Scanning near-field optical coherent anti-Stokes Raman microscopy (SNOM-CARS) with femtosecond laser pulses in vibrational and electronic resonance

Abstract

Accessing ultrafast photoinduced molecular dynamics on a femtosecond time-scale with vibrational selectivity and at the same time sub-diffraction limited spatial resolution would help to gain important information about ultrafast processes in nanostructures. While nonlinear Raman techniques have been used to obtain highly resolved images in combination with near-field microscopy, the use of femtosecond laser pulses in electronic resonance still constitutes a big challenge. Here, we present our first results on coherent anti-Stokes Raman scattering (fs-CARS) with femtosecond laser pulses detected in the near-field using scanning near-field optical microscopy (SNOM). We demonstrate that highly spatially resolved images can be obtained from poly(3-hexylthiophene) (P3HT) nano-structures where the fs-CARS process was in resonance with the P3HT absorption and with characteristic P3HT vibrational modes without destruction of the samples. Sub-diffraction limited lateral resolution is achieved. Especially the height resolution clearly surpasses that obtained with standard microCARS. These results will be the basis for future investigations of mode-selective dynamics in the near field.

Nonlinear spectroscopy in the near-field: time resolved spectroscopy and subwavelength resolution non-invasive imaging

“…Unauthenticated Download Date | 5/12/18 6:19 AM electronic and vibrational resonance is capable of yielding highly resolved images of P3HT in different environments [14]. It has to be pointed out, that the goal of our work was not to improve existing CARS imaging techniques, but rather to make an important step towards a timeresolved application in the near-field.…”

“…[14]) was calculated using WSXM software developed by Horcas et al [31]; it is shown in Figure 7C. In order to guarantee that the AFM topography matches the SNOM image as precisely as possible, the images were recorded using a SNOM tip having an aperture of ≈200-300 nm and a metal coating of 0.2 μm thickness.…”

“…The full width at half maximum (≈150 fs) of the near-field and the far-field cross correlation traces reflects the respective instrument response functions, which are equal for microCARS and SNOM-CARS [14].…”

“…In the following, we would like to review some of our own works, which already have been published elsewhere [8,14,15]. Besides this, the review will also give an insight into the state of art in the field of nonlinear optical spectroscopy with sub-diffraction limited spatial resolution.…”

Mapping Sub‐Surface Structure of Thin Films in Three Dimensions with an Optical Near‐Field

“…Scanning near‐field optical microscopy (SNOM) has proved to be a powerful tool to achieve sub‐wavelength resolution in both imaging and fabrication7a,8 of a variety of nanostructures, in particular for its capabilities in obtaining simultaneous topographical and photoluminescence images7b,9—and often further information such as the local photoconductivity, polarization,1a,11 surface plasmons, mechanical stress, Raman signal,13b,14 thickness, dichroism or the chemical composition of the sample. As with other members of the scanning probe family of techniques, SNOM is primarily aimed at surface characterization, although the nature of the evanescent optical field allows a degree of sub‐surface probing.…”