WO2019096751A2

WO2019096751A2 - Method and device for determining the refractive index of a medium

Info

Publication number: WO2019096751A2
Application number: PCT/EP2018/080981
Authority: WO
Inventors: Karl Joachim Ebeling; Rainer Michalzik
Original assignee: Universität Ulm Institut Für Optoelektronik
Priority date: 2017-11-14
Filing date: 2018-11-12
Publication date: 2019-05-23
Also published as: DE102017126708A1; WO2019096751A3

Abstract

The present invention relates to a method for measuring at least one feature in a medium to be examined, preferably for determining the refractive index of the medium. The invention also relates to a device suitable for determining the refractive index of a medium, comprising an optical element suitable for carrying out the method. The optical element comprises a defining surface which is designed to come into direct contact with the medium to be examined during measurement, said defining surface of the optical element being provided with a layer system acting as a resonator, which enables multiple reflections for incident electromagnetic radiation.

Description

Method and device for determining the refractive index of a medium

description

The present invention generally relates to a method for measuring at least one feature in a

investigating medium, preferably for determining the refractive index of the medium. The invention further relates to a device suitable for determining the refractive index of a medium, comprising an optical element suitable for carrying out the method.

For determining the refractive index of transparent media are various devices, in particular

Refractometer, known. The underlying principle of a refractometer generally uses the behavior of light at the junction between a prism with known properties and the medium to be examined.

The diverse applications of refractometers in the chemical, pharmaceutical and optical industries, including microelectronics, fluidics, biotechnology and medicine have produced a wide range of refractive index measurement techniques. Known methods use, for example, light deflection in prisms, Brewster angle, critical angle of total reflection, material-dependent

Interference effects in Fabry-Perot, Mach-Zehnder or

Michelson arrangements or integrated optical

Waveguides, substance-dependent propagation of evanescent fields in photonic crystals, dielectric photonic bandgap waveguides or microresonators, finally Plasmon resonances in dielectric loaded

metallic waveguides and much more.

The document DE 10 2008 014 335 A1 describes about a device and a method for determining a

Refractive index of a test object. This is an opto

presented electronic sensor.

The document DE 10 2009 055 737 A1 also describes an optical device for generating a disturbing internal total reflection.

A device that is still frequently used for measuring refractive indices is the Abbe refractometer, which, however, even in its newer variants has measuring surfaces of typically well over one square millimeter.

In many applications in the chemical, food, plastics, paper, pulp or pharmaceutical industry, the substances to be investigated have refractive indices ranging between 1.3 and 1.7. For example, the concentration and temperature-dependent refractive indices of the electrolytic solutions NaCl, KCl and CaCl 2, the polar glucose solution, the nonpolar ethyl acetate solution and the protein solution bovine serum albumin are common

Concentrations between n = 1.32 and n = 1.42, such as the document Chan-Yuan Tan, Yao-Xiong Huang: "Dependence of Refractive Index on Concentration and Temperature in

Electrolyte Solution, Polar Solution, Nonpolar Solution, and Protein Solution, J. Chem. Eng. Data, 2015, 60, pages

2827-2833, can be seen. The refractive indices of biological tissue can vary over a fairly wide range. Gray cells in the

Brain, for example, has an average refractive index of n = 1,395 and white cells are at n = 1,410, such as Steven L Jacques: "Optical properties of biological tissues: a review", Phys. Med. Biol. 58 (2013)

R37-R61.

Due to illness, higher refractive indices up to n = 1.48 are also observed. Even higher refractive indices occur

Example in glasses, ceramics and semiconductors. For metals, in addition, the strong absorption of

Meaning .

Against this background, it is an object of the present invention to provide a method and apparatus for measuring at least one feature in a subject to be examined

Medium, preferably to determine the refractive index of the medium to provide.

The determination of the refractive index of extremely small

Material volumes should be possible. The device should also be integrated micro-optically.

Furthermore, the method should compensate by scattering or absorption occurring optical losses in the measurement volume by an optical gain in the measuring system.

To increase the sensitivity is a high

Photon density can be generated in the measurement volume. Finally, a high two-dimensional lateral spatial resolution is to be made possible.

This object is surprisingly simple by a method and a device for measuring at least one feature in a medium to be examined, preferably for

Determination of the refractive index of the medium, solved according to one of the independent claims.

Preferred embodiments and further developments of

Invention are to be taken from the respective subclaims.

The device for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, preferably comprises an optical element according to the invention, which is suitable for the

Use in the device for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, is suitable.

In this case, the optical element preferably comprises at least one interface, which is designed to form a flat area of direct contact with the medium to be examined during the measurement, wherein

at least part of this interface of the optical

Elements represents a measuring surface, and

wherein this interface of the optical element is equipped with a layer system which acts as a resonator and which acts on an electromagnetic radiation incident obliquely on the boundary surface in the case of the radiation, which is at an angle greater than that

Limit angle of the total reflection at the interface between the optical element and the medium to be examined causes multiple reflections of this radiation, so that resonances can form in the layer system.

The device for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, may comprise, in addition to the optical element, a radiation source and a receiving device, wherein

the radiation source is designed to emit electromagnetic radiation and couple it

Radiation in the optical element, and wherein

the receiving device is designed to receive electromagnetic radiation reflected at the interface.

The method according to the invention for measuring at least one feature of a medium to be investigated, preferably for measuring the refractive index of the medium, may comprise the abovementioned apparatus and the aforementioned optical element. The method can be used for

Performing the measurement includes the following steps:

for the measurement, the optical element is in direct contact with the medium at the interface,

electromagnetic radiation is coupled into the optical element in such a way that it impinges at the interface at an angle which is greater than the limit angle of total reflection at the interface, a total reflection of this coupled radiation arises at the interface between the optical element and the medium,

- Which leads to multiple reflections and which forms resonances in the layer system of the optical element whose spectral and

directional situation by the

Receiving device is measured.

Advantageously, an evaluation unit is provided, which is used to determine the refractive index of the medium to be examined. This can be integrated in the receiving device. To receive the electromagnetic

Radiation, the receiving device with a suitable sensor, such as a photosensor, be equipped.

The electromagnetic radiation emitted by the radiation source is also referred to below as light.

So if one speaks of light or light waves, this is to be understood as electromagnetic radiation, which therefore does not necessarily have to lie only in the visible wavelength range, but for example also in the

adjacent wavelength ranges. Preferably, the radiation source is designed so that it can emit monochromatic light and more preferably a cylindrically divergent or convergent monochromatic light beam in the direction of the optical element.

