Microwave resonance cavity

Abstract

Microwave resonance cavities and associated methods and apparatus are described. In one example, a cavity (100) comprises a first and a second input port (102, 104) for inputting microwave radiation at a first and a second frequency respectively. The microwave radiation at the first frequency may be to excite a sample in the cavity whereas the microwave radiation at the second frequency may be to interrogate a sample in the cavity for analysis. The cavity has dimensions such that it resonates at both the first and the second frequency.

Claims

1. A microwave resonance cavity comprising: a first input port for inputting microwave radiation at a first frequency, the microwave radiation at the first frequency being to excite a sample in the microwave resonance cavity; a second input port comprising a waveguide configured to: input the microwave radiation at a second frequency in the microwave resonance cavity, the microwave radiation at the second frequency being to interrogate the sample in the microwave resonance cavity for analysis; transmit a microwave return signal from the microwave resonance cavity; and filter radiation at the first frequency from the microwave return signal; wherein the microwave resonance cavity has dimensions such that it resonates at both the first frequency and the second frequency; wherein the waveguide has a cut-off frequency greater than the first frequency and less than the second frequency.

2. The microwave resonance cavity of claim 1, wherein an excitation caused by the microwave radiation at the first frequency is heating.

3. The microwave resonance cavity of claim 1, wherein the microwave resonance cavity has a sample receiving region and is dimensioned such that: the microwave radiation at the first frequency results in a high electric field in the sample receiving region; and the microwave radiation at the second frequency results in a high magnetic field in the sample receiving region.

4. The microwave resonance cavity of claim 3, wherein the microwave resonance cavity is dimensioned such that the first frequency results in a TE.sub.101 resonant mode and the second frequency results in a resonant TE.sub.102 mode.

5. The microwave resonance cavity of claim 3, wherein the first input port is proximal to a maximum of the magnetic field of a resonant mode of the microwave radiation at the first frequency and the second input port is proximal to a maximum in the magnetic field of a resonant mode of the microwave radiation at the second frequency.

6. The microwave resonance cavity of 1, wherein the waveguide connects the microwave resonance cavity to a microwave analysis unit.

7. The microwave resonance cavity of claim 1, wherein the second input port is further configured to receive radiation output by the excited sample.

8. The microwave resonance cavity of claim 1, wherein the first input port is a coaxial input port.

9. The microwave resonance cavity of claim 1, wherein the microwave resonance cavity is an elliptic cylinder.

10. The microwave resonance cavity of claim 9, wherein the elliptic cylinder comprises first and second ends and the microwave resonance cavity comprises two separable parts, the parts separable in a plane perpendicular to the ends of the elliptic cylinder.

11. The microwave resonance cavity of claim 9, wherein the elliptic cylinder is dimensioned such that the second frequency is an X-band microwave frequency.

12. A method of performing microwave analysis in a microwave resonance cavity comprising: irradiating the microwave resonance cavity at a first frequency to excite a sample; irradiating the microwave resonance cavity at a second frequency to perform an interrogation of the sample; transmitting a microwave return signal from the microwave resonance cavity; and filtering radiation at the first frequency from the microwave return signal; wherein the steps of irradiating the microwave resonance cavity to excite the sample and irradiating the microwave resonance cavity to perform an interrogation of the sample are carried out at least partially concurrently.

13. The method of claim 12, wherein: the first frequency excites the microwave resonance cavity at a mode having a maximum electric field at a location of the sample; and the second frequency excites the microwave resonance cavity at a mode having a maximum magnetic field at the location of the sample.

14. The method of claim 12, wherein performing a microwave radiation absorption analysis comprises measuring an electron paramagnetic resonance spectrum.

15. The method of claim 12, further comprising performing microwave resonance spectroscopy of the sample.

16. The method of claim 15, wherein the microwave resonance spectroscopy is one of electron paramagnetic resonance spectroscopy and nuclear magnetic resonance spectroscopy.

17. The method of claim 12, further comprising determining a change in absorption of the radiation at the second frequency.

