PLASMONIC DEVICE, SYSTEM AND METHOD

Abstract

A plasmonic device amplifies an optical signal of a sample positioned subsequently thereto comprises a high refraction index dielectric element, a low refraction index dielectric element with a modifiable width and a layer of metal. When light producing plasmon resonance is received at a dielectric metal interface, a plasmonic field is generated in the sample. A system comprises the plasmonic device, a holder for placing the sample, an optical circuit with a light source and a photodetector and rotatable supports for the plasmonic device. A magneto optical signal is produced according to an incident light angle, a distance of the plasmonic device and the sample and the inner width of a dielectric in the plasmonic device. A method obtains an amplified magneto optical signal from the sample, modifies an angle near a total reflection, adjusts distance of the sample to the plasmonic device or internal width of second dielectric such that it produces a maximum plasmonic field.

Claims

1. A plasmonic device for amplifying an optical signal of a sample comprising: a first dielectric element of a high refraction index, the first dielectric element comprising a first surface for receiving light and a second opposite surface; a second dielectric element of a low refraction index, the second dielectric element comprising a first surface and a second opposite surface, defining a width therebetween, wherein the width is modifiable and wherein the first surface of the second dielectric element is arranged proximate the second surface of the first dielectric element; a layer of metal, the layer of metal comprising an internal side proximate the second surface of the second dielectric element, and an external opposite side; wherein the plasmonic device is configured to receive light producing plasmon resonance at a region interface of the second dielectric element and the layer of metal and to generate a plasmonic field in the sample, when the sample is positioned subsequently to the plasmonic device in relation to the region of plasmon resonance.

2. The plasmonic device of claim 1, where the first dielectric element is a prism.

3. The plasmonic device of claim 1, further comprising a width adjustable support, such that the distance between the first dielectric element and the layer of metal is adjustable, wherein the second dielectric element is gas or vacuum formed therebetween.

4. The plasmonic device of claim 1, wherein the second dielectric element comprises an elastic material having a width modifiable by applying external pressure.

5. The plasmonic device of claim 1, wherein the second dielectric element comprises a piezoelectric material having a width modifiable by applying a voltage.

6. The plasmonic device of claim 1, further comprising an additional layer of metal internally arranged between the first dielectric element and the second dielectric element, the additional layer of metal being a second layer of metal.

7. The plasmonic device of claim 6, further comprising a width adjustable support for the metal layer and the additional metal layer, wherein the second dielectric element is gas or vacuum formed therebetween.

8. The plasmonic device of claim 1, further comprising a third dielectric element of a low refraction index, externally arranged to the layer of metal, the third dielectric element comprising an internal side proximate the external side of the layer of metal, and an opposite external side distant the layer of the metal, and wherein the width between internal side and external side is modifiable, wherein the plasmonic device is further configured to receive light producing plasmon resonance at a region interface of the third dielectric element and the layer of metal and to generate an additional plasmonic field in the sample.

9. The plasmonic device of claim 8, further comprising a fourth dielectric element of a high refraction index, the fourth dielectric element comprising a first surface and a second opposite surface, wherein the first surface of the fourth dielectric element is arranged proximate the second surface of the first dielectric element, and wherein the second surface of the fourth dielectric element is proximate the first surface of the second dielectric element.

10. The plasmonic device of claim 1, further comprising a third dielectric element of a high refraction index, the third dielectric element comprising a first surface and a second opposite surface, wherein the first surface of the third dielectric element is arranged proximate the second surface of the first dielectric element, and wherein the second surface is proximate the first surface of the second dielectric element.

11. The plasmonic device of claim 1, wherein the first dielectric element comprises Zinc Selenide (ZnSe) or Rutile (TiO2).