The inventive device uses oblique

Light wave propagation in the ultra-short, high-refractive The term "ultrashort" in this context means a very small length of the resonator, the optical system layer system may advantageously comprise a Fabry-Perot resonator for this purpose, based on the underlying principle of a Fabry-Perot resonator An incident light beam is guided through these mirrors only if it fulfills the resonance condition of the mirrors In this layer system, a targeted optical loss or gain can be set surprisingly simply, so that the measurement accuracy can be significantly improved.This principle makes use of the invention.

The optical element may advantageously be another

Include surface through which the electromagnetic radiation can be coupled. This surface can be equipped with a dielectric reflector.

The intensity of the total optical element

reflected light then depends on the wavelength, the angle of incidence and in particular the phase shift in the total reflection. By multiple reflections can be in the resonator in a particularly advantageous manner

Form resonances, their spectral and

Direction-dependent position allows the determination of the refractive index of the medium to be examined.

With increasing attenuation and associated decreasing half-width of the resonance grows in the highest Advantageously, the measurement accuracy of the

Device according to the invention. In the medium to be examined penetrates only the evanescent part of

coupled light wave, which is particularly small

Measuring volumes are guaranteed. In resonance, there is a significant increase in the photon density or the electric field strength in the evanescent part of the

Light wave, so that the arrangement is very sensitive to near-surface changes in the refractive index, which is particularly important for applications in the field of biosensors of great importance.

By optical imaging of the interface, the determination of the lateral refractive index distribution with high resolution is possible. For this purpose, the receiving device is provided, in which the evaluation unit can be integrated, which allows using algorithms stored an evaluation of the received by the receiving device images.

For the application of the method according to the invention, the refractive index n _{j of} the high refractive index material of the optical element is assumed to be known. By high-refractive index is meant a refractive index n _j which is at least at n _j = 1.8, preferably at n _j > 3.0 and more preferably at n _j >

3.5 lies. The refractive index of the medium to be examined is advantageously lower than the refractive index of the optical medium

Element. Therefore, for the ratio of the refractive index of the medium to be examined n to the refractive index of the optical element: n <n _j . The refractive index of the examined

Medium can, for example, in a range between n =

1.3 and n = 1.7 are. In the layer system can in a preferred

Embodiment of the invention optical losses or optical amplification can be introduced selectively. At the interface with the low refractive medium, under certain circumstances, total internal reflection of the coupled electromagnetic radiation occurs. The incidence of light preferably takes place at a predetermined, oblique angle to the optical axis of the optical element. In other words, the coupled-in light is incident on the layer system at a predetermined angle, this angle preferably being selected such that the conditions for a total reflection at the interface are given. In this way, the multiple reflection of this radiation can be effected, so that resonances can form in the layer system.

The layer system preferably comprises a dielectric mirror, that is to say a Bragg reflector, at the interface of the optical element. The resonator length, that is to say the thickness of the layer system forming the resonator, is

preferably only a few microns.

The medium to be examined may be, for example, a transparent substance and is located in the

evanescent field in the case of one at the interface

totally reflected light wave. In the resonator, losses or amplification may occur during a measurement resulting in characteristic intensity changes in the resonator

lead reflected measuring beam and so the

Improve evaluation options. As a measuring beam is doing understood that light which leaves the optical element after reflection at the interface and of the

Receiving device can be received. The measured intensity changes depend on the wavelength, the

Angle of incidence of the light wave and in particular of the refractive index-dependent phase shift of the light wave in the total reflection from.

The layer system at the interface of the optical element may comprise gallium arsenide layers, which are particularly suitable because of their high refractive index. The Bragg reflector can be advantageously made of a semiconductor layer system such as AlGaAs GaAs

InGaAsP-InP (indium gallium arsenide phosphide indium phosphide) or InAlGaN GaN (indium aluminum gallium nitride gallium nitride) may be formed and thus have GaAs layers and / or AlAs layers available (aluminum gallium arsenide gallium arsenide). Such a Bragg reflector shows a very high reflectivity.

In a preferred embodiment, it may be formed on the basis of the material system GaAs-AlAs (AlAs = aluminum arsenide) and have, for example, 15 GaAs-AlAs Bragg mirror pairs. In general, a suitable Bragg reflector can use alternating AlGaAs layers

include different Al content. Other embodiments of the Bragg reflector are also conceivable, for example a varying composition from layer to layer or multi-stepped or graduated instead of simply stepped transitions at the interfaces. The layer system may advantageously have a so-called

Quantum film include, so a semiconductor layer in the

Nanometer thickness range. The following is from a

If the layer thickness is so small that the energy levels of the charge carriers in the film, ie electrons or holes, are appreciably quantized, the particles are thus quantified by the quantum film

Presence of interfaces no longer everyone

Can take energy state. In the case of the layered semiconductor material systems used according to the invention, this is the case for thicknesses below 15 nm.

The quantum film forming layers may be based on the material system InGaAs GaAs, GaAs AlGaAs, InGaAsP InP or InAlGaN GaN and have a small thickness, preferably a thickness of 15 nm or less, more preferably a thickness of 10 nm or less. By the quantum film, that is, the layers forming the quantum film, an absorption coefficient in the layer system can be adjusted. The absorption coefficient is dependent on the wavelength of the electromagnetic

Radiation. For high wavelengths, for example, the value drops to almost zero, then the material is transparent. For wavelengths significantly shorter than the band edge wavelength, it can rise to some 10,000 / cm. By

Energy supply, for example by optical pumping with an external laser source, but the quantum well can also be in the range of the gain, ie in a range of negative absorption, brought. The achievable gain values can range from zero to a few thousand / cm. The quantum film can Accordingly, have an excitation-dependent positive or negative absorption coefficient of, for example +/- 400 / cm, +/- 800 / cm or +/- 1.600 / cm.

On the layer system can also be a metallic layer as a contact surface to the medium to be examined

be upset. In other words, the measuring surface comprises a metallic surface coating on which

Lightly attach and functionalize biomolecules. This layer may comprise, for example, gold, silver, copper or aluminum. Preferably, a gold-containing layer is provided as the metallic layer in the contact region.

The optical element can have different shapes

respectively. An advantage is about training as a prism, since a particularly simple radiation control is possible here. In a preferred embodiment, the

Prisma plane surfaces, so also a plane interface.

