WO2015087257A2

WO2015087257A2 - Optical storage medium, oam-light generating device comprising an optical storage medium, hyperpolarization device comprising an oam-light generating device and magnetic resonance system comprising a hyperpolarization device

Info

Publication number: WO2015087257A2
Application number: PCT/IB2014/066762
Authority: WO
Inventors: Hans-Aloys Wischmann; Michael Harald Kuhn; Julius Simon Cohen; Khalid Shahzad; Lucian Remus Albu; Ramon Quido ERKAMP
Original assignee: Koninklijke Philips N.V.; Philips Deutschland Gmbh
Priority date: 2013-12-10
Filing date: 2014-12-10
Publication date: 2015-06-18
Also published as: WO2015087257A3

Abstract

The present invention relates to an OAM-light generating device for generating light endowed with an orbital angular momentum (OAM) comprising a light source (12) configured to emit at least one light beam (14, 50, 52) and an OAM generating arrangement (16) configured to generate OAM in said at least one light beam (14, 50, 52), said OAM generating arrangement (16) comprising at least one OAM generating element (18) configured to individually receive and diffract said at least one light beam (14, 50, 52) and to impart OAM with an OAM-charge number to said at least one light beam (14, 50, 52), wherein said OAM generating arrangement (16) is configured as an optical storage medium (22).

Description

Optical storage medium, OAM-light generating device comprising an optical storage medium, hyperpolarization device comprising an OAM-light generating device and magnetic resonance system comprising a hyperpolarization device

FIELD OF THE INVENTION

The present invention relates to the field of photonics. In particular, it relates to light carrying orbital angular momentum (OAM). It finds applications in free space optical communication, quantum information for possible future quantum cryptography or quantum computation. It also finds particular applications in magnetic resonance imaging (MRI).

BACKGROUND OF THE INVENTION

Orbital angular momentum (OAM) is an intrinsic property of light which possesses an azimuthal phase angle, independent of the choice of axis about which the OAM is defined. This property was verified in 1992 by Allen et al., "Optical angular momentum", ISBN 0 7503 0901 6. Both theoretical and experimental studies were carried out to explore ways of creating light endowed with OAM and its applications. In the field of photonics, light endowed with OAM (or OAM-light) can be used to interact with matter in order to achieve optical cooling of atomic systems to extremely low temperatures in oder to investigate a special state of the atomic system called Bose-Einstein condensates (BEC). OAM-light can also be used to study the entanglement of photons for exploring their properties for quantum information and/or quantum computation. OAM-light can further play a role in optical communication, in particular in OAM multiplexing which promises very significant improvement in bandwidth when used in combination with other existing modulation and multiplexing schemes.

In the field of medical imaging, OAM-light can be applied to hyperpolarize an object, for instance, a human body tissue, a contrast agent or compound, or a lumen, by irradiating the object with an OAM-light beam. In this way, the nuclear spins of the irradiated regions of the object become strongly aligned and the object is hyperpolarized. Such a hyperpolarized object can subsequently be used for NMR spectroscopy to generate MRI images.

There are a number of well-established techniques for generating light endowed with OAM. Using a spiral phase plate, a light beam can acquire OAM by crossing the varying thickness of the spiral phase plate. More conveniently, light can also be endowed with OAM based on diffraction at a phase hologram, in particular a phase hologram comprising a pitchfork structure. Another method for generating OAM is based on the coupling between spin angular momentum and OAM that may occur in a medium which is both anisotropic and inhomogeneous. Last but not least, OAM-light can also be generated by converting a Hermite-Gaussian mode into a Laguerre-Gaussian mode of a light beam, for instance by using an astigmatic system with two well-aligned cylindrical lenses.

WO 2009/090610 Al discloses a device for analyzing a sample consisting of molecules, the analysis being based upon nuclear magnetic resonance spectroscopy, wherein the device comprises means for introducing OAM into light. The means comprises a spatial light modulator (SLM) which produces a computer generated phase hologram designed to change the Gaussian incident beam to a Laguerre-Gaussian (LG) beam carrying both OAM and spin angular momentum (SAM). In an embodiment, the SLM is configured as a liquid crystal on silicone (LCoS) panel. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an OAM-light generating device, a hyperpolarization device comprising such an OAM-light generating device and a magnetic resonance system comprising such a hyperpolarization device that enable an easy and efficient generating of light endowed with OAM.

In a first aspect of the present invention, an OAM-light generating device for generating light endowed with an orbital angular momentum (OAM) is provided comprising a light source configured to emit at least one light beam and an OAM generating arrangement configured to impart OAM to said at least one light beam, said OAM generating arrangement comprising at least one OAM generating element configured to individually receive and diffract said at least one light beam and to impart OAM with an OAM-charge number to said at least one light beam, wherein said OAM generating arrangement is configured as an optical storage medium.