12. A system for detecting a magneto optical signal of a sample comprising: a plasmonic device for amplifying an optical signal of a sample comprising: a first dielectric element of a high refraction index, wherein the first dielectric element is a prism comprising a first surface for receiving light and a second opposite surface; a second dielectric element of a low refraction index, the second dielectric element comprising a first surface and a second opposite surface, defining a width therebetween, wherein the width is modifiable and wherein the first surface of the second dielectric element is arranged proximate the second surface of the first dielectric element; a layer of metal, the layer of metal comprising an internal side proximate the second surface of the second dielectric element, and an external opposite side; wherein the plasmonic device is configured to receive light producing plasmon resonance at a region interface of the second dielectric element and the layer of metal; the system further comprising: a holder for placing the sample, wherein the holder is configured to set an adjustable distance of the sample with respect to the plasmonic device; an optical circuit comprising: a light source and a polarizer, so as to emit a polarized light beam; a first rotatable support for placing the holder and the plasmonic device at a selectable incident angle with respect to the light beam, so as to allow obtaining a total reflection of the polarized light beam on the plasmonic device; a photodetector for detecting light exiting from the plasmonic device; and a second rotatable support for placing the photodetector at a selectable exiting angle with respect to the light exiting from the plasmonic device; wherein a magneto optical signal is produced from the sample exposed to a plasmonic field generated by the plasmon resonance, wherein the magneto optical signal is controlled by the incident angle, the distance of the sample with respect to the plasmonic device and/or the second dielectric width.

13. The system of claim 12, further comprising a magneto optical setup comprising an electromagnet for applying an external magnetic field having one of the following orientations: an orientation perpendicular to the plane of incidence thereby producing a transversal magnetic optic Kerr effect, TMOKE, signal, or an orientation parallel to the plane of incidence and to the external opposite side of the layer of metal of the plasmonic device thereby producing a longitudinal magnetic optic Kerr effect, LMOKE, signal, or parallel to the plane of incidence and perpendicular to the external opposite side of the layer of metal of the plasmonic device thereby producing a perpendicular magnetic optic Kerr effect, PMOKE, signal.

14. The system of claim 12, wherein the second dielectric element comprises a piezoelectric material and wherein the second dielectric element width is modifiable by applying a voltage.

15. The system of claim 12, wherein the second dielectric element comprises a elastic material and wherein the second dielectric element width is modifiable by applying an external pressure.

16. A method for obtaining an amplified magneto optical signal from a sample comprising the steps of: placing a sample in a holder having an adjustable distance with respect to a plasmonic device, the plasmonic device comprising an internal surface of a first dielectric element of a high refraction index, wherein the internal surface is opposite to an external surface for receiving light and a second dielectric element of a low refraction index; setting an incident angle for a polarized light beam on the external surface of the plasmonic device and emitting the polarized light beam; applying a coarse adjustment by: varying, with a rotatable support coupled to the holder, the incident angle of the emitted light beam; detecting, with a photodetector, a corresponding reflected light beam intensity; obtaining, based on the reflected light beam intensity, a total reflection angle producing a total reflection of the polarized light beam on the internal surface of the first dielectric element; increasing the incident angle over the total reflection angle until the minimum reflectivity is reached, thereby generating a plasmonic field; and producing an amplification of the magneto optical signal from the sample.

17. The method of claim 16, further comprising the step of applying a fine adjustment by: modifying the incident angle over the total reflection angle until a maximum plasmonic field is generated; and/or varying the distance of the sample with respect to the plasmonic device until a maximum plasmonic field is generated; and producing an additional amplification of a magneto optical signal from the sample;

18. The method of claim 16, further comprising the step of applying a fine adjustment by: varying the width of a second dielectric element comprised in the plasmonic device by applying a pressure or voltage until a maximum plasmonic field is generated; and producing an additional amplification of a magneto optical signal from the sample.

19. The method of claim 16, further comprising a step of sweeping an external magnetic field produced by a magneto optical setup over the sample and producing a magnetic hysteresis loop thereof.

20. The method of claim 19, wherein the external magnetic field orientation is selected among transversal, parallel, or longitudinal thereby producing a TMOKE signal, PMOKE signal, or LMOKE signal respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] A series of drawings which aid in better understanding the disclosure and which are presented as non-limiting examples and are very briefly described below.