In another embodiment, the optical includes

Element additionally an optical grid for

Radiation steering. This can be applied to the light entrance side and the coupled light in the

predetermined angle to the interface of the optical

Lead element. In this embodiment, the

optical element may be formed, for example, plane-parallel, which allows cost-effective production.

In one embodiment of the invention, the interface is not flat, but wavy. In a particularly preferred embodiment of the

The device according to the invention is the optical element directly with the tip of a polarization-maintaining

Single-mode optical fiber connected, which is a very compact

Design allows.

In another, also particularly preferred

Embodiment of the device according to the invention, the optical element is directly on an obliquely formed

Light exit surface of a polarization-maintaining

Single-mode fiber applied. The angle of the oblique light exit surface to the optical axis of the glass fiber is preferably selected so that the conditions for a total reflection at the interface are given.

A major advantage of the invention is the fact that the measuring surface of the device can be made very small, for example, less than 1 mm ² , preferably less than 0.1 mm ² and more preferably less than 0.01 mm ² . The depth resolution of the device can be in a range of a few 100 nm up to a range of less than 100 nm.

In this way, the device according to the invention is very compact to produce and thus versatile. Against this background, the device according to the invention is particularly suitable for, for example, minimally invasive examinations. The device can also be integrated micro-optically. At a high quality, among which in terms of resonance, a high photon density at the totally reflecting

Interface, sharp results

Resonance curves with low half-width, from which the refractive index of the zu. For a given wavelength of the injected light and known angle of incidence of

can be determined with high precision.

By an optical image of the total reflecting interface, a high lateral resolution can be achieved. Due to the low penetration depth of the evanescent wave in the medium to be examined extremely small amounts of substance are sufficient to a

Make refractive index determination.

Occurring optical losses in the measuring volume, such as by scattering or absorption, can be achieved by an optical

Gain can be compensated in the measuring system. For this purpose, it is advantageous that the device for increasing the

Sensitivity can generate a high photon density in the measurement volume.

To further illustrate the invention, reference is made to the preferred embodiments shown below with reference to the accompanying figures.

The drawings show:

Fig. 1 is a schematic representation of an apparatus for illustrating the inventive Measuring method for determining the refractive index of a substance using an exemplary embodiment,

Fig. 2 exemplified by total reflection

caused dependence of the phase rotation DF of the refractive index n of a medium to be examined,

Fig. 3a, 3b calculated reflection factors R as a function of the phase angle F for different

Profit factors

Fig. 4 exemplarily calculated half-widths and

Resonance dips or resonance peaks as a function of the gain factor in one

Embodiment,

Fig. 5 exemplifies the intensity reflection spectrum of a Bragg structure with 15 AlAs GaAs

Mirror pairs, embedded in GaAs, for TE and TM waves with light incidence below 30 °,

Fig. 6 shows the refractive index profile of the AlAs-GaAs Bragg reflector from FIG. 5 and the location-dependent progression of the field strength amplitude in the region of the Bragg reflector,

FIG. 7 schematically shows a Bragg reflector according to FIG. 6 in front of an interface at which total reflection occurs, in a further embodiment in which a quantum film is inserted into the spacer layer of the Fabry-Perot resonator formed, via which optical losses or amplification to adjust,

Fig. 8 exemplarily calculated intensity reflection spectra of the structure of FIG. 7 for different refractive indices n of the adjacent medium to be examined,

Fig. 9a 9b refractive index profiles and typical location-dependent

Waveforms of the electric field strength for a wavelength outside the resonance and for a resonance wavelength,

10 shows exemplary calculated intensity reflection spectra for a structure according to FIG. 9 with a quantum film with a moderate gain of 400 / cm for different refractive indices of the adjacent medium to be examined, FIG.

FIG. 11 shows by way of example calculated intensity reflection spectra for a refractive index profile according to FIG. 9 with an additional gold coating at different thickness, FIG.

FIG. 12 shows exemplary calculated intensity reflection spectra for gold layers on a GaAs Abstandsschicht,

13 shows a schematic representation of a measuring system for the examination of laterally structured bilayers, as they occur, for example, in biological cells or biosensors _V ,

Fig. 14 calculated intensity reflection spectra of a

Structure according to FIG. 13 with a 100 nm thick transition layer with different refractive indices on water with n = 1,330,

FIG. 15 shows a refractive index profile and a field distribution in FIG

Resonance for an AlAs-GaAs layer system, which is formed with a 0.2 nm thick gold layer (n = 0.26), and adjoining water-based transition layer, FIG. 16 shows intensity reflection spectra calculated from a refractometer according to FIG. 15 with a 100 nm thick transition layer for refractive indices between n = 1,330 and n = 1,350, FIG.

Fig. 17 is a refractive index profile for an AlAs GaAs

Bragg reflector with InGaAs-GaAs reinforcing spacer layer and quasi-waveguide as well as calculated field distribution in resonance,

Fig. 18 calculated intensity reflection spectra for a

Structure according to FIG. 17 with vacuum, air and CO2 in the evanescent field,

19 is a diagram of a measuring system, which a

cylindrical divergent monochromatic

Uses light bundles for irradiation into the resonator,

Fig. 20 shows an embodiment of a refractometer with

Light coupling into the highly refractive material via an optical grating,

Fig. 21 shows schematically a device for measuring the

Refractive index of a medium with a prism as a refractometer at the tip of a single-mode glass fiber and with feedback of the measuring beam via a multimode fiber,

Fig. 22 shows schematically a device for measuring the

Refractive index of a medium with a directly applied refractometer at the tip of a single-mode glass fiber and with the return of the

Measuring beam via a multi-mode fiber, and

Fig. 23 schematically shows an embodiment of a

Refractometer with sinusoidal wavy interface. Detailed description of preferred embodiments

In the following detailed description

For the sake of clarity, preferred embodiments denote substantially identical parts in or on these embodiments. For better

Clarification of the invention, however, the preferred embodiments shown in the figures are not always drawn to scale.

Fig. 1 shows a schematic representation of a

Section of a device 1 for measuring at least one feature of a medium to be examined,

preferably for measuring the refractive index of the medium, according to a first embodiment of the invention. This illustration serves to illustrate the measuring method according to the invention for determining the refractive index of a medium or substance 10 to be examined.

For the sake of clarity alone, the radiation source and the receiving device are not in the figures

shown, as well as the evaluation unit. The

Receiving device is intended to provide an optical image of the interface for the determination of the lateral refractive index distribution with high resolution

enable. In this, the evaluation can be integrated, which by means of stored algorithms a

Evaluation of the images received by the receiving device allows. The ones described below Mathematical relationships and rules for analysis are advantageously integrated into the evaluation unit.