In a further aspect of the present invention, an optical storage medium is provided comprising at least one OAM generating element configured to individually receive and diffract at least one light beam of a light source and to impart orbital angular momentum (OAM) with an OAM-charge number to said at least one light beam.

In a further aspect of the present invention, a hyperpolarization device for creating nuclear hyperpolarization in a compound is provided comprising an OAM-light generating device according to the first aspect for generating light endowed with an orbital angular momentum (OAM) and at least one optical arrangement configured to direct said light to at least partially interact with said compound, thereby leading to nuclear

hyperpolarization of said compound.

In yet a further aspect of the present invention a magnetic resonance system is provided, comprising a hyperpolarization device according to the preceding aspect and an RF system configured to induce magnetic resonance and detect resonance signals.

In yet a further aspect of the present invention, a method for generating light endowed with orbital angular momentum (OAM) is provided comprising emitting at least one light beam and generating OAM in said at least one light beam, said generating comprising individually receiving and diffracting said at least one light beam and imparting OAM with an OAM-charge number to said at least one light beam, said imparting being carried out with an OAM generating arrangement configured as an optical storage medium.

In yet another aspect of the present invention there is provided a computer program comprising program code means for causing a computer to carry out the steps of the method for generating light endowed with orbital angular momentum according to the preceding aspect when said computer program is carried out on said computer.

Preferable embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed optical storage medium, hyperpolarization device, magnetic resonance system, method and computer program have similar and/or identical preferable embodiments as the claimed OAM-light generating device and as defined in the dependent claims.

One advantage of the present invention is that light endowed with OAM can be generated by exploiting the existing hardware and software technologies of optical storage media, enabling inexpensive manufacturing of OAM-light generating devices.

Another advantage of the invention is that the inherent high resolution of existing optical storage media can be made use of, which allows a large number of holographic images to be arranged on an optical storage medium.

A further advantage of the invention lies in the possibility of generating OAM- light representing different values of OAM-charged numbers using one single OAM-light generating device.

Still further advantages of the present invention will be appreciated by those of ordinary skill in the art upon reading and understanding the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the accompanying drawings:

Fig. 1 schematically shows a side view of a first embodiment of an OAM-light generating device according to the present invention;

Fig. 2 schematically shows a top view of an embodiment of an optical storage medium according to the present invention;

Fig. 3 a top view of a phase hologram arranged on the optical storage medium in Fig. 2;

Fig. 4 schematically shows a side view of a second embodiment of an OAM- light generating device according to the present invention;

Fig. 5 schematically shows a side view of a third embodiment of an OAM- light generating device according to the present invention;

Fig. 6 shows a schematic representation of an embodiment of a hyperpolarization device according to the present invention;

Fig. 7 shows a schematic representation of an embodiment of a magnetic resonance system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In quantum mechanics, the spin angular momentum (SAM) and the orbital angular momentum (OAM) are intrinsic properties of quantum particles such as electrons, protons and neutrons as well as particles based on these, such as atoms and molecules. In an atomic or a molecular system consisting of such quantum particles, the SAM and the OAM of different particles can interact with each other leading to coupling effects which are fundamental for the atomic or molecular structures and the physical and chemical properties of the atomic or molecular system. During interactions between different atomic and/or molecular systems, both the SAM and the OAM can be transferred from one to the other atomic and/or molecular system.

Light is also known to possess these two intrinsic properties. Circularly polarized light has an SAM associated with its circular polarization. For a single photon, which is the quantum particle of electromagnetic waves, the SAM has a value of ± fl (fl being Planck's constant) depending on the direction of the circular polarization. For clockwise circular polarization, the SAM takes the "+" sign, while for anticlockwise circular polarization, the SAM takes the "-" sign. Besides SAM, circularly polarized light can also possess OAM. To the contrary of SAM, the OAM of circularly polarized light is independent of the sign of its circular polarization. OAM arises from helical phase fronts of a light beam, whose Poynting vector, which is the vectorial (cross) product of the electric field vector and the magnetic field vector of the light beam, is not parallel to the light beam's propagation axis. With a fixed radius starting from the light beam's propagation axis, the Poynting vector of the light beam follows a spiral trajectory rotating around the propagation axis. Such a light beam is also said to contain an optical vortex. The mode function of the light beam possesses an azimuthal phase dependence which is proportional to εχρ(-ί φ), wherein φ is the azimuthal phase angle and I is any integer value, which can be zero, any positive or negative integer. The value of the OAM is the product of I and the Planck's constant ft. In lieu of the + and the - sign for SAM, which are viewed quantum mechanically as the SAM charge number, the integer I is viewed as the OAM charge number for an OAM with a value of ft normalized for each photon.

