Tip-Enhanced Two-Photon Light Absorption and Emission in/from Soft Materials

Two-Photon Absorption (TPA) is a non-linear optical process in which the simultaneous absorption of two photons takes place in order to promote a molecule from its ground state to excited state^1,2. Because there is a simultaneous absorption of two photons, the probability of such a process is proportional to the square of the light intensity. Two-Photon Emission (TPE) is the follow-on process whereby a molecule or material that has been excited via TPA, then fluoresces light. There are numerous studies on combining TPE with AFM, whereby the TPE signal becomes tip-enhanced, leading to Tip-Enhanced TPE (TE-TPE) signals with a greatly improved spatial resolution—on the order of the tip-diameter. To the best of our knowledge, here we show for the first time that AFM can also be combined with the TPA spectroscopy technique, to achieve co-located topographic information and simultaneously tip-enhanced non-linear TPA signals. The Tip-Enhanced TPA (TE-TPA) technique is more general than TPE because it can work even on non-photoluminescent materials. In this work, TPA and TPE measurements were carried out using an amplified fsec laser with a central wavelength of 1028 nm. The TPA was generated in a transmission geometry using a tuning-fork based AFM from Nanonics that was situated between upright and inverted microscopes. We developed different approaches to observe the tip-enhancement. We then used tip-enhanced TPA and TPE spectroscopy to map the optical and topographic properties of soft semiconductor materials such as organic dyes, thin films of CsPbBr₃ perovskites, a few layered WSe₂, amongst other soft-semiconductors. We observe strong TE-TPE when the material being studied has a prominent excitonic absorption peak centered at near the TPA wavelength of 514 nm, being nearly in “2-photon resonance” with the fsec pulsed excitation. We use a balanced detection scheme with a boxcar integrator to measure differential absorption. We also use an ultra-fast detector to measure TPE. The variation of the TPE signal was measured at different powers for different laser repetition rates. With the TE-TPA technique, we were able to characterize thin films of soft materials, and identify changes in the optical properties at the nanoscale grain boundaries and interfaces.<br/><br/>1. M. Rumi and J. W. Perry, <i>Adv. Opt. Photonics</i>, 2010, <b>2</b>, 451.<br/>2. C. Lee, B. G. Jeong, S. J. Yun, Y. H. Lee, S. M. Lee and M. S. Jeong, <i>ACS Nano</i>, 2018, <b>12</b>, 9982–9990.