Theoretical Study On Fano Resonance/Spaser For The Detection Of Single Molecules And Single Nanoparticles

Surface plasmon polaritons,with the compact storage of optical energy in electron oscillations concentrated at a metal/dielectric interface,are the key to breaking down the diffraction limit of conventional optics,with large electromagnetic field enhancements,high photothermal conversion efficiencies,and rich spectral responses.They have opened up a wide range of application,from nonlinear optics,lasing,biosensing,ultrafast dynamics,to solar cells and so on,with the birth of an important branch of nanooptics,known as plasmonics.We mainly study the property of Fano resonance caused by plasmon coupling and spaser being one kind of plasmonic nanolasers.Their applications for the label-free detection of single molecules and single nanoparticles are also investigated.The results are as follows:1.We study on the narrow Fano resonance of a 3D nanocrescent and its application in single-molecule detection.The Fano resonance is attributed to the interference between the quadrupolar mode supported by the horizontal crescent and the dipolar mode supported by the nanotip oscillating along the height direction.The Fano-shape resonance is closely related to the gap size and height and highly depends on the incident light polarization.The linewidth of Fano dip is as narrow as 10 nm as the height of 3D nanocrescent is 30 nm.The narrow linewidth is caused by the strong narrow resonant absorption coming from the dipolar mode of nanotip overlapping with the quadrupolar mode of nanocrescent.The narrow Fano resonance is highly sensitive to a single nanoparticle trapped by the nanocrescent.The wavelength shift is 4.03 nm as a protein nanoparticle with radius of 2.5 nm is trapped by the nanocrescent.The wavelength shift is larger than the detection limit(10-5 nm)by 5 orders in the magnitude,which can provide a real-time label-free detection system for a single biology/dielectric nanoparticle in the future.2.We report spasers based on multipolar Fano resonances in disk-ring nanostructures(DRN)covered with a silica layer doped with gain material of ytterbium–erbium.The electric field amplitudes at the quadrupolar mode(lasing wavelength)and the octupolar mode(pumping wavelength)are simultaneously enhanced by tens of times.Moreover,the spaser operates in a dark mode,which can reduce the radiation loss and enhance the confinement effectively.These factors work together to greatly decrease the critical gain coefficient and threshold.By adjusting the elliptic partial degrees of the nano-ring,the spasers can be tuned in the range of 1550 to 1650 nm,while the pumping light remains at the 980 nm absorption band of Yb3+ ions.Moreover,the spasers at three Fano resonance wavelengths of the disk-ring nanostructures appear in sequence with increasing the gain coefficient,which may be beneficial to the spaser applications.3.We study a lasing plasmon based on the multipolar resonances of a split-ring filled with silica doped with Yb3+:Er3+ ions as gain material and the detection of single molecule and single nanoparticle.The lasing mode(1500 nm)is far away from the pump mode(980 nm),which can depress the detection noise from the pump light.The hot spots of the two modes overlap well in spatial distribution and are enhanced simultaneously.These factors work together to greatly decrease the critical gain coefficient and threshold.The lasing plasmons have special benefits for biomolecular detection.The shift is 0.031 nm when a protein nanoparticle of r=1.25 nm is attracted in the SRR gap.A small molecule with several atoms can be detected by this method under the experimental detection limit of spectral shift of 10-5 nm.Moreover,the lasing intensity is also very sensitive to the trapped nanoparticle,which indicates the simulation will be a good guidance for the biosensing.