The most common form of a helically phased light beam is the so-called

Laguerre-Gaussian (LG) mode. In general, a light beam, for example generated by a laser, gradually expands as it propagates. The magnitude and the phase of the electric field vector at different positions in a cross section of the light beam are described by a mode function. For light beams without helical phase fronts, this mode function is a product of a Hermitian polynomial and a Gaussian function. A light beam with such a mode function is hence called a Hermite-Gaussian (HG) mode. To the contrary, cylindrical LG modes have an explicit phase factor εχρ(-ί φ), making them a natural choice for describing light beams carrying OAM.

There exist a number of well-established techniques for generating light endowed with OAM. Besides the afore-mentioned method of using a spiral phase plate,

OAM can also be generated by coupling SAM with OAM. In particular, the so-called q-plate enables generating OAM wherein the OAM charge number is controlled by the sign of the circular polarization of the associated light beam. Such a q-plate can be realized using liquid crystals, polymers or sub-wavelength gratings. Light can also be endowed with OAM by converting an HG mode into an LG mode. This can be done by using an astigmatic system with two well-aligned cylindrical lenses placed at a specific distance in order to introduce a well-defined relative phase difference between horizontal and vertical light beams in an HG mode. Such a method exploits the analogy to the conversion of linearly polarized light into circularly polarized light by using a quarter-wave plate. Although this conversion process is known for its efficiency, the creating of each LG mode requires the conversion of a particular initial HG mode. This specific requirement limits the range of LG modes that can be produced in this way. For this reason, the most convenient way of generating light endowed with OAM is by using an OAM generating element, such as a phase hologram with a pitchfork structure, based on the property of diffraction. As described in the afore-mentioned prior art document, an incident light beam is diffracted by a phase hologram containing a pitchfork structure. The diffracted light beam carries an OAM with an OAM charge number which depends on the details of the phase hologram. Such OAM generating elements containing a pitchfork structure can be fabricated using the deposition of metallic pitchforks on a transparent substrate such as borosilicate glass. However, this technique requires relatively expensive processing procedures. Besides, assessing different OAM-charge numbers on such a substrate requires rather accurate scanning and positioning systems that are in general very expensive.

Furthermore, the accuracy of this technique is usually limited by the accuracy of the computer-controlled scanning on a micron scale.

Alternatively, the OAM generating elements can also be provided by numerical computations. Such OAM generating elements can be used to generate light beams with any desired value of OAM-charge number. In addition, this method can take advantage of the high-quality spatial light modulators (SLMs) that have recently become available. The numerically calculated pitchfork structures can be displayed directly on an SLM, e.g. an

LCoS panel. However, the SLM typically suffers from flickers. When an SLM is addressed digitally, flicker noise at frequencies typically above 100 Hz can be observed. This can lead to electro-optical response of the SLM that is not constant over the frame of the OAM generating element. Subsequently, this can result in relatively large fluctuations in the reflected and/or transmitted amplitudes of the OAM-light.

At least some of these problems are solved by the present invention. With reference to Fig. 1, a first embodiment of an OAM-light generating device 10a for generating light endowed with OAM is shown. The OAM-light generating device 10a comprises a light source 12 configured to emit a light beam 14. Preferably, the light source 12 is configured as a white light source and the light beam 14 is a white light beam. Alternatively, the light source 12 can be configured, preferably as a laser, to emit a light beam from any part of the optical spectrum, such as visible light, ultraviolet, far and near infrared, etc. In this preferable embodiment, the OAM-light generating device 10a comprises further an optical arrangement 20, through which the light beam 14 propagates after being emitted by the light source 12. The light beam 14 can be a linearly polarized light beam. In this case, the optical arrangement 20 is configured to comprise one or more circular polarizers to convert the linear polarization of the light beam 14 into circular polarization. For this purpose, the optical arrangement 20 preferably comprises a quarter wave plate. Alternatively, the light beam 14 can also be a circularly polarized light beam when emitted by the light source 12. In this case, the optical arrangement 20 is configured to comprise one or more lenses and/or mirrors to guide the light beam 14 in a specific direction after traversing the optical arrangement 20. Further alternatively, the light beam 14 can also be an unpolarized light beam. In this case, the optical arrangement 20 is configured to comprise one or more linear polarizers to first linear-polarize the light beam 14, and one ore more circular polarizers to subsequently circular-polarize the light beam 14 after being linearly polarized. In this preferable embodiment of the OAM-light generating device 10a, the light beam 14 comprises an HG mode after traversing the optical arrangement 20. Preferably, the HG mode is realized as the TEM00 laser mode, while other HG modes are also possible. In general, the optical arrangement 20 can further comprise one or more beam shutters and/or one or more beam splitters and/or one or more further optical elements in order to achieve desired optical effects in the applications.