To explain the method is shown in FIG. 1

For example, assume that an incident linearly polarized monochromatic plane light wave 30 of the electric field intensity light source Ei (x, y, z) in a high refractive index lossless optical element 20

after passing through a Bragg reflector 22 with amplitude reflection factor r _B and amplitude transmission factor t _B and a plane spacer layer 23 of thickness L at the angle of incidence 0 _j at z = 0 from above onto an in the example flat boundary surface 24 adjacent to this interface 24, too

investigating medium 10 of lower refractive index n

meets.

The optical element 20 is embodied in the example as a high refractive prism and thus comprises one in the

illustrated embodiment, planar interface 24, which is in direct contact with the medium 10 to be examined during the measurement.

This boundary surface 24 is formed by the Bragg reflector 22 and also planar in the example

Spacer layer 23 is formed. The Bragg reflector 22, the spacer layer 23 and the interface 24 together form a Fabry-Perot resonator 21.

For sufficiently large angles of incidence 0 _j > 9 _gr with sm 6 _gr = n / n _i , the incident wave at the interface 24

totally reflected, as in the presentation with the

Light wave 30 is shown at the reflection point 25. By 6 _gr is here meant the critical angle to the optical axis 34, under which the condition for a total reflection is given. Accordingly, a reflected light wave 31, which represents the measuring beam, results from the coupled-in light wave 30. In this case, this reflected light wave 31 undergoes a polarization-dependent phase rotation.

For a lossless interface 24 and lossless medium 10 with weakly x-dependent refractive index n (x) the complex amplitude ^reflection factor is to be written as phase factor r (x) = b ^{ίDf (c)} = _b ^ί2Af tE ^(c) , where for transversal electrical

Polarization, ie for TE waves, DF and DF _{T £}

and as described, for example, in the publication KJ Ebeling: Integrated Optoelectronics, in: Springer, Berlin 1992.

By measuring the phase rotation at the interface can be at a known angle of incidence 0 _j and known

refractive index

of the optical element determine the refractive index of the adjacent medium 10.

In Fig. 2 is an example by total reflection

caused dependence of the phase rotation DF of

Refractive index n of the adjacent medium 10 is shown where rii = 3.6 (eg for GaAs), 0 _j = 30 ° and TE polarization was assumed for the incident wave.

An obliquely incident monochromatic wave

location-dependent electric field strength

with the wave vector components / g. = / c sin0 _j and k _z = k cos 9 _ir the wavenumber in the medium k = 2ph _; / l and the vacuum wavelength l is reflected many times in the Fabry-Perot resonator 21, whereby interference phenomena form, which are characteristic in the intensity curve of the

precipitate reflected light wave 31. The electric field of the resulting, total of the resonator

returning plane light wave 31 can be written as

E _r = E _r (x, z) = E _r0 e ^ik ^ - ^ik ^, where the complex electric field amplitude E _r0 of the orbital phase of the wave in the resonator

4 pp; L COS Q

F = - - - + DF

l and the intensity reflection coefficient of the Bragg reflector R _B = | r | | depends. In addition, an optical damping or gain inside the

Resonator by a positive or negative

Considers intensity absorption coefficient a, then gives the complex field amplitudes, the relationship

as, for example, the publication Eugene Hecht, Alfred Zajak: Optics (p. 305). Addison-Wesley, London 1974, can be seen. The absorption coefficient, also as

Damping constant known, is a measure of the

Reducing the intensity of electromagnetic radiation when passing through a given medium. By

Squared squaring is obtained with the intensities I _r = \ EA ² , h = ² for the intensity reflection factor R the formula

FIGS. 3a and 3b show calculated reflection factors R as a function of the phase angle F for different ones

Gain factors g = exp (-aV), in Fig. 3a for R _B = 0.99 and in Fig. 3b for R _B = 0.999.

In the lossless case a = 0 ff = / _r // j = l is valid for all phase angles F. For small losses 0 <aL «1 and highly reflective Bragg structures with R _B ~ 1, sharp resonance decays occur in the reflection spectrum

phase angles

with integer m up. For given parameters n _i e L and A, the occurring at a change in the refractive index of the adjacent layer to be examined depends

Shift of the resonance phase position άF _th / άh only by the differential change of the phase rotation at the

Total reflection off. We have άF _pi / dn = άAF / dn. This makes it possible to use the phase shift άF _th

Refractive index change dn of the adjacent layer

determine directly.

The resonance wavelengths are included

The relative shift of the resonance wavelength with change of the phase rotation άAF increases

with increasing order m, so also increasing

Resonator length, ie layer thickness, L from.

For exp (-aL) <l, ie predominant losses in the resonator, the reflection minimum applies

The height of the resonance dip is h = d ₀ l / b _0, and for the phase position F ₁₇₂ to half the height (h _{_1/2} = h / 2) of the Resonance collapse (ί? (Fi / ₂ ) = lh / 2) follows

Thus, the phase of half the half width is through

given, and applies to the full width at half maximum

In the limiting case adapted losses, ie exp (-aL) - »R _B

("Matched losses"), the intensity of the reflected wave disappears with R _min ^ 0, and it follows

F1 / 2

sin ² (1 - RB) ²

2 4 R _B The full half width with adjusted losses is thus

where the approximation on the right for R _{B «} 1

true. For the half-width on the wavelength scale one obtains accordingly approximately

By DF / ₂ or AA ₁₇₂ are in the classical sense

Defined resolution limits with which changes άAF / άh the phase rotation by total reflection at the adjacent layer can still be detected.

For high resolution, low losses (due to scattering or absorption) in the resonator core and at the same time Bragg structures with a high reflection factor are required. The resonator core is formed by the total reflection interface and the first layer boundary of the Bragg reflector. Due to the penetration depth of the light wave into the Bragg reflector, these adjacent layers can also be counted with the resonator core.

For adjusted losses R _B = 0.99,

=

3.6, 0 _j = 30 °, L = 1 mhi and A = 1 mhi half-widths of DF ₁₇₂ = 0.02 and AA _Xj2 ~ 0.5 nm and correspondingly smaller values for higher reflection _factors R _B.

