Magneto-optical Kerr Reversal In Magnetic Nanogratings And The Research On Reversal-based Sensors

With the development of nano-micromachining technology,researchers are expected to tune the interaction of light and matter on the nanoscale.Based on the large electromagnetic field enhancement of surface plasmons and the sensitivity to surrounding dielectric medium,plasmon-based sensors are developing rapidly.At present,researchers are studying on the use of plasmon to make biosensors to detect biological proteins and antibodies of cancer cells,etc.The figure of merit(FOM)is an important parameter in the sensor design.In the current research,researchers adopted multiple methods to improve FOM of sensors.However,researchers mainly use light intensity as the sensing signal and ignore the phase of the light.In the physical,the light intensity is always greater than zero,so no matter what method is adopted,the full width at half maximum(FWHM)can't reach to zero,which limits the further improvement of the FOM.In fact,the FOM can be further improved by introducing magnetoplasmons to make a light phase-based sensor.However,current light phase-based sensors mainly use the randomly distributed nanodisk structures.Although to some extent the FOM has been improved,the improvement of the FOM is still limited due to the small slope at the phase zero point.In addition,one of the fatal flaws in current study is that the reflectance or transmittance of the light at the Kerr zero point is minimal,which is detrimental to the sensor fabrication.Therefore,theoretically and experimentally this paper discusses how to further improve the FOM from three aspects:1.Tuning the magneto-optical Kerr reversal in magnetic nanogratingsObtaining a magneto-optical phase reversal point and obtaining a large slope near the reversal point are prerequisites for making an excellent magnetoplasmon-based sensor.However,in many cases,the magnetoplasmon can only bring about the enhancement of magneto-optical Kerr effect but a reversal point.And it is still difficult to obtain a large slope near the reversal point even if the reversal point is achieved.In terms of structure,we abandoned the conventionally used magnetic nanodisk structure and instead used a one-dimensional magnetic nanograting.We calculated SPP excitation wavelength by the SPP wave vector matching conditions.In addition,the relationship between the resonance intensity and the cross section of the grating was calculated by Comsol,which indicates the avenues of the experiment.Then using the interference lithography method,a sample with certain period and cross-sectional shape was prepared.All the two samples achieved Kerr zero and achieved a large slope at Kerr zero.Our slope at the Kerr zero point is improved by 4 orders of magnitude compared to the slope obtained in the literature which uses randomly distributed nanodisk structures.2.Enhancing the figure of merit of refractive index sensors by magnetoplasmons in one-dimensional nanogratingsThe further improvement of the FOM of the light intensity-based sensor will be limited,so we give up using light intensity as the sensing signals,but instead using the magneto-optical Kerr angle as the sensing signals.So one-dimensional periodic magnetic nanogratings with a period of 285 nm were prepared by using interference lithography.And a Kerr zero was obtaind by our nanogratings.In addition,magnetron sputtering was used to sputter materials with different refractive index.We eventually obtained a FOM of up to 1728/RIU by detecting the Kerr null point,which is one order of magnitude larger than the results reported in the literature.In addition,our sensing system exhibits a good linear relationship over a wide range of refractive index.Moreover,we studied the relationship between the Kerr null point and the thickness of the dielectric layer.The results show that the sensitivity based on thickness detection can reach 1.6.Theoretically and experimentally,we have confirmed that the Kerr null point originates from the resonant coupling of the localized surface plasmons(LSPs)and surface plasmon polaritons(SPPs).3.Separation of magnetic and optical signals and the sensing researchWhether for traditional randomly distributed nanostructures or periodic structures,the big disadvantage of current research is that the position of magneto-optical enhancement or reversal often corresponds to the position where the reflectance is the smallest.Due to the asymmetry of the grating,the reflectivity is still large when the incident light is p-polarized the strip extending direction is parallel to the incident surface.If one can still get Kerr zero under such incident configurations,then the problem has been solved.Since the dipole oscillation in the vertical direction affects the magneto-optical effect,and the dipole oscillation in the original direction mainly determines the optical effect,theoretically,there is such a possibility.Experimentally,we prepared samples with appropriate parameters by adjusting the width of the gratings by positive resist lithography.When the incident light was p-polarized light and the strips extending direction were parallel to the incident surface,the Kerr zero point was obtained by our nanograting and the reflectivity near the zero point was still as high as 0.4.The sensitivity of the Kerr null point to the surrounding medium shows that the Kerr zero point has a redshift when the refractive index of the medium increases.Moreover,we adjusted the depth of the strip by negative resist lithography,and also obtained the Kerr zero point in the same incident configuration(the reflectivity near the zero point is still as high as 0.25).Similarly,we studied the change of Kerr null point with the surrounding dielectric medium.It shows that the Kerr zero is also sensitive to the refractive index of the surrounding dielectric medium.