DE102011054512A1 - Method and apparatus for high-frequency analysis of liquid systems - Google Patents

Abstract

The present invention relates to a method and a device for guiding electromagnetic radiation in the terahertz frequency range in a liquid. Due to the strong absorption of aqueous solutions, biomolecular systems have hitherto been preferably dried, pressed and characterized in powder form by means of THz radiation, ie outside their natural aqueous environment. It is therefore an object of the present invention to provide a method and a device which make it possible to expand the propagation distance of electromagnetic radiation in the THz frequency range in liquids, in particular in water and aqueous solutions. To achieve this object, a method for guiding electromagnetic radiation in the THz frequency range in a liquid is proposed with the steps: generating electromagnetic radiation in the THz frequency range, providing the liquid, coupling the electromagnetic radiation into the liquid and exciting a surface polariton mode in the liquid or at their interfaces.

Description

The present invention relates to a method and a device for guiding electromagnetic radiation in the terahertz frequency range in a liquid.

The present invention furthermore relates to a method and a device for analyzing a liquid with the aid of electromagnetic radiation in the terahertz frequency range.

• – Viele optisch undurchsichtige Materialien sind im THz-Frequenzbereich transparent.
• – THz-Strahlung ist nicht ionisierend und wird daher im biomedizinischen Bereich als sicher betrachtet.
• – Bestimmte rotatorische, vibronische oder libratorische Molekülanregungen weisen eine Resonanzfrequenz im THz-Frequenzbereich auf.
• – THz-Strahlung liefert wesentliche Informationen über Ladungsträgerdynamiken, insbesondere in solchen Strukturen, die eine essentielle Rolle in zukünftigen photonischen und elektronischen Komponenten spielen.
• – THz-Strahlung zeigt eine geringere Streuung verglichen mit optischen Frequenzen und ist daher insbesondere zur Verwendung in industriellen Umgebungen, in denen es beispielsweise vermehrt zur Staubbildung kommt, geeignet.
• – Betrachtet man Kommunikationssysteme, so ermöglichen höhere Frequenzen größere Übertragungsbandbreiten.

The terahertz (THz) frequency range or submillimeter wavelength range is one of the last "dark" regions of the electromagnetic spectrum. It is only slowly that commercial applications for this area of the electromagnetic spectrum are opening up, even though the THz frequency range has intrinsic advantages over other frequency bands in the electromagnetic spectrum:

• - Many optically opaque materials are transparent in the THz frequency range.
• - THz radiation is non-ionizing and therefore considered safe in the biomedical field.
• - Certain rotatory, vibronic or libratory molecular excitations have a resonance frequency in the THz frequency range.
• THz radiation provides substantial information about charge carrier dynamics, especially in those structures that play an essential role in future photonic and electronic components.
• THz radiation exhibits less scattering compared to optical frequencies and is therefore particularly suitable for use in industrial environments in which, for example, more dust is generated.
• - Looking at communication systems, higher frequencies allow greater transmission bandwidths.

THz radiation is basically suitable in particular for analytical investigations on biomolecular systems. However, it has been found that it is almost impossible to analyze these biomolecular systems in their natural, ie aqueous, environment using THz radiation. This limitation is due to the fact that water has very high absorption coefficients for THz radiation, so that this region of the electromagnetic spectrum is completely absorbed in water and aqueous solutions after only a few hundred micrometers. A detection of the interaction of the THz radiation with the biomolecular systems after a suitable interaction path is therefore not possible, because on this route, the aqueous solution completely absorbs the THz radiation.

Therefore, biomolecular systems have hitherto been preferably dried, pressed and characterized in powder form by means of THz radiation, ie outside their natural aqueous environment.

It is therefore an object of the present invention to provide a method and a device which make it possible to expand the propagation distance of electromagnetic radiation in the THz frequency range in liquids, in particular in water and aqueous solutions. In addition, it is the object of the invention to provide a method and a device for the analysis of liquids and substances in liquids, in particular for the analysis of biomolecular systems in an aqueous environment, with the aid of THz radiation.

At least one of these objects is achieved by a method for guiding electromagnetic radiation in the THz frequency range in a liquid, comprising the steps of generating electromagnetic radiation in the THz frequency range, providing the fluid, coupling the electromagnetic radiation into the fluid and exciting a surface polariton. Mode in the liquid or at their interfaces.

At least one of these objects is also achieved by a method for analyzing a liquid, comprising the steps of: generating electromagnetic radiation in the THz frequency range, providing the liquid, coupling the electromagnetic radiation into the liquid, exciting a surface polariton mode in the Liquid or at their interfaces and detecting the coupled into the liquid to be analyzed electromagnetic radiation.