[0028] FIGS. 1A-1H illustrate different embodiments of a plasmonic device.

[0029] FIG. 2 illustrates a plasmonic generation using an embodiment of the plasmonic device.

[0030] FIGS. 3A-3C illustrate different types of generation of magneto-optic plasmonic Kerr effects.

[0031] FIG. 4 illustrates an embodiment of a plasmonic device used in experimental tests.

[0032] FIGS. 5A-5B and 5C illustrate schematic diagrams of two embodiments of a system according to the disclosed teachings.

[0033] FIG. 6 is a flow diagram of steps of a method according to the disclosed teachings.

[0034] FIGS. 7A-7B show angular reflectivity and TMOKE signals obtained in experimental tests and theoretical prediction for a sample in Otto configuration.

[0035] FIG. 8 shows amplification obtained in experimental tests of the magnetic hysteresis loops for the sample under the influence of plasmon resonance fields and in the absence of plasmon fields.

[0036] FIGS. 9A-9B show reflectance and corresponding TMOKE signal vs angle that demonstrates how it changes as function of the angle of incidence.

[0037] FIGS. 10A-10G illustrate different hysteresis loops found at each angle during variation step for finding a maximum amplification based on a hysteresis technique.

DETAILED DESCRIPTION

[0038] A set of embodiments of a device, a system and a method will be described in detail by reference to the appended drawings. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.

[0039] FIG. 1A schematically shows a structure of layers in an embodiment 101 of a device for amplifying an optical signal in a sample. From top to bottom there are a dielectric element of a high refractive index 121 in the form of a semi-spherical prism, a dielectric layer 124 of a low refraction index and a bottom layer of metal 125. The width of the dielectric layer 124 is modifiable. For instance, due to its piezoelectric or an elastic properties (e.g. polydimethylsiloxane, PDMS). A sample is to be positioned below the plasmonic device either in contact with the device or close to it. The upper element 121 receives light beam under certain conditions a plasmon resonance region may be produced.

[0040] FIG. 1B schematically shows an embodiment 102 similar to FIG. 1A, the main difference lies in there is an upper dielectric element 121a that is not in form of a prism but of a layer. This embodiment saves space and weight. On the other hand, it requires to be optically coupled to an external prism (not shown in FIG. 1B) to be functional.

[0041] FIG. 1C schematically shows an embodiment 103 similar to FIG. 1B, the main difference lies in the dielectric layer 124 being gas, e.g. air, or vacuum instead of a non-gaseous material. This layer forms a gap between the prism element 121 and the metal layer 125. A support 126 is provided for the metal layer. This support 126 may be adjustable to modify the properties of the device, in particular, the formation of plasmon resonance.

[0042] FIG. 1D schematically shows an embodiment 104 similar to FIG. 1A, the main difference lies in an additional metal layer 123 is provided below the upper dielectric element 121 and above the dielectric layer 124.

[0043] FIG. 1E schematically shows an embodiment 105 similar to FIG. 1D, the main difference lies in the dielectric layer 124 being gas, i.e. air, or vacuum instead of a non-gaseous material.

[0044] FIG. 1F schematically shows an embodiment 106 similar to FIG. 1A, the main difference lies in an additional dielectric element 126 of a low refraction index. The dielectric element 126 is in form of a layer and is placed on the bottom, below the layer of metal 125.

[0045] FIG. 1G schematically shows an embodiment 107 similar to FIG. 1A, the main difference lies in a further intermediate dielectric element 122 of a high refraction index. The intermediate dielectric element 122 is in form of a layer and is placed below the upper dielectric prism 121 of a high refractive index and above the dielectric layer 124 of a low refractive index. The bottom layer is also a layer of metal 125.

[0046] FIG. 1H schematically shows an embodiment 108 similar to FIG. 1F, the main difference lies in a further intermediate dielectric element 122 of a high refraction index. The intermediate dielectric element 122 is in form of a layer and is placed below the upper dielectric prism 121 of a high refractive index and above the dielectric layer 124 of a low refractive index. The bottom layer is also a dielectric layer 126 of a high refraction index.

