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

Abstract

Abstract:The combination of near-field microscopy along with nonlinear optical spectroscopic techniques is presented here. The scanning near-field imaging technique can be integrated with nonlinear spectroscopic techniques to improve spatial and axial resolution of the images. Additionally, ultrafast dynamics can be probed down to nano-scale dimension. The review shows some examples for this combination, which resulted in an exciton map and vibrational contrast images with subwavelength resolution. Results of two-color femtosecond time-resolved pump-probe experiments using scanning near-field optical microscopy (SNOM) on thin films of the organic semiconductor 3,4,9,10 Perylenetetracarboxylic dianhydride (PTCDA) are presented. 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 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.

Organic Optoelectronic Materials: Mechanisms and Applications

“…Materny and co-workers , developed a near-field pump–probe (PP-NSOM) spectroscopy, which they applied to PTCDA films. In PP-NSOM, pump and probe laser pulses, time-delayed with respect to each other, are collinear and are incident on the sample via a microscope objective.…”

“…In the near-field CARS, the signal is detected through the NSOM tip. The authors were able to demonstrate NSOM-CARS imaging of P3HT films and surface-enhanced CARS in pyridine films …”

“…795,845,846 recent studies of organic semiconductors, NSOM was used to establish the effects of aggregation on the device performance; 847 it enabled domain spectroscopy in domains with different molecular order and crystallinity in oligomeric PQT-12 films 848 and in P3HT:p(NDI2OD-T2) copolymer blends, 847 and it revealed various polymorph phases in rubrene microcrystals. 849 Materny and co-workers 850,851 developed a near-field pump− probe (PP-NSOM) spectroscopy, which they applied to PTCDA films. In PP-NSOM, pump and probe laser pulses, time-delayed with respect to each other, are collinear and are incident on the sample via a microscope objective.…”

“…The authors were able to demonstrate NSOM-CARS imaging of P3HT films and surface-enhanced CARS in pyridine films. 851 7.2.2. Optical Absorption and PL Imaging.…”

Near‐Field Nano‐Optical Imaging of van der Waals Materials

“…[52][53][54] For instance, pulse-broadening due to the optical nonlinear and dispersion effects of the fiber-based probe complicates dynamic studies as it alters the ultrafast (e.g., femtosecond) optical pulses, where the periodic thermal expansion/contraction of fiber probe exacerbates time-resolved SNOM applications by generating artifacts in the near-field signal. [50,[55][56][57] Subsequently, the introduction of scattering-type scanning near-field optical microscopy (s-SNOM) and then the realization of the technique by Wickramasinghe and co-workers [58] in 1995 [52] enabled optical imaging beyond the diffraction limit at the atomic resolution by accessing evanescent waves. While SNOM is limited to specific wavelength regions considering the transmission coefficient, the apertureless approach of s-SNOM widens the application range from visible to infrared light.…”