Microwave resonator with distributed bragg reflector (=DBR)

Abstract

An NMR (nuclear magnetic resonance) probe head has a microwave resonator with at least two elements which are reflective in the microwave range, at least one of which is focusing. The reflective elements at least partly delimit a resonance volume of the microwave resonator. At least one of the reflective elements is a DBR (Distributed Bragg Reflector), and the NMR probe head has at least one NMR coil integrated into the DBR. The NMR detection coil can thereby be positioned particularly near to the sample and the distortions of the static field by resonator components are reduced, such that the detection sensitivity and the spectral resolution of the experiment are significantly improved.

Claims

1. An NMR (nuclear magnetic resonance) probe head, the probe head comprising: a microwave resonator having at least two reflective elements which are reflective in the microwave range, wherein at least one of said two reflective elements is focusing, said at least two reflective elements at least partly delimiting a resonance volume of the microwave resonator, wherein at least one of said at least two reflective elements is a DBR (distributed Bragg reflector); and at least one NMR coil which is integrated in said DBR.

2. The NMR probe head of claim 1, wherein at least two reflective elements are focusing.

3. The NMR probe head of claim 1, wherein all reflective elements are focusing.

4. The NMR probe head of claim 1, wherein one reflective element is focusing and one is planar.

5. The NMR probe head of claim 1, wherein one reflective element is focusing and one is defocusing.

6. The NMR probe head of claim 1, wherein a surface of at least one of said reflective elements has a spherical or elliptical shape.

7. The NMR probe head of claim 1, further comprising a coupling element for microwave radiation disposed on a side of said DBR facing away from said resonance volume of said microwave resonator.

8. The NMR probe head of claim 1, wherein a sample position of the probe head is disposed in said resonance volume.

9. The NMR probe head of claim 1, wherein a sample position of the probe head is disposed at a minimum of an electric field of said microwave resonator during measuring operation.

10. The NMR probe head of claim 8, wherein the NMR probe head comprises at least one NMR coil which induces a magnetic RF (radio frequency) field at the sample position.

11. The NMR probe head of claim 10, wherein the NMR probe head further comprises an element for supplying a sample to the sample position.

12. The NMR probe head of claim 1, wherein said at least one NMR coil which is integrated in said DBR is disposed on a surface of said DBR.

13. The NMR probe head of claim 12, wherein said DBR has a plurality of layers and said NMR coil is designed as an RF coil, wherein said NMR coil is disposed on a first layer of said DBR and is transparent to microwave radiation.

14. The NMR probe head of claim 13, wherein said NMR coil is designed as a grid.

15. The NMR probe head of claim 1, wherein said at least one NMR coil is designed as an RF coil and said DBR has a plurality of layers, wherein said NMR coil is disposed on a rear side, facing away from an other reflective element, of a first layer of said DBR.

16. The NMR probe head of claim 1, wherein said DBR comprises a plurality of dielectric layers and channels are provided in a front side, facing an other reflective element, of at least a first layer of said DBR, said channels enabling exact spatial positioning of an NMR sample.

17. The NMR probe head of claim 16, wherein channels are provided in a plurality of layers of said DBR for spatially positioning an NMR sample.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic view of an inventive microwave resonator designed as a Fabry-Prot resonator, in which a planar metallic mirror is replaced by a DBR;

(2) FIG. 2 shows the simulation of an ideal FP resonator with a metallic mirror in comparison with a simulated reflection curve of a model with DBR instead of the metallic mirror;

(3) FIG. 3 shows a schematic view of a Fabry-Prot resonator according to prior art consisting of a spherical and a planar mirror;

(4) FIG. 4 shows a schematic view of a Fabry-Prot resonator consisting of a spherical mirror and a DBR in which sample channels and an RF coil are integrated;

(5) FIG. 5 shows a schematic view of a DBR in which a sample channel and an RF coil are integrated on the side facing the reflective element; and

(6) FIG. 6 shows a schematic view of a DBR in which a sample channel and an RF coil are integrated between the layers of the DBR.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) The main application of the invention is in the field of electron resonance spectroscopy (ESR), nuclear magnetic resonance spectroscopy (NMR) and in the field of dynamic nuclear polarization. The invention comprises a sub-THz resonator with a so-called distributed Bragg reflector (DBR).