18. The method of claim 12, further comprising applying a rotating magnetic field to the sample.

19. The method of claim 12, wherein the sample comprises magnetic nanoparticles.

20. The method of 17, further comprising determining a rate of rotation of nanoparticles in the sample based on the determined change in absorption.

21. The method of claim 20, further comprising determining a presence of specific biological matter in the sample based on the rate of rotation of the nanoparticles.

22. A microwave resonance apparatus comprising: a microwave resonance cavity with a first resonance mode at an exciting microwave frequency and a second resonance mode at an interrogating microwave frequency; an interrogation unit comprising an interrogating microwave source and a microwave analysis unit; an exciting microwave source; and a waveguide with a cut-off frequency greater than the exciting microwave frequency and less than the interrogating microwave frequency, and configured to: input microwave radiation at the interrogating microwave frequency to the microwave resonance cavity; transmit a microwave return signal from the microwave resonance cavity to the microwave analysis unit; filter radiation at the exciting microwave frequency from the microwave return signal; and wherein the interrogating microwave source and the exciting microwave source are configured to excite the microwave resonance cavity at a first and second resonant mode of the microwave resonance cavity respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Non-limiting examples will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of a microwave resonance cavity.

(3) FIG. 2A is a plot of the magnetic field strength of a TE.sub.101 resonance mode.

(4) FIG. 2B is a plot of the electric field strength of a TE.sub.101 resonance mode.

(5) FIG. 2C is a plot of the magnetic field strength of a TE.sub.102 resonance mode.

(6) FIG. 2D is a plot of the electric field strength of a TE.sub.102 resonance mode.

(7) FIG. 3 is schematic diagram of a microwave resonance apparatus.

(8) FIG. 4 is a flowchart of an example method of performing microwave analysis in a microwave resonance cavity.

DETAILED DESCRIPTION

(9) FIG. 1 shows a microwave resonance cavity 100. The microwave resonance cavity comprises a first input port 102 for inputting microwave radiation at a first frequency. The microwave resonance cavity also comprises a second input port 104 for inputting radiation at a second frequency. The radiation at the first frequency is to excite a sample 106 in the cavity 100 and the radiation at the second frequency is for analysis of the sample 106. The cavity 100 has dimensions x.sub.m, x.sub.n, x.sub.l such that it resonates at both the first and second frequency.

(10) In this example, the cavity 100 is configured such that it has a sample receiving region 106, for example, as shown in the Figure in dotted outline as it does not comprise part of the cavity 100, a glass walled tube, such a NMR tube. The sample receiving region 106 is shown here as a vertically orientated cylinder parallel to the x.sub.m dimension, however in other examples the region may be orientated in other directions, for example parallel to the x.sub.n dimension, or indeed there may be more than one sample receiving region, each having a different orientation.

(11) As shown in FIG. 1, the cavity 100 has dimensions x.sub.m, x.sub.n, x.sub.l. In this example, the cavity 100 is dimensioned such that the microwave radiation at the first frequency results in a high electric field in the sample receiving region and radiation at the second frequency results in a high magnetic field in the sample receiving region. The shape and dimensions of the cavity 100 will determine the frequency of its resonant modes.

(12) In this example, the dimensions of the cavity 100 are selected such that the first frequency corresponds to a TE.sub.101 resonant mode of the cavity 100 and the second frequency corresponds to a TE.sub.102 resonant mode of the cavity 100, as described in more detail with respect to FIGS. 2A-D.

(13) FIG. 1 shows the first input port 102 and second input port 104. The input ports 102, 104 may be any type of input port. For example they may be waveguide input ports or may be coaxial input ports, and/or may receive waveguide or coaxial cables. The input ports may be different types of input ports, for example the first input port may be a coaxial input port and the second input port may be a waveguide input port. The location of the ports 102, 104 may be selected to provide energy transfer at a particular resonant mode of the chamber.

