Adaptive tip-enhanced nano-spectroscopy

Abstract

Tip-enhanced nano-spectroscopy and -imaging, such as tip-enhanced photoluminescence (TEPL), tip-enhanced Raman spectroscopy (TERS), and others (1-4), have become indispensable from materials science (5-7) to singlemolecule (8-11) studies. However, the techniques suffer from inconsistent performance due to lack of nanoscale control of tip apex structure, which often leads to irreproducible spectral, spatial, and polarization resolved imaging. Instead of refining tip-fabrication to resolve this problem (12-14), we pursue the inverse approach of optimizing the nano-optical vector-field at the tip apex via adaptive optics. Specifically, we demonstrate dynamic wavefront shaping of the excitation field to effectively couple light to the tip and adaptively control for enhanced sensitivity and polarization-controlled TEPL and TERS, with performance exceeding what can be achieved by conventional tip-fabrication

Tip-induced nano-engineering of strain, bandgap, and exciton dynamics in 2D semiconductors

“…We prepare the wrinkled ML crystal on the thin Au film and excite it with a radially polarized beam. Furthermore, we use a spatial light modulator (SLM) for wavefront shaping of the excitation beam, which allows us to obtain the additional enhancement of TEPL signals (see Methods for details) [34]. Figure 1B describes a conventional TEPL experiment excited by a flat wavefront (left) and 𝑎-TEPL experiment excited by the optimal wavefront (right).…”

“…The optimal phase mask of SLM was found via the stepwise sequential algorithm before we perform the distance-dependence TEPL experiment. Once we find the optimal wavefront of the specific Au tip, the wavefront switching can be performed dynamically with a temporal resolution of ∼20 ms [34]. Thus, we obtain 𝑎-TEPL spectra at 𝑑 = 3 nm in Figure 2A and B (purple) by immediately switching the wavefront without affecting to shear-force feedback conditions.…”

“…Figure 2D shows a change in the measured 𝑎-TEPL intensity of a WSe 2 ML during the wavefront shaping process for the different Au tip, which gives ∼200 % larger additional enhancement compared to normal TEPL intensity (see Section S3 for details), with illustration to find the optimal phase mask. As we described in detail in the Method section, we set a random phase first for the 64 segments to prevent the local minimum problem [34] and find the optimal temporal phase of each segment by monitoring 𝑎-TEPL intensity. By repeating this feedback process sequentially for the 64 segments, we can find the optimal phase mask giving the maximum 𝑎-TEPL response.…”

“…To obtain an additional signal enhancement from the conventional TEPL signals, we used the adaptive optics technique, i.e., wavefront shaping of the excitation beam, we recently developed [34]. Basically, since the apex structure of an electrochemically etched tips differs from tip-to-tip, the adaptive wavefront of 𝑎-TEPL gives rise to a larger field enhancement than that of a conventional tip-enhanced nano-spectroscopy using a flat wavefront.…”

“…Here, we present a hyperspectral tip-enhanced photoluminescence (TEPL) nano-imaging approach, combined with nano-optomechanical strain control, to investigate and control the nano-optical and -excitonic properties of naturally-formed wrinkles in a WSe 2 monolayer (ML). In this approach, the excitation field is highly localized at the plasmonic tip by adopting the near-field wavefront shaping technique [34] and the tip selectively probes Purcell-enhanced TEPL emission at wrinkles with <15 nm spatial resolution. From hyperspectral TEPL imaging of nanoscale wrinkles, we observe that uniaxial tensile strain locally alters the radiative emission properties, e.g., an increased quantum yield, decreased PL energy up to ∼10 meV, and a symmetry change in spectral shape.…”