The OAM-light generating device 10a further comprises an OAM generating arrangement 16 configured to impart OAM to the light beam 14. The OAM generating arrangement 16 comprises at least one, preferably a plurality of OAM generating elements 18 configured to individually receive and diffract the light beam 14 and impart OAM with an OAM charge number to the light beam 14. In this way, the incident light beam 14 is transformed into at least one outgoing light beam 14' by diffraction at one of the OAM generating elements 18. In this preferable embodiment, the at least one outgoing light beam 14' comprises an LG mode with a helical phase front, thus carrying OAM with the OAM- charge number. In general, a plurality of outgoing light beams departing from one of the OAM generating elements 18 and corresponding to different diffraction orders can be created, from which those except the one corresponding to the zero^th diffraction order carry non-zero OAM. For reasons of clarity, only one outgoing light beam 14' corresponding to one of the diffraction orders with non-zero OAM is shown here.

The OAM generating arrangement 16 comprising more than one OAM generating element 18 can be realized when it is configured as an optical storage medium 22. Preferably, the optical storage medium 22 comprises a substrate 24 and an active layer 26 which is arranged adjacently to the substrate 24. In this preferable embodiment, the plurality of OAM generating elements 18 are arranged in the active layer 26. Besides significantly increasing the number of the OAM generating elements 18 arranged within one OAM-light generating device 10a, existing optical storage technology can be exploited, leading to easy fabrication of the OAM-light generating device 10a.

Fig. 2 shows schematically a preferable embodiment of the optical storage medium 22 in a top view. In this embodiment, the optical storage medium 22 is configured as an optical disk 28 having a center 30 about which the optical disk can rotate. The optical disk 28 can be realized in any existing form of optical storage media, including but not restricted to compact disk (CD), digital versatile disk (DVD), high density digital versatile disk (HD- DVD), Blu-ray disk (BD), minidisk (MD), magneto-optical disk (MOD), multiplexed optical data storage disk (MODS-disk), universal media disk (UMD), protein-coated disk (PCD), holographic versatile disk (HVD), etc. Such an optical disk 28 is generally configured such that the active layer 26 (Fig. 1) comprises a storage track 32, preferably comprising a plurality of spiral sections 33 which are arranged in a spiral form with increasing radius in anticlockwise direction in respect to the center 30. Alternatively, the radius of the spiral sections 33 can also increase in clockwise direction in respect to the center 30. Each of the plurality of spiral sections 33 comprises a plurality of first segments 35 and second segments 36 in an alternating arrangement, as is more clearly shown in the enlarged detail view in Fig. 2. The first segments 35 ("lands") are shown as white fields and the second segments 36 ("pits") are shown as shaded fields. The first segments 35 possess in general higher reflectivity levels for the light beam 14 than the second segments 36. This can be realized preferably by larger heights of the first segments 35 than the second segments 36 in a direction essentially perpendicular to the plane of the optical disk 28 facing the light source 12. This is, however, non-restricting since there exist also other methods, for instance thermal and/or optical and/or structural treatments, to create such first and second segments 35, 36. It is noted that the encircled region of the optical disk 28 corresponding to the enlarged detail view shows in general only the first segments 35 and the second segments 36 when corresponding data have been recorded to the optical disk 28 at this site. In the case shown here, the encircled region is outside of the OAM generating elements 18 for the sake of easier presentation. However, it is to be understood, that segments 35 and 36 are at the sites of the OAM generating elements 18 as recorded portions of the optical disk 28.

The storage capability of the optical disk 28 is characterized by its disk pixels essentially defined based on the second segments 36, and their size. The disk pixels of the optical disk have disk pixel sizes which depend on the actual type of the optical disk 28. For instance, if the optical disk 28 is realized as a CD, the size (length x width) of a pixel is approximately 800 nm x 600 nm; in case of a DVD it is approximately 400 nm x 320 nm; in case of an HD-DVD it is approximately 200 nm x 200 nm; and in case of a Blu-ray it is approximately 150 nm x 130 nm. The storage capability of the optical disk 28 is further characterized by its pitch 37, which is essentially defined by the distance between two radially adjacent spiral sections 33. The actual value of the pitch 37 also depends on the actual type of the optical disk 28. For instance, for a CD/DVD/HD-DVD/Blu-ray the pitch 33 is approximately 1.6 μιη/1.1 μιη/620 nm/480 nm, respectively. With the afore-mentioned disk pixel sizes and pitch sizes, the storage track 32 can be configured to possess a length of several kilometers. For instance, for a CD the length of the storage track 32 is approximately 5.38 km, whereas it is even larger for the other afore-mentioned types of optical disks 28.