(14) The cavity is constructed from any suitable material, for example a highly conducting metal, which may be selected to have a low concentration of magnetic impurities, such as aluminium, copper or silver. The cavity may be constructed primarily from a first material and coated with a second material. The second material may be a metal such as silver, which is a desirable material to use due to its physical properties, however its high cost means it may be impractical to use in the construction of the whole chamber. Alternatively the whole chamber can be constructed from a material such as aluminium which provides adequate performance, lower cost and is relatively easy to machine.

(15) The cavity 100 shown in FIG. 1 is a rectangular cavity, but the cavity may be other shapes, for example the cavity may have a square or circular cross section. In some examples the cavity has an elliptical cross section, as described in more detail with reference to FIG. 3. Although not shown, the cavity 100 may have an opening, or comprise a lid, or be fabricated as separable parts or the like, to allow a sample to be inserted and removed.

(16) The resonant frequencies of a TE.sub.mnl mode of a rectangular microwave cavity are determined by the following equation:

(17) $f_{\mathrm{mnl}}=\frac{c}{2\sqrt{{\mu }_r{ϵ}_r}}\sqrt{{\left(\frac{m}{x_m}\right)}^2+{\left(\frac{n}{x_n}\right)}^2+{\left(\frac{l}{x_l}\right)}^2}$
where f.sub.mnl is the frequency of the TE.sub.mnl mode, μ.sub.r and ϵ.sub.r are the relative permeability and permittivity of the cavity respectively, m, n, and l are integers, and x.sub.m, x.sub.n and x.sub.l are the dimensions of the cavity.

(18) In some examples, radiation used in microwave resonance spectroscopy may be in a frequency band of around 8-12 GHz (so-called ‘X-band’ radiation). The dimensions of the cavity may be selected such that a resonant mode results in a maximum in the magnetic field at the centre of the cavity where a sample under test is located. For example, this may be a so-called TE.sub.102 resonance mode of a rectangular cavity.

(19) This is a desirable configuration as, as noted above, it is the magnetic component of the radiation which is used to perform the electron paramagnetic resonance measurements.

(20) FIGS. 2A-D shows plots of the magnitude of the magnetic and electric fields in a rectangular cavity such as the rectangular cavity 100 shown in FIG. 1 in the TE.sub.101 and TE.sub.102 resonant modes. In these figures lighter shades correspond to higher field strength and darker shades correspond to lower field strength.

(21) FIG. 2A shows the magnetic field magnitude in the TE.sub.101 resonant mode and FIG. 2B shows the electric field magnitude in the TE.sub.101 resonant mode. Microwave radiation at the first frequency may be used to excite the TE.sub.101 resonant mode. As can be seen from FIGS. 2A and 2B the magnetic field is at a minimum and the electric field is at a maximum at the centre of the cavity in this mode. The electric field component of the radiation may be used to excite a sample 106, therefore if a sample 106 is located at the centre of the cavity and the cavity is irradiated with radiation at the first frequency the sample 106 will be excited. It can be seen in FIG. 2B the TE.sub.101 resonance provides a relatively uniform electric field at the centre of the cavity. This feature results in a mode particularly well suited for exciting samples as it will provide uniform excitation throughout the sample.

(22) FIG. 2C shows the magnetic field magnitude in the TE.sub.102 resonant mode and FIG. 2D shows the electric field magnitude in the TE.sub.102 resonant mode. Microwave radiation at the second frequency may be used to excite the TE.sub.102 resonant mode. As can be seen from FIGS. 2C and 2D the magnetic field is at a maximum (FIG. 2C) and the electric field is at a minimum (FIG. 2D) at the centre of the cavity in this mode. The magnetic field component of the radiation may be used for analysis, or interrogation, of the sample 106. Therefore if the sample 106 is located at the centre of the cavity and the cavity is excited with radiation at the second frequency, the sample 106 may be interrogated using any of a number of different microwave absorption analysis techniques. For example the sample may be interrogated using a microwave resonance spectroscopy technique such as electron paramagnetic resonance spectroscopy or nuclear magnetic resonance spectroscopy. In other examples the sample may be interrogated using a magnetic nanoparticle polarization analysis technique.