For optical amplification in the resonator core, negative values of a are in the range

1 / jRÜ> e ^{~ aL} > 1 consulted. In the reflection spectrum occur

Resonance breaks now resonant peaks with maximum values R _max ⁼ d _p / b _p at the wavelengths X _m . The

Resonance overshoot is

it goes in the limit of self-excitation, so for

exp (-aL) - »1 / j R _B , towards infinity. The phase at half

Half-value width determined by RF _{_1/2)} = l + h / 2, supplies again

and accordingly

As the gain increases, the half-value width decreases more and more, until, in the limit, it goes beyond the classical resolution limit for exp (-αL) - »1 / ^ JR _B and according to

DF _{_1/2} 0 tends to zero, which (endless ideally) is associated with increasing resolution. Practical limits result from the diffraction-related divergence of a finite extended wave, the not perfect linear polarization and the finite spectral width of the measuring beam. As a measure of the possible measurement accuracy or the quality of the measurement, the ratio of resonance peaking or resonance breakdown to full width at half maximum, ie Q = h / DF _1/2 ,

be used.

Calculated half-widths and resonance decays or

Resonance peaks are in Fig. 4 for R _B = 0.99 as

Function of the gain factor g = exp (-aV).

Only the branches of the curve, for which g <l / HH = 1.0101, have physical significance. The circle and arrow markers shown in the graphs indicate the reference to the corresponding ordinate axis, i. the arrow points to the current ordinate axis.

Planar layer systems, as present in the inventive Fabry-Perot resonator of the optical element 20, are expediently using the transfer matrix method

analyzed, cf. R. Michalzik: VCSELs: Fundamentals, Technology and Applications of Vertical-Cavity Surface-Emitting Lasers, Springer-Verlag, 2013.

In particular, field intensity and intensity reflection spectra as well as spatial characteristics of field strength, intensity and photon density can be used with this method

to calculate. The following numerical

By way of example and without limitation, calculations relate to layer structures in an InGaAlAs-GaAs semiconductor material system (InGaAlAs = Indium gallium aluminum arsenide). They can be carried out in the same way for other material systems.

In an InGaAlAs layer system, the reflector on the light input side can preferably be realized as an AlAs-GaAs Bragg reflector which is embedded in GaAs as a spacer layer 23.

Fig. 5 illustrates the intensity reflection spectrum of such a mirror structure for TE and TM waves

Light incidence below 30 °. The term TE-wave here means a transverse electric wave and the term TM-wave means a transverse-magnetic wave. In

Propagation direction disappears in the former wave, the electrical component in the propagation direction, while the magnetic component values can assume unequal to 0, in the latter wave disappears only the magnetic component in the propagation direction, while the electrical component can assume values not equal to zero.

The reflector is designed in this embodiment for a center wavelength of about 1060 nm and consists of 15 AlAs-GaAs mirror pairs with thicknesses of 110 nm for the individual AlAs layers of refractive index 3.0 and 80 nm for the individual GaAs layers of the refractive index 3.6.

In the wavelength range from 900 nm to 1200 nm, the absorption is negligible. The diminished

Maximum reflection for TM waves is at oblique Incidence compared to TE waves due to lower reflection at the AlAs-GaAs interfaces. at

negligible absorption applies to the spectral

Intensity transmission factor T is the relationship T (l) = 1 R (A) for both TE and TM polarization.

FIG. 6 shows the course of the location-dependent electric field strength amplitude calculated in the region of the Bragg reflector 22 for maximum reflection. Also marked is the refractive index profile. The reflection forms in front of and in the Bragg structure

Standing wave field whose fluctuations decrease with increasing penetration into the Bragg reflector 22. The

Fluctuations completely disappear when the shaft exits the Bragg area. The square of the electrical

Field strength is proportional to the spatial photon density, so that from FIG. 6, the distribution of the temporal

averaged photon distribution in the structure.

FIG. 7 schematically shows a Bragg reflector 22 according to FIG. 6 in a further embodiment of the optical element 30 in front of an interface 24 on which total reflection takes place.

In the spacer layer 23 of the formed Fabry-Perot resonator 21, a quantum well (QW) 26 may be inserted, which is responsible for optical loss or gain. In the example, the spacer layer consists of GaAs and comprises an 8 nm-thick InGaAs-GaAs quantum film.

The amount of loss or gain, for example, by optical pumping or embedding of the structure in a pn-heterojunction as in a VCSEL (VCSEL = vertical-cavity surface-emitting laser);

Vertical laser diode) also by electrical stimulation

be set.

FIG. 8 shows exemplary calculated intensity reflection spectra of the structure of FIG. 7 for FIG

different refractive indices n of the adjacent medium 10. The spacer layer in the present example has a thickness of 25 nm, the quantum film has an absorption coefficient of 600 / cm. There are sharp resonance decays, which become longer with increasing refractive index n

Move wavelengths.

FIGS. 9a and 9b show refractive index profiles and typical location-dependent courses of the electric field strength

in FIG. 9a for a wavelength outside the resonance and in FIG. 9b for a resonance wavelength.

Outside the resonance, a characteristic of Bragg reflectors attenuation of the wave field with increasing penetration depth can be found. In contrast, the field strength increases with a maximum in the vicinity of the interface in resonance. The resonance peaking leads to a high photon density in the evanescent field in the adjacent, to be examined medium, which in the sense of

Perturbation theory is decisive for the sensitive

Dependence of the resonance wavelength on the refractive index of the adjacent medium 10. In Fig. 10 are calculated intensity reflection spectra for a structure of Fig. 9a or 9b with a

Quantum film with a moderate gain of 400 / cm for different refractive indices of the adjacent medium 10 shown. There are significant gains in resonance. The resulting extremely low

Halfwidths allow a precise determination of the

Refractive index, for example, in the range of 1.30 to 1.36, which for many chemical and biological systems of

is of particular interest.

For precise adjustment of losses, the

Semiconductor material in a preferred embodiment in the spacer layer n- or preferably p-doped. For the same purpose, alternatively, a few nanometers thick, weakly absorbing passivation layer,

For example, be used from silicon oxynitride, in a further preferred embodiment, the

Semiconductor surface simultaneously against the attack

protects against aggressive substances.

If biomolecules are to be functionalized on the surface, a further one is offered

Embodiment, a thin metallic

Surface coating, for example of gold, whose effect on the resonance behavior of FIG. 11

evident. There are also other metallic ones

Surface coatings of the measuring surface possible, comprising the materials silver, copper or aluminum. In the case of a gold-containing surface coating, a clear blue shift of the resonance wavelength and increased attenuation can be observed with increasing thickness of the gold layer. The example calculated in FIG. 11

Intensity reflectance spectra are shown for a refractive index profile of Figure 9a or 9b with an additional gold coating having a thickness of 0.1 nm, 0.3 nm, 0.6 nm, 1.0 nm and 1.5 nm.