For the purposes of the present application, the liquid is referred to as "liquid to be analyzed", if it is a method for analyzing a liquid.

For the purposes of the present application, the THz frequency range of the electromagnetic spectrum extends in a range from about 10 GHz to about 10 THz.

It has been found that the excitation of a, in particular coupled, surface polariton mode at the interfaces of a THz radiation strongly absorbing liquid, such as water, the propagation distance of the THz radiation considerably extended without the radiation thereby completely from the Liquid would be absorbed. This means in particular that the interaction length of the THz radiation with a to be analyzed Liquid and preferably with substances in the liquid can be considerably expanded.

In one embodiment, exciting a coupled surface polariton mode in the liquid succeeds when the liquid forms two interfaces with dielectric material transparent to the THz radiation, the THz radiation liquid having a greater absorption than the dielectric Material. In this case, the greater the absorption of the liquid compared with the dielectric material, the better the propagation of the THz radiation by means of the surface polariton mode.

In one embodiment of the method according to the invention, the liquid is provided in the form of a layer whose thickness is smaller than the wavelength of the electromagnetic radiation used in the THz frequency range.

In another embodiment, the liquid is provided in the form of a layer whose thickness is less than 1.5 times the skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used, the thickness of which is preferably smaller is more preferably 1.25 times the skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used and the thickness thereof, and less than the skin depth for at least one of the wavelengths of the radiation source generated electromagnetic radiation in the liquid to be used.

In those embodiments where the liquid is in at least one direction, i. H. between the two opposing interfaces has a dimension or thickness which is smaller than the wavelength of the electromagnetic radiation or smaller than the skin depth of the electromagnetic radiation in the liquid, the electric field of the electromagnetic radiation coupled into the liquid falls on two sides exponentially into the dielectric material surrounding the liquid. The majority of the field lies outside of the strongly absorbing liquid in the lossless or low-loss material, which forms the two interfaces with the liquid.

In one embodiment, the thickness of the liquid layer in a direction perpendicular to the two interface between the liquid and the dielectric material is less than about 50 μm, preferably less than about 40 μm.

In one embodiment of the method, the liquid is provided as a layer between a first and a second element of dielectric material. In this way, surface polariton modes can form at two interfaces of the liquid to dielectric material.

In a further embodiment of the method according to the invention, an evanescent field of the electromagnetic radiation is coupled into the liquid.

To accomplish this, in one embodiment, an evanescent field is first coupled into the dielectric material, for example, which forms an interface with the liquid, with the evanescent field extending into the liquid.

For coupling an evanescent field into the liquid, for example, an internal total reflection at an interface between a body transparent to the THz radiation, preferably a prism, and the dielectric material forming the interface to the liquid can be used.

For this purpose, the transparent body for the THz radiation used has a greater refractive index than the dielectric material, which forms the interface with the liquid. If the electromagnetic radiation hits the interface between the transparent body and the dielectric material at an angle which is greater than the critical angle, a total reflection occurs at this interface and only an evanescent, ie. H. The exponentially decaying electric field of the electromagnetic radiation extends into the dielectric material which forms the interface with the liquid.

Excitation of the coupled surface polariton mode in the liquid or at its interfaces is then detectable as a weakening of the total internal reflection at the interface between the transparent body and the dielectric material, which forms the interface to the liquid. Interactions of the electromagnetic radiation in the THz frequency range with the liquid also manifest themselves in changes in the total internal reflection and thus the intensity of the reflected radiation detected by a detector.

In another embodiment, where the liquid is provided as a layer between a first or second dielectric material element, an evanescent field of electromagnetic radiation is coupled into the first dielectric material element evanescent field through the liquid extends into the second element of dielectric material.

In one embodiment of the invention, the coupling of the electromagnetic radiation into the liquid takes place substantially parallel to the interface of the liquid with the dielectric material.

In a further alternative embodiment, the coupling of the electromagnetic radiation into the liquid takes place substantially perpendicular to the interface between the liquid and the dielectric material.

Depending on the nature of the coupling of the electromagnetic radiation into the liquid, the detection of the excitation of the surface polariton mode and / or its interaction with the liquid takes place directly or indirectly.

As the propagation direction of the surface polariton mode in the liquid, a direction parallel to the interface between the dielectric material and the liquid is referred to in the sense of the present application.