(23) The input ports 102, 104 may be located to enhance transfer of radiation to the cavity. They may be located to be proximal to a maximum of the magnetic field in a particular mode. For example the first input port may be located proximal to a maximum in the magnetic field at the resonant mode at the first frequency 202 and the second input port may be located proximal to a maximum in the magnetic field at the resonant mode at the second frequency 204. As used herein, the term ‘proximal to’ a maximum of magnetic field is used to a position along an axis. For example a maximum in the magnetic field may be located along a central axis of the cavity, however a location proximal to the maximum in magnetic field may be a location on a side of the cavity sufficiently close, for example substantially aligned with, the maximum so as to allow effective coupling of the fields.

(24) Considering some example dimensions for the cavity 100, this may have dimensions of approximately x.sub.m=5.5 cm and x.sub.l=2.9 cm (x.sub.n is unconstrained for the sake of determining TE.sub.101 and TE.sub.102), in which case, the TE.sub.101 mode may be excited by radiation at 7.3 GHz and TE.sub.102 mode may be excited by radiation at 9.6 GHz. A cavity with dimensions x.sub.m=35.6 mm, x.sub.l=33.6 mm may have TE.sub.101 and TE.sub.102 resonant modes at approximately f.sub.101=6.2 GHz and f.sub.102=9.6 GHz. The cavity may have dimension x.sub.n≈10 mm, however the value of this dimension will not affect the resonant frequencies of interest.

(25) This lower frequency mode could not generally be excited using an example X-band radiation source (for example having an operational range of 9.1-9.9 GHz within the X-band range of around 8-12 GHz), the low frequency mode is never encountered. According to the present invention, this mode may be excited by specifically providing microwave radiation at a first frequency of 7.3 GHz, which results in heating of the sample. A filter may be used to filter this radiation such that it is prevented from being transmitted to analysis apparatus.

(26) In other examples, different resonant modes may be excited, and/or the cavity may have dimensions to result in modes having different frequencies.

(27) FIG. 3 shows a microwave resonance apparatus 300. The apparatus comprises a microwave cavity 302 with a first resonance mode at an exciting microwave frequency and a second resonance mode at an interrogation microwave frequency. The apparatus further comprises an interrogation unit 308, which comprises an interrogating microwave source 310. The apparatus also comprises an exciting microwave source 312. The interrogating microwave source and the exciting microwave source 312 are configured to excite the cavity at a first and second resonant mode of the cavity. In some examples the interrogating microwave source 310 and the exciting microwave source 312 are independent units. In other examples exciting and interrogating microwaves may be provided a single broadband source.

(28) In this example, the interrogation unit 308 further comprises a microwave analysis unit 314. The second input 304 may be a waveguide port for connecting the cavity to the microwave analysis unit 314. The microwave analysis unit 314 is configured to measure absorption of microwave radiation at the second frequency.

(29) Microwave radiation at the second frequency is input via a waveguide 316. Microwave radiation may also be output from the excited sample through the second input port through the waveguide 316 to the microwave analysis unit 314.

(30) As described previously, waveguides may be associated with a cut-off frequency and will not transmit significant amounts of radiation with a frequency below the cut-off frequency. In this example, the dimensions of the cavity 302 and in this example, the waveguide 316 is selected such that the cut-off frequency of the waveguide 316 is greater than the first frequency and less than the second frequency.

(31) This means that, if microwave radiation is input to the cavity at both the first and second frequency partially concurrently, the waveguide will act as a filter, filtering out the first frequency. For example a sample may be excited by inputting radiation at the first frequency and interrogated using radiation at the second frequency at least partially concurrently without radiation at the first frequency being transmitted by the waveguide 316 to the microwave analysis unit 314. The waveguide 316 passes radiation at the second frequency but acts like a high pass filter and limits radiation at the first frequency from interfering with the interrogation of the sample.

(32) The cut-off frequency, f.sub.c, of a hollow rectangular waveguide operating in the n, m mode can be determined by the following equation:

(33) $f_c=\frac{c}{2}\sqrt{{\left(\frac{n}{x_n}\right)}^2+{\left(\frac{m}{x_m}\right)}^2}$
where x.sub.n and x.sub.m are the cross sectional dimensions of the waveguide and c is the speed of light.