The data storage in the optical disk 28 is essentially realized by arranging the first segments 35 and the second segments 36 in a desired way. More precisely, a plurality of logical bits comprising a plurality of "0"s and/or "l"s are stored in such an optical disk 28 as a standardized sequence of the first and second segments 35, 36 of varying lengths. Each logical bit "0" is represented by no change from a first segment 35 to a second segment 36 or vice versa ("pit to pit" or "land to land") , while each logical bit "1" is represented by a change from a first segment 35 to a second segment 36 or vice versa ("pit to land" or "land to pit"). The arrangement of the first segments 35 and the second segments 36 can be precisely controlled and scanned along the storage track 32, preferably using an optical sensor and/or an optical microscope. It is also possible to create a mapping from a specific bit in the data to coordinates, such as x,y coordinates in a Cartesian coordinate system, by an appropriate coordinate transformation.

With help of existing optical storage technology, it is thus possible to create the OAM generating arrangement 16 with high precision and efficiency. In particular, both the disk pixel size and the pitch size possess fairly precise values so that the sequence of the first and second segments 35, 36 can be well defined. As a result, the OAM generating arrangement 16 can be easily created by appropriately arranging the logical bits "0" and "1". A further advantage of the optical storage medium 22 is that mass production with high cost efficiency is possible for the OAM generating arrangement 16 due to the well-established optical storage technology.

As can be seen in Fig. 2, the plurality of OAM generating elements 18 are arranged in the optical disk 28, preferably in the active layer 26 (Fig. 1) of the optical disk 28. It is noted that the relative scaling between the OAM generating elements 18 and the storage track 32 shown in Fig. 2 does not represent the actual scaling, since the dimensions of the storage track 32 and the OAM generating elements 18 are far apart from one another. For example, for a CD, the pixel size typically is 800 nm x 600 nm, while the size of an OAM generating element 18 is of the order of 200 μιη x 200 μιη in an example. The OAM generating elements 18 are arranged preferably radially (18b) and/or circumferentially (18a) in respect to the center 30. The OAM generating elements 18 each comprise a plurality of element pixels 38 with an element pixel size. For reasons of plausibility, the element pixels 38 are shown as a dash inside of one of the OAM generating elements 18. The dash, however, does not represent the real form, scale or other physical or structural properties of the element pixels 38. In this preferable embodiment, each element pixel 38 comprises at least a fraction of the plurality of disk pixels 34. Hence, the element pixel size is correlated to the disk pixel size of the optical disk 28. Preferably, the minimum of the element pixel size is limited by the minimum of the disk pixel size. Further preferably, each element pixel 38 comprises essentially one disk pixel 34 so that the element pixel size of the OAM generating elements 18 is essentially equal to the disk pixel size. Hence, depending on the actual type of the optical disk 28, the element pixel size can take different values.

In comparison to existing OAM-light generating devices, the OAM-light generating device 10a according to the present invention enables OAM generating elements 18 with significantly smaller element pixel sizes. In particular, the afore-mentioned LCoS panel comprises an element pixel size of approximately 10 μιη xlO μιη. The relatively large element pixel size poses a limitation to the number of element pixels that can be brought onto the LCoS panel. With the afore-mentioned actual types of the optical disk 28, the element pixel size of the OAM generating elements 18 according to the present invention can be reduced by almost two orders of magnitude compared to the LCoS panel. A much larger number of element pixels 38 can be thus allocated in each OAM generating element 18 so that the OAM-light generating device 10a possesses an inherently high resolution.

The OAM generating elements 18 comprise preferably each a phase hologram 40. Fig. 3 shows a preferable embodiment of the phase hologram 40 which comprises a pitchfork structure 42. The phase hologram 40 further comprises a plurality of dark stripes 44 and a plurality of bright stripes 46. A general form of such a phase hologram 40 is disclosed in WO 2009/09061 OA 1, the content of which is incorporated herein by reference. The plurality of dark and bright stripes 44, 46 create a contrast in brightness. The advantage of creating the phase hologram 40 in the optical disk 28 is that such a contrast in brightness can be easily achieved by choosing the appropriate arrangement of the first segments 35 and the second segments 36, whose alternating reflectivity levels for the light beam 14 lead to such a contrast. In particular, the alternating heights of the first and second elements 35, 36 naturally result in such a contrast. Such an arrangement of the first and second segments 35, 36 can be easily realized using an appropriate sequence of the logical bits "0"s and "l"s, so that the phase hologram 40 can be fabricated with high cost-efficiency, high precision and high resolution according to the present invention.

The phase hologram 40 imparts OAM with an OAM charge number to the light beam 14 by converting the HG mode of the light beam 14 into an LG mode. The value of the OAM charge number depends on the actual structure of the phase hologram 40, particularly the pitchfork structure 42. The lateral dimension of the phase hologram 40 is a quantity that depends on its element pixel size, which in turn is correlated to the disk pixel size of the optical disk 28. The beam size of the light beam 14 is adequate for being reflected and diffracted by the phase hologram 40. Preferably, the beam size of the light beam 14 is a function of the lateral dimensions of the phase hologram 40.

