variation relative or derived from the plasmonic signal, with the consequent increased sensitivity of the method A synchronized analysis can be performed using, for example, an amplifier "lock-in"

The step of analyzing light coming from said plasmonic medium may comprise performing a Fourier analysis of at less a characteristic of that light. Fourier analysis allows determine the relative variation of the plasmonic signal, by example, the relative variation of light intensity, and there is commercial software that easily allows you to implement this measure, using, for example, FFT (Fast-Fourier Transform)

The wavelength of the laser diode light is it can vary between a minimum wavelength (λ) and a maximum wavelength (λmax), the difference between the minimum wavelength and the maximum wavelength less than 5 nm, or even less than 3 nm. It is considered that with more than 5 nm of difference the signal derivative would no longer be calculated very well plasmonic In many conventional laser diodes, the difference between the maximum wavelength and the minimum wavelength is order of 2 nm.

The step of periodically varying the length of light wave emitted by the laser diode may comprise varying said wavelength by a variation of the power of emission of the laser diode. There are conventional laser diodes whose emission wavelength varies with the emission power, which which allows to vary the wavelength by varying the current of feeding, which can be easily varied depending on a default modulation pattern, which can be a signal sine with a certain frequency, a square signal, etc.

The variation of said shape wavelength periodic between a minimum wavelength (λ min) and a maximum wavelength (\ lambda_ {max}) can be realized with a frequency greater than 1 kHz, preferably greater than 2 kHz A high frequency improves the signal-to-noise ratio, although may involve more expensive devices, especially in the case of very high frequencies.

The plasmonic medium can be configured to that the wavelength corresponding to plasmon resonance surface corresponds substantially to the wavelength emission center of the laser diode. Ideally, the length of central emission wave of the laser diode exactly matches the wavelength corresponding to plasmon resonance, but what important is that it does not deviate too much from said wavelength. For example, a difference between the central wavelength of emission of the laser diode and the wavelength at which the surface plasmon resonance of the order of, for example, 1 nm, 2 nm, 3 nm, or 5 nm, may be acceptable.

Plasmonic media are tunable, it is that is, you can tune the wavelength at which it occurs the phenomenon of surface plasmon. For example, in the case of thin layers of metal, the wavelength at which the resonance (i.e. the peak in resonance, which can be, by example, a minimum in reflectivity in the case of SPR) can be modify by changing the angle of incidence of light, the index of refraction of the coupling prism, or the metal used. If of nanostructures, the wavelength at which it occurs said resonance (for example, maximum dispersion or absorption) can be tuned by varying the shape, size, composition (the material) and the distribution of nanostructures.

The step of analyzing light coming from said plasmonic medium may comprise analyzing a plasmonic signal associated with said light. With plasmonic signal it is understood, for example, the reflectivity in the case of SPR with metallic layer, and, for example, dispersion or absorption, in the case of LSPR.

The step of analyzing a plasmonic signal may comprise determining the variation of the value of the plasmonic signal induced by the variation of the wavelength of the light emitted by the laser diode, divided by the value of said plasmonic signal. Said plasmonic signal can be R pp in the case of SPR, and it can be a measure of absorption or dispersion, for example, in the LSPR.

Another aspect of the invention relates to a device for the analysis of the refractive index of a medium dielectric, comprising:

- a light source configured to direct a beam of light towards a plasmonic medium, to excite the resonance of superficial plasmon; Y

- a detector to analyze light coming from said plasmonic medium. As is conventional, the detector can comprise, for example, a photodetector that produces a signal output depending on the characteristics of the incident light on it, for example, the intensity of light, and a system of analysis of the output signal, etc.

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According to the invention, the light source it comprises a laser diode, and the device comprises a subsystem of periodic variation of the wavelength of the light emitted by The laser diode.

What has been said in relation to the method applies, mutatis mutandis.

The detector can be configured to analyze the light coming from said plasmonic medium in a way synchronized with a variation of the wavelength of the light of the laser diode

The detector can be configured to perform a Fourier analysis of at least one characteristic of the light coming from said plasmonic medium.

