Patent Publication Number: US-2021190682-A1

Title: Plasmonic device, system and method

Description:
FIELD 
     The present disclosure generally refers to plasmonic techniques for use in optics and magneto optics. 
     In particular, the present disclosure proposes amplifying a magneto-optical signal as result of placing a sample in contact or close to a particular plasmonic device. 
     BACKGROUND 
     The interaction of electromagnetic field with electrons at metal dielectric interface leads to the so-called Surface Plasmon-Polariton (SPP) resonance. This is the result of the frequency and linear momentum match of the evanescent incident radiation and the electrons collective oscillation at the interface. Advantageously, SPPs enhance the magneto-optical activity of nano-structured or layered systems. 
     Such an enhancement can be used for magneto optical signals. In the prior art, two configurations are mainly used for optically producing SPP. The first is Otto&#39;s configuration rarely found in the literature. The second is Kretschmann&#39;s configuration most commonly used. 
     Both configurations have a layered structure and can be used to optically excite surface plasmon waves. These configurations produce a giant electric field or plasmonic field, at a layer of a material (sample) located around the plasmon resonance region. An important limitation of these configurations for real applications lies on the fact that said sample needs to be located internally. The sample is placed between two metallic layers, in the Kretschmann&#39;s configuration, which makes it impossible to separate it from the measuring device. 
     On the other hand, when simply using MOKE effect directly to study the magnetic properties, it demands the sample to be directly illuminated with the incident light. This is a further disadvantage. For instance, these configurations cannot be used in case of the sample being in the form of liquid, powder, micro or nano structures, as these disperse the light going out of the sample making very difficult to detect and measure its magnetic properties accurately. 
     Finally, just placing the sample around the plasmonic device adds an additional complication as this modifies the effective dielectric constant of the medium. Depending on the sample thickness, magnetic properties surface and nano structure, the resonance condition is modified, thereby affecting the linear momentum match of the evanescent incident radiation and the electrons collective oscillation at the interface. 
     SUMMARY 
     It would be desirable to overcome or at least mitigate the limitations found in the prior art related to the sample; in particular its location where magnetic materials are part of the plasmonic device and the modification of the effective dielectric constant that modifies the plasmon resonance of the structure. Also, it would be desirable to further improve signal amplification. 
     Thus the main aspect of the present disclosure is aimed at a plasmonic device capable of amplifying an optical signal of an external sample coupled to the device. 
     In an embodiment, the plasmonic device comprises a first dielectric element of a high refraction index. Following the first dielectric element, a second dielectric element of a low refraction index is arranged. The width of second dielectric element can be modified. Following the second dielectric element, a layer of metal is arranged. The sample is to be located near or in contact with the layer of metal. When incident light is received on the first dielectric element, a plasmon resonance can be produced at the interface region between the second dielectric element and the layer of metal. This leads to a plasmonic field generated in the sample. By modifying the width, the plasmonic field can be adjusted. 
     In some embodiments, the first dielectric element is typically a prism. 
     In some embodiments, second dielectric element may be air o vacuum and the plasmonic device may include a frame or support to hold the layer of metal separating it from the first dielectric element. 
     In other embodiments, the plasmonic device may also include additional components. 
     For instance, a third dielectric element of low refraction index and having a modifiable width can be arranged after the layer of metal to generate an additional plasmonic field in the sample. 
     For instance, in addition to the third dielectric element, a fourth third dielectric element of high refraction index can be arranged between the first and second dielectric elements. 
     Another aspect of the present disclosure is a system for detecting a magneto optical signal in a sample is also proposed. The system comprises a plasmonic device as those mentioned above and a holder for placing the sample, an optical circuit to emit a polarized light beam on the plasmonic device. 
     The system allows an indirect observation of the magnetic optic Kerr Effect (MOKE) enhanced by a plasmon resonance field generate at the plasmonic device with a sample located in contact with or close to said plasmonic device. 
     The system also comprises a rotatable support for placing the holder and the plasmonic device at a selectable angle with respect to the emitted light beam. The rotatable support facilitates obtaining a total reflection of the light on the plasmonic device. The optical circuit also includes a photodetector placed on a further rotatable support. Thus, the photodetector may detect light exiting from the plasmonic device. The magneto optical signal may be produced from the sample exposed to a plasmonic field generated by the plasmon resonance. Advantageously, the system can detect the magneto optical signal that is generated and also allows select the incident angle, the distance between the sample and the plasmonic device and even the inner structure of the plasmonic device itself by means of modifying width in a dielectric material in the plasmonic device. 
     In an embodiment, the system also includes one or more electromagnets for applying an external magnetic field. Depending on the electromagnet(s) field orientation, different types of signals can be produced. If the field orientation is perpendicular to the plane of incidence, a transversal magneto optical Kerr-Effect (TMOKE) signal is obtained. If parallel to the plane of incidence and to the external side of the plasmonic device, a longitudinal magneto optical Kerr-Effect (LMOKE) signal is obtained. If parallel to the plane of incidence and perpendicular to the external side of the plasmonic device, a perpendicular magneto optical Kerr-Effect (PMOKE) signal is obtained. 
     A further aspect of the present disclosure is a method for obtaining an amplified magneto optical signal from a sample using the plasmonic device as those mentioned above. 
     The method comprises placing the sample in a holder, setting an incident angle theta for a polarized light beam and detecting at a two theta angle the reflected light on a photodetector. Setting the distance between a metal layer internal surface and a first dielectric element of a plasmonic device by adjusting the width of a second dielectric. 