For comparison, in FIG

Intensity reflection factor R of a gold layer on GaAs semiconductor material shown as a spacer layer, from which the increase in losses is derived with increasing metal layer thickness. In addition to the above

Layer thicknesses are by way of example still a 2.0 nm thick

Gold coating shown. It should be noted that the calculations for FIGS. 11 and 12 as well as FIGS. 8, 9 and 10 for TE polarization and 30 ° angle of incidence have been performed.

The above-described embodiments of a Fabry-Perot resonator 21 with amplifying quantum films 26 and totally reflecting interface 24 are suitable for

Examination vertically and laterally structured

Layer systems as orkommen example in biosensors, biological cells or biochips _V, and as described for example in HH Nguyen, J. Park, S. rank, M. Kim: Surface Plasmon Resonance: A Versatile Technique for

Biosensor Applications, Sensors 2015, 15, 10481-10510, or in LK Chin, AQ Liu, CS Lim, XM Zhang, JH Ng, JZ Hao, S. Takahashi: Differential single living cell refractometry using grating resonant cavity with optical trap, Applied Physics Letters 91, 243901 (2007),

are described.

Fig. 13 shows schematically a representation of a

Section of a device 1 for measuring at least one feature of a medium 10 to be examined,

preferably for measuring the refractive index of the medium 10, according to a further embodiment of the invention, which is suitable for examining laterally structured

Double layers 11 as they come for example in biological cells or biosensors _V. By projection or imaging, information about the two-dimensional refractive index distribution of the medium to be analyzed near the interface can be obtained. The rapid decline of

evanescent field allows a depth resolution of a few 100 nm.

In the example shown, a prism 20 turns off

high-refractive material a collimated, even

monochromatic shaft 40 coupled. With the

_Z calibrate reference 42 is a projection of the reflected wave 41 shown. In the exemplary illustration, the medium 11 to be examined has two regions 12 with a refractive index n to be analyzed. This refractive index n thus differs from the refractive index n _{buik of} the surrounding material of the medium 11 to be investigated, which in the example is a water _layer .

Fig. 14 shows calculated intensity reflection spectra of a structure of Fig. 13 having a thickness of 100 nm Transitional layer, which is in front of an extended

Water _layer with n _buik = 1,330 is located. These

Transition layer may be, for example, a biological cell, the refractive index of which is the medium of the 11th

different. For the transitional layer

different refractive indices from n = 1,330 to n = 1,350

accepted. At very moderate gain by the

Quantum film of 400 / cm, refractive indices on the surface can be determined significantly better than the third place after the decimal point, ie significantly better than 0.1%.

Similarly precise results can be obtained if, for example, an approximately only 0.2 nm thick (absorbing) gold layer is applied to the semiconductor surface, on which biomolecules readily accumulate and

functionalize. A typical resonant

Field distribution including refractive index profile for such an embodiment of the invention is shown in FIG.

Fig. 15 shows a refractive index profile and a

Field distribution in resonance for a device with an AlAs-GaAs layer system, which is formed with a 0.2 nm thick gold layer of refractive index n = 0.26, and an adjacent water-based transition layer.

Fig. 16 shows - similar to Fig. 14 - a calculated intensity reflection spectra of 100 nm thick layers with refractive indices of n = 1,330 to n = 1,350 on water. In order to compensate for the absorption of the gold layer, in this embodiment, a higher gain by the Quantum film provided, which is in the example at 1,500 / cm.

Fig. 17 shows a refractive index profile for an AlAs-GaAs Bragg reflector with InGaAs GaAs reinforcing

Spacer layer, wherein the quantum film on the

Layer material InGaAs (InGaAs = indium gallium arsenide), hereinafter also referred to as InGaAs quantum film

and quasi-waveguides and the calculated field distribution in resonance. It may also be advantageous, according to the illustrated embodiment in FIG. 17, to provide on the semiconductor surface a lossless, lower refractive dielectric quasi-waveguide layer several hundred nanometers thick, in which a resonant-like excessive photon density is established, also in the evanescent field of the adjacent, yet

of lower refracting medium.

The refractive index profile shown is designed for an AlAs GaAs Bragg reflector with 15 mirror pairs, GaAs spacer layer with 10 nm thick InGaAs quantum film and 20 ° light incidence angle and TE polarization. The field distribution of the TE polarized electric field is calculated for resonance.

FIG. 18 shows calculated intensity reflectance spectra corresponding to structures of FIG. 17 and various

Refractive indices of the adjacent medium, in the example vacuum, air and CO2, were calculated in the evanescent field. Compared to vacuum, air (n = 1,0003) or CO2 (n = 1,001) at normal pressure results in displacements of the

Resonant wavelength of about 33 and 100 .mu.m, which when amplified by the quantum film of 1,550 / cm in

Quantum film can still be resolved clearly. It is also possible to prove pressure dependencies. Measurements in the spectral environment of absorption lines additionally provide information about the gas composition.

Devices according to the invention for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, ie resonant refractometers as explained above, are not limited to the incidence of plane light waves.

In one embodiment of the invention is therefore a

Cylindrical divergent monochromatic light beam 50 used for irradiation in the resonator.

Fig. 19 shows schematically a representation of a

Section of a device 2 for measuring at least one feature of a medium 10 to be examined,

preferably for measuring the refractive index of the medium 10, according to an embodiment of the invention, which

Cylindrical divergent monochromatic light beam 50 for irradiation in the Fabry-Perot resonator 21 uses.

The projection 52 of the reflected light wave 51 produces, depending on the resonance collapse or resonance peaking in the reflected light wave 51, angle-dependent dark or light stripes. The same applies to a convergent one incident light beam, in particular for

lateral scanning of the measurement object 10 offers.

Convergent beam path devices 1, 2 can also be used as a reinforcing element in a laser with external mirrors.

In addition to prisms for coupling the light at a predetermined angle of incidence Q _ir which according to the invention is preferably greater than the critical angle of the

Total reflection, optical grating structures offer as an optical element, which use diffracted light in a higher order for analysis.

FIG. 20 therefore shows schematically a representation of a section of a further embodiment of a

Device 3 for measuring at least one feature of a medium to be examined 10, preferably for measuring the refractive index of the medium 10, which with a

Light coupling into the high refractive index material of the optical element 60 via an optical grating 62a, 62b. The optical element 60 is made high-refractive in the example and includes GaAs as the base material.