The periodic variation subsystem of the Wavelength of the light emitted by the laser diode can be configured to vary the wavelength of the diode light laser between a minimum wavelength (λ) and a maximum wavelength (λmax), the difference between the minimum wavelength and the maximum wavelength less than 5 nm, or even less than 3 nm. It is considered that with more than 5 nm of difference the signal derivative would no longer be calculated well plasmonic In many conventional laser diodes, the difference between the maximum wavelength and the minimum wavelength is order of 2 nm.

The device may additionally comprise said plasmonic medium.

Description of the figures

Then it goes on to describe very brief a series of drawings that help to better understand the invention and some of which expressly relate to a embodiment of said invention presented as an example illustrative and not limiting it.

Figure 1 schematically illustrates the distribution of the electromagnetic field of a plasmon superficial.

Figure 2 illustrates, schematically, a conventional configuration to excite the plasmon with light superficial, based on the coupling with prism.

Figures 3A and 3B illustrate, in a way schematic, a system of detection of changes in the index of refraction of a dielectric medium, according to the state of the technique.

Figure 4 is a diagram that reflects two curves that relate the intensity R_ {pp} of reflected light TM with different angles of incidence? of light, for two refractive indices (n_ {d1}, n_ {d2}) different from the medium dielectric, according to the state of the art.

Figure 5 schematically reflects a detail of a surface plasmon resonance sensor in a Biosensor application, according to the state of the art.

Figure 6A schematically reflects a detail of a localized surface plasmon resonance sensor in a biosensor application, according to the state of the art.

Figure 6B reflects the displacement of the Plasmon signal curve before and after index change of refraction by the union of analyte molecules to receptors immobilized in the nanostructure, according to the state of the technique.

Figure 7 shows a reflectivity curve angular for two wavelengths separated 2 nm.

Figures 8A and 8B reflect, for a case of SPR, theoretical wavelength modulation curves for one layer 50 nm of Au when the wavelength of light varies 1 nm.

Figure 9 shows experimental curves of emission of the laser diode ML101J27.

Figure 10 shows the effect of frequency of modulation in the experimental SPR resonance curve for a Au layer with not optimized thickness.

Figure 11 schematically reflects a device according to a possible embodiment of the present invention.

Preferred Embodiment of the Invention

It can be said that the invention is based on the principle of increasing the sensitivity of resonance sensors of surface plasmon by introducing a system of modulation that varies the wavelength of the incident beam, making use that plasmonic metals are highly dispersive, is that is, its dielectric constant changes a lot with λ.

Therefore, and if the case of the SPR is considered in metal layers (the situation in the case of LSPR is analogous, so it is not necessary to explain it here in more detail), a small change in the wavelength of the incident light can induce a considerable variation of the plasmon wave vector superficial:

5

When it is struck with monochromatic light, the surface plasmon excitation condition is given by:

6

where n_ {a} is the index of refraction of the incident medium (prism) and the is the angle of incidence of light. Therefore the angle at which the resonance is given by

7

Therefore, a small change in length incident wave will produce a small offset of the angle of resonance. This effect can be seen in Figure 7, where shows the angular reflectivity curve for two lengths of 2 nm separated wave.

The small change in wavelength incident translates, therefore, into an angular displacement of the resonance curve As what is measured in SPR sensors is the reflectivity, the reflectivity change can be expressed due to the wavelength variation of the following way:

8

That is, the differential change of the reflectivity can be seen as the slope of the angular curve multiplied by the displacement of the resonance when the wavelength. Therefore, a small change in the length of wave allows access to measure the derivative of the curve angular.