It is advantageous to configure the OAM generating elements 18 as phase holograms 40, since it is possible to create OAM-light in an efficient way. In particular, numerically computed phase holograms can be created to impart OAM with any desired OAM charge number to the same incident light beam. Hence, according to the present invention, the OAM generating arrangement 16 can be realized to impart OAM with desired OAM charge numbers to the light beam 14. In addition, due to the small disk pixel sizes of typical optical disks, the phase holograms 40 can be configured to comprise a fairly small element pixel size. For instance, phase holograms 40 arranged on a CD can possess an element pixel size of approximately 800 nm x 600 nm. Consequently, the size of the phase holograms 40 can also be significantly reduced in this way to the order of 200 μιη x 200 μιη, whereas the afore-mentioned phase hologram realized in an LCoS panel has a much larger size of 2 mm x 2 mm.

Furthermore, as shown in Fig. 1 and Fig. 2, the OAM generating arrangement 16 comprises a plurality of OAM generating elements 18 (shown as squares). Preferably, the OAM generating elements 18 correspond to OAM charge numbers, from which at least two are different. Further preferably, the OAM generating elements 18 are arranged in a radially increasing circumferential order, so that their associated OAM charge numbers are arranged in an order increasing circumferentially (clockwise or anticlockwise) and/or radially

(outwards or inwards) in respect to the center 30. Such an OAM generating arrangement 16 enables imparting OAM with at least two different OAM charge numbers to the light beam 14. The different OAM charge numbers can thereby be selected by changing the incidence direction of the light beam 14, thus switching from one OAM generating element to a different one which receives and diffracts the light beam 14.

Alternatively, the at least two different OAM charge numbers are selectable by rotating the optical disk 28 around the center 30. As shown in a second embodiment of an OAM-light generating device 10b in Fig. 4, the optical disk 28 is rotatable around an axis 48 which coincides with the center 30 of the optical disk 28. The light beam 14 emitted by the light source 12 propagates through the optical arrangement 20 and irradiates one of the OAM generating elements 18. The light beam 14 is diffracted by the OAM generating element 18 and results in the outgoing light beam 14'. The plurality of OAM generating elements 18 can be arranged circumferentially in respect to the center 30, such as is the case of the optical disk 28 shown in Fig. 2. By rotating the optical disk around the axis 48 by an angle a, the light beam 14 will irradiate another OAM generating element 18 with a different OAM charge number from the previous one. The possible values the anlge a can take depend on the actual arrangement of the plurality of OAM generating elements 18. For instance, the optical disk 28 shown in Fig. 2 comprises 16 OAM generating elements 18a which are circumferentially arranged with essentially equidistance between any two adjacent OAM generating elements 18a. In order to switch from one OAM generating element 18 to a different one, the optical disk 28 can be rotated by α=η*22.5°, n being an integer value except n=16 or any multiple thereof.

The advantage of such an optical disk 28 is that the center 30 is generally well defined so that the rotation around the axis 48, which is essentially perpendicular to the plane of the optical disk 28, can be precisely controlled. This enables a precise and easy switching between different OAM charge numbers imparted by the plurality of OAM generating elements 18 to the light beam 14. In particular, such an OAM generating arrangement 16 is highly efficient and suitable for applications that require high precision and efficiency in switching between OAM charge numbers, such as applications in signal processing, optical image processing and telecommunications.

The light source 12 can also emit a plurality of light beams 14, 50, 52. Fig. 5 shows a third embodiment of an OAM-light generating device 10c, wherein the light source 12 emits a first light beam 50 and a second light beam 52. Preferably, the first and the second light beams 50, 52 comprise each an HG mode. Both the first and the second light beams 50, 52 propagate through the optical arrangement 20 and irradiate two different OAM generating elements 18 individually at the same time. The first and the second light beams 50, 52 are diffracted by the two OAM generating elements 18. After being diffracted, the first and the second light beams 50, 52 are transformed into a first outgoing light beam 50' and a second outgoing light beam 52', respectively. The HG mode of each of the two incident light beams 50, 52 is thereby converted into an LG mode, hence carrying OAM with an OAM charge number. The two OAM generating elements 18 correspond preferably to two different OAM charge numbers, so that they impart OAM each with a different OAM charge number to the first and second light beams 50, 52. The optical disk 28 is rotatable around the axis 48 so that by a rotation of the angle a the pair of achievable OAM charge numbers can be replaced by another pair of OAM charge numbers. Similarly to the embodiment shown in Fig. 4, the possible values of a depend on the actual arrangement of the OAM generating elements 18 in the optical disk 28.