• a) Die Anregung von inter- und intramolekularen Vibrations- und Rotationszuständen von Substanzen in wässriger Lösung. Diese führen zu einer Abschwächung der durch die zu analysierende Flüssigkeit propagierenden Oberflächen-Polariton-Mode.
• b) Bindungsereignisse an oberflächenimmobilisierten Molekülen, d. h. in solchen Molekülen, die an der Grenzfläche zwischen dielektrischem Material und zu analysierender Flüssigkeit lokalisiert sind. Diese führen zu einer Brechungsindexänderung der zu analysierenden Flüssigkeit und damit zu einer Änderung des Absorptionsverhaltens der zu analysierenden Flüssigkeit.
• c) Brechungsindexänderungen der zu analysierenden Flüssigkeit beispielsweise infolge einer Änderung der Konzentration der in der Flüssigkeit enthaltenden Biomoleküle. Diese führen ebenfalls zu einer Abschwächung der Anregung der Oberflächen-Polariton-Mode.

With the aid of the method disclosed in this application, in particular the following events can be detected in a liquid to be analyzed:

• a) The excitation of inter- and intramolecular vibrational and rotational states of substances in aqueous solution. These lead to a weakening of the propagating through the liquid to be analyzed surface polariton mode.
• b) Binding events on surface-immobilized molecules, ie those molecules located at the interface between dielectric material and liquid to be analyzed. These lead to a refractive index change of the liquid to be analyzed and thus to a change in the absorption behavior of the liquid to be analyzed.
• c) refractive index changes of the liquid to be analyzed, for example due to a change in the concentration of the biomolecules contained in the liquid. These also lead to a weakening of the excitation of the surface polariton mode.

While one possible application of the guidance of electromagnetic radiation in the THz frequency range is the analysis of liquids or substances contained in a liquid, the guiding may also serve to guide electromagnetic THz radiation, for example on an analysis chip, to a sample chamber. Such a liquid-based waveguide has the advantage that it can be manufactured with the same process steps as a sample chamber for the liquid to be analyzed. When the method or apparatus of the present invention is used as a waveguide, the chamber for receiving the liquid is extended in one direction to connect an injection point to a target point for the electromagnetic radiation.

In addition, at least one of the above-mentioned objects is also achieved by a device for guiding electromagnetic radiation in the THz frequency range in a liquid having a chamber for receiving the liquid, wherein the chamber has two opposite wall sections of one transparent to the electromagnetic radiation in the THz frequency range dielectric material, wherein the wall sections each form an interface with the liquid to be accommodated in the chamber and wherein the liquid to be accommodated in the chamber extends as a layer between the two interfaces, and means for coupling the electromagnetic radiation into the chamber, the like is set up that during operation of the device, a surface polariton mode is excited along the interfaces.

In addition, at least one of the above objects is achieved by a device for analyzing a liquid having a radiation source adapted to produce in operation electromagnetic radiation in the THz frequency range, a chamber for receiving the liquid, the chamber having two opposite wall sections comprising a dielectric material which is transparent to the electromagnetic radiation in the THz frequency range, the wall portions each forming an interface with the liquid to be accommodated in the chamber, and wherein the liquid to be accommodated in the chamber extends as a layer between the two boundary surfaces, means for coupling electromagnetic radiation into the chamber adapted to excite a surface polariton mode along the interfaces during operation of the device, and an electromagnetic radiation detector for detecting the radiation injected into the chamber opposed electromagnetic radiation.

For the purposes of the present application, the chamber for the liquid is a sample chamber, if it is a device for analyzing a liquid.

Suitable radiation sources for the electromagnetic radiation in the THz frequency range are all radiation sources that emitted in this frequency range. These are on the one hand continuous wave sources and on the other hand pulse-shaped sources. In Depending on the liquid to be used, the radiation source may generate electromagnetic radiation at only a single or a plurality of discrete frequencies. However, for many applications it is desirable for the radiation source to provide broadband THz radiation, as do, for example, optoelectronic sources emitting pulsed radiation.

The electromagnetic radiation detector must be sensitive to the electromagnetic radiation in the THz frequency range.

In this case, for certain applications, the detector is a simple power detector, such. B. may be a bolometer or a Golay cell, while it may be important in other embodiments, in addition to the intensity of the electromagnetic radiation in the THz frequency range to detect their phase position. For detecting the phase position, coherent detection methods are suitable in which the electromagnetic radiation is mixed with a reference signal in order to be able to detect the phase position of the radiation.

In one embodiment of the invention, the radiation source is adapted to produce in operation electromagnetic radiation of at least one wavelength, the chamber having a dimension at least in a direction perpendicular to the interfaces between the dielectric material of the wall portions of the chamber and the liquid is smaller than at least one of the wavelengths of the electromagnetic radiation generated by the radiation source.

In one embodiment, in a direction perpendicular to the interfaces between the dielectric material of the wall portions of the chamber and the liquid, the chamber has a dimension that is less than 1.5 times the skin depth for at least one of the wavelengths of the Radiation source generated electromagnetic radiation in the liquid to be used, which is preferably less than 1.25 times the skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used and is particularly preferably smaller than that Skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used.