If a small change in wavelength occurs in the measurement range, the reflectivity R_ {pp} and the reflectivity variation ΔR_ {pp} can be measured simultaneously (Fig. 8A). As accessed simultaneously both quantities can be obtained in each measurement the quotient \ R {Delta p} / {R} pp. This ratio is completely independent of the fluctuations in intensity of the light source. Furthermore, the measure \ Delta R {p} / R {p} has an angular resonance very narrow because, when the surface plasmon is excited, \ Delta R {p} is maximized, while R {p} decreases dramatically (Fig. 8A). Therefore,? R pp / R pp has an angular curve with a very narrow resonance (Fig. 8B) that allows to increase the sensitivity.

According to the invention, to achieve a small change in the wavelength the incident light, a laser diode and you can take advantage of changing the wavelength that occurs when the emission power in a diode is varied To be. With commercially available laser diodes you can achieve a variation greater than 1 nm, as illustrated in the Figure 9, showing experimental diode emission curves ML101J27 laser.

With this modulation method can achieve a change in reflectivity \ Delta R} {pp almost 10 times higher than that obtained by the magneto-optical modulation described in WO-A-2005/121754. The maximum frequency that can modulate the wavelength of the laser light diode is about 2 kHz (see Fig. 9, reflecting the value of \ Delta R {p} / R {p} as angle of incidence, for modulations with frequency of 2 kHz, 1 kHz, 500 Hz, 250 Hz and 100 Hz). The frequency of 2 kHz is much higher than those currently available with sensors with magneto-optical modulation of the type described in WO-A-2005/121754 . Thanks to this, the signal-to-noise ratio of the sensor with wavelength modulation is considerably higher.

Detecting \ Delta R {p} / R {p}, which is a measure for increasing the sensitivity, it can be performed easily, through a Fourier analysis of the intensity of light reflected by the layer metallic, since both the emission power and the laser wavelength are modulated at the same Ω frequency. As the reflectivity depends on the wavelength, the intensity reflected by the metal sheet can be expressed by the formula:

9

whereby I {0} is the intensity of the laser diode unmodulated and \ Delta I is the intensity modulation. The reflectivity R_ {pp} (\ omega) can be developed in Fourier series, so the reflected intensity will be:

10

Taking the ratio of the first harmonic between the continuous value (0 \ omega) is obtained:

eleven

Thus, we can easily access the value \ Delta R} {pp / pp R {} which is based on the measurement sensor, according to an embodiment of the invention. To obtain this amount, the signal can be analyzed by software, calculating the FFT (Fast Fourier Transform) of the signal, or it can be obtained through a lock-in amplifier.

Figure 11 schematically reflects a system according to an embodiment of the present invention, and comprising a light source 1 in the form of a laser diode controlled from a controller module 2 that regulates the current of laser diode power, to periodically vary the length wavelength of the light beam emitted by the laser diode 1 plus or minus 1 nm in each direction from a central wavelength (the module controller can also control the temperature, which also may influence the wavelength). It is considered that in some cases it may be preferable to use a laser diode whose wavelength without modulation is preferably in the range 650-800 nm, for example, the laser diode ML101J27 (λ? 660 nm). The light is affected by a medium plasmonic 4, which can be a thin layer of metal (for example, a Au layer about 50 nm thick and optionally with 1 nm Ti to increase adhesion on the glass substrate) or a medium nanostructured metal (for example, metal nanoparticles, nano holes in a metallic layer, or combinations of these) (in the case of a metal layer, a means of coupling, such as glass prism 41 illustrated in the figure eleven). On the other hand, there is a fluid management system 5 that carries a fluid to be analyzed in proximity to the plasmonic medium, to that possible changes in the refractive index can be studied of the dielectric associated with the plasmonic medium and caused by analytes in the fluid, as is conventional in this type of Applications.

On the other hand, a detector is planned for analyze light from said plasmonic medium, that is, by example, in the case of the SPR in a metallic layer, to determine the reflectivity of said layer (which may represent the signal plasmonic in the case of SPR) or to determine the level of dispersion or absorption of light in the case of LSPR in a medium nanostructured metal. In this case, the detector comprises a photodiode 7 and a detection and analysis system that receives the signal photodiode output (indicative of the intensity of the light that affects the photodiode) and that analyzes it in synchronization with the power signal of the laser diode.