The optical element 60 is further with two

formed on opposite, preferably plane-parallel surfaces, wherein the first surface is the one over which during the measurement electromagnetic radiation 70 of the wavelength A _{t is} coupled, and wherein the

opposite, second surface during the measurement, the interface 64 to the medium to be examined (not shown). The second surface accordingly comprises a Fabry-Perot resonator 61. In the illustrated

Embodiment, the second surface is additionally formed with an absorber layer 65, which surrounds the Fabry-Perot resonator 61.

To avoid unwanted radiation by reflection of the radiation 70 at the first surface, the first one is

Surface of the optical element 60 is provided with an antireflection coating 63, which surrounds the optical grating 62 a, 62 b in the example.

In this case, the Fabry-Perot resonator 61 can be designed as shown using the exemplary embodiments explained above. In particular, in a preferred embodiment, it comprises a Bragg reflector and a

Quantum film. For the optical excitation of the gain or control of the losses in the quantum well, for example, a vertically incident, unpolarized or a TE or TM polarized pump wave l _r 75 can be used.

The light wave 70 from the radiation source (not

shown) applies in the example perpendicular to the optical grating 62a and undergoes a diffraction here. The

Lattice constant is preferably chosen so that the deflection of the light wave 70 is so strong that the

deflected light wave at an angle which is greater than the critical angle of total reflection, is directed to the Fabry-Perot resonator 61. The reflected light wave becomes two when passing the optical grating 62b

Split part waves 71, 72, wherein the partial wave 71 with the receiving device (not shown) can be detected.

Particularly preferred embodiments of a

Device according to the invention for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium

fiber optic light guides, also as optical fibers

designated include. These optical fibers can guide the electromagnetic radiation from a light source to the optical element 20 and / or from this back to the receiving device. Due to their compact construction, such embodiments are particularly suitable for minimally invasive examinations.

In FIGS. 21 and 22, two such embodiments are shown purely by way of example.

Fig. 21 shows schematically a section of a

Device 4 for measuring the refractive index of a medium with a prism as a refractometer at the tip of a single-mode optical fiber and with the return of the measuring beam via a multimode fiber.

The optical element 20 according to the invention is at the tip of a single-mode optical fiber 81 in the example

trained optical fiber arranged. The device 4 further has an arrangement for returning the coupled and reflected light wave over a

Multimode fiber 84. The optical element 20 is formed in this example as a high refractive prism and at the Tip of the polarization-maintaining single-mode optical fiber 81 mounted. The light beam 90 coupled into the single-mode optical fiber 81 is reflected after reflection

Measuring beam 91 is fed back via the multimode fiber 84 and can be detected by means of the receiving device (not shown).

The optical element 20 is based on the material GaAs. The embodiment based on a GaAs prism also allows optical amplification in the resonator region of the optical element when a pumping wave is irradiated by the single-mode optical fiber 81. The optical element 20 has in the example a Fabry-Perot resonator 21 and an AlGaAs-GaAs Bragg reflector 22. Further, the optical element 20 is at its light entrance and at its

Light exit side provided with an anti-reflection layer 87.

The single-mode optical fiber 81 comprises a core 82, which is surrounded by a jacket 83. Similarly, the multi-mode fiber 84 includes a core 85 and a cladding 86. To get an impression of the

achievable, small dimensions of the miniature refractometer, in Fig. 21 essential sizes are indicated by the letters A, B and C. Without

Restriction to the given embodiment, relevant size ratios are as follows: A = 10 ym, B = 50 ym and C = 250 ym.

The light entrance surface of the multimode fiber 84 has an angle to the optical axis of the multimode fiber 84 of 45 ° so that radially incoming, reflected light from the optical element 20 can be exactly coupled into the core 85. The reflectivity of this interface can be significantly increased for example by a few nm thick gold layer. If the reflection surface of the GaAs prism has an angle f not equal to 45 ° to the optical axis of the single-mode fiber, the reflection surface of the multimode fiber should be set to the angle 90 ° -cp.

Fig. 22 shows schematically a section of a

Further, the illustrated device 5 for measuring the refractive index of a medium comes out without an additional optical element.

Instead, the tip of a single mode optical fiber 101 comprising a core 102 and a cladding 103 is provided with a light exit surface inclined at 45 ° to the optical axis. The light exit surface comprises a directly applied Fabry-Perot resonator 21 and

expediently a planar dielectric Bragg reflector 22 of TiCg SiCg l / 4-layer pairs and a

Distance layer of SiC> 2, in the example by

Doping with silver ions optical losses are introduced. In this way, the light exit surface of the single-mode optical fiber 101 is already particularly advantageously the measuring surface of the device 5 available. The return of the measuring beam 91 takes place via a multimode fiber 84.

In this embodiment, the optical element as a dielectric refractometer is directly on the oblique ground light exit surface of the single-mode glass fiber can be applied. It is advantageous in this

Embodiment the possible better adaptation of the

Refractive indices of the resonator to the glass fiber material.

Regarding the angle f and the improvement of the

Reflectivity is the same as above to the

Embodiment executed under Fig. 21.

A likewise advantageous development of the invention lies in another embodiment of the interface to the medium to be examined 10. The losses for a

For example, totally reflected waves can also be achieved by a relief-like design of the interface

produce. The interface is thus not planed in the contact area to the medium to be examined, but for example in the form of a sine wave.

Fig. 23 shows schematically a section of a

Device 6 of such an embodiment of a

Refractometer. In the example shown, the optical element 120 is formed with a sinusoidally wavy boundary surface 124.

The light deflected into the 1st diffraction order can be used to image the interface. If the amplitude a of the sine wave is small, that is to say a l (ϊΐ _ί - n) / (nn _t ), then the disturbance can be approximated as optical

Phase gratings are described. If the period d of the sine wave d <l / {2n), then higher orders of diffraction can be used only propagate in the higher refractive material, so only in reflection.

Furthermore, for lattice constants in the range

l / (2hi) <d <l / (2ή) the angle of incidence 0 _j > 9 _gr, and the lattice constant at the same time so small that only positive orders of diffraction occur and the +1.

Diffraction order propagates in the direction of the optical axis orthogonal to the Bragg planes. About that in the +1.

Diffraction order deflected light can thus be the

Sharpen interface with high resolution.

