Surface-phonon-polariton-enhanced photoinduced dipole force for nanoscale infrared imaging

Abstract

The photoinduced dipole force (PiDF) is an attractive force arising from the Coulombic interaction between the light-induced dipoles on the illuminated tip and the sample. It shows extreme sample-tip distance and refractive index dependence, which is promising for nanoscale infrared (IR) imaging of ultrathin samples. However, the existence of PiDF in the mid-IR region has not been experimentally demonstrated due to the coexistence of photoinduced thermal force (PiTF), typically one to two orders of magnitude higher than PiDF. In this study, we demonstrate that, with the assistance of surface phonon polaritons, the PiDF of c-quartz can be enhanced to surpass its PiTF, enabling a clear observation of PiDF spectra reflecting the properties of the real part of permittivity. Leveraging the detection of the PiDF of phonon polaritonic substrate, we propose a strategy to enhance the sensitivity and contrast of photoinduced force responses in transmission images, facilitating the precise differentiation of the heterogeneous distribution of ultrathin samples.

Keywords: nanoscale infrared imaging; photoinduced dipole force; surface phonon polariton; ultrathin sample.

Figure 1.

Schematics of photoinduced thermal force (PiTF) and photoinduced dipole force (PiDF). The tip is modeled as an ellipsoid with a length of 2L. The plane wave light is illuminated to the sample with an angle of θ. z is the gap distance from the tip to the sample surface and d is the thickness of the sample.

Figure 2.

PiFs of quartz. (a) Real (dashed black line) and imaginary (solid black line) parts of permittivity of quartz from library data [35] and the calculated effective absorption coefficient (red solid line). (b) Tip-enhanced (red solid line) and global (red dashed line) thermal expansions, analytically calculated by implementing the finite dipole method. (c) PiDF (blue solid line) and PiTF (red solid line) for the surface response, analytically calculated by implementing the finite dipole method. (d) Measured resonance-enhanced contact mode PTIR (AFM-IR) spectrum on pure quartz. (e) The PiFM spectrum, measured by a driving amplitude of 1 nm with the 90% setpoint of the amplitude on pure quartz, and the previously measured s-SNOM amplitude spectrum on pure quartz (blue dashed line) [40]. All the calculation parameters are given in the supplementary data.

Figure 3.

Phonon-polariton-enhanced nano-IR contrast imaging platform. (a) Sketch of the substrate-enhanced nano-IR contrast imaging platform based on a polar crystal substrate under the metallic tip. (b) Typical PiFM spectra were observed on the sample near its IR resonance and on the substrate near the tip-induced near-field resonance. (c, d) Schematics for the imaging of (c) the PiTF of the sample, and (d) the PiDF of the substrate, and a layered sample deposited on a PiDF-dominant substrate, as depicted in (a).

Figure 4.

Nano-IR contrast imaging of PDMS on quartz. (a) AFM image of a PDMS wedge on the quartz surface. (b, c) PiFM images were recorded at 1260 and 1130 cm⁻¹ within the same region, respectively. (d) Comparison of the line cuts of PiFM signals at 1260 and 1130 cm⁻¹ along the green arrow in (c). The scale of the PiFM signal at 1260 cm⁻¹ is inverted to directly compare with the one at 1130 cm⁻¹. (e) COMSOL (dots) and analytical calculation (solid lines) of near-fields at the tip end with respect to the PDMS thickness at 1260 and 1130 cm⁻¹. The calculation parameters can be found in Supplementary Section 5.

Figure 5.

(a) AFM image of layered COFs on c-quartz surface, scale bar: 200 nm. The green arrow indicates the direction for signal extraction in (e). (b, c) PiF images of the same region recorded at 1470 cm⁻¹ (PiTF of COF is dominant) and 1130 cm⁻¹ (PiDF of quartz is dominant), scale bar: 200 nm. (d) PiF spectra collected on the different layers of the COFs layers. (e) Signals along the green arrow of topography and PiF images as indicated in (a).

Figure 6.

Subsurface imaging of deposited block copolymer clusters. (a) Topography of a clustered PS-b-PMMA on a quartz surface. Scale bar: 1 μm. PiFM images recorded at (b) 1730 cm⁻¹ and (c) 1130 cm⁻¹ in the same region shown in (a). The insets show the zoomed-in area corresponding to the red rectangle, with 256 × 256 resolution. (d) Diagnosed nanostructure with nano-IR contrast images. (e) Point PiFM spectra on BCP (blue cross on topography) and quartz substrate (red cross on topography).