It is necessary that during operation of the device, the liquid has an absorption which is greater than the absorption of the dielectric material.

The excitation of a coupled surface polariton mode in the liquid requires phase matching between the exciting THz radiation and the excited surface polariton mode. That is propagation constants of the THz radiation and the excited surface polariton mode must largely match. Since the surface polariton mode to be generated always has a larger momentum than the dielectric material of the chamber, it is necessary to compensate for this pulse offset with the aid of the device for coupling the electromagnetic radiation into the chamber.

For such a phase-adapted coupling of the electromagnetic THz radiation to the surface polariton mode, in particular the arrangements of the device disclosed below for coupling the electromagnetic radiation into the chamber are suitable.

It is true that the coupling of the exciting THz radiation to the excited surface polariton mode is the better, the greater the spatial overlap of the fields of the exciting radiation and the excited mode.

In one embodiment of the invention, the means for coupling the electromagnetic radiation into the chamber comprises an electromagnetic radiation transparent body, preferably a prism, arranged to form an interface with the dielectric material of the chamber, the transparent one Body has a greater refractive index than the dielectric material of the chamber and wherein the device comprises a beam guide which couples the electromagnetic radiation into the body during operation of the device such that total reflection occurs at the interface between the body and the dielectric material of the chamber. wherein the detector is arranged to detect the electromagnetic radiation emerging from the body after reflection at the interface between the body and the dielectric material of the chamber.

With such an arrangement, an evanescent field can easily be created in the chamber which extends into the liquid and excites the surface polariton mode along the interfaces between the liquid and the dielectric material of the wall portions of the chamber. Thereby, the evanescent field which extends from the interface between the body and the dielectric material of the chamber into the dielectric material of the chamber is coupled into the chamber or the liquid contained therein.

It is particularly important to note that in one embodiment, the dielectric material of the chamber between its interfaces with the transparent body and the liquid has a thickness that is significantly smaller than the exponential decay of the evanescent field. The greater the overlap between the exciting mode of electromagnetic radiation and the excited surface polariton mode, the better the excitation of the surface polariton mode.

In an alternative embodiment, the means for coupling the electromagnetic radiation into the chamber comprises a grid for the electromagnetic radiation arranged to enable coupling of the electromagnetic radiation into the chamber.

In one embodiment, the apparatus further comprises means for coupling the electromagnetic radiation out of the chamber, the means for coupling preferably comprising a grid for the electromagnetic radiation arranged to allow outcoupling of the electromagnetic radiation from the chamber.

For this purpose, in one embodiment of the invention, the gratings are arranged such that they couple the electromagnetic radiation into or out of the dielectric material of the chamber.

In another alternative embodiment, the means for coupling the electromagnetic radiation into the chamber is configured to include focusing the electromagnetic radiation into the chamber such that the electromagnetic radiation has substantially the same propagation direction as the surface polariton mode, i , H. substantially parallel to the interface between the dielectric material of the chamber and the liquid.

In one embodiment of the invention, the chamber is integrated into a resonator for the excitation electromagnetic radiation used in the THz frequency range, so that it in the chamber to an increase of the electric field and thus to improve the coupling of the exciting THz radiation to the excited Surface Polariton Mode is coming.

In a further embodiment, the chamber has a resonator for the excited surface polariton mode.

Such a resonator for the excited surface polariton mode leads to an increase of the electric field of the mode in the liquid and thus to an enhancement of the effective interaction of the surface polariton mode with the liquid to be analyzed. Thus, an increase in the sensitivity of the surface polariton mode to changes in the liquid can be achieved.

This is particularly expedient in embodiments in which the coupling of the electromagnetic THz radiation takes place substantially parallel to the propagation direction of the surface polariton mode.

In this case, an embodiment is advantageous in which Bragg gratings are provided as reflectors for the surface polariton mode on the chamber.

The Bragg gratings, in one embodiment, are provided as periodic or quasi-periodic structures in the dielectric material of a wall portion of the chamber which forms an interface to the liquid. In one embodiment, the periodic or quasi-periodic structure of the Bragg gratings is formed by recesses in the dielectric material of a wall portion of the chamber.

A resonator for the surface polariton mode serves in particular to reinforce the field within the liquid.

In one embodiment, the effective refractive index of the surface polariton mode in the liquid depends not only on the surrounding materials and geometrical dimensions but also, in particular, on the liquid itself and its composition. However, a change in the refractive index of the liquid also leads to a change in the effective refractive indices of the Bragg gratings. In particular, a change in the effective refractive index of the surface polariton mode in one embodiment results in a change in the contrast between the effective refractive index of the surface polariton mode in the regions of dielectric material and the effective refractive index of the surface polariton mode in the regions the interspaces, which in their entirety represent the structure of the Bragg lattice. Such a change in the contrast of the effective refractive indices of the Bragg gratings of the resonator for the surface polariton mode in turn leads to a change in the stop band or the transmission band of the gratings and the resonator. This change in the band properties of the Bragg gratings, in turn, can in one embodiment serve for the highly sensitive detection of the properties of the liquid itself.

