Micro/nano Dielectric Structures For Luminescence Modulation Of Quantum Dots

Fluorescent substances are widely used in biosensing,display,anti-counterfeiting,energy and communication fields,so have an important impact on people's lives.With the development of micro/nano photonics,the modulation of micro/nano structures on the light-emitting process of fluorescent materials has received extensive attention.The micro/nano structures can affect the emission of fluorescence through a unique electromagnetic action mode,which will greatly improve the applicability of the fluorescent substances and has a revolutionary effect on the fluorescence technology.The specific micro/nano structures not only enhance fluorescence,but also inhibit the emission of fluorescence.In terms of fluorescence enhancement,the micro/nano structures not only increase the total radiant energy of the fluorescence,but also modulate the fluorescence toward the signal detector by changing the radiation distribution pattern,thereby finally achieving the directional fluorescence enhancement.In terms of fluorescence suppression,the micro/nano structures can weaken the fluorescence radiation by a fluorescence quenching phenomenon or a special dielectric resonance mode.The noble metal micro/nano structures based on plasmon resonance are conventionally used to modulate the fluorescence emission.However,the inherent defects of metal limit its further application.Therefore,the use of low loss dielectric micro/nano structures to achieve the luminescence modulation of fluorescent materials has become the focus of research.In order to optimize the fluorescence modulation performance of dielectric micro/nano structures,three micro/nano structures based on dielectric materials are studied in this paper.First,a tubular microfluidic channel formed by the enveloping of two dielectric microlenses is proposed.When the quantum dots?QDs?are inside the microfluidic channel,their directional luminescence intensity is enhanced.Thus,when the biological sample solution bound to the QDs flows through the microfluidic channel,high-sensitivity fluorescence detection of the biological sample can be achieved.By the finite difference time domain?FDTD?method,it is first studied that the effect of thickness of two microlenses in the microfluidic channel and refractive index of the microlens on the directional emission enhancement of fluorescence.Next,the enhancement of the fluorescence excitation rate of QDs in the structure is studied in the fluorescence excitation process by means of plane wave illumination.When both microlenses have a thickness of 1?m and a refractive index of 1.5,the best directional emission enhancement effect can be obtained.For the quantum dot on the vertical central axis of the microfluidic cavity and 0.8?m from the bottom of the microfluidic cavity,the directional enhancement factor in the fluorescence collection angle of 8.8°is about 378;for quantum dots at different positions in the microfluidic cavity,the directional enhancement in the direction of 90°above the substrate is about 15 times.Secondly,a dielectric spheres hybrid nano-antenna composed of silicon nanospheres dimer and TiO₂ microsphere is proposed.The directional emission enhancement of a single quantum dot?QD?can be realized by the hybrid nano-antenna,and the nano-antenna has a promising application in the light-emitting devices such as a single photon emission source.By the FDTD method,the quantum yield enhancement,fluorescence collection efficiency enhancement and fluorescence excitation rate enhancement are all studied to illustrate the fluorescence enhancement effect of the hybrid nano-antenna.The results show that the hybrid nano-antenna can enhance the quantum yield of CdSe QDs by about 4 times and the fluorescence collection efficiency by about2 times.And due to the superposition of enhancement effect of the silicon nanospheres dimer and the TiO₂ microsphere on the fluorescence excitation process,a directional fluorescence enhancement of about 3064 times can be finally obtained.Finally,a 3D silicon nanostructure composed of 12 silicon nano-cylinders is proposed.When the QDs with a specific wavelength are placed in such a hollow 3D silicon nanostructure,their luminescence is suppressed due to the excitation of the anapole resonance mode.By using this luminescence suppression effect,we can expand the ratio of the two-wavelength radiation intensity of dual-emitting QDs,thereby improving the sensitivity of the fluorescence ratiometric sensing.In application,to achieve high sensitivity ratiometric fluorescence sensing,many such 3D silicon nanostructures can be placed in the biological sample solution bound to the dual-emitting QD fluorescence probes.By the FDTD method,the anapole resonance mode of the3D silicon nanostructure at 621 nm is first studied.Then,we choose another reference wavelength of 582nm and combine it with 621nm to form the two emission wavelengths of dual-emitting QDs.The effect of the 3D structure on the ratio of two-wavelength radiation intensity is next studied.It can be obtained that when the QD is 0 nm,25 nm and 50 nm from the center of the 3D structure,the double radiation intensity ratio can be enhanced by a factor of about 14.3,6.1,and 2.0,respectively.