     The method further comprises applying a coarse adjustment and, optionally, a fine adjustment. In the coarse adjustment, a rotatable support and photodetector are used to produce a theta two theta scan reflectivity measurement. The rotatable support is coupled to the holder for varying the incident angle of the emitted light beam. On the other hand, the photodetector may detect the amount of reflected light. Thus, a total reflection of the polarized light beam can be reached. Then, by slightly increasing the incident angle over the total reflection angle, a minimum reflectivity can be detected with the photodetector. When the minimum reflectivity is achieved, a plasmonic field is generated which amplifies the magneto optical signal from the sample. The distance between the metal layer internal surface and the first dielectric is varied and a theta two theta reflectivity scan is performed. The former procedure is repeated self consistently until the minimum angle of reflectivity is found. 
     For the fine adjustment MOKE signal is obtained and the above theta two theta scan procedure is repeated until the maximum MOKE signal is found. 
     The present teachings also allow a better characterization and study of the MOKE signal as function of the incident light angle around and at the peak of plasmon resonance. 
     According to the present teachings, among many other possible applications, these techniques can improve standard MOKE characterization techniques including Magnetic Surface Microscopy, sensing of magnetic elements, detection of micro and nano particles, quality control and reading of memory storage devices and Magnetic Spectroscopy. 
     More in particular, retrieval of magnetic hysteresis loops, characterization of magnetic properties as function of magnetic field, detection of magnetic response as function of magnetic field and related. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIGS. 1A-1H  illustrate different embodiments of a plasmonic device. 
         FIG. 2  illustrates a plasmonic generation using an embodiment of the plasmonic device. 
         FIGS. 3A-3C  illustrate different types of generation of magneto-optic plasmonic Kerr effects. 
         FIG. 4  illustrates an embodiment of a plasmonic device used in experimental tests. 
         FIGS. 5A-5B and 5C  illustrate schematic diagrams of two embodiments of a system according to the disclosed teachings. 
         FIG. 6  is a flow diagram of steps of a method according to the disclosed teachings. 
         FIGS. 7A-7B  show angular reflectivity and TMOKE signals obtained in experimental tests and theoretical prediction for a sample in Otto configuration. 
         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. 
         FIGS. 9A-9B  show reflectance and corresponding TMOKE signal vs angle that demonstrates how it changes as function of the angle of incidence. 
         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 
     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. 
       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. 
       FIG. 1B  schematically shows an embodiment  102  similar to  FIG. 1A , the main difference lies in there is an upper dielectric element  121   a  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. 
       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. 
       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 . 
       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. 
       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 . 
       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 . 
       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. 
     The usual values for the refraction index in each of the dielectric elements are the following: 
     In  FIGS. 1A-1C : For the upper dielectric element  121 , preferably n≥1.5; for the intermediate dielectric element  124 , 1≤n≤1.5. 
     In  FIG. 1E : For the upper dielectric element  121 , preferably n≥1.5; for the intermediate dielectric element  124 , n˜1. 
     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. 
     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. 
     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. 
     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&lt;1.5, reducing such difference in exchange of lower performance. 
     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. 
       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. 
     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. 
     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. 
     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. 
       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:
         polar direction  301  as in  FIG. 3A , a P-MOKE signal is obtained;   longitudinal direction  302  as in  FIG. 3B , a L-MOKE signal is obtained;   transversal direction  303  as in  FIG. 3C , a T-MOKE signal is obtained,       

       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. 
     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. 
       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 . 
       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. 
     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. 
     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 . 
     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). 
     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 . 
     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. 
     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 . 
     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. 
       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. 
     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 . 
     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 . 
     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. 
     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 . 
     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. 
       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 . 
     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. 
     In an emitting step  604 , an initial incident angle is set for a polarized light beam to be emitted on the plasmonic device  100 . 
     In a varying step  606 , the incident angle of the emitted light beam is progressively modified. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
       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. 
       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. 
       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. 
       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. 
     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. 
     On the other hand, at an angle of 46° in  FIG. 10E  there is a great increase of the reflectance for any magnetic field. 
     This particular behavior is used for finding a maximum amplification. 
     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.