For example, a given liquid can be detected reliably if it is assumed that only this one liquid is the one Resonator tuned so that it is in resonance with the surface polariton mode. An excessively high signal is then detected for this one fluid, while only a weak or no signal is detected for all other fluids that do not resonate with the surface polariton mode resonator.

Further advantages, features and applications of the present invention will become apparent from the following description of embodiments and the associated figures.

1 shows a schematic diagram of a device for analyzing a liquid according to the present invention.

2 shows an embodiment of a sample chamber according to the invention.

3 shows a first embodiment of the coupling of electromagnetic radiation in the liquid to be analyzed.

4 shows an alternative embodiment of the coupling of the electromagnetic radiation in the liquid to be analyzed.

5 shows a further alternative embodiment of the coupling of the electromagnetic radiation in the liquid to be analyzed.

6 shows an embodiment of a sample chamber according to the invention with a resonator.

7 schematically shows the field distribution of the coupled surface polariton mode.

In the following description of the figures, identical elements are designated by identical reference numerals.

7 schematically shows the field distribution of the x component of the electric field 50 and the y-component of the magnetic field 51 the coupled surface polariton mode in a chamber according to the invention. The chamber is made of a first dielectric material 52 which has a low absorption for the electromagnetic radiation used in the THz frequency range, ie is substantially transparent to the radiation used. Furthermore, the chamber consists of a second dielectric material 53 which likewise has a low absorption for the electromagnetic radiation used in the THz frequency range, ie is substantially transparent to the radiation used. Between the dielectric materials, a gap is formed in which a THz radiation strongly absorbing liquid 54 is included. On both sides of the gap an interface forms 55 . 56 between the dielectric wall sections 52 . 53 the chamber and the liquid 54 out. The distance of the interfaces 55 . 56 from each other, ie the gap width, is just chosen so that it corresponds to the skin depth of the electromagnetic radiation in the THz frequency range in the liquid.

1 shows the schematic structure of a device according to the invention for the analysis of a liquid with the aid of THz radiation.

In the illustrated embodiment, a coherent THz optoelectronic device, as known in the art, is used to generate and detect the THz radiation in the device 1 used to analyze a liquid.

A short pulse laser system 2 is used to generate short electromagnetic pulses in the infrared spectral range with a pulse duration in the femtosecond range. The output power 3 of the short pulse laser system 2 is using a beam part 4 split so that part of the radiation is on an optoelectronic element 5 for converting the infrared radiation in THz radiation can be passed and another part of the radiation to an optoelectronic element 6 to capture the from the element 5 generated THz radiation.

In this case, in the illustrated embodiment, the optoelectronic elements 5 . 6 electro-optic crystals, here from zinc telluride. However, alternative embodiments are conceivable in which the optoelectronic elements are fast optoelectronic switches with an associated antenna structure for emitting the THz signal.

While in the electro-optical crystal 5 The THz radiation is generated by the second electro-optic crystal 6 the time-resolved detection of the electric field of the crystal 6 incident THz radiation 8th , To detect the THz radiation is exploited that the electro-optical crystal 6 the polarization state of the crystal 6 propagating radiation 7 in the infrared range as a function of the field strength of the electric field of the likewise propagating through the crystal, electromagnetic THz radiation 8th changed. With the help of an analyzer 9 becomes the change of the polarization state of the electromagnetic short pulse radiation 7 through the crystal 6 converted into a change in intensity, using a conventional detector 10 for the infrared electromagnetic radiation 7 can capture.

For the purposes of the present application forms the short-pulse laser 2 together with the electro-optical crystal 5 out 1 the radiation source for the electromagnetic radiation in the THz frequency range. The electro-optical crystal 6 , the analyzer 9 and the infrared detector 10 together form the detector for the electromagnetic radiation 8th in the THz frequency range.

Because the pulse of infrared radiation 7 short of the pulse of electromagnetic THz radiation 8th short, lets the field strength of the on the electro-optical crystal 6 Detect the corresponding THz pulse time-resolved when the time lag between the THz pulse 8th and the impulse in the infrared spectral range 7 with the help of a delay line 11 can be adjusted and passed.

While only the summary briefly describes the functionality of an optoelectronic measurement setup, as it is also known from the prior art as a THz-time domain setup (THz-TDS), the measurements can alternatively be carried out with systems that instead of the pulsed THz radiation use continuous wave radiation at one or more discrete frequencies.