The number of light beams emitted by the light source 12 is not limited to two but can be scaled up to any number. The advantage of the light source 12 emitting multiple light beams 50, 52 is that, depending on the actual number of the emitted light beams, a plurality of OAM charge numbers can be imparted to the multiple light beams

simultaneously. With the help of the rotatable optical disk 28, the switching between different groups of OAM charge numbers is easily and precisely realizable. This is particularly advantageous for medical imaging applications, in particular for hyperpolarizing different materials using OAM-light with different OAM charge numbers simultaneously. The efficient switching between different OAM charge numbers is furthermore particularly advantageous for signal processing as well as optical image processing applications, just to name a few.

In a preferable embodiment of the OAM-light generating device lOa-c, the optical storage medium 22 is configured to comprise a recordable optical storage format. Such an optical storage format can be realized in two ways: a write-once optical storage format or a rewritable optical storage format. The write-once optical storage format such as CD-R and DVD-R comprises a reflective layer that is covered by a photo-sensitive dye layer. During the writing process, the absorption properties of storage track are regionally altered by modulating the output of a writing apparatus, preferably a writing laser, as it drives along the storage track 32. After thermal and/or optical treatments induced by the writing laser, a plurality of first and second segments 35, 36 can be created along the storage track 32. An arrangement of the first and the second segments 35, 36 with varying reflectivity levels for the light beam 14 can thus be formed. The rewritable optical storage format further allows multiple recording of data. Using an optical storage medium 22 comprising a recordable optical storage format has the advantage of high efficiency and convenience in creating an OAM generating arrangement 16 with desired properties, such as the OAM charge numbers and their arrangement. In order to acquire the information about the arrangement of the first and the second segments 35, 36 contained in the optical disk 28, the user only needs to insert the optical disk 28 into a conventional optical disk player, wherein the choice of the optical disk player depends on the choice of the optical disk 28. In addition, the writing process (also known under the term "burning") can advantageously be easily and precisely controlled by applying an appropriately chosen sequence of the logical bits "0"s and "l"s. This sequence can be generated in form of a computer program, preferably with an ISO disk image file format. Other disk image file formats such as IMG and NRG formats are also possible. Such a computer program containing the afore-mentioned sequence of logical bits can be conveniently run on a computer using of-the-shelf consumer optical writing devices in cooperation with or containing a writing laser. Modifications of the arrangement of the first and the second segments 35, 36 are easily possible in this way. In general, the writing process also requires a writing software which is widely available.

The recordable optical storage format also allows the phase hologram 40 to comprise a plurality of reflectivity levels that are continuously variable. Further, the plurality of element pixels 38 can be configured without limitation in length. Both can be achieved by using a writing device that allows custom modulation of the writing laser, such as its beam size, mode and power.

In the following, two possible applications of the OAM-light generating device lOa-c are presented with reference to Fig. 6 and Fig. 7, respectively.

Fig. 6 shows an embodiment of a hyperpolarization device 54, which comprises an OAM-light generating device lOa-c and an optical arrangement 56 comprising at least one optical element 57. The OAM-light generating device lOa-c generates a light beam 14', 50', 52' endowed with OAM. The light beam 14', 50', 52' is subsequently guided by the optical arrangement 56 to at least partially interact with a compound 58. During the interaction, the light beam 14', 50' 52' transfers its OAM to the compound 58, in particular to the valence electrons of its atoms and/or molecules, under preserving the energy and momenta (including SAM and OAM) of the light beam 14', 50' 52' and the compound 58. The hyperfine interaction between the valence electrons and the nuclei of the compound 58 leads finally to an alignment of a fraction of the nuclear spins of the compound 58. When the fraction is significantly higher than under thermal equilibrium, a nuclear hyperpolarization of the compound 58 is realized.

In the preferable embodiment shown in Fig. 6, the optical arrangement 56 further comprises a spatial filter 60. The spatial filter 60 is configured so that it filters out a first portion of the light beam 14', 50', 52' and to allow a second portion of it to pass.

Preferably, the spatial filter 60 is configured to block the zero^th diffraction order of the light beam 14', 50', 52', so that the incident light arriving at the compound 58 carries non-zero OAM.

Such a hyperpolarization device 54 is particularly advantageous. With help of the OAM-light generating device lOa-c which makes use of the optical disk 28, the hyperpolarization device 54 enables hyperpolarizing the compound 58 with OAM-light with high efficiency and precision. A wide range of different OAM charge numbers is

simultaneously available due to the small disk pixel size of the optical disk 28 incorporated in the OAM-light generating device lOa-c. Furthermore, it also enables efficient and precise switching between the different OAM charge numbers. Such a hyperpolarization device 54 can be fabricated with high cost efficiency, since the fabrication involves existing optical storage technology which is well-established.

