Ultrafast Coherent Control Of Tip-enhanced Raman Spectra And Fano-resonance-enhanced Coherent Anti-stokes Raman Spectra

Raman spectra can obtain the information of molecular vibration and rotation by analyzing the scattering spectrum whose frequency is different from the incident light.It is widely used in the study of molecular structure,and it has effectively promoted the development of chemistry,physics,biology,medicine and other fields.However,due to the weak Raman spectrum signal,many kinds of Raman spectrum enhancement technologies have been studied,including surface-enhanced Raman spectroscopy（SERS）,tip-enhanced Raman spectroscopy（TERS）and coherent anti-Stokes Raman spectroscopy（CARS）.Multiple peaks are usually excited simultaneously in enhanced Raman spectroscopy.In addition to the background noise,the adjacent peak will also produce great interference to the target peak,and it is strong noise.How to improve the signal-to-noise ratio of Raman spectrum is a key in this field.By femtosecond pulse shaping and coherent control,the selective excitation of target Raman peaks and the suppression of adjacent peaks can be realized,and the signal-to-noise ratio can be improved.Unfortunately,ultrafast coherent control to realize the selective excitation and surface-enhanced Raman spectroscopy to realize the signal enhancement are two independent research directions.In order to improve the intensity of the target Raman peak,to suppress the adjacent Raman peak and to improve the signal-to-noise ratio of the Raman spectrum,the combination of the surface-enhanced Raman spectroscopy of metal nanostructures and the Raman spectrum coherently controlled by shaping femtosecond laser pulse is proposed,and the ultrafast selective excitations of TERS and CARS are theoretically studied.The main research results are as follows:1.Combining ultrafast coherent control with surface-enhanced Raman spectroscopy,selective excitation of two among the three excited states in tip-enhanced Raman spectroscopy is studied theoretically by shaping femtosecond laser pulse.The pump pulse shaped byπ-phase scanning and Stokes pulse shaped by rectangleπ-phase modulation and rectangle cutting are transformed into time domain,illuminate the TERS nanostructure containing single-layer molecules obliquely,and the time evolution of the electric field of the probe point in the gap of the TERS structure is calculated.The time domain,frequency spectrum and phase distribution of the response pulses are discussed respectively.It is found that the frequency components and relative phase are basically the same as the corresponding input pulses,but the intensities are enhanced by more than 10 times,and the probabilities of Raman transition are increased by more than 4 orders of magnitude.The results show that the selective excitation and enhancement of two among the three excited states in stimulated Raman scattering can be realized simultaneously by shaping femtosecond laser pulse in the TERS structure.2.The selective excitation and enhancement of one among the three Raman peaks is more difficult than that of two among the three Raman peaks,and the modulation methods of pump and Stokes pulses are more complex.The different methods of pulse shaping and ultrafast coherent control are further studied.The pump and Stokes pulse shaped by the rectangularπ-phase modulation and spectrum cutting are transformed into time domain,discretized and reconstructed,and then irradiated obliquely to the TERS structure containing single-layer molecules.The spatial distribution of transient electric field on the TERS structure,the time domain,frequency spectrum and phase distribution of response pulses at the different probe points are calculated.Through comparison,it is found that the spectra of the response pulses oscillate and broaden,which is caused by phase modulation and spectrum cut-off.The phases of the response pulses are shifted as a whole,but the relative phase among frequency components remains unchanged.The results show that by shaping the pump and Stokes laser pulses,the enhancement and selective excitation of one among the three Raman peaks can be realized simultaneously in the TERS structure.3.In order to further enhance the signal intensity,to improve the signal-to-noise ratio and to realize the selective excitation of narrow spacing energy levels at the single molecule detection level,selective excitation and enhancement of CARS are studied theoretically.Optimizing the plasmonic substrate to ensure that all the hot spots of the pump,probe,Stokes,and anti-Stokes light are at the same position is the key to increase the CARS signal to reach the level of single molecule detection.At the same time,the selective excitation of the target CARS peak and the suppression of the adjacent peaks are the key to improve the signal-to-noise ratio.Therefore,the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure are studied theoretically by shaping pump,Stokes and probe femtosecond laser pulse.Firstly,by optimizing the phase modulation and spectrum cutting of the pump,Stokes,and probe pulses,one CARS peak is maximized,while the other two are suppressed to zero.Then by designing and optimizing the size of the disk-ring structure,it is ensured that Fano resonance is applied to simultaneously enhance the surface plasmon modes of the four lights,and that all the hot spots are located at the same position in the disk-ring gap with a deviation distance of less than 2 nm.The intensity of CARS is enhanced by 1.43×10¹² times,which is much higher than the requirement of single molecular detection.The time,frequency,and phase distribution of the input and the response of the four pulses are studied in detail.It was found that the selective excitation and spectra of CARS are both well preserved.Through the combination of ultrafast coherent control and surface-enhanced Raman spectroscopy,we have theoretically realized the selective excitation and enhancement of two or one among the three Raman peaks in TERS,and those of any one of the three CARS peaks based on Fano resonance of disk-ring structure.These theoretical results have guiding significance for improving the intensity and signal-to-noise ratio of Raman signal in experiment.