Tip-Enhanced Coherent Anti-Stokes Raman Scattering for Vibrational Nanoimaging

An electric field enhanced by a metallic nanoprobe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in a spatial resolution far beyond the diffraction limit of light. Our tip-enhanced CARS microscope visualized the DNA network structure at a specific vibrational frequency ($\sim 1337\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$) corresponding to the ring-breathing mode of diazole of adenine molecules.

Figure 1

(a) Energy diagram of coherent anti-Stokes Raman scattering process. (b) Tip enhancement of CARS polarization of molecules near the metallic tip in a tightly focused spot.

Figure 2

Schematic of the tip-enhanced CARS microscope.

Figure 3

A spontaneous Raman spectrum of the DNA of poly(dA-dT)-poly(dA-dT). The two frequencies adopted for our TE-CARS imaging are indicated by the downward arrows. The on-resonant frequency at $1337\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$ can be assigned to the ring-breathing mode of diazole adenine molecule in the DNA.

Figure 4

Tip-enhanced CARS images of the DNA clusters. (a) TE-CARS image at on-resonant frequency ($1337\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$) and (b) the simultaneously obtained topographic image. (c) TE-CARS image at the off-resonant frequency ($1278\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$). (d) The same image as (c) shown with a different grayscale. (e) Far-field CARS image of the corresponding area obtained without the silver tip. The scanned area is $500\mathrm{nm}\times 300\mathrm{nm}$. The number of photons counted in 100 ms was recorded for one pixel. The acquisition time was $\sim 3\min$ for the image. The average powers of the ${\omega }_1$ and ${\omega }_2$ beams were 30 and $15\mu \mathrm{W}$ at the 800 kHz repetition rate.

Figure 5

Tip-enhanced CARS images of the DNA network. (a) Topographic image of the DNA network. (b) TE-CARS image at on-resonant frequency ($1337\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$). (c) TE-CARS image at the off-resonant frequency ($1278\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$). (d) Cross sectional line profiles of $y=270\mathrm{nm}$ (indicated by the solid arrows). The scanned area is $1000\mathrm{nm}\times 800\mathrm{nm}$. The number of photons counted in 100 ms was recorded for one pixel. The acquisition time was $\sim 12\min$ for the image. The average powers of the ${\omega }_1$ and ${\omega }_2$ beams were 45 and $23\mu \mathrm{W}$ at the 800 kHz repetition rate.