High photon density at the interface and thus high

Intensity in the +1. Diffraction order is obtained only in case of resonance. If the refractive index in the evanescent field of the adjacent medium changes only slightly in the lateral x or y direction, ie | degree n (x, y) | «N (x, y) / (5ln _; ), noticeable light scattering takes place in the +1.

Diffraction order only at those points in the x, y interface plane at which the resonance condition

is satisfactorily fulfilled. This could be through

Figure the interface in the direction of the optical axis Locations (x, y) Identify the same refractive index as it does

For example, for the study of biological tissue or histological sections may be of interest. To avoid optical aberrations caused by the Bragg grating, its stop band should surround the

Concentrate angle of incidence, which can be achieved by suitable design of the Bragg reflector, especially by low refractive index differences in the constituent Bragg mirror pairs. The measuring surfaces of the devices for measuring at least one feature of a medium to be examined,

preferably for measuring the refractive index of the medium, are designed very small. In the exemplary embodiments according to FIG. 21 or FIG. 22, for example, they can be significantly less than 1 mm ² . Likewise it is also possible, the

Measuring surface smaller than 1 mm ² , preferably smaller than 0.1 mm ² and more preferably smaller than 0.01 mm ² to make, the depth resolution of the device is in a range of a few 100 nm up to a range of less than 100 nm.

Against this background, the devices according to the invention explained above are particularly suitable for minimally invasive examinations. The devices can also be integrated micro-optically.

With a known wavelength of the coupled-in light and known angle of incidence, the refractive index of the medium to be investigated can be determined with high precision.

By an optical image of the total reflecting interface can still be a high lateral

Achieve resolution. Due to the low penetration depth of the evanescent wave in the medium to be examined extremely small amounts of substance are sufficient to a

Make refractive index determination. Occurring optical losses in the measuring volume, such as by scattering or absorption, can be compensated by an optical gain in the measuring system. For this purpose, it is advantageous that the device for increasing the

Sensitivity can generate a high photon density in the measurement volume.

Claims

claims

1. An optical element for use in a device for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, wherein

the optical element comprises at least one interface which is designed to form a flat contact with the medium to be examined during the measurement,

at least a part of this interface of the optical element is a measuring surface, and

this interface of the optical element with a layer system acting as a resonator

which is at an obliquely impinging on the interface electromagnetic

Radiation at that radiation, which at an angle which is greater than the critical angle of

Total reflection at the interface between the optical element and the medium to be examined causes multiple reflections of this radiation, so that form resonances in the layer system.

2. Optical element according to claim 1, characterized

characterized in that the measuring surface is less than 1 mm ² , preferably less than 0.1 mm ^2, and more preferably less than 0.01 mm ² .

3. Optical element according to one of the preceding

Claims, characterized in that the

incident beam from a medium of refractive index penetrates the resonator so that n <

, where n is the refractive index of the medium to be examined.

4. Optical element according to one of the preceding

Claims, characterized in that the optical element comprises a high refractive index material, preferably with a refractive index n _j , which is at least at n _j = 1.8, preferably at n _j > 3.0 and particularly preferably at n _j > 3.5.

5. Optical element according to one of the preceding

Claims, characterized in that the

Layer system comprises a Bragg reflector.

6. Optical element according to the preceding claim,

characterized in that the Bragg reflector is formed of a semiconductor layer system, preferably AlGaAs GaAs, InGaAsP InP or InAlGaN GaN.

7. Optical element according to one of the preceding

Claims, characterized in that the

Layer system comprises at least one quantum film.

8. Optical element according to the preceding claim,

characterized in that the quantum film is preferably based on the material system InGaAs-GaAs, GaAs-AlGaAs, InGaAsP-InP or InAlGaN-GaN and has a small thickness, preferably a thickness of 15 nm or less, more preferably 10 nm or less.

9. Optical element according to one of the two preceding claims, characterized in that the

Quantum film has an excitation-dependent positive or negative absorption coefficient of, for example +/- 400 / cm, +/- 800 / cm or +/- 1.600 / cm.

10. Optical element according to one of the preceding

Claims, characterized in that the

Layer system comprises a Fabry-Perot resonator.

11. Optical element according to one of the preceding

Claims, characterized in that the measuring surface comprises a metallic surface coating, preferably comprising gold, silver, copper or

Aluminum.

12. Optical element according to one of the preceding

Claims, characterized in that the optical element for radiation steering comprises a prism.

13. Optical element according to one of the preceding

Claims, characterized in that the optical element for radiation steering comprises an optical grating.

14. Device for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, comprising a radiation source,

an optical element according to any one of claims 1 to 13, and a receiving device,

in which

the radiation source is designed to emit electromagnetic radiation and to couple this radiation into the optical element,

and where

the receiving device is formed at the

Interface to receive reflected electromagnetic radiation.

15. Device according to the preceding claim,

Furthermore, comprising an evaluation unit, which allows using algorithms stored an evaluation of the received by the receiving device images.

16. Device according to one of the two preceding claims, characterized in that the

Depth resolution of the measurement in a range of a few 100 nm is up to a range of less than 100 nm.

17. Device according to one of the three preceding

Claims, characterized in that the

Radiation source is designed for emitting electromagnetic radiation, preferably for emitting monochromatic light and particularly preferably for emitting a cylindrically divergent or convergent monochromatic light beam.

18. Device according to one of the four above

Claims, characterized in that the optical Element is connected directly to the tip of a single-mode optical fiber.

19. Device according to the preceding claim,

characterized in that the layer system is applied directly to an oblique light exit surface of a single-mode glass fiber.

20. Using a device according to one of

preceding claims 14 to 19 for minimally invasive examinations.

21. A method for measuring at least one feature of a medium to be examined, preferably for measuring the refractive index of the medium, comprising a device according to one of the preceding claims, with a device according to one of the preceding

Claims 14 to 19, comprising the following steps: for the measurement, the optical element at the interface in direct contact with the medium, electromagnetic radiation is coupled into the optical element such that it at the

Impinges at an angle greater than the critical angle of total reflection at the interface,

a total reflection of this coupled radiation arises at the interface between the optical element and the medium,

which results in multiple reflections and whereby resonances are formed in the layer system of the optical element whose spectral and direction-dependent position is measured by the receiving device.

22. The method according to the preceding claim, characterized in that an evaluation unit is provided, which is used to determine the refractive index of the medium to be examined and a

Evaluation of the by the receiving device

images received.

23. Method according to one of the two preceding

Claims, characterized in that the

Receiving device with a suitable sensor, preferably a photosensor, is equipped.