(34) For example a rectangular waveguide designed to transmit X-band microwaves may have cross sectional dimensions 22.86 mm by 10.16 mm and have a recommended frequency band of operation of 8.20 GHz to 12.40 GHz, with a cut-off frequency of 6.56 GHz for the lowest order mode, TE.sub.10.

(35) In other examples, a separate filter may be provided. However, by arranging the modes to have a suitable separation and selecting a waveguide 316 with a cut off frequency between the first and second frequency, the waveguide 316 itself can act as a filter and no additional apparatus is required.

(36) In some examples the second input port 306 is a coaxial input port configured to input exciting microwaves from the exciting microwave source to the microwave cavity. A coaxial input port can be easily adjusted to allow for the coupling to be adjusted to reduce reflected power.

(37) In the example of FIG. 3, the cavity 302 is an elliptic cylinder. The resonant modes of an elliptic cylinder cavity are determined by the dimensions of the cavity. The difference in frequency between resonant modes may be varied by varying eccentricity of the ellipse. An elliptic cylinder of comparable dimensions to a rectangular cylinder can provide a greater difference in frequency between resonant modes suitable for exciting and interrogating a sample. For example an elliptic cylinder cavity with dimensions of 39.4 mm by 35.6 mm will have suitable resonances at frequencies 6.1 GHz and 9.6 GHz. The cavity may have a third dimension of around, for example, 10 mm, however this will not affect the resonant frequencies of interest.

(38) Considering for example the rectangular cavity described previously above with TE.sub.101 and TE.sub.102 resonant modes at 7.3 GHz and 9.6 GHz respectively, if a standard X-band waveguide was used, for example as described above with a cut-off frequency of 6.56 GHz, radiation corresponding to both resonant modes would be transmitted by the waveguide. Therefore exciting radiation from the TE.sub.101 mode at 7.3 GHz would be transmitted to the measurement apparatus and may interfere with the microwave analysis measurements. If the shape of the cavity was instead an ellipse, the cavity may have a resonant mode suitable for microwave analysis within the X-band, for example at 9.6 GHz, with a different resonant mode suitable for exciting at a wavelength below the cut-off frequency of the waveguide. In the other example rectangular cavity described above with TE.sub.101 and TE.sub.102 resonant modes at 6.1 GHz and 9.6 GHz respectively, the frequency of the TE.sub.102 resonant mode is above the cut-off frequency and would therefore be transmitted and the frequency of the TE.sub.101 resonant mode is below the cut-off frequency and would therefore not be transmitted by the waveguide.

(39) In this example, the microwave resonance apparatus 300 further comprises a magnetic field source 318. The magnetic field source 318 may be a permanent magnet or may be an electromagnet. In some examples coils of conducting wire are used to provide a magnetic field. An electromagnet may provide a magnetic field that varies with time. For example the current supplied to electromagnet coils may be varied to provide a magnetic field that varies in strength. In other examples multiple electromagnets may be provided which can provide a field that varies in both strength and/or direction by coordinating time varying voltages supplied to the different coils.

(40) The magnetic field source 318 may be configured for magnetic resonance spectroscopy. For example, for electron paramagnetic resonance spectroscopy, there may be two electromagnet coils to provide a uniform magnetic field in the cavity 302 which varies in strength with time. In other examples the magnetic field source 318 may provide a magnetic field for magnetic nanoparticle polarization analysis. For such an analysis a rotating magnetic field may be provided by four coils arranged around the cavity, each with a time varying voltage applied.

(41) FIG. 3 shows a cavity which comprises two separable parts which are separable in a plane perpendicular to the ends of the elliptic cylinder (a ‘clam shell’ configuration). Although shown in FIG. 3 for an elliptic cylinder cavity, any shape cavity may be constructed to be separable in a similar manner, such that the cavity is separable into two parts. This construction may simplify construction and may also allow for easier access to a sample located within the cavity. Access to a sample may be useful for physical access to a sample or for interrogating a sample for example by using other non-microwave forms of radiation, for example visible light. Such interrogations may take place during irradiation of the cavity at the first frequency to excite the sample and/or during irradiation of the cavity at the second frequency to interrogate the sample.