The central element of the device according to the invention for analyzing a liquid is the sample arrangement 12 , in the 1 only schematically shown as a rectangular box.

The sample arrangement 12 contains a sample chamber 13 as they are schematic in 2 is shown. The sample chamber is designed to be a liquid layer 14 defines, wherein the liquid layer has two interfaces 15 . 16 with the surrounding dielectric material 17 . 18 formed. The dielectric material 17 . 17 forms the upper and lower walls of the sample chamber 13 ,

Along the interfaces 15 . 16 the propagation takes place for the achievement of long interaction distances between the electromagnetic THz radiation 8th and the liquid to be analyzed 14 necessary surface polariton modes.

In the described embodiment, the liquid layer has 14 a thickness measured between the two interfaces 15 . 16 of about 40 μm or less.

The dielectric walls 17 . 18 the sample chamber 13 are made of a cyclo-olefin copolymer (COC), here TOPAS 5013. Also well suited for the production of the dielectric walls of the sample chamber is COP.

Around the sample chamber with a defined thickness of the liquid layer 14 is provided in the in 2 In the exemplary embodiment shown, a COC substrate having a thickness of 3 mm is first spin-coated with a negative photoresist (SU-8) to produce a layer thickness of the resist that is approximately the thickness of the liquid layer 14 in the finished sample chamber 13 equivalent. With the help of a UV mask exposure those areas of the paint are cured, which the side walls of the sample chamber (in 2 not shown) form and after the development of the paint, the paint is removed where in the sample chamber, the liquid layer is to be recorded. Finally, the top wall 17 the sample chamber 13 applied to this arrangement and glued to the remaining lacquer layer.

To prevent parts of the top wall 17 on the substrate or the lower wall 18 Lower, are in the illustrated embodiment in the liquid layer 14 ie in the actual free volume for receiving the liquid in the sample chamber 13 , Columns of lacquer integrated, which is the top wall 17 wear.

To excite the surface polariton mode in the liquid layer 14 the THz radiation must be in the liquid layer 14 in the sample chamber 13 be coupled. If it is possible to excite the surface polariton mode, it propagates along the interface 15 or the interface 16 between liquid layer 14 and the upper or lower wall 17 . 18 the sample chamber 13 ,

So that for the long interaction lengths between the THz radiation and the liquid layer 14 necessary surface polariton mode can be excited, the liquid layer needs 14 have a higher absorption than the walls 17 . 18 which the interfaces 15 . 16 along which propagate the surface polariton modes.

In order to achieve a phase matching between the exciting THz radiation and the surface polariton mode, different ways of coupling the THz radiation into the liquid layer occur 14 in the sample chamber 13 in question. The 3 to 5 show three alternative embodiments of the coupling of the THz radiation in the liquid layer 14 ,

3 shows the coupling of the electromagnetic THz radiation 8th as an evanescent field 27 to the coupled surface polariton mode 19 at the interface 15 between the wall 17 and the liquid layer 14 , This is on the wall 17 a prism 20 placed. The material of the Prism has a higher refractive index than the refractive index of the dielectric material 17 the sample chamber 13 , Now couple the optoelectronic THz radiation 8th in the prism 20 such that the radiation 8th at an angle to the interface 21 between prism 20 and wall 17 the sample chamber 13 impinges, so occurs at the interface 21 Total reflection on and the electromagnetic radiation 8th will be at the interface 21 reflected.

The reflected part 22 The electromagnetic THz radiation is then detected by means of a detector, ie, for example, with the arrangement of electro-optical crystal 6 , Analyzer 9 and detector 10 out 1 , detected. Only an evanescent 27 extends into the wall 17 into it and choosing a suitable thickness of the wall 17 also in the liquid film 14 in the sample chamber and through it into the second, here lower wall 18 the sample chamber.

The evanescent field 27 is in the present case perpendicular to the film of the liquid 14 directed. The excited anti-symmetric coupled longitudinal electric field 19 falls exponentially into the liquid film 14 adjacent walls 17 . 18 from. Due to the high absorption of the liquid film 14 is the bulk of the anti-symmetric coupled field outside the liquid film 14 in the almost lossless material of the walls used for the THz radiation 17 . 18 localized. In this way, propagation distances for the surface polariton mode excited by the THz radiation in the range of centimeters can be achieved and thus significantly above the previously achieved penetration depths of THz radiation in liquids, in particular water.

Capturing the successful excitation of a surface polariton mode at the interface 15 respectively. 16 between liquid film 14 and the walls 17 . 18 the sample chamber 13 is indirect as a change in the intensity of the interface 21 between the prism and the upper wall 17 the sample chamber 13 totally reflected electromagnetic radiation detected.

