Research On Anti-counterfeiting Tri-color Shift Device Based On Subwavelength Structure

The subwavelength bi-color shifting technology has been applied in anti-counterfeiting field successfully for more than ten years.However,with the popularity of bi-color shifting technology,its anti-counterfeiting function has declined.Our research is focused on the subwavelength tri-color shifting anti-counterfeiting technology.First,the diffractive property of the dual-layer waveguide grating is analyzed.Then,a tri-color shifting device based on a subwavelength one dimensional?1D?single-period rectangular structure is designed by overcoming the multi variables and the polarization property of 1D subwavelength gratings.Finally,the tri-color shifting device is produced by the micro-nano-fabrication and vacuum evaporation.Such a device realizes an azimuth-tuned tri-color shift for natural light using a simple structure.All these break through the limit of subwavelength filter technology in terms of design difficulty,optical performance and visual effect.The main research results are as follows.1.A tri-color shift device based on an embedded 1D single-period subvavelength rectangular structure is proposed,and the diffractive property of the device is numerically analyzed.Because the commercial software Rsoft dosen't support the optimizing of subwavelength structure,which is turned by azimuths and incidented simultaneously by TE and TM polarizations,the calculation of diffraction efficiency is programed.The program lays the foundation for optimal designing of subwavelength tri-color shift device in the next step.2.The tri-color shift device based on Si₃N₄ and ZnS film and polyethylene terephthalate?PET?polyester substrate are designed respectively.The structure parameters and azimuths producing blue,green and red lights are optimized by combining RCWA and genetic algorithm?GA?.Meanwhile,the natural light is considered as a superposition of TE and TM polarizations and the scientific and effective evaluation function is established.Simulation results show that,with a specific incident angle,high reflection efficiencies can be achieved respectively in blue,green and red region for both TE and TM modes..3.Dispersion characteristics of the tri-color shift device are analyzed.Analysis shows that the dispersion mainly affects the locations of resonance reflection and has less impact on their efficiencies.Since the offsets of resonance peaks are in desired wavebands,visual effects of the device are not affected.Moreover,the laws of infulence of key parameters on reflection characteristics are exposed.That is,with the increase of device period,groove depth,coating thickness,and the decrease of incident angle,the reflection peaks of blue,green,and red lights get red shift.Refletion peaks are especially sensitive to period,so periodic error should be controlled strictively during fabrication.Also,the tolerance of the key parameters are investigated.The results prove the producing feasibility and provide the guidance to design and manufacture of the device.4.The tri-color shift device with Si₃N₄ film is fabricated and tested.During the fabrication,errors are controlled depending on the tolerance of the parameters,and the coating thickness is optimiaed by the actual coating technology.Test results show that structure parameters are all in tolerance scope.With the natural incident light,the device exhibits blue-green-red color shifting by changing the azimuths.Testing results are consistent with simulation results.5.The physical mechanism of the tri-color shift device is explained.When a parallel whitelight is incident on a wave-guide grating,the high-order spatial sub-waves are coupled and propagate through the wave-guide layer expect the zero-order.When guided waves interact with the grating layer,they diffract out of the layer.Under specific conditions,the destructive interference happens between the zero order transmission lights and the leaky guided waves.This leads to a guided mode resonance.Further analysis shows that the equivalent refractive indices of the subwavelength grating are dependant on the azimuths of incident light.So when the azimuth changes,the resonant wavelength will also change which corresponding to different equivalent refractive indices.When the grating parameters and the incident angle change,the reflective peaks get red or blue shift.Physically,this is due to coupling of the externally propagating diffrated fields to the modes of the waveguide.When some guides modes without maching phase enter the external diffraction field,the response degradate,which is shown as reflection degradation.Compared with the bi-color shift technology,the tri-color shift device proposed in this thesis has higher design difficulty,better optical performance and more abundant visual effect.Its simple structure is useful for fabrication of high-precision master mask.And it can be mass produced at low-cost by matured embossing and coating technique.The general public can conveniently observe the unique visual effects in daylight.Also,the trained examiners can readily identify the distinctive spectral behaviours by machine.Thus,the tri-color shift device may have great theoretical significance and application prospects in field of the optically variable image security.