(42) FIG. 4 shows a method of performing microwave analysis in a microwave resonance cavity. In block 402 the microwave cavity is irradiated at a first frequency to excite the sample. In block 404 the cavity is irradiated at a second frequency to perform an interrogation a sample. The steps of irradiating the microwave resonance cavity to excite the sample and irradiating the microwave resonance cavity to perform an interrogation of the sample are carried out at least partially concurrently.

(43) The microwave radiation at the first frequency may excite the cavity at mode having a maximum electric field at the location of the sample and the microwave radiation at the second frequency may excite the cavity at a mode having a maximum magnetic field at the location of the sample. The sample may be located in the centre of the cavity. The sample may be prepared in a glass tube such as an NMR tube or similar and inserted in to the cavity. The cavity may have an opening to insert a container containing the sample. In some examples the cavity may be separable into parts to allow access to the sample.

(44) Microwave radiation at the first frequency may be input to the cavity through a first input port proximal to a maximum of the magnetic field at a resonant mode at the first frequency and microwave radiation may be input to the cavity through a second input port proximal to a maximum of the magnetic field at a resonant mode at the second frequency.

(45) In the examples described above the analysis performed may be microwave resonance spectroscopy. The microwave resonance spectroscopy may for example comprise electron paramagnetic resonance spectroscopy or it may be nuclear magnetic resonance spectroscopy.

(46) In other examples the method may comprise determining a change in polarization of the radiation of at the second frequency. This may be achieved by applying a polarizing filter to the radiation output from the cavity. The polarization state of the output radiation may vary with time and measurements of how the polarization changes may be used to determine properties of the sample.

(47) In some examples the sample may be exposed to a rotating magnetic field. Such a field may induce a periodic variation in the polarization state of the radiation. If the sample comprises magnetic nanoparticles this effect may be enhanced. In a liquid comprising magnetic nanoparticles, rotation of the particles can be induced by applying a rotating magnetic field. The magnetic nanoparticles tend to align with an applied field and can act as a polarizer and so alter the polarization state of the radiation.

(48) By measuring the change in absorption the rate of rotation of nanoparticles can be determined. The rate of rotation of the nanoparticles can provide information about the particles or the liquid in which they are contained. For example the dimensions of the particles, the material which they are made from or the viscosity of the liquid they are contained in may affect their movement when they are exposed to a time varying magnetic field.

(49) Nanoparticles such as nano-rods may be configured or selected such that they bind with certain chemical or biological molecules such as amino acid sequences, nucleotides, nucleosides, DNA or RNA. When they bind to molecules it may affect their motion. For example nanoparticles which bind to large molecules may rotate more slowly. Therefore by measuring the rate of rotation of nanoparticles the presence of specific biological or chemical matter may be detected. Therefore such techniques may be useful in diagnostics or testing chemical or pharmaceutical samples.

(50) To measure biological or chemical compounds it may be preferable to process the sample by exciting the sample, which may comprise heating the sample. For example biological samples may be heated to break down cells into their constituent components. Therefore the sample may be heated in the cavity by exciting the cavity at the first frequency to prepare the sample. The cavity may also be excited at the second frequency for analysis of the chemical or biological matter in the sample. In other examples the excitation may be non-thermal, for example an electroporation excitation, wherein genetic information is exchanged between the sample and its surroundings, triggered by microwaves of the excitation mode. In some examples, after excitation, there may be an incubation period (in some cases, of a few minutes) before analysis of the nanoparticles is carried out.

(51) More generally, while in some methods of use of the apparatus described herein, excitation and analysis may be carried out at least partially concurrently, in other examples, these steps may be carried out at different times, for example with a delay between excitation and analysis (albeit that, in some examples, the sample may remain in the cavity during any incubation or delay between the steps).

(52) While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.

(53) The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and features of the dependent claims may be combined in any practical combination.