(8) The invention concerns a new system for recording microwave reflection spectra.

(9) FIG. 1 shows an inventive microwave resonator in which the planar metallic mirror in an FP resonator was replaced by a DBR:

(10) aspherical mirror body,

(11) bspherical mirror with radius of curvature R.sub.1,

(12) cmicrowave supply,

(13) diris coupling,

(14) eresonant volume and sample location,

(15) fdistributed Bragg reflector.

(16) In order to clearly illustrate the inventive principle, a comparative study with two geometries was performed:

(17) A first model consisted of an ideal FP resonator with a metallic mirror, a second model consisted of a DBR.

(18) The simulated reflection curve (s.sub.11 parameter) is illustrated in FIG. 2. The same spherical mirror was used in both cases.

(19) FIG. 2 shows in detail:

(20) (Continuous line) a simulated s11 parameter for an FP resonator with R.sub.1=10 mm, =2.95 mm and a TEM.sub.005 resonance at 263 GHz;

(21) (Dashed line) the same simulation with a 5-layer DBR (.sub.r1=3.75, .sub.r2=1).

(22) In both cases, the TEM.sub.005 resonance occurs at the same frequency. The coupling iris was not changed in the simulation, which indicates a similar resonance Q-factor. The invention is therefore particularly suited as a resonator for DNP experiments.

(23) The present invention comprises i.a. the following aspects:

(24) 1. A Fabry-Prot resonator with a metallic and a distributed Bragg reflector for magnetic resonance experiments.

(25) 2. A DBR consisting of a stack of planar, curved, non-metallic plates with a refractive index of 1.

(26) 3. An optimized DBR with internal channels in order to position an NMR/EPR or NMR/DNP sample.

(27) 4. A DBR with internal or external coil which were optimized for optimum detection of the NMR signal or for generating defined field gradients.

(28) 5. A DBR which is designed such that distortions of the static field in the sample volume are minimized, either through selection of suitable materials, through a special geometry, or through a combination of both.

(29) 6. A DBR which can be used as an adjustable coupler in order to couple-in a microwave/THz beam into the FP resonator.

(30) FIG. 3 shows a Fabry-Prot resonator according to prior art consisting of a combination of a spherical and a planar mirror with a high reflectivity. A flat liquid EPR/DNP sample can be placed directly on the surface of the planar mirror where the electric microwave field is small.

(31) FIG. 4 shows, by way of example, an inventive Fabry-Prot resonator consisting of a spherical mirror (thick curved line) and a DBR consisting of layers with alternating refractive indices n.sub.1 and n.sub.2 (hatched and white rectangles). The microwave beam is illustrated by enveloping lines at the same field amplitude (thin curved lines). The sample channels (A) and (B) are introduced into the DBR. The conductor paths of the RF coil (C) are located on the side facing the mirror.

(32) In contrast to the view of FIG. 4, FIG. 5 shows a DBR with a sample channel (A) and an RF coil (C), the conductor paths of which are located within the microwave beam (not shown). Through suitable selection of conductor width and separation, the majority of the microwave power can also be transmitted through the RF coil in this case.

(33) In contrast to the view of FIG. 5, FIG. 6 shows a DBR into which a sample channel (A) was introduced directly on the side facing the spherical mirror. The RF coil (C) is introduced between the layers of the DBR.

ABBREVIATIONS

(34) ESR electron spin resonance

(35) (N)MR (nuclear) magnetic resonance

(36) DNP dynamic nuclear polarization

(37) MAS magic angle spinning

(38) DBR distributed Bragg reflector

(39) FP Fabry-Prot resonator

(40) PBS photonic band-gap structure

(41) hrNMR high resolution NMR

(42) RF in NMR spectroscopy the electromagnetic frequency range from 1 MHz to 1000 MHz

(43) Microwave electromagnetic frequency range between 1-300 GHz

(44) THz tera hertz, electromagnetic frequency range of 0.3-3 THz

EXPLANATION OF TERMS

(45) TABLE-US-00001 Fill factor the ratio between field-filled space and sample volume; weighted with the field amplitude Q-factor ratio between the EM energy stored in the resonator and the energy which is converted during a frequency period through (material) losses or radiation

LIST OF REFERENCES

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