Selective Excitation on Tip-Enhanced Raman Spectroscopy by Pulse Shaping Femtosecond Laser

In this paper, we propose a scheme for achieving ultrafast coherent control of the selective excitation among three excited states in stimulated Raman scattering process on a tip-enhanced Raman spectroscopy (TERS). The center frequencies of the pump and Stokes laser pulses are 14,000 cm−1 and 12,500 cm−1, and their spectral bandwidths are both 700 cm−1. By properly modulating the spectral phase distribution and cutting the frequency components, the stimulated Raman transition probabilities of two excited state keep maximal, while the other one can be suppressed to zero. The shaped pump and Stokes pulse irradiate obliquely into the TERS nanostructure containing a single layer molecule. The impulse response in temporal and frequency domain is calculated by using finite-difference time-domain (FDTD) simulation followed by Fourier transform. The frequency components and the relative phase are same with the corresponding input pulses, but the intensities are enhanced by more than 10 times. Compared with the case without the TERS nanostructure, the probability of selective excited Raman transition increases by more than 4 orders of magnitude, and the selective depressed Raman peak keeps at 0.