4 shows an arrangement in which the coupling of the electromagnetic radiation 8th in the liquid layer 14 using a first grid 23 takes place while the coupling out of the electromagnetic radiation 8th from the liquid layer 14 with a second grid 24 he follows.

In the embodiment of 5 the coupling of the THz radiation takes place 8th in a side facet 25 of the liquid film 14 and the decoupling takes place at the other end of the sample chamber, namely through a side facet 26 ,

In the embodiment of 3 coupling the evanescent field to the surface polariton mode as a decrease in the intensity at the interface 21 between prism 20 and dielectric material 17 the wall of the sample chamber 13 reflected electromagnetic radiation 8th measured. Ie. the measurement is done indirectly. Demgegnüber is in the embodiments of the 4 and 5 the power of using the surface polariton mode through the water film 14 propagated THz radiation directly detected.

6 shows an alternative embodiment of the sample chamber 30 , As before, this represents the liquid to be analyzed as a film in a gap 31 between two wall sections 32 . 33 dielectric material transparent to the THz radiation, here COC. Each of the wall sections 32 . 33 forms an interface 34 . 35 with one in the gap 31 to be absorbed from the liquid.

The sample chamber 30 additionally has a resonator for the surface polariton mode. This one has a pair of Bragg grids 36 . 38 and 37 . 39 at the first and second end of the sample chamber. Here are the grids 36 . 37 in the first dielectric wall portion 32 trained while the grid 38 . 39 in the second dielectric wall portion 33 are formed.

The Bragg gratings are as periodically arranged recesses in the dielectric material of the wall sections 32 . 33 intended. Between the depressions remain stripes 41 made of dielectric material. The periodicity of the Bragg gratings 36 . 37 . 38 . 39 is selected to be in consideration of the effective refractive indices of the dielectric material, the air between the dielectric material and the liquid layer in the gap 31 forming a stop band for the surface polarization mode. In this way, the surface polariton mode between the pair of grids 36 . 38 at the first end of the sample chamber 30 and the second pair of grids 37 . 39 oscillate at the second end of the sample chamber. Here is the distance of the grid pair 36 . 38 at the first end of the resonator of the grid pair 37 . 39 at the second end of the sample chamber 30 just chosen so that the resonance condition for the surface polariton mode is satisfied in the resonator.

For purposes of the original disclosure, it is to be understood that all such features as will become apparent to those skilled in the art from the present description, drawings, and claims, even if concretely described only in connection with certain other features, both individually and separately any combinations with other of the features or feature group disclosed herein are combinable, unless it was expressly excluded or technical conditions make such combinations impossible or pointless. On the comprehensive, explicit representation of all conceivable combinations of features is omitted here only for the sake of brevity and readability of the description.

While the invention has been shown and described in detail in the drawings and the foregoing description, such illustration and description are exemplary only and not intended to limit the scope of the protection as defined by the claims. The invention is not limited to the disclosed embodiments.

Variations of the disclosed embodiments will be apparent to those skilled in the art from the drawings, the description and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. Reference signs in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

1

Apparatus for analyzing a liquid with THz radiation

2

Short-pulse laser system

3

infrared radiation at the output of the laser 2

4

beamsplitter

5, 6

electro-optical crystal

7

infrared radiation

8th

THz radiation

9

analyzer

10

detector

11

delay path

12

sample arrangement

13

sample chamber

14

liquid layer

15, 16

Interfaces between liquid 14 and wall

17, 18

Wall of the sample chamber

19

x component of the E-field of the coupled surface polariton mode

20

prism

21

Interface between prism 20 and wall 17 the sample chamber

22

lens

23, 24

grid

25, 26

Side facets of the sample chamber

27

evanescent field of stimulating THz radiation

30

sample chamber

31

gap

32, 33

dielectric wall portion of the sample chamber 30

34, 35

interface

36, 37

Bragg grating

38, 39

Bragg grating

40

deepening

41

material web

50

x component of the E-field of the coupled surface polariton mode

51

y-component of the H-field of the coupled surface polariton mode

52, 53

dielectric wall section

54

strongly absorbing fluid

55, 56

interface

Claims (16)