[0047] The usual values for the refraction index in each of the dielectric elements are the following:

[0048] In FIGS. 1A-1C: For the upper dielectric element 121, preferably n≥1.5; for the intermediate dielectric element 124, 1≤n≤1.5.

[0049] In FIG. 1E: For the upper dielectric element 121, preferably n≥1.5; for the intermediate dielectric element 124, n˜1.

[0050] In FIG. 1F: For the upper dielectric element 121, preferably n≥1.5; for the intermediate dielectric element 124, 1≤n≤1.5; for the bottom dielectric element 126, 1≤n≤1.5.

[0051] In FIG. 1G: For the upper dielectric element 121, preferably n˜1.5; for the upper intermediate dielectric element 122, n≥1.5, for the lower intermediate dielectric element 124, 1≤n≤1.5.

[0052] In FIG. 1H: For the upper dielectric element 121, preferably n˜1.5; for the upper intermediate dielectric element 122, n≥1.5, for the lower intermediate dielectric element 124, 1≤n≤1.5; for the bottom dielectric element 126, 1≤n≤1.5.

[0053] It is desirable to keep the difference of refractive indices between the high and low index materials of dielectric elements as high as possible. However it is possible to use slightly lower index of refraction for first dielectric element n<1.5, reducing such difference in exchange of lower performance.

[0054] For superior performance materials such as Zinc Selenide (ZnSe) or Rutile (TiO2) can be used as upper dielectric element with high index of refraction, when used in conjunction with low index of refraction materials.

[0055] FIG. 2 schematically shows the embodiment 105 of the device in FIG. 1E under operation with a sample 200 placed below and near (or in contact with) the bottom layer of metal 125. It suffices that the sample be magnetic. Due to the fact the incident light does not directly impact on the sample, it does not need to be solid or continuous or flat as required by conventional techniques.

[0056] The magneto optical Kerr effect, MOKE, describes the changes in intensity and polarization experienced by a light wave after being reflected from the surface of a material. Said material is exposed to an external magnetic field. The changes depend on the orientation of the magnetic moments of the material under the influence of the applied magnetic field and the plane of incidence.

[0057] When incident polarized light 201 is received with a certain angle on the dielectric prism 121, plasmon resonance regions 203, 205 are produced at each of the interfaces of dielectric and metal. The sample is also exposed to such giant plasmonic field 204 due to its proximity to the plasmonic device. This electric field is produced by collective surface charge oscillations at a metal dielectric interface. These oscillations, also known as Surface Plasmon-Resonance (SPR), are the result of coupling a parallel component of the wave vector of the charge oscillations and frequency between an external electric field and surface charges present in the metal.

[0058] The magneto optical response of a sample in the proximity to the plasmonic device can be enhanced as result of the presence of said giant electric field. Given that the magnetic sample is not directly shown with light it can be in different states of matter such as solid liquid, or powder phase or geometries such as nano structured samples.

[0059] FIGS. 3A-3C illustrate three different configurations according to the direction of an external magnetic field in relation to a device 100 representing anyone of the embodiments 101 to 108 previously described. When the magneto-optical Kerr effect, MOKE, is produced under an external magnetic field having a: [0060] polar direction 301 as in FIG. 3A, a P-MOKE signal is obtained; [0061] longitudinal direction 302 as in FIG. 3B, a L-MOKE signal is obtained; [0062] transversal direction 303 as in FIG. 3C, a T-MOKE signal is obtained,

[0063] FIG. 4 is a schematical representation of an embodiment 103 in the Otto configuration used for experimental tests. It has been proved that a T-MOKE signal coming from a magnetic sample can be amplified 8 times or more.

