Sensitive Detection,ultrafast Coherent Control And Periodic Ripples Formation Based On Surface Plasmon Enhancement

Surface plasmon resonance is a kind of surface bound electromagnetic wave formed by free electrons on the surface of material under the action of external incident light field.Because of its strong local field enhancement,it has a wide range of applications in biosensor,single molecule detection,nano lithography and so on.The surface plasmons are mainly divided into local surface plasmons and propagating surface plasmons.In terms of the properties and applications of the local surface plasmon,the ultrafast selective excitation of the plasmon ruler and the surface enhanced Raman spectrum is studied.In the aspect of propagating surface plasmons,the femtosecond laser excites the surface plasmons on the semiconductor surfaces,which results in the periodic distribution of light field intensity and the periodic deposition of energy,forming subwavelength periodic ripples.The main research results are as follows:?1?A Fano resonance plasmon ruler based on the trimer nanostructure is designed.It consists of a concentric square ring disk and an external nano cube.The coupling,Fano resonance intensity,linewidth and contrast of nanostructures with different plasmon modes are studied by adjusting the gap between the concentric ring and the outer cube,or by fixing the center of the outer cube to rotate in a fixed axis.The results show that the sensitivity of the ruler based on the Fano resonance is 20-80 times higher than that of the ordinary one.It can realize the high-precision measurement of 1? translation and 1o rotation of the external nano cube.?2?Raman spectroscopy,which provides information about the vibration and rotation of molecules,is called“molecular fingerprint”.It is widely used in chemistry,physics,biology and medicine.Surface enhanced Raman scattering has been attracted wide attention as an important technology for studying and monitoring molecular behavior.Shaping femtosecond pulse is an effective method for coherent control of molecular vibration and rotation.Based on the femtosecond pulse shaping technique and the coherent control theory,we analyzed and studied the selective excitation conditions of two Raman levels of single porphyrin molecules by?phase modulation and spectral shear of the pump pulse and Stokes pulse.The selective excitation of 327 cm^-1 and the suppression of 367 cm^-1 are realized.The ultrafast dynamics of the enhanced Raman spectra of the single porphyrin in the middle of the silver nanosphere dimer was studied by using the numerical simulation of Comsol and Fourier transform.The results show that our theoretical model achieves both Raman enhancement and selective excitation.The surface-enhanced Raman scattering probability is enhanced by more than 6 orders in magnitude compared with the case in air environment,and the FWHMs of the enhanced and the depressed Raman peaks are both larger than17 cm^-1 despite of the narrow gap of only 40 cm^-1 between the two Raman peaks.?3?By optimizing the phase and amplitude modulation of the incident pump and Stokes pulse,the selective excitation of any one of the three Raman energy levels of 150 cm^-1,250 cm^-1 and 350cm^-1 is realized.By adjusting the size of the nano-crescent structure,the Fano dip can be tuned to the wavelength near the center of the 800 nm.In the range of 610 cm^-1,the electric field is enhanced greatly.It is found that the electric field of the pump pulse and the Stokes pulse increases more than500 times,while the relative phase of different spectral components remains unchanged.?4?By using the ultrafast pump-probe imaging technology,the formation process of subwavelength periodic ripples on the GaP surface with nanogrooves under the irradiation of a single 800 nm,50 fs femtosecond laser pulse are observed.It is found that the formation of periodic ripples can be observed in tens of picoseconds after the pump pulse reaches the sample surface,and then the ripples start to disappear due to the ablation,and disappear completely in hundreds of picoseconds,leaving an overheated transient solid surface.After hundreds of picoseconds,new ripples appear and gradually become clear.Some of the ripples after solidification,and the period of the ripples is equal to that of the transient ripples.In order to understand the mechanism of periodic ripples induced by femtosecond laser pulses,the surface plasma thin layer is used as a multilayer model.The dynamics of carrier excitation,carrier temperature,lattice temperature and the temporal and spatial evolution of transient dielectric constant are studied by using the two-temperature model and the Drude model.The theoretical results show that the periods of electric field distributions at different depths of the plasma layer are the same,and agree well with the experimental results.The formation of the two kinds of transient ripples and the remained ripples are all related to the periodic energy deposition due to the SPP excitation at the air-plasma interface.