Method for guiding electromagnetic radiation in the THz frequency range in a liquid with the steps of generating electromagnetic radiation ( 8th ) in the THz frequency range, providing the liquid ( 14 ), Coupling the electromagnetic radiation ( 8th ) into the liquid and exciting a surface polariton mode in the liquid ( 14 ). A method according to claim 1, characterized in that a coupled surface polariton mode is excited. A method according to claim 1, characterized in that the liquid is provided so that it has at least two interfaces to dielectric, transparent to the electromagnetic radiation in the THz frequency range material and the liquid extends as a layer between the two boundary surfaces, wherein the liquid has greater absorption than the dielectric material. A method according to claim 3, characterized in that the liquid is provided in the form of a layer whose thickness between the two interfaces is smaller than the wavelength of the electromagnetic radiation used in the THz frequency range and whose thickness is preferably smaller than the 1.5- times the skin depth for at least one of the wavelengths of the radiation source generates electromagnetic radiation in the liquid to be used, preferably less than 1.25 times the skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used and which is particularly preferably smaller than that Skin depth for at least one of the wavelengths of the electromagnetic radiation generated by the radiation source in the liquid to be used. Method according to one of claims 1 to 4, characterized in that an evanescent field of the electromagnetic radiation is coupled into the liquid. Method according to one of claims 3 to 5, characterized in that an evanescent field of the electromagnetic radiation is coupled into the dielectric material, wherein the evanescent field extends into the liquid. Method according to one of claims 3 to 6, characterized in that an evanescent field of the electromagnetic radiation is coupled into the dielectric material, which forms a first interface with the liquid, wherein the evanescent field through the liquid to the second element of the dielectric Material stretches. Method for analyzing a liquid ( 14 ) with electromagnetic radiation in the THz frequency range with the steps of guiding the electromagnetic radiation in the THz frequency range in the liquid to be analyzed ( 14 ) with a method according to one of claims 1 to 7 and detecting the liquid to be analyzed ( 14 ) coupled electromagnetic radiation ( 8th ). Contraption ( 1 ) for guiding electromagnetic radiation in the THz frequency range in a liquid having a chamber ( 13 ) for receiving the liquid ( 14 ), whereby the chamber ( 13 ) two opposite wall sections ( 17 . 18 ) from one for the electromagnetic radiation ( 8th ) transparent dielectric material, wherein the wall sections each have an interface ( 15 . 16 ) to that in the chamber ( 13 ) liquid ( 14 ) and wherein the liquid to be accommodated in the chamber extends as a layer between the two boundary surfaces, and a device ( 20 . 22 . 23 . 24 ) for coupling the electromagnetic radiation ( 8th ) into the chamber ( 13 ) which is set up such that, during operation of the device, a surface polariton mode along the interfaces ( 15 . 16 ) is stimulated. Apparatus according to claim 9, characterized in that it comprises a radiation source ( 1 . 5 ) which is adapted to operate in use electromagnetic radiation ( 8th ) having at least one wavelength, wherein the chamber between the interfaces has a thickness which is smaller than at least one of the wavelengths of the electromagnetic radiation generated by the radiation source and which is preferably smaller than the skin depth of the electromagnetic radiation in a liquid to be accommodated in the chamber , Apparatus according to claim 9 or 10, characterized in that the means for coupling the electromagnetic radiation into the chamber comprises a body transparent to the electromagnetic radiation, which is arranged to form an interface with a wall portion of the chamber, the transparent body a larger refractive index than the dielectric material of the wall portion of the chamber and wherein the Vorrrichtung has a beam guide, which coupled in the operation of the device, the electromagnetic radiation in the body such that at the interface between the body and the dielectric material of the sample chamber, a total internal reflection occurs, wherein a detector is arranged so that it detects the exiting from the body after the reflection at the interface between the body and the dielectric material of the sample chamber electromagnetic radiation. Device according to one of claims 9 to 11, characterized in that the means for coupling the electromagnetic radiation into the chamber comprises a grating for the electromagnetic radiation, which is arranged so that it allows a coupling of the electromagnetic radiation in the chamber. Device according to one of claims 9 to 12, characterized in that it comprises a device for coupling out the electromagnetic radiation, wherein the device for coupling preferably comprises a grid for the electromagnetic radiation, which is arranged so that it is a decoupling of the electromagnetic radiation the chamber allows. Device according to one of claims 9, 10 or 13, characterized in that the means for coupling the electromagnetic radiation into the chamber comprises a beam guide with a focus, so that the electromagnetic radiation is coupled with a propagation direction in the chamber, which is substantially parallel to the interfaces between the liquid and the dielectric material of the wall sections of the chamber. Device according to one of claims 9 to 14, characterized in that it comprises a resonator for the surface polariton mode, wherein the resonator preferably comprises two Bragg reflectors. Device for analyzing a liquid with a radiation source ( 1 . 5 ), which is set up so that, during operation, it can emit electromagnetic radiation ( 8th ) in the THz frequency range, a device according to one of claims 1 to 15, and a detector ( 6 . 9 . 10 ) for the electromagnetic radiation ( 8th ) for detecting in the chamber ( 8th ) coupled electromagnetic radiation ( 8th ).

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