[0064] The embodiment 103 includes a prism BK7 121, followed by an air gap 124 of ˜300 nm of width and a layer of metal Ag 125 of 20 nm, followed by a sample 200 made of a layer of Co of 10 nm deposited on Si substrate i.e. the effective structure studied was prism/Air (˜300 nm)//Ag (20 nm)/Co(10 nm)/Si (substrate). In this configuration several tests have been performed revealing a T-MOKE signal coming from the magnetic sample has been amplified about 8 times or more.

[0065] FIGS. 5A-5B and 5C shows two setups for finding a maximum amplification of a MOKE signal that have been used for the experimental tests. The system 500 allows an indirect observation of the magneto-optic Kerr effect (MOKE) enhanced by exciting a plasmon surface resonance near a sample 200.

[0066] FIG. 5A is a schematical representation of a top view along with FIG. 5B, which is a partial perspective view of an embodiment of a system 500 with a Lock-in technique.

[0067] The system 500 includes an optical circuit. On a side, a laser emitter 501 as a light source coupled to a polarizer 502. The light beam is directed to the plasmonic device 100. On the other side, part of the incident light beam is reflected and can be detected by a photodetector (photodiode) 505. The sample 200 is located in contact with, or close to, the plasmonic device 100. The system provides a holder 507 for a suitable positioning of said sample 200 in relation to the plasmonic device 100. For instance in an embodiment of FIG. 1C the holder allows to position the sample and at the same time to apply pressure thus changing the average width of the second dielectric.

[0068] To allow easily changing the angle of incident light beam, a rotation system 506 composed of two goniometers for placing the holder at a certain angle θ and detecting reflected light, using a photodetector 505, at an angle 2θ. When correctly aligned, the photodetector 505 detects light exiting from the plasmonic device 100.

[0069] A MOKE signal can be produced from the sample 200 when exposed to a plasmonic field that can be controlled by several factors, namely, the incident light beam angle, the distance between the sample 200 and the plasmonic device 100 and, modifiable structural features in the plasmonic device 100 (e.g. the width of an internal layer).

[0070] An oscillatory magnetic field 303 is applied via electromagnet or coil 504 driven by a power source 513 (e.g. KEPCO bipolar). The current through the coil 504 is varied periodically using an external reference signal supplied by a wave function generator 512. The sample experiences an alternating magnetic field where the magnetization is periodically flipped resulting in a square signal response detectable by the photodiode 505.

[0071] Small variations in the reflected light can be extracted with a Lock-in amplifier 511 in phase with the oscillatory magnetic field 303. The light variations serve to form a reflectivity signal solely as result of magnetic field 303 variations.

[0072] In this example, the direction of the magnetic field 303 that causes the magnetization changes in the sample, is normal to the plane of light incidence and is in the same plane of the sample 200. Thus the signal to be obtained is of a T-MOKE type as result of changes in the reflectivity of the incident light due the magnetic state of the sample 200.

[0073] Likewise shown by FIG. 3A-3C, a different alignment of the magnetic field 303 is also valid, so as to produce another kind of MOKE amplification such as PMOKE or LMOKE.

[0074] FIG. 5C is a schematical representation of a top view of another embodiment of a system 500 that alternatively uses a set of electronics so called hysteresis-technique.

[0075] The lock-in amplifier is replaced with several components. A low-pass filter 522 is used for pre-amplification. A compensator 523 matches a DC signal coming from the low-pass filter 522. A differential amplifier 521 multiplies several times the subtraction of the two signals from the filter 522 and from the compensator 523.

[0076] For this setup, the wave generator 512 produces a sinusoidal wave to control the power source 513 and thus the current that is fed in the coil 504.

[0077] For a particular example, the low-pass filter may have a cutoff frequency at 30 Hz, the coil may be controlled by a sinusoidal wave with frequency of 1 Hz, the gain of the differential amplifier may be 200. These values provide a low-noise AC signal coming from the photodiode.

[0078] A magnetic hysteresis loop is obtained by sweeping the magnetic field. The electromagnet and coil 504 serve for applying a sweeping magnetic field while reflectivity value is sensed by the photodetector 505.