After being hyperpolarized, the compound 58 can be applied for magnetic resonance imaging (MRI). Fig. 7 shows an embodiment of a magnetic resonance system 62 comprising a hyperpolarization device 54' and an RF system 64. The hyperpolarization device 54' is preferably the embodiment shown in Fig. 6. The RF system 64 comprises at least one RF coil. The compound 58 is first hyperpolarized by the hyperpolarization device 54' before it is further introduced to the RF system 64. The RF system 64 is configured to induce magnetic resonance in the hyperpolarized compound 58. In a preferable embodiment, the RF system 64 emits electromagnetic waves from a frequency band covering the resonance frequencies associated with the hyperpolarized compound 58. These

electromagnetic waves induce oscillations of nuclear spins of the hyperpolarized compound 58 which decay within a decay time depending on the individual resonance frequency. Using Fast Fourier Transformation (FFT), the different decay times can be transformed back into the resonance frequencies of the hyperpolarized compound. In this way, the resonance signals of the hyperpolarized compound 58 can be detected. This technique is known as the Free Induction Decay (FID) technique, which is advantageous in particular for its low noise level and high signal-to-noise ratio (SNR). By incorporating the OAM-light generating device lOa-c according to the current invention, the magnetic resonance system 62 is distinguished from conventional magnetic resonance systems by allowing the acquisition of magnetic resonance signals of a compound in a significantly more efficient way. A wide range of different OAM charge numbers that are simultaneously available for hyperpolarizing the compound 58 due to the small disk pixel size of the optical disk 28 incorporated in the OAM-light generating device lOa-c. Furthermore, it also enables efficient and precise switching between the different OAM charge numbers.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, 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. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An OAM-light generating device for generating light endowed with an orbital angular momentum (OAM), comprising

a light source (12) configured to emit at least one light beam (14, 50, 52); and an OAM generating arrangement (16) configured to generate OAM in said at least one light beam (14, 50, 52), said OAM generating arrangement (16) comprising at least one OAM generating element (18) configured to individually receive and diffract said at least one light beam (14, 50, 52) and to impart OAM with an OAM-charge number to said at least one light beam (14, 50, 52), wherein said OAM generating arrangement (16) is configured as an optical storage medium (22).

2. The OAM-light generating device according to claim 1, wherein said optical storage medium (22) is configured as an optical disk (28) having a center (30), said optical disk (28) comprising a plurality of disk pixels (32).

3. The OAM-light generating device according to claim 2, wherein said at least one OAM generating element (18) comprises a plurality of element pixels (38), each comprising at least a fraction of said plurality of disk pixels (32).

4. The OAM-light generating device according to claim 2, wherein said at least one OAM generating element (18) is arranged radially and/or circumferentially in respect to said center (30).

5. The OAM-light generating device according to claim 2, wherein said OAM generating arrangement (16) comprises a plurality of OAM generating elements (18) imparting OAM with at least two different OAM-charge numbers to said at least one light beam (14, 50, 52).

6. The OAM-light generating device according to claim 5, wherein said at least two different OAM-charge numbers are selectable by rotating said optical disk (28) around said center (30).

7. The OAM-light generating device according to claim 1, wherein said at least one OAM generating element (18) comprises a phase hologram (40), said phase hologram (40) comprising a pitchfork structure (42).

8. The OAM-light generating device according to claim 1, wherein said optical storage medium (22) comprises a recordable optical storage format.

9. The OAM-light generating device according to claim 1, wherein said light source (12) is configured to emit a plurality of light beams (14, 50, 52).

10. Optical storage medium comprising at least one OAM generating element (18) configured to individually receive and diffract at least one light beam (14, 50, 52) of a light source (12) and to impart orbital angular momentum (OAM) with an OAM-charge number to said at least one light beam (14, 50, 52).

11. A hyperpolarization device for creating nuclear hyperpolarization in a compound (58), comprising:

an OAM-light generating device (lOa-c) as claimed in claim 1 for generating light endowed with an orbital angular momentum (OAM); and

at least one optical arrangement (56) configured to direct said light to at least partially interact with said compound (58), thereby generating a nuclear hyperpolarization of said compound (58).

12. A magnetic resonance system, comprising:

a hyperpolarization device (54, 54') as claimed in claim 11; and - an RF system (64) configured to induce magnetic resonance and detect resonance signals.

13. A method for generating light endowed with orbital angular momentum

(OAM), comprising: emitting at least one light beam (14, 50, 52); and

generating OAM in said at least one light beam (14, 50, 52), said generating comprising individually receiving and diffracting said at least one light beam (14, 50, 52) and imparting OAM with an OAM-charge number to said at least one light beam (14, 50, 52), said imparting being carried out with an OAM generating arrangement (16) configured as an optical storage medium (22).

14. Computer program comprising program code means for causing a computer to carry out the steps of the method as claimed in claim 13 when said computer program is carried out on said computer.