Although the previous description has focused especially in SPR systems, the description is directly also applicable to LSPR systems, although these, instead of be based on the reflectivity of the plasmonic medium for polarized light TM, are based on the corresponding media characteristics nanostructured, that is, in the analysis of absorption or light scattering, or any combination of these, both in reflection or transmission settings.

In this text, the word "understand" and its variants (such as "understanding", etc.) should not be interpreted excluding, that is, they do not exclude the possibility that described include other elements, steps etc.

On the other hand, the invention is not limited to the specific embodiments that have been described but also include, for example, the variants that can be made by the expert medium in the field (for example, in terms of the choice of materials, dimensions, components, configuration, etc.), inside from what follows from the claims.

Claims (15)

1. Method for analyzing the index of refraction of a dielectric medium adjacent to a plasmonic medium (4), which includes the steps of: - direct light from a light source (1) towards the plasmonic medium, to excite a plasmon resonance superficial; - and analyze light coming from said medium plasmonic (4); characterized because - as a light source (1) a laser diode is used, Y - the wavelength of the the light emitted by the laser diode.

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2. Method according to claim 1, wherein the step of analyzing light from said plasmonic medium (4) comprises analyzing at least one characteristic of said light in function of the wavelength variation of the light emitted by The laser diode. 3. Method according to claim 2, wherein the step of analyzing light from said plasmonic medium (4) comprises analyzing said light from said plasmonic medium in synchronization with the wavelength variation of the laser diode light. 4. Method according to claim 1, wherein the step of analyzing light from said plasmonic medium (4) comprises performing a Fourier analysis of at least one characteristic of said light. 5. Method according to any of the previous claims, wherein the wavelength of light of the laser diode is varied between a minimum wavelength (λ) and a maximum wavelength (λ), being the difference between the minimum wavelength and the wavelength maximum less than 5 nm. 6. Method according to any of the previous claims, wherein the step of varying periodically the wavelength of the light emitted by the diode laser comprises varying said wavelength by a variation of the emission power of the laser diode. 7. Method according to any of the previous claims, wherein the wavelength is varied periodically between a minimum wavelength (\ lambda_ {min}) and a maximum wavelength (λ max), with a frequency greater than 1 kHz, preferably greater than 2 kHz. 8. Method according to any of the previous claims, wherein the plasmonic medium (4) is configured so that the wavelength corresponding to the Superficial plasmon resonance corresponds substantially to the central wavelength of emission of the laser diode. 9. Method according to any of the previous claims, wherein the step of analyzing light from said plasmonic medium (4) comprises analyzing a plasmonic signal associated with said light. 10. Method according to claim 9, wherein the step of analyzing a plasmonic signal comprises determining the variation of the value of the plasmonic signal induced by the variation of the wavelength of the light emitted by the laser diode, divided by the value of said plasmonic signal. 11. Device for analyzing the index of refraction of a dielectric medium, comprising: - a light source (1) configured to direct a beam of light (3) towards a plasmonic medium (4), to excite the surface plasmon resonance; - a detector (6) to analyze light coming from of said plasmonic medium (4); characterized because the light source (1) comprises a diode To be, and because the device comprises a subsystem (2) of periodic variation of the wavelength of the light emitted by the laser diode

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12. Device according to claim 11, in which the detector (6) is configured to analyze the light coming from said plasmonic medium in a synchronized manner with a variation of the wavelength of the laser diode light. 13. Device according to claim 11, in which the detector (6) is configured to perform an analysis Fourier of at least one characteristic of the light coming from said plasmonic medium. 14. Device according to any of the claims 11-13, wherein the subsystem (2) of periodic variation of the wavelength of the light emitted by The laser diode is configured to vary the wavelength of the laser diode light between a minimum wavelength (λ) and a maximum wavelength (λ), being the difference between the minimum wavelength and the wavelength maximum less than 5 nm. 15. Device according to any of the claims 11-14, further comprising said plasmonic medium.