[0079] The system presented in both figures allows the sample 200 be illuminated at θ angle while the photodetector 505 is at 2θ angle (so called θ-2θ configuration). At an angle greater than the total reflection angle, plasmon surface resonance may be excited thereby modifying optical behavior, such as an effect of reduction of reflected light. The detection of reflected light reduction allows the study the MOKE signal as function of angle around and at the plasmon resonance peak.

[0080] FIG. 6 schematically represents steps of an embodiment of a method for obtaining an amplified magneto optical signal from a sample. The method can be used in a system 500 as explained above with reference to FIGS. 5A-5C.

[0081] In a placing step 602, a sample 200 is placed at an adjustable distance with respect to a plasmonic device 100. In this case, firstly a distance is set. Other embodiments may allow for the distance be further modified during the method.

[0082] In an emitting step 604, an initial incident angle is set for a polarized light beam to be emitted on the plasmonic device 100.

[0083] In a varying step 606, the incident angle of the emitted light beam is progressively modified.

[0084] In a detecting step 608, a light beam reflected from the plasmonic device 100 is detected until a total reflection angle θ′ producing a total reflection of the polarized light beam is obtained. This value θ′ is important since it indicates the proximity of a maximum amplification condition.

[0085] In a tuning step 610, the total reflection angle θ′ is increased until a minimum reflectivity is reached. By doing so, amplification of the magneto optical signal is optimized since plasmon resonance around the minimum reflectivity. This step 610 can be viewed as a reflectivity vs angle scan.

[0086] The above sequence of steps 606, 608 and 610 can be considered a coarse adjustment and permits quickly finding an enhanced amplification of a magneto-optical signal from the sample 200. In particular, to a MOKE signal.

[0087] Optionally, several supplementary steps may be carried out for a fine adjustment that enables further amplification of the magneto optical signal. Such a fine adjustment may be achieved as follows.

[0088] In a sample distance adjusting step 612, distance of the sample 200 with respect to the plasmonic device 100 is modified, until a maximum plasmonic field is generated.

[0089] An additional o alternative fine adjustment to 612 can also be obtained by a core distance adjusting step 614 in the plasmonic device 100, where the width of an internal dielectric element is modified until a maximum plasmonic field is generated. Thus the inner structure of the plasmonic device 100 is changed. By analyzing reflectivity vs dielectric width, (e.g. using an amplifier 521) a further amplification can be obtained. If the plasmonic device 100 includes a dielectric layer with piezoelectric properties, a voltage may be applied to modify its width.

[0090] FIGS. 7A and 7B show angular reflectivity and TMOKE signals obtained in experimental tests depicted as solid spheres in the Otto configuration while solid line indicates theoretical prediction. FIG. 7A correspond to reflectivity measures while FIG. 7B shows the variation of the TMOKE signal as function of angle. These results are compared with the theoretical prediction depicted in solid line.

[0091] FIG. 8 shows the actual amplification of the magnetic hysteresis loops obtained in experimental tests for the sample under the influence of plasmon resonance fields (solid spheres) and in the absence of plasmon fields (hollow spheres) where the sample has been directly illuminated with light.

[0092] FIGS. 9A and 9B show the reflectance and corresponding TMOKE signal vs angle that depicts how it changes as function of the angle of incidence with a Lock-in technique.

[0093] FIGS. 10A-10G show a sequence of how the hysteresis technique is applied in a sequence of seven reflection angles θ. It can be seen how the reflectance and magnetic field B change in dependence upon the value of reflection angle θ.

[0094] At angles of 43.2° and 58.4° in FIG. 10C and FIG. 10G there is a drastic reduction of the reflectance for any magnetic field.

[0095] On the other hand, at an angle of 46° in FIG. 10E there is a great increase of the reflectance for any magnetic field.

[0096] This particular behavior is used for finding a maximum amplification.

[0097] It is to be understood that the specific embodiments and applications of the concepts disclosed herein are merely illustrative. Numerous modifications may be made to the present teachings without departing from the spirit and scope of the disclosure.