  Coherent anti-Stokes Raman scattering (CARS) and, in particular, femtosecond
adaptive spectroscopic techniques (FAST CARS) have been successfully used for
molecular spectroscopy and microscopic imaging. Recent progress in ultrafast
nanooptics provides flexibility in generation and control of optical near
fields, and holds promise to extend CARS techniques to the nanoscale. In this
theoretical study, we demonstrate ultrafast subwavelentgh control of coherent
Raman spectra of molecules in the vicinity of a plasmonic nanostructure excited
by ultrashort laser pulses. The simulated nanostructure design provides
localized excitation sources for CARS by focusing incident laser pulses into
subwavelength hot spots via two self-similar nanolens antennas connected by a
waveguide. Hot-spot-selective dual-tip-enhanced CARS (2TECARS) nanospectra of
DNA nucleobases are obtained by simulating optimized pump, Stokes and probe
near fields using tips, laser polarization- and pulse-shaping. This technique
may be used to explore ultrafast energy and electron transfer dynamics in real
space with nanometre resolution and to develop novel